YELLOW PERCH IN SAGINAW BAY 



403 



taken place in the yellow perch population of 

 Saginaw Bay. The numbers of legal-sized perch 

 in the two periods were closely similar but this 

 similarity is largely the result of the decline in 

 growth rate that permitted so few perch to reach 

 8V2 inches in 1943—55. In total numbers of fish 

 taken, the rate of capture in 1943-55 was nearly 

 7 (6.78) times that of 1929-30. The actual in- 

 crease in numbers was surely even higher than 

 the ratio indicates. Because of their slow growth 

 rate many fish of the 1943-55 population could 

 escape from the nets at ages which were ade- 

 quately sampled in 1929—30. A ratio of 10 rather 

 than 6.78 might be much nearer the truth. 



Despite the possibility that other factors may 

 have been of some consequence, it is believed that 

 the increase in population density of Saginaw 

 Bay yellow perch is in itself sufficient to explain 

 the observed decrease of growth rate. 



The status of rate of growth of fish as a 

 density-dependent variable is too well established 

 to require presentation of argument or any ex- 

 tensive review of the voluminous literature on the 

 subject. Evidence in the matter has come mainly 

 from two sources : Changes in growth following 

 transplantation from densely to thinly populated 

 areas; changes in growth accompanying change 

 of population density within a stock. Changes 

 of population density that have been accompa- 

 nied by change of growth rate have included 

 fluctuations resulting from fluctuations in rate 

 of exploitation or in the strength of year classes, 

 from destruction of a considerable portion of the 

 stock by a catastrophe, such as an epidemic or a 

 winterkill, and by a deliberate, experimental de- 

 struction of the stock. A single example of each 

 type of observation should provide adequate 

 illustration. 



The classic example of results from moving 

 fish from crowded to thinly populated grounds 

 is provided by plaice (Pleuronectes platessa) of 

 the North Sea area. These experiments started 

 early in the present century. Borley (1912) re- 

 ported on the improved growth of plaice trans- 

 ferred from inshore waters to the Dogger Bank 

 in 1904—08. These and later studies proved bene- 

 fits to growth to be so substantial that the Danes 

 developed an economically profitable enterprise 

 in the transfer of young from the crowded nurs- 

 ery grounds to the broads of the Limf jord. A 

 comprehensive review of the plaice transplanta- 



tion experiments was published by Blegvad 

 (1933). 



The plaice also provided an early clear-cut 

 illustration of change of growth rate with fluc- 

 tuation in the density of the stocks. Prior to 

 World War I excessive exploitation had greatly 

 thinned the stock but during the war the drastic 

 curtailment of operations permitted a substantial 

 accumulation of fish on the grounds, and with 

 it came not only an increase of average age but 

 also a sharp decline in growth rate. The re- 

 sumption of heavy fishing in post-war years led 

 in turn to a thinning of the plaice, a decrease 

 of average age, and an increase in growth rate. 

 A thorough discussion of various aspects of the 

 problem of population density and growth, cen- 

 tered largely about observations on plaice, was 

 given by Buckmann (1932), Russell (1932), and 

 other contributors to volume 80 of Rapports et 

 Proces-Verbaux, Conseil Permanent International 

 pour l'Exploration de la Mer. 



Evidence on the effects of destruction of a 

 large segment of the population by catastrophe 

 was provided by Beckman's (1950) study of the 

 growth of fishes in four Michigan Lakes that 

 suffered severe winterkill in early 1945. With 

 the exception of a single species in a single lake, 

 all stocks exhibited an immediate marked im- 

 provement of growth. The rapid growth was 

 soon lost, however, as reproduction restored 

 population density to a high level. 



The effects of an experimental reduction of 

 population size was demonstrated by Beckman 

 (1941 and 1943) for the rock bass (Ambloplites 

 rupestris) of Standard Lake (Mich.). The de- 

 struction of the entire stock of fish in one basin 

 of this hourglass-shaped lake in 1937 produced 

 an immediate sharp improvement in growth. 

 This good growth was still largely maintained 

 at least as late as 1942, the year of the last 

 sampling. 



Critical reviews of the literature and refer- 

 ences to publications on the relation between 

 population densities and growth rates of fishes 

 have been given by Hile (1936), Van Oosten 

 (1944), and Watt (1956). 



Changes of growth rate with increase or de- 

 crease of population density have mostly been 

 related to the availability of food. In many 

 situations, as notably with the plaice, evidence 

 in support of this view is good. Some authors 

 have suggested, however, that space available per 



