The ecology of flounders, Platichthys flesus and other major fish species in the Severn Estuary and the Bristol Channel.

Abstract

1. Although flounders are present throughout the year in the estuary and the Bristol Channel, it was observed that sexually mature animals occurred in late summer/autumn and only in small numbers during the winter. Young animals (0+ and 1+), however, were present at all times but in varying numbers. The absence of sexually mature animals in late spring/early summer is occasioned by breeding factors, while in the case of the younger fish the variation in numbers is probably due to feeding migration, Sea snails and five bearded rocklings, on the other hand, are winter residents in the study area (except in the case of five bearded rocklings at Minehead), arriving in small numbers in autumn which build up to peak numbers by mid-winter, only to diminish by late winter/early spring. From year to year fish populations vary in this region, but the patterns of behaviour within species remains reasonably constant. 2. The age structure of the Severn fish population has been studied by otolith readings combined with length-frequency distributions and it was observed that the majority of the fish population in this area belong to the first and second year classes, indicating that this region is a nursery ground for a number of different species of fish. 3. Despite the fact that the Severn region is heavily industrialised, no disparity in growth in length and weight was observed for animals obtained from this region when compared with those from areas where little industrial activity exists and our results show rapid growth for young fish during their period of residency in the estuary and the Bristol Channel. 4. We note that there is generally a preponderance of females (in some cases, vary marked) over males in the three main species of teleosts investigated, but so far we can offer no explanation for this observation. 5. Flounders, sea snails and five bearded rocklings spawn from spring to early summer, but there is some evidence for species differences. Further, it is concluded that the rate of growth of the gonads is similar at all the sites within the estuary and the channel. Immediately prior to the onset of the spawning migration, the gonads of female sea snails, five bearded rocklings and flounders grew rapidly and eventually accounted for at least 10-13% of the total body weight, but the increase in weight of the testis was less dramatic and at the time of departure from the study area formed about 1,0-1.5% of the total body weight of the fish. 6. The spawning migratory pattern is the same for male and female flounders and five bearded rocklings, but in the case of sea snails there was a tendency for the males to leave for the spawning grounds a few weeks earlier than the females. 7. The utilisation of food organisms by teleosts has not always been closely correlated with the relative abundance of such organisms, but where many dietary organisms are present, food preferences may be observed. Thus, Gammarus spp. were found to be an important and relatively constant food organism for all the teleosts studied, except for mature flounders taken in the Barnstaple area, where the bivalve, Macoma baltica was found to be widely ingested. Despite the abundance of shrimps, Crangon erangon, throughout the year, there was an actual decrease in the uptake of this decapod in all the three species investigated since 1973 in the Severn Estuary and the Bristol Channel. 8. Although there was variation in the percentage of nematode infestation from species to species, it was seen however, that in any one species the percentage of infection did not vary from site to site. 9. The rate of uptake of heavy metals in the vertebrate and invertebrate populations of the study area depend largely on such factors as salinity, water temperature, type of food uptake, and age, as well as differing from species to species. As a result of continuous monitoring a considerable amount of data has been collected concerning the uptake of zinc, lead and cadmium by fish and their prey. Concentrations of cadmium and lead in fish tissues are lower than in their food organisms. Thus, the ratios of concentrations in predator and prey varied from 1 r 25,0 for cadmium and 1 : 2-3 for lead. On the other hand, zinc concentrations in predator and prey are similar. Present studies suggest that while lead and zinc are to some extent present in high concentrations in the natural run off of the region, cadmium is an industrial by-product, since fish from the less industrialised areas contain much lower concentrations of this element compared with those from sampling sites closer to industrial activity.

Date of Award	1977
Original language	English
Awarding Institution	University of Bath