Maturation, growth and mortality of Clupeid and Engraulid stocks in relation to fishing
The response of a stock to fishing, on a “per recruit” basis, depends essentially on the form and magnitude of three characteristics - natural mortality, growth, and the size at which fish first enter the exploited phase (i.e. the catch). The first two of these, although they may be influenced to a degree by fishing through the mediation of population density, are intrinsic biological features of the species or stock in question; the third, on the other hand, can be determined wholly or in part by the selective action of the gear or of the fishing operations. In many species, however, the immature fish are largely or wholly segregated in space from the mature, and this may give rise to commercial fisheries based exclusively on concentrations which are either approaching maturation for the first time or are already mature. This feature, which is perhaps especially characteristic of clupeid and engraulid species, causes the size at which fish enter the exploited phase to be effectively the same as that at which first maturation is reached and so also to be biologically determined. It is therefore of interest to see whether the relationships between the parameters of natural mortality, growth and maturation size among the various stocks of clupeid and engraulid species have any properties in common; if they have, then it follows that the response of the stocks to fishing may also have certain generalized characteristics.
A preliminary investigation along these lines covering a wide range of fish species was made by Beverton and Holt (1959), who concluded that there is a relationship between longevity and growth pattern which differs quantitatively between certain of the main families. The present paper is offered as a more detailed study within the two families Clupeidae and Engraulidae; while it cannot claim to be exhaustive it includes the majority of the important commercial fisheries based on members of these two families, and covers some forty-two stocks of clupeid species and seven stocks of engraulid species. There are, however, two reservations which must be made - perhaps inevitably in a comparative study of this kind. One is that because most of the parameters required are seldom quoted as such in the published accounts, a considerable element of personal judgement has been involved, not only in their estimation but also in the interpretation of the supporting data, for which I take sole responsibility. The other is that some degree of selection of examples has been unavoidable although this has been kept to the minimum. Thus, estimates have not been attempted in cases where there are evident uncertainties in age determination or where the age composition data are too few or are clearly unrepresentative of the mature stock. In particular, I have not included the anadromous clupeoids (e.g., Caspian shad and Atlantic menhaden) whose migratory habits make a proper interpretation of age compositions difficult in the absence of expert knowledge of the stocks and fisheries concerned. It is hoped that the range of examples quoted may nevertheless be large enough to give a reasonably representative picture for the present purposes.