Fourhorn sculpin COSEWIC assessment and status report: chapter 6

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Biology

Reproduction

Very little is known about the reproduction of the freshwater fourhorn sculpin. Fallis et al. (1987) determined the sex and maturity of 27 Garrow Lake sculpins collected by gillnets under the ice in May. The sample was predominantly females (22) with 12 having eggs in the early stages of development. The number of eggs per female was not recorded. None of the mature females was in spawning condition though Fallis et al. (1987) proposed spawning would occur within a few months. The age at first maturity is unknown. However, Hammar et al. (1996), after examination of the otolith, determined sexually mature Lake Vättern fourhorn sculpins, ranging in length from 82 - 110 mm TL, to be possibly between 4+ and 6+ years of age.

The reproductive cycle of the marine form has been well described by Morrow (1980) and some of the general features would apply to the freshwater form. Fertilization is internal. Fourhorn sculpins are territorial nest-builders with only the males defending the eggs until the pelagic larvae hatch. Nyman and Westin (1968) stated the lake populations of fourhorn sculpin in Sweden have been found to differ in roe colour. In Lakes Vättern, Orsasjön, and Siljan, the egg colour is yellow. The sculpin population in Lake Mälaren has a greenish-blue egg colour similar to those found in the Baltic. In Lake Fryken, fourhorn sculpin with both egg colours have been reported. The two different roe colours led Svärdson (1961) to believe that the populations were distinct subspecies; those with green-to-greenish-blue eggs were M. quadricornis relictus whereas those with yellow roe were M. q. quadricornis. Nyman and Westin (1968) compared blood proteins of three forms, consisting of the Baltic fourhorn sculpin (M. quadricornis), M. q. quadricornis, and M. q. relictus, and concluded that there was no significant difference amongst the various forms.

Little is known concerning the growth of the freshwater fourhorn sculpin. Fallis et al. (1987) reported the Garrow Lake sculpin as having a very slow growth rate compared to marine specimens from the Beaufort Sea and Strathcona Sound areas based on length-weight regressions. According to Bond and Erickson (1989), marine fourhorn sculpin from Phillips Bay, YK, grew slowly to a maximum otolith age of 14 years. Young-of-the-year (YOY) sculpin were 10-14 mm TL in late June and by September total lengths of 15-39 mm TL were observed. Fourhorn sculpins ranging in size from 40-99 mm TL probably represent age groups 1 to 3 (Bond and Erickson 1989). Subsequent growth of the marine form is as follows: age 5+ and 6+ sculpin are 21-24 cm; age 7 and 8 sculpins are 24-27 cm; age 10+ sculpins are, on average, 30-31 cm (Morrow 1980). Garrow Lake fourhorn sculpins are generally smaller and Fallis et al. (1987) suggested that the slower growth of the Garrow Lake fourhorn sculpin be attributed to the lack of suitable habitat and available prey items in the lake. The maximum age and time for three generations in the freshwater form are unknown.

Survival

Predation on the freshwater fourhorn sculpin has largely been attributed to piscivorous fishes and birds. Fish species include burbot (Lota lota), lake char (Salvelinus namaycush), northern pike (Esox lucius), and Arctic char (Salvelinus alpinus) (Dickman 1995; Hammar et al., 1996).  Due to its shallow water distribution, the fourhorn sculpin can be significantly predated by avian predators, including gulls, loons, herons, cormorants, grebes, and mergansers. The swimming ability is relatively poor making it susceptible to predation (Bengtsson 1993). However, in the marine environment, the sculpin inhabits very shallow water where its cephalic spines are presumed to disrupt the body outline thereby making it less discernible to avian predation. The freshwater form, having absent or diminished cephalic spines, typically has a deeper depth distribution, usually beyond the depth where effective avian predation occurs (Dickman 1995).

Little is known concerning the parasites of freshwater fourhorn sculpin, but Scott and Scott (1988) stated that the species does not appear to be heavily parasitized in either freshwater or seawater. Hammar et al. (1996) observed acanthocephalan parasites in three of 19 fourhorn sculpin collected. The parasitized individuals were small (39-41 mm TL) and had a maximum of one worm/individual.

Physiology

Virtually nothing is known about the physiology of the freshwater fourhorn sculpin.

Movements/dispersal

Little information is available on movements of fourhorn sculpins in Canadian lakes, but seasonal migrations have been demonstrated by the Lake Vättern population that migrates to depths greater than 40 m from August through September to avoid layers affected by summer warming (Hammar et al., 1996).  Furthermore, 19 small-sized fourhorn sculpins (27-110 mm TL; mean = 53.9 mm TL) were collected by Hammar et al. (1996) while conducting nocturnal pelagic midwater trawls for European smelt (Osmerus eperlanus) and European cisco (Coregonus albula) in Lake Vättern. Hammar et al. (1996) suggested that the nocturnal vertical migration of these YOY and small sexually mature individuals into the warmer pelagic layers enabled the fourhorn sculpins to maximize their metabolic rates and growth, as a direct or indirect way of decreasing cannibalism from profundal concentrations of large-sized conspecifics, as well as interspecific predation and competition from Arctic char and burbot.

Nutrition and interspecific interactions

The fourhorn sculpin is an ambush predator that is capable of crushing large food items before swallowing (Leonardsson et al., 1988). Its feeding activities are largely nocturnal, but such activities become diurnal from November to April (Froese and Pauly 2002). The general diet consists of invertebrates, such as priapulids, mysids, isopods, amphipods, copepods, annelids, chironomids, and mollusks, as well as small fishes and fish eggs, particularly that of conspecifics. Invertebrates inhabiting the benthos appear to be the most important prey items and routinely consist of the isopod Mesidotea entomon, the amphipod Pontoporeia affinis, and the mysid Mysis relicta. The stomach contents of the sculpins have also been found to contain insects, plant material, sand, gravel, and unidentified animal material (Morrow 1980; Muus et al., 1999).

Fallis et al. (1987) examined the stomach contents of 27 Garrow Lake sculpins and found that approximately half (14) were empty. An abundant food item, in terms of presence (30.8%) and biomass (69.1%), was unidentified eggs and these were possibly sculpin eggs because the fourhorn sculpin is the only freshwater fish collected in the lake (BC Research 1978; Fallis et al., 1987; Dickman 1995). Some of the stomachs contained plant material (15.4%) and copepods (7.7%), but the biomass was low at 3.3% and 2.2%, respectively (Fallis et al. 1987). Limnocalanus species, according to BC Research (1978), was the dominant prey item with respect to occurrence. Fallis et al. (1987) discovered that some of the fourhorn sculpins, approximately 7.7%, had ingested amphipods, representing a negligible biomass. The majority of stomachs held unidentified materials (53.8%), representing 25.4% of the total biomass.

Svärdson et al. (1988) felt the fourhorn sculpin was the most specialized feeder on the relict deepwater crustacean fauna of Lake Vättern with M. relicta and Pallasea quadrispinosa dominating in sculpin smaller than 200 mm, and Sadurai entomon, P. quadrispinosa, and Gammaracanthus lacustris dominating in sculpin larger than 200 mm. The stomach contents of smaller Lake Vättern fourhorn sculpins (27-45 mm TL) were predominantly large sized cyclopoid copepods and M. relicta while slightly larger sculpin (82-110 mm TL) also consumed P. quadrispinosa, M. affinis, and G. lacustris. S. entomon, chironomid larvae and pupae, and ostracods were found in stomachs, but were low in terms of abundance and biomass (Hammar et al. 1996).

Behaviour/adaptability

The adaptability of the fourhorn sculpin to natural disturbance is evident in that it is a glacial relict. During the last ice age its distribution is thought to have extended much further south. When the ice retreated, many local populations were left behind in freshwaters since their access to the sea had been cut off. These managed to become adapted to the changed conditions, and so the fourhorn sculpin is found today as a relict in many deep cold freshwater lakes in northern North America and northern Europe (Muus and Dahlstrøm 1999; Muus et al. 1999).