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COSEWIC Assessment and Update Status Report on the Deepwater Sculpin (Western and Great Lakes-Western St. Lawrence Populations) in Canada

Limiting Factors and Threats

Historically, deepwater sculpin were limited by the availability of suitable habitat (deep, cold, highly oxygenated water) that had postglacial links (Parker 1988). Lakes where deepwater sculpin occur must reside within the former boundaries of proglacial lakes, as the present distribution of the species indicates no secondary dispersal from glacial lake boundaries throughout Canada (Fig. 4) (Sheldon et al. in prep.). In fact, dispersal of deepwater sculpin has not occurred since the late stages of the proglacial lake phase of the Wisconsinan glaciation. Therefore, even if potential habitats become available, deepwater sculpin will be unable to exploit these habitats. According to fish survey and physical lake characteristics gathered in 2004 (T. Sheldon, unpubl. data), it is possible that populations in Lac des Iles and Heney Lake Quebec may be declining, or have disappeared, due to changing lake conditions (eutrophication) in the past 20 years (Sheldon et al. unpubl. data.). Most information on the limiting factors and threats of deepwater sculpin, however, is from the Great Lakes.

Index-netting programs in the upper Great Lakes indicate that deepwater sculpin have remained relatively abundant over a fairly long period of time. Dynamics in Lake Michigan suggest that their abundance is directly affected by predation by burbot (Madenjian et al. 2002) and probably by lake trout. The deepwater sculpin was a particularly important forage fish for lake trout before this important commercial species was greatly reduced and extirpated from much of the Great Lakes. In Lake Ontario, the deepwater sculpin was particularly important prey for burbot, as well as lake trout: some deepwater lake trout had large numbers of sculpin in their stomachs when both were abundant (Scott and Crossman 1973). Similar heavy predation has been reported from Lake Michigan, where deepwater sculpin are very abundant. In particular, temporal trends in the abundance of deepwater sculpin in Lake Michigan during the 1960s through 1980s are best explained by alewife and burbot predation (Madenjian et al. 2002; Madenjian et al. 2005).Alewife and rainbow smelt are also considered to be important predators of the pelagic larval stage. Rapid increase in population size of deepwater sculpin in Lake Michigan in the 1970s and early 1980s was most likely attributable to a decrease in alewife abundance at that time (Madenjian et al. 2002). As well, a decline in deepwater sculpin abundance during the 1960s was considered to be related to an increase in alewife numbers.

It has been speculated that in Lake Ontario, the population decline after the 1940s was the result of DDT pollution (Scott and Crossman 1973). However, the true cause of this decrease is not well understood. It occurred when lake trout were declining dramatically and eventually became extirpated (Casselman and Scott 2003). This resulted in increased abundance of smelt and alewife, important exotic predators of sculpin larvae, and most likely further contributed to the general disappearance of deepwater sculpin. As in Lake Michigan, alewife predation was undoubtedly important, but the reciprocal relationship between smelt abundance and deepwater sculpin presence suggests that smelt must also have been involved (J. Casselman, unpubl. data).

Finally, a recent decline of Diporeia spp. (possibly related to zebra mussel invasion) in the lower Great Lakes may represent a threat to deepwater sculpin populations. Diporeia spp. were the main prey item of lake whitefish (Coregonus clupeaformis) in Lake Michigan and the decline of this amphipod has adversely affected the body condition and growth of lake whitefish in Lake Michigan (Pothoven et al. 2001). Because Diporeia spp. also compose a majority of the deepwater sculpin diet, their decline could potentially affect deepwater sculpin in the same manner.

Habitat-related issues, e.g., deepwater oxygen levels, and climate change have not been investigated, but may be worth future study. The presence of exotic species, e.g., round goby (Neogobius melanostomus) might affect deepwater sculpin through interactions at larval or other stages, and this too should be investigated. However, the disappearance of deepwater sculpin from Lake Ontario preceded the appearance of the round goby.

A detailed study of the sporadic occurrence of deepwater sculpin in Lake Ontario would no doubt provide considerable insights into the factors limiting and threatening the species. Nevertheless, a remnant population of deepwater sculpin exists in Lake Ontario and although reintroduction has been proposed, it now seems inappropriate given recent catches. In the case of Lake Erie, it may simply be too shallow to support a self-sustaining population, although larval drift from Lake Huron has occurred from time to time (Roseman et al. 1998).