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Oceanographic Variability Oceanography (e.g. water temperatures, currents, productivity, etc) and climate dynamics have major influences on fish population dynamics and fisheries (Lehodey et al., 2006) in addition to any fishery impacts. Ocean-climate systems have been shown to strongly influence tuna fisheries in the Western and Central Pacific Ocean (WCPO) at various spatiotemporal scales and in different ways (Bour et al., 1981, Lehodey et al. 2003, Lehodey et al., 2006). Changes in oceanography may influence vertical and horizontal movements of tunas and other species, as well as eggs and larval survival. Individual tuna species display different preferences (e.g. preferred temperature) and thus will respond differently to changes in oceanography and climate (Fromentin and Fonteneau, 2001). While beyond the control of fishery managers, it is important to take into account the influence of oceanography and climate in order to better manage fisheries. Globally, tuna catches are highest in the western equatorial Pacific warm pool, a region characterized by low primary productivity and the warmest surface waters of the world’s oceans. However, the WCPO displays remarkable dynamics in oceanography mostly linked to climatic changes (such as El Niño Southern Oscillation, ENSO). In response, variations in tuna catches are reported at both regional and domestic scales both seasonally and inter-annually. The major oceanographic processes that impact on tuna distribution and abundance include in the WCPO are briefly discussed below. 1) Ocean processes that induce movement of water masses play on tuna distribution. Areas of divergence or convergence of currents are of major importance as they induce physical phenomena (upwellings, thermal fronts, eddies) that enhance local productivity and create zones of forage availability. These, in turn, attract and concentrate tuna. A better knowledge of these processes and their spatio-temporal variability remains a key issue for fishery management. 2) Tuna movement is linked to horizontal displacement of surface isotherms and vertical change in mixed layer depth that determine their surface habitat. In particular, east-west migration of the warm pool-cold tongue pelagic ecosystem, should be considered carefully by fisheries managers of the different countries. Analyses have demonstrated that inter annual variability in the environmental conditions linked to climatic oscillation (ENSO) is evident in the operation of the main tuna fisheries and the population dynamics of the tuna species. ENSO affects tuna fisheries through environmental changes in the warm pool-cold tongue system. During El Niño conditions, the distribution of purse-seine catch in the western and central Pacific is generally displaced eastwards, indicating a spatial shift in the distribution of tuna. There is an obvious impact for the Pacific Island Nations through variations in the level of tuna catches in their Exclusive Economics Zones and consequently on their economical revenue, according to the ENSO situation. 3) Contraction or extension of the warm pool also has a major impact on tuna recruitment which varies among species. For tropical species, such as skipjack and yellowfin, El Niño events would favour recruitment through extension of warm water spawning habitat, whereas La Niña events would restricted the favourable spawning areas and reduce the recruitment. For subequatorial species, such as South Pacific albacore, the opposite trends are found. These phenomenons may be taken into account to provide some indication of likely future catches within individual EEZs. 4) Global warming is also likely to affect regional tuna fisheries by raising average sea surface temperature to levels currently experienced during El Niño and by increasing year to year variability. Possible change in primary productivity in the tropical Pacific is also hypothesized. These factors would affect distribution, abundance and catchability of tuna fisheries, but further investigations are required to validate these assumptions. El iño Southern Oscillation El iño-Southern Oscillation (ESO; commonly referred to as simply El iño) is an irregular climatic oscillation of 3–7 years that evolves under the influence of the dynamic interaction between atmosphere and ocean (Philander 1990). “El Niño” usually refers to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific (between approximately the date line and 120oW). In contrast, La Niña is characterized by anomalously cool water in the central and east central equatorial Pacific. ENSO also results in changes in the intensity and distribution of rainfall in the Tropics and in changes in the patterns of sea level pressure and atmospheric circulation that affect many areas worldwide. Every El Niño is somewhat different in magnitude and in duration and this magnitude is typically measured as the difference in air pressure between eastern and western Pacific (the Southern Oscillation Index or SOI) (Picaut et al. 1996.). Prolonged periods (usually more than 3 months) of increasingly negative SOI values (below-normal air pressure at Tahiti and above-normal air pressure at Darwin) coincide with El Niño episodes whereas prolonged periods of positive SOI values coincide with La Niña episodes.The SOI has been recorded since the late 1800s, and Figure 1 provides a record of the SOI since 1950. Figure 1 Southern Oscillation Index (SOI) based on the differences in air pressure anomaly between Tahiti and Darwin, Australia. Dashed lines correspond to +/- standard deviation over January 1951 to march 2009 period (Source: http://www.cpc.noaa.gov/data/indices/). Ocean processes, fish distribution and movement The ocean is comprised of horizontally and vertically moving water masses, each of which can differ in their temperature, salinity, water pressure and other factors. These different properties create different conditions or habitats for fish in different parts of the ocean. Each fish species has evolved particular physiological and behavioral characteristics which allow them to exploit specific conditions/habitats, rather than all conditions. Typically, if they move outside these “habitats” then growth and survival can decline, hence fish tend to aggregate to and move with their preferred habitats. For example, skipjack tuna are physiologically adapted to warmer surface waters while bigeye tuna have physiological features that allow them to exploit deeper colder waters (e.g. Brill et al., 2005). Outside the influence of physiological restrictions, tuna and other fish will typically aggregate to areas of high food (prey) density, whose occurrence are also determined by specific oceanographic processes, which in turn are coupled to climatic processes. Areas where currents meet and converge or diverge tend to have concentrations of food (or prey of large pelagic fishes). Divergence of currents brings nutrient-enriched water from underlying cold layer to surface to compensate the deficit induced by water displacement. The mixing of warm and cool nutrient rich waters under the influence of sunlight leads to a process of enrichment that result in the creation of local areas of high productivity (planktonic “blooms”). The intense concentration of planktonic communities attract plankton feeding fish which in turn attract the pelagic predators (tuna, billfish and sharks) that feed upon them. Under the influence of convergent currents, the organisms from the divergence zone remain aggregated in a large zonal band usually associated with a thermal front (Figure 2). These features create zones of high food availability in otherwise barren tropical oceans (Grandperrin, 1978). Therefore, seasonal and annual variations in current patterns and the associated variation in the location and timing occurrence of upwellings, convergences and divergences, have a major effect on tuna distribution. Oceanographic conditions also influence the survival of eggs and larvae and therefore impact on recruitment to fish populations (e.g. Bakun, 1996; Lasker et al. 1981). Results from statistical population dynamics modeling (Lehodey et al., 2004) and related analyses (e.g. Langley et al., 2006) point to a link between tuna recruitment, climatic fluctuations and oceanography. Warm water Cold salt water t Figure 2 Dynamics of currents and productivity (modified from Grandperrin, 1978) The warm pool and cold tongue system Tropical Convergence zone Warm pool Cold tongue Figure 3 Sea surface temperature (AVHRR Pathfinder) and chlorophyll a concentration (Seawifs) in Western and Central Pacific Ocean detected by satellite in January 2000. Data source: http://oceanwatch.pifsc.noaa.gov/las/servlets/dataset The western equatorial Pacific is characterized by low primary production and warmer surface waters (>28˚C) that typically show little seasonal variability (<1˚C). This water mass is referred to as the “warm pool”. In the eastern and central Pacific Ocean, an upwelling extends westward along the equator from the coast of South America. The water mass is characterised by cold nutrient-enriched waters rising to the surface. This results in the formation of a large zonal band in which there is high chlorophyll concentration that correspond to the primary production. This band is commonly referred to as the “cold tongue”. The warmer, low nutrient, low salinity surface saline waters from the warm pool move seasonally eastwards under the influence of westerly wind events, and the area on the eastern edge of the warm pool which encounters the westward advection of cooler waters (cold tongue) is referred to as the tropical convergence zone. This is identified by a salinity front or more approximately by the 29°C isotherm (see Figure 3). The location and extent of the convergence zone is determined by the relative movements of the warm pool and cold tongue waters, which are driven by equatorial winds, which in turn vary in direction and strength under different climate conditions. Impact of El ino phenomena on the warm pool-cold tongue system and related fisheries Sea Surface Temperature January 1999 January 1998 Chlorophyll a January 1999 January 1998 Figure 4 Thermal structure and productivity of the equatorial Pacific during example El Niño (January 1998) and La Niña conditions (January 1999), including representation of mean sea surface temperatures (Source: http://poet.jpl.nasa.gov/) and chlorophyll a ( Source: http://www.giovanni.gsfc.nasa.gov) In an average situation, the convergence zone on the eastern edge of the warm pool oscillates around longitude 180°, but very large zonal displacements (i.e. horizontal or longitudinal shifts) occur in correlation with changes in the ENSO signal, sometimes over 50° of longitude. During El iño conditions – these are characterized by the expansion of the warm pool eastwards (Figure 4), resulting in warmer than average waters in the central and eastern Pacific, higher rainfall in that region, and cooler than average waters in the western Pacific. During La iña conditions – these are characterized 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 (Figure 4). The equatorial divergence that occurs within the SEC induces a shallow thermocline (~ 50 m) in the eastern Pacific that deepens progressively westward (~150 m in the warm pool). However, during the eastward displacement of the warm water masses accompanying an El Niño, the thermocline deepens in the central and eastern Pacific while it rises abnormally in the western Pacific (Figure 4). Conversely, during La Niña the warm pool is confined to the extreme west of the equatorial Pacific (Picaut et al., 1996), the thermoclime deepens in the western part while it rises in the eastern part. The largest proportion of the tuna catch (mainly skipjack) in the Pacific Ocean is taken within the warm pool area. Surface tuna fisheries, particularly purse-seine fisheries targeting skipjack, appear to respond to the movement of the warm pool (Figure 5). Large scale migration of tropical tuna in the western central equatorial Pacific have been correlated with the position of the oceanic convergence zone, produced where the warm pool meets the cold tongue. This nutrient-rich zone supports high concentrations of secondary productivity (small fishes) in a band several hundred kilometres wide along the eastern edge of the warm-water pool. Tuna are likely to seasonally follow this convergence zone to remain in waters with relatively high concentrations of prey species in conditions suitable for reproduction. Variations in tuna catches are reported at regional and domestic scales both seasonally and inter-annually and are related to east-west migration of the warm pool-cold tongue pelagic ecosystem. For longline fisheries, the vertical change in the thermal structure during El Niño (La Niña) events results in the rising (deepening) and vertical extension (contraction) of the temperature habitats of yellowfin and bigeye, thereby affecting the catchability of the species. Figure 5 Examples of purse seine catches distribution in the equatorial Pacific during an El Nino episode (top) and La Niña episode (bottom). Blue-unassociated sets, red-drifting FADs, yellow-logs, green-anchored FADs. Climate change and risks for fisheries Climate change is currently one of the most scientifically debated and controversial issues. While understanding climate change will be important for fisheries, it is clearly an even larger issue for many Pacific Island states. In this section we will briefly discuss two of the more widely accepted outcomes of climate change for oceanography and tuna fisheries. Clearly this is an area of importance and a considerably active research so we expect to be able to greatly expand upon the potential impacts in the coming years. In considering the relationship between tuna, tuna fisheries and the climate-ocean system, we must also consider the possibility that this relationship will change (or is changing) as a result of climate change. The key question currently is – “How will it change?”. While there is currently significant uncertainty about the implications of climate change for tuna fisheries, considerable research is underway, and a number of observations and hypotheses have been proposed. Firstly, increases in global average sea temperatures have been observed. Sea surface temperatures affect the patterns in atmospheric pressure, which in turn are responsible for wind generation. It has been hypothesized that 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 early 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). The implication of this is that a decline in the upwelling system of the central and eastern equatorial Pacific may lead to reduced productivity in that region which currently moves with currents to the western equatorial Pacific and is a critical feature upon which our tuna 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). A second observation is that El Niño events appear to have become more frequent in recent decades (possibly linked to climate change) (Timmermann et al., 1999; Timmermann et al., 2004) and are associated with an eastward shift of the major tuna resource (e.g. skipjack) in the WCPO (Lehodey et al. 1997). Currently total catch does not appear effected by ENSO related shifts in the stocks. It has been suggested that 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 and bigeye tuna suggest a strong shift in the populations towards the eastern Pacific (see Figure 5). This is related to the weakening of the equatorial upwelling and equatorial current systems predicted by the IPCC models. There are implications of this scenario for purse seine fleets. 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. More research is required to confirm current theories regarding the implications of climate change for tuna fisheries. Figure 6 Global warming and predicted changes in spatial distribution of bigeye and skipjack adult biomass under the IPCC (SRES A2) scenario. (From Lehodey et al. 2008.). REFERECES Bakun A. 1996. Patterns in the ocean: Ocean processes and marine population dynamics. California Sea Grant College System, National Oceanic and Atmospheric Administration. 320 pp. Bopp, L., P. Montfray, O. Aumont, J.C. Orr, G. Madec, J.L. Dufresne, S. Valcke, L.Terray and H. 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