STOCK, RECRUITMENT AND MODERATING PROCESSES IN FLATFISH

A difficulty that frequently arises when stock-recruitment relationshi ps are fitted to historical data of fish populations is the high degre e of variation in recruitment so that the relationship is difficult to identify with any precision. The purpose of this paper is to describe refinements that can be made to the model by incorporating informatio n on parallel environmental factors that also affect recruitment. The identification of the stock-recruitment relationship can be made with greater precision because of the reduction in the unexplained variabil ity. Many investigations on the effects of environmental changes on re cruitment have been published in the fisheries literature. It is, howe ver, comparatively rare for the simultaneous effects on recruitment of environmental factors and stock size to be analysed. Here we describe the formulation of an appropriate mathematical relationship to descri be these effects. The framework of this formulation is F.E.J. Fry's cl assification of environmental factors into one of five kinds: controll ing limiting, lethal, masking and directive, following the work of Nei ll et al. (1994) [Neill, W.H., Miller, J.M., Van der Veer, H.W., Winem uller, K.D., 1994. Ecophysiology of marine fish recruitment: a concept ual framework for understanding interannual variability. Neth. J. Sea Res. 32, 135-152.]. An examination of some of the theory underpinning stock-recruitment relationships indicates how independent experimental evidence on the effects of environmental factors on recruitment can b e incorporated into the relationship in an appropriate mathematical fo rm. A method is described for the graphical illustration of the relati onship between, on the one hand, stock and recruitment allowing for th e effects of environmental factors and, on the other hand, the relatio nship between environmental factors and recruitment allowing for chang es in stock levels. The method is based on the idea of partial residua ls (or adjusted variables) derived from statistical regression methodo logy. The scatter of the adjusted variables around the fitted relation ship is often considerably less than that of the raw data. The method is illustrated by two examples of flatfish stocks, plaice (Pleuronecte s platessa) in the North Sea and English sole (Pleuronectes vetulus) i n the Hecate Strait off the coast of British Columbia. (C) 1998 Elsevi er Science B.V. All rights reserved.