196 OCEANOGRAPHY 



yiucp yoii cannot count the stock under your charge you have to estimate 

 the count by indirection. One of the most useful tools to do this with is to tag 

 a population of a certain size, turn those loose in the area and see what i)er- 

 centage <tf these get caught by the fishery. Another way is to compare the 

 percentage of each year class present in the population. This will give an esti- 

 mate of the total mortality. This will be related to both the fishery mortality 

 and the natural mortality in a manner which you have to discover by other 

 means. Another way is to assume that the catch per unit of effort bears a con- 

 stant relationship to the size of the population, then by keeping track of the 

 amount of fishing effort being used you can calculate the changes in the 

 relative abundance of the fish stock and relate this to the amount of fishing effort 

 and determine when the fishing is exceeding the point of maximum sustainable 

 yield. 



The trouble with all of these indirect methods of determining the size of 

 your fish stock and the effect of the fishery upon it is that they are dependent 

 upon a mathematical treatment of data derived from fish catches. The fish catch 

 is dependent, however, on two different factors (leaving aside the skill of the 

 fisherman and hoping that this is taken care of by averaging all the fishermen 

 together). These two factors are the actual abundance of the fish in the stock 

 and the availability of those fish to the fisherman. 



in some kinds of fish, like the I'acific halibut, the availability factor does 

 not change sharply from year to year and the scientist and conservationist is 

 a lucky fellow. He can predict and regulate with some precision. In the tropi- 

 cal tunas, and a good many other kinds of fish, however, slight changes in the 

 ocean currents, surface temperatures, etc. cause wide seasonal and yearly dif- 

 ferences in the availability of fish to the fisherman that may be quite indei)endent 

 from the actual abundance of the fish in the stock. The scientist has to measure 

 this factor too and be able to evaluate it in his abundance calculations. To 

 do this he must become an oceanographer as well as a biologist. 



This, incidentally, was the very way Dr. Schaefer, Director of the Inter- 

 American Tropical Tuna Commission, who testified before you last week became 

 such an eminent oceanographer. He and his staff are hired to find out the rela- 

 tion between our fishery and the stocks of yellowfin and skipjack tuna we fish, 

 to determine these points of maximum sustainable catch, and see that we do 

 not exceed that in our fishing. Very early in the game he found that availability 

 related to changes in ocean climate was such a large factor in the catch rate for 

 tropical tunas by the methods we used that he had to put in as much effort 

 and time in evaluating what the ocean was doing as he did in evaluating changes 

 in the fish abundance and the fishing activity. 



All of this applies only to a homogeneous stock. Fish (as well as other 

 animals and plants) have a tendency to break up into separate interbreeding 

 stocks which inhabit adjacent geographic areas but do not mix much with each 

 other. Each of these stocks present a separate problem of the kind noted above. 

 As an example the Pacific halibut north and west of Cape Spencer, Alaska, mix 

 very sparsely, if at all, with those south of that point along the coast of south- 

 east Ala.ska, British Columbia, Washington, and Oregon. Yet the same fleets 

 of fishing vessels fish both of these areas. It would do no good to regulate in 

 one of these areas alone because the fleet would just shift to the other area and 

 overfish the other quite independent stock. 



The difi'erentiatiou of these separate stocks within a fishery is never easy, 

 has always taken a number of years to accomplish, and to the great discourage- 

 ment of the scientists a sharp change in ocean climate for a few years may cause 

 the stocks to intermingle for that period of years in a most confusing manner, 

 and then separate out again in a subsequent climate cycle. 



This highly oversimplified picture of the task of the fishery scientist and con- 

 servationist in the ocean fisheries has described the veiy simplest case, where 

 the fishery operates on a single species of fish (or nearly so). Such cases 

 are the menhaden fishery of the east and gulf coasts the halibut fishery of 

 the northeast Pacific, or the pilchard fishery of South Africa. Unfortunately, 

 however, many very important marine fisheries depend upon catching several 

 species of fish at the same time in a mixed fishery. 



Our fisliery for tropical tunas is the simplest example of this sort of mixed 

 fishery. We fish for yellowfin tuna and for skipjack tuna and for a number 

 of years they were aliout equally important in our catches. Individual boat- 

 loads composed roughly half and half of the two species were more common 

 than not ( wc catch a third species occasionally, too. the liig-eye tuna but this 

 is not sufficiently abundant in our fishing to be a problem to Dr. Schaefer). It 



