The Effects of Fish Predation on Benthic Invertebrates in a Lake Erie Coastal Wetland

Many lake fish species (e.g., channel catfish and yellow perch) breed in Great Lakes coastal wetlands that are connected to the adjoining lake.  Other fish species (e.g., crappie, brown bullhead, bluegill) live in coastal wetlands year-round.  Juvenile fish in these habitats are dependent on invertebrates as a key food resource.  In June 2007, we built wire mesh exclosures to examine how fish predation affected benthic invertebrate density and diversity in a Lake Erie coastal wetland.  We built large mesh (2.54-cm mesh; 5-m diameter size), and small mesh (0.64-cm mesh; 1 m x 1 m size) exclosures that prevented access by different sized fish.  We also built large and small mesh control treatments with 1-m openings cut into the sides, and sham treatments marked off by posts that allowed access by all fish.  Benthic invertebrates were sampled in June, July, September and October 2007 with core samplers, and potential fish predators were captured with fyke nets set adjacent to exclosures. Large fish were initially most commonly trapped in fyke nets, but small and medium sized fish became numerically abundant later.  Gut contents of the most common Young of the Year fish species were examined by dissection, and their diets included macroinvertebrates and zooplankton.  Total invertebrate densities were over twice as high in exclosures than control or sham treatments in July to October indicating that fish predation reduced invertebrate numbers throughout the study.  Large fish had the greatest overall impact because total invertebrate densities were not different between small vs. large mesh exclosures.  The most common benthic invertebrates were chironomid midges, sphaeriid clams and oligochaete worms, which are collector-gatherers.  Sphaeriid and oligochaete densities were 2 to 8 times higher in exclosures than in control and sham treatments, and they were greater in small mesh exclosures than large mesh exclosures.  Therefore, medium-sized fish were probably important predators of these taxa.  Chironomid midges were twice as abundant in exclosures than control or sham treatments in July.  However, midge densities were lowest in small mesh exclosures in October.  This suggests that competition between midges and other invertebrates may be altered in the absence of fish predation.  MRPP and NMS analysis found that species composition of invertebrate communities also differed in areas with fish access than exclosures without fish predators.  Overall, these results indicate that fish predation strongly impacts invertebrate density and community structure in coastal marshes, which can affect management goals for these habitats.