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

Biology

Little is known of the biology of deepwater sculpin, mainly because they are normally found at great depth (see Habitat). Most studies have focused on the biology of deepwater sculpin from single lakes, such as Lake Michigan, Lake Superior, or Lake Ontario within the Great Lakes or Burchell Lake in northwestern Ontario (Black and Lankester 1981; Brandt 1986; Kraft and Kitchell 1986; Selgeby 1988; Geffen and Nash 1992).


Age and Growth

Selgeby (1988) reported a maximum age of seven years in Lake Superior, while Black and Lankester (1981) reported a maximum age of five years in Burchell Lake, Ontario. Length increment is largest during the first year, and decreases by 40% during the second and third years in deepwater sculpin in Lake Superior (Selgeby 1988). In following years, length increment is only 35 to 40% of that in the first year (Selgeby 1988). Weight increment, on the other hand, increases with each succeeding year up to six years of age (Selgeby 1988). Weight increase in deepwater sculpin is significantly higher than isometric growth (Selgeby 1988).

There has been discussion of size variation in deepwater sculpin with latitude (McPhail and Lindsey 1970; Scott and Crossman 1973; Black and Lankester 1981; Selgeby 1988). Based on a large individual from Lake Ontario (235 mm in total length (TL)), and the relatively smaller sizes of deepwater sculpin from Great Slave Lake (maximum of 69 mm), these authors suggest that the maximum length of deepwater sculpin decreases as latitude increases from the Great Lakes. However, no such trend was recorded in the 2004 survey (T. Sheldon unpubl. data). The largest specimens captured during the 2004 survey were from Wollaston Lake, northern Saskatchewan at over 100 mm TL (up to 110 mm TL), while those specimens in Great Slave Lake reached lengths of 75 mm TL, and an individual from Alexie Lake, NT (just north of Great Slave Lake) measured 98 mm TL (T. Sheldon, unpubl. data).

However, deepwater sculpin in the Great Lakes are relatively large individuals compared to all other populations, including those in inland lakes of the same latitude. A typical size distribution of fish caught in routine indexing programs in Lake Huron is illustrated in Figure 6. The modal size was in the 100-110 mm range, with a few individuals approaching 200 mm. Historically, the species reaches a larger size in Lake Ontario than in any of the other Great Lakes (Scott and Crossman 1973).


Figure 6: Frequency distribution of total length of deepwater sculpin, by 10-mm length intervals, caught in indexing programs in Lake Huron

Figure 6: Frequency distribution of total length of deepwater sculpin, by 10-mm length intervals, caught in indexing programs in Lake Huron.


Reproduction

The reproductive cycle of the species is not fully understood. Age at maturity was estimated as three years for females and two years for males from individuals from Burchell Lake, Ontario by Black and Lankester (1981). Spawning period of deepwater sculpin is unknown. McAllister (1961), McPhail and Lindsey (1970), and Scott and Crossman (1973) hypothesized that spawning occurs in late summer or early fall based on nearly ripe eggs found in females in the Great Lakes in July and August. Black and Lankester (1981) suggested spawning most likely occurs in late fall or winter. Based on the appearance of eggs and ovaries, as well as the collection of young-of-the-year deepwater sculpin caught while trawling during early spring, Selgeby (1988) suggested that spawning occurs in Lake Superior from late November to mid-May, peaking in January.

In Lake Michigan, larval deepwater sculpin hatch in deep water in March, then move to the surface and are transported inshore (Geffen and Nash 1992). The larvae then move offshore and are found at depth by late fall. In Lake Ontario, a gravid female was, however, caught in relatively shallow water (30 m) on June 22, 1996 (Casselman, unpublished data).


Diet

The deepwater sculpin almost always occurs with the relict crustaceans Mysis relicta and Diporeia spp. (Dadswell 1974) and these species compose a large part of their diet. The stomach contents of individuals from Burchell Lake revealed Diporeia spp. occurring in 71% of the deepwater sculpin examined, while chironomid larvae and Mysis relicta occurred in 41% and 3% of the stomachs (Black and Lankester 1981). Diporeia spp.and Mysis relicta composed 73% and 26%, respectively, of the biomass of stomach contents of deepwater sculpin from Lake Superior while chironomid larvae composed 1% of the diet of these deepwater sculpin (Selgeby 1988). Diporeiaspp. have dominated the deepwater sculpin diet in Lake Michigan (Davis et al. 1997).   Preliminary stomach content analysis of deepwater sculpin captured during the 2004 survey indicated that the amphipod Diporeia spp. composed the vast majority of the diet, followed by Mysis relicta (T. Sheldon, unpubl. data). Chironomid larvae were the only other food item found on a somewhat regular basis. Zooplankton are probably the primary diet during the pelagic larval stage (<22 mm).


Parasitism

The relationship between parasitism and the health of deepwater sculpins is unknown. However, parasites reported in deepwater sculpin from Burchell Lake, Ontario include trematodes (Diplostomulum spp.), cestodes (Cyathocephalus truncatus, Bothriocephalus spp.), and nematodes (Cystidicola stigmatura, Spirurine larva) (Black and Lankester 1981). Parasites reported from deepwater sculpin across their range include copepods (Ergasilus spp.) on the gills, cestodes (Bothriocephalus spp., Proteocephalus spp.) in the intestine, digeneans in the intestine, nematodes in the liver (Raphidascaris spp.), and acanthocephalans (Echinorhynchus spp.) in the stomach and intestine (J. Carney, unpubl. data).


Predation

Deepwater sculpin are an important item in the diet of piscivores, such as lake trout (Salvelinus namaycush) and burbot (Lota lota) (Scott and Crossman 1973; Stewart and Watkinson 2004).


Physiology

There is virtually no information on the physiology of deepwater sculpin. Stapleton et al. (2001) reported that deepwater sculpin are able to reduce their polychlorinated biphenyl (PCB) load by as much as 10% by forming MeSO2-PCBs through a biochemical pathway which is novel for freshwater fish species.


Dispersal/Migration

Historically, dispersal of deepwater sculpin occurred via proglacial lakes. Presently, there is virtually no potential for migration or dispersal between inland lakes due to the ecological requirements of the species (occurring only at significant depths in lakes). Drift of larvae occurs between Lake Huron and Lake Erie (Roseman et al. 1998).


Interspecific Interactions

Brandt (1986) suggested that the disappearance of deepwater sculpin from Lake Ontario during the 1950s may have been due to the loss of piscivores (lake trout and burbot) from the lake, resulting in monopolization of benthic habitats by sympatric slimy sculpin (Cottus cognatus). Brandt (1986) hypothesized that this would have resulted in increased competition or predation on young deepwater sculpin by slimy sculpin. Present trends of increasing appearance in Lake Ontario do not support this contention. Slimy sculpin are quite abundant in deepwater trawls in which deepwater sculpin have been collected recently (J. Casselman, unpubl. data). Smith (1970) suggested that the disappearance of deepwater sculpins in Lake Ontario may have been due to alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax)predation on the eggs and larvae of deepwater sculpin. In the 2004 survey of inland lakes, spoonhead and deepwater sculpin were rarely found in the same lakes (T. Sheldon, unpubl. data), perhaps suggesting competitive exclusion between the two species.


Adaptability

The adaptability of deepwater sculpin is relatively unknown, but evidence suggests it is extremely limited. Although downstream transport of larval individuals into new habitats may occur (i.e. from Lake Huron into Lake Erie), reproducing populations of deepwater sculpin are not known from locations other than their preferred deep, cold, highly oxygenated habitats. Further, deepwater sculpin have not been kept in captivity.