402 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The opening of stomachs of the larger Saginaw 

 perch in samples of the present study indicated 

 smelt to be an important, possibly the principal, 

 item of diet. Perch sometimes take smelt of 

 surprising size (often the tail of the smelt pro- 

 trudes from the perch's mouth). 



The alewife can be dismissed as a significant 

 factor in the slow growth of yellow perch of the 

 1943—55 samples because it has become plentiful 

 too recently. The current great abundance of 

 alewives in Saginaw Bay is new. It is to be 

 questioned whether alewives were present in con- 

 sequential numbers before 1954 or even 1955. 

 Should alewives continue to be plentiful, they 

 could supply additional forage for the larger 

 perch. The degree of food competition with 

 small perch is not known. 



The abundance indices for other associates of 

 the yellow perch in Saginaw Bay (table 45) are 



Table 45. — Average abundance and production of yellow 

 perch and associated species in Saginaw Bay in 1929-30 

 and 1943-55 



1 In thousands of pounds. 



based on records of the commercial catch per 

 unit of fishing effort and were computed by the 

 procedure described by Hile (1937) ; the base of 

 100 is the mean abundance for 1929-43. It should 

 be understood that the indices are based on the 

 catch of only the legal- or marketable-sized fish 

 and that they have been biased to some degree 

 by changes of regulations. In the most recent 

 years the abundance of suckers and carp prob- 

 ably was underestimated because of the failure 

 of fishermen to land their entire catch (weak 

 market conditions). Despite their defects, these 

 indices are our best information on the changes 



in population level of the several species, except 

 perch for which additional information is given 

 later. 



The mean abundance index of yellow perch 

 and of all its associates except the walleye in- 

 creased from 1929-30 to 1943-55. The average 

 index for perch in 1943-55 was 1.19 times that 

 for 1929-30. For the other species the ratio 

 ranged from 0.73 (walleyes — only value below 

 1.00) to 3.85 (carp). These changes of the in- 

 dex, the addition of smelt as an important mem- 

 ber of the population, and the recent great abun- 

 dance of alewives all point toward a substantial 

 rise in the fish population of Saginaw Bay. 



The records of sizes of yellow perch in random 

 samples from commercial gear in 1929-30 and 

 1943^5 makes possible a more discriminating 

 estimate of the change in the population density 

 of that species. The nature of the computations 

 can be illustrated with the data for 1929-30. 

 From the length-frequency distribution pub- 

 lished by Hile and Jobes (1941) and from their 

 length-weight equation it was determined first 

 that the 640 legal-sized fish (then 9 inches or 

 longer) in their sample had a total weight of 

 239.5 pounds. In the same 2 years the average 

 catch of yellow perch per lift of one trap net 

 was 17.83 pounds. It is then calculated that the 

 lifting of 100 trap nets in 1929-30 yielded 



640 X ' '° r or 4,765 legal-sized fish. The same 



239.5 

 sample, however, contained also 302 undersized 

 perch which correspond to a rate of capture of 

 2,248 per 100 lifts. The same procedure applied 

 to the size distribution of all samples and the 

 average catch per trap net for 1943-55 leads 

 to an estimated take of 5,062 legal (now 8i/ 2 

 inches or longer) and 42,502 undersized perch 

 per 100 trap nets. 



The comparison of the estimates of numbers 

 of legal- and undersized yellow perch taken per 

 100 trap-net lifts in 1929-30 and 1943-55 (table 

 46) brings out the enormous change that has 



Table 46. — Estimated numbers of legal-sized and undersized 

 yellow perch captured in Saginaw Bay per lift of 100 

 shallow trap nets in 1929-30 and 1943-55 



[Minimum legal size: 1929-30, 9 inches; 1943-55, 8H inches] 



