Who Killed Cock Robin? 285 



nate all other kinds and reserve the resources of the lake for 

 bass only. They proceeded to drain the lake and to remove 

 all other fish. Two or three years passed and my informant 

 received a request to visit the lake and to determine, as a 

 parasitologist, why the bass had become wormy and unfit 

 for consumption. His first task was to identify the parasite; 

 his next was to determine which of the fish that had been 

 removed had controlled the pest. After a series of trials he 

 discovered which species had been responsible. Convinced 

 that he had found the source of the trouble he told the lake 

 owners to put these fish back and their troubles would proba- 

 bly be over. This proved to be the case. To him, it was one 

 of many proofs, he said, that the disturbance of natural bal- 

 ance was often a dangerous and delicate business. 



Another case along similar lines involved the harbor seal 

 on the Pacific Coast, an animal which both the commercial 

 and sports fishermen consider a great destroyer of salmon. 

 A few men who had spent their lives studying such situations 

 were not so sure that the seals were as guilty as charged. One 

 of them deplored the fact when the state finally put a con- 

 siderable bounty on harbor seals. When I remarked that 

 there seemed to be plenty of evidence that the seals took 

 salmon, he agreed, adding that the salmon had many other 

 enemies, notably dogfish. What he would like to know, he 

 declared, was what the harbor seals fed on during the months 

 when salmon were not numerous on the coast. When I asked 

 the reason for his curiosity he said that if, during that period, 

 the seals fed on dogfish and on other predators of the salmon, 

 it might be that the good they did by reducing these other 

 predators was far greater than the damage caused by direct 

 attacks on the salmon. At any rate, he considered this factor 

 the crux of the whole problem and said that it was possible 

 that the action taken was not only a financial waste but also, 

 by relieving the pressure on the other important salmon 

 predators, was causing a net loss in the salmon crop. He 



