The effect of fishing mortality on the variation of yield is re-examined, with particular attention to the changes of stock size from one year to the next. Using the SHOT model of the stock dynamics, we show that the year-on-year variability of the exploitable biomass increases roughly in proportion to the fishing mortality. The theory is further developed by means of an age-structured model which takes account of the exploitation pattern (the fishing mortality as a function of age). We determined the condition for minimum variability. The corresponding exploitaion pattern is not unique but depends on the fishing mortality on the recruiting year class. We discuss the relationship between the exploitation pattern and the selectivity of the fishing gears used to exploit the stock. As an example, the theory is applied to the cod stock in the North Sea. We show how the variability of the yield depends on the fishing effort through the number of year-classes contributing to the fishery. The effect of changing the exploitation pattern is considered. It appears that the optimum gear selectively function for the lowest variability of yield depends on the level of exploitation. When the exploitation is high, it is better to have a selectivity function which increases steadily with age, rather than the S-shaped to give which is commonly believed to be more desirable. We conclude that the reduction of stability due to heavy exploitation, and the improvement available through effort reduction and/or technical measures to control the selectivity of fishing gear, are much larger than indicated by previous work which considered only the variation of yield about its mean level.
|Journal||ICES (International Council for the Exploration of the Sea) Journal of Marine Science|
|Publication status||Published - 1992|