16
J . A. CIJLLAND AND J. E. CARROZ
and assuming that the catch rates of the individual vessels might be
rather lower, a t least in terms of value, than when fishing for cod in the
north-east Atlantic, their total catch would still be probably around
half a million tons. Since the north-east Atlantic catch would remain
the same, this half million tons would be a net addition to the total
world catch taken a t no extra cost.
The wrong type of management can also inhibit the development of
fishing on unexploited stocks by discouraging or prohibiting the use of
the most effective type of gear. Thus on the west coast of North
America restrictions on the use of trawls, introduced to protect the
catches of the highly priced halibut and the interests of sports fishermen,
have hindered the exploitation of the very large stocks of hake and
other species. Also the general failure to achieve good management is
very likely to discourage governments or commercial interests from
investing large sums in developing fisheries whose long-term future is
uncertain. For these and other reasons proper management implies far
more than merely ensuring, through appropriate regulations on the
amount of fishing or the size offish caught, that the maximum sustained
yield is taken from those stocks which would otherwise be overfished,
important though this is.
B. Theoretical studies
The effect of fishing on a stock of fish has been described by means
of a range of models of varying mathematical complexity (see, for
instance, Ricker, 1958; Beverton and Holt, 1957; Schaefer, 1954, 1967a;
Schaefer and Beverton, 1963; Gulland, 1965, 1967); but the implications for rational inanagement are much the same. I n the absence of
fishing a stock of fish will be large, including a relatively high proportion
of big. old individuals, and the increase in the total biomass due to
growth of the individuals arid recruitment of young fish will be balanced
over a period by the losses due to natural deaths. When fishing begins
the large stock gives large catches to each vessel, even though the total
catch is small. Any fishing will tend to reduce the stock abundance, but
at the reduced stock levels the losses due to natural deaths will be less
than the gains due to growth and recruitment. If the catch taken is
equal to this surplus, the stock will not change : any catch greater than
this sustainable yield will decrease the stock, while a smaller catch will
allow the stock to increase. This sustainable yield is small for very
large stocks, because natural deaths are only just less than the growth
and recruitment; equally it is small for very small stocks, where the
absolute value of the increase due to growth and recruitment will be
small. Thus the greatest sustainable yield will be taken a t some inter-
J . A. CIJLLAND AND J. E. CARROZ
and assuming that the catch rates of the individual vessels might be
rather lower, a t least in terms of value, than when fishing for cod in the
north-east Atlantic, their total catch would still be probably around
half a million tons. Since the north-east Atlantic catch would remain
the same, this half million tons would be a net addition to the total
world catch taken a t no extra cost.
The wrong type of management can also inhibit the development of
fishing on unexploited stocks by discouraging or prohibiting the use of
the most effective type of gear. Thus on the west coast of North
America restrictions on the use of trawls, introduced to protect the
catches of the highly priced halibut and the interests of sports fishermen,
have hindered the exploitation of the very large stocks of hake and
other species. Also the general failure to achieve good management is
very likely to discourage governments or commercial interests from
investing large sums in developing fisheries whose long-term future is
uncertain. For these and other reasons proper management implies far
more than merely ensuring, through appropriate regulations on the
amount of fishing or the size offish caught, that the maximum sustained
yield is taken from those stocks which would otherwise be overfished,
important though this is.
B. Theoretical studies
The effect of fishing on a stock of fish has been described by means
of a range of models of varying mathematical complexity (see, for
instance, Ricker, 1958; Beverton and Holt, 1957; Schaefer, 1954, 1967a;
Schaefer and Beverton, 1963; Gulland, 1965, 1967); but the implications for rational inanagement are much the same. I n the absence of
fishing a stock of fish will be large, including a relatively high proportion
of big. old individuals, and the increase in the total biomass due to
growth of the individuals arid recruitment of young fish will be balanced
over a period by the losses due to natural deaths. When fishing begins
the large stock gives large catches to each vessel, even though the total
catch is small. Any fishing will tend to reduce the stock abundance, but
at the reduced stock levels the losses due to natural deaths will be less
than the gains due to growth and recruitment. If the catch taken is
equal to this surplus, the stock will not change : any catch greater than
this sustainable yield will decrease the stock, while a smaller catch will
allow the stock to increase. This sustainable yield is small for very
large stocks, because natural deaths are only just less than the growth
and recruitment; equally it is small for very small stocks, where the
absolute value of the increase due to growth and recruitment will be
small. Thus the greatest sustainable yield will be taken a t some inter-