Are Salmonid Foraging Costs Increasing in Lake Ontario?

Great Lakes phytoplankton, benthic invertebrate, and pelagic forage fish communities have undergone considerable changes since salmonid stocking programs were initiated in the 1970's. Additionally, introductions of non-indigenous species have substantially altered both community structure and production dynamics.  Unfortunately, little is known of the ability of salmonids to efficiently track and use available prey resources under such changed conditions.  In this study, lake trout (Salvelinus namaycush) biomonitoring data collected from Lake Ontario were used to examine changes in this species' bioenergetics over the period from 1989 - 2008.  Since 1995, the dominant age class in lake trout collections declined from 7 to 5 years of age with a concomitant 30% decrease in average body mass. Importantly, lipid content has declined by an average of 20% for 3 - 10 year old lake trout in the period (post 1999) defined by the introduction and establishment of non-indigenous round goby in the food web. When lake trout age classes were categorized into low, median, and high growth cohorts, accumulated pollutant body burdens of mirex were not observed to be significantly different between the three cohorts but body masses were.  These results indicate that the low weight at age individuals maintained similar consumption rates as larger fish but had, on average, lower growth conversion efficiencies.  Given that lake trout body mass and lipid content are decreasing in Lake Ontario over time, it is concluded that the energetics of lake trout foraging in this system is becoming less efficient.  Diet composition analyses indicate that round gobies (Apollonia melanostomus) and smaller alewives (Alosa pseudoharengus) than historically consumed have been more recently incorporated into this species' diet indicating a switch to less profitable prey in response to the changes in the Lake Ontario pelagic forage base.  In this manner, the impacts of non-indigenous species introductions and the foraging behaviour of salmonid species in Great Lakes food webs may be creating trophic bottlenecks for large predators in these systems.