404 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



individual may in itself affect growth, inde- 

 pendently of food conditions. The possibility of 

 an influence of space on growth was recognized 

 many years ago. Semper (1880) demonstrated 

 a strong positive correlation between water vol- 

 ume per individual and the growth of snails 

 (Lymn-aeus stagnalis) reared from the same 

 batch of eggs. Wilier (1929) applied the term 

 "Raumfaktor" to this influence of space on 

 growth and offered the view that it is important 

 in determining growth rate in natural populations 

 as well as under experimental conditions. Nu- 

 merous authors since have mentioned the space 

 factor as of possible importance in observed cor- 

 relations between growth rate and population 

 density, but have not been able to separate its 

 effects from those of food competition (see Hile 

 1936). 



The sevenfold or greater increase from 1929-30 

 to 1943-55 in the numbers of yellow perch in 

 Saginaw Bay suggests at once that competition 

 for food has greatly increased, for we can hardly 

 assume that food production has increased in a 

 corresponding ratio. On the other hand, we are 

 quite without evidence that an actual scarcity of 

 food for perch has existed in recent years. Fur- 

 thermore, the fish give no indication of starva- 

 tion. They lack altogether, the large-headed, 

 thin-bodied appearance of the stunted perch de- 

 scribed and figured by Esehmeyer (1937), but, 

 on the contrary, appear plump and healthy. This 

 impression as to the well-being of fish of the 

 1943-55 samples is supported by the facts, for 

 the earlier data on the length-weight relation 

 proved them to be heavier, length for length, 

 than the rapidly growing perch of the 1929-30 

 samples. The evidence, then, supports the con- 

 clusion that crowding in itself, not a scarcity of 

 food, was the principal factor in the decline in 

 growth. 



POSSIBLE FACTORS OF FLUCTUATIONS 

 IN YEAR-CLASS STRENGTH AND 

 GROWTH RATE 



The literature on the relation between environ- 

 mental factors and the fluctuation of year-class 

 strength and growth in fishes is extensive. No 

 general review of this subject is undertaken 

 here since excellent and detailed reviews were 

 given by Hile (1936 and 1941), Van Oosten 

 (1944), and more recently by Watt (1956). 



Evidence from the literature suggests that fac- 

 tors not only vary, but conditions that control 

 growth and success of reproduction are not the 

 same for any particular species in different wa- 

 ters. It appears also that under natural condi- 

 tions, the year classes and growth rate are not 

 controlled by single factors, but by a number of 

 interacting ones, some of which may be beneficial 

 while others are harmful. The interrelations 

 among these complex factors are unknown and 

 indeed it is to be suspected that some important 

 factors have not been discovered or tested. Most 

 work done on this subject has been only ex- 

 ploratory. 



The present inquiry is also exploratory and 

 not conclusive. It is limited to possible effects 

 of population density and to certain environ- 

 mental factors about which information was 

 available ; temperature, precipitation, water level, 

 and turbidity. The study is restricted also to 

 growth in length. More intensive study must 

 await the accumulation of information covering 

 a greater span of calendar years. 



Data on precipitation and on monthly air tem- 

 peratures at Bay City (Mich.) published by the 

 United States Weather Bureau, were taken as 

 indicative of fluctuations of rainfall and the 

 water temperature of the Bay itself. Doan (1942) 

 showed a significant correlation between average 

 temperatures of the air and water for Lake Erie. 

 Similar correlations were determined between 

 average air temperature and the temperature of 

 water, for April through October 1939 to 1950. 

 The data were collected at the intake (3,400 feet 

 from shore) of the Bay City filtration plant, 

 The values of r for the different months are as 

 follows : 



Month 



April. 

 May.. 

 June.. 

 ■Tuly-. 



0.748 

 .887 

 .S46 

 .699 



Month 



August 



September. 

 October — 



0.607 

 .640 

 .781 



The preceding values of r are statistically sig- 

 nificant at the 1-percent level except for July, 

 August, and September which are significant at 

 the 5-percent level. 



Data on the average monthly turbidity were 

 obtained from the Bay City water plant for the 

 years 1939-52 (after 1952 the intake was changed 

 to a different location) and expressed as parts 

 per million (table 47). 



