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Effect of size-selective mortality on growth of coastal cod (Gadus morhua L.) illustrated by tagging data and an individual-based growth and mortality model

Kristiansen, Tore S.; Svåsand, Terje
Working paper
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CM_1996_G_48.pdf (1.348Mb)
URI
http://hdl.handle.net/11250/105580
Date
1996
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  • ICES CM documents authored by IMR scientists (1949-2011) [3139]
Original version
This report is not to be cited without prior reference to the authors  
Abstract
Since 1983 more than one million reared cod juveniles have been tagged and released in

different areas along the Norwegian coast. The goal of the programme is to evaluate the

ecological and economical potential in sea ranching with cod. In this evaluation use of

correct growth rates and growth models are essential to estimate optimum yield and fishing

pattems. In one of the release areas the apparent growth rates of 3 year old cod, calculated by

sampling the population at different ages, were very slow (0.08 mm/day). However, when

measuring individual growth rates of individual tagged cod of the same size, the mean growth

rates were much faster (0.24 mm/day). These observations were attributed to size-selective

fishing mortality and were illustrated by an individual based simulation model of a

population of cod with variable individual growth rates. The effects on mean length at age of

the surviving population of increasing fishing intensity and mesh sizes were demonstrated.

The model showed that size selective fishing with the observed individual growth variation,

removed the fastest growing individuals at proportionally higher rates than the slower

growing ones, leading to decreased mean population growth rate. Also the fishing mortality

values which gave the largest yields, changed when individual variation was included, and

when we used the apparent growth rate in the model the yield per recruit was dramatically

reduced. This study has shown that individual growth variation and size selective mortality

are factors which should be considered in future fisheries management and ecosystem

models.
Publisher
ICES
Series
ICES CM Documents;1996/G:48

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