Mapping Lake Trout Spawning Habitat Along Minnesota's North Shore

Abstract

Lake Superior's surface covers more area than any other body of fresh water in the world. While it is the largest of the Great Lakes, less is known about it than any of the other lakes in the chain. Lake Superior supports a variety of life and its nearshore area is vital to its overall ecosystem as well as to many fish species that inhabit the lake. Lake trout (Salvelinus namaycush) have historically been the top predator in the Lake Superior fish community and are the primary species caught by anglers. Lake trout are well adapted to the cold, clear, infertile waters of Lake Superior and generally require boulder and cobble substrates at depths less than 30 meters for spawning and early survival of eggs and fry (Marsden et al., 1995). An important component of lake trout management in Lake Superior has been the protection of known spawning areas. Stocking suitable habitat with hatchery reared lake trout is a management strategy based on the belief that adult fish returning to these areas will have increased early survival of eggs and fry. Biologists also believe that early life stages of lake trout stocked on appropriate spawning substrates will imprint and re-colonize these spawning areas more quickly than if left to normal population expansion (Krueger et al., 1986). These approaches require site specific knowledge of the distribution and areal extent of bathymetric features and substrate type so that efforts can be concentrated in specific areas where success is likely. An important information need, discussed in the Fisheries Management Plan for the Minnesota Water of Lake Superior (MNDNR 1995), is the identification and quantification of lake trout spawning habitats. Unfortunately, detailed maps of Lake Superior's benthic habitats sufficient for identifying potential lake trout spawning habitats are largely nonexistent in Minnesota waters. With the exception of embayments and ports extensively used for shipping, contemporary bathymetric maps of the lake are built from data consisting of a few depth measurements per square kilometer. While these maps are sufficient for describing the general shape of the lake 's bottom for general navigation purposes, they are insufficient to depict detailed fish habitat. Furthermore, the substrate of the lake is largely unknown. When looking out over any large body of water, it is difficult to tell what might lie underneath. The shoreline geology can provide a clue, but what is on shore is not always the same as what lies a few hundred meters, or even just a few meters, offshore. Very few systematic surveys of substrate type have been conducted and no maps exist. To date, the long length of the shoreline and cost associated with conducting such surveys have prohibited extensive surveys. Even mapping just a square kilometer of near shore area with traditional methods would have been a major undertaking.