Some factors affecting size distribution and density of grass shrimp (Palaemonetes pugio) populations in two New Jersey estuaries

Abstract

Mature Palaemonetes pugio, from a polluted estuary, Piles Creek (PC) are larger than those in a more pristine estuary in Tuckerton (TK). Possible causes for the differences in size-structure could be differences in environmental factors at the two creeks, differential competition at the two sites, inherent factors causing a greater growth rate in the PC population, differences in reproductive timing, or differential predation at the two sites. Lab microcosm studies were used to examine the possibility of inherent population differences or environmental conditions (including differences in salinity) causing faster growth in PC. There was no evidence that PC shrimp grew faster than TK shrimp, nor that PC conditions fostered greater growth compared to TK. Salinity adjustments in PC and TK tanks also had no effect on growth. Therefore, it appears that genetic and environmental factors did not play a significant role in greater shrimp growth in PC. In the field, data on relative abundances of Fundulus heteroclitus and P. pugio, and size-frequency distributions of P. pugio were collected from the two estuaries. Gravid females and recruits of young shrimp physically appeared at approximately the same time within both systems, eliminating earlier reproduction as a cause of the size discrepancy. There were three times as many shrimp in PC than in TK (eliminating the possibility of increased population density and competition at TK as a cause) and three times as many F. heteroclitus present in TK than in PC. Since it has already been established that the Fundulus at PC are smaller than at TK and that they are poor predators, differences in Fundulus predation would appear to be an important factor in determining the number and size-frequency of the grass shrimp.