Diets of Lake Michigan Salmon and Maximum Size of Alewife Prey

Abstract

The diets of Lake Michigan salmon were determined by examining stomach contents of sport-caught salmon. The study was conducted from 2001 through 2008 when the population of principal forage (i.e., the alewife) was declining and there was a decrease in salmon stocking. The largest alewives were measured to determine upper size limits utilized by salmon. Sixty-four charter boat trips were taken with a total catch of 713 salmon (430 coho salmon, 210 chinook salmon, 34 rainbow trout, 27 lake trout, 11 brown trout, and one pink salmon). Small alewives were the predominant prey of salmon. Of the coho salmon stomachs with prey, 97.7% contained alewives, and for chinook salmon, 99.5 % contained alewives. Other occasional prey items were smelt, sticklebacks, yellow perch, and blackflies. No round gobies were found in the diet. The largest alewives in the diets were 178 mm and only 9.6% of the alewives were greater than 152 mm. Coho salmon had fewer empty stomachs (13%) than chinook salmon (51.8%), and coho salmon had a more diverse diet than chinook salmon. INTRODUCTIONThe diets of sport-caught Lake Michigan salmon were examined a s the alewife population was declining and there was a decrease in salmon stocking. There was a potential collapse of the salmon population (Madenjian et al. 2005). Since small alewives (Alosa pseudoharengus) were the principal forage of salmon (Madenjian et al. 2002), the analyses included measurement of the largest alewife in individual salmon diets in order to determine the size range of alewives that supported the salmon populations. As a response to the alewife population decline, fewer salmon were stocked (Trudeau 2005). It was hoped that with declining alewife populations and increasing round goby (Neogobius melanostomus) populations, salmon would start to forage on round gobies (Alexander, 2009). Similar declines in the alewife population had occurred in the past without resulting in salmon foraging on alternate prey (Madenjian et al. 2002); however, the round goby was not found in Lake Michigan during the previous alewife population decline.The sport-caught salmon in this study were chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kitsutsch), rainbow trout (Oncorhynchus mykiss), pink salmon (Oncorhynchus gorbuscha), lake trout (Saluelinus namaycush), and brown trout (Salmo trutta). The prey fishes were alewives, rainbow smelt (Osmerus mordax), yellow perch (Perca flauescens), and three-spine stickleback (Gasterosteus aculeatus). Blackflies ( Simulidae) were also in the diet.

Angler‐Caught Piscivore Diets Reflect Fish Community Changes in Lake Huron

“…Comparisons of the diets of Atlantic Salmon, Coho Salmon, Pink Salmon, and Rainbow Trout between our study and other studies in the Great Lakes were limited because those species have generally been a small proportion of angler catches relative to Chinook Salmon and Lake Trout. But in prior studies of angler-caught fish diets all four species were dominated by Alewives or Rainbow Smelt, although Coho Salmon and Rainbow Trout had low numbers of invertebrates present (Diana 1990;Savitz 2009). Rand et al (1993) found high dietary proportions of insects in Rainbow Trout, but predominantly in age-1 and age-2 fish that were likely smaller than those sampled in our study.…”

“…We found little evidence that piscivores were able to compensate for reduced size by consuming more individuals. Following the prey (Diana 1990) and during 2009-2011, and they did not change their foraging strategy, as has been seen in other studies (Stewart et al 1981;Jude et al 1987;Warner et al 2008;Savitz 2009). This failure to alter their strategy occurred despite the near absence of Alewives compared with the years prior to 2004 and lower Rainbow Smelt biomass compared with the 1980s (Riley et al 2014).…”

“…Surprisingly, salmonids were encountered in about 5% of Lake Trout and comprised almost 10% of dietary mass; we FIGURE 7. Comparison of species composition, number of prey in stomachs, and size distributions of Alewives and Rainbow Smelt in predator stomachs between samples taken during 2009-2011and those obtained during 1983-1986(Diana 1990. See Figure 2 for species abbreviations.…”

“…Temporal differences in fishing regulations may also have influenced our results. We compared diets and prey size during 1983-1986 with those observed during 2009-2011. During 1984-1986 the Lake Trout fishery closed on August 15, but during 2009-2011 it remained open until October.…”

Chinook Salmon Foraging Patterns in a Changing Lake Michigan

“…Similarly, our nonlinear regression analysis of average Alewife TL versus predator TL indicated that the asymptotic mean Alewife size consumed by the largest Chinook Salmon was 49 mm shorter in 2009-2010 than in 1994-1996. This result seems to mirror the differences in maximum prey sizes reported by diet studies in the 1990s versus the 2000s (Rybicki and Clapp 1996;Madenjian et al 1998aMadenjian et al , 1998bSavitz 2009) and certainly parallels changes in Alewife size and abundance found in prey fish surveys between these time periods (Warner et al 2011;Madenjian et al 2012).…”