Winter distributional overlap facilitates lake whitefish (Coregonus clupeaformis) piscivory on invasive round gobies (Neogobius melanostomus) in Green Bay, Lake Michigan

Abstract

The prevalence of round goby (Neogobius melanostomus) in the diet of lake whitefish (Coregonus clupeaformis) >350 mm total length was investigated during winter, when round gobies occupy deeper water and have distributional overlap with lake whitefish. On average, round gobies were the most important diet item (46.6% by dry weight), followed by nonmollusk benthic invertebrates (21.3%) and unidentifiable fish parts (10.6%). Of whitefish that contained food, round gobies were the most frequently occurring diet item (49% occurrence); 44.3% of whitefish examined had empty stomachs. Although lake whitefish are not generally considered piscivores, altered lake whitefish diets have been observed during a recent period of high lake whitefish density and reduced abundance of historically important prey like Diporeia spp., which declined in the Great Lakes following the invasion of Dreissena polymorpha and D. bugensis. There is evidence that fish provide an important summer diet item for lake whitefish in some areas of the Great Lakes, and our study suggests round gobies are also an important component of lake whitefish diets during winter.

Keywords:

• Coregonus clupeaformis
• Neogobius melanostomus
• Great Lakes
• Dreissena spp.
• zebra mussel
• diet

Lake whitefish (Coregonus clupeaformis) are benthivorous fishes that support the largest commercial fishery on the Laurentian Great Lakes (Ebener et al. 2008). However, recent observations of reduced lake whitefish growth rate, body condition, and egg lipid content have raised concerns about the health of this fishery, with several studies suggesting that these changes stem from a combination of high whitefish abundance and reduced availability of a preferred prey item (Pothoven et al. 2001; Madenjian et al. 2002; Kratzer et al. 2007). Diets of Great Lakes lake whitefish were historically dominated by zoobenthos and amphipods (Diporeia spp.; 89.9% by volume, meta-analysis by Vander Zanden and Rasmussen 1996). However, the abundance of Diporeia spp. declined in Lakes Michigan and Huron during the late 1990s and 2000s, coinciding with the proliferation of invasive Dreissena polymorpha and D. bugensis mussels (Nalepa et al. 2007, 2009). In addition, lake whitefish reached and sustained high levels of abundance in several areas of Lakes Michigan and Huron over roughly the same timeframe (Madenjian et al. 2002; Kratzer et al. 2007). High whitefish density, combined with shifts in forage availability, likely led to an altered diet concomitant with low growth rates observed by some studies. In Lakes Michigan and Huron, for example, average lake whitefish consumption of nonmollusk invertebrates (Diporeia spp., Mysis relicta, and Chironomidae) declined by 46–96% relative to pre-1990s data, while consumption of mollusks (including Dreissenid mussels) increased 200–500%, corresponding with an average 38% reduction in weight-at-age (Pothoven and Madenjian 2008).

Although lake whitefish are not known for piscivory, several studies suggest that altered diets may also include fish. For example, Pothoven (2005) noted that April–October diets of lake whitefish >430 mm were dominated by fish in three areas of northern Lake Michigan and that fish comprised a larger portion of whitefish diet in nearshore areas (<30 m depth). Similarly, Pothoven and Nalepa (2006) noted that spring and summer diets of lake whitefish >350 mm total length were occasionally dominated by fish (primarily nine-spine stickleback Pungitius pungitius and invasive round goby Neogobius melanostomus) in certain areas of Lake Huron. In addition, round gobies have been frequently observed in lake whitefish stomach contents during late summer surveys since 2005 in northern Green Bay, Lake Michigan (Dale Hanson, personal communication). Round gobies may be particularly susceptible to lake whitefish predation due to their small size, benthic habitat, and large abundance in areas of the Great Lakes. Although round gobies are present at depths frequented by lake whitefish in warmer seasons, distributional overlap between these two species is likely greatest during winter when round gobies migrate from shallow nearshore habitats to deeper offshore waters (Miller 1986; Walsh et al. 2007). Lake whitefish also move into shallower water during winter in Green Bay and may pursue round gobies to help counteract reduced caloric intake during summer (Scott Hansen, personal communication). To our knowledge, no study has examined the potential for a winter trophic linkage between lake whitefish and round gobies due to the challenges associated with winter sampling.

We evaluated the importance of round gobies to winter (January–March) lake whitefish diets by examining gut contents from 83 individuals >350 mm total length. All individuals were captured through ice angling offshore of Henderson Point and Little Sturgeon Bay in Green Bay, Lake Michigan, during winter 2010 and 2011. Whitefish stomachs were collected from 0.5 to 4.5 km offshore and from depths of 3–20 m (Bret Alexander and John Propsom, personal communication). Stomachs were frozen until gut contents could be evaluated in the laboratory. Diet items from each fish were identified if possible and sorted into categories. For each fish, individual prey categories were weighed after being dried at 60°C for 24 h.

Of all the whitefish examined, 43.4% had empty stomachs, possibly because angled samples can be biased towards hungry fish. Round gobies were the most frequently observed diet item in lake whitefish (48.9% occurrence in fishes with nonempty stomachs) and were the most important diet item by dry weight (average of 46.6% in such fish; Figure 1). These values may be underestimates, as round gobies likely contributed to the average 10.6% of diet attributed to unidentifiable fish parts. Nonmollusk benthic invertebrates (primarily Chironomidae) were the second most important diet item (21.3% by dry weight), while Dreissena spp. were surprisingly found only in 4.3% of feeding individuals, averaging 0.7% of diet by dry weight.

Figure 1. Winter lake whitefish diets from Green Bay, Lake Michigan, USA. Values represent the mean percent contributions, by dry weight, of prey items to diet of 47 lake whitefish. Thirty-six lake whitefish had empty stomachs and were not included in the percentage calculations used in this figure. Individuals captured from 2010 and 2011 were pooled together and all individuals were greater than 350 mm total length. Bars are ± 1 standard error.

These data highlight the potential for winter piscivory from Great Lakes lake whitefish >350 mm, which may frequently encounter round gobies overwintering in offshore benthic habitat. Round gobies are also expanding into deeper waters as populations increase (Dietrich et al. 2006; Walsh et al. 2007), which may lead to increased encounters with lake whitefish during warmer seasons. Many Laurentian Great Lakes fishes utilize round gobies as prey (Kornis et al. 2012), including several species known primarily from offshore areas (e.g., lake trout, brown trout, and burbot). Slow lake whitefish growth rates have been linked to the poor quality of Dreissenid spp. as forage (Pothoven and Madenjian 2008) and fish offer a higher quality prey to individuals large enough to avoid gape limitation. The difficulty of sampling deepwater areas during the winter ice-on period necessitated several study limitations, including a low number of sample sites and the use of angling as a sampling method. However, we have identified a winter trophic linkage between lake whitefish and round gobies that could be relevant to other areas of the Laurentian Great Lakes with high round goby abundance, such as Lake Erie, Saginaw Bay, and the Bay of Quinte. Further research could help determine the extent of winter piscivory in other lake whitefish populations and probe implications for individual growth and condition.

Acknowledgements

We extend our gratitude to Bret Alexander, John Propsom, and Hans Martin for collecting lake whitefish stomach samples for this study. We also thank Jake Vander Zanden for providing laboratory infrastructure and advising both authors on related projects. Finally, we thank two anonymous reviewers for their constructive feedback on an earlier version of this manuscript. This work was funded by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, NOAA, the U.S. Department of Commerce, and from the State of Wisconsin. Federal grant number NA100AR4170070, project number R/AI-3.