Now, let's finish the Lake Michigan salmonine management efforts story...
All salmon and trout are susceptible to EMS (early mortality syndrome), which is induced via ingestion of high thiaminase content forage fish, specifically alewife and rainbow smelt. Nearly a decade ago NWF/USGS researchers documented that lake trout are particularly susceptible to EMS to the point that their principal forage base components, smelt and alewife, were likely the major negative impact factor on natural reproduction rates in the lower Great Lakes. In this background, highly altered food-web dominated by invasive species introduced Pacific salmon have slowly developed naturally reproducing stocks/races/demes, with these cohorts of naturally reproducing fish slowly expanding since the 1980s (Yes, that's right, right through the BKD era.) to current levels where they comprise roughly 60% of the open lake stock. These fish originate primarily from Michigan tributaries on the eastern side of the Lake Michigan basin, but there is also a large cohort of swim-over fish from Lake Huron's Canadian tributaries.
The Lake Michigan Management Committee, as well as the USFWS/ USGS recognize Atlantic salmon as a Great Lakes endemic species despite the historical records only indicating that they never existed west of the Niagara falls complex, limited to Lake Ontario. As early as the 1860s, alewife were described to exist in large numbers in Lake Ontario's basin as well, yet they are deemed an invasive...
There have been two principal drivers of the decline in forage fish abundance within Lake Michigan, mirrored as well in Lake Huron, eventually contributing to the forage base's collapse over the 2003-2006 interval in Lake Huron. The major characteristic displayed by the alewife stock which is the dominant forage stock component, both numerically as well as on a biomass basis is the slow truncation of sexually mature age-classes within the stock array over time. Lake Michigan had eight alewife age-classes documented in 2007, declining to one and some "change" in 2016 trawl samples conducted lakewide by the USGS . This has been attributed to over-consumption of alewife from a top down perspective, principally by Chinook salmon stocks, with additional forage pressure from the remaining salmonine stock array. The other issue influencing Lake Michigan's productivity is the bottom-up effects tied to driessenid mussel filter feeding and colony proliferation impacts, which have been profound. This has influenced alewife year-class success rates largely via reductions in the spring diatom blooms lake wide. Diatoms are a high omega free fatty acid content phytoplankton that larval alewife feed on at swim-up, after the have absorbed their yolk sac energy. Declines in the Deep Chlorophyll Layer of diatoms have likely also contributed to declines in Mysis diluviana abundance, and also the near total loss of Diporeia sp. deepwater amphipods I strongly suspect, based on preliminary data that declines in free fatty acid content in Diporeia sp. on a seasonal basis have markedly influenced their basin wide decline.
Two years ago the USFWS reported on a large sample size, multiple salmonine species stable isotope food habits analysis at the Great Lakes Fishery Commission Lake Committee annual meetings. The over-riding value of stable isotope food habits analysis is that the ratios exhibited in samples are essentially a multi-month snapshot of the subject's food habits, rather than the several weeks at a shot data derived from stomach content analysis done at multiple intervals over the course of a year. Matt Kornis presented the data, essentially interpreting it from both a 'this is where this species eats within the food web' perspective as well as doing an analysis of degree of dietary niche overlap for Lake Michigan salmonines. Stable isotope "signatures" of carbon and nitrogen were first determined for forage web constituents, both invertebrate and vertebrates, to enable interpretation of ratios obtained from sample fish. Lake trout were determined to have one of the broadest forage arrays among the salmonine stocks in Lake Michigan, while Chinook salmon were determined to have the most narrow forage base, essentially consisting of invertebrates in their first nine months in the open lake environment, shifting to a nearly entirely alewife dependent diet from that point on.
One of the most intriguing outcomes form the stable isotope analysis food habits study was the data on alewife stock consumption by the salmonine array. USFWS researcher obtained stable isotope signatures were submitted from other researcher's for BOTH adult and juvenile alewife. What the data actually indicated, post-analysis, was that all of the existing salmonine stock component species have largely shifted over to consume the more numerous juvenile alewife array as the alewife adult stock component has declined. Obviously, this has negative repercussions on age-class abundance as these fish move toward sexual maturity and the consequent opportunity to replace themselves in the stock and/or expand it. The standout finding from the S-I food habits data was the conclusion that lake trout fed preferentially on the rapidly declining adult alewife stock component. Do Pacific salmon eat adult alewife? Yes, but not on a preferential basis, which is key, given the reality that lake trout became the numerically dominant stock component in the in the 2007-2008. So, after a management program that has consisted of single species reductions of Chinook salmon stocking, with no beneficial stabilization of alewife stocks you would conclude that this finding would further support the rationale of a mixed species salmonine stocking reduction.
Now, let's toss-in that MSU's Quantitative Fisheries Center was contracted by the Lake Michigan Comm. of the Great Lakes Fishery Commission to develop a Decision Analysis Model that via multi-year runs of various stocking scenarios determined, from a very broad initial array, that four management options were the most viable to offer to the Public for input to guide fishery managers stocking efforts to maintain a biologically viable alewife stock within specific upper and lower bounds to maintain a stable sport fishery, while not allowing them to again overpopulate the lake basin. Each of these stocking options was loaded into the DA Model and 100 simulations of that stocking option were analyzed to be a best fit for the pre-determined Sport fishery stock management model. Option 4: A mixed species stocking reduction consisting primarily of Chinook salmon, but with proportional lake trout, steelhead stocking reduction components as well, had the best fit. Public respondents voted. Most of them voted for Option 4, by a narrow margin. Fishery managers enacted Option 1- a Chinook only reduction. Why? They had predetermined that no fish in multi-year hatchery rearing cycles would be tossed out! Economics trumps science once again!
Well, after much Public shaming and push-back over the last nine months, the Lake Michigan Management Committee has now backed away from enacting another Chinook only reduction in salmonine stocking, to now include reductions in lake trout stocking and liberalization of creel limits in southern basin waters. So much for science based management of an 8 billion dollar sport fishery!
No, to my knowledge no Public presentation of the USFWS Stable Isotope Salmonine food habits analysis has contained the data on differential preferential consumption of adult alewife by lake trout.
It is incumbent on all resource management organizations to periodically sit-down and assess whether the paradigm that guides their approach, management programs, and philosophies is realistic, based on current science, and relevant!