Introduction

Background - 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 then any of the others 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 and 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's 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 the cost associated with conducting such surveys has prohibited extensive surveys. Even mapping just a square kilometer of near shore area with traditional methods would have been a major undertaking.

Objectives - The purpose of this project was to conduct a series of benthic habitat surveys along Minnesota's shoreline in water depths suitable for lake trout spawning using remote hydroacoustic survey techniques, which are distinctly less expensive than conventional methods. These surveys were designed to provide a more complete understanding of the bottom characteristics of nearshore areas which could be used for identification and quantification of potential lake trout spawning and nursery areas. It was expected that the information collected would also be used for a variety of other purposes. Hydroacoustic methods have been used to examine specific spawning reefs in other Great Lakes (e.g. Edsall et al. 1989, 1992) although they have not been fully integrated with other digital technologies. Our objective was to create a series of maps of Lake Superior through the integration of recent advancements in hydroacoustic methods as well as Geographical Information Systems (GIS) and Gobal Positioning System (GPS) technologies. All of these technologies have become readily available over the last decade but have not been fully evaluated for the purposes of quantifying fisheries habitat, specifically lake trout spawning and nursery habitats.

Report Format - This report consists of a series of maps created over 65 square kilometers of Minnesota's nearshore region with data collected during 1995, 1996, and 1998. The resulting maps can be seen in the map Viewer section. The Methods section describes how the maps were produced. Basic statistical information from the survey data and a brief discussion of lake trout spawning and nursery habitats are located in the Results and Discussion section. The GIS files subdirectory on this CD-ROM contains both the raw and processed data suitable for importing into a GIS. The people and entities that helped to fund and implement this project are listed in the Acknowledgments section.

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