CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

REPORTS VOLURlE SI, 1 JULY 1963 TO 30 JUNE 1966 9.5measure recruitment-strength as accurately as possibleso as to get a better basis for forecasting yieldfluctuations.Effects of the Fisbery on the StockThere is little doubt that a fishery of about 6 milliontons per year must have some effects on the anchovypopulation. The total mortality rate must have increased.By how much we do not know, but from arough assessment of predation of birds and fish it isnot unreasonable to think that the mortality has atleast been doubled. We should expect that it wouldbe possible to demonstrate the effects of this increaseof mortality on the size composition of the fish. Suchdemonstrations are, however, complicated by the fluctuationsin recruitment. A longer series of data isnecessary to compensate for the natural fluctuationsin the size composition, and thus provide a normalaverage basis for a comparison.The comparisons of recruitment and adult fishabundance shown in Figure 8 have one feature whichmay indicate an increase of total mortality. The expectationof a straight-line relationship passing zerois only valid if the total mortality has been unalteredduring the period of observation. If mortality increasesone would expect the abundance of the adultgroup to fall off more than that of the recruits. Atendency of this nature can be seen in the dataplotted in Figure 8.We expect to be able to make more definite statementsconcerning the effects of the fishery on thestock after the season November 1964 to May 1965.If the abundance of the population of adult fish andthus the success of fishing do not rise to the expected“normal” level during this season as a result of thc.high recruitment in 1964, then it must be concludedthat the fishing mortality influences the stock to ilmarked degree.In practice the effects of increased mortality andthe resulting lower average size and age of the fishin the stock would be a corresponding decrease of themean size of the fish caught in the fishery. The totalstock abundance would also on the average be lowerwith a decrease of the catch per unit of effort. Theyield would be more variable from year to year becauseit would to a greater extent depend upon thefluctuating abundance of the recruit fishes. This ofcourse would make the operation of the industrymore difficult with higher costs of raw material andless continuity of operations. It is, however, notthought that any lasting or permanent harm will bemade to the stock by this form of over-exploitation.The recruitment in big oceanic fish stocks does notseem to be directly related to the size of the spawningstock : big-year-classes can result from the spawningof poor-ones. The direct economic effects on the industrywill be those arising from expensive and inperiods scarce raw material. These effects can, however,be serious enough as we have seen from thestate of the industry during 1963-64.REFERENCESSaetersdal, G., and J. E. Valdivia. 1964. A study of growth, sizeand recruitment of the anchovy ( Engradis ringens J.) basedon length frequency data. Inst. Invest. Recurs. Mar., Bol.,1 (4) ~121-136.Saetersdal, G., I. Tsukayama and B. Alegre. 1965. Fluctuationsin the apparent abundance of the anchovy stock in 1959-1962.Imt. Mar. Peru, BoZ. l(2) :87-104.