Shifts in Marine Species

Shifts of marine species are particularly noticeable around Ireland, which stands at a kind of boundary between ‘northern’ and ‘southern’ fish and other animals.

Most fish prefer a specific temperature range and will try to stay within it. Teams of scientists in Galway, Mayo and Maynooth, led from the Marine Insitute and studying the wide range of ocean impacts, suggest that Irish species such as cod, salmon, eels and other native cold-water fish may be forced into deeper, colder and more northerly waters, to be replaced with warm-water species, such as sea bass, red mullet and John Dory. There have been summer migrations of anchovies from the Bay of Biscay as far north as Donegal and Scotland, helped by the strong northward current along the edge of the continental shelf.

The ocean ecosystem includes seabirds that look to it for food, and changes in the mix of fish can have disastrous impacts. For example, rising sea surface temperature is credited with a recent population explosion in the north-east Atlantic of the snake pipefish, a bony relative of the seahorse. Looking much the same as the sandeel, familiar food of puffins and kittiwakes (and itself in a severe decline linked to loss of its planktonic food), it has been brought to chicks in the nest. They find its horny body impossible to swallow. A litter of discarded fish has turned up at seabird colonies from Ireland to the Faroes and Iceland. At Great Skellig, off County Kerry, many puffins failed to fledge any young in 2007.

The substitution of sandeels with pipefish could be called a ‘mismatch’ arising from climate change. But the term is more usually applied to failures of synchrony in nature – mismatches between the life cycles of interacting species. Nature’s ecosystems, whether marine or terrestrial, are full of such essential links: predators with prey, insects with their flowering plants, parasites with their hosts.

A classic example is the food chain that begins with the earlier bud-burst of the oak tree. This affects the peak supply of winter moth caterpillars that feed on its young leaves. These are brought to the young of many small woodland birds (a 10-strong brood of blue tit nestlings in Killarney’s oakwoods may eat up to 1,000 in a day). When responses to warming get out of phase, adaptation can be imperfect: great tits in an English oakwood are laying earlier, but too early for the bulk of the caterpillars; in a Dutch wood the tits haven’t changed their dates and are too late for the peak supply.

The nesting of many birds is triggered genetically by day-length, rather than temperature, so adaptation to earlier springs can be gradual. In Scotland, for example, the average advance in nesting has been assessed at about one day per decade. The appearance of aphids, however, a prime insect food for birds, has advanced by almost five days per decade.

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