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Oceanography of the WCPO and its influence on tuna dynamics 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 • Currents, Warm pool/cool tongue system • ENSO Relationship between oceanographic processes and fish populations • 5. Ocean properties, movement, productivity Survival, Growth, Movement, Recruitment Oceanographic impacts on fisheries • Catchability and catch rates • Movement and distribution 6. Implications for stock assessments 7. Climate change 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 better 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 climatic and atmospheric processes, and conversely have a strong influence upon these…ocean and atmosphere are a coupled dynamic system What are the key properties and structure of oceanic waters? Key properties of sea water - Temperature • The temperature of the oceans varies 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 (warm) Deep Layer (cold) Key properties of sea water - Density • • • • The density of oceanic waters varies by depth Low density waters (due to heating and precipitation) lie 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. (warmer, less salty and less dense) Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/ 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: • Gravity • Wind • The following section will discuss how these factors drive the movement of water in the ocean 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 is created to stabilize the atmospheric pressure and moves from the high to low pressure area • Wind-driven currents - As wind moves across the water, it drags on the water. Water moves at about 3-4% of the wind speed. 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 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) Primary producers represent the base of the oceanic food chain and their abundance is critical to the abundance of animals higher in the chain 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/ Oceanography and fish populations in the Pacific ……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. 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 currents play a major role in the location and size of the warm-pool/cold tongue convergence zone. Pacific Ocean – Warm Pool / 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”. 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”. These two water masses meet at the “convergence zone”. Convergence zone Warm pool Cold tongue 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 – Inter-annual Primary Production During El Nino conditions, the Eastern upwelling is suppressed, but coastal upwelling and productivity in the far western area (PNG, Phillipines, Palau) can be enhanced. 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. Sea Surface Temperature El Nino (Jan 98) La Nina (Jan 99) Chlorophyll a 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 on these). These are play a large role in 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 – Growth What environmental factors impact GROWTH? 1. 2. 3. Primary production (Food availability) Water temperature (Metabolic rate) Current/turbulence (Energy expenditure ex: holding position in water column) Oceanographic/environmental impacts on fish populations – Recruitment What environmental factors impact RECRUITMENT? 1. 2. 3. 4. 5. 6. Primary production/Food availability (Parental condition, larvae survival) Water temperature Current speed (turbulence) and advection Salinity Oxygen Predation/cannibalism (of eggs, larvae) Oceanographic/environmental impacts on fish populations – Survival What environmental factors impact SURVIVAL? 1. 2. 3. 4. 5. 6. Primary production/Food availability Water temperature Current speed and advection Salinity Oxygen Predation Impacts vary depending on stage of development • Oceanography impacts on fish populations Numerous oceanographic factors have a significant influence on the recruitment, growth and survival of pelagic fish species; the degree of influence often varies between different life history stages Spawning and fertilisation Adults Eggs Maturation Hatching Larvae Survival/growth Juvenile stages Survival/growth Oceanographic/environmental impacts on fish populations – Movement 1- Horizontal Movement 2- Vertical Movement 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) La Nina period Skipjack tuna and climate/oceanographic processes– Basin wide and EEZ impacts 2000 (+) 2002 (-) Changes in the depth of the thermocline also impacts on catchability of fish by longline fisheries 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 Climate related changes in habitat volume in the Pacific There are implications of habitat volume and variation in species horizontal and 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. Impacts on Stock Assessment The stock assessments conducted by SPC have 3 aspects that might be impacted by oceanographic and climatic processes: 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 fish appropriately 2. Incorporation of movement –tuna might move into/out of stock assessment regions in response to oceanographic/climatic shifts. 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 SA. 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. Climate Change 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 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. Hypothesis: Climate change may 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 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 it moves in a complex manner, and that it has different properties (of temperature, salinity, density) in different areas and at different times. These sets of conditions constitute mobile, sometimes transient, but differing habitats within the ocean Target tuna species have different habitat preferences (which vary over life history), and different behaviours evolved to exploit these habitats. 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. Session Summary and Conclusions The impacts of oceanographic phenomena on species recruitment, survival and growth have very large implications for population dynamics and status, irrespective of the fisheries impacts. Furthermore, the movement of tuna, 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. THANKS ! ENSO Variability Time series of skipjack biomass is lagged by 8 months potential for forecasting Impacts of Climate Change 1950 1950 2000 2000 2050 2050 2099 2099 Skipjack Bigeye Change in adult distribution Using IPCC Scenarios to force SEAPODYM