Numerical Changes and Population Regulation in Brook Trout Salvelinus fontinalis

Publisher Website
Google Scholar
Abstract
A brook trout population in a stable stream environment was studied for 14 years. Population estimates were made each September, the catch of fish by anglers was recorded, and collections of fish were made to determine sex ratio, age at sexual maturity, and fecundity.The sex ratio was close to 1:1 for all age-groups. Some male trout were sexually mature at the end of the 1st year of life, most were mature at the end of the 2nd year, and all were mature at the end of the 3rd. No females were mature at the end of their 1st year, most were mature at the end of the 2nd, and all were mature at the end of the 3rd year of life. The regression of egg content on total length of female trout was determined.On the average 132,000 eggs were produced annually by the population in the study area, from which 4813 fish survived to the end of the 1st year; 1966 to the end of the 2nd; 356 to the 3rd; 29 to the 4th; and 0.6 to the end of the 5th year.The growth rate of the fish varied little during the period of study. Annual egg production varied between 80,000 and 212,000; the number of age-group 0 fish between 2809 and 6703; age-group I fish between 1589 and 2653; age-group II between 175 and 685; age-group III between 13 and 68; and age-group IV fish between 0 and 2.During the 14 years of observation, survival of adult fish tended to increase, leading to larger numbers of spawners. The larger egg complements which resulted tended to experience lower survival, thus largely preventing substantial increases in population size. Positive serial correlations exist in some of the series of estimates of numbers and of survival. No biological explanation is advanced for these serial correlations.Life tables and age-specific fecundity rates were calculated for 11 successive cohorts of brook trout. Net reproduction rates varied from 0.74 to 1.88, averaging 1.14. The lx components were about three times as variable as the mx components, indicating that changes in death rates rather than changes in birth rates have been the principal immediate cause of fluctuations in the population.The mx values were not related to population density in any way that would tend to regulate population size. Survival from the 12th to the 24th month of life was directly density-dependent. The pattern of survival from the 24th to the 36th month of life suggested an inverse density-dependent relationship.The number of 12-month-old progeny surviving varied from 2809 to 6703, and the parental egg complement varied from 80,000 to 212,000. The form of the relationship between these two variables could not be defined, partly because of the relatively small range of values of parental egg complement.Even though the form of the density-survival relationship was defined for several stages of the life history, no relationship between the size of a parental egg complement and the lifetime egg production of the progeny could be established, again partly because of the small range of values of the former variable.