Exploring the BOFFFF hypothesis using a model of Southern African deepwater hake (Merluccius paradoxus)

Abstract

The Big Old Fat Fecund Female Fish (BOFFFF) hypothesis is explored quantitatively using an age-structured stock production model in which the usual stock:recruit relationship is replaced by a three-stage recruitment process based on maternal age. The recruitment process is decomposed into three age-dependent stages: 1) number of eggs spawned per batch per female, 2) number of batches spawned per year (related to the length of the spawning season), 3) increased survival of young stages with maternal age. The first two stages together give the stock recruitment potential and can be well justified by experimental observation in many fish species. The third stage is affected by both maternal age and also environmental conditions. The results indicate that the number of batches spawned per year is the most important component. They also indicate that a few old fish potentially contribute many times more recruits to the population than many younger fish. It is argued that the concept of spawner stock biomass (which is heavily weighted by the large numbers of young first time spawners, especially in heavily exploited populations) should be replaced by the concept of stock recruitment potential, which takes the age-structure of the population in to account. The conclusion is that for many long-lived fish populations such as hake and cod, management should aim to maintain the age-structure of the population, rather than encouraging, or even targeting, exploitation of large, old fish.