20
J . A. GULLAND AND J. E. CARROZ
of younger stages-to estimate the basic population parameters both
of the young fish themselves and also of other relevant populations,
especially the food of the larvae (Ahlstrom, 1954, 1965 ; Baranenkova,
1965), and work in the laboratory-estimation of food requirements of
the larvae, the effects of reduced food on growth and survival, etc. This
adds up to a substantial volume of research, but it is probably the field
in which marine biological research has most to offer in improving the
scientific advice on fishery management.
Using the techniques outlined previously of estimating the population abundance, its rate of change, and the growth, mortality and
recruitment rates, the biologist can draw up, with a greater or less
degree of precision, sets of curves relating the total catch from a stock
of fish to the amount of fishing and to the sizes of fish caught. These
curves form the essential basis of proper management. I n drawing up
these curves the biologist has to ignore, in the first instance, many of
the complexities of the real situation. Thus the simplest, and most
frequently used form of the Beverton and Holt yield equation ignores
possibIe changes in the growth pattern, or in the natural mortality rate.
However as the authors show, any likely changes in these will not alter
the general shape of the yield curves, and hence will not affect the
nature of the biologists’ advice concerning a fishery which is to any
extent “ overfished ”. When, perhaps as a result of this advice, the
fishery approaches to what may be its optimum condition, then these
complexities, and others such as the interaction between different
species, etc., may well have to be taken into account in order to define
the optimum more accurately. There are other effects, in particular the
precise nature of the relation between adult stock and the subsequent
number of recruits, which, as discussed above, may alter the whole
shape of the yield curves. However, the direction of the change will be
generally known-at larger stocks (i.e. with reduced fishing mortality
or increased size of fish caught) the recruitment (and hence future stocks
and future catches) will almost certainly be greater than predicted on
the basis of constant recruitment. Thus the yield curves, and assessments based on them, assuming constant recruitment, will be conservative, in the sense that they will provide a lower limit to the benefits
from conservation actions.
These relations between yield and effort and between yield and size
of fish caught are interdependent, so that the form of the curve relating
the yield to the size of fish caught depends on the amount of fishing. It
will, except a t very low rates of fishing, have a maximum, and the
position of this maximum will depend on the amount of fishing. The
greater the amount of fishing, the greater will be the size of fish a t
J . A. GULLAND AND J. E. CARROZ
of younger stages-to estimate the basic population parameters both
of the young fish themselves and also of other relevant populations,
especially the food of the larvae (Ahlstrom, 1954, 1965 ; Baranenkova,
1965), and work in the laboratory-estimation of food requirements of
the larvae, the effects of reduced food on growth and survival, etc. This
adds up to a substantial volume of research, but it is probably the field
in which marine biological research has most to offer in improving the
scientific advice on fishery management.
Using the techniques outlined previously of estimating the population abundance, its rate of change, and the growth, mortality and
recruitment rates, the biologist can draw up, with a greater or less
degree of precision, sets of curves relating the total catch from a stock
of fish to the amount of fishing and to the sizes of fish caught. These
curves form the essential basis of proper management. I n drawing up
these curves the biologist has to ignore, in the first instance, many of
the complexities of the real situation. Thus the simplest, and most
frequently used form of the Beverton and Holt yield equation ignores
possibIe changes in the growth pattern, or in the natural mortality rate.
However as the authors show, any likely changes in these will not alter
the general shape of the yield curves, and hence will not affect the
nature of the biologists’ advice concerning a fishery which is to any
extent “ overfished ”. When, perhaps as a result of this advice, the
fishery approaches to what may be its optimum condition, then these
complexities, and others such as the interaction between different
species, etc., may well have to be taken into account in order to define
the optimum more accurately. There are other effects, in particular the
precise nature of the relation between adult stock and the subsequent
number of recruits, which, as discussed above, may alter the whole
shape of the yield curves. However, the direction of the change will be
generally known-at larger stocks (i.e. with reduced fishing mortality
or increased size of fish caught) the recruitment (and hence future stocks
and future catches) will almost certainly be greater than predicted on
the basis of constant recruitment. Thus the yield curves, and assessments based on them, assuming constant recruitment, will be conservative, in the sense that they will provide a lower limit to the benefits
from conservation actions.
These relations between yield and effort and between yield and size
of fish caught are interdependent, so that the form of the curve relating
the yield to the size of fish caught depends on the amount of fishing. It
will, except a t very low rates of fishing, have a maximum, and the
position of this maximum will depend on the amount of fishing. The
greater the amount of fishing, the greater will be the size of fish a t