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Malia Chevolot Jim R. Ellis Galice Hoarau Adriaan D. Rijnsdorp Wytze T. Stam Jeanine L. Olsen 《Journal of Sea Research》2006,56(4):305-316
Prior to the 1950s, thornback ray (Raja clavata L.) was common and widely distributed in the seas of Northwest Europe. Since then, it has decreased in abundance and geographic range due to over-fishing. The sustainability of ray populations is of concern to fisheries management because their slow growth rate, late maturity and low fecundity make them susceptible to exploitation as victims of by-catch. We investigated the population genetic structure of thornback rays from 14 locations in the southern North Sea, English Channel and Irish Sea. Adults comprised < 4% of the total sampling despite heavy sampling effort over 47 hauls; thus our results apply mainly to sexually immature individuals. Using five microsatellite loci, weak but significant population differentiation was detected with a global FST = 0.013 (P < 0.001). Pairwise Fst was significant for 75 out of 171 comparisons. Although earlier tagging studies suggest restricted foraging distances from home areas, the absence of genetic differentiation between some distant populations suggests that a substantial fraction of individuals migrate over wide areas. Autumn/winter locations appear to have a lower level of differentiation than spring/summer, which could be due to seasonal migration. Management and conservation of thornback ray populations will be challenging as population structure appears to be dynamic in space and time. 相似文献
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Big fish (and a smallish skate) eat small fish: diet variation and trophic level of Sympterygia acuta,a medium‐sized skate high in the food web 
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下载免费PDF全文 The bignose fanskate, Sympterygia acuta, is a small‐to‐medium‐sized species endemic to shallow coastal waters of the Southwest Atlantic. Sympterygia acuta displays a clear seasonal reproductive cycle, characterized by maximum egg‐laying activity in spring and hatching in summer. We hypothesized that diet and feeding activity change with maturity stage and season and that, given its smallish size, the trophic level is low. Using a multiple‐hypothesis modeling approach, the diet of S. acuta in relation to sex, body size, maturity stage, region (i.e. north and south) and season was analysed; and a potential relationship between feeding activity and the seasonal reproductive cycle was assessed. Sympterygia acuta fed on a broad spectrum of prey, but teleosts were more important (47.97% index of relative importance, %IRI), followed by decapods (39.84%IRI), cumaceans (8.31%IRI) and isopods (1.89%IRI). Maturity stage was a strong determinant of the ontogenetic diet shift of S. acuta, and relationships between number of prey consumed with season and region were found.. Feeding activity was higher in the cold season than in the warm season, and was less important in the south region than in the north region. Unexpectedly, the specific trophic level was high (3.87). Sympterygia acuta shifts its diet with maturity stage, possibly by a combination of an improved ability to capture prey and a change in energy demand of mature individuals. Despite being a small‐to‐medium‐sized skate, S. acuta showed a trophic level similar to that of large‐bodied marine predators. It reduces its feeding activity seasonally because in the warm season this species may experience an increased predation risk from large sharks. 相似文献
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