stock size providing that reasonably accurate data on current stock abundance 

 and age structure are available, and assuming that the recruiting age class is 

 a small proportion of the average harvest. The theory underlying these models 

 is well developed, and the models are being utilized at or near their full poten- 

 tial wherever adequate fishery data bases are available. The limitations of 

 these models are that they do not take account of species or environmental 

 interactions and they are of limited value where annual recruitment represents 

 a large fraction of potential harvest. 



Pre-recruit (i.e., juvenile fish) estimates of abundance often are correlated 

 with year-class strength and thus can provide useful forecasts of recruitment. 

 Recruitment estimates together with the traditional population models add a 

 further improvement to short-term stock projections. These approaches are also 

 being utilized at or near full potential given sufficient time series of data 

 for establishing a regression. Improvements in sampling procedures can sometimes 

 help, but the general priority for research in this area is low. The lead time 

 achieved with pre-recruit abundance estimates is usually only a few years, and 

 precision of estimates is seldom high. 



Single species and raultispecies Virtual Population Assessment (VPA) methods 

 are very useful for estimating parameters needed for traditional population 

 dynamics models. The VPA is a calculation procedure more than a model, but it 

 provides good "hindcasting" ability in terms of mortality parameters and stock 

 sizes in previous years, and it is still under development. VPA calculations 

 do not provide forecasts, and estimates of the most recent year or two are 

 subject to the greatest errors due to uncertainties of fishing mortality rates 

 applicable in the most recent years. On the other hand, multispecies VPA 

 methods do provide a method for estimating natural mortality of pre-recruit 

 stages caused by predation. Further advances along these lines are clearly 

 feasible given the data on abundance of pre-recruits and predation rates, and 

 these results would have useful applications to both short-term forecasts and 

 long-term harvest strategies. 



Since the major natural fluctuations in fish populations result chiefly 

 from variation in recruitment, a top priority for improving predictions of 

 future population abundances is the development of models incorporating definitive 

 insight into the physical and biological mechanisms controlling the recruitment 

 process. It is conceivable that empirical relationships might be derived from 

 a long time series of recruitment and general environmental data (e.g., 

 temperature, timing of spawning), but it is unlikely these will ever be adequate 

 for anything but gross predictions (e.g. , recruitment always poor after abnormally 

 cold spawning seasons). More likely, extensive field and laboratory studies 

 will be needed to clarify the relative importance of mortality processes 

 including food supply, predation, physical transport, and disease. At present, 

 models of these processes do not have much predictive ability because our 

 understanding of the mortality processes is still largely qualitative (even the 

 timing of first year mortality is seldom known let alone the causes). Recruit- 

 ment process models offer the largest potential improvement in predictive 

 capability, but the logistic problems are enormous for unraveling such complex 

 large-scale phenomena as the growth, dispersal, and survival of larvae even 

 for one species. A number of conceptual models of the process are available, 

 but empirical measurements for testing the validity of the models are extremely 

 rare. 



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