The biological characteristics of the adult sea trout stock in the River Tywi were examined between 1988 and 1992 by sampling catches from a temporary main-river trap, and the commercial seine, commercial coracle and recreational rod fisheries. Radio tracking of adults during the spawning season investigated behavioural activities and their possible effects on stock characteristics. Catch effort data from angler log books assessed the annual performance of the rod fishery between 1990 and 1993. The effects of ameliorating the impact of acidification on water quality, benthic macro-invertebrates and juvenile salmonids, through reservoir liming, was studied.

The crabs Cancer pagurus (L) and Necora puber (L) are exported live, in bulk, from the UK to various continental countries. The success of this relatively new trade is marred by the incidence of mortalities and impaired quality of the delivered product. These studies addressed various causes - procedural and biological - of these events.Descriptions are given of detailed examinations of handling and other marketing protocols for both species from point of capture to arrival at continental dealer's premises. Such examinations were made with the help of a number of major dealers in the UK, Spain and France and included studies of handling, packing, holding and transportation methods, physical damage assessments before and after consignment, and chemical and biochemical analyses of seawater and blood samples.Dissolved ammonia levels were found to increase greatly in the fixed volume water of vivier tanks and this was found to be matched by correspondingly high blood ammonia values of the contained animals. The measurement of both free ammonia and ionic ammonia efflux rates of juvenile and adult Cpagurus and Npuber in media with high dissolved ammonia levels was investigated and was found to be related to concentration gradients between the internal and external media. The fluxes could be explained on the basis of diffusion down concentration gradients. When animals were transferred to media with higher ammonia levels than those in blood, a cessation of efflux, or even a net influx of ammonia (NH4+) occurred.During emersion, blood ammonia concentration rose. Such accumulated ammonia was very rapidly off loaded when the animals were re-immersed.The data produced has been discussed in the context of crustacean physiology and of improving the expectations of delivering a live, quality product after journeys of several days.

The breakdown of reproductive isolation leading to inter-specific hybridization is a widespread phenomenon amongst cyprinid fishes. There are seventeen cyprinid species occcurring in the British Isles, within five sub-families, giving rise to some ten different types of hybrid. Most of these belong to the sub-family Leuciscinae. The most commonly occurring hybrids are probably those between roach, Rutilus rutilus (L.), and common bream, Abramis brama (L.). In this study the phenomenon of hybridization amongst species of the Cyprinidae was investigated through: an experimental breeding programme to investigate the nature of inter-species and hybrid gamete compatibilities; the identification of species and hybrids from the experimental breeding programme through the analysis of morphometric, meristic and genetic characters (genetic characters were analysed using enzyme electrophoresis); the comparison of morphmetric, meristic and genetic information of natural fish with similar features of fish from the experimental breeding programme to identify the occurrence of post Fl hybridization in natural populations; the use of restriction enzyme analysis of mitochondrial DNA to elucidate the importance of maternal ancestry in a natural hybrid population. The breeding programme found, for the species in this study, that there was no success in cross-fertilization of taxa between different sub-families. Interspecific gamete compatibility was only found within the leuciscine sub-family. In cases where a hybrid cross produced progeny it was also noted that the reciprocal cross was successful. This suggests that there is not a genetic barrier to gamete compatibility resulting from the sexual directionality of a hybrid cross. Female roach/common bream hybrids also produced progeny when crossed with males of leuciscine species. Identification of the progeny of the experimental breeding programme showed that the genetic techniques of enzyme electrophoresis was more reliable than the statistical analysis of meristic and morphometric traits in the identification of species and their Fl hybrids. However, genetic information alone cannot establish precisely the nature of post Fl hybrids and in the identification of backcrossed roach/common bream hybrids it was noted that meristic information was needed to support genetic data. In the two natural hybrid populations of roach/common bream and rudd/common bream, from the Forty Foot Drain and Essex University Lake respectively, the analysis of morphometric, meristic and genetic characters found no evidence of post Fl hybridization in these waters. It is suggested that absence is due to either the limitations of the sampling methods or biological processes. Possible biological processes include factors such as the inappropriate mating behaviour of Fl hybrids or the inferior fitness of post Fl hybrids. The analysis of mitochondrial DNA did not yield sufficient results to elucidate the importance of maternal ancestry in hybridization. It is suggested that this aspect of hybridization is of such critical importance that it must become the subject of a future research programme. The importance of the causes and consequences of inter-specfic hybridization in fishes are discussed. It is suggested that, because they are rarely investigated in hybrid studies, these become incorporated into research programmes in the future. These areas of investigation will have implications for fisheries management, freshwater ecology, genetic conservation and species integrity.