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Marine mammal depredation on IPHC standardized setline surveys: a look at killer whales and sperm whales as major depredators in Alaska waters Nicholas Wong1 Abstract Depredation, or the plundering of hooked fish, presents operational challenges and economic loss for fishers. Its suspected rise in prevalence spawns concerns as depredation has important socioeconomic and scientific implications for commercial fishers and resource managers worldwide. Data collected from the International Pacific Halibut Commission (IPHC) standardized longline survey that details marine mammal encounters from 2009 to 2014 are used to analyze spatial and temporal distribution of depredators and reveal depredation characteristics and trends across IPHC charter regions. Assessment of the impact of depredation on catch is exploratory in nature. Killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) remain dominant depredators encountered by IPHC vessels. Killer whales are commonly found in Gulf of Alaska, the Aleutian Islands, and the Bering Sea, while sperm whales are generally distributed along the continental shelf break stretching from the eastern Gulf of Alaska to northern B.C. No trends in depredation rates were observed from 2009-2014. Killer whale-depredated sets were likely to show halibut damage (70% of sets), record a reported low catch (28% of sets), and a decrease both legal and sublegal halibut WPUE (26% and 39%), while sperm whales showed less clear depredating signals and do not appear to significantly affect halibut WPUE. Bent, broken, or missing hook data indicated sperm whales to be potentially more destructive to the gear. A sperm whale-depredated set was up to four times more likely to have damaged gear compared to a non-depredated set, while killer whale-depredated sets received up to three times the normal level of hook damage. Increased gear damage yet less apparent fish damage suggest considerable undetected depredating behavior by sperm whales. Current analysis of depredation’s effect on catch composition shows confounding results; and refinement of data analysis, especially hook counts at the skate level, are discussed for future studies. Outcomes of this study can contribute to an understanding of depredation and shed light on possible solutions to this issue that affects fishers, resource management agencies, and depredating marine mammals. Introduction Background Marine mammal depredation, the removal of fish off fishing gear by marine mammals, has been a well-documented and emerging phenomenon worldwide. Existing literature shows different marine mammal species as principal depredators in different regions of the world. For example, bottlenose dolphins are the main regional depredators for artisanal fisheries in the Mediterranean marine reserves (Rocklin et al. 2009), and the Hawaiian longline fisheries are frequently depredated The IPHC provides opportunities for undergraduate student interns and encourages them to prepare reports of their projects conducted at the IPHC. Reports by interns are included here with only minor editing. 1 418 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 by false killer whales (Pseudorca crassidens) and pilot whales (Globicephala spp.) (Tec Inc. 2009). Anecdotal reports over the past few decades indicate that depredation behavior may be on the rise. There are indications of increased depredation frequency in the 1990’s in Alaska (Peterson et al. 2013, Thode et al. 2006). In other areas, no trend was found in whale-gear interaction rates, which remained stable from 2003 to 2008 in Crozet Islands Exclusive Economic Zone, where frequent depredation occurs (Tixier et al. 2010). In Alaska, toothed whales (suborder Odontocetes) are notorious for feeding off of sablefish and halibut longlines. In particular, killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) are pervasive in Alaskan fisheries (Peterson et al. 2014, Hill 1999; Sigler 2008, Yano and Dahlheim 1994). In the Southern Ocean, depredating sperm whales were more frequently encountered than killer whales in recent years for the toothfish fishery (Clark and Agnew 2010). Occurrence studies indicate Odontocete depredation to be highly variable from year to year in Alaska (Sigler 2008, Hamer 2012). In addition to economic loss for fishers in the affected fisheries, depredation has the potential to affect estimates used for fishery stock assessments and cause injuries or death with depredating marine mammals via gear entanglement. Impact Traditionally, the sablefish (Anoplopoma fimbria), and halibut industries have been most affected by whale depredation since sablefish are part of the natural diet of toothed whales, as are halibut (Sigler and Lunsford 2008, Hill et al. 1999). Toothed whales have also been documented depredating on Pacific cod, arrowtooth flounder (Atheresthes stomias), and Greenland turbot (Reinhardtius hippoglossoides) (Yano and Dahlheim 1994). Information gathered from interviews indicate whales normally appear during gear haul back. Whether it is fishers suffering direct loss in catch or in employing avoidance strategies, depredating behavior costs fishers time, energy, money, and gear. The estimated direct cost from fish loss ranged from $2,982 to $34,571 per 4-month season in the Prince William Sound sablefish fishery from 1982-1988 (Yano and Dahlheim 1994). Peterson et al. (2014) estimated additional fuel and gear costs of $433 per depredation event and $522 in opportunity cost per additional vessel day due to depredation for sablefish fisheries. Killer whale depredation significantly lowered overall catch rate for six groundfish species from 9-28%. When killer whales were present, up to 72% of sablefish and 51% of halibut were removed from the fishing gear (Peterson et al. 2013). There are various possibilities that may contribute to the suspected rise of depredation. First, whales elevated status as protected species, along with conclusion of whaling may have recently increased whale population worldwide even though they are still at risk (Taylor et al. 2008). A trend towards long individual fishing quota (IFQ) fishing seasons as opposed to short term derbystyle fishing exposes more hooked fish over a longer season, thus allowing more opportunities for depredation. Lastly, whale transmission of intelligent and learned behavior may have also made vessels more vulnerable to depredation. Whale intelligence was demonstrated as sperm whales were shown to spread depredation behaviour via social transmission through the Gulf of Alaska region (Schakner et al. 2014). In addition, video footage shows sperm whales interacting with fishing gear on bottom and therefore out of view, in which fish are shaken off the fishing lines, and leaving no obvious evidence (SEASWAP2, unpublished). For toothed whales, the inherent risk of gear entanglement can lead to injuries and/or death (Garrison 2007). Meanwhile, whales face the indirect risk of fishers’ aggression or retaliation in response to depredation. Ecologically, Sperm whale and longline interaction. http://seaswap.info/ 2 419 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 this continual behaviour can potentially shift their normal feeding habit outside their natural parameters, and may even lead to an artificial enhancement of their populations (Plaganyi and Butterworth 2005, Gilman et al, 2006). As a management body for the halibut stock, the IPHC also experiences a potentially growing challenge as the IPHC setline survey program takes place on the same grounds as the fishery and where killer whales and sperm whales are prominent. Sperm whales were shown to display novel foraging (depredating) behaviour in the Gulf of Alaska (Shackner 2011), while killer whales are distributed along the western Gulf of Alaska, Aleutian Islands, the Bering Sea, and Prince William Sound (Yano and Dalheim 1995, Zerbini et al. 2007). Depredation by marine mammals can affect estimates of stock abundance when it interferes with the fundamental method of collecting fish data by taking fish off longline gear. When depredation rates are dynamic or changing, there are further long-term implications for annual stock assessments. Finally, costs associated with fish loss, gear damage, and avoidance increase for the survey program. IPHC standardized annual setline surveys Since 2009, the IPHC has been collecting depredation data during its annual standardized setline survey. An initial reporting of depredation occurrences summarized the first two years (2009 and 2010) of data collection (Dykstra and Soderlund 2010). Adding in subsequent years of depredation data has provided a more sufficient sample size to analyze the extent of depredation and offers an opportunity to identify trends. The IPHC depredation monitoring process provides useful information on this widespread phenomenon as its setline surveys resemble commercial fisheries in some important ways. Targeting Pacific halibut, IPHC research vessels have extensive coverage that overlaps with productive fishing grounds, where most commercial fisheries take place. The method of survey fishing is by longline gear, which is also used by the sablefish and halibut fisheries. However, the standardized survey differs from commercial fisheries in that it only fishes at each station once per year, whereas commercial fishing may take place at the same location multiple times in a given year. Moreover, commercial fishers may be constrained to continue hauling gear (weather, crew availability, etc) when whales arrive to maintain profitability, which deviates from the research objectives and protocols of IPHC survey vessels. The IPHC similarly became increasingly aware of depredation though communication with research vessels as part of an annual setline survey, as well as with commercial fishers. Therefore, it is the objective of this data review to 1) assess temporal and spatial distribution of depredation events across all survey stations, 2) determine if there are trends in depredation rates and general depredator tendencies, and 3) investigate the impact depredation has on gear damage and catch rates during the IPHC setline surveys. The results generated from these data may better address the ever-rising concern over its prevalence, shed light on future mitigation and avoidance strategies and in so doing, add to a fuller understanding of marine mammal depredation. Methods Depredation forms Since 2001 samplers aboard IPHC survey vessels have been completing marine mammal sighting forms for the U.S. National Marine Mammal Laboratory (NMML), documenting each time that marine mammals were sighted from survey vessels, including during transit. Minimal information regarding mammal interaction with the actual fishing operations can be obtained from 420 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 this data set; however, general spatial patterns of the types and numbers of mammals encountered by the survey can be generated. In light of depredation events, the IPHC staff and the Research Advisory Board (RAB) concluded that fishers’ logbooks capturing depredation information are subjective in nature and cannot reliably be used to estimate the impact of depredation on catch rate. Thus, the Board advised against the use of logbooks for estimating depredation (Hutton 2008). Starting in 2009, samplers on IPHC survey vessels began tracking depredation systematically, which included samplers completing a form that details observed or suspected depredation events at each occurrence. Specifically, during gear haul back, samplers recorded 1) a marine mammal (Pinniped or toothed whale) approach within 100m of the vessel, even if there were no signs of depredation and 2) suspected gear interference. An IPHC depredation form contains the following information: Depredator o species o number of depredators sighted o closest approach and whether it was within 100 m of the sector (location relative to vessel) in which depredator(s) were observed o time, visibility, hook numbers at first and last sightings Vessel (Fig. 3b) o sector in which the gear was being hauled (sectors are additive) o sector in which offal was discarded (sectors are additive) Catch o number of damaged halibut (those with physical bite marks or tissue missing) o bycatch species showing damage o existence of apparent reductions in halibut catch rate relative to perceived rate of capture prior to the arrival of the depredator(s) o whether actual depredation of halibut or bycatch was witnessed o whether the depredator was seen feeding on discards o whether depredator was caught, snagged, or entangled in gear A vessel sector chart is provided on the depredation set form that defines sectors of boat using an additive number system. Numbers are added together to describe sectors. As well, there is space for samplers to leave comments. An example of the depredation set form can be found in the 2015 standardized stock assessment survey manual (IPHC publication). Chi-squared analysis and Fisher’s exact test are used as standard statistical tests to assess occurrences within regions. Gear damage and catch information In conjunction with the depredation form, the number of bent, broken, and missing hooks is recorded by skate across all survey stations. This information provides a background level of gear damage frequency. The established baseline can then be used to compare to depredated sets to analyze depredation impact on gear. As part of standardized survey data collection, bycatch species are recorded on a 20-hook count, in which information about the first 20 hooks of each skate is recorded. To examine if halibut weight-per-unit-effort (WPUE) drops off when a whale arrives, only depredated sets in which the whale arrives after the gear has begun hauling are utilized. WPUE is calculated for both O32 legal size fish and U32 sublegal size fish at the skate level. Subsequently, the WPUE while whales are present and WPUE while whales are absent can 421 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 be compared. For catch analysis, a one-tailed, paired t-test is utilized to examine the hypothesis that whale presence does not cause lower WPUE. In addition, because of variability in the number of skates fished per year, slight variations in hook numbers on each skate (~100 hooks), and some stations being whole-hauled (recorded for each hook), a standardized value is assigned through extrapolation using actual hook counts and an effective set number. The adjusted number yields an extrapolated value per skate for each recorded hook in examining hook information between depredated sets and non-depredated sets. Considerations A set is the deployment of longline gear at a particular time at a particular station; each station is fished once. Occasionally, some survey stations may be reset if they are deemed ineffective using standard, pre-determined survey criteria. In this study, number of stations fished will be accounted for. A station is determined to be ineffective when gear did not fish properly, i.e. if catch is not representative of what might have been caught under normal circumstances. Currently, for the IPHC to consider a set’s data ineffective due to whale depredation, whales must be present during gear haul back and the sum of damaged gear and catch are greater than 10% of hooks set. Each species is considered as an observed depredation event, even if they are present on the same station. In 2014, two charter regions were expanded, leading to more stations in those regions. As a result, charter region Unalaska was divided into Four Mountains (East and West) and Unimak (East and West), and 4A Edge was divided into 4A Edge North and 4A Edge South. Also due to the expansion, some stations fished at deeper depths (~400 fm) as opposed to typical depth (20-275 fm). 2014 expansion stations in Unalaska and 4A Edge will not be used for trend analysis but are still considered in describing depredation events. As they are still the most commonly observed group seen interacting with vessel gear, killer whales and sperm whales remain the primary focus of this study. Data regarding the two species are analyzed separately as there are discernible differences. Statistical analysis uses Microsoft Excel and the statistical software program R (v. 3.2.1) for current study. Results Spatial and temporal IPHC chartered vessels conducted the annual survey from May to September of each year. From 2009 to 2014, there were 337 instances where a marine mammal species was seen during gear hauling across all stations fished. Killer whales and sperm whales were the most frequent depredators on IPHC setline surveys, comprising more than half of all observed depredation events (54.9%, Table 1). Other species observed were Dall’s Porpoise (Phoncoenoides dalli) and occasionally Pacific white sided dolphins (Lagenorhynchus obliquidens). Of the Pinniped depredators, Steller sea lions (Eumetopias jubatus) were the most frequently seen and showed relatively overt damage signals. Steller sea lions were considered a distant third major depredator behind the Odontocetes. Historically, sea lions have occasionally presented a depredation problem (Dykstra and Soderlund 2010). Of the 337 depredation events, seven were co-occurrences of two mammal species at the same station, one of which saw both killer whale and sperm whale presence. Over the six years, there were four ineffective stations because of whale depredation. Overall depredation rates remained stable with depredation seen on 3.2% to 5.2% of all stations for each year (Table 2). There was an average of 16.7 killer whale depredation events (n=100, SD=8.3) 422 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 and 14.2 sperm whale depredation events (n=85, SD=3.4) per year, i.e., 0.84% to 2.44% of all stations per year for killer whales, and 0.63% to 1.41% of all stations per year for sperm whales (Table 3a). It should be noted that variability for killer whales was generated by a high occurrence of depredation due to expansion in killer whale prominent regions of Unalaska and 4A Edge in 2014. Killer whales are commonly encountered in the Aleutian Islands, Bering Sea, western Gulf of Alaska, and Prince William Sound (PWS). Sperm whales have a wider distribution, stretching from the eastern Gulf of Alaska to northern B.C., along the continental shelf break (Fig. 1a). This spatial distribution generally agrees with existing literature for natural whale distribution (Thode et al. 2006). Over six years, there was one instance of gear entanglement with harbor seals (Phoca vitulina) and sperm whale, and two for Steller sea lions during IPHC standardized setline surveys. While both killer whales and sperm whale depredation occurred on around 1-2% of all stations per year, occurrences ranged from 0% to 16.9% for killer whales (Unalaska) and 0% to 16.7% for sperm whales (Seward) in certain charter regions in a given year (Table 3a). When adjusted for expansion stations, sperm whales showed more variability in their occurrences. In 2014, the number of killer whale interactions increased dramatically (Fig. 2a). This coincided with the survey station expansion in the Unalaska and 4A Edge charter regions, which is a documented active killer whale area. When adjusted for all expanded stations from 2009 to 2014 (Fig. 2b), there was no evidence that the proportion of stations with whale depredation varied among years for either killer whales (χ25= 2.7643, p=0.7363) or sperm whales (χ25=5.4038, p=0.3686). IPHC charter regions 4A Edge, 4D Edge, PWS, Unalaska, Fairweather, Seward, and Yakutat represent Odontocete hotspots, where 69% (128 of 185) of whale depredation events took place (Table 3a). No hotspot regions3 except Seward showed evidence that the proportion of whale depredated stations was dependent upon the year. (Fisher’s exact test, KW depredation in: 4A Edge (p=0.118), 4D Edge (p=0.3976), PWS (p=0.2043), Unalaska (p=0.1408); SW depredation in Fairweather (p=0.4556), Yakutat (p=0.1153)) (Table 3b). The rate of depredation was dependent on the year and highly variable for number of sperm whale depredation events in Seward (Fisher’s exact test, p<0.001). However, there was no apparent trend over time. Whale depredation characteristics Generally, sperm whales approached closer to vessels than killer whales. Killer whales appeared in pods, whereas sperm whales tended to be more solitary when they approached vessels (Table 4). During depredation events, sperm whales tended to be in sector 4 or the lower right quarter between the starboard and the stern of the vessel, while killer whales tended to be at the starboard side (sector 1) and the bow (sector 6) more often (Fig. 3a). The individual sectors are 1,3,5, but the observations of a whale that moves through multiple sectors are additive. So for example if the whale was on the starboard (1) and the stern (3) during a depredation event, it would be recorded as sector 4. Qualitative in nature, the five main criteria used to document depredation are: 1. observed halibut damage 2. observed feeding on halibut 3. observed bycatch damage 4. observed feeding on bycatch 5. reported sudden, significant drop in halibut catch Analyses done after excluding expansion stations. 3 423 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 In all, 75% of killer whale depredated sets showed at least one of the criteria, compared to 39% of sperm whale depredated sets, and 32% of all other marine mammal depredated sets by killer whales showed a higher incidence of halibut damage (70%) than sperm whales (24%) during their respective depredation events (Table 4; χ21=37.879, p<0.001). There was no significant speciesspecific difference in bycatch damage between killer whale (19%) and sperm whale (16%) (Table 5; χ21=0.18742, p=0.6651). However, there is evidence that killer whale depredation events were more likely to have a reported decline in catch (28%) than sperm whale events (9%) (Table 6; χ21= 9.295, p=0.002). Sablefish, arrowtooth flounder, and Greenland turbot were the top bycatch species showing damage, with sablefish as the most common. Killer whales were associated with damaged arrowtooth flounder, Greenland turbot, grenadier, Pacific cod and sablefish. Damaged bycatch was often observed alongside halibut damage on these sets. Sets that were depredated by sperm whales tended to have less diverse damage, with only sablefish damage and one instance of longnose skate. Halibut damage was less frequently seen with sablefish damage for sperm whales. Gear and catch evidence In areas of high whale depredation, depredated sets showed more gear damage than background levels. Killer whale depredated sets had 1.61% damaged gear per skate in 4A Edge, 4D Edge, Unalaska, and Sitka compared to a non-depredated 0.46% baseline level in those same areas, i.e. three times as likely to encounter a bent, broken, or missing hook when whales were present (Fig. 4). Sperm whale depredated sets showed 2.08% damaged gear per skate in Fairweather, Seward and Yakutat compared to a 0.43% baseline in those same areas, i.e. it is four times as likely to encounter a bent, broken, or missing hook. Sets exhibited more gear damage when both whale species were present. In Seward, where most of the sperm whale depredation took place, bent, broken, or missing hooks were 4.4 times more likely to occur (Fig. 4). Other mammals appeared to show high instances of gear damage but sample sizes varied greatly in those areas with high whale occurrences. When only whale-depredated sets were analyzed, killer whale depredation negatively impacted halibut catch on skates when for which they were present during gear haul-back. Legal sized (O32) halibut WPUE dropped 38.8% (Fig. 5; t= -2.902, df= 76, p=0.0024) on skates when killer whales were present. The same was true for killer whales’ effect on U32 halibut, where the average WPUE dropped 26.3% when they were present (Fig. 5; t=-2.822, df= 76, p=0.003). In sperm whale depredated sets, their presence seemed to not affect halibut WPUE. WPUE for U32 halibut dropped (11.0%) but not significantly when sperm whales were present on the set (Fig. 5; t=-1.383, df= 63, p=0.0858). WPUE increased for O32 halibut when whales were present on those sets (Fig. 5). Catch composition obtained from extrapolated hook counts in Unalaska showed killer whale depredated sets had lower numbers of sablefish, halibut, and Pacific cod, more prominent damaged (BBM) hooks, fewer hooks with bait, but lower empty hook counts than non-depredated sets. In the 4A Edge region, killer whale depredated sets had lower numbers of sablefish and Pacific cod but higher numbers of halibut, more empty hooks returned, and fewer hooks with skin and bait. In both regions, damaged gear and grenadier count were higher for killer whale depredated sets (Fig. 6a). Likewise, catch composition data in Regulatory Area 3A, where high sperm whale activity was reported, were examined to yield an extrapolated hook count. In Seward, sperm whale depredated sets exhibited noticeably higher numbers of sablefish and Pacific cod, a lower halibut count, fewer 424 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 hooks with bait, and fewer empty hooks than non-depredated sets. In the Fairweather region, sperm whale depredated sets had noticeably higher counts of empty hooks, a comparable number of halibut, a higher sablefish count, and a lower Pacific cod count than non-depredated sets. In both regions, the depredated sets, again, were associated with more damaged (BBM) hooks and fewer hooks with bait (Fig. 6b). Only two visible patterns from the catch composition data can be inferred in relation to depredation. First, whale depredated sets continually showed a higher proportion of damaged (BBM) gear (Figs. 6a and 6b). Second, sperm whales depredated sets showed a higher proportion of sablefish (Fig. 6b). Discussion Primary findings For the years 2009 to 2014, the impact of depredation on IPHC setline surveys appears to be low. With no apparent signs of increasing frequency, overall depredation appears to be stable. Similarly, whale depredation occurrences in regions with high whale activity showed no significant signs of increasing, even when depredation rates varied. With four confirmed sets deemed ineffective due to depredation by whales and one ineffective station in 2014 due to unknown depredation while having whales present, data loss due to depredation appears minimal. While depredating behaviour can decrease WPUE, it is generally not extensive enough to rate most affected stations ineffective under current IPHC criteria. Continual presence of killer whales in Area 4A and 4D and of sperm whales in Area 3A along the shelf break of the Gulf of Alaska agrees with existing literature on whale distribution and can give insights about specific problematic stations. Halibut WPUE analysis suggests catch reductions at whale-depredated sets are species specific. For killer whales, their presence in confirmed depredation events reduced catch by 26.3% for sublegal and 38.8% for O32 halibut. Halibut WPUE associated with sperm whale depredation showed mixed signals, and other indicators such as bycatch damage and percentage of damaged gear should be considered when assessing sperm whale depredation. Previous IPHC studies have noted the difference in depredation between killer whales and sperm whales (Dykstra and Soderlund 2010). Like the 2010 IPHC report, killer whales generally showed clear depredation signals. Killer whale associated depredation consistently left more evidence of fish damage (especially halibut), led to decreased catch, more bent, broken, and missing hook counts, and a distinct drop in WPUE (in both O32 and U32 halibut). It should be noted that the degree of damaged fish is not subjectively recorded, hence only the presence or absence of damaged fish is known. For example, 70% of killer whale-depredated sets showed overt signals of damaged halibut, but the extent of damage was not detailed. Thus, WPUE was used to understand the impact on catch quantitatively. Damage signals as recorded on depredation forms categorized marine mammal depredation, but only provided limited catch information. That being said, all evidence surrounding killer whale depredation events pointed to killer whales having an undeniable impact on halibut catch. In contrast, sperm whales show significantly less halibut damage, but comparable levels of bycatch damage, and fewer reports of lowered catch. The less obvious damage on catch by sperm whales relative to killer whales was similarly documented in the depredation study in the southern ocean (Kock et al. 2005, Clark and Agnew 2010). It is noteworthy that sablefish was the most often damaged bycatch associated with sperm whales, whereas killer whales showed more diversity in bycatch damage. A possible preference for sablefish and relatively variable depredation agrees with existing knowledge (Sigler et al. 2008). 425 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Preliminary hook analysis of sperm whales showed that sperm whale depredated sets had a higher proportion of sablefish (Fig. 6b). Hence, it is very possible that sperm whales congregate around areas that have more sablefish. Killer whale-depredated sets showed both noticeably lower proportions of halibut and sablefish, which may attribute to a lower halibut WPUE. The combination of the greater number of empty hooks and lower numbers of hooks with baits and skin may be a rudimentary gauge for fish loss due to depredation. However, current analysis does not demonstrate any relationship between empty hooks, hooks with bait, and hooks with skin with different species catch rates. Still, it is advisable that proportion of BBM and hooks with skin be weighed against adjacent catch rate data when undertaking depredation analysis. It may also be useful to take the amount of empty hooks, hooks with bait and hooks with skin into consideration when studying sperm whales, due to their tendency to leave weak fish damage evidence. From this study, it continues to be challenging to estimate the real extent of depredation and even harder to quantify fish loss. Although a 20-hook count provides an accurate representation of full-set information (Menon 2004), there are confounding variables in assessing coastwide depredation rate (Table 2). The problem of the hook count analysis is two-fold. First, depredation generally happened in concentrated areas along the stations found on the outer edge of a chartered region, while the remainder of the region is not depredated but are characterized jointly (Figs. 1c and 1d). Any difference seen among the depredated stations and the non-depredated stations can also be due to landscape/environmental dissimilarities. That could mean geographic patchiness, varying depths, different habitat types, and perhaps variable productivity. Hence, all stations within a chartered region cannot be assumed to be equal. Second, a depredator’s evidence that is left behind cannot reliably be used to deduce a set’s appeal to whales because of certain traits. For one, pre-existing differences may be due to geographical differences in species distribtutions, as mentioned. It’s possible that having certain species in higher proportion was what attracted the whales, or whales may have caused the change in proportion of species by depredating before observations could be made. There is simply no way to tell what a set looked like before a whale had depredated on it. More analysis will be needed to account for factors like environmental variation to investigate the true impact of depredation. Limitations There are multiple limitations to this study. Overall depredation rates include some expansion stations that only exist for certain years for the survey program4. Ineffective stations due to all reasons were not excluded as there were depredated stations that were ineffective because of reasons other than whale depredation. Depredation analysis did not account for soak time, depth, and geographical/regional disparity. Additionally, whale depredated sets with perpetual depredator presence did not provide a relevant whale-absent WPUE and were omitted. Furthermore, halibut WPUE analysis was coarse, as depredator presence was recorded at the exact hook number but halibut was tracked only to the skate level. For example, a whale depredator is observed at hook 75, but the entire skate (hooks 1-100) would be considered “whale-present”. IPHC Stock Assessment Besides decreased profitability and lowered catch rate, the removal of fish off longline gear during retrieval is inherently problematic in several ways for survey programs trying to estimate 2010 and 2011: special tagging stations; 2011 and 2013: expansion in 2A; 2014: expansion in 2A and 4A. 4 426 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 stock abundance. The most conspicuous way depredation can seriously affect assessment is that when damage is extensive, a set becomes ineffective and will not be included in the stock assessment. Over time, the IPHC might repeatedly lose data on stations that are heavily depredated over and over again. It may also lead to loss of data on particularly productive fishing grounds. Damaged fish provides incomplete information about length, sex, and maturity. When damaged gear is retrieved, it is unknown whether depredation has taken place or damage was a consequence of other environmental factors. Even more so, when an empty hook is returned, one cannot eliminate the possibility of a removed fish by depredator. Sperm whales have been observed shaking fish off the line without direct hook contact, hence contributing to the possible unseen effect (SEASWAP, unpublished). Sperm whale presence corresponded to less apparent fish damage, increased gear damage, and insignificant effects on halibut WPUE (Figs. 5 and 6), making sperm whale impact more difficult to decipher. In contrast, killer whales’ propensity to leave strong evidence provides straightforward signals making them generally reliable to identify depredators. In some ways, even though they incur more damage, killer whale depredating behavior is obvious and therefore more quantifiable, which can have value as a potential correction factor. Whether depredation occurred obviously or inconspicuously, the resulting fish mortality is accounted for in the stock assessment. This is because any mortality affects the number of fish at each age. Nevertheless, the source of mortality may not be attributed correctly. Since fish were already hooked, fish loss due to depredation should first be considered as fishing mortality and not necessarily natural mortality. Moreover, depredation mortality can contribute to a depleted stock estimate. The removal of an unknown quantity of fish may bias an original stock estimate, thus leading to a lower perceived estimate of the stock. Another complication of depredation is whether the rate of depredation is dynamic. Should there be a rise in proportion of whale depredating behaviour, an assumption that depredation rates remain stable can lead to stock overestimates. A changing dynamic of depredation occurrences makes effort to account for depredation difficult on a yearly basis. As of 2014, the rate of depredation appears to be stable and no specific trend can be indicated as encountered by IPHC survey vessels. Still, this does not preclude the possibility of a future rise in depredation. There are also complex and unforeseen effects that depredation can exert on the stock assessment. One such example is the potential that the proportion of adult and juvenile halibut being depredated is different, and that can have further complications on stock abundance indices. Modification Sets that suffer from depredation, even if effective, can be difficult to assess. The IPHC’s criteria of deeming a station ineffective due to depredation may need to be re-visited due to possible unseen effects, especially from findings of sperm whale’s ambiguous signals. The predetermined rule that designates ineffective due to whale depredation (the sum of damaged catch and damaged gear exceeding 10%) may need to be adjusted downward. Further review of depredation data and evaluation of unseen effects may call for a more stringent threshold in light of sperm whale’s weak fish damage signal but strong gear damage signals. A more rigorous guideline may help account for unseen damage by sperm whales’ deep-diving, depredating behavior. Furthermore, since killer whales and sperm whales display noticeably different patterns, it may be beneficial to have different criteria that screen each species separately, such as incorporating damage effects for sperm whales. Similarly, as depredators may appear only for a portion of a set, individual skates could be given ineffective codes so as to avoid total discard of whole data sets. 427 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 A possible way to monitor whales is the use of hydrophones when setting or hauling gear. Refinement of data collection such as keeping track of damaged bycatch such as sizes and halibut characteristics can shed further light on species-specific differences. Natural variation in landscapes (i.e., patchy areas) is one of the factors that sometimes causes catch rates to drop. Large scale data analysis must first be done to distinguish “patchy” or areas that traditionally have low yield before one can deduce a catch rate drop is specific to depredation. A background WPUE at a skate level that accounts for patchiness can then be used to compare to depredated levels to precisely assess real impact of depredation on halibut WPUE. Moving forward, it would be feasible to include bent, broken, or missing hook data, and proportion of empty hooks in combination with fish damage when deciding a set’s effectiveness. Mitigation and prospect for depredation Often, fishers are faced with the tough decision between continuing to haul gear despite depredation, or dropping the set until depredating whales disappear. When fishers stop hauling they are incurring extra costs in time and energy to avoid whales and there is no guarantee the whales will not reappear when hauling resumes. Noises produced by vessels’ engines, gear hauling, and particularly propeller cavitation may serve as a “dinner bell” for whales as an opportunity to forage off fishing gear (Thode et al. 2006). It would be in the fishers’ best interest to implement effective, yet safe avoidance or mitigation methods. At the same time, whales would face less risk with gear entanglement and injuries. Recently, efforts have been initiated that show potential to deter depredating marine mammals such as an Australia Department of Environment project that tested different mitigation methods of depredation (Hamer et al. 2010). Beginning in 2003, the Southeast Alaska sperm whale Avoidance Project (SEAWAP) initiated a project to study and lower the impact of sperm whale depredation on demersal sablefish longlines. The findings of the project showed that satellite tagging and acoustic tracking of whales were highly viable avoidance methods (Thode et al, 2006). Acoustic deterrent methods have not yielded any significant results; it has also been suggested that mammal intelligence would overcome temporary deterrence created by acoustic devices (Werner et al. 2015). Overall, although acoustically tracking whale depredating behavior remains feasible, active acoustic harassment devices are not preferred because of an adverse effect on the overall noise level in the marine environment (Hamer et al. 2010) On the other hand, depredation mitigation devices (DMD’s) using physical means to protect hooked fish have been tested for efficiency. The DMD’s tested were called the the spider and the sock, and are both meant to slide down and cover the catch once fish is hooked. Although both were effective in protecting catch, with the spider performing better, entanglement dramatically increased hauling time (Rabearisoa et al. 2009). Again, while physical modification or active deterrent devices (ADD’s) show some favorable results for the Patagonian toothfish fishery in reducing killer whale depredation (Moreno et al. 2008), it might ultimately be expensive and difficult to implement over the long term. Werner et al. (2015) comprehensively evaluated and summarized current depredation mitigation strategies involving prevention, monitoring, acoustic, and physical methods as they apply to pelagic or demersal longline fisheries. Criteria that should be used that consider a method’s mitigating potential includes: degree of depredation reduction, effect on target species catch, technological viability, and overall practicality (Werner et al. 2015). It has been communicated through internal discussions that whales are shown to avoid snarls, which are tangled gear along the longline. This effect may be a product of unusual acoustic signal 428 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 that snarls create for some whale species (McPherson 2003). There have been reports where hooked fish within a snarl stay untouched while fish outside the snarl were depredated (IPHC, internal discussions). Possible deterrents may center on the snarl-avoiding observation that combines acoustic measures and physical gear alterations. As suggested by Werner et al. (2015), the best process for research and development of an effective mitigation method should be a multidisciplinary collaboration between fishers, scientists, and industry engineers, much like the tangible work generated by SEASWAP. Additionally, as shown by the different feeding pattern of sperm whales and killer whales, mitigation methods may be tailored to a specific depredating species as well as for different targeted fisheries. Fundamentally, food is a major motivator for the repeated depredating behaviour. When whales are present, it is suggested that fishers not feed or discard offal, not set or haul gear and stop hauling and drop gear until whales are gone. It is also advisable to set fake gear and “dummy” stations. Finally, a general avoidance strategy that couples tagging and an online whale-notification network among fishers can be immensely useful in avoiding extra costs and in avoiding any gear interaction with marine mammals. With the recent passage of rules to allow for sablefish pot fishing to combat the rising concern of whale depredation in Alaska, fishers have suggested a possible depredation shift towards other fisheries, in particular the halibut longline fishery. The pot fishing method largely protects the catch from whales. The rising concern will again be a topic of interest, especially to the remaining longliners. The IPHC will continue to gather depredation data and monitor depredation in the immediate future. It should be emphasized that the current study focuses on depredation as encountered during IPHC setline survey. Even though fishing grounds are overlapped with the survey, IPHC vessels had the added option of moving to different fishing stations or cease fishing for the day when whales are encountered, that commercial fishers may not afford. Commercial fishers likely suffer more from depredation as they tend only to be in certain productive fishing grounds and have longer gear haul time. When no overt damage is shown, the quantity of fish being taken is unknown unless underwater cameras capture it. Sperm whale depredation is uncertain, for the most part, when it comes to returning empty hooks, as is their true impact on the stock. Future studies may involve case studies of depredated whole-hauled set (hook-by-hook information) that allow depredator presence to correspond to exact hooks for halibut WPUE analysis. In that way, unlike the current study, analysis is more precise in that may produce clear-cut depredator-absent WPUE and depredator-present WPUE. There is a clear need to employ generalized linear models for future studies when determining true impact on catch, as well as analyses involving linear regression. For all fisheries, even having non-significant losses does not mean zero losses; for smaller fishing operations, damage could be more devastating. Further examination of existing data sets can prove to be meaningful in engaging the whale depredating problems as encountered during IPHC setline surveys, as well as contributing to an understanding of depredating behaviour. Current study results can better inform future depredation studies and even offer insights into possible solutions. References Clark J. M. and Agnew, D. J. 2010. Estimating the impact of depredation by killer whlaes and sperm whales on longline fishing for toothfish (Dissostivhu eleginodes) around South Georgia. CCAMLR Science. 17:163-178. 429 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Dykstra, C. L. and Soderlund, E. 2010. Categorizing marine mammal depredation on IPHC standardized setline surveys. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2010: 435-444. Garrison, L. P. 2007. Interactions between marine mammals and pelagic longline fishing gear in the U.S. Atlantic Ocean between 1992 and 2004. 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CCAMLR Science. 15: 79-91. Peterson, M. J., Mueter, F., Criddle, K., and Haynie, A. C. 2014. Killer Whale Depredation and Associated Costs to Alaskan Sablefish, Pacific Halibut and Greenland Turbot Longliners. Plos One. 9(2): 1-12. Peterson, M. J., Mueter, F., Hanselman, D., Lunsford, C., Matkin, C., and Fearnach, H. 2013. Killer whale (Orcinus Orca) depredation effects on catch rates of six groundfish species: Implications for commercial longline fisheries in Alaska. ICES Journal of Marine Science. 70(6): 1220-1232. 430 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Plagnyi E. E. and Butterworth D. S., 2005. Indirect Fishery Interactions, Reynolds J. E., Perrin, W. F., Reeves, R. R., Montogomery, S., Ragen, T. J., ed., Marine Mammal Research, John Hopkins University Press, Maryland, p.19-21. Rabearisoa N., Bach, P., Lucas, V., Giroux, F., Vely, M., Romanov, E., Tixier, P., and Guinet C. 2009. 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Interactions of Patagonian toothfish fisheries with killer and sperm whales in the Crozet Islands exclusive econmic zone: an assessment of depredation levels and insights on possible mitigation strategies. CCAMLR Science. 17: 179-195. Werner, T. B., Northridge, S., Press, K, and Young, N. 2015. Mitigating bycatch and depredation of marine mammals in longline fisheries. ICES Journal of Marine Science. 72(5): 1576–1586. Yano, K., and Dahlheim, M. E. 1994. Killer whale, Orcinus orca, depredation on longline catches of bottomfish in the southeatern Bering Sea and Adjacent waters. Fishery Bulletin 93:355-372. Yano, K., Dahlheim, M. E. 1995. Killer whale (Orcinus orca) depredation on longline catches of bottomfish in the southeastern Bering Sea and adjacent waters. Fish. Bull. 93(2): 355-373. Zerbini, A. N., Waite, J. M., Durban, J. W., LeDuc, R., Dahlheim, M. E., and Wade, P. R. 2007. Estimating abundance of killer whales in the nearshore waters of the Gulf of Alaska and Aleutian Islands using line-transect sampling. Marine Biology. 150(5): 1033-1045. 431 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Table 1. Number of instances where marine mammal species were present at a station during gear hauls, from 2009 to 2014. Year Marine Mammals 2009 2010 2011 1 2 California sea lion 2012 2013 2014 Total 3 Dall’s porpoise 21 10 15 7 7 3 63 killer whale 14 14 11 14 12 35 100 8 2 8 7 2 6 33 18 17 12 8 14 16 85 5 6 10 5 11 5 52 Pacific white sided dolphin 1 7 8 Harbour seal Total 1 2 3 51 67 Northern fur seal sperm whale Stellar sea lion 66 41 46 65 337 Table 2. Overall coast wide depredation rate, by year1. Year 2009 2010 2011 2012 2013 2014 Total Depredation rate 5.2% 4.1% 5.1% 3.2% 3.5% 4.5% 4.3% Includes ALL IPHC setline survey stations. 1 Table 3a. Percentage of all IPHC setline survey stations at which killer whales or sperm whales were observed, by charter region and year. Whale killer whale sperm whale Region 4A Edge 4D Edge PWS Unalaska Coastwide Seward Yakutat Fairweather Coastwide 2009 7.0% 1.5% 6.7% 4.5% 1.09% 16.7% 11.8% 2.0% 1.41% 2010 0% 4.4% 2.2% 12.1% 1.11% 10.4% 5.9% 2.0% 1.35% 432 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Year 2011 3.5% 1.5% 0.0% 9.1% 0.84% 0% 2.0% 4.0% 0.91% 2012 7.0% 1.5% 2.2% 4.5% 1.10% 0% 0% 0% 0.63% 2013 1.8% 0% 0% 7.6% 0.92% 2.1% 3.9% 6.1% 1.08% 2014 8.7% 0% 0% 21.8% 2.44% 6.3% 5.9% 6.1% 1.11% Table 3b. Percentage of IPHC setline survey stations at which killer whales or sperm whales were observed, by charter region and year, excluding special tagging and expansion stations. Whale killer whale sperm whale Region 2009 7.0% 1.5% 6.7% 4.5% 1.10% 16.7% 11.8% 2.0% 1.42% 4A Edge 4D Edge PWS Unalaska Coastwide Seward Yakutat Fairweather Coastwide 2010 0% 4.4% 2.2% 12.1% 1.03% 10.4% 5.9% 2.0% 1.35% Year 2011 3.5% 1.5% 0.0% 9.1% 0.86% 0% 2.0% 2.0% 0.86% 2012 7.0% 1.5% 2.2% 4.5% 1.10% 0% 0% 0% 0.63% 2013 1.8% 0% 0% 7.6% 0.94% 2.1% 3.9% 6.1% 1.09% 2014 8.9% 0% 0% 16.9% 1.48% 6.3% 5.9% 6.1% 1.25% Table 4. Average and nearest approach to IPHC setline survey vessels, and average and maximum number of individuals present when whales were observed. Depredator Avg approach (m) Closest approach (m) Avg. no. whales per event Max. no whales at one set Tot. No. interactions killer whale 61.7 3 4.2 11 100 sperm whale 47.2 1 1.9 12 85 433 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Table 5. Percentage of depredated sets with damaged catch when whales are present during haulback by regulatory area. Excludes instances of co-occurrence. 4A 3A Coastwide Depredator Dam. Halibut Dam. bycatch Dam. Halibut Dam. bycatch Dam. Halibut Dam. bycatch killer whale 76% 19% 58% 8% 70% 19% sperm whale 0% 0% 35% 20% 24% 16% Table 6. Percentage1 of depredated sets with reported halibut catch decline after whales arrive, for all regions combined. Depredator killer whale sperm whale 1 % sets with whale activity and reported decline in catch 28% 9% One killer whale depredation instance where catch decline information was not provided. 434 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Figure 1a. Marine mammal sightings from 2001-2014 collected by IPHC samplers using NMML’s form, by regulatory area. 435 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Figure 1b. Marine mammal depredation from 2009-2014 collected by IPHC samplers using depredation forms, by regulatory area. Figure 1c. Marine mammal depredation from 2009-2014 in charter regions with substanstive killer whale presence. 436 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Figure 1d. Marine mammal depredation from 2009-2014 in charter regions with substansive sperm whale presence. 437 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Whaledepredationoccurences(allstations) 40 35 30 25 KillerWhale 20 SpermWhale 15 10 5 0 2009 2010 2011 2012 2013 2014 Figure 2a. Number of killer whale and sperm whale depredation events per year across all stations, from 2009-2014. Whaledepredationoccurences 20 18 16 14 12 10 8 6 4 2 0 KillerWhale 2009 2010 2011 2012 2013 2014 Figure 2b. Number of killer whale and sperm whale depredation events per year excluding expansion stations, from 2009-2014. 438 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Boatsectors 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 9 8 6 5 4 3 1 KillerWhale SpermWhale Figure 3a. Proportion of sectors relative to the vessel where whales were seen. Figure 3b. Sector chart as found on IPHC depredation tracking form. 439 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 Bent,Broken,orMissinghookcountsby charteredregion 2.50% 3.00% Bent,Broken,orMissinghookcountsby charteredregion 2.50% 2.00% allsets allsets 2.00% 1.50% non depredated sets 1.00% 1.50% nonͲ depredated sets 1.00% SW KW 0.50% 0.50% 0.00% 0.00% Figure 4. Mean percentage of bent, broken, or missing hooks observed during IPHC setline survey hauls, by charter region, from 2009-2014. PWS indicates the Prince William Sound charter region. O32HalibutWPUEondepredated sets 40 160 35 140 30 120 25 100 20 Present 15 absent WPUE WPUE U32HalibutWPUEondepredated sets 80 Present 60 Absent 10 40 5 20 0 0 KillerWhale SpermWhale KillerWhale SpermWhale Figure 5. Halibut Weight per Unit Effort for whale depredator presence or absence on depredated sets. 440 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015 60 AveragehookcountperskateinSeward AVeragehookcounts 50 40 30 NonͲdepredated SW 20 10 0 Figure 6a. Average hook count per skate for killer whales in regulatory area 4A, from 2009-2014. 60 AveragehookcountperskateinFairweather Averagehookcounts 50 40 30 20 NonͲdepredated SW 10 0 Figure 6b. Average hook count per skate for sperm whales in regulatory area 3A, from 2009-2014. 441 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015