Habitats of 0+ fry in an English lowland river

Abstract

The fish fry communities of the upper River Thames and a connected backwater were sampled quantitatively by a fry seine net in July-August when river flows were very low and the natural marginal vegetation was fully developed. At this time of year, the habitats available to them were (1) clumps of channel plants at the water edge of the littoral vegetation and (2) deep erosion banks without plants. A third characteristic habitat in this river were (3) shallow organically-rich sandy-gravel beaches produced by watering cattle. These three habitats were sampled along a 25 km stretch of the main channel in order to estimate species composition, densities and sizes of their fry communities. A comparable study in a flow-through backwater sited on the original river course showed that fast water flows from an open weir-gate influenced water depths, produced steeper inshore gradients and affected the distribution of the marginal vegetation and the fry communities. The most species-rich and abundant fry communities were found in habitats with a plant cover and in the shallow beaches because they offered both a good food resource and protection from predation. Neither of these was available in the deep erosion banks where few 0+ fish occurred. No 0+ fry occupied the plantless sites in the backwater which were subject to high velocities although even a poor plant cover could offer a refuge against flow for a few species. Roach was the predominant species in the abundant fry communities and its relative abundance explained differences in community structure estimated by Simpson's and Shannon Diversity Indices.