Researchers use a beach seine to collect forage fish as a storm approaches. - FWS photo

Researchers use a beach seine to collect forage fish as a storm approaches. - FWS photo

Ted Eggebraaten prepares to bring the trawl aboard. - FWS photo by Dale Hanson

A sample of alewives captured from a gill-net set at Clay Banks Reef. - FWS photo by Dale Hanson

Figure showing the separation of fatty acid signatures for Lake Michigan forage fish species. - courtesy graphic created by Jacques Rinchard

The instability of Lake Michigan’s food-web, brought about by the establishment of non-native species (alewives, zebra and quagga mussels, etc.), is a leading theory as to why lake trout are still not reproducing naturally in the lake. 

Scientists already know that nutritional deficiencies associated with inadequate levels of thiamine (vitamin B₁) in the eggs result in high mortalities of yolk sac stages of several salmonid species in the Great Lakes.

Mortality driven by thiamine deficiency, commonly referred to as early mortality syndrome (EMS), is a likely effect of high levels of thiaminase, an enzyme degrading thiamine, found in prey species such as alewife and rainbow smelt.

Because alewife is a major component of the lake trout diet in Lake Michigan, EMS may be a significant bottleneck in the natural reproduction of this species.  However, researchers have been unable to determine the specific source for EMS since thiaminase levels in alewives and other forage are highly variable and the direct link to high egg mortality is poorly understood.

Recently, Service biologist Dale Hanson has teamed up with researchers from the USGS-Great Lakes Science Center and Illinois Natural History Survey on emerging research to use fatty acid signatures to examine food web dynamics and reproductive success among lake trout in Lake Michigan. 

Fatty acids are essential to the physiological processes for all animals, and there are certain types of fatty acids that fish must obtain from their diet.  Remarkably, the structure of these fatty acids remains intact as they pass through the food-web and therefore a fish’s diet history can be inferred by comparing its fatty acid composition to that of potential prey items. 

Principal investigators Jacques Rinchard (USGS) and Sergiusz Czesny (INHS) have preliminary results showing that forage fish have unique fatty acid signatures which can be used as a tool to assess food-web dynamics (see Figure 1). 

Fatty acid compositions are also indicators of fish health and deficiencies in essential fatty acids may lead to serious health risks including high mortality.  Preliminary data indicate that certain fatty acids in the eggs correlate with survival of lake trout embryos and in concert with thiamine levels, explain up to 60% of lake trout embryonic and early life stages mortality.

Rinchard and Czesny are in the initial stages of the project funded by the GLFC which involves assessing the fatty acid signatures of potential lake trout prey in a northern and southern location of Lake Michigan.  In May, both researchers accompanied Hanson and FWS biologist Ted Eggebraaten on a forage sampling trip (photo 1) near Clay Banks Reef in Lake Michigan. 

Gill-nets, trawls, beach seining, and zooplankton netting were all employed to sample different habitats near Clay Banks Reef to enable a diverse collection of forage species.  The group will collect seasonal samples of forage fish and lake trout for fatty acid composition analysis.  In fall, the group will also collect lake trout eggs from spawning females.  Some of the eggs will be run for fatty acid signatures while the rest will be fertilized and hatched in a laboratory to assess egg and larval survival. 

This project highlights the Service’s commitment to developing cooperative partnerships that will advance the goal of lake trout restoration in Lake Michigan.  While this work is currently in the initial stages, it is anticipated that additional field collections and lab studies will provide new information on the interactions between lake trout diet, reproductive success, and the role of food-web disruption brought about by non-native species.