PRODUCTION BY THREE POPULATIONS OF 



WILD BROOK TROUT WITH EMPHASIS ON 



INFLUENCE OF RECRUITMENT RATES 



Robert F. Carline 1 



ABSTRACT 



Populations of wild brook trout, Salvelinus fontinalis, in three small ponds in northern Wisconsin 

 were studied for 4 yr to determine annual production with particular emphasis on influence of 

 recruitment rates. Recruitment included trout hatched in ponds and immigrants from adjacent 

 waters. Age-specific growth rates and densities of trout were estimated in spring and fall. Harvest 

 of trout was estimated through partial creel surveys. 



Among populations annual production ranged from 26 to 331 kg/ha and was directly related 

 to recruitment rates. Production was most influenced by population biomass. Instantaneous growth 

 rates did not vary significantly within or among populations despite large differences in population 

 densities; hence, variations in production appeared unrelated to growth rates. Among populations, 

 yield of trout ranged from 25 to 72 kg/ha and fishing pressure ranged from 154 to 1,405 h/ha. 

 Proportion of annual production that was harvested was directly related to fishing pressure. 



Production of fry during the first 9 mo of life may have been overestimated because mortality 

 rates from emergence to fall were assumed constant. Estimates of production of adult trout could 

 have been positively or negatively biased depending upon immigration patterns. Despite these 

 possible errors, it was clear that recruitment was the most important factor affecting production. 



Estimation of fish production has gained wide- 

 spread acceptance because it provides some 

 measure of a system's capacity to support species 

 of interest (Gerking 1967). Production is defined 

 as the total elaboration of tissue by a population 

 during a specified time interval, regardless of the 

 fate of that tissue (Ivlev 1945). Unlike standing 

 crop estimates, production is a dynamic popula- 

 tion parameter that is useful in evaluating the 

 environmental performance of a fish population 

 (Le Cren 1972). Studies by Ricker and Foerster 

 (1948), Allen (1951), and Hunt (1971) are good 

 examples of how fish production has been related 

 to predation, the food supply, and habitat suit- 

 ability. While many studies have considered the 

 effects of standing crops, growth rates, and mor- 

 tality on production, the importance of recruit- 

 ment has not been well defined. 



In northern Wisconsin, standing crops of wild 

 brook trout, Salvelinus fontinalis, in spring- fed 

 ponds vary greatly. Some ponds have filled-in 

 naturally and living space is limiting. In others, 

 living space appears to be adequate, but spawning 



'Wisconsin Department of Natural Resources, Route 1, 

 Box 203, Waupaca, WI 54981; present address: Ohio Cooperative 

 Fishery Research Unit, Ohio State University, 1735 Neil 

 Avenue, Columbus, OH 43210. 



Manuscript accepted April 1977. 



FISHERY BULLETIN: VOL. 75, NO. 4, 1977. 



areas are small or nonexistent and recruitment 

 seems to be limiting standing crops of trout. The 

 objective of this study was to determine annual 

 production by three populations of wild brook 

 trout with particular emphasis on the influence 

 of recruitment rates. Recruitment includes all 

 trout hatched in the ponds plus all immigrant 

 trout. 



The ponds were chosen because they differed 

 greatly in areas available for spawning and 

 numbers of immigrating trout. Ponds were sim- 

 ilar in size and watershed characteristics, and 

 springs were the primary sources of water. Outlet 

 streams, which flowed into larger streams and/or 

 lakes, provided convenient sampling boundaries, 

 but did not impede movement of trout into or out 

 of the ponds. I estimated densities and growth 

 rates of trout every spring and fall from 1968-72 

 and conducted partial creel surveys during 3 yr 

 of the study to estimate trout yields. 



DESCRIPTION OF STUDY AREA 



The study ponds, situated in a terminal mo- 

 raine, are located within 7 km of each other in 

 Langlade County, north central Wisconsin. The 

 moraine is composed of glacial till ranging in size 

 from sand to large boulders. These permeable 



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