Long-Term Harvest Strategies 



Short-term predictions of abundance are the bases of short-term decisions, 

 e.g., what should the catch be for next year? The specific management decisions 

 are based on a combination of predicted abundances and long-term exploitation 

 strategies which, in turn, are based on balancing tradeoffs between harvesting 

 now or later and between target population sizes and target species. 



Recruitment is also the critical issue in the development of models in 

 support of long-term harvesting strategies. Here we are more concerned with 

 long-term trends in abundance (long-term average abundance) rather than annual 

 fluctuations, and the average relationship between spawning stock size and 

 recruitment is the feature of major concern. Traditional stock/recruitment 

 models (e.g., Ricker, Beverton, and Holt) are not adequate because the parameters 

 essential to long-term management strategies are not estimated with adequate 

 precision. New approaches are needed which include stochastic elements, i.e., 

 assessment of the risk of transition from the condition where recruitment is 

 sufficient to support fishing to a condition where probability of collapse of 

 the fishery is high. The new paradigm for stock/recruitment models probably 

 should be based on a world-wide analysis of the empirical behavior of various 

 classes of exploited fish populations. At the same time, experimental studies 

 on mortality mechanisms are needed on these same classes of fish populations 

 because changes in the environment can alter the relative importance of certain 

 mechanisms, thereby confounding the relationship between stock size and 

 recruitment. 



Another type of model in current, if limited, use for evaluating long-term 

 strategies is the surplus production model. This approach has been applied to 

 both single species and multispecies situations. The predictive capability of 

 this model is low because it does not explicitly take account of the recruitment 

 process, nor does it incorporate any definitive environmental linkages. In 

 essence, all the complex physical and biological processes controlling fish 

 production are assumed to follow a certain pattern, but the model provides no 

 basis for testing the validity of the assumptions. 



Once the stock/ recruitment problem has been clarified, multispecies 

 fisheries models incorporating recruitment effects will have significantly 

 improved predictive value for charting long-term management strategy. For the 

 time being, multispecies extensions of traditional single species models can be 

 used to help evaluate alternative strategies. For example, multispecies yield- 

 per-recruit analysis (post-recruit stages only) is feasible given the same data 

 required for a single species VPA. With this approach, tradeoffs between 

 exploitation of different species can be evaluated to a significant degree in 

 advance of solutions to the multispecies stock/recruitment problem. The most 

 recent multispecies VPA models are also designed to incorporate pre-recruit 

 stages as well. However, predation rates and absolute abundances of pre-recruit 

 stages are necessary to model this aspect, and these data are seldom available 

 in any fishery. 



Biomass balance models and holistic ecosystem models have also been used 

 to help develop guidance for management of multispecies fisheries. However, 

 these approaches make assumptions about energy transfers and linkages which are 

 not firmly grounded in our quantitative understanding of the actual control 

 mechanisms. Hence, their predictive power is unknown and unverif iable. 



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