Population management in fisheries enhancement: Gaining key information from release experiments through use of a size-dependent mortality model

Population management, the optimisation of release and harvesting regimes, is a key element of effective fisheries stock enhancement. Population dynamics theory and assessment methods are becoming available to support management decision-making in enhanced fisheries. To apply such approaches in practice, it is necessary to estimate growth and mortality parameters for the target fishery. Here I show how this information can be extracted from release experiments through use of a size-dependent mortality model. The model, which describes natural mortality as an inverse function of fish length, has been derived in an earlier study and enjoys strong empirical support. Analysis of release experiments using this model provides direct estimates of size-dependent natural and fishing mortality patterns, which can be used to predict recapture and yield under alternative release and harvesting regimes. Furthermore, by expressing size-dependent mortality in terms of a single parameter, the mortality rate Mr at reference length Lr, the model facilitates comparative analyses of data from experiments in which mortality rates have been measured for different fish sizes. A preliminary comparative analysis shows that natural mortality is more variable, and substantially higher on average, in released hatchery-reared fish (median Mr = 13.3 year−1) than in wild fish (median Mr = 3.3 year−1) for the same reference length (Lr = 5 cm). Such comparative information provides a benchmark against which the effectiveness of releases may be evaluated. Application of the approach is illustrated through the analysis of published data from a cod (Gadus morhua) release experiment in Norway.