Influence of Buoyancy and Vertical Distribution of Sardine Sardinops Sagax Eggs and Larvae on Their Transport in the Northern Benguela Ecosystem

important components affecting recruitment of fish populations. Successful recruitment depends on the planktonic stages being retained in, or transported to, nursery areas favourable for growth and survival. Advective loss of larvae into less favourable areas would clearly have a negative impact on recruitment (Iles and Sinclair 1982), particularly in upwelling systems where offshore loss of larvae may be considerable (Bakun and Parrish 1982). In his triad theory, Bakun (1996) identified three major groups of environmental processes of importance to fish recruitment in upwelling areas: enrichment of the food chain, retention of the offspring in suitable habitats, and concentration of the food organisms suitable for larvae within these habitats. Based on Bakun’s triad, it seems reasonable to assume that fish with pelagic eggs and larvae will spawn in upwelling areas at certain times of the year and in specific regions that provide optimal conditions for growth and survival of the larvae. Such conditions might be found in coastal indentations where windinduced offshore transport and turbulence are reduced and the width of the continental shelf tends to be greater (Parrish et al. 1983). In the northern Benguela upwelling system, transport within the upper layer will fluctuate with the intensity of upwelling as a result of variation in offshore surface Ekman transport caused by variations in the wind/ pressure field (Shannon 1985). Modelling studies in the northern Benguela (Sundby et al. 1999, Stenevik et al. in prep.) have shown that the thickness of the offshore-moving upper layer is about 20 m and depends on distance offshore and wind force. The same depth was mentioned by Parrish et al. (1981) as a reasonable one for the Ekman layer. Below this depth the water moves inshore. Historically, sardine have been one of the major commercial fish species in the northern Benguela and are typical of upwelling systems in general. The sardine stock in the northern Benguela declined drastically between the 1960s and the 1980s, and has remained low ever since (Beckley and van der Lingen 1999, Schwartzlose et al. 1999). At present the fishable biomass is estimated at around 250 000 tons (Boyer and Boyer 2000). Sardine eggs and larvae are considered to be pelagic, which generally means that they accumulate in the upper mixed layer and are found in increasing concentrations towards the surface, depending on the degree of wind-induced mixing (Sundby 1991). In upwelling systems this means that eggs would have a tendency to be transported offshore within the upper layer and potentially be lost. On the other hand, increased upwelling also favours plankton production and consequently the production of food for larvae (Bakun 1973, 1990). Based on these two opposing processes, Cury and Roy (1989) suggested that optimal recruitment is a trade-off between strong winds, that 85