A decades-long program to reestablish self-sustaining stocks of lake trout (Salvelinus namaycush) in portions of the four lower Great Lakes, where the species was once abundant, produced excellent fisheries supported by stocked fish. Stocked fish spawned widely and small numbers of their offspring were collected intermittently from Lakes Michigan, Huron, and Ontario, but no self-sustaining stocks were established. Two major hypotheses offered to explain the problem were:

(1) the spawning habitat used succcessfully by the native fish had become degraded by eutrophication and was no longer capable of supporting the production of large numbers of viable fry; and

(2) the stocked fish were homing to and spawning at stocking sites instead of at sites and on substrates used successfully for reproduction by the native fish.

Methods
We used sidescan sonar, a remotely operated underwater vehicle equipped with a color video camera, and a four-person submarine to map the lake bed (Figure 1) and pinpoint the best egg and fry habitat on 30 historical lake trout spawning grounds in the Great lakes. We also conducted egg-incubation bioassays on several of these reefs.

Figure 1. Computer-drawn map showing substrate distribution and bathymetry of Mischley Reef in Lake Huron. The reef supports natural reproduction by stocked lake trout. The best substrate is BR at 1012 m on the SW corner of the reef. Water depths are in meters.

Substrate Type, Area (hectares)

BR = Bedrock and rubble, 7.8
D = Dredge spoils, 0.7
RS = Rubble and sand, 5.4
S = Sand, 21.1

Total Area = 35.0 hectares

Results
Collectively these surveys and bioassays indicated that all but 2 of the 30 sites could support reproduction by stocked lake trout. They also showed that some of the best spawning, egg, and fry habitat in the lower Great Lakes occurred at deeper, offshore sites.

Significance of This Study to Great Lakes Fisheries
Our results encouraged fishery managers to stock fish on these deeper, offshore sites instead of at shoreline sites of convenience. By pinpointing the best substrates on these historical spawning grounds, we also encouraged managers to specifically target those substrates for stocking. As a result, fish from these targeted stockings have produced substantial numbers of offspring in at least one area in the offshore waters of Lake Huron and egg deposition was recently discovered at an offshore site in southern Lake Michigan.