Evaluating Regulations Imposed on Wild Brook Trout Lakes to Restore Age and Size Quality


By Forrest R. Bonney

Maine Department of Inland Fisheries and Wildlife
Fisheries and Hatcheries Division
Augusta, Maine

March, 2002
JOBS F-103, F-104, F-204

Interim Summary Report No. 1 (1994-2001)


Twenty-four Maine lakes with wild brook trout populations were studied from 1994 to 2001 to gather biological information for statewide averages and to evaluate the effectiveness of new, restrictive regulations imposed on 334 of Maine’s 1,103 brook trout lakes in 1996. This report incorporates the results of work conducted on five study ponds in 2001 with the work previously completed and reported in Progress Report No. 5 (Bonney 2001).

Brook trout abundance1, estimated from 54 population estimates, averaged 4.1 brook trout/acre and biomass2 averaged 4.0 lb/acre. The average size of the 4,858 brook trout aged was 9.6 inches and 6.6 ounces. The proportion of brook trout age III+ and older increased from 22% in 1994-1995 to 41% in 1999-2001, indicating that the restrictive regulations are meeting their goal of restoring older-age fish to the population.

Brook trout abundance is limited by competition from other fish species. For lakes with high levels of competition, brook trout accounted for only 4% of the total weight of all fish trapnetted. Brook trout from waters with high levels of competition were also smaller; however, the proportion of older brook trout (age III+ and greater) was highest in these waters.

A total of 24 lakes was studied from 1994-2001 to determine brook trout population abundance and age structure, and to determine the effects of interspecific competition and regulatory severity (Table 1). This report incorporates the population estimates determined at five study ponds in 2001 to the work previously conducted from 1994-2000 and reported in Progress Report No. 5. Two of these waters – Horseshoe Pond and Round Pond in Chase Stream Township, Somerset County – were sampled for the first time by trapnetting (Tables 2 and 3). The other three ponds – Little Moxie Pond, Rum Pond, and Trout Pond – have been sampled in previous years. As in the past, ages of all brook trout sampled were determined from scales by experienced readers. Differences between mean fish sizes were tested using ANOVA and Duncan’s multiple range test. Chi-square analysis was used to compare age structures, and Pearson’s test was used to determine correlations. Significance level was set at P=0.05 for all tests.

The mean 1994-2001 post-fishing season population estimate of age I+ and older brook trout for all study waters was 4.06 fish/a (Table 4) and biomass averaged 4.03 lb/a. Abundance estimates varied widely from water to water; however, variance decreased markedly when waters were grouped by the degree of interspecific competition, suggesting that this indicator may be a useful discriminator in improving accuracy of brook trout abundance estimates.

The average size of the 4,858 brook trout aged by scale reading since the inception of the study in 1994 was 245 mm (9.6 inches) and 187 g (6.6 ounces) (Table 5). Using the average size of age I+ fish as a reference, the greatest incremental increase in length occurred between their second and third year, when they increased an average of 65 mm (2.6 inches); the greatest weight increment occurred between their third and fourth year, when they increased an average of 188 mm (6.6 ounces). On average, brook trout grew at a rate of 45 mm (1.8 inches) and 103 g (3.6 ounces) per year.

Brook trout age III+ and older represented 30 % and age IV+ and older represented 7 % of all fish sampled (Table 6). The proportion of brook trout age III+ and older increased from 22 % in 1994-95 to 41 % in 1999-2001. The proportion of brook trout age IV+ and older increased from 1 % to 12 % during this period. There was no trend in changes of lengths or weights of brook trout. The increases in the proportion of older-age fish is attributed to increased regulatory protection imposed in 1996. This contention is supported by the fact that waters with greater regulatory severity had significantly more older-age fish (Table 7). For this analysis, waters with ‘low’ and ‘moderate’ regulatory restrictions were grouped as low and waters with ‘high’ and ‘severe’ regulatory restrictions were grouped as high.

The increase in the proportion of older-age fish is considered to be important, because historical data indicated a decline in the proportion of older-age brook trout sampled from statewide lakes over time. Fifty-one percent of the brook trout sampled from 1939-44 were age III+ and older. This figure declined to 40% for those fish sampled from 1989-93. The proportion of brook trout age IV+ and older declined from 19% to 9% for the same year-groups (MDIF&W 1995). The reduction in the proportion of these fish resulted in less angler satisfaction and concern that stocks were being genetically compromised. There was also a significant relationship between regulation severity and brook trout biomass (correlation coefficient = 0.47; P = 0.005), confirming that protection from harvest resulted in greater standing stocks.

The study waters were grouped by brook trout maturity and regulation severity (Table 8) to determine whether differences in the proportion of mature fish could be attributed to regulations in effect. The proportion of mature trout age III+ and older increased from 23% of those sampled in waters with low to moderate severity to 29% of those with high to severe regulatory severity. For brook trout of ages IV+ and older, the respective percentages were 4 and 7. Actual values of the percentage of older-age fish sampled in earlier years are not comparable to those sampled in this study because sampling techniques were different. However, these data indicate that the more restrictive regulations imposed in 1996 were effective in reversing the decline in the numbers of older brook trout.

In ponds with low rates of competition, brook trout accounted for over 98 % of the biomass trapnetted. The proportion of brook trout declined rapidly as interspecific competition increased (Table 9); for those with moderate rates of competition, they accounted for 18%; and for those with high competition, they made up only 4% of the biomass trapnetted. For all lakes, brook trout accounted for an average of 45% of the biomass trapnetted. Clear Lake, which is oligotrophic, had a relatively high proportion of brook trout biomass despite having a large number of competing species. Brook trout represented an average of 20% of the biomass for this water.

Estimates of brook trout biomass ranged from 6.1 lb/a for lakes with low interspecific competition to 1.7 lb/a for those with high interspecific competition (Table 10). Clear Lake, which had enough interspecific competition to warrant a ‘severe’ rating, nonetheless had a greater brook trout biomass (2.5 lb/a) than more typical trout ponds with fewer competitors. The proportion of age III+ and older brook trout was highest (31%) in waters with high levels of interspecific competition; the value for those with low competition was 27% (Table 11). For Clear Lake, which had the highest rate of interspecific competition, 53% of the brook trout were older-age fish.

Mean sizes of brook trout sampled from the study ponds during fall trapnetting were also compared by the degree of interspecific competition (Table 12). Comparison of size and age frequencies among the study waters indicated that there is not a simple inverse relationship between brook trout growth rates and the presence of competing species. Overall, mean sizes of brook trout from waters with high competition were significantly smaller than those with low, moderate, or severe competition. Lakes with severe competition tended to have some of the larger fish, suggesting that abundance values for oligotrophic lakes need to be further evaluated. For other categories, however, there was little correlation between average brook trout size and interspecific competition.

Not surprisingly, there were significant inverse relationships between the degree of interspecific competition and brook trout abundance and biomass (correlation coefficient = 0.66; P = 0.0003). The unexpected positive relationship between interspecific competition and the proportion of older individuals in brook trout populations suggests that future analysis should continue to consider this variable as a factor in determining the effects of regulatory protection.

1 - Number of brook trout per acre.
2 - Lbs. of brook trout per acre.

More Information, Please Contact:

Forrest Bonney, Regional Fishery Biologist
689 Farmington Road
Strong, Maine 04983-9419
Telephone: (207) 778-3322 Ext. 22
Email: forrest.bonney@maine.gov