Something to have a think about.....Most attempts to assess stocks of elasmobranchs have suffered from insufficient data and the use of inappropriate models (Anderson, 1990). Assessments of Spurdog, in contrast, have benefitted from excellent syntheses by Wood et al. (1979) and Silva (1993) and from growth information of Nammack et al. (1985). Earlier work by Holden (1974) on S.acanthias in the Northeast Atlantic laid a firm theoretical foundation for the utility of life history analyses. A major obstacle for further improvements in assessments of spiny dogfish stock in the Northwest Atlantic is the absence of age composition of the catch. Sharks are difficult to age but recent advances (see Cailliet (1990)) may permit routine updates of contemporary growth rate models. A greater dependence has to be placed on approaches which incorporate known information about the life history parameters of Spurdog. Clearly, the deductions from the life history models can provide useful information to management (see (Fogarty et al., 1989; Smith and Abramson, 1990; Cailliet, 1992; Cailliet et al., 1992; Cortes, 1995, 1996; Sminkey and Musick, 1996)). With an F of 0.26 and assuming a minimum length at entry to the fishery of 84 cm, the estimated number of pups per recruit is about 1, with a yield per recruit of less than 0.9 kg. Maximum yield per recruit (1.2 kg) occurs at an F of about 0.17 and a minimum size of 70 cm. Yield per recruit decreases with increasing minimum sizes, owing to the very slow growth rate at these ages. However, since reproduction in females occurs primarily in animals >_80 cm, fishing mortality rates in excess of 0.1 on animals >_80 cm results in negative female pup replacement. At an 84 cm minimum size, the maximum F that would ensure replacement recruitment is about 0.25. However, based on sea sampling, it is apparent that a substantial amount of fishing mortality also occurs on Spurdog as small as 50 cm. Consequently, the minimum size at entry to the fishery is less than 84 cm. Thus, it is even more likely that current fishing mortality is at a level which will result in negative replacement. Under these conditions, the stock will eventually decline.
Posted on: Sun, 25 Jan 2015 07:43:18 +0000
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