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|Title:||On the precision of fish stock parameters from the pseudo-cohort analysis VIT|
|Authors:||RAETZ Hans-Joachim; CHEILARI Anna; LLEONART Jordi|
|Citation:||SCIENTIA MARINA vol. 74 no. 1 p. 155-162|
|Publisher:||INST CIENCIAS MAR BARCELONA|
|Type:||Articles in periodicals and books|
|Abstract:||The comparative analyses of the VIT model results with those of the agreed VPA assessment of the cod stock in the Skagerrak, North Sea and Eastern Channel reveal that overall trends in fishing mortality and stock size are well captured over a period of three decades (1963-1992). However, the implications of the equilibrium assumption (steady state) by the VIT model are huge as it forces strong inter-annual variations in the estimated fishing mortality, especially in those years when the age composition shows strong year class effects. Estimated VIT output parameters should be carefully interpreted when the assessed stock suffers from reduction in abundance and high exploitation, as such condition is likely to deviate from steady state. The VIT estimates of the virgin stock biomass appear very variable, extremely high and rather dependent on annual stock size estimates. Their use in fisheries management context as realistic biological stock biomass reference is not recommended. The VIT estimates of F0.1 and Fmax are found in line with the ICES estimates. Therefore, model results should allow rather qualitative conclusions regarding the status of marine living resources in relation to a target reference of exploitation rate. Quantitative conclusions regarding the assessment of the exploitation status are only recommended when the model is applied to short time series of consecutive annual data and the resulting variation of the estimated stock parameters appears reasonably low. As such, the model results may form the basis for scientifically sound fisheries management advice covering the actual state of the stock as well as medium term forecasts of catch and biomass under different options|
|JRC Institute:||Institute for the Protection and Security of the Citizen|
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