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Effects of Diporeia Declines on Fish Diet, Growth and Food Web Dynamics in Southeast Lake Michigan
Diporeia, the dominant benthic macroinvertebrate in offshore waters of the Great Lakes, decreased in abundance in southern Lake Michigan by 89%, 91% and 45% at sites < 30 m, 31-50 m, and 51-90 m between 1993 and 2002. The declines of Diporeia have been attributed to the spread of dreissenid mussels, but the exact mechanism is uncertain despite extensive study. Diporeia feed on organic material settled from the water column, especially diatoms. In turn, Diporeia are eaten by most offshore fish species in Lake Michigan. Therefore, prior to their decline, Diporeia represented an efficient link between primary production and fish production in the Great Lakes. Our objectives for this study were:
The benthic amphipod Diporeia were historically the dominant benthic macroinvertebrate in the offshore waters of the Laurentian Great Lakes (Nalepa 1998). Diporeia served as an important pathway between pelagic primary production and fish production in the Great Lakes. Diporeia depend on pelagic inputs of organic materials to the benthic region (Dermott and Corning 1988), and in turn, are fed on by most offshore fish species (Wells 1980). Declines of Diporeia have been documented in all the Laurentian Great Lakes except Lake Superior, and vast areas of Lake Michigan, Huron, Erie, and Ontario are now devoid of this macroinvertebrate (Nalepa et al. 1998, 2003, 2006, Dermott and Kerec 1997, Dermott 2001). The declines of Diporeia have been attributed to the spread of dreissenid mussels, but the exact mechanism is uncertain despite extensive study (Nalepa et al. 2006).
Fish species that depend on pelagic pathways are expected to suffer following dreissenid invasions (Fahnenstiel et al. 1995, Johannsson et al. 2000, Strayer et al. 2004). On the other hand, dreissenid invasions lead to increased benthic primary and secondary production (Stewart and Haynes 1994, Fahnenstiel et al. 1995) and production of fish that depend on benthic pathways or directly consume dreissenids might increase (Karatayev et al. 1997, Johansson et al. 2000). However, dreissenids may negatively impact zoobenthos such as Diporeia that directly depend on pelagic pathways (Strayer et al. 2004). Given the importance of Diporeia as a food item for most Great Lakes fish species, we expected that its decline could impact growth, condition, abundance, and diets of both benthivorous fish and pelagic planktivorous fish.
Relating changes in fish populations to dreissenid mussel invasions has been difficult due to the lack of long-term data sets and complications from other factors (Strayer et al. 2004). For this study we used a combination of 1) several long-term data sets on fish growth, 2) laboratory experiments and 3) short-term field studies across a gradient of Diporeia densities to examine how the loss of Diporeia might be impacting Great Lakes fishes.
Diet Composition, Relative Abundance, and Energy Density of Planktivorous Fish (2000-2001)
Diporeia densities at St. Joseph, Michigan were near 0/m2 over much of the bottom, but averaged more than 3800/m2 at Muskegon and Little Sable Point, Michigan. Consistent with these differences in Diporeia density, fish diet composition, relative abundance, and energy density varied spatially. For example, alternative prey types comprised a larger fraction of the diets of bloater Coregonus hoyi, large (> 100 mm TL) alewife Alosa psuedoharengus, and slimy sculpin Cottus cognatus at St. Joseph than at Muskegon and Little Sable Point. The pattern was seasonally dependant for alewives and bloater because Diporeia were eaten mainly in June. Food biomass per stomach was not lower at St. Joseph than elsewhere, suggesting that the spatial variation in diet composition was due to greater consumption of alternative prey by fish at St. Joseph. Although slimy sculpin and bloaters were able to feed on alternative prey, the relative abundance of these species at certain depths was lower at St. Joseph than at Muskegon or Little Sable Point, indicating that Diporeia availability may also influence fish abundance and distribution. Finally, a link between Diporeia density and fish energetics was suggested by the comparatively low energy density of deepwater sculpin Myoxocephalus thompsonii and large alewives at St. Joseph (Figure 1), a result that may reflect the low energy content of other prey relative to Diporeia.
Lake Whitefish Diets (1998-2001)
Lake whitefish were collected for diet analysis from Michigan waters of Lake Michigan during 1998-2001. When the benthic amphipod Diporeia was available it was an important item in the diets of both small (< 430 mm) and large (> 430 mm) lake whitefish. In southern Lake Michigan, the most common prey consumed in the absence of Diporeia included zebra mussels (Dreissena polymorpha), gastropods, chironomids, and Mysis relicta. In northern regions of the lake, alternative prey included chironomids, isopods, Bythotrephes, and fish. Following the decline of Diporeia in southeastern Lake Michigan between 1998 and 2001, the contribution of Diporeia to the diet of small lake whitefish fell from 57% to 1% (dry weight). The contribution of Diporeia to the diet was similar for small fish captured in nearshore (9-30 m) and offshore (31-46 m) waters. Mysis were more common in the diets of fish collected at offshore stations whereas chironomids and zebra mussels were more common in fish from nearshore stations.
Lake Whitefish Growth, Condition and Diets
We evaluated the long-term trends of the benthic macroinvertebrate community (1980-1999) and biological attributes of lake whitefish Coregonus clupeaformis (1985-1999) in southeastern Lake Michigan. We also determined what food types were important to lake whitefish in an area where Diporeia had not yet declined in 1998 and how the diet of lake whitefish changed as Diporeia declined in 1999-2000. Dreissena polymorpha invaded the study area in 1992 and the amphipod Diporeia began to decline in 1993 and was nearly absent by 1999. The body condition of lake whitefish decreased after 1993 and remained low thereafter. Length and weight-at-age of lake whitefish was lower in 1992-1999 than 1985-1991. After declines of Diporeia at Muskegon, Michigan between 1998 and 1999-2000, the proportion of Diporeia in the diet by weight fell from 70% to 25%, and the proportion of lake whitefish that ate Diporeia decreased from 81% to 45%. In contrast, the proportion of lake whitefish that ate other prey, such as Mysis relicta, Ostracoda, Oligochaeta and zooplankton increased in the same period. At sites south of Muskegon where the density of Diporeia has been low since 1998, Chironomidae, D. polymorpha, and Shaeriidae were the most important diet items of lake whitefish. Decreases in body condition and growth are associated with the loss of the high energy prey resource Diporeia, the consumption of prey with lower energy content such as D. polymorpha, and possible density-dependence.
Sculpin Diets and Prey Selection
Many characteristics of benthic fish and their prey differ from the planktivore-zooplankton model on which fish foraging theory is based, but few studies have examined the processes underlying prey selection in benthic fishes. In this study, diet and prey selection dynamics were analyzed for a pair of benthic fish from southeast Lake Michigan, the sculpin Cottus cognatus and Myoxocephalus thompsonii (Family Cottidae). Both field and laboratory studies were employed to investigate the effects of prey species composition, abundance and behavior on prey selection by these fish. Results were consistent with the hypothesis that prey selection in C. cognatus and M. thompsonii is mainly a passive process wherein all prey are equally suitable but differ in vulnerability (Figure 2). Thus, more vulnerable prey types are preferentially consumed when sculpin feed opportunistically. Study results suggest that prey vulnerability is a function of prey micro-habitat use, motility, and evasiveness which influence predator-prey encounter rates and predator capture success. Chironomids (Order: Diptera; Family: Chironomidae) were found to be an example of a highly vulnerable prey type due to their limited motility, and were preferentially consumed by C. cognatus. In contrast, the freshwater shrimp Mysis relicta was considerably less vulnerable to fish predation as a result of its highly effective predator escape response (Figure 3). The vulnerability of a given prey type to predation appears to depend on sculpin species due to interspecific variation in sculpin search behaviors and reactive volumes that impact predator-prey encounter rates. Changes in prey selectivity were not observed with variation in prey density or sculpin size, suggesting that diet choice is not actively modified in response to food limitation or ontogeny. This implies that sculpin in the Great Lakes may not “switch” their diet in response to declines of a key prey, the benthic amphipod Diporeia spp. Active predator choice could not be entirely ruled out as a factor influencing the prey-size selection of these fish, but results from this and other studies suggest that much of the variation in prey-species and prey-size selection by benthic fish can be explained solely on the basis of passive mechanisms.
Alewife Energy Density
The dreissenid mussel invasion of Lake Michigan during the 1990s has been linked to a concomitant decrease in the abundance of the amphipod Diporeia. We tracked the seasonal energy dynamics of alewife (Alosa pseudoharengus) in Lake Michigan during 2002-2004, and compared our findings with previously published results for years 1979-1981. Adult alewife energy density exhibited a pronounced seasonal cycle during both the pre-invasion and post-invasion periods, with energy density in October or November nearly twice as high as that in early summer (Figure 4). However, on average, adult alewife energy density was 23% lower during the post-invasion period compared with the pre-invasion period. This significant decline in energy density was attributable to decreased importance of Diporeia in adult alewife diet. In contrast, energy density of juvenile alewives did not significantly differ between the pre-invasion and post-invasion periods. To attain a weight of 8 kg by age 4, bioenergetics modeling indicated that a Chinook salmon (Oncorhynchus tshawytscha) in Lake Michigan would have to consume 22.1% more alewives during the post-invasion period compared with the pre-invasion period.
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