YELLOW PERCH OF LAKE ERIE 



251 



strength. Although the fish assigned to age 

 group III dominated the late-season commercial 

 catch of yellow perch by gill nets in 1947, the 

 evidence that the 1944 year class was exceptionally 

 strong is not conclusive. As group-II fish the 

 1944 year class strongly dominated (71.5 percent) 

 the commercial catch in 1946 and exliibitcd the 

 second-strongest dominance in the 6-year period 

 1943^8, hut the class appeared sparingly (1.:? 

 percent) as age group I in 1945. Data from the 

 gill nets add strength to the suggestion based on 

 trap-net catches that the year class of 1944 was 

 stronger than that of either 1943 or 1945. 



The occurrence of rather wide fluctuations in 

 the abundance of year classes has been observed 

 in a large number of species, both marine and 

 fresh-water. Despite the extensive studies that 

 have been made of the fluctuations in abundance 

 of year classes, relatively little is known concerning 

 the imderlying causes. It is agreed rather gen- 

 erally, however, that the fluctuations "have their 

 origin in certain conditions prevailing at a very 

 early period in the fife of the fish" (Hjort 1914). 

 The belief is general also that fluctuations depend 

 on variations in meteorological-hjdi-ographical 

 conditions, although biological conditions (for 

 example, competition for food among the young 

 and increase in predators) may at times be 

 important." 



Under conditions of a stabilized fishing inten- 

 sity, it is believed that the causes of fluctuations 

 in the abundance of year classes in the fishes of 

 Lake Erie are most probably to be found in the 

 meteorological-hydrographical conditions. It is 

 recognized that overfishing and other factors also 

 may be involved. The simultaneous occurrence 

 in 1926 of a strong year class in seven species 

 strongly suggests that competition for food among 

 the young is not normally a limiting factor. The 

 comparatively low yield of the fishery in 1926, a 

 year that produced a strong year class, indicates 

 that as long as the population is maintained at a 

 reasonable strength the number of spawners may 

 not be the primary determining factor. 



The weather records from the Sandusky, Ohio, 

 station of the U. S. Weather Bureau (1919-48) 

 have been examined in an effort to detect a 

 possible correlation between weather conditions 



■> Jensen (1933) gave a detailed review of the literature and a critical dis- 

 cussion of the causes of fluctuations in the abundance of marine fish of the 

 North Sea and neighboring waters. 



and the strength of the year classes. It has been 

 assumed that conditions in 1926 and 1942 and 

 probably in 1936 and 1944 were exceptional as 

 those years produced the strongest year classes 

 of 3'ellow perch found within the data, and that 

 the causes for the strength of those year classes 

 should be found in the extent and manner in 

 which the meteorological conditions of those j^ears 

 (lifTered from other years. It was expected 

 further that conditions would be more comparable 

 in the years 1926 and 1942 than in any other j'ears. 



Because of the previously mentioned impossi- 

 bilit}' of evaluating accurately the strength of each 

 j-ear class it is possible to speak only in general 

 terms concerning the effects of weather, hence 

 detailed weather data will not be presented. The 

 temperature data that were examined referred to 

 air temperatures. As mentioned previously, trends 

 in air temperatm-e no doubt indicate approximate 

 trends in water temperature, especiafly in such 

 shallow water as is found in western Lake Erie. 

 It was found that the winter of 1925-26 (Novem- 

 ber to February) was cold and that the following 

 prespawning period (March and April) was the 

 coldest for the years 1919 to 1948. However, 

 both the winter of 1941-42 and the prespawning 

 period in 1942 were warmer than average. The 

 2 years probablj^ producing strong jear classes 

 (1936 and 1944) differed m that the winter of 

 1935-36 was exceptionally cold and that of 1943-44 

 was warmer than average. The prespawning 

 period in 1936 had above average temperatures 

 but in 1944 temperatures were below average. 

 In other months of the year temperature exhibited 

 no relation to the strength of the year classes. 



Although all of the 4 years that apparently 

 produced strong year classes had less than average 

 rainfall in May and June, the total precipitation 

 in both 1942 and 1944 was only slightly below 

 normal and amounted to between two and tliree 

 times that in either 1926 or 1936. Wind velocities 

 and percentage of possible sunshine appear to bear 

 even less relation to the strength of year classes 

 than the other factors considered. Van Oosten 

 (1948) pointed out that there was no relation 

 between turbidity and strength of year classes. 



The contradictory evidence of the effects of 

 temperature during the winter and prespawning 

 period and total precipitation during May and 

 June on the strength of year classes makes it 

 appear that no siinjile n^lation exists. Although 



