CARLINE: PRODUCTION BY WILD BROOK TROUT 



trout. If changes in growth rates of trout fry in 

 my study were similar to those in Lawrence 

 Creek, then assumptions of constant growth rates 

 are much less serious than those regarding 

 mortality rates. 



To estimate production with the Ricker formula 

 (G x B) one assumes that no emigration or im- 

 migration occurred (Chapman 1967). Effects of 

 emigration on production are similar to those of 

 mortality. Recognition of emigration allows one 

 to demonstrate the fate of production, but does 

 not directly affect calculated values. Immigration, 

 however, can have serious effects upon production 

 estimates. The Ricker formula integrates two 

 simultaneous processes, growth and mortality. 

 Numbers offish are assumed to decrease exponen- 

 tially and their mean weights are assumed to 

 change in a similar fashion. When immigration 

 occurs and an age-group increases in number, the 

 Ricker formula treats this increase as an exponen- 

 tial one. 



To assess the influence of immigration on pro- 

 duction, I simulated three different immigration 

 patterns in which year class density increased 

 from 1 ,400 trout/ha in April to 3,600/ha in October 

 (Figure 9). Curve B represents an exponential 

 increase in density, i.e., that assumed in the 

 Ricker formula. Production was calculated at 

 monthly intervals and the same growth rate was 

 used for each simulation. If all immigration had 



4500 



3500 



o 



o 



2500 



1500 



500 



A. 57 



C 26 



APRIL 



JUNE 



AUG 



OCT 



FIGURE 9.— Three hypothetical immigration patterns for a 

 single age-group. Production for each curve was calculated 

 monthly using the same instantaneous growth rate i G = 0.99, t 

 = 0.5 yr>. Total production for each curve is given next to letter 

 designation. 



occurred in the first half of the interval (A), 

 estimation by the Ricker formula would have 

 underestimated production by 307c , and if trout 

 had immigrated in the latter half of the interval 

 (C), production would have been overestimated 

 by 549c. This increase in cohort size was similar 

 to that of age 1 trout in Hoglot Springs in 1970, 

 the largest increase that occurred in either Hoglot 

 or Clubhouse springs. Therefore, potential errors 

 in production estimates for other intervals would 

 have been less serious. 



Recruitment, via immigration and spawning 

 within ponds, appeared to be the most important 

 factor influencing production. Even though pro- 

 duction by age trout could have been over- 

 estimated, production by age 1 and older trout 

 was closely tied to recruitment rates. In other 

 studies, only a few attempts have been made to 

 link production to recruitment. Backiel and 

 Le Cren (1967) analyzed data from Lawrence 

 Creek (Hunt 1966) and Cultus Lake (Ricker and 

 Foerster 1948) and showed that production was 

 directly related to numbers of emerging fry. 

 Highest annual production of sockeye salmon, 

 Oncorhynchus nerka, in Lake Dal'neye occurred 

 in years of highest egg deposition (Krogius 1969 1. 



In this study population biomass was deter- 

 mined by annual recruitment. Among popula- 

 tions, production was most influenced by trout 

 biomass because age-specific growth rates were 

 not significantly different. As a result, production 

 increased linearly with biomass. Hunt (1974) 

 found similar linear relationships for brook trout 

 in Lawrence Creek. Backiel and Le Cren (1967) 

 reviewed density effects on production and illus- 

 trated both linear and curvilinear associations 

 between production and biomass. Curvilinear 

 relationships resulted when growth rates were 

 severely depressed at high fish densities and in 

 all of these studies fish were stocked and move- 

 ment was restricted. I am not aware of any study 

 of wild fish populations in which inverse density- 

 dependent growth caused curvilinear relation- 

 ships between production and biomass. Rather, 

 in wild populations of salmonids, fish densities 

 appear to be maintained at levels that do not 

 result in seriously depressed growth rates and 

 production increases directly with biomass. 



Standing crops of harvestable trout (age 1 and 

 older) in the three populations declined over a 

 year's time because total mortality exceeded 

 growth rates, even though immigration bolstered 

 density of some age-groups (Table 12). The actual 



763 



