© 1987 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A simple method for analysing trends in recruitment
MAFF Directorate of Fisheries Research, Fisheries Laboratory Conwy, Gwynedd LL328UB, Wales
In the absence of sufficient, or adequate, data to fit a stock and recruitment model, it is suggested that trends in recruitment might be analysed by expressing recruitment (R) as a function of the replacement recruitment level (
), the number required to balance the annual losses from a stock. The method requires estimates of stock in number, catch in number, number of recruits, and the natural mortality rate, then: surplus recruitment is calculated as (R – ) or a relative recruitment index may be expressed as (R – )/. The results from both are shown as recruitment anomalies. It is demonstrated that even in the absence of a commercial fishery, the North Sea mackerel stock produced too few recruits to maintain the status quo stock between 1970 and 1980. For the Western mackerel stock, it is shown that between 1972 and 1983 the average recruitment exceeded the losses due to natural mortality, but fell short of the number removed by total mortality; i.e., catches were greater than the stock could sustain.