Comprehensive Description

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Comprehensive Description

Ruud (1954) published the first detailed report on several Antarctic fish species in the family Channichthyidae, including the Mackerel Icefish (Champsocephalus gunnari), that have nearly colorless blood as a consequence of a total lack of hemoglobin (many icefish species, including the Mackerel Icefish, also do not produce the related myoglobin proteins in their muscles [Grove et al. 2004] and, like many related Antarctic fishes, carry compounds in their blood that prevents it from freezing [Cheng and Detrich 2007; di Prisco et al. 2007]). This lack of hemoglobin results in an oxygen-carrying capacity in icefishes that is less than 10% of that seen in closely related red-blooded species. However, the problem of obtaining sufficient oxygen is at least somewhat reduced because the solubility of gases is inversely proportional to temperature, making oxygen content in Antarctic waters extremely high. Nearly all icefishes live only in the cold-stable and oxygen-rich environment of the Southern Ocean. The Mackerel Icefish is one of the few icefishes with a low-Antarctic range of distribution, nearly all the others being confined to high Antarctic latitudes (Kock and Everson 2003). It is a prominent member of the coastal fish fauna of the Seasonal Pack-Ice Zone and the islands north of it (Kock and Everson 1997). Mackerel Icefish are generally found above 300 m in depth and have never been found below 700 m (Kock and Everson 2003 and references therein).

A range of anatomical and physiological features of icefishes appear to be adaptations associated with the lack of hemoglobin. Notably, modifications of the cardiovascular system of icefishes help compensate for the absence of oxygen-binding hemoglobin. Denser vascularization supplying tissues with especially high oxygen demand, such as retinal tissue, has been shown in some icefishes, including the Mackerel Icefish. More generally, icefishes possess large capillaries and have blood volumes that are two to four times larger than those of red-blooded fishes. Hearts are larger in icefishes than in red-blooded fishes of similar size, resulting in mass-specific cardiac outputs that are several times greater than those of red-blooded species. As a consequence of these features, icefishes are able to circulate large blood volumes at relatively high flow rates. This is achieved at low arterial blood pressures due to decreased systemic resistance to flow. The combination of high-throughput cardiovascular systems, waters of high oxygen content, and relatively low absolute metabolic rates enables icefish to deliver sufficient oxygen to their tissues. (Kock 2005 and references therein; Wujcik et al. 2007 and references therein; Garofalo et al. 2009)

Mackerel Icefish are active bentho-pelagic fish which migrate up and down the water column in a daily cycle (Kock and Everson 2003 and references therein). In contrast to many Antarctic fishes, when Mackerel Icefish form dense aggregations, they often stay in the water column 20 to 30 m or even farther above the bottom during the day (Kock and Jones 2005). Mackerel Icefish mature at three to four years of age (Kock 2005) and may exceed 60 inches in length and live to 15 years. Sexually mature males have a significantly higher first dorsal fin than do females (Kock and Jones 2005 and references therein). Mackerel Icefish were targeted by commercial fisheries in the 1970s and 1980s and most stocks had been overexploited by the beginning of the 1990s (Kock 2005).

Main et al. (2009) studied the stomach contents of more than 2000 Mackerel Icefish individuals. They found that diet varied significantly between year and age classes. In general, the diet was dominated by Antarctic Krill (Euphausia superba) and by the amphipod Themisto gaudichaudii. Smaller (younger) individuals tended to include in their diets a higher proportion of T. gaudichaudii and smaller euphausiids (such as Thyanoessa sp.) and fewer Antarctic Krill.

Kock and Everson (2003) review what is known about the ecology of this species. Kock et al. (2007) review the history of the Mackerel Icefish fishery.

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