Location of seagrass beds in estuaries: effects on associated fish and decapods

Abstract

This study examined whether there were differences in abundance, species composition, and species richness of juvenile fish and decapods associated with Zostera capricorni Aschers, beds at different distances (zones) along an estuary. It also determined whether such variation was consistent between two arms of an estuary and if it occurred in different seasons. Total abundance of fish did not differ among zones or arms but abundance of decapods was often greatest near the mouth of each arm. More species of fish and decapods were caught in the lower and middle reaches of the estuary than in the upper reaches, although this depended on the arm. There were significant differences in abundances among zones for 13 of the 16 species studied separately in May, and for 17 of the 20 species examined in November/December. These differences were usually expressed as zone × arm interactions, and often occurred because individuals were absent or scare in one arm. Patterns of mean abundance among zones and arms were different for most species. Each combination of zone and arm had a distinct assemblage of fish and decapods. Artificial seagrass units (ASUs) were used to control for variation in physical complexity of Zostera among zones. There were striking similarities in patterns of abundance among zones and arms for the majority of species abundant enough for analysis in both Zostera and ASUs. We conclude that 1. (1) the location of a seagrass bed within an estuary has a significant effect on abundances of juveniles of many species of fish and decapods, 2. (2) for most species, these effects are due to the position of the bed, not to its size, shape, leaf height or leaf density, and 3. (3) the most likely cause of these effects is variation in distribution and availability of competent larvae.