Hydrographic features related to pilchard and anchovy spawning in the northern Benguela system, comparing three environmental regimes

Abstract

Seasonal and spatial trends in spawning of pilchard Sardinops ocellatus and anchovy Engraulis capensis in the northern Benguela system were examined in relation to hydrographic features within the spawning grounds, comparing cooler and warmer regimes and an El Niño-type event in 1983/84. The comparisons were based on 53 cruises from nine seasonal surveys during the periods 1972–1974 and 1978–1985. Within the spawning grounds there were two localities of main reproductive activity, associated with different hydrographic features and differing spawning strategies. Between 22 and 24°S, a locality in which there is usually only a thin layer of oxygen-rich water above the shallow thermocline (<20 m), heavier spawning was associated with moderate thermal stratification in the upper 20 m, and there was little spawning when stratification was either absent or extensive for a long period. Between the Cunene River and 22°S, a locality where the upper mixed layer of well-oxygenated water is often 100 m thick, the presence or absence of thermal stratification in the upper 20 m did not apparently affect spawning. During the El Niño-type event, when nutrient concentrations, even deep in the water, were low and strong stratification was prevalent over a large area for several months, spawning was reduced. Variation in egg distribution in relation to salinity and temperature could be explained by differences in the locality and the timing of spawning during the three regimes. Pilchard and anchovy spawned mainly in water with salinities of 35,1 – 35,6 × 10⁻³ and temperatures at 20 m of 13–19°C.