Guidelines for harvesting species of different lifespans

Background

The idea that long-lived, slow-growing species have less potential to provide a sustainable yield than short-lived, fast-growing species was first encapsulated in Gulland's 1971 formula (Y = 1/2 MB0), where potential yield is equal to half natural mortality(M) multiplied by the virgin exploitable biomass (B0). Subsequent re-examinations of this have indicated that maximum sustainable yield is often lower than 1/2 MB0 but may vary considerably around this value. However, these analyses have not accounted for both population age structures and spawning stock-recruitment relationships. This study aimed to examine in a general way the potential yield of species with different life histories and stock recruitment relationships across a whole range of life spans: from annual species (e.g. squid) to the very long-lived (e.g. whales, orange roughy).

Approach

Yield was examined as a proportion of the unexploited biomass, of which there are two interrelated levels: the proportion of the total biomass and the proportion of the exploitable biomass. The latter is most useful to fisheries managers, the intended users of the guidelines, who are interested in the limits of commercial exploitation, and have the ability to adjust the age or length at which exploitation commences. Thus, most attention was placed on determining potential yield as a proportion of virgin exploitable biomass.

The study concentrated on cases where there is no stochastic recruitment variation, plus two situations where environmental variation is involved. The first examined the relationship between mean and variance of yield for different parameter combinations and the way this relates to the deterministic results. The second examined the ability of species of different life-spans to respond to and recover from catastrophic events.

Findings

A computer program has been developed which implements an extremely flexible, fully age-structured simulation model of the dynamics of exploited fish stocks, and incorporates density dependence in the form of a stock-recruitment relationship. The mathematical structure of the model is similar to Beverton and Holts'. Key biological parameters are the rate of natural mortality (M), the growth rate of the species (K) and the size at which exploitation begins and the fishing mortality rate imposed by harvesting.

Analysis of the model has revealed simple and practical guidelines for calculating the maximum sustainable yield available from a stock in terms of the virgin exploitable biomass and the natural mortality rate. Similar guidelines are also available for the fishing mortality rate producing maximum yield as a proportion of the natural mortality rate. The computer program also allows explicit calculation of the yield-biomass ratios given estimates of key biological and technical parameters. This has been used in conjunction with fisheries databases such as FISHBASE (WorldFish Centre) and comparable compilations of estimated stock-recruitment relationships to obtain estimates of yield-biomass ratios for a number of important fish stocks.