Geology topics

Timothy B. Johnson

Publications and source records attributed to Timothy B. Johnson.

22 records · Page 2Linked to original sources

USGS2005

Hydroacoustic estimates of abundance and spatial distribution of pelagic prey fishes in western Lake Superior

Lake herring ( Coregonus artedi ) and rainbow smelt ( Osmerus mordax ) are a valuable prey resource for the recovering lake trout ( Salvelinus namaycush ) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha −1 with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha −1 ), followed by the Duluth Minnesota region (20.22 kg ha −1 ), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha −1 ). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.

Journal of Great Lakes Research↗

USGS2004

Spatial patterns in assemblage structures of pelagic forage fish and zooplankton in western Lake Superior

We assessed abundance, size, and species composition of forage fish and zooplankton communities of western Lake Superior during August 1996 and July 1997. Data were analyzed for three ecoregions (Duluth-Superior, Apostle Islands, and the open lake) differing in bathymetry and limnological and biological patterns. Zooplankton abundance was three times higher in the Duluth-Superior and Apostle Islands regions than in the open lake due to the large numbers of rotifers. Copepods were far more abundant than Cladocera in all ecoregions. Mean zooplankton size was larger in the open lake due to dominance by large calanoid copepods although size of individual taxa was similar among ecoregions. Forage fish abundance and biomass was highest in the Apostle Islands region and lowest in the open lake ecoregion. Lake herring ( Coregonus artedi ), rainbow smelt ( Osmerus mordax ) and deepwater ciscoes ( Coregonus spp.) comprised over 90% of the abundance and biomass of fishes caught in midwater trawls and recorded with hydroacoustics. Growth and condition of fish was good, suggesting they were not resource limited. Fish and zooplankton assemblages differed among the three ecoregions of western Lake Superior, due to a combination of physical and limnological factors related to bathymetry and landscape position.

Michigan, Minnesota, Wisconsin↗

USGS2004

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) diets in western Lake Superior

We describe the diets of lake herring ( Coregonus artedi ) and rainbow smelt ( Osmerus mordax ) in western Lake Superior during the summers of 1996 and 1997. Both species consumed predominantly (> 71% by number) zooplankton, showing a preference for larger taxa. Diet overlap between the two species was low (Schoener's index = 0.42). Mysis was most important in rainbow smelt diets, whereas Diaptomus sicilis was most important in lake herring diets. Rainbow smelt selected larger taxa, and larger individuals within a taxon when compared to lake herring, although rainbow smelt tended to be smaller fish. Fish diets have changed relative to previous studies and may be reflecting changes in the zooplankton community. Continued changes in the fish and zooplankton community will alter predatorprey and energetic pathways, ultimately affecting growth and production of the ecosystem.

Michigan, Minnesota, Wisconsin↗

USGS1998

Estimation of invertebrate production from patterns of fish predation in western Lake Superior

We used bioenergetic models for lake herring Coregonus artedi , bloater Coregonus hoyi , and rainbow smelt Osmerus mordax to estimate consumption of zooplankton, Mysis , and Diporeia in western Lake Superior for selected years between 1978 and 1995. Total invertebrate biomass consumed yearly ranged from 2.5 to 38 g/m 2 with nearly 40% consumed between August and October in all years. Copepod zooplankton represented the largest proportion of biomass collectively consumed by the three species (81%), although rainbow smelt consumed almost twice as much Mysis as zooplankton. Growth efficiency was highest for rainbow smelt (3.84–16.64%) and lower for the coregonids (1.91–12.26%). In the absence of quantitative secondary production values, we suggest our estimates of predatory demand provide a conservative range of the minimum invertebrate production in western Lake Superior during the past 20 years.

Transactions of the American Fisheries Society↗