Trout, Ciscoes, and Shrimp: News from the Serrated Edge of Science
Scientists are developing a better understanding of pelagic predator-prey relationships.
Sometimes answering a question like, "Why do trout travel great distances through Lake Superior's water column?" can be as tough as slicing basalt. In these hard-as-rock cases, the cutting edge of science is serrated—where many sharply angled points of inquiry cut incrementally and cumulatively into the core of a complex situation.
Sea Grant's researchers often use a serrated edge approach. For example, Tom Hrabik, associate professor of biology at the University of Minnesota Duluth, and his colleagues are working to explain predator-prey relationships in Lake Superior and how these relationships influence the whereabouts of particular species. Their progress has involved studies of fish eyes, fish stomachs, sampling protocols, computer modeling, and around-the-clock and nearly around-the-year fieldwork.
The resulting suite of information includes insights into the difficulties of keeping up with shrimp and the challenges of working with creatures that don’t sleep.
Mysis on the Move
Opossum shrimp* (Mysis relicta) are so numerous in Lake Superior that Jack Kelly, the branch chief of ecosystem assessment research at the EPA’s Mid-Continent Ecology Division, estimates that if you rounded them up and plunked them on a scale, they would outweigh the 650,000 people in the Lake Superior basin by five to six times.
Despite their vast numbers, these inch-long-at-best creatures can be elusive. Those that dare to leave the floor of Lake Superior shun daylight, hide from the moon, and flee from beams of light cast by research equipment. What makes them especially challenging to study is that their daily round-trip treks up and down through the water column can exceed a distance of well over three football fields (equivalent to a person commuting about 34 miles to work and back each day).
The surface waters of Lake Superior team with life at night. Tyler Ahrenstorff, UM Duluth, unpublished data.
Diel Vertical Migration (DVM) is the phraseology scientists use to describe the behavior of Mysis and other oceanic and freshwater species that move through the water column in response to ecological gradients such as light and food. During the growing season in the northern hemisphere, the commuter Mysis population sprints to the surface just after dusk to gorge on algae and other zooplankton. At dawn, they plunge toward the floor, and sometimes into it, to reduce the chances of being found and eaten by sharp-eyed fish.
Mysis like their water cold (below 14 ºC, 57 ºF) and dark (less than 10-4 lux, less than a moonless overcast night) according to a review by Dr. Lars Rudstam, of Cornell University, in Encyclopedia of Inland Waters (2009). Preferring an environment that is warmer and brighter than their subjects, researchers have traditionally used vertical net tows (circa 1890) managed from the decks of ships to determine the whereabouts and numbers of wild mysids. Hrabik and his colleagues were hopeful that hydroacoustics or laser optical plankton counters would provide a faster, easier, and cheaper way to spatially characterize the vertical distribution of Mysis in the Great Lakes.
No such luck. Research funded by the Minnesota and Wisconsin Sea Grant Programs suggests that low-level ship noise interferes with hydroacoustic data drawn from deeper water and the abundance of suspended Mysis-sized matter skews the data from laser optical plankton counters. It appears as though the most thorough way to ascertain where Mysis are and how many there are requires acoustic, optic, and physical sampling as well as an additional component of sampling the part of the population opting to spend their days and nights eating benthic organisms and detritus on the lake floor.
"Mysis are likely a keystone species in Lake Superior’s pelagic food web, much as krill are in the ocean around Antarctica," said Hrabik. "It's clear that to understand Mysis DVM, we need to consider light, the seasons, prey, predators, and the best methods for answering specific questions."
Fear, Desire, Fish
Mysis aren't the only creatures yo-yoing through Lake Superior's water column. Hrabik and his collaborators were the first to discover that the daily Mysis commute is echoed by kiyi, a large-eyed deepwater cisco, and siscowet, a fat-bodied strain of lake trout. Like Mysis, both kiyi and lake trout are native to and thriving in Lake Superior.
Until recently, scientists didn't have much of an opportunity to study the relationships that evolved among Lake Superior's native species. Overfishing and the arrival of smelt and sea lamprey put the food web into a tangle during the 20th Century. Now that some of Lake Superior's native fish populations are recovering or recovered, it is simpler to track what is going on, but it’s still not easy.
"I just returned from an eight-day research cruise," said Hrabik. "Since kiyi and lake trout don’t sleep, we've been gathering data day and night. I’m pretty tired."
This was not Hrabik's first Lake Superior research cruise, nor his last. His studies, which span about a decade, indicate that predation pressure by lake trout on the ciscoes (a.k.a. lake herring) and kiyi in the middle of the food web influences the depth of their vertical plunge during the daylight hours over the growing season. It is unclear if kiyi practice DMV primarily to follow their favorite food (Mysis), but they and other ciscoes certainly move to escape the hungry jaws of lake trout. Siscowets are currently abundant and well-adapted for moving easily through the water column and finding prey in very low light. Hrabik and his colleagues have evidence that suggests a sizable proportion of the current Mysis population, most kiyi, and about three quarters of the siscowet population make a daily dietary leap from the benthic "table scrap" realm to the pelagic gravy train for at least some of the year.
Hrabik's group is also investigating the DVM patterns of siscowet in relation to movement of ciscoes, which dine primarily on copepods. They're field-testing mathematical models and remain curious about how invasive species affect the system's energy transfer efficiency. They're planning to test the sensitivity of kiyi eyes to different wavelengths of light (they already know that lake trout see well in the green spectrum, which are about the only wavelengths that can penetrate past 200 feet — approx. 60 meters — below the lake’s surface).
Quantifying the duration and extent of DVM patterns across trophic levels sets the stage for advancing fisheries management and our understanding about energy transfer. These advances make it more likely that decisions, such as whether commercially harvesting siscowet could be sustainable (see Stout Trout Eyed for Market, page 5), will incorporate other parts of Lake Superior's biotic communities.
Hrabik and his colleagues are producing results that are indicative of how complementary fields of inquiry can cumulatively reveal the workings of the Great Lakes. Their work, which spans multiple disciplines including biology, physiology, animal behavior, and theoretical ecology, is ongoing.
"Some really interesting information is coming to light that is allowing us to create a more robust picture of Lake Superior's food web," said Hrabik. "We're collaborating to add primary production information to the current understanding of energy transfer in Lake Superior. It’s exciting, complex, and it will be fascinating to see how all the pieces fit together."
*Despite their name and their superficial resemblance, opossum shrimp are only distantly related to true shrimps.
For more details, read Minnesota Sea Grant journal articles:
JR557. Jensen, O.P., P.M. Yurista, T.R. Hrabik, and J.D. Stockwell. 2009. Densities and diel vertical migration of Mysis relicta in Lake Superior: a comparison of optical plankton counter and net-based approaches. Verh. Internat. Verein. Limnol. 30(6): 957-963.
JR564. Hrabik, T.R., O.P. Jensen, S.J.D. Martell, C.J. Walters, and J.F. Kitchell. 2006. Diel vertical migration in the Lake Superior pelagic community: I. Changes in vertical migration of Coregonids in response to varying predation risk. Canadian Journal of Fisheries and Aquatic Sciences 63(10): 2286-2295.
See also, Jensen O.P., T.R. Hrabik, S.J.D. Martell, C.J. Walters, and J.F. Kitchell. 2006. Diel vertical migration in the Lake Superior pelagic community. II. Modeling trade-offs at an intermediate trophic level. Canadian Journal of Fisheries and Aquatic Sciences 63(10):2296-2307.
By Sharon Moen