Use of Outlet Barriers to Prevent Fall Emigration of Brook Trout Stocked in Adirondack Lakes

—Outlet barriers in stocked lakes of the Adirondack Mountains were used to test the hypothesis that preventing fall emigration by adult brook trout Salvelinus fontinalis would increase trap-net and angling catch rates as well as the number of large trophy fish. Outlet barriers were maintained during the fall spawning season for 6 years on one lake and 10 years on another lake. We compared prebarrier and postbarrier trap-net and angler catches (1978–1998) in two blocked and two unblocked lakes. Postbarrier availability of brook trout to anglers and trap nets increased significantly in Rock Lake (blocked) but not in Lower Sylvan Pond (blocked) or the two unblocked lakes. However, highly significant increases in angler catches of trophy fish occurred from both blocked lakes. Variables other than emigration, such as number of fish stocked and angling mortality, influenced our ability to detect significant increases in the availability of age-2 and older brook trout in Lower Sylvan Pond. The outlet barriers elicited a population response by stocked brook trout consistent with the hypothesis that emigration losses of mature adults depleted olderaged fish from unblocked lakes. Outlet barriers are a potential management tool; however, managers must consider their specific management goals and the problems associated with seasonally interrupting migratory corridors. Emigration by mature brook trout Salvelinus fontinalis during the fall spawning season can reduce the abundance of adult fish in lakes where fisheries depend on stocking for population recruitment. Fall emigration by stocked brook trout reduced adult populations in two Michigan lakes by 40–90% (Alexander et al. 1990); in Woods Lake, New York, by 21–86% (Schofield and Keleher 1996); and in three lakes in the Adirondack Mountains by 33–69% (Josephson and Youngs 1996). These lakes had little or no spawning habitat, so mature fish presumably emigrated in search of spawning habitat (Warrillow et al. 1997). These studies concluded that fall emigration could seriously deplete populations of mature fish in lakes that lack outlet barriers. Stocking is essential to maintaining brook trout sport fisheries in the majority of lakes in the Adirondack Mountain region of New York State. Since the early 1800s, human-induced perturbations in this region have resulted in the fragmentation of habitats by dams, logging operations, and acid precipitation, and changes in fish communities have followed introductions of exotic fishes (Schofield 1976; George 1980). These perturbations have eliminated the ability of brook trout to sustain wild populations within most lakes of the region. Most remnant populations of brook trout * Corresponding author: [email protected] Received June 28, 2000; accepted April 11, 2001 exist primarily in isolated headwaters of lakes and streams (Keller 1979; Perkins et al. 1993). This constrains reproduction to such an extent that brook trout recruitment in 90% of the lakes depends on stocking (Pfeiffer 1979). Management of stocked brook trout in the region focuses on individual lakes as the spatial units of management. Lake management practices emphasize stocking and regulations (e.g., seasons, creel limits), and in a few lakes, liming (to neutralize acid precipitation) or chemical reclamation (to remove competitor and predator fish species) to improve conditions for brook trout growth and survival. A fall fingerling stocking program has also been instituted to produce put–grow–take fisheries or to establish self-sustaining wild brook trout populations (Keller 1979). Brook trout may establish self-sustaining populations in some reclaimed lakes (Schofield 1993) but rarely in limed lakes (Flick and Webster 1992), and many of these lakes require routine stocking. The current brook trout management plan for the region does not address the potential effects of emigration losses of adult fish on fisheries in stocked lakes. We hypothesized that preventing fall emigration of adult brook trout would increase trap-net and angling catch rates and the number of large trophy fish in populations. To examine that hypothesis, we compared indices of population abundance and angling catch of brook trout populations in blocked and unblocked lakes. Comparison of brook trout 844 JOSEPHSON ET AL. TABLE 1.—Physical characteristics, location, and brook trout stocking history of Goose Lake, Otter Lake, Lower Sylvan Pond, and Rock Lake in the Adirondack Mountains of New York. Study site Drainage area (ha) Lake area (ha) Coordinates Stocked strains of brook trouta Control lakes: no barriers Goose Lake Otter Lake 134.5 540.0 5.3 9.6 438239N, 748439W 438239N, 748449W 2, 5, 6, 7, 8, 9, 10, 14 2, 4, 5, 6, 7, 8, 10, 14 Treatment lakes: with barriers Lower Sylvan Pond Rock Lake 48.7 393.6 6.5 78.9 438379N, 758569W 438579N, 748529W 4, 5, 6, 7, 8, 9, 10, 14 1, 2, 3, 4, 5, 6, 10, 11, 12, 13, 14 a See Table 2 for an explanation of the number code for stocked brook trout strains. populations before and after installation of fish barriers, as well as to control lakes without barriers, provided direct evidence of emigration effects on stocked brook trout populations.