Northern pike and yellow perch (hereafter pike and perch, respectively) are often stocked in shallow lakes in an attempt to maintain clear-water states by controlling undesirable fish, but to date impacts of stocking are largely unknown. It is also unknown whether food webs in lakes in clear-water states derive most of their carbon (C) from littoral sources, while food webs in lakes in turbid-water states obtain most their C from phytoplankton. We assessed impacts of stocking pike and perch on lake ecosystems and food web sources of C by sampling δ13C and δ15N in ecosystem components of four large shallow lakes in southern Minnesota (USA) in summer of 2017. We used δ15N to test whether trophic position of fish species differed in each lake, and whether patterns of trophic position for each species across the four lakes were linked to fish community characteristics. We also tested whether reliance on littoral carbon differed across the four lakes for each fish species using baseline-corrected δ13C.
Results/Conclusions
Results showed trophic position differed among multiple fish species in just three of four lakes, and only two lakes (Geneva and Rice) had trophic positions of perch and pike generally higher than other non-piscivorous fish. Trophic position of all fish species was also positively related to catch-per-unit-effort (CPUE) of perch, such that trophic position of individual fish species was highest in lakes with highest CPUE of perch. Perch effects on trophic position are likely driven by perch serving as a forage base for larger piscivores and by perch reducing competition for macroinvertebrates by preying on smaller planktivorous fish. Our δ13C results showed fish were more reliant on littoral energy in the two lakes (Geneva and Pickerel) with higher abundance of submerged plants relative to the other two lakes. Overall our results indicate higher densities of perch facilitate upward shifts in trophic positions of all fish species, and basal sources of energy flow differ between lakes in turbid and clear states. Thus, management activities inducing state shifts or favoring high densities of perch will result in major changes in predator-prey interactions and energy flow in shallow lakes.