Fine-scale spatial variability of different stages of pelagic fish eggs over the western Agulhas Bank, South Africa

Abstract

Stage-dependent spatial distributions of anchovy Engraulis encrasicolus, sardine Sardinops sagax and round herring Etrumeus whiteheadi eggs over the western Agulhas Bank South Africa were examined from samples collected at a fine-scale (1.8 km) resolution using a continuous underway fish egg sampler (CUFES). Spatial structure in egg distributions was assessed using variograms for three developmental stage groups (early-, middle- and late-stage eggs), and indices of aggregation were also computed for those stage groups. No early- or middle-stage anchovy eggs were collected, but indices of aggregation showed that early-stage sardine eggs were more aggregated than those of round herring, suggesting that adult sardine have a more contiguous distribution during spawning than have round herring. Middle-stage eggs were equally aggregated, whereas late-stage eggs were aggregated differently in all three species. Fitted empirical variograms showed that early-stage sardine eggs have low small-scale variability (nugget effect) (≤10% of total variance) compared with round herring eggs (≥50% of total variance). The autocorrelation range of both those species extends to 3.7 km. Middle-stage sardine eggs have a low nugget effect (≤10%) compared with round herring eggs (≤50%), with autocorrelation ranges of 7.4 km and 5.6 km respectively. Late-stage anchovy eggs have a lower nugget effect (≤20%) than that of sardine (≤50%) and round herring (≥100%) eggs, with autocorrelation ranges of 3.7 km for all three species. The spatial structures of the eggs of the three species were heterogeneous at the sampling scale used, indicating differences in spatial organisation of the adults during spawning. The spatial variability of the eggs of the three species is not well captured using the sampling strategy during current ichthyoplankton surveys in the southern Benguela, in which stations are spaced 18.5 km apart.