Fourhorn sculpin COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

The relict populations of fourhorn sculpins exist in some freshwater lakes on the islands of the Arctic Archipelago, chiefly on Victoria, Cornwallis, and Ellesmere Islands (Johnson 1964, McAllister and Aniskowicz 1976) and others. Some of these lakes are fresh while others are meromictic with water layers of different salinities. McAllister and Aniskowicz (1976) considered that the fourhorn sculpins were introduced by marine inundation following glaciation and isolated in freshening lakes during subsequent isostatic rebound. Occurrences of the species in these lakes are concurrent with that of a relict isopod, Mesidosethra entomon glacialis, which is not found in inland freshwaters on the mainland (McAllister and Aniskowicz 1976).

The suggestion is that these freshwater relict populations arose independently and more recently from the marine fourhorn sculpin than did the deepwater sculpin and should still be referred to as M. quadricornis; although morphologically different, the variations are not sufficiently significant to be subspecifically distinct (McAllister and Aniskowicz 1976). Variation in such relict populations would be greater than in the sea because there would be little, if any, gene flow between lake populations and genetic differences would accumulate in lakes but not in the open sea where gene flow would be free. Differences in body lengths and mean meristic values suggest there may be genetic differences. Lake temperatures and salinity in meromictic lakes would fluctuate more [especially during incubation, thereby influencing morphological variability such as number of vertebrae (see McAllister and Aniskowicz 1976)]. It is interesting to note that the relict populations can occur in lakes lacking other freshwater fishes (e.g. Garrow Lake) and sometimes containing the isopod M. entomon glacialis.

Little has been reported concerning the habitat restrictions of the freshwater fourhorn sculpin, but the habitat range appears to be narrow. Much of what is known resulted from research conducted in Garrow Lake, NU, on Little Cornwallis Island, approximately 95 km from Resolute (Dickman 1991). Discovered in 1974, this small coastal basin is the northernmost recorded hypersaline meromictic lake (Dickman and Ouellet 1987) with a maximum depth of 47 m (Page et al. 1984). Meromictic lakes are normally characterized by their lack of complete vertical mixing, a continuing absence of oxygen in the monimolimnion, and an increase in relative salinity and density with depth below the chemocline (Stewart and Platford 1986). Such conditions prevail in Garrow Lake as it can be divided into four distinct layers: [described by Dickman and Ouellet (1987); summarized by Dickman (1995)]: (1) a 0-5 m deep, oxygen saturated mixolimnion with low salinity (< 1 ppt) attributed to freshwater runoff and ice-melt water; (2) a 5-12 m brackish-water zone with saturated and supersaturated dissolved oxygen and increasing salinity; (3) a 12-20 m deep chemocline with a strong salinity gradient and rapidly decreasing dissolved oxygen; and (4) a 20-47 m anoxic monimolimnion that can be extremely high in salinity (up to 82 ppt, Dickman 1991) and hydrogen sulfide. The majority of Garrow Lake fourhorn sculpin, the only vertebrate in the lake, reside in the brackish-water layer. There is a shallow outflow to the Arctic Ocean (Stewart and Platford 1986) that was dammed by a nearby lead-zinc mining company thereby increasing the lake level by approximately 2.5 m (Donald, pers. comm., 2003).

Garrow Lake fourhorn sculpins have been collected at a depth range of 3.8-15 m (BC Research 1978; Fallis et al. 1987; Dickman 1991, 1995) and a salinity range of approximately 3-35 ppt (Dickman 1995). Fourhorn sculpins found in lakes are considered the freshwater form, but some of these forms are capable of inhabiting saline and even hypersaline conditions. The Garrow Lake fourhorn sculpin is restricted to a narrow depth range corresponding to its temperature, oxygen, and dissolved oxygen preference ranges. Below 15 m the temperature rapidly increases reaching 8.9°C at 20 m and the dissolved oxygen rapidly declines to zero at 20 m. The majority of specimens collected by Dickman and colleagues were caught at 7-12 m (Dickman and Ouellet 1987; Dickman 1995). BC Research (1978), using beach seines, gill nets, and SCUBA observations, observed no sculpins below 13 m in Garrow Lake. Fallis et al. (1987) reported no fourhorn sculpins captured below 15 m with the majority (92%; n = 51) collected in the depth range 3.8-9.3 m. Divers of Arctic Divers Limited observed fourhorn sculpins in 6-9 m of water with the abundance highest in shallower depths (Gzowski, pers. comm., 2003; Leger, pers. comm., 2003). It is not known whether fourhorn populations inhabiting other Canadian Arctic lakes exhibit similar depth distributions or whether individualistic and/or mass migrations occur. Some populations of the European freshwater fourhorn sculpin have a much deeper depth distribution. Nyman and Westin (1968) collected all specimens in Lakes Vättern and Mälaren at a depth greater than 40 m, whereas in Lakes Orsasjön and Siljan they were found only between 80 and 90 m.

Temperature may also be a factor contributing to the depth distribution of freshwater fourhorn sculpin. Hammar et al. (1996) indicated that the Lake Vättern fourhorn sculpin population appeared restricted to depths greater than 40 m in late summer (August-September) to avoid warm water (approximately 8-17°C). The majority of juveniles were collected within or below the thermocline in temperatures below 10°C. The sculpins are usually found on the bottom at a temperature of 5°C or less. Westin (1968) determined the lethal upper temperature of the Baltic fourhorn sculpin to be 14°C; however, the freshwater form may have a higher tolerance for warmer temperatures as Hammar et al. (1996) caught one specimen near the surface at 17°C. 

Trends

Little is known about the stability of freshwater habitat for the fourhorn sculpin in North America. However, some research has been conducted concerning the limnology of Garrow Lake, NU (Page et al. 1984; Stewart and Platford 1986; Dickman and Ouellet 1987; Fallis et al. 1987; Dickman 1991, 1995). Garrow Lake, because of the high sulfide concentration in the hypersaline profundal layer, was determined to be an ideal water body for the disposal of mining waste, consisting primarily of lead and zinc tailings, from the Polaris mine. An environmental assessment stated that lead and zinc mine tailings entering the monimolimnion would come in contact with sufficient concentrations of sulfide to result in the rapid precipitation of metal sulfides into the lake’s sediments. This process was essential in keeping the metals out of the surface waters where they pose a threat to the natural biota of both the lake and the sea from which Garrow Lake is separated by a shallow stream (Dickman 1991). From November 1981 to August 2002, roughly 15 million metric tonnes of lead and zinc mine tailings entered Garrow Lake from the mine (Donald, pers. comm., 2003). This pollution severely impacted the sulfide generating bacteria, the lake’s major primary producer. Sulfide concentrations in the lake’s anaerobic zone declined as lead and zinc concentrations increased in the surface waters (Dickman 1991) though zinc levels remained below the 0.5 mg/L permissible limit (Donald, pers. comm., 2003). The entire food chain has been impacted, and it was a concern of Dickman (1991) that the fourhorn sculpin, which survived the last 3000 years in Garrow Lake, would become extirpated within the next twenty years. However, the Polaris Mine was decommissioned in 2002 and Teck Cominco Limited has initiated plans to return the lake to its once pristine condition. Activities, such as removal of tailings disposal pipes and docking wharves, have been initiated while the dam has been removed so that the lake can resume its normal seasonal discharge cycles. Studies will also be conducted to determine the effects the mine has had on the resident fourhorn sculpin population (Donald, pers. comm., 2003).   

Page details

Date modified: