Are fish stocks food-limited in the southern Benguela pelagic ecosystem?

Abstract

Estimates of phytoplankton standing stock from satellite ocean color measurements are in good agreement with ship observations in the southern Benguela Current system. Monthly variation of phytoplankton biomass was within 50% of the mean value. Available data on phytoplankton primary production in the system are reviewed and indicates, that an average value of 2.8 g C m-2 day-1 is appropriate for the productively active area of the system, defined from chlorophyll measurements. Consumer zooplankton and pelagic fish biomass are estimated and used with realistic P/B [production/biomass] ratios, growth efficiencies and daily ration requirements to calculate upper and lower production and consumption limits. The estimates agree wtih independent calculations of pelagic fish production. Combined with zooplankton production estimates, these give consumer production/primary production ecological efficiencies of 5-18% (mean 9.5%). Considerations of food and spatial suitability within the productively active area (match-mismatch in a pulsed upwelling system) indicate that only some 12% of total primary production is likely to be utilizable by small pelagic shoal fish, and that a production of some 2 million tonnes wet mass of pelagic shoal fish is the maximum that can be supported in the southern Benguela system. Estimates of the carrying capacity of the spawning area during spring and summer indicate that the maximum spawning biomass which could be supported is litle more than 500,000 tonnes wet pelagic fish. Consideration of food requirements and availability suggests that spawning and recruitment areas and times are most likely to limit fish production in the southern Benguela area.