Science: Lake Michigan prey fish populations still near historic lows

Since 1973, the Great Lakes Science Center has performed daytime bottom trawl surveys in Lake Michigan to measure the relative abundance and estimate biomass of prey fish populations.  The unit of sampling is 10-minute tow of a standard 12-m bottom trawl at depths of 9 to 110 m in 9-m increments.  The sampling is executed at 7 established index transects throughout Lake Michigan.  The resulting estimates are used by fisheries managers to set stocking and harvest rates of salmonines and harvest levels for commercial anglers.

The lake-wide prey fish biomass has trended downward since 1989 and although lake-wide prey fish biomass increased from 2008 to 2009, 2009 was the third lowest year on record.  From 2008 to 2009, Alewife Alosa pseudoharengus numeric density increased by factor of just less than two and biomass density increase from 1.7 kilograms (kg) per hectare (ha) to 3.2 kg per ha.  This increase is attributable to slightly less predation by Chinook salmon Oncorhynchus tshawytscha and a strong year class from 2005 becoming fully recruited to the gear.  Bloater Coregonus hoyi numeric density increased from 21 fish per ha to 72 fish per ha and biomass density increased by more than threefold.  The bloater population has been decreasing since 1989 and this decrease is attributed to poor recruitment that is possibly part of a 30 year cycle.  Rainbow smelt Osmerus mordax numeric density increased from 8 fish per ha to 45 fish per ha and biomass density increased from 0.07 kg per ha to 0.30 kg per ha.  The cause of the rainbow smelt decline is unclear.  For deepwater sculpin Myoxocephalus thompsonii numeric density (92 fish per ha) and biomass density (1.1 kg per ha) were the lowest values recorded for this species since sampling began.  This decrease is possibly because a large proportion of the population shifted to depths deeper than those sampled during the study.  Slimy sculpin Cottus cognatus numeric density increased by 16% and biomass density increased by 72%.  The slimy sculpin population has been increasing since 1990 probably because fishery managers have focused on stocking lake trout on offshore reefs instead of nearshore where lake trout feed on slimy sculpin.  Ninespine stickleback Pungitius pungitius numeric density was 63 fish per ha and biomass density was 0.11 kg per ha.  Ninespine stickleback densities have been increasing since the introduction of dreissenid mussels in the mid 1990’s and the resulting prevalence of green algae in Lake Michigan.  Round goby Neogobius melanostomus numeric density decreased from 158 fish per ha to 25 fish per ha.  Round goby abundance has been increasing since 2003 when they first appeared in the trawls but abundance may be leveling off due to predation.  In 2010, the survey was conducted but a full report was not made available.  The 2010 survey indicated that total prey fish biomass was the third lowest recorded and that the lake-wide biomass of every species except bloater and round goby decreased from 2009 to 2010.

Implications for anglers:  The results of the Lake Michigan bottom trawl survey are mixed news for anglers.  Prey fish populations appear to have grown from 2008 to 2009 but declined in 2010.  Improved prey fish populations should mean better foraging opportunities for game fish and therefore better fishing opportunities for anglers.  Increases in prey fish, especially alewives, could result in increased survivability and growth of trout and salmon.  The fact that rainbow smelt appears in the trawls is encouraging and smelting seasons should continue to improve.  Continued predation on gobies should bring there numbers down but it appears as though the goby will continue to be a nuisance.  The total lake-wide prey fish biomass is hovering near 40 year lows, so any improvement is good news but it will probably be a long time until prey fish populations are as strong as they were in the 1980’s. 

Selected definitions:

Abundance:  the relative representation of a species in a particular ecosystem and is usually measured as the number of individuals per sample

Biomass:  the dry weight of organic matter of a group of organisms in a particular habitat.

Recruitment:  The process by which fish enter the exploitable stock and become susceptible to fishing.

  

Lots of good information in these reports, check out the link: 
http://www.glsc.usgs.gov/_files/reports/2009LakeMichiganPreyfish.pdf

http://www.glsc.usgs.gov/_files/reports/2010LakeMichiganPreyfish.pdf

Madenjian, C.P., D.B. Bunnell, J.D. Holuszko, T.J. Desourcie and J.V. Adams. 2010. Status and trends of prey fish populations in Lake Michigan in 2009. USGS report presented to Lake Michigan Committee, March 2010.

Bunnell, D.B., C.P. Madenjian, J.D. Holuszko, T.J. Desorcie and J.V. Adams. 2011. Status and trends of prey fish populations in Lake Michigan in 2010. USGS report presented to Lake Michigan Committee, March 2011.