Dynamics of Great Lake food webs in a changing environment

2007 Impact statement
  • Rudstam, Lars Gosta

abstract

We investigated several hypotheses for why Mysis relicta may be declining in Lake Ontario, including changes in migration pattern due to increased water clarity, decreased growth rates, and increased predation rates from alewife. More alewife now feed on mysids, possibly due to increased water clarity making them more visible. Mysids are high in essential fatty acids, and increased predation on these animals could improve overwinter survival of forage fish as long as the population remains at high levels. Development of whole lake assessment using hydroacoustics allows us to follow the mysids population over time. The population declined by 50 percent from 2005 to 2006, suggesting that current feeding rates on this resources may not be sustainable.

submitted by

issue being addressed

Mysids are small shrimp that are a dominant predator on zooplankton, and they are prey for forage fish in Lake Ontario. A decline was observed in 2002, and the possibility of a crash of another major invertebrate prey in Lake Ontario could severely impact sport fisheries in the region. Several hypotheses could explain this decline, among them increased water clarity affecting distribution and predation rates on mysids by alewife and decreased growth rates caused by decreased productivity in Lake Ontario. Because the shrimp is difficult to sample, we also needed to develop sampling methods for effective abundance estimates using remote sensing, in this case with sound (hydroacoustics).

response

We obtained funds to study migration patterns of mysids in the laboratory, developed predictive models of their distribution, developed algorithms for assessing the mysid using standard fisheries acoustic surveys, compared alewife feeding rates on mysids over time, studied mysid diets and fatty acid content, and tested different growth indices for application to field-caught animals. Laboratory-based probability functions of distributions predicted mysid distribution changes associated with temperature and light (moon phases). Fish distributions varied, but alewives were caught below the thermocline feeding on mysid in all seasons. Fatty acids templates were developed and showed high PUFA (polyunsaturated fatty acids) in mysids. Whole lake acoustics can now be used for mysid abundance estimates.

impact assessment

Mysids have not declined as much as originally feared. But populations have declined and are now more heavily used by the primary forage fish and the base for the salmonid sport fishery—the alewife. Mysids can now be estimated with hydroacoustics that are collected for fisheries assessment. This method is also being used in other Great Lakes by the U.S. Geological Survey and the Ontario Ministry of Natural Resources and for large lakes in Europe. The importance of mysids in the Lake Ontario food web is substantial, because they are both the dominant predator on zooplankton in the offshore of the lake and an important prey for fish species like alewife. Mysids provide an alternative food source to alewife that is more available now because of interactions between mysids` response to light and alewives feeding rates at different light levels. A crash of the mysids population could severely affect alewife production and therefore the salmonid sport fishery.

academic priority area

has geographic focus

funding source description

New York Sea Grant

collaborators

  • USGS-Great Lakes Laboratory
  • Environment Canada
  • Simrad A/S
  • Limnological Institute
  • Swedish Fisheries Board
  • University of Oslo
  • Ontario Ministry of Natural Resources
  • USGS-Lake Ontario Laboratory
  • Department of Fisheries and Oceans

key personnel

  • Helge Balk
  • Ellis Loew
  • E. L. Mills
  • David Warner
  • Frank Knudsen
  • Robert O`Gorman
  • Gideon Gal
  • Ora Johannsson
  • Thomas Axenrot
  • Michael Arts

department, unit, division

mission focus

From CALS annual faculty reporting. Imported on August 5, 2008