Download Marine mammal depredation on IPHC standardized setline surveys

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.
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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/
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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
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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
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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)
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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.
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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
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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).
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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
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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.
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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
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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
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Dykstra, C. L. and Soderlund, E. 2010. Categorizing marine mammal depredation on IPHC
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Garrison, L. P. 2007. Interactions between marine mammals and pelagic longline fishing gear in
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Peterson, M. J., Mueter, F., Criddle, K., and Haynie, A. C. 2014. Killer Whale Depredation and
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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:
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Plagnyi E. E. and Butterworth D. S., 2005. Indirect Fishery Interactions, Reynolds J. E., Perrin,
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Rocklin, D., Santoni, M., Culioli, J., Tomasini, J., Pelletier, D., and Mouillot, D. 2009. Changes
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Schakner, Z. A., Lunsford, C., Straley J., Eguchi, T., and Mesnick S. L. 2014. Using Model of
Social Transmission to Examine the Spread of Longline Depredation Bahvior among sperm
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Patagonian toothfish fisheries with killer and sperm whales in the Crozet Islands exclusive
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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%
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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
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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.
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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.
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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.
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IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015
Figure 1d. Marine mammal depredation from 2009-2014 in charter regions with substansive
sperm whale presence.
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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.
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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.
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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.
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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.
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IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2015