**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**

MORPHOLOGICAL DIVERSITY OF LAKE TROUT: DIFFERENTIATION

BETWEEN DEEP AND SHALLOW FORMS

Andrew Muir¹, Charles C. Krueger¹, Randy L. Eshenroder¹, Mara S. Zimmerman²

¹ 2100 Commonwealth Blvd. Suite 100, Ann Arbor, Michigan 48105

² Washington Department of Fish and Wildlife, 600 Capitol Way N, Olympia,

WA 98501

December 2011

ABSTRACT:

Rehabilitation of deepwater lake trout in the Laurentian Great Lakes requires a scientific basis for understanding deepwater forms and how they differ from those found in shallow water. Efforts in this regard were fueled by recent discoveries of multiple lake trout forms in Great Bear Lake (Alfonso 2004, Blackie et al. 2003) as well as from our own field work in Great Slave Lake. We requested funding to broaden the geographic range of lakes included in our analysis of lake trout phenotypic diversity to better define the diversity and differentiation expected in rehabilitated deepwater communities. Our research objectives were as follows: (1) determine the prevalence of humper- , siscowet-, and lean-like morphologies in deepwater communities at a broad geographic scale; (2) determine whether buoyancy is associated with deepwater morphologies of lake trout at a broad geographic scale; and (3) compare the depths of capture and gut contents of forms within and among these five lakes. We found evidence of similar patterns of diversity in Great Slave Lake, Lake Mistassini, and Great Bear Lake with shallow and deepwater morphs occurring, but the patterns were not parallel as we predicted. In Great Slave Lake, shallow-water (<50m), large lake trout were light in color, low buoyancy, streamlined, and possessed short pectoral fins. In deep water (>50m), small lake trout were dark in color, high buoyancy, deep bodied, and had long pectoral fins. In Lake Mistassini, a shallow-water form (< 50-m depth) was identified by streamlined shape, dark body coloration, and high buoyancy. A second deep-water form (> 50-m depth) was identified by a deep anterior-body profile, light body coloration, and low buoyancy. Low buoyancy tissue was characteristic of the deep-water morphotype in Great Slave Lake but not Great Bear Lake or Lake Mistassini. Ontogenetic changes in habitat, depth, and buoyancy supported the conclusion that “humper” and “siscowet” deep-water forms are ecologically unique from each other and from sympatric shallow-water forms. Humper-like trout fed in marginal, deep-water habitats and grew slowly; siscowet-like trout transitioned at ~ 40 cm length to feed on deep-water pelagic prey and grew fat. Invertebrate eaters and piscivores, shallow-water morphotypes unique to Great Bear Lake, may be maintained by ready shallow-water access to both fish and invertebrate resources in this cold lake.