FISHERY BULLETIN: VOL. 75. NO. 4 



TABLE 12. — Comparison of annual yield of brook trout with potential yield and biomass 

 loss to natural mortality. Data are for trout age 1 and older. All values are in kilograms 

 per hectare. 



biomass loss includes both the change in standing 

 crops from one year to the next and the production 

 during that interval. In all three populations, the 

 actual annual loss in biomass exceeded average 

 standing crops. This loss in biomass may be 

 viewed as the potential yield (Table 12). Biomass 

 lost to natural mortality was calculated as the 

 difference between potential and actual yields. 

 Fate of potential yields appeared dependent upon 

 fishing pressure. In Clubhouse Springs fishing 

 pressure was highest (Table 9), and yield in 1970 

 was 859c of the potential. Only 16 and 25% of 

 potential yields were taken in Maxwell Springs, 

 where fishing pressure was lowest. The relatively 

 low level of exploitation in Maxwell Springs 

 resulted in substantial biomass losses to natural 

 mortality. 



Estimates of fish production in lentic waters 

 have varied from less than 1 g/m 2 to 64 g/m 2 , 

 but in most studies they were <20 g/m 2 (Le Cren 

 1972). Highest reported values were for juvenile 

 sockeye salmon in Lake Dal'neye (Krogius 1969). 

 Production estimates for Maxwell Springs (21- 

 33 g/m 2 ) are among the highest values currently 

 available. Even if contributions of age trout in 

 Maxwell Springs are ignored, production esti- 

 mates still rank high (11-22 g/m 2 ). Carline and 

 Brynildson (1977) suggested that high levels of 

 trout production in ponds similar to Maxwell 

 Springs were due to extensive littoral areas and 

 high standing crops of benthic organisms. While 

 prevailing food densities determine the level of 

 potential fish production, attainment of this poten- 

 tial level is dependent upon annual recruitment 

 of some minimum number offish. 



In this study differences in spawning areas 

 among ponds were obvious and trout production 

 varied accordingly. In many instances quantity 

 and quality of spawning sites are unknown or 

 cannot be readily determined. Where recruitment 

 is limiting, fish production will be relatively low, 



regardless of the water's general productivity. 

 If production is to be used as a measure of a 

 system's capacity to support species of interest, 

 recruitment of that species should be at or near 

 maximum levels. 



ACKNOWLEDGMENTS 



I am indebted to O. M. Brynildson and 

 R. L. Hunt for their guidance throughout the 

 study. K. Neirmeyer and H. Sheldon provided 

 much technical assistance. J. J. Magnuson made 

 many valuable suggestions during data analysis. 

 D. W. Coble and R. A. Stein ably reviewed earlier 

 manuscripts. This study was supported by the 

 Wisconsin Department of Natural Resources and 

 by funds from the Federal Aid in Fish Restoration 

 Act under Project F-83-R. 



LITERATURE CITED 



ADAMS, L. 



1951. Confidence limits for the Petersen or Lincoln Index 

 used in animal population studies. J. Wildl. Manage. 

 15:13-19. 

 ALLEN, K. R. 



1951. The Horokiwi stream, a study of a trout population. 

 N.Z. Mar. Dep. Fish. Bull. 10, 238 p. 



Backiel, T., and E. D. Le Cren. 



1967. Some density relationships for fish population 

 parameters. In S. D. Gerking (editor), The biological 

 basis of freshwater fish production, p. 261-293. Blackwell, 

 Oxf. 

 BRASCH, J. 



1949. Notes on natural reproduction of the eastern brook 

 trout (S. fontinalis) with a preliminary report on several 

 experiments on the subject. Wis. Conserv. Dep., Div. 

 Fish. Biol., Invest. Rep. 653, 9 p. 

 CARLINE, R. F., AND O. M. BRYNILDSON. 



In press. Effects of hydraulic dredging on the ecology of 

 native trout populations in Wisconsin spring ponds. 

 Wis. Dep. Nat. Resour. Tech. Bull. 

 CHAPMAN, D. W. 



1967. Production in fish populations. In S. D. Gerking 

 (editor), The biological basis of freshwater fish produc- 

 tion, p. 3-29. Blackwell, Oxf. 



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