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Clupeoid Population Variability, the Environment and Satellite Imagery in Coastal Upwelling Systems

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Clupeoid Population Variability, the Environment and Satellite Imagery in Coastal Upwelling Systems Reviews in Fish Biology and Fisheries 8, 445±471 (1998) Clupeoid population variability, the environment and satellite imagery in coastal upwelling systems JAMES COLE1Ã and JACQUELINE MCGLADE2 1Paci®c Fisheries Environmental Laboratory, NOAA±NMFS, 1352 Lighthouse Avenue, Paci®c Grove, CA 93950, USA 2Centre for Coastal and Marine Sciences (NERC), Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, United Kingdom Contents Abstract page 445 Introduction 446 Background to coastal upwelling systems 447 Clupeoids and the environment: a general perspective 451 The management problem 454 Recruitment variability and the environment 455 Mechanistic theories Synthesis theories Problems with predicting recruitment 459 Non-linearity Scale The multiplicity and complexity of processes that in¯uence recruitment Environmental indicies Satellite oceanography: a way forward? 462 Conclusion 465 Acknowledgements 466 References 466 Abstract Sardine, pilchard and anchovy stocks form the basis of commercially important purse seine ®sheries in eastern boundary upwelling regions. High levels of environmentally driven recruitment variability have, however, made them especially dif®cult to manage. Reliable forecasts of recruitment success would greatly help with the setting of catch quotas prior to each ®shing season. Theories of how environmental conditions in¯uence recruitment success, according to survival=mortality of the early life-history stages, can be divided into mechanistic and sythesis theories. Mechanistic theories are concerned with speci®c physical processes, whereas synthesis theories attempt to unite the various ÃAuthor to whom correspondence should be addressed (e-mail: [email protected]). 0960±3166 # 1998 Chapman & Hall 446 Cole and McGlade mechanistic processes within a single conceptual framework. Despite the successful testing of some theories, there has been little success in reliably predicting recruitment success from a knowledge of environmental conditions. Possible reasons include the following: non-linearity in the relationship between environmental parameters and recruitment; the poor spatial and temporal resolution of much oceanographic data; the wide range of different factors involved in determining recruitment success; and the choice of environmental index. The recent compilation of time series of satellite images for these regions offers a solution to some of these problems, and in doing so reopens the possibility of ®nding suf®ciently good relationships between environmental conditions and recruitment success for management purposes. In particular, the high resolution of these time series allows for the construction of environmental indices across many different spatial and temporal scales. These time series also open up the possibility of quantifying the behaviour of upwelling systems according to the evolution of their spatial structure through time, using pattern analysis techniques. Keywords: clupeoids, coastal upwelling, population dynamics, recruitment, satellite imagery Introduction Clupeoid ®shes are found in pelagic environments throughout the world's oceans, and are especially abundant in the productive coastal upwelling regions found along the eastern margins of the Atlantic and Paci®c oceans (Fig. 1). Species of sardine, pilchard and anchovy (Sardinops spp. and Engraulis spp.) typically form the largest stocks. The relative ease with which they are caught by purse seiners, owing to their schooling behaviour, combined with the enormous biomasses reached by some populations, has made them important economic resources. For instance, at the peak of the Peruvian anchoveta (E. ringens) ®shery, in the early 1970s, clupeoids contributed roughly a third of total world catches, which at the time were in the region of 65 million tonnes. The Atlantic herring (Clupea harengus) aside, clupeoids tend to be short lived, rarely living beyond 5±10 years, and typically recruit to the adult stock between 1 and 3 years old (Blaxter and Hunter, 1982). High levels of recruitment variability have made clupeoid ®sh especially dif®cult to manage. Because they are usually short lived, any ¯uctuations in recruitment success translate rapidly into ¯uctuations in population size, and what may be a conservative level of exploitation during years with good recruitment may during unfavourable years result in over®shing. Despite evidence that ocean conditions play a major role in determining recruitment success, according to their in¯uence on the survival of the early life-history stages (Lasker, 1975; Cury and Roy, 1989; Bakun, 1996), ®sheries scientists remain unable to anticipate environmentally driven ¯uctuations in recruitment, and to adjust management policy accordingly. This article has three main goals: ®rstly, to review the physical and ecological factors relevant to the dynamics of clupeoid populations in eastern boundary coastal upwelling systems, including a synopsis of theories which address the causes of recruitment variability according to the survival of early life-history stages; secondly, to highlight speci®c problems with the use of traditional management techniques for these stocks; Clupeoid stock dynamics in coastal upwelling areas 447 1808 1508 1208 908 608 308 08 308 608 608 608 N Oregon/ Northwest 308 California Africa 308 08 08 Peru 308 Benguela 308 608 608 S 1808 W 1508 1208 908 608 308 08 308 608 E Fig. 1. Eastern boundary coastal upwelling regions, adapted and redrawn from Mann and Lazier (1991). Arrows indicate prevailing winds. and ®nally, to examine why there has been widespread failure in recruitment forecasts, and to suggest new research avenues whereby this situation might be remedied. For a thorough review of clupeoid biology and ecology, please refer to Blaxter and Hunter (1982). Background to coastal upwelling systems Coastal upwelling in eastern boundary systems (Figs 1 and 2) is driven by the action of the prevailing equatorward winds on the surface waters combined with the Coriolis effect (see Mann and Lazier, 1991 for a full discussion). Biologically it is important because often it brings fresh supplies of nutrient into the surface layers. This, in turn, fuels high levels of primary production, resulting in a productive food web which can support large populations of ®sh, marine mammals and sea birds (Lalli and Parsons, 1993). Fluctuations in the strength of upwelling favourable winds, changes in bathymetry along the coast, dynamic instabilities in current ¯ows, and remotely forced processes, such as coastally trapped waves and warm water intrusions, typically cause a high degree of spatial and temporal variability in upwelling activity such as illustrated by the three images in Fig. 3. Strong interannual variability in upwelling and current ¯ows are related to oscillations in the atmospheric pressure ®elds over the equatorial Paci®c and Atlantic Oceans (Hisard, 1988; Mann and Lazier, 1991; Bakun, 1996). 448 Cole and McGlade Thermal upwelling SURFACE WINDS front North East Warm ocean water Cool LAND water Ekman Drift Upwelling Thermocline SEA BED Fig. 2. Schematic representation of eastern boundary coastal upwelling in the southern hemisphere, adapted and redrawn from Mann and Lazier (1991). El NinÄos, and the less publicised Benguela (or `Atlantic') NinÄo (Shannon et al., 1986), are the most dramatic manifestation of these atmospheric oscillations. In coastal upwelling systems they are typically characterized by intrusions of warmer water masses from the edges of the system, a deepening of the thermocline, and subsequent reductions in the upwelling of cool, nutrient-rich water from below the thermocline. A longshore intrusion of warm surface tropical Angolan water into the northern Benguela during the strong Benguela NinÄo in 1984 (Shannon et al., 1986) is shown in Fig. 3(b). The biological impacts of these events on coastal upwelling systems can be dramatic, and are often associated with greatly reduced levels of productivity throughout the entire ecosystem and the invasion of biota from neighbouring regions (Valdivia, 1978; Boyd et al.,1985). Ecologically, these systems are generally characterized by high biomasses, high productivities, low biodiversity, and a low number of trophic exchanges between Fig. 3. Contrasting conditions in the northern Benguela: (a) strong winter upwelling activity throughout the region and a well developed upwelling ®lament between 258 and 278 S; (b) a strong intrusion of tropical Angolan water along the Namibian coast during the 1984 Benguela NinÄo event; and (c) warming off central Namibia combined with the onshore movement of South Atlantic surface water. The arrows indicate intrusions of warmer water into the system and represent potential mechanisms for the retention of clupeoid eggs and larvae, and the concentration of food particles across the associated thermal fronts. Areas contaminated by cloud cover were interpolated according to the distance-weighted mean of the surrounding non-contaminated pixels. These SST composites form part of the Cloud and Ocean Remote Sensing around Africa (CORSA) dataset, held at the Space Applications Institute of the European Commission's Joint Research Centre. Clupeoid stock dynamics in coastal upwelling areas 449 450 Cole and McGlade primary production and ®sh production (Ryther, 1969). The species composition and trophic organization of one coastal upwelling region to another is similar, and for the pelagic layer this is much as illustrated in Fig. 4. The `bottom-up' perspective on these ecosystems places high
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