22
J. A. GULLAND AND J. E. CARROZ
size a t first capture (Beverton and Holt, 1957 ; Dickie and McCracken,
1955 ; Mako, 1961 ; Misu, 1964).
Such curves describe the biological situation, but this is not all that
is required for management. I n particular, the value of the catch and
the cost of catching it must also be considered. The value of the catch
will depend on biological factors such as the size of fish (small fish
being nearly always less valuable than medium or large fish), and on
their condition-fish which have just spawned, for example, often
being not very valuable-as well as on economic factors such as the
variation of price with supply. The factors can be introduced into the
equation to give expressions for the catch in value, but to a first
approximation it is normally sufficient to take the value as being
proportional to the weight caught, though most conservation measures
tend to increase the average size of the fish caught, and hence their
average price.
Other things being equal (and the results of regulation may well be
to make them not equal) the cost of fishing will be closely proportional
to the amount of fishing, but little affected by the sizes of fish caught.
Therefore, if the size of fish caught can be regulated (and it may not be
practicable for some gears, e.g. purse seines), the size should be regulated
to give the maximum yield for the current amount of fishing, being
adjusted as necessary if the amount of fishing changes.
It is not so certain that, for rational management, the amount of
fishing on a particular stock should be adjusted to give the maximum
catch from that stock, even if such a maximum exists a t a practicable
level of fishing. The fact that the well-being of a fishery cannot be
measured solely by the total weight of the catch, and that there is a need
for an objective of management less narrow than the maximum catch
has been pointed out by many authors, both biologists and economists
(e.g. Graham, 1943; Beverton and Holt, 1957; Schaefer, 1957b; Bell,
1959; Gordon, 1954; Christy and Scott, 1965; Gulland, 1968b).
I n the absence of regulation, the fishery will tend to stabilize a t a
level where the value of the catch is about equal to the total costs of
catching. If the fish are valuable, and easy to capture, and also if small
fish cannot be protected, then it is likely that this equilibrium will be
reached a t a level of fishing greater than that giving the maximum
sustainable yield. A reduction of effort to the level of the maximum
yield should result in an increase in the value of the catch, and reduction
of costs, and must therefore be beneficial, but a further reduction is also
likely to be desirable. Close to the maximum the curve of catch against
effort is very flat, so that a small to moderate reduction in effort will
result in a negligible reduction of catch. That is, perhaps 98% of the
J. A. GULLAND AND J. E. CARROZ
size a t first capture (Beverton and Holt, 1957 ; Dickie and McCracken,
1955 ; Mako, 1961 ; Misu, 1964).
Such curves describe the biological situation, but this is not all that
is required for management. I n particular, the value of the catch and
the cost of catching it must also be considered. The value of the catch
will depend on biological factors such as the size of fish (small fish
being nearly always less valuable than medium or large fish), and on
their condition-fish which have just spawned, for example, often
being not very valuable-as well as on economic factors such as the
variation of price with supply. The factors can be introduced into the
equation to give expressions for the catch in value, but to a first
approximation it is normally sufficient to take the value as being
proportional to the weight caught, though most conservation measures
tend to increase the average size of the fish caught, and hence their
average price.
Other things being equal (and the results of regulation may well be
to make them not equal) the cost of fishing will be closely proportional
to the amount of fishing, but little affected by the sizes of fish caught.
Therefore, if the size of fish caught can be regulated (and it may not be
practicable for some gears, e.g. purse seines), the size should be regulated
to give the maximum yield for the current amount of fishing, being
adjusted as necessary if the amount of fishing changes.
It is not so certain that, for rational management, the amount of
fishing on a particular stock should be adjusted to give the maximum
catch from that stock, even if such a maximum exists a t a practicable
level of fishing. The fact that the well-being of a fishery cannot be
measured solely by the total weight of the catch, and that there is a need
for an objective of management less narrow than the maximum catch
has been pointed out by many authors, both biologists and economists
(e.g. Graham, 1943; Beverton and Holt, 1957; Schaefer, 1957b; Bell,
1959; Gordon, 1954; Christy and Scott, 1965; Gulland, 1968b).
I n the absence of regulation, the fishery will tend to stabilize a t a
level where the value of the catch is about equal to the total costs of
catching. If the fish are valuable, and easy to capture, and also if small
fish cannot be protected, then it is likely that this equilibrium will be
reached a t a level of fishing greater than that giving the maximum
sustainable yield. A reduction of effort to the level of the maximum
yield should result in an increase in the value of the catch, and reduction
of costs, and must therefore be beneficial, but a further reduction is also
likely to be desirable. Close to the maximum the curve of catch against
effort is very flat, so that a small to moderate reduction in effort will
result in a negligible reduction of catch. That is, perhaps 98% of the