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CHANGES IN NUTRIENT STATUS AND ENERGY FLOW THROUGH LOWER TROPHIC LEVELS: IMPLICATIONS FOR GREAT LAKES FISHERY MANAGEMENT
Tom Stewart1, Andy Todd2, Brian C. Weidel3, Lars G. Rudstam4, David “Bo” Bunnel5 and Julie Hinderer6
1 39 Elm St. Kingston, Ontario, K7K 1M8
2 Ontario Ministry of Natural Resources and Forestry, Lake Ontario Management Unit, RR#4, Picton, Ontario, K0K 2TO
3 USGS, Great Lakes Science Center, Lake Ontario Biological Station, 17 Lake St., Oswego, NY 13126
4 Department of Natural Resources, Cornell University Biological Field Station, 900 Shackelton Point Road, Bridgeport, NY 13030
5 USGS Great Lakes Science Center, 1451 Green Road, Ann Arbor, MI 48105
6 Great Lakes Fishery Commission, 2100 Commonwealth Blvd., Suite 100, Ann Arbor, MI 48105
The Great Lakes Fishery Commission (GLFC) Science Transfer Board commissioned a workshop process to better understand and communicate relationships among lower trophic level change and fish community and fisheries change in the Great Lakes. Through synthesis of published and unpublished data, literature review, analysis and facilitated discussion among Great Lakes technical experts, a novel conceptual model was developed. Syntheses of the literature and direct observations from Great Lakes studies confirmed a strong positive relationship between the total fish biomass and total phosphorus concentration. Variability around this relationship is high and several additional factors can modify the total amount of fish biomass that can be sustained for a given concentration of phosphorus. Modifiers interact and are expressed at the species, community, and food web level of organization. Modifiers include traditional fisheries management activity such as stocking, managing predator-prey balance, fishery regulation, and habitat protection and rehabilitation. Changes in water clarity influence fish communities, fisheries and fish assessments, by changing the catchability of fish, their vulnerability to predation, habitat, distribution and feeding behavior. In shallower waters, increased light penetration induced by reduction in nutrients and dreissenid filtering causes a shift from turbid-phytoplankton dominated system to a clear-macrophyte dominated system with associated shifts in fish community structure. Additionally, food web structure modifies how efficiently, and among what fish species, energy and material is transferred from lower trophic levels. An index of overall transfer efficiency for the Great Lakes overtime, ranged from 3.4 -12.7% with an average of 8.9%. Primary production required (PPR) is a food web metric that estimates the accumulated species-group life-cycle acquisition of primary and detrital production. For fully described Great Lakes food webs, PPR is a strong predictor of species-specific biomass. The conceptual model represents testable hypotheses supported by observations informed by expert opinion. Literature, data trends, expert discussions, and preliminary model concepts are detailed in an appended workshop proceedings. A mock-up fact sheet for Lake Huron was developed using the conceptual model as a guide.
· A conceptual model was developed describing the influences of changes in water quality, food web structure, and fisheries management activities on Great Lake fish and fisheries.
· Synthesis of the literature and direct observations from Great Lakes studies confirmed a positive relationship between the total amount of total fish biomass and nutrient concentration.
· Food web structure, fish management activities and increased water clarity can aggravate or mitigate the influence of declining nutrients.
· The concepts articulated here may facilitate further discussions among Great Lakes stakeholders to refine the concepts, find mutually agreeable ecosystem goals, and the means to achieve them.
The synthesis and workshop process was successful in developing concepts useful to fisheries managers for communicating the influence of lower trophic levels on fish and fisheries. The findings are supported by the literature and the syntheses of published and unpublished Great Lakes research reported herein. However, more research is warranted to challenge and refine these concepts. The comparative food web studies provided much insight and aided in the development and application of the concepts of trophic transfer efficiency and primary production required. Many of the mass-balanced descriptions of Great Lakes food webs remain unpublished, others are outdated, and this needs to be corrected. Food web studies requires the integration of large amounts of multi-trophic level, multi-scale data which introduces considerable uncertainty. Methods exist for adequately accounting and understanding the consequence this uncertainty, such as linear inverse modelling (van Oevelen et al. 2010), and can be adapted to existing Great Lakes food web data (Hossain et al. 2017). Applying these methods to existing mass-balance descriptions of Great Lakes food webs would allow a more fulsome exploration of uncertainty and its consequences. Integrating isotope and biomass size spectra approaches may be another independent method to assess Great Lakes food web attributes (Jennings et al., 2002, Trebilco et al., 2013). Given the ubiquitous nature of its influence, more research is needed to better understand the effects of increased water clarity on Great Lakes ecosystems and associated fish and fisheries.
The management implications of this work can be summarized in the statement “cleaner water means less fish”. However, less fish does not necessarily mean low-valued unsustainable fisheries. Potential fish production is lower in nutrient poor systems, and as many Great Lakes ecosystems shift from mesotrophic to oligotrophic, not all fish species can be supported at historical levels, while others may thrive. However, highly functioning ecosystems and productive and diverse quality fisheries are still possible. It will be a challenge for managers to adjust their expectations and those of their clients, and to innovate and adapt their fisheries management practices. The concepts articulated here may facilitate further discussions among Great Lakes stakeholders to refine the concepts, find mutually agreeable ecosystem goals, and the means to achieve them.