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Stock Assessment Workshop 19th June -25th June 2008 SPC Headquarters Noumea New Caledonia Day 1 Session 3 How do oceanographic and climatic processes impact upon tuna fisheries (and stock assessment) Overview 1. Introduction 2. Basic Principles of Physical Oceanography • 3. 4. Oceanography of the Pacific • Ocean structure, Currents, Warm pool/cool tongue system • ENSO Relationship between oceanographic processes and fish populations • 5. 6. Ocean properties, structure, movement, productivity Survival, Growth, Movement, Recruitment Oceanographic impacts on fisheries • Catchability and catch rates • Movement and distribution Scales of impact • WCPO v. EEZ v. eddies, current lines etc 7. Climate change 8. Implications for stock assessments Introduction • Oceanographic and climatic factors influence the distribution and abundance of pelagic fish (through impacts on recruitment, growth and mortality), and subsequently, the distribution and activity of the fisheries that target them • Understanding the relationship between fish abundance/distribution and oceanographic and climatic factors can provide fishers, managers and scientists with an understanding of fishery variability. • This may allow management and development plans to consider fluctuations in fish biomass and availability. • With climate change, understanding these relationships may become even more important. Basic principles • There are four key features of the ocean about which we need to be aware if we are to understand how fish populations and fisheries are influenced by the ocean: • The oceans are not uniform water masses but have a physical structure, both vertically and horizontally • This structure results from spatial differences in the properties (e.g. temperature, salinity, water pressure and other factors) of the oceans water • Oceanic waters are constantly moving and this movement can be horizontal (e.g. wind driven surface currents) or vertical (e.g. upwellings or downwellings). • The properties, structure and movement of oceanic waters are strongly influenced by climate and atmospheric processes, and conversely have a strong influence upon these…ocean and atmosphere are a coupled dynamic system Basic principles • Subsequently….. • The structure of the water column is constantly changing depending on prevailing climatic conditions, and hence the ocean provides a heterogeneous and dynamically changing environment within which fish must survive. • We are going to now briefly look in more detail at: • Oceanic water properties, • Ocean structure, and • Movement of water within oceans. • Climatic and atmospheric influences • Each has significant implications for fish population ecology, abundance, distribution and catchability What are the key properties and structure of oceanic waters? Key properties of sea water - Temperature • The temperature of the oceans varies by depth and by latitude • Ocean surface temperature strongly correlates with latitude because insolation, the amount of sunlight striking Earth’s surface, is directly related to latitude, and is highest in the tropics, hence tropical waters are warmer. Key properties of sea water - Temperature Warmer waters are generally less dense than cooler water and therefore “sit on top” of the cooler waters (i.e. so temperature decreases with depth). In tropical and subtropical waters, there is a thermocline (depth at which rapid temperature change) Surface Mixed Layer Deep Layer Why are surface waters typically warmer, less salty and less dense? Key properties of sea water - Temperature • In the tropical Pacific Ocean, warmer waters are typically distributed westward, but can shift with climatic conditions (discussed further later). The mean, upper-ocean, thermal structure ALONG the equator in the Pacific from 140E to 100W for November 1997 The mean, upper-ocean, thermal structure ACROSS the equator in the Pacific from 8N to 8S for November 1997 Images from NOAA Pacific Marine Environmental Laboratory Key properties of sea water - Temperature • Tropical/subtropical oceans are permanently layered with warm, less dense surface water separated from the cold, dense deep water by a thermocline. • Temperate regions have a seasonal thermocline, polar regions have none. Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ Key properties of sea water - Salinity The salinity of the oceans waters also varies by depth. More saline waters are typically denser than less saline waters and therefore tend to sink beneath less saline water (i.e. salinity increases with depth) and by latitude (relating to precipitation and evaporation) Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ Key properties of sea water - Density Density of sea water is a function of (related to) temperature, salinity and water pressure. Density increases as temperature decreases and salinity increases as pressure increases. Pressure increases regularly with depth, but temperature and salinity are more variable. Higher salinity water can rest above lower salinity water if the higher salinity water is sufficiently warm and the lower salinity water sufficiently cold. Pycnocline is a layer within the water column where water density changes rapidly with depth. Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ Key properties of sea water - Density The density of oceanic waters varies by depth Low density waters (due to heating or low salinity) lye at the surface, denser waters below. Pycnocline is a rapid change in density with depth (similar principle to thermocline) The pycnocline is transitional between the surface and deep layers In the low latitudes (tropics), the pycnocline coincides with the thermocline. Surface water in high latitudes cools, becomes dense, sinks (convects) to the sea floor and flows outward (advects) across the ocean basin. Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ Why do oceanic waters move? Ocean water movement • The waters of the ocean are constantly in motion….why is this? • There are different types of ocean water movement, including: • horizontal currents, gyres and eddies, and • vertical upwellings and downwellings. • These movements are caused by two main factors: • Wind • Gravity • The following section will discuss how these factors drive the movement of water in the ocean Ocean movement – Currents Eddies Surface Mixed Layer Surface currents Gyres Coastal Upwelling/ Downwelling Seamount/ Ridge Upwelling Subsurface currents Divergence based upwelling Convergence/ Gravity based downwelling Ocean movement – Horizontal currents High pressure (Cooler air) Solar radiation and wind creation Low pressure (Warmer air) High pressure (Cooler air) Ocean movement – Horizontal currents Wind-driven currents - As wind moves across the water, it drags on the water. Water moves at about 3-4% of the wind speed. Zonal wind flow is wind moving nearly parallel to latitude as a result of Coriolis deflection. Currents carry warm water poleward on the western side of basins and cooler water equatorward on the eastern side. Westerlydriven ocean currents in the trade winds, easterly-driven ocean currents in the Westerlies and deflection of the ocean currents by the continents results in a circular current, called a gyre, which occupies most of the ocean basin in each hemisphere. Source: http://www.tulane.ed u/~bianchi/Courses/ Oceanography/ Ocean movement – Vertical currents Ocean water can also move vertically, with the two most common processes called upwelling (movement of deep water to the surface) and downwelling (movement of surface waters to the deep). Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ Ocean movement Thermohaline circulation is a density driven flow of water between ocean basins that appears largely driven by waters of the North Atlantic, where, poleward of 45 degrees (north), the density of water increases because of declining temperature and increased salinity because of evaporation or ice formation. The water sinks to a density-appropriate level and then slowly flows outward in all directions across the basin until they are blocked by a continent. Ocean basins interconnect and exchange water with each other and with the surface. Source: http://www.tulane.ed u/~bianchi/Courses/ Oceanography/ Oceanographic structure and movement summary Differences in solar heating between areas causes differences in air pressure which results in the air moving, i.e. wind. Winds at the surface of the ocean drags on the water, causing the water to move also, i.e. creating surface currents and gyres. Some wind patterns can cause divergence of surface waters which can result in upwelling of deeper waters. Subsurface topography can also divert deep currents to the surface, as can the convergence of currents. Evaporation leading to increased salinity and water density, or cooling leading to increased density, can result in downwelling (sinking) of surface waters. The location, strength and duration of all these water movement features varies dynamically in time and space, being driven by climatic and atmospheric factors. Oceanography and Primary Production Oceanographic and Primary production Primary production refers to the amount of inorganic C (mainly carbon dioxide) converted to organic C (e.g. simple sugars) by microscopic algae in a process known as photosynthesis (photo – (sun) light; synthesis = to make something). Land plants also do a similar thing Primary producers represent the base of the oceanic food chain and their abundance is critical to the abundance of animals higher in the chain Primary production Plants require sunlight, nutrients, water and carbon dioxide for photosynthesis. Phytoplankton blooms occur when light and nutrients are abundant. Other factors that influence phytoplankton growth include upwelling, turbulence, grazing intensity and turbidity. Tropics/subtropics - sunlight is abundant, but strong thermocline restricts upwelling of nutrients and results in lower productivity. High productivity can occur in areas of coastal upwelling, in the tropical waters between the gyres and at coral reefs. In temperate regions productivity is distinctly seasonal. Polar waters are nutrient-rich all year but productivity is only high in the summer when light is abundant. Oceanography and Primary production Low productivity in the gyres, due to downwelling (opposite of process needed to bring nutrients to surface) Source: http://www.tulane.edu/~bianchi/Cours es/Oceanography/ Why is oceanography important to fisheries? “Habitat – the place where an organism (or a community of organisms) lives, including all living (biological) and nonliving (environmental) factors or conditions of the surrounding environment.” The ocean is not a homogenous uniform water mass. It is structured, it moves, and it has different properties (of temperature, salinity, density) in different areas and at different times. As a result, some areas are productive and some aren't. What does this mean for fish? The ocean also has habitats within it. An area or mass of water within the ocean which is warm, shallow, with low salinity, low nutrients and productivity, constitutes a very different set of living conditions to one which is shallow, cold, high salinity and high productivity. Deep waters constitute a very different habitat to shallow waters. Fish species have evolved different physiologies and morphologies to exploit different habitats in the ocean Tuna species are no different. e.g. Skipjack - exploit warm, shallow tropical waters. Bluefin tuna - exploit deeper, colder more temperate habitats. Oceanography and fish populations ……the following sections are going to discuss: 1. Some of the key features of oceanographic and climate processes (and ocean habitats) of the Pacific Ocean in more detail. 2. What we currently know about the relationship between oceanographic processes and the key species targeted by pelagic fisheries in the Pacific 3. The implications of these relationships for the fisheries targeting them, and, 4. For assessments of their status and subsequent management of the fisheries. Oceanography and climate of the Pacific Pacific Ocean - Surface currents Three major current features are the North Pacific Subtropical Gyre, the South Pacific Subtropical Gyre, and the equatorial currents The strength and direction of the equatorial and subequatorial currents is dependant on the prevailing winds and climatic conditions 60o Major shifts occur in currents due to changes between South East Trade Wind and North West Monsoon seasons warm pool cold tongue Subarctic Gyre convergence 40o KUR divergence Subtropical Gyre 20o NEC NECC 0o SEC SECC 20o EAC Subtropical Gyre HBT 40o 60o 120o 140 o 160 o 180o 160 o 140o 120 o 100 o 80o The strength and direction of the wind driven equatorial and subequatorial currents play a major role in the location and size of another major oceanographic feature of the Pacific, the warmpool/cold tongue convergence zone. Pacific Ocean – Warm Pool / Cold Tongue In the eastern and central Pacific, wind driven movement of currents along the equator creates an upwelling that extends westward from South America…this feature is called the “cold tongue” Western equatorial Pacific - low primary production, extreme uniformity of high sea surface temperatures (SST) (up to 28° C year-round). This water mass is referred to as the “warm pool”. These two water masses meet at the “convergence zone”. Convergence zone Warm pool Cold tongue Pacific Ocean – Warm Pool / Cold Tongue Cooling in the WCPO is absent due to high rainfall causing haline stratification that prevents mixing of surface and deeper cold nutrient rich waters. However, the low nutrient, low salinity surface waters from the warm pool can move eastward on a seasonal basis under the influence of westerly wind events. The eastward movement of warm pool meets the western movement of the cold tongue, creating a convergence zone where they meet. Pacific Ocean - El Nino Southern Oscillation Phenomena (ENSO) La Nina Warm pool c) ENSO • Results from complex interaction between surface layers of equatorial Pacific and overlying atmosphere • Has a very significant influence on the fisheries in the Pacific Ocean • Irregular cycle of 3-7 yr • Has 3 states – El Nino, Neutral, La Nina Cold tongue a) El Nino Warm pool • Pacific Ocean - ENSO El Nino conditions – expansion of the warm pool eastwards, resulting in warmer than average waters in the central and eastern Pacific, higher rainfall in that region, a deepening of the thermocline in the east and rising of thermocline in the western region, and cooler than average waters in the western Pacific. Neutral conditions, and La Nina conditions –characterised by stronger Pacific Trade winds, the contraction of the warm pool into the equatorial western Pacific, higher rainfall in the western Pacific and lower rainfall in the eastern Pacific, a deepening of the thermocline in the west and rising of thermocline in the eastern region. Pacific Ocean – ENSO and PDO 0 -2 -4 Measured by difference in air pressure between EPO and WPO – negative values (<-2) signify El Nino and positive values (>2) La Nina. SOI 2 El Nino magnitude/duration varies 1910 1920 0 SOI -2 1930 1940 1950 1960 1970 0 -2 SOI 2 1920 -4 PDO operates on a scale of 20-30 years, is characterised by long term shifts in average SST between the EPO and WCPO. ENSO is a pattern that can be thought of as lying on top of the large scale temperature distribution determined by the Pacific Decadal Oscillation. 1900 -4 Another climate phenomena exists for the Pacific – the Pacific Decadal Oscillation (PDO) - characterized by abrupt, but infrequent changes that are described as “climate regime shifts” 1890 2 1880 1970 1975 1980 1985 1990 1995 2000 2005 (*dotted line is the standard deviation calculated over the entire time series) Pacific Ocean – Primary Production The convergence zone between the warm pool and cold tongue in equatorial Pacific is the meeting point of cool nutrient rich upwelled waters with warm surface waters from the west. Under the influence of sunlight, these conditions promote the rapid growth and reproduction of phytoplankton (i.e.; blooms). The explosion in phytoplankton population has a cascading effect through the food chain…zooplankton>>small fish>>big fish (e.g. tunas) This process takes weeks to months, during which time equatorial waters are diverging away from the equator The regions of high tuna density and catch rates can therefore be displaced somewhat from where primary productivity was initiated Pacific Ocean – Inter-annual Primary Production During La Nina, upwelling induced primary production is enhanced in the equatorial Eastern pacific and brought by wind driven surface waters across to central and Western Pacific, which at the same time diverge north and south of the equator. During El Nino conditions, the Eastern upwelling is suppressed, but upwelling and productivity in the far western area (PNG, Phillipines, Palau) can be enhanced. Sea Surface Temperature El Nino (Jan 98) La Nina (Jan 99) Chlorophyll a El Nino (example year 1997) 1st Quarter ENSO – EEZ impacts There are very clear differences in seasonal surface current directions and strength that result from interannual climatic state variations, and these cause large changes in the oceanography associated with any given EEZ in the WCPO For example, current speed and direction in the FSM region during El Nino and La Nina events La Nina (example year 1999) Surface currents – 1st Quarter Seasonal and interannual variation 2nd Quarter 2nd Quarter 3rd Quarter 3rd Quarter 4th Quarter 4th Quarter Source data: www.oscar.noaa.gov ENSO – EEZ impacts 1997 There are very clear differences in seasonal sea surface temperatures that occur in Pacific EEZs, that result from interannual climatic state variations, For example, SST in the FSM region during El Nino and La Nina events January March May July August October 1998 1999 ENSO – EEZ impacts 2006 28 29 30 31 2001 2000 2002 2004 2006 1998 2000 2002 2004 2006 1998 2000 2002 2004 2006 100 140 180 20 60 100 140 1998 60 For example, SST, Depth of 27C and 22C thermoclines in 5 subregions within the FSM EEZ Depth (m) of 22C thermocline Thermal structure of the water column also changes dramatically with changes in climate state. Depth (m) of 27C thermocline 27 Thermal structure 1996 Sea surface temp. 1991 Summary – Oceanography of the Pacific There are three key and interacting features of the Pacific ocean-climate system that have a large influence on the distribution and abundance of the target tuna species. These are: 1. The direction and strength of the major surface currents, 2. The size and location of the warm-pool-cold tongue interaction, and 3. The overall influence/interaction of prevailing climatic conditions (in particular ENSO and PDO) on these. These are play a large role in tuna distribution, abundance and fishing success, as will be see in following sections How are oceanographic and climatic processes relevant to fish population dynamics and stock assessment in the Pacific? WCPO tuna stock assessments take account of: 1. Growth 2. Recruitment 3. Survival 4. Movement 5. Catch rates (Abundance indices) Oceanographic/environmental impacts on fish populations – Introduction What are the key processes that determine population biomass? 1. Growth 2. Reproduction (Recruitment) 3. Survival (or mortality) Oceanographic/environmental impacts on fish populations – Introduction What environmental factors impact GROWTH? 1. Food availability 2. Water temperature 3. Energy expenditure (holding position in water column, so currents and turbulence etc) 4. Other Oceanographic/environmental impacts on fish populations – Recruitment What environmental factors impact RECRUITMENT? 1. 2. 3. 4. 5. 6. 7. 8. Parental condition (Nutritional status>>Food availability) Food availability Water temperature Predation/cannibalism (of eggs, larvae) Disease Current speed (turbulence) and advection Salinity Oxygen Oceanographic/environmental impacts on fish populations – Recruitment Source: Adam Langley, Karine Briand, David Seán Kirby, Raghu Murtugudde (In press) Influence of oceanographic variability on recruitment of yellowfin tuna Thunnus albacares in the western and central Pacific Ocean Oceanographic/environmental impacts on fish populations – Recruitment YFT BET Primary Productivity SKJ Secretariat of the Pacific Community Oceanography impacts on fish populations - Survival • Few fish species can survive in all oceanographic conditions. Instead, fish species have evolved physiological and behavioural characteristics which allow them to exploit specific conditions. • Typically, if they move outside these “habitats” then condition and survival decline. • For example, bigeye tuna have specialised circulatory features that allow them to exploit deeper colder waters than some other species, such as skipjack tuna, are capable of. Oceanographic/environmental impacts on fish populations – Survival What environmental factors impact SURVIVAL? 1. 2. 3. 4. 5. 6. 7. Predation Disease Food availability Water temperature Current speed and advection Salinity Oxygen Impacts vary depending on stage of development Oceanography impacts on fish populations - Survival • Numerous oceanographic factors have a significant influence on the survival of pelagic fish species, and the degree of influence often varies between different life history stages Adults Spawning and fertilisation Eggs Maturation Hatching Larvae Juvenile stages Vertical Movement Climate related changes in habitat volume in the Pacific There are implications of habitat volume and variation in species vertical movements for interpretation of CPUE data and its use in stock assessments. The effect of increasing/decreasing thermocline depth on catch rates, for example, is very much species specific, and also dependant on the gear type being used. Bigeye and yellowfin tuna movement 1. 2. Bigeye tuna show strong diurnal vertical movement (deep during day, shallower at night) due to tracking of mesopelagic prey. Bigeye physiology enables exploitation of deeper colder lower oxygen waters. FAD associated Yellowfin show diurnal pattern, but mostly within mixed layer above the thermocline. YFT lack key physiological adaptations of BET that enable BET to exploit deeper waters 3. Movements differ on/off FADs for both species 4. Complicates use of CPUE data in stock assessments Source: Bruno Leroy, SPC, 2007 BIGEYE TUNA FAD associated YELLOWFIN TUNA Bigeye tuna vertical movements and purse seine susceptibility Implications for CPUE analyses – catch rates will vary according to depth/volume of habitat, prey movement, time of day and depending on whether tuna are associated to a “feature” (seamount, FAD, log etc) Implications for catchability by purse seine….bigeye more vulnerable when associated to floating objects From: MICHAEL K. MUSYL,1,* RICHARD W. BRILL,2 CHRISTOFER H. BOGGS,2 DANIEL S. CURRAN,1 THOMAS K. KAZAMA2 AND MICHAEL P. SEKI2 Vertical movements of bigeye tuna (Thunnus obesus) associated with islands, buoys, and seamounts near the main Hawaiian Islands from archival tagging data. Fish. Oceanogr. 12:3, 152–169, 2003 Bigeye and yellowfin vertical movements and longline susceptibility BET CPUE higher on deep sets (in WCPO) 5S 50 YFT CPUE higher on shallow sets 5N 150 Catch rates depend on depth of longline gear relative to the thermocline depth. 15N 15S 250 Yellowfin predominantly above thermocline, bigeye predominantly below (during the day) 140E 5 10 160E 15 20 25 180 30 160W 140W 120W Bigeye tuna movement Bigeye tuna follow the vertical migration of mesopelagic prey, which move up into shallow waters above the thermocline during the night, and return to deeper waters at dawn. Figure shows movement of a bigeye tuna in relation to prey (sound scattering layer) over the course of an hour during the night Source: Erwan Josse, Pascal Bach & Laurent Dagorn. Simultaneous observations of tuna movements and their prey by sonic tracking and acoustic surveys Hydrobiologia 371/372: 61–69, 1998. Bigeye tuna movement is closely related to their physiological capacity to cope with different environmental conditions From: Richard W. Brill1, Keith A. Bigelow, Michael K. Musyl, Kerstin A. Fritsches, Eric J. Warrant BIGEYE TUNA (THUNNUS OBESUS) BEHAVIOR AND PHYSIOLOGY AND THEIR RELEVANCE TO STOCK ASSESSMENTS AND FISHERY BIOLOGY Col. Vol. Sci. Pap. ICCAT, 57(2): 142-161 (2005) Recent findings on yellowfin tuna movement SOURCE: Laurent Dagorn1,a, Kim N. Holland2, Jean-Pierre Hallier3, Marc Taquet4, Gala Moreno5, Gorka Sancho6, David G. Itano7, Riaz Aumeeruddy8, Charlotte Girard1, Julien Million9 and Alain Fonteneau10 Deep diving behavior observed in yellowfin tuna (Thunnus albacares) Aquat. Living Resour. 19, 85–88 (2006) Oceanography, gear impacts and depth of capture Temperature-depth timer study of species caught by longline off New Caledonia Above thermocline Below thermocline Beverly, S (2005) Notes on a longline trip in the New Caledonia EEZ using TDRs in combination with remote sensing data (SSH and SST) , WCPFC–SC1 FT IP–4 Problem for interpretation of CPUE data (where hooks per basket is assumed an indicator of fishing depth) – hook depth can vary very significantly during the course of a single fishing operation – this variation will depend on prevailing local oceanographic conditions, current shear etc Scientists need to be careful regarding assumptions pertaining to fishing depth when incorporating CPUE data into assessments William Sokimi, SPC, 2007 Depth and catch rates Japanese study indicates that catch rates vary by depth, reflecting the differing movements and depth habitats of different species BET YFT BSH Marlins 1.0 Depth layer (m) Figures courteousy of Kotaro Yokawa (2007) – National Institute of Far Seas Fisheries 350-375 325-350 300-325 275-300 250-275 225-250 200-225 175-200 150-175 125-150 100-125 75-100 50-75 25-50 0.0 0-25 Scaled index 2.0 Depth and catch rates 0 0.2 Ratio to the total 0.4 0.6 Absolute depth layer (m) 0-25 25-50 50-75 75-100 100-125 125-150 150-175 175-200 Vertical CPUE Vertical distribution (PAT tag) 200-250 Striped marlin caught deeper layers than expected by tag data Figures courteousy of Kotaro Yokawa (2007) – National Institute of Far Seas Fisheries 0.8 Influence of thermocline (and hypoxic layer) on movement Recent study compared vertical movement of marlin and sailfish in Eastern Pacific with that of same species in the western Atlantic Oceans. Both marlin and sailfish in the Atlantic study (Red lines) area had a significantly different mean depth distribution to the same species tagged in the Eastern Pacific (Blue lines). Evidence presented suggested that this difference in depths was due to the differing depth of the cold hypoxic layer between the two areas. In the Atlantic this layer was considerably deeper than in the Pacific, allowing surface layer dwelling species greater vertical habitat range. Source: ERIC D. PRINCE1,* AND C. PHILLIP GOODYEAR2 Hypoxia-based habitat compression of tropical pelagic fishes. Fish. Oceanogr. 15:6, 451–464, 2006 Influence of thermocline (and hypoxic layer) on movement Australia 3/29 0 4/3 4/8 4/13 4/18 4/23 4/28 5/3 Similarly, research into striped marlin has revealed that the average dive by striped marlin appears closely related to the depth of the mixed layer in the region inhabited by the fish 5/8 50 100 150 200 250 mixed layer deepest dive average dive 300 350 400 Costa rica 2/7 2/12 2/17 2/22 2/27 3/4 0 50 3/9 e.g. Average dive is much shallower in the eastern Pacific (off Costa Rica) than in the Western Pacific (off Australia) Implications for habitat volume and catch rates within stock assessments 100 150 200 Mixed layer 250 Deepest dive Average dive 300 Figures courteousy of Michael Domeier (2006) – Pfleger Institute. Horizontal Movements Horizontal movements and oceanographic shifts in the Pacific Horizontal shifts in the warm pool correspond to changing thermocline depth and together may influence the movement of tropical tuna. Bigeye and Yellowfin tuna movements in the Pacific in relation to Oceanographic/ Climatic factors. There are a number of theories regarding these mechanisms. Typically, a decreasing in thermocline depth is associated with higher LL CPUE for bigeye Source: Lu et al. (2001) Oceanography impacts on fish populations - SKIPJACK Horizontal movement Displacements of tagged skipjack tuna during representative El Nino (top) and La Nina (bottom) periods. Thick arrows indicate the direction and magnitude of displacement of the skipjack CPUE gravity centre during the tag recapture periods. El Nino period From (Lehodey et al, 1997) These ENSO related changes in movement pattern are not yet captured in the movement parameterisation for MFCL assessments La Nina period Skipjack tuna and climate/oceanographic processes– Basin wide and EEZ impacts 2000 (+) DFADs: allow fishing in east in non-El Niño periods AFADs: allow fishing in west in all years 2002 (-) Changes in the depth of the thermocline also impacts on catchability of fish by longline fisheries Skipjack tuna – horizontal movement and association with the Warm Pool a) Skipjack tuna CPUE for US purse seine fleet (88-95) b) Monthly mean CPUE; pink line is the longitudinal gravity centre of CPUE (G) blue line is the 29.8C sea surface temperature (SST) isotherm broken red line is the southern oscillation index (SOI) (Source: Lehodey et. al , 1997 - NATURE |VOL 389 | 16 OCTOBER 1997) ***Implications for understanding environmental effects on movement estimation in population models Bigeye and yellowfin tuna Bigeye tuna Yellowfin tuna 0 10 20 30 40 50 Albacore CPUE (kg/hhhooks) 35 S Latitude of isotherm 25 S 15 S 0 10 20 30 40 50 Albacore CPUE (kg/hhhooks) 35 S Latitude of isotherm 30 S 25 S 20 S 15 S Albacore tuna movement and environmental forcing 1998 1998 2000 2000 2002 25 C SST isotherm 2002 2004 2006 2004 2006 Summary – Impacts on tuna fisheries Four likely key impacts of oceanographic and climatic processes on WCPO stock assessments: 1. Calculation of CPUE index – as an index of abundance to which the model is fitted, is a critical component of stock assessments. Variation in oceanographic processes effect catch rates of pelagic tuna due to: a. Changing the volume of the species habitat (effects their density) b. Effecting the ability of fishers to target their gear appropriately c. Movements of tuna into/out of stock assessment regions in response to oceanographic/climatic shifts 2. Incorporation of movement – Fish move in response to changes in their physical environment (one reason). Stock assessment models need to incorporate variation in fish movement and migration that occurs due large scale oceanographic and climatic changes 3. Recruitment Estimation – the least easily estimated but most important biological factor in SAs. Variations in recruitment can be very substantial and are predominantly due to environmental processes. Current efforts are attempting to use oceanographic analyses to predict recruitment to feed into stock assessment models. 4. Growth rate impacts – variation in productivity of oceanic waters may relate to variation in growth rates by cohorts. Climate Change Climate Change In considering the relationship between tuna, tuna fisheries and climate-ocean system, we must also consider the possibility that this relationship will change (or is changing) as a result of climate change. How will it change? There is currently significant uncertainty about the implications of climate change for tuna fisheries. However, research is underway, and the following observations and hypotheses have been made……… Climate Change Fact: Increases in global average sea temperature have been observed. Sea surface temperatures affect the patterns in atmospheric pressure, which in turn are responsible for wind generation. Hypothesis: Changes in wind generated surface currents would not only modify the weather conditions but also alter the timing location and extent of the upwelling processes upon which much oceanic primary productivity is reliant. Some studies suggest that primary productivity in tropical oceans would decline due to increased stratification between warmer surface waters and colder deeper water (and consequent reduction in upwelling) (Bopp et al., 2001), but further research is required. Implication: Decline in the upwelling system of the central and eastern equatorial Pacific may lead to reduced productivity that is normally advected westwards and upon which pelagic fish stocks depend. This decreasing production could lead to a decline in tuna abundance, particularly in the bigeye and adult yellowfin population (the species targeted by the longline fleet). Climate Change Fact: El Nino events appear to have become more frequent in recent decades, (possibly linked to climate change) and are associated with an eastward shift of major tuna resource (e.g. skipjack) in the WCPO. Total catch does not appear effected by ENSO related shifts in the stocks. Hypothesis: Climate change may, however, imply more permanent El Niños, which are likely to increase the annual fluctuations of the spatial distribution and abundance of tuna. First simulations of global warming on skipjack suggest a global improvement of its habitat conditions east of the date line and a spatial redistribution of this species to higher latitudes (Loukos et al., 2003). Distant water fishing fleets should be able to adapt to changes in the spatial distribution and abundance in tuna stocks. But domestic fleets would be vulnerable to fluctuations of tuna fisheries in their Exclusive Economic Zones. Climate Change Uncertainty remains on the change in the productivity of the western equatorial Pacific. The impact of climate change on tuna recruitment and spawning migration is also poorly known. Current thinking is that climate change may lead to a shift in the spatial tuna distribution, as well as possible changes in productivity, total abundance and total catch in different regions. Session Summary and Conclusions The ocean is not a homogenous uniform water mass. We have seen that the water in the ocean is structured, it moves in a complex manner, and that it has different properties (of temperature, salinity, density) in different areas and at different times. Furthermore, some areas possess properties and conditions that are more productive than other areas. Effectively these sets of conditions constitute mobile, sometimes transient, but differing habitats within the ocean Few if any fish species, including tuna species, can thrive and survive across all the sets of conditions (habitats) in the ocean. Each species has evolved to exploit specific sets of conditions (or specific ocean habitats), and if placed for long periods outside these conditions, will do poorly or die. Target tuna species have different habitat preferences (which vary over life history), and different behaviours evolved to exploit these habitats. Session Summary and Conclusions Finding and exploiting the ideal habitat has very large implications for species recruitment, growth and mortality, and population dynamics. Because each species differs in its relationship to the ocean environment, each species will be impacted differently by large scale changes in climate and oceanography. For humans and our fisheries, the impacts of oceanograhic phenomena on species recruitment, survival and growth have very significant implications for population dynamics and status, irrespective of our own impacts. Furthermore, the movement of habitats and therefore the tuna that associate to them, as a result of changes in oceanography, have significant implications for catchability of these species. It is imperative that we understand the relationship between oceanographic and climatic phenomena and fisheries catches for ensuring more reliable assessments, and for ensuring management decision making processes that take account of this.