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tion the catch per unit of effort goes steadily (and perhaps sharply) down, and 
the cost per unit of production goes up. 
This concept of maximum sustainable yield is basic to fishery management but 
it has a great many pitfalls. Variations in the environment (and these may be in 
the biological as well as the physical environment) may cause wide ‘swings in 
the level of maximum sustainable yield available from a particular population 
through changes in survival rate attributable to natural conditions. These changes 
may come in succeeding years Or move in more or less a cyclic manner. The point 
of maximum sustainable yield is always difficult to estimate with the best of 
scientific data and understanding, and must often be a well educated guess based 
on the best data and understanding available. 
In the case of animals of temperate regions (where most fishery population 
dynamics research has been done) a rough rule of thumb is talked about by fishery 
scientists, but not very strongly relied upon because of continued uncertainty and 
doubt. That is to the effect that the point of maximum sustainable yield is 
reached about where the stock is half as abundant as it was in the state of nature, 
and that at this point of lowered abundance about half of the remaining popula- 
tion can be removed by the fishery each year without causing gross further 
perturbations in population levels. 
The above very generalized suggestions applies to temperate stocks where a 
number of year classes are available at any one time to the fishery. Some re- 
sources supporting major fisheries are, however, almost annual crops, and this 
condition is more common in the tropics where life cycles tend to move more 
swiftly than under boreal or arctic conditions. For instance, most of the Penaeid 
shrimps that support most present ‘shrimp production in the world live only 
fifteen to eighteen months in a state of nature. The anchovy that support the 
mammoth fishery of Peru live only two or three years in a state of nature. The 
tropical tunas (yellowfin and skipjack) at least come to maturity and adult 
size in two or three years, etc. In the case of the shrimp (and probably many 
other tropical resources) if the crop is not taken in the year it dies, like the 
field of wheat, and is not available for harvest the next year. 
The great variety in the nature of success or failure among the various com- 
merical fisheries of the United States has been noted. There is just as great 
variety in the natural capability of particular resources to support fisheries. 
Some (most mammals and sharks) will support only a very light relative fishing 
mortality without the population going downhill until it is eliminated as a 
commercial proposition, or is eradicated entirely. Others (such as tuna, cod, 
ocean perch, anchovy) have such enormous natural reproductive resilience that 
ordinarily the population gets below the level of commercial practicability in 
harvest before the stock is in much danger. The fishery goes broke before the 
population does. In other cases (salmon, fur seal, oysters, clams, Penaeid shrimp) 
a characteristic of the life history of the species may make it particularly 
liable to being damaged by man’s activities. All salmon, for instance, must come 
into fresh water to reproduce (pink salmon may be a partial exception under par- 
ticular circumstances) and if access to their fresh water range is cut off by ob- 
structions the population is killed off entirely. 
The specificity may be more detailed. Most races of sockeye salmon must 
spawn in relation to a lake so that their young can spend a year or so in the 
lacustrine habitat, or the race dies. Fur seal require a hauling out ground that 
is cool for reproductive activity which means in practice, fog enshrouded. These 
locations are few in the world. The young of most commercial Penaeid shrimp 
must have a brackish water habitat for a few months or they will not survive 
in good number. The number of such criteria of living resources are legion, and 
must be learned about (and heeded) to provide the basis for management of the 
use of the resource. 
B. Jurisdiction over fisheries 
Man may (and does) intervene in these complicated natural situations of 
living resources in order to make the particular resource produce what he wants 
in the manner he wants it. To do this in a conscious, regular, and continuous 
manner some entity must have jurisdiction over the resource and its use, whether 
this is called ownership, sovereignty or just jurisdiction. This situation is just 
as compjex in fisheries as the natural conditions of the resources and the variabil- 
ity in economic success of their commercial harvesting. 
Under international law there are four sorts of waters juridically, from the 
standpoint of fisheries. 
