Browsing Theses & Dissertations by Author "Clarke, Maurice"
Aspects of the biology of three exploited deepwater sharks Centrophorus squamosus, Centroscymnus coelolepis and Deania calceus (Elasmobranchii: Squalidae) from the continental slopes of the Rockall Trough and Porcupine BankClarke, Maurice (National University of Ireland, 2000)Aspects of the biology of three exploited sharks (Elasmobranchii: Squalidae) from the continental slopes of the Rockall Trough and Porcupine Bank (NE Atlantic) were examined using specimens taken during trawl and long-line surveys (December 1995 to December 1999) and commercial samples obtained from trawlers (March 1997 to June 1998). The species under study were Centroscymnus coelolepis, Centrophorus squamosus, and Deania calceus. The entire depth range (300 m – 1,800 m) of each species was sampled. C. squamosus and D. calceus were abundant between 700 m and 900 m. C. coelolepis was more abundant deeper (1,300 m) but gravid females were more abundant in shallower waters. Small specimens of each species were absent from the study area. Trawls and long-lines selected for different size ranges of C. coelolepis and D. calceus, though not C. squamosus. Deepwater bobbin and French high headline trawls did not select for different size ranges of C. coelolepis. Unlike the other species D. calceus is not landed. Discard rates from long-liners exceeded 40 % of total catch depending on depth fished. While percentage discard rates from trawlers were lower, an estimated 745 t of this species were discarded in 1996 alone. Sexual maturity was achieved at large size, more than 75 % of maximum length in each species. No evidence of seasonal cycles in reproduction was found from spermatogenic analysis or the occurrence of maturity stages. Mean ovarian fecundity was low; C. squamosus, 8, C. coelolepis and D. calceus 13. Dorsal spine sections were used for age estimation. Significant positive regressions of spine width and total length were found and the growth rates of first and second spines were not significantly different. Estimates of 21-70 years (C. squamosus) and 11-35 years (D. calceus) were obtained. Agreement within 1 year was found for more than 93 % of D. calceus and 88 % of C. squamosus first and second spines. Empirical and von Bertalanffy growth data suggest that growth had slowed down or ceased before the onset of maturity. Estimates of natural mortality (M) for C. squamosus (0.066 – 0.087) and D. calceus (0.116-0.177) were low. Fishing mortality (F) for D. calceus was estimated to have reached the level of M for females but below M for males. This study suggests that these species have conservative life-history strategies. No management plan currently exists for the multi-species deepwater fishery in the northeast Atlantic. In the absence of suitable abundance indices, stock assessment should be based on known life history parameters of target and by-catch species. Such an approach will facilitate the rational management of the fishery. Correspondance à: Maurice Clarke, Marine Institute, Abbotstown, Dublin 15, République d’Irlande. Aspects de la biologie de trois requins profonds, Centrophorus squamosus, Centroscymnus coelolepis et Deania calceus (Elasmobranchii: Squalidae) du talus continental du Fossé de Rockall et du Banc de Porcupine.