THE ECOLOGY OF PALAEMON PACIFICUS (STIMPSON) ASSOCIATED WITH ZOSTERA CAPENSIS SETCHELL

SUMMARY

The shrimp Palaemon pacificus was sampled in Zostera capensis beds in three eastern Cape estuaries (Sundays, Swartkops and Kromme). In all three estuaries shrimp numbers and biomass in the Zostera generally exhibited seasonal fluctuations, with summer/autumn peaks from November to March (density 200–400 m⁻²; dry mass 3–6 g m⁻²) declining to winter/spring minima from June to September (20–100 m⁻²; 0,3-1,5 g m⁻²). The Kromme had the highest mean annual shrimp biomass (dry mass 2,86-3,10 g m⁻²), followed by the Sundays (2,69 g m⁻²) and the Swartkops (2,11-2,22 g m⁻²). Shrimp size-frequency distributions displayed a pattern of bimodality in summer/autumn from November to March, becoming polymodal during the year. A temporal variation of sex ratio was evident with overall M:F ratios of 1:2,27 in the Sundays and 1:1,96 and 1:1,82 at two Swartkops stations. A mean of 4 cohorts per annum was resolved for the Sundays and 3,1 for the Swartkops, with mean residence times in the Zostera of 11,9 months and 10 months respectively. Mean summer growth (0,104 mm d⁻¹) was double that of winter (0,055 mm d⁻¹). A highly significant association was found between shrimp and Zostera both spatially and temporally. At low tide significant correlations were found between shrimp mass and water depth in the Zostera. The mean annual somatic production of P. pacificus was highest in the Sundays (15,30 g m⁻²) followed by the Swartkops (12,42 g m⁻²). The production/biomass ratios were similar, varying from 5,62 to 5,83 in the Swartkops to 5,69 in the Sundays.