Degree Name

Master of Science (MS)

Semester of Degree Completion

2006

Thesis Director

Robert U. "Bud" Fischer

Abstract

The predator-prey base of the Great Lakes has been altered since the early 1900's, with the majority of these changes occurring due to invasive species such as the sea lamprey (Petromyzon marinus), alewife (Alosa pseudoharangus), and rainbow smelt (Osmerus mordax). These changes have forced large piscivorous fish to find alternate prey species. One predator, the lake trout (Salvelinus namaychush) has experienced a series of diet shifts in the last century, due to a changing prey base. In recent years the Great Lakes, particularly Lake Michigan has experience a new invader, the round goby (Neogobius melanostomus), which has colonized the near shore habitat of the entire lake.

Knowing that the round goby possesses characteristics of a successful invader and has established itself throughout Lake Michigan, and that lake trout have a history of diet shifts caused by changes in the prey base of Lake Michigan, this study was designed to determine if round gobies have been incorporated into the diets of lake trout since their initial invasion. In particular our objectives were to 1) determine if overall diet assemblages changed spatially or temporally in lake trout diets in Lake Michigan, 2) document when round gobies were first incorporated into lake trout diets in Lake Michigan, and 3) determine of what importance round gobies were in relation to other prey items in Lake Michigan lake trout diets.

Lake trout began incorporating this newly abundant prey species in their diets in all regions of the lake in 2000. Indices of relative importance (%IRI) which incorporate %occurrence, % weight and% number, were placed in a multi-dimensional scaling program to ascertain observable patterns between prey importance values throughout the lake. ANOSIM (Analysis of Similarity) showed that prey composition of lake trout has been relatively consistent over the last ten-years over the entire lake with alewife composing the majority of lake trout diets. Other species of importance included rainbow smelt, bloater chub, round gobies, and sculpin. However, in 2004 and 2005, in the Southeast and East regions of the lake, prey assemblage in lake trout diets changed with traditional prey species such as alewife and rainbow smelt being replaced in high proportions by round gobies, sculpin, trout perch and various shiners.

Changes in the prey fish community have had and continue to have profound effects on the diets of lake trout. These initial lake-wide observations provide baseline measures regarding foraging shifts that have occurred in the last ten years. Subsequent diet observations combined with bioenergetics modeling will determine the influence that shifts in prey assemblages are predicted to have on growth and survival of lake trout populations in Lake Michigan.

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