FLUCTUATIONS IN ABUNDANCE OF MARINE FISHES 187 



creased growth of plankton which in turn would provide increased food for the 

 mackerel and attract them to the fishing grounds. This theory was sustained by 

 the findings of another investigator whose data showed a close relation between 

 the catches of mackerel and amounts of zooplankton (animal plankton as dis- 

 tinguished from plant plankton) observed in the same area. 



In another well-documented study a considerable relation was shown between 

 the catches of plaice and mackerel off the coast of Denmark and the local surface 

 temperature of the water. In a study of the plaice fishery of the North Sea it was 

 found that the percentage frequency of winds from certain directions was asso- 

 ciated with the production of successful year classes of the fish. In this case the 

 theory was that the proper winds would carry the young plaice from the spawning 

 grounds to the good nursery areas where their chances for survival would be 

 considerably enhanced. The temperature near the bottom was found to be related 

 to the survival of cod fry in Scandinavian waters. When the bottom temperatures 

 for 3-year periods occurring 4 years before the commercial catches (to account 

 for the period of growth of the fish) were compared with the catches in question, 

 it was found that there was a rather high correlation. 



Populations of other species of fish in the same region also constitute part 

 of the environment of a given marine fish, and the abundances are often inter- 

 related. For instance, the Scandinavian cod feed rather heavily on the herring of 

 the same region, and it was found that the catches of herring were inversely re- 

 lated to the catches of cod one year earlier. In other words it appeared that un- 

 usually high abundance of cod would cause excessive feeding on the herring pop- 

 ulation and a reduction of the latter. 



Diseases and parasites have their eff^ect on fish populations, just as they do on 

 human, although great fluctuations from these causes appear to be rather rare. 

 An example which has had considerable publicity recently concerns the decline 

 of the catches of trout in Lake Huron. This occurred simultaneously with the ap- 

 pearance on the fish of extensive marks indicating attacks by the parasitic sea 

 lamprey, which had entered the Great Lakes through the Welland Canal (built 

 to permit navigation around Niagara Falls ) . 



Long term or short term climatic changes affect the abundance of fish in par- 

 ticular areas in that they cause them to migrate to new regions. A recent example 

 is the appearance off the Middle Atlantic Coast of considerable numbers of 

 southern species not ordinarily found there. This was believed to be due to an 

 unusually far northward extension of the warm waters of the Gulf Stream. 



Man-Made Causes. First among the man-made causes affecting marine fisheries 

 is undoubtedly the actual fishing itself. Conclusive proof has been found, for 

 instance, that changes in the abundance of Atlantic haddock are associated with 

 sizes of catches taken from the population. By means of the statistical analysis 

 of catch and catch-per-unit-of-effort data alone, or in combination with tagging 

 data, it is possible to estimate the actual percentage take of the fishery. In general 

 it is found that a fishing rate which will keep the population not at its highest, 

 nor at its minimum level, but rather at a middling level is the one which will 

 result in the greatest continuing average yield. 



In the case of anadromous fish anything which affects the streams in which the 

 fish spawn will in turn affect the survival both of the upstream migrating adults 

 and the dowoistream migrating young. Among these causes dams, irrigation diver- 



