Mackerel icefish
Champsocephalus gunnari
FAMILY
Channichthyidae
TAXONOMY
Champsocephalus gunnari Lönnberg, 1905, Cumberland Bay, South Georgia.
OTHER COMMON NAMES
English: Crocodile icefish, pike glassfish; French: Poisson des glaces; Spanish: Draco rayado; Russian: Ledyanaya ryba.
PHYSICAL CHARACTERISTICS
Elongated, scaleless, pikelike body (length/depth ratio, about 7:2) with an elongated snout. The jaws do not protract. In coloring it is silvery gray, darker on the back, and silvery on the belly, with dark vertical stipes on the sides, reminiscent of those seen in mackerel. The gills are a pale yellowish; the blood is colorless, completely lacking hemoglobin. The absence of hemoglobin is a remarkable feature of all 15 icefish species, apparently caused by a single massive mutation that deleted the β-globin gene. Far from being an adaptation to Antarctic conditions, this has been termed a "disaptation." Several factors have enabled channichthyids to survive this evolutionary catastrophe: frigid seawater and blood plasma can carry more dissolved oxygen, and low body temperatures limit metabolic requirements. Subsequent evolution has compensated for the loss of an oxygen-binding pigment: blood volume, heart, vessel, and gill size and the perfusion rate of gills and blood vessels all have increased. As a consequence, channichthyids are surprisingly active, competing successfully with other notothenioids.
DISTRIBUTION
South Georgia and the islands of the Scotia Arc, southward to the northern Antarctic Peninsula; it also inhabits Bouvet, Kerguelen, and Heard Islands. A closely related species, C. esox, occurs in the Falklands Islands and Patagonia and is the only channichthyid to occur outside Antarctic/subantarctic waters.
HABITAT
This is a coastal species, found mainly between 330 and 1,140 ft (100–350 m). Mature adults are found offshore in summer, moving inshore to spawn in the fall (March–May).
BEHAVIOR
The species aggregates in locations with dense krill populations, remaining near the bottom during the day and migrating upward with the krill at night.
FEEDING ECOLOGY AND DIET
The mackerel icefish feeds mainly on krill (Euphausia superba); it takes other euphausiids, mysids, and hyperiid amphipods (Themisto gaudichaudii) when krill numbers are low.
REPRODUCTIVE BIOLOGY
Reproductive parameters vary considerably between the different stocks. Maturation of oocytes takes less than a year, producing 1,300–31,000 eggs that are 0.12–0.16 in (3–4 mm) in diameter. When krill are scarce, oocytes may be resorbed, and as many as 60% of mature females may be nonreproductive. A marked three-year periodicity in reproduction is reported from the Kerguelen stock. Near South Georgia, spawning occurs from March to May; it takes place later in South Orkney and South Shetland and as late as July to August in the Kerguelen stock. Eggs are deposited in the depth range of 330–660 ft (100–200 m) and hatch after 30–180 days, depending on the stock. Newly hatched larvae are 0.05–0.07 in (12–17 mm) long and feed on copepods within 12 mi (20 km) offshore; larger juvenile fish move further offshore but remain over the continental shelf. Juveniles grow at 2.5–3.5 in (6.4–8.9 cm) per year, reaching sexual maturity at three years at a length of 10 in (25 cm).
CONSERVATION STATUS
Not listed by the IUCN. The various stocks of mackerel icefish have been targeted by minor trawl fisheries since 1974. Catches fluctuate from year to year, but they appear to be sustainable for the South Georgia and Kerguelen stocks. In South Orkney and South Shetland, the stock was depleted rapidly below sustainable levels and is no longer being fished.
SIGNIFICANCE TO HUMANS
A total catch of 4,295 tons (3,896 tonnes) was reported from the Commission for the Conservation of Antarctic Marine Resources management area in 1999–2000.
