Nekton community structure of the Rio Grande estuary

Abstract

Spatial and temporal trends in nekton community structure of the Rio Grande estuary were characterized quarterly during May 1989-February 1990. Bag seine, otter trawl, experimental gill net and hydrographic sampling was conducted at nine stations located between the mouth of the Rio Grande (river mile 0) and near Brownsville, Texas (river mile 51). Ninety-eight taxa (77 fishes and 21 macroinvertebrates) represented by 38, 958 nekton constituents were grouped according to estuarine utilization into estuarinedependent (45 taxa), resident (23), marine (17), and freshwater components (15). Five estuarine-dependent taxa-brown shrimp, white shrimp, gulf menhaden, bay anchovy, and striped mullet-comprised 78% of the study-wide harvest and dominated quarterly catches. Nekton community structure indices (species diversity, evenness, and richness) were calculated for quarterly seine and trawl collections at each station and pooled to provide seasonal values. Greater species utilization of the estuary during warm months (May, August, and November) and recruitment of seasonally dominant species significantly affected species diversity and richness. Significant reductions in species diversity and richness during February coincided with water temperature minima and a notable increase in gulf menhaden and striped mullet abundance. Species diversity and richness near the river mouth were significantly greater than those at upstream stations (river miles 40 and 5 1). Principal component analysis partitioned the study area into three salinity-related zones-marine, mixing/brackish, and freshwater- which explained 94% of the variance in nekton distribution. Dominant taxa comprising each zone were estuarine-dependent species exhibiting seasonal utilization of the lower Rio Grande.