Scientia Marina, Vol 67, No S1 (2003)

Annual trends in catchability and fish stock assessments


https://doi.org/10.3989/scimar.2003.67s163

Marchal Paul
IFREMER, France

Ulrich Clara
DIFRES, Denmark

Korsbrekke Knut
IMR, P.O, Norway

Pastoors Martin
RIVO-DLO

Rackham Brian
CEFAS, United Kingdom

Abstract


A key assumption of many fish stock assessment models is that catchability is constant over time. We assume here that trends in catchability may occur through fishing power creeping. The tuning fleets, which are prone to fishing power development, may be identified using the Hybrid method. A range of catchability trends, including values derived from the Hybrid method, is then implemented to standardise the fishing effort of some tuning fleets used in the stock assessments performed by XSA (eXtended Survivors Analysis). Stocks being assessed are the North Sea cod, saithe, plaice and sole. The performances of the new and traditional XSA assessments are compared using criteria based on the precision of catchability estimates, stationarity of Log-catchability residuals and retrospective patterns relative to fishing mortality, spawning stock biomass and recruitment estimates. The performances of the North Sea cod, plaice and sole assessments could be enhanced by accounting for an overall annual increase in the catchability of some of the tuning fleets. No significant trends could be detected in the catchability of the tuning fleets relative to the assessment of the North Sea saithe. By contrast with the traditional assessment, the spawning biomass of cod is expected not to have increased between 1997 and 1998, while the fishing mortality of sole is expected to have increased over the same period.

Keywords


hybrid method; extended survivors analysis; fish stock assessment; catchability; fishing power; North Sea; trends; retrospective pattern

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