Seasonal abundance, growth and production of Palaemon pacificus (Stimpson) in eastern Cape tidal pools

Abstract

The shrimp Palaemon pacificus was sampled monthly from six tidal pools for two years. The shrimp populations varied seasonally with peak numbers and biomass found in summer (December to March) following a major influx of juveniles in November. These recruits were composed of discrete modal groups (bimodal) which spread and became less obvious by April (polymodal). Females had a greater mean size range than males. The sex ratio varied temporally and overall male: female ratios varied from 1:1,66 to 1:2,96. Average shrimp total length was significantly correlated to the average and maximum pool depth and to log pool volume while the average number of cohorts per year was found to be negatively correlated to these parameters. In smaller pools the shrimp disappeared above 20-30 mm but remained up to reproductive age in the deeper pools. Average residence time was positively correlated to pool depth, but not to log volume. Mean summer growth rates were almost double those in winter and were combined in the von Bertalanffy growth equation L_t = 64,51 (1-e^{-0,08902(t-0,1001)}). P. pacificus   reaches first maturity from egg at six months for males and 12 months 2 for females. Mean annual production varied between pools from 0,22 to 2,0 g m ^-2 and biomass from 0,06 to 0,5 gm^-2. The highest P/B ratio of 4,24 was obtained from the smallest pool which had a low biomass but high productivity and shrimp turnover.