Myoxocephalus thompsonii
deepwater sculpin

Although there are similar Nearctic and Palearctic species, as well as similar European freshwater species, deepwater sculpin, Myoxocephalus thompsonii are limited to North America. Once abundant in the Great Lakes and most deep lakes of Canada (especially Nipigon in Ontario, Great Slave in Manitoba, Waterton in Alberta, and Great Bear in the Northwest Territories), the geographic range of deepwater sculpin is rapidly shrinking.

At present, deepwater sculpin are plentiful in lakes Huron, Michigan and Superior, and rare in Ontario and Erie. Despite their deepwater habitat requirements, they are also found occasionally in the inland waterways that connect the Great Lakes, such as the St. Claire River.

During their first year, deepwater sculpin are pelagic, rather than benthic organisms. They live in the water column feeding on pelagic phytoplankton and small invertebrates. As they age they undergo physiological changes and become benthic organisms.

Adult deepwater sculpin are generally found in waters deeper than 20 meters and are particularly abundant between 70 and 90 meters. In Lake Superior they have been found at depths of 407 meters. Although all adults are benthic, the largest fish of a population are found in the deepest waters. Deepwater sculpin live only in cold water, 40 degrees Celsius or less.

Due to their benthic habitat, deepwater sculpin spend their adult lives in complete darkness where the temperature ranges between 3 and 6 degrees Celsius. At this depth, the bottom substrate is fine particulate matter, mud and clays of a uniform size. When they were much more plentiful, they could also be found on substrates consisting of combinations of sand, silt, clay and mud intermixed with rocks, coal and cinders. During one study in 1952, they were found in the waters off Munising, Michigan, on thick beds of aquatic vegetation (Jacoby 1953).

Deepwater sculpin have been known to reach 9 inches (23 cm) in length, although the average is between 2 and 5 inches (5 to 10 cm). The body is slender with a large flattened head and blunt snout with a large mouth. Eyes are set close together on top of the flattened head. There are four preopercular spines, the two upper ones so close together as to be mistaken as one (Brandt, 1986). There are two dorsal fins, the second significantly larger than the first. In mature males, this second dorsal fin often overlaps the base of the caudal fin, and is one of the distinguishing characteristics of deepwater sculpin (Page et al., 1991). Although deepwater sculpin have no scales, they do have prickles on top of the body.

Coloration is largely characteristic of what one would expect in a habitat with very little if any light. The dorsal region is gray-brown, while the ventral is several shades lighter. The top and sides are speckled and there are thin oval shaped-marks dotting the back (Page et al., 1991).

Sexual dimorphism is apparent only in sexually mature individuals. The largest fish tend to be the most sexually dimorphic in the development of the fins. “In order of decreasing magnitude, it is most pronounced in the second dorsal, pelvic, anal, first dorsal, pectoral, and caudal fins” (Jacoby, 1953). (Jacoby, 1953; Page and Burr, 1991)

Although research is ongoing, due to the deepwater sculpin’s inaccessible habitat, there is little concrete documentation of their developmental stages.

Ontogenetic changes occur during the deepwater sculpin’s first year of life, transforming them from pelagic larva to benthic organisms as juveniles, and eventually as adults (Bruch, 1986).

Size increase is greatest in the first year. During the second and third years of life, size increase is typically three-fifths that of the first year. In successive years, the length increase was less than two-fifths that of the first year. On the other hand, weight gain is inversely related to length increase. Deepwater sculpin put on the least weight in the first year, despite the greatest gain in length. In successive years, as elongation slows, weight gain increases (Selgeby, 1988). The largest fish tend to be the oldest.

The general method for determining age has been to count the annual ‘rings’ on the otolith. According to data collected in the summer of 1973, the maximum age for deepwater sculpin in the Great Lakes was seven years. More recent research suggests that alternating clear and dark bands on the otolith reflect rapid growth during the summer and much slower growth during the winter. (Selgeby, 1988)

These fish live in cold, deep waters and very little is known about their behavior.

Deepwater sculpin live in very dark environments. Little is known about their modes of perception and how they might communicate, but it is likely that they use tactile and chemical perception in their dark habitat.

Data on food preference is based on stomach contents of captured fish. The most prominent food item in all specimens examined was the amphipod Pontoporeia hoyi. Also often consumed were opossum shrimp, Mysis relicta (Black et al., 1981). An overwhelming proportion of the diet of large deepwater sculpins is made up of P. hoyi compared to the diet of smaller sculpins which feed fairly equally on P. hoyi and M. relicta (Brandt, 1986 and Wojcik et al., 1986). (Brandt, 1986)

Predators of deepwater sculpin include lake trout (Salvelinus namaycush) and burbot (Lota lota) (McAllister et al., 1979). By the early 1950s, lake trout and burbot were nearly extirpated from Lake Ontario after falling victim to over-fishing and sea-lamprey parasitism. Shortly after the population declines of these two keystone predators, deepwater sculpin disappeared from Lake Ontario for almost the next 50 years. It is believed that the loss of these keystone predators resulted in huge disruptions to this freshwater fish community and deep population declines of sculpin.

Anti-predation mechanisms have barely been studied in deepwater sculpin. The prickles on the top of the body and the four spines on top of the head may serve as deterrents to predators. In addition, the fact that deepwater sculpin spend most of their time under conditions few other species can withstand, limits their interaction with potential predators.

Deepwater sculpin are the number one prey item for lake trout (Salvelinus namaycush), a once widely available, commercially harvested fish found in all the Great Lakes. Although a concerted effort has been made since 1950 to restore lake trout populations to the Great Lakes, success has been spotty and limited. Natural reproduction of lake trout is currently occurring on a widespread basis only in Lake Superior. In Lakes Huron, Michigan and Ontario, only limited natural reproduction has occurred. Of these three, only in Lake Huron do the larval fish survive into adulthood (USGS, 2001).

Due to their role as key forage items for lake trout and burbot and because M. relicta and P. hoyi are their main food sources, deepwater sculpin are thought to be responsible for facilitating the majority of the energy movement from benthic organisms to higher trophic levels. This obscure but critical role is likely to have far ranging influences on the overall productivity of the entire Great Lakes ecosystem.

It may be that competition between sculpin species contributed to a decline in deepwater sculpin. Data on competitive feeding habits of various sculpin species are limited, however, juvenile deepwater sculpin and slimy sculpin (Cottus cognatus) have over-lapping food preferences and habitat use, creating the potential for competition. There is also evidence that slimy sculpin prey on the eggs and larvae of deepwater sculpin. (Brandt, 1986)

There are no known negative affects of deepwater sculpin on humans.

Deepwater sculpin currently are not seen as having any commercial value or economic importance on a local, national or international scale. They are important members of the native Great Lakes and northern lakes ecosystem.

From 1942 to 1972, no deepwater sculpin were captured in Lake Ontario so far as is known. Between 1972 and 2002, only six have been caught there, despite continued government mandated trawls aimed at determining the extent of their extirpation from the Great Lakes. All six were caught in Canadian waters, three in 1972 and three in 1996 (Black et al. 1981).

Despite their disappearance from Lake Ontario and steady decline in the other Great Lakes, the United States Fish and Wildlife Service (USFWS) does not list deepwater sculpin as even being a species of concern for the Great Lakes region (NYSDEC, 2002). They are however, listed by the New York State Department of Environmental Conservation and the Canadian government as being threatened and monitored (NYSDEC, 2002, COSEWIC, 2001). Research efforts are underway to determine how many deepwater sculpin are left in the Great Lakes and the other deep water lakes of Canada. The decline of deepwater sculpin may have been linked to the introduction of two non-native species in the Great Lakes: alewives and rainbow smelt. Both of those fish species eat deepwater sculpin eggs and compete with them for food.

There are currently two known threats to the continued survival of deepwater sculpin. The first is the loss of the amphipod Pontoporeia from the majority of the bottom of Lake Michigan. As one of their main food sources, the loss of this important food source will substantially impact the recruitment of deepwater sculpin. The second threat is the presence of invasive round gobies in off-shore waters that overlap the distribution of spawning sites for deepwater sculpin. Round gobies are fierce fighters, and often out-compete sculpin species in the same area (Jude et al., 2002).