MANAGEMENT OF FISHERY RESOURCES
19
caerulea (Girard)) fishery, which declined from over half a million tons
annually around 1940 to only 3 000 tons in 1963, due to failure of
recruitment. It is possible that this failure of recruitment was caused at
least in part by the reduction in the adult stock by the heavy fishing of
the 1930s and 1940s (MacGregor, 1964 ; Murphy, 1966).
I n other major fisheries uncertainty concerning the relation between
adult stock and subsequent recruitment is of immediate practical
relevance in considering the present need for management. Thus for
the Arcto-Norwegian stock of cod a reduction in the amount of fishing
to half the high level occurring in 1965 would, if recruitment were
independent of adult stock, result in a slight increase in sustained
catch (I.C.E.S., 1966). However, there are definite signs that the
recruitment is reduced at the low stock corresponding to the 1965 level
of fishing (Garrod, 1966) ; if so, continued fishing at the 1965 level could
very greatly reduce the stock, and result in sustained catches very
substantially below those that could be obtained at lower levels of
fishing. Again, in the Peruvian anchoveta fishery, it is fairly clear that
increased fishing will not appreciably increase the yield per recruit and
any small increase in yield would be accompanied by a substantial fall
in catch per unit effort (Boerema et al., 1965), and therefore it is
desirable to discourage any further increase in effort. It is, however,
possible that increased fishing, by reducing the spawning stock, will
reduce the future recruitment ; this reduction in recruitment could well
cause a loss in total catch of several million tons per year. In this case
it would be not only desirable, but virtually essential, to prevent any
appreciable increase in fishing. It is also conceivable, and consistent
with the data a t present available, that reduced adult stocks would,
because of reduced competition for food between adults and young,
result in increased recruits ; increased fishing might therefore substantially increase the total catch.
Having stated the uncertainty, it is not easy to resolve it. Certain
theoretical models relating stock and recruitment have been developed
and curves relating recruitment to adult stock proposed which ranged
from the asymptotic to the parabolic (Ricker, 1954; Beverton and
Holt, 1957 ; Radovich, 1962). However, the observed data, in the form
of pairs of values of adult stocks and subsequent recruitment, are
usually few (accumulating at the rate of only one pair per year), and
are often very variable, so that the form of the stock/recruitment curve
often remains very uncertain (Gulland, 1967). Probably the problems
can only be resolved by a better basic understanding of the processes
determining the survival from egg to adult, particularly the early larval
stages. This will probably require both work at sea-detailed surveys
19
caerulea (Girard)) fishery, which declined from over half a million tons
annually around 1940 to only 3 000 tons in 1963, due to failure of
recruitment. It is possible that this failure of recruitment was caused at
least in part by the reduction in the adult stock by the heavy fishing of
the 1930s and 1940s (MacGregor, 1964 ; Murphy, 1966).
I n other major fisheries uncertainty concerning the relation between
adult stock and subsequent recruitment is of immediate practical
relevance in considering the present need for management. Thus for
the Arcto-Norwegian stock of cod a reduction in the amount of fishing
to half the high level occurring in 1965 would, if recruitment were
independent of adult stock, result in a slight increase in sustained
catch (I.C.E.S., 1966). However, there are definite signs that the
recruitment is reduced at the low stock corresponding to the 1965 level
of fishing (Garrod, 1966) ; if so, continued fishing at the 1965 level could
very greatly reduce the stock, and result in sustained catches very
substantially below those that could be obtained at lower levels of
fishing. Again, in the Peruvian anchoveta fishery, it is fairly clear that
increased fishing will not appreciably increase the yield per recruit and
any small increase in yield would be accompanied by a substantial fall
in catch per unit effort (Boerema et al., 1965), and therefore it is
desirable to discourage any further increase in effort. It is, however,
possible that increased fishing, by reducing the spawning stock, will
reduce the future recruitment ; this reduction in recruitment could well
cause a loss in total catch of several million tons per year. In this case
it would be not only desirable, but virtually essential, to prevent any
appreciable increase in fishing. It is also conceivable, and consistent
with the data a t present available, that reduced adult stocks would,
because of reduced competition for food between adults and young,
result in increased recruits ; increased fishing might therefore substantially increase the total catch.
Having stated the uncertainty, it is not easy to resolve it. Certain
theoretical models relating stock and recruitment have been developed
and curves relating recruitment to adult stock proposed which ranged
from the asymptotic to the parabolic (Ricker, 1954; Beverton and
Holt, 1957 ; Radovich, 1962). However, the observed data, in the form
of pairs of values of adult stocks and subsequent recruitment, are
usually few (accumulating at the rate of only one pair per year), and
are often very variable, so that the form of the stock/recruitment curve
often remains very uncertain (Gulland, 1967). Probably the problems
can only be resolved by a better basic understanding of the processes
determining the survival from egg to adult, particularly the early larval
stages. This will probably require both work at sea-detailed surveys