Research Document - 2004/092
Description of the methods used for estimating the abundance of Sebastes fasciatus and S. mentella in Units 1 and 2.
By Méthot, R., B. Morin, D. Power
Abstract
Efforts have been made to assess the situation of Sebastes fasciatus and S. mentella in Units 1 and 2. Because no visual distinction is possible between those two redfish species, methods have been developed to estimate indices of abundance of each species from the historic databases. The discrimination criteria used, coming mainly from the Redfish Multidisciplinary Research program 1995-99, were anal fin ray counts (AFC) and the analysis of the malate dehydrogenase (MDH). These characters have been used to split the redfish sp. into species or genotypes. The main method use AFC to evaluate the number of each species caught by tow. A correction was calculated at different depth intervals to reflect the difference in depth distribution for the two species. Proportions of species abundance by depth zone have been calculated from MDH analysis and applied for sets when no discrimination criterion had been recorded. Another method using AFC-length keys subsequently converted into species using MDH was also elaborated. Those methods have been used to analyze different surveys in Units 1 and 2.
The estimates of S. fasciatus abundance can be different depending of the method used. These gaps are more important for years when no discrimination character were recorded. In Unit 1, S. fasciatus was dominant at lower depth zone and S. mentella in deep water, the mixing zone being mainly between 183-274 m. Each species showed different length frequencies, S. fasciatus is generally smaller then S. mentella. Because of this, the AFC-length key method was found appropriate to separate species. In Unit 1, the Cod Sentinel summer survey, the GEAC survey and the survey in 4T showed decreasing trends in the last years or low historic abundance for both species. Only the Cod Sentinel fall survey showed stable but low level indices of abundance from 1995 to 2002. In Unit 2, little information was available. Like in Unit 1, S. fasciatus dominates in shallow areas (less than 366 m). Moreover, a survey covering only division 4V showed indices of abundance at a low level in the last years.
The discrimination by AFC recorded at each tow and corrected to genotype is simple to perform and could give good estimates of species composition. In years with no criteria of separation recorded, genotype proportions by depth or AFC-length keys could give acceptable estimates of the two species, but should be interpreted with caution because fluctuations of abundance due to mortality, recruitment and/or switch in depth preference could have affected the results of these methods. However, abundance of S. fasciatus evaluated by whatever methods show the same trend, so both methods were found useful to get general trends of the redfish species.
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