Comparative Performance of Two Genetic Groups of Stocked Brook Trout in Maine
By Forrest R. Bonney
Maine Department of Inland Fisheries and Wildlife
Fisheries and Hatcheries Division
Final Report (1997-2001)
The performance of two new genetic groups of hatchery-reared brook trout was studied in eight Maine lakes from 1998 to 2001. These groups are being developed to replace the older domestic strains which, due to inbreeding, exhibited high mortality rates prior to hatch-out, and were short-lived in the wild. Paired stockings of Kennebago and Sourdnahunk fish, identified by different fin clips, were evaluated for catch rates, growth rates, and fall abundance. Anglers fished the study ponds at an average rate of 29 angler trips/ac/season, kept 0.14 fish/angler, and caught a legal-size brook trout for every 3.7 hours of fishing. The estimated harvest was equally comprised of Kennebago and Sourdnahunk fish. Older (age II+ and III+ fish) accounted for 31% of the Kennebago and 25% of the Sourdnahunk harvest. Because the older fish were heavier, Kennebago fish provided a harvest of 1.39 lb/a, compared to 0.83 for the Sourdnahunk fish.
Population estimates, determined for only the three ponds with low interspecific competition, averaged 11 brook trout/ac, or 5.0 lb/ac. Older-age fish represented 17% of the number and 27% of the weight of the population. There was no difference in the incidence of hooking injuries between the Kennebago and Sourdnahunk fish. However, the Kennebago fish were more abundant, were larger than the Sourdnahunk fish and matured at an earlier age. There were differences in growth rates among ponds. Age II+ fish of both groups had a higher rate of hooking injuries than age I+ fish. Fish from a pond with an artificial-lures-only regulation also had significantly more hooking injuries than those from a pond with a fly-fishing-only regulation. Fish with hooking injuries were less robust than those without hooking injuries. For ponds with a similar number of competing fish species, older-age fish of the Kennebago and Sourdnahunk strains represented 33.5% of those captured, compared to only 4.3% for the domestic strains evaluated in an earlier study. To date, the new strains have higher hatching rates and better survival rates to older age than the domestic strains. This study is scheduled to be continued one more year.
Eight Maine lakes, located in Cumberland, Franklin, Kennebec, Oxford, and Washington counties, were studied in 1998-2000 to evaluate the performance of two new genetic groups of hatchery-reared brook trout (Salvelinus fontinalis). Paired stockings of Kennebago and Sourdnahunk trout, identifiable by differential fin excision, were evaluated for returns to the angler, growth rates, and post-angling-season abundance. Anglers fished the study ponds at an average rate of 29 angler trips/ac/season (72 angler trips/ha/season), kept 0.14 fish/angler, and caught a legal-size brook trout for every 3.7 hours of fishing. The estimated harvest was comprised of 50% Kennebago fish and 50% Sourdnahunk fish. Age II+ and age III+ fish accounted for 31% of the Kennebago and 25% of the Sourdnahunk harvest. Older-age Kennebago fish were harvested at a rate of 1.39 lb/ac (1.56 Kg/ha), compared to 0.83 lb/a (0.93 Kg/ha) for the Sourdnahunk fish. Population estimates, determined only for three ponds with low interspecific competition, averaged 11 brook trout/ac (27/ha), or 5.0 lb/ac (5.6 Kg/ha). Older-age fish represented 17% of the number and 27% of the weight of the standing stock. There was no significant difference in incidence of hooking injuries by genetic group. Kennebago fish were recaptured by trapnetting at higher rates, were larger (in both length and weight), and matured at an earlier age than Sourdnahunk fish. Age II+ fish of both groups had significantly more hooking injuries than age I+ fish. The incidence of hooking injuries was inversely correlated to regulatory severity. Fish with hooking injuries had significantly lower conditions than those without. Brook trout accounted for 48% of the fish biomass in a pond with low interspecific competition, but less than 1% of the biomass in ponds with severe interspecific competition. For ponds with comparable levels of interspecific competition, older-age (ages II+ through IV+) fish of the Kennebago and Sourdnahunk strains stocked at the same age represented 33.5% of those captured compared to only 4.3% for the domestic strains evaluated in an earlier study.
Of Maine's 1,135 principal brook trout lakes, 476 are dependent on stocking to provide a fishery. Over the past century, the Department of Inland Fisheries and Wildlife's hatchery system has reared several strains of brook trout to meet the stocking needs of Maine's public waters. Most of these strains, including the so-called 'Maine Hatchery Strain', originated outside of the state. Despite periodic infusions of genes through the introduction of new strains, including the Assinica strain, domestic brook trout have exhibited poor longevity and high egg mortality. A 4-year study comparing performance of the Maine Hatchery and F1 hybrid (Maine Hatchery/Assinica) strains, which have accounted for the majority of the production stocking, indicated that holdover from age I+ to age II+ was only 6 and 8%, respectively (MDIF&W 1993). Furthermore, declining and erratic rates of egg survival (Appendix 1) have rendered these strains unreliable as sources of production fish. The inbreeding and domestication of these strains is attributed to crossings made with inadequate numbers of brood fish.
In an effort to reduce egg mortality and to increase the longevity of stocked brook trout, the Department's Hatchery Division undertook a program to replace domesticated stocks with two genetic groups of wild brook trout. Both groups were taken from river drainages with few or no records of having been stocked by the Department, and emphasis was placed on acquiring enough brook trout to assure that genetic variability was maintained. Brook trout eggs have been taken from Sourdnahunk Lake, located in Piscataquis County, since 1995; and from the Kennebago River, located in Franklin County, since 1996. Analysis of microsatellite DNA variation confirmed that these two populations represent distinct genetic units (Bernatchez 1996).
The protocol for the establishment of these two new hatchery strains stipulates that a minimum of 100 female and 100 male brook trout be mated annually from each of these waters for a minimum of three years to establish a pool of brood fish; thereafter, an infusion of wild gametes will be made every four to six years in an effort to maintain heterozygosity.
In addition to establishing two populations of brood fish, progeny of these brook trout were also stocked experimentally in selected lakes to evaluate their relative performance. This report documents the relative performance of these two genetic groups of hatchery-reared brook trout.
For more information, please contact:
Forrest Bonney, Regional Fishery Biologist
689 Farmington Road
Strong, Maine 04983-9419
Telephone: (207) 778-3322 Ext. 22