CARLINE: PRODUCTION BY WILD BROOK TROUT 



winter of 1968-69 fall 2-yr-olds increased by 50% 

 (Figure 2). Mean population densities were higher 

 in fall than in spring (4,480 vs. 3,200/ha) because 

 of recruitment by age trout and age 1 trout. 



Trout migrating into Hoglot Springs had a 

 marked effect on population biomass. Biomass 

 was highest in fall 1970 because of the large stock 

 of yearlings (91 kg/ha), most of which were recent 

 immigrants (Table 3). Little immigration oc- 

 curred oversummer in 1971 and overwinter in 

 1971-72. As a result, population biomass in spring 

 1972 reached its lowest level of the 4-yr period. 



APR OCT 



1970 



APR OCT 



1971 



APR 

 1972 



FIGURE 2. — Estimated numbers of brook trout in Hoglot 

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

 areas separate calendar years. 



Maxwell Springs 



Except for 1972, Maxwell Springs supported 

 the largest of the three populations, and natural 

 reproduction accounted for nearly all recruitment. 

 Two experiments were conducted to evaluate the 

 extent of immigration from Maxwell outlet into 

 the pond. In June 1969 and April 1972, a total of 

 602 ages and 1 trout were captured in the outlet 

 and marked. In subsequent surveys of the pond, 

 I examined over 4,000 trout, only 3 of which had 

 been marked in the outlet. Hence, I concluded 

 that trout reared in the outlet did not materially 

 affect recruitment in the pond. 



From April 1969 to September 1972 trout densi- 

 ties in Maxwell Springs declined markedly (Fig- 

 ure 3). Spring densities steadily decreased from 

 7,300/ha in 1969 to 1,810/ha in 1972. Fall 

 populations followed a similar trend. This decline 

 was due in part to decreasing numbers of fall 

 fingerlings. Densities of age trout ranged from 

 4,085/ha in October 1969 to 1,940/ha in Septem- 

 ber 1972. However, even the 1969 year class, 

 which was larger than the succeeding three year 

 classes, had to be smaller than the 1968 and 1967 

 year classes, based on their densities as ages 1 

 and 2 fish in April 1969 (Figure 3). I estimated 

 numbers of fall fingerling for the 1967 and 1968 

 year classes by using average mortality rates of 

 succeeding year classes. The 1967 year class was 

 estimated at 16,000/ha and the 1968 year class 

 at 8,300/ha. Thus, numbers of fall fingerlings had 

 steadily declined from 1967 to 1972 with one 

 exception, the 1971 year class. 



The reduction in year class strength in Maxwell 

 Springs may have been related to the installation 

 of a weir in the pond outlet in 1968. The weir, 

 which was used to monitor discharge, was located 

 132 m downstream from the pond and it created 



APR OCT 



1969 



FIGURE 3.— Estimated numbers of brook trout in Maxwell 

 Springs, 1969-72. Numbers designate age-groups and hatched 

 areas separate calendar years. 



755 



