Seasonal changes of the pelagic fish assemblage in a temperate estuary

The pelagic (i.e. fishes that spend most of their time swimming and feeding in the water column) fish assemblage was examined within a temperate estuary to determine the seasonal patterns of abundance, biomass, richness, and species composition. These measures were related to abiotic measures including temperature, salinity, water transparency, and depth. Pelagic fish (n<350 000) were sampled weekly from August 1995 through December 1996 in Great Bay in New Jersey, USA, with a pop net (5m diameter, 5m length, 3mm cod end). The most frequently occurring and abundant species included Clupea harengus (frequency of occurrence ¼ 27.4%; abundance ¼ 80.5% of total catch), Menidia menidia (83.5, 10.6%), Anchoa mitchilli (66.5, 6.0%), and Anchoa hepsetus (23.1, 1.9%). There were strong seasonal trends in species richness, total abundance, and total biomass with peaks in spring, summer, and autumn, and very low values for these parameters in winter. In addition, there were five seasonal groupings within this pelagic fish assemblage that were primarily, but not exclusively, represented by young-of-the-year (age 0þ). Winter (December–March) was represented by adult Gasterosteus aculeatus and age 1þ by M. menidia. Spring (April–early June) was overwhelmingly dominated by age 0þ C. harengus. Early summer (late June–early July) was relatively species rich and characterized by age 0þ of Brevoortia tyrannus, Pomatomus saltatrix, Alosa pseudoharengus, and age 1þ of Anchoa mitchilli. Late summer (late July– August) remained relatively species rich and included mostly age 0þ of A. mitchilli, A. hepsetus, and M. menidia. Autumn (September– November) continued to have high catches of age 0þ of M. menidia and A. mitchilli. As expected for temperate estuaries, the best of the measured abiotic predictors of these groupings included temperature, salinity, and degree of change in temperature. However, timing of life history characteristics, such as location, and timing of reproduction and migration also influenced the seasonal occurrence of these pelagic species. The large abundance and biomass of a relatively few species, as observed in this study, are typical of estuaries, although this is one of the few studies that have focused solely on the pelagic component of estuarine fish assemblages.