STATE OF THE LAKE SUPERIOR ECOSYSTEM IN 201720 Bryan G. Matthias21, Thomas R. Hrabik, Joel C. Hoffman, Owen T. Gorman, Michael J. Seider, Michael E. Sierszen, Mark R. Vinson, Dan L. Yule, and Peder M. Yurista The Lake Superior ecosystem, near pristine in comparison to the other Laurentian Great Lakes, has seen major biological changes during the past two decades. Starting in the late 1990s, pelagic prey-fish biomass has been declining in both nearshore and offshore waters (Pratt et al. 2016; Vinson et al. 2016). Declines have been observed in native coregonines, including Cisco, Bloater, and Kiyi along with Deepwater Sculpin and non-native Rainbow Smelt (Gorman 2012; Pratt et al. 2016; Vinson et al. 2016). These species comprise a substantial proportion of the diets of native predators like the lean and siscowet forms of Lake Trout (hereafter, siscowet) and Burbot and introduced migratory salmonines (15-80% of total diets; Matthias and 20Complete publication including maps of place names, abstract, other chapters, scientific fish names, and references is available at http://www.glfc.org/pubs/SpecialPubs/Sp21 02.pdf. B.G. Matthias. U.S. Fish and Wildlife Service, 850 South Guild Avenue, Suite 105, Lodi, CA 95240, USA. T.R. Hrabik. University of Minnesota Duluth, 207 Swenson Science Building, 1035 Kirby Drive, Duluth, MN 55812, USA. J.C. Hoffman, M.E. Sierszen, and P.M Yurista. U.S. Environmental Protection Agency, Center for Computational Toxicology and Ecology, Great Lakes Toxicology and Ecology Division, 6201 Congdon Boulevard, Duluth, MN 55804, USA. O.T. Gorman, M.R. Vinson, and D.L. Yule. U.S. Geological Survey, Great Lakes Science Center, Lake Superior Biological Station, 2800 Lake Shore Drive East, Ashland, WI 54806, USA. M.J. Seider. U.S. Fish and Wildlife Service, Ashland Fish and Wildlife Conservation Office, 2800 Lakeshore Drive East, Ashland, WI 54806, USA. ^Corresponding author (e-mail: brvan.matthias@fws. gov-!. 55 ------- Yule 2020; also see Kitchell et al. 2000; Negus et al. 2007; Gamble et al. 201 la, 201 lb; Isaac et al. 2012). The declines in prey resources are troubling given lean and siscowet Lake Trout populations have remained relatively stable during this time. We built an EcoPath with EcoSim (EwE) model to quantify how the Lake Superior ecosystem changed from 2005 to 2016 and to predict how the ecosystem might change if 2016 commercial and recreational harvest levels on all targeted species are sustained until 2055. The EcoPath model was parameterized to the first lakewide Cooperative Science and Monitoring Initiative (CSMI), a binational intensive monitoring and assessment program conducted in 2005-2006 (https://www.epa.gov/great-lakes- monitoring/coopcrativc-sciencc-and-monitoring-initiativc-csmi). The model includes nearshore and offshore food webs, including 59 groups of producers and consumers and 3 detrital groups (Fig. 25; for model details and input data see Matthias and Yule 2020). The model represents a significant increase in the breadth of lower trophic levels when compared to past Lake Superior models (i.e., Kitchell et al. 2000; Cox and Kitchell 2004) and recent models of Lakes Michigan and Huron (Langseth et al. 2012; Rogers et al. 2014; but see Kao et al. 2016). We incorporated multiple levels in the microbial loops representing significant sources of biomass and carbon cycling from detrital sources. This model includes greater detail in the offshore fish communities and all zooplankton communities than in prior ecosystem models. Fig. 25. Configuration of the Lake Superior EcoPath model representing both nearshore (generally left side) and offshore zones (generally right side), benthic- pelagic coupling, and nearshore-offshore coupling. Node size is proportional to total biomass, and line thickness represents biomass flow between groups. 56 ------- Nearshore Nearshore Offshore Offshore benthic pelagic pelagic benthic The EcoSim model was fit to 2005-2016 data from the CSMI (e.g., Yurista et al. 2009; Isaac 2010; Yule et al. 2013; Pratt et al. 2016), U.S. Environmental Protection Agency's Great Lakes National Program Office (e.g., Barbiero et al. 2019), U.S. Geological Survey trawl survey (see Vinson et al. 2016), coordinated siscowet surveys (see Status of Siscowet Lake Trout in Lake Superior in 2017 chapter), Minnesota and Wisconsin DNR gillnet and acoustics surveys (C. Goldsworthy, unpublished data; B. Ray, unpublished data), acoustics and fish community surveys in Ontario (Fisch et al. 2019a; E. Berglund, unpublished data), and statistical catch-at-age models from the Michigan, Minnesota, and Wisconsin DNRs (see Modeling Subcommittee, Technical Fisheries Committee 2018). This model represented data from across Lake Superior (Fig. 26) and encompassed all trophic levels. Agency surveys indicate declines in biomass across the prey- fish community (Cisco, Bloater, Kiyi, and Deepwater Sculpin); relatively stable populations of Lake Whitefish, Rainbow Smelt, and nearshore Slimy 57 ------- and Spoonhead Sculpins; lean and siscowet fonns of Lake Trout; and Burbot. The EcoSim model for 2005-2016 estimated large declines (>20% since 2005) for siscowet, Bloater, Kiyi, and sculpins. Unlike the survey trends, Cisco biomass was estimated to remain stable along with that of Lake White fish and lean Lake Trout (Fig. 27). Biomass of Burbot and Rainbow Smelt was estimated to increase (see Fig. 27). Fig. 26. Map showing political jurisdictions and spatial areas where EcoSim model fitting procedures occurred. 92°0'0"W 90°0'0"W 88D0'0"W 86°0'0"W 84°0'0"W WI-2 0 50 100 200 O O to *3- Kilometers 92°0'0"W 90°0'0"W 88°0'0"W 86°0'0"W 58 ------- Fig. 27. Relative biomass trends for the Lake Superior fish community (points and thin lines) and trends averaged over all surveys (thick dashed black line), 2005 to 2016 for management units or surveys. Model predicted biomass from EcoSim (thick solid black line) for major fish species or groups. Nearshore sculpin includes Slimy and Spoonhead Sculpins (USGS = U.S. Geological Survey; SCAA = statistical catch-at-age analysis; OMNRF = Ontario Ministry of Natural Resources and Forestry). ro QJ Q. OC V) ) TO E o CO 4.0 3.0 2.0 1.0 0.0 1.5 1.0 0.5 0.0 1.0 0.5 0.0 4.0 3.0 2.0 1.0 0.0 Whitefish >430 mm) Smelt (> 130 mm) 4.0 1.0 0.5 0.0 2006 2(ni 2016 Cisco { > 300 mm) A Nearshore Sculpin EwE predicted Average trend CSMI acoustic survey USGS trawl Siscowet survey SCAA assessment OMNRF index Thunder Bay acoustic survey MN summer acoustic survey MN fall acoustic survey MN small mesh Wl-l spring survey Wl-l summer survey WI-2 spring survey WI-2 summer survey WI-2 fall survey Deepwater Sculpin Lean Lake Trout ( > 500 mm) 4.0 2.0 0.0 1.5 1.0 0.5 0.0 Siscowet ( > 500 mm) 2006 2011 2016 Year 2006 2011 2016 59 ------- Predicted long-term trends (i.e., 2005-2055) appear consistent with the estimated 2005-2016 EwE trends, provided harvest remains at the 2016 level. Biomass is predicted to decline for most coregonines, sculpins, and Burbot; increase for Rainbow Smelt; and remain stable for Cisco and both forms of Lake Trout. The prediction that Cisco will remain stable is contrary to data provided for this species in this reporting period (see Status of Prey Fish in Lake Superior in 2017 chapter). Cisco appears to decline in most surveys (Fig. 27), but there is high variability within these trends. In addition, we have not yet been able to account for the high variability observed in Cisco recruitment, which influenced biomass trends. Future work should seek to assess the drivers of Cisco recruitment and incorporate recruitment variability in EcoSim. Model development will continue into future reporting periods, and we will be able to utilize future CSMI efforts and agency surveys to better inform the model and test predictions of population trajectories over time. For example, there are concerns by Lake Superior biologists that the biomass of large-sized (>500 mm total length) siscowets and leans generated from the bottom-trawl surveys are misleading because the trawl itself does a poor job of capturing these large-sized fish. The bottom-trawl biomass inputs to the EwE were downweighted relative to other data sources because of these concerns. Studies of the selectivity and catchability of large-sized Lake Trout forms and other species to the bottom trawls would better inform future EwE simulations. The long-term goal is to provide a reliable forecasting tool that can be used to predict outcomes of management actions on various fish community objectives, given observed trends in the Lake Superior ecosystem (sensu Kitchell et al. 2000). 60 ------- |