Fall Diets of Red-Breasted Merganser (Mergus Serrator) and Walleye (Sander Vitreus) in Sandusky Bay and Adjacent Waters of Western Lake Erie

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ABSTRACT.-

Although published studies indicate the contrary, there is concern among many sport anglers that migrating red-breasted mergansers (Mergus serrator) and other waterbirds pose a competitive threat to sport fish species such as walleye (Sander vitreus) in Lake Erie. We quantified the diet of autumn-migrant mergansers and walleye during 1998-2000 in Sandusky Bay and adjacent waters of western Lake Erie. We hypothesized that the diets of both predators would be similar in species composition, but because of different foraging ecologies their diets would differ markedly in size of prey consumed. In addition to predator samples, we used trawl data from the same general area as an index of prey availability. We found that mergansers fed almost exclusively on fish (nine species). Gizzard shad (Dorosoma cepedianum), emerald shiner (Notropis atherinoides) and round goby (Neogobius melanostomus) were consumed in the greatest numbers, most frequently and comprised the greatest biomass. Walleye fed exclusively on fish: gizzard shad, alewife (Alosa psuedoharengus) and emerald shiner were consumed in the greatest numbers, most frequently and comprised the greatest biomass. Diet overlap between mergansers and walleye was 67% by weight and 66% by species frequency. Mean total lengths of gizzard shad, emerald shiner and round goby found in walleye stomachs exceeded those captured in trawls by 47%, on average. Mean total lengths of gizzard shad, emerald shiner and round goby were greater in walleye stomachs than in merganser stomachs. Mean total lengths of emerald shiner and round goby were less in merganser stomachs than in trawls. Our results suggest that although the diets of walleye and mergansers overlapped considerably, mergansers generally consumed smaller fish than walleye. Given the abundance and diversity of prey species available, and the transient nature of mergansers on Lake Erie during migration, we conclude that competition for food between these species is minimal.

INTRODUCTION

In North America, red-breasted mergansers (Mergus serrator, hereafter: mergansers) feed in a variety of water bodies, including the ocean and associated bays, estuaries, large rivers and lakes. Western Lake Erie provides an important rest stop for migrating mergansers because of its location (Bellrose, 1976) and abundant prey-fish population (Hartman and Margraf, 1992; Deller et al., 2003). As many as 210,000 mergansers migrate through Lake Erie annually (Peterjohn, 1989), and many of these birds stage for several weeks or more in western Lake Erie during the spring and autumn (M. Shieldcastle, Ohio Department of Natural Resources [ODNR], pers. comm.). However, little is known about the feeding ecology of migrating mergansers staging in this region.

Given the importance of sport fisheries to the economy of the Great Lakes region (Talhelm, 1988), potential foraging competition between piscivorous waterbirds (e.g., mergansers and double-crested cormorants, Phalacrocarax auritus [Bur et al, 1999; Madenjian and Gabrey, 1995]) and sport fish is a concern. Lake Erie sustains a rather large and diverse predatory fish community, including walleye (Sander vitreus), yellow perch (Perca flavescens) and other species (Nepszy, 1999). In western Lake Erie, Hartman and Margraf (1992) calculated that walleye consumed, on average, 88,200 metric tons of prey fish during each growing season (May through Nov.) from 1986 through 1988. In comparison, Madenjian and Gabrey (1995), using a bioenergetics model, estimated that piscivorous waterbirds on Lake Erie consumed 13,368 metric tons of fish per year. Of the total tonnage, mergansers ate an estimated 37%, almost twice the quantity estimated as consumed by any other bird species, including double-crested cormorants (Madenjian and Gabrey, 1995). However, there are limited data available on the species, quantities and sizes of the fishes consumed by mergansers on Lake Erie.

Previous authors have described the diet of mergansers as consisting of a wide variety of fish (Cottam and Uhler, 1937; Munro and Clemens, 1939; White, 1957; Cronan and Halla, 1968; Feltham, 1990). …