FISHERY BULLETIN: VOL. 75, NO. 4 



nets and seines showed that collecting trout with 

 just electrofishing gear did not yield biased 

 estimates (Carline unpubl. data). Efficiency of the 

 electrofishing gear usually increased with trout 

 size (Table 2). Mean proportions of marked trout 

 captured during the second electrofishing sample 

 for age to 3 fish were 0.18, 0.31, 0.35, and 0.39, 

 respectively. Recapture efficiencies were always 

 lowest for age trout and values ranged from 

 0.05 to 0.30. For age 1 and older fish, precision 

 of estimates depended mostly upon sample size 

 and confidence limits for the oldest age groups 

 were generally broad because of their low densi- 

 ties (Table 2). 



TABLE 2. — Examples of trout population estimates and 95% 

 confidence limits by age-groups. Data were collected in fall 1970. 



Item 







1 



Clubhouse Springs: 

 Mean length (mm) 

 Proportion of marked 



fish recaptured 

 Population estimate 



(no./ha) 

 95% confidence limits 



Maxwell Springs: 

 Mean length (mm) 

 Proportion of marked 



fish recaptured 

 Population estimate 



(no./ha) 

 95% confidence limits 



99 



0.30 



386 

 234 

 782 



92 



0.05 



175 



0.40 



363 

 279 

 466 



147 



0.43 



2,195 1,572 

 1,183 1,408 

 3,944 1,778 1,003 



211 



0.41 



84 



47 



124 



182 



0.53 



909 



845 



274 



0.50 



6 







40 



220 



0.34 



433 

 367 



507 



287 



0.22 



28 

 17 

 56 



Clubhouse Springs 



The brook trout population in Clubhouse 

 Springs was the smallest of the three populations. 

 Because no spawning areas were present, this 

 population was entirely dependent upon immigra- 

 tion from downstream areas. Trout densities 

 usually declined from spring to fall and only age 

 trout appeared to immigrate in substantial 

 numbers oversummer (Figure 1). Total trout 

 numbers in 3 of 4 yr increased overwinter due 

 to immigration. Numbers of trout in spring 

 ranged from 390 to 1,750/ha and densities in fall 

 ranged from 390 to 840/ha. Age structure of the 

 population was at times atypical because young 

 age groups were less numerous than older ones, 

 owing to differential rates of immigration. 



Changes in population biomass closely paral- 

 leled numerical changes. Biomass in spring 

 averaged 45 kg/ha and in fall 26 kg/ha (Table 3). 

 In all years, population biomass increased from 

 fall to spring, the period when immigration 

 appeared greatest. 



APR OCT 



1968 



FIGURE 1. — Estimated numbers of brook trout in Clubhouse 

 Springs, 1968-72. Numbers designate age-groups and hatched 

 areas separate calendar years. 



TABLE 3. — Estimated biomass (kilograms per hectare) by age- 

 group of brook trout in study ponds, 1968-72. Mean weights of 

 individuals in each age-group were multiplied by estimated 

 density of the age-group to calculate biomass. 



Site and date 



Total 



Clubhouse Springs: 



27 Mar. 1968 



28 Aug. 1968 

 8 Apr. 1969 



8 Sept. 1969 1.8 



1 Apr. 1970 



8 Sept. 1970 3.5 



29 Apr. 1971 



8 Sept. 1971 2.1 



21 Apr. 1972 

 Hoglot Springs: 



2 Apr. 1 968 



26 Aug. 1968 6.8 



8 Apr. 1969 



8 Sept. 1969 15.9 



13 Apr. 1970 



8 Oct. 1970 16.9 



28 Apr. 1971 



21 Sept. 1971 7.0 



2 May 1972 

 Maxwell Springs: 



9 Apr. 1969 



13 Oct. 1969 27.0 



26 Mar. 1970 



6 Oct. 1970 22.0 



26 Apr. 1971 



20 Sept. 1971 24.6 



26 Apr. 1972 



29 Sept. 1972 14.8 



Hoglot Springs 



Although some fingerlings were hatched in 

 Hoglot Springs, numbers of immigrating trout, 

 particularly age 1 fish, had the most impact on 

 population size. In 3 of 4 yr, densities of yearling 

 trout increased oversummer, and during the 



754 



