Common dab

Abstract: Data from the Scottish participation in the International Young Fish Survey for the period 1976–1993 were analysed to examine the effect of light level and depth on catch rates. The species selected for this study were common dab, herring, haddock and whiting. Differences between day and night were observed for the juvenile common dab and herring and for the adult common dab and haddock. Differences between shallow and deep water were observed for the juveniles of all the species and for the adults of common dab, haddock and whiting. The mean lengths and the composition of the common dab and whiting catches were affected by both light and depth, indicating behaviour differences between the juveniles and adults of these species. Whilst light seems to have no effect on the mean length and the catch composition of herring, differences were observed between shallow and deep waters. In the case of haddock, neither the light level nor the depth had any noticeable effect on the mean length and catch composition of the catches. We conclude that the diel behaviour and the geographic distribution are important factors in determining the quantity and composition of trawl catches, but their effects are species dependent. When trawl surveys are not exact replicates in terms of fishing times and areas, the estimation of catch indices should allow for the possible bias introduced by these factors. There is a need for models of the capture process that take account of such effects.

Abstract: Salmonid alphaviruses (SAVs), which include the aetiological agents of salmon pancreas disease (SPD) in farmed Atlantic salmon Salmo salar L. and sleeping disease (SD) in rainbow trout Oncorhynchus mykiss (Walbaum), are significant viral pathogens of European salmonid aquaculture. SAV is horizontally transmitted and the virus can survive for extended periods in seawater. A lack of convincing evidence for vertical transmission coupled to the fact that the SPD virus (SPDV) occurs in historically infected sites irrespective of fallow period duration suggests that a substantial reservoir of infection exists in the marine environment. We used a highly sensitive real-time PCR (qPCR) assay targeting a region of the SAV nsP1 gene to screen wild marine fish species for the presence of SAV in an attempt to identify such a potential reservoir. Screened fish species were caught in the vicinity of aquaculture activity in an area with a previous history of SAV infection (Shetland Isles, Scotland). SAV RNA was detected in internal organs (kidney and heart) from the flatfish species common dab Limanda limanda, long rough dab Hippoglossoides platessoides, and plaice Pleuronectes platessa. Based on these findings, sampling was extended to an area remote from aquaculture activity (Stonehaven Bay, NE coast of Scotland) from where heart tissues obtained from common dab also tested positive. While no virus could be cultivated from these samples, qPCR detections were shown to be SAV-specific by sequencing of an alternative gene region (E2) to that targeted by the qPCR assay. Analysis of these nucleotide sequences revealed minor differences to those previously obtained from farmed salmon, and subsequent phylogenetic analysis of an E2 dataset demonstrated a subtype V-like sequence.

Abstract: A survey of fish diseases in the common dab Limanda limanda L. was conducted in 4 areas (the German Bight, 2 areas along the west coast of Denmark and the Skagerrak) in May during the years 1983 to 1993. A total of 53302 dab were examined for the presence of the diseases lyrnphocystis, epldermal papilloma and skin ulcers. The present study describes long-term variations in the prevalence of these 3 chseases. The German Bight and the 2 areas along the west coast of Denmark showed significant similarities in the temporal trend of the diseases investigated with peaks in 1985 and 1988 for both lymphocystis and epidermal papilloma. A similar trend was not observed in the Skagerrak. Skin ulcerations did not reveal any specific temporal trends in any of the 4 areas. The highest prevalence of lymphocystis was observed in the most offshore area along the Danish west coast with a peak value of 14.9% in 1988; epidermal papilloma in the German Bight showed a peak value of 9 4 % in 1988 and skln ulcers in the Skagerrak a peak of 4.4 % In 1989. Areas in the German Bight and along the west coast of Denmark had suffered from oxygen deficiency in late summer during the years 1981 to 1983. These events may have been an important factor triggering the outbreak of lymphocystis and epidermal papilloma, as the prevalence of these diseases increased from 1983 until 1985 followed by a decline until 1988. Although it was not possible to establish significant correlations between the oxygen levels at the bottom and the disease prevalence in the present case, the disease pattern showed evident similarities with formerly described oxygen deficiency-induced outbreaks of lymphocystis and epidermal papllloma in dab in the Kattegat. In 1988, the prevalence of lymphocystis and epidermal papllloma increased significantly without any apparent reason. The disease pattern observed in the Skagerrak differed significantly from the other 3 areas with skin ulcerations being the most prevalent disease. A part of the explanation for this may be that the Skagerrak has not suffered from the same impaired environmental conditions as the other areas.