Antarctic fishes have a limited ability to acclimate to any elevations in temperature. Therefore the survival of icefish is likely to be jeopardised if temperatures in the Southern Ocean continue to rise in accordance with the current trends (O’Brien and Mueller, 2010). Atmospheric temperatures in the Western Antarctic Peninsula have risen by 3ºC (Atkinson et al., 2004), and summer sea surface temperatures have risen by around 3ºC in the last 50 years, which works out as an average of around 0.56ºC per decade (Vaughan et al., 2003). However, according to Gille (2002), temperatures in the Southern Ocean have risen by a maximum of 0.4ºC per year (Figure 8). Upper water layers (down to 50m depth) off the Antarctic Peninsula have seen temperature increases of around 1ºC since 1955 and are predicted to rise by a further 2ºC by the end of the century (Murphy and Mitchel, 1995; Meredith and King, 2005). Increases in sea temperature will inevitably result in a reduction in the extent and duration of sea ice which could lead to changes in oceanic salinity and density (Jacobs and Comiso, 1997; Moline et al., 2004). The depth of the surface mixed layer; predominantly determined by salinity and surface water stratification, drive phytoplankton blooms and therefore overall ecosystem production (Moline et al., 2004). Any changes that affect the phytoplankton community will lead to changes in the food web by bottom-up control.
Figure 8. Changes in temperature (°C yr–1) in the Southern Ocean from data collected in the 1930’s to 2000 at 900m depth. Data was collected from shipboard profiles and Autonomous Lagrangian Current Explorer float data. The greatest increases in temperature occurred around the Antarctic land mass, with lesser degrees of warming at higher latitudes (Gille, 2002).
Icefish are an integral part of the Antarctic marine ecosystem. Any changes in their abundances will act as a bio-indicator of climate change and of any resulting indirect effects on other members of the food web (Dulvy et al., 2008). Fish as a whole occupy a central position in the ecological network as they fill a wide number of trophic niches, are the primary consumers of plankton and benthos, and are an important food source for species higher up the food web (Mintenbeck et al., 2012) (Figure 9). Trophic mismatches are one potential impact on the Antarctic food web and would occur as a result of the reduced sea ice content and higher water temperatures earlier in the year. This would lead to peaks in the abundances of prey and consumer species occurring at different points during the season (Mintenbeck et al., 2012). An alteration in the prey species composition would also involve changes in the type of prey available to Antarctic fish species in terms of size structure and the energy content of available prey. For example, a shift from diatoms to cryptophytes is generally accompanied by shifts in the size structure of primary producers (Mintenbeck et al., 2012).
Figure 9. Fish species form a crucial component of the Antarctic food web, upon which the majority of marine mammals and penguins depend on in the Southern Ocean. Therefore any changes in oceanic temperature which reduce the abundance of Antarctic fish is likely to have repercussions higher up the food web. (Image taken from: http://www.coolantarctica.com/Antarctica%20fact%20file/wildlife/whales/food%20web.htm).
The effects of an increase in water temperature are likely to vary on an individual or species-specific basis, as some species illustrate a greater degree of physiological plasticity than others. One of the ways in which icefish have a low thermal tolerance is due to their phospholipid-rich muscles (Scandalios, 2002). Mitochondrial membranes of Antarctic fishes are rich in fatty acids, which accelerate the formation of reactive oxygen species (ROS). These can damage proteins, lipids and DNA through an auto-catalytic mechanism. The tissues of icefish have a high density mitochondria, resulting in an increase in the sensitivity of icefish to elevations in temperature compared to other red-blooded fish species (Girotti, 1985).
As Antarctic fishes are stenothermic, they have a limited ability to acclimate to any increases in water temperature (Hofmann et al., 2000). The upper lethal temperature of red-blooded fish species is about 6ºC, however the value for icefish is thought to be lower due to the reduced oxygen-carrying capacity of their blood (O’Brien and Mueller, 2010). Holeton (1970) found that one species of icefish, Chaenocephalus aceratus, died at oxygen tensions below 50mmHg. The loss of haemoglobin in the blood of Antarctic icefish and the relatively stable environment in which they have evolved, mean that any changes in temperature that result in lower levels of dissolved oxygen in the water column, are likely to place greater stress on the aerobic respiratory pathways of icefish. Therefore, in order to conserve both icefish and the charismatic marine mammals and penguins that directly or indirectly depend on icefish as a food source, action needs to be taken to reduce CO2 emissions and slow current trends in climatic and oceanic temperature increases.