Penguins and sea-ice
Sea-ice probably has the most significant effect on penguin numbers. Sea-ice in Antarctica can fluctuate markedly over both annual and longer periods.
Too much sea-ice increases foraging effort by limiting access to the sea; not enough reduces the foraging habitat of this sea-ice-dependent species. Hence, the extent of sea-ice cover far outweighs any direct effect of absolute prey density on ease of food acquisition.
Spring sea-ice cover
Extensive sea-ice in spring reduces the length of the growing season and the area for phytoplankton growth. Primary production (algal growth) can be at least 40% down on a normal year. This can affect the abundance and behaviour of upper trophic (feeding) level animals that time their life cycles to the annual and normally predictable supply of food in spring and summer.
Despite this, a greater than normal extent of sea-ice can lead to an increase in the growth of ice algae. Consequently the numbers of Euphausia crystallorophius that forage on the ice algae also increase. In 2000/01, for example, a large iceberg delayed the break-up of sea-ice around Ross Island. Stomach sampling of Adélies showed that the proportion of E. crystallorophius in the diet had doubled from the usual 35% to 72%.
Winter sea-ice cover
Some years, winter sea-ice can cover up to 40% of the Southern Ocean! Since 1970, satellites have recorded changes in the cycling of winter sea-ice with a trend towards two good years followed by five warmer years with less ice.
In winter, Adélies feed in a 200-km-wide band around Antarctica where the concentration of pack ice is between 15 and 80%. This region normally coincides with the food-rich waters lying south of the southern boundary of the Antarctic Circumpolar Current (ACC). In years of more extensive sea-ice, access to this food-rich region is limited. Thicker winter sea-ice also limits the penetration of light into the water column and as Adélies are visual feeders, the availability of prey may be reduced. Decreased food resources cause higher than usual juvenile mortality.
Although it would appear that penguins are advantaged in years when there is less winter sea-ice, the situation is not that simple. Warmer winters and less extensive sea-ice do not favour the recruitment, breeding and survival of krill, Adélie penguins' main prey over winter. As krill are the basis of almost the entire Antarctic food web, a drop in krill numbers could see other populations soon following.
An increasingly significant influence on Adélie populations breeding on Ross Island is the recent appearance of a number of very large icebergs.
- Icebergs can change currents and reduce primary productivity
- Icebergs occupy foraging habitat and block access to colonies affecting immigration/emigration rates
- Icebergs affect the extent and nature of sea-ice
In 2000/01 and 2001/02 the effect of a huge iceberg, B15a, has been to delay the loss of spring sea-ice from around Ross Island. This dramatically delayed the onset of the usual phytoplankton bloom by up to 2 months in some regions, reducing primary production by up to 41%. This in turn reduced the availability of food for penguins.
The presence of such a large iceberg also blocked the normal migration routes of birds returning from their wintering feeding grounds to their breeding colonies. In some areas, the movements of icebergs, and tide and wave action, have forced sea-ice into a high, jumbled barrier of push-ice along the shoreline. This became an impenetrable barrier, preventing both Adélie and emperor penguins from reaching their breeding colonies.
The delay in the sea-ice breaking up meant that thousands of breeding penguins at Capes Crozier, Bird and Royds were having to walk across the 90 km of sea-ice between their feeding grounds and breeding colony. Parent birds were expending more energy on the journey than they could store as food. Consequently, many penguins leaving for sea simply did not return to the colony. This resulted in a high level of egg desertion and chick mortality. The numbers of chicks raised in 2001/02 were between 2% and 10% of the usual number depending on colony.
In May 2002, two further huge icebergs calved from the Ross Ice Shelf. One of these, C19, was 200 km long and 35 km wide. These certainly added to the dramatic effects icebergs had on the breeding success of Ross Island Adélies in the 2002-05 seasons.