Another Life: Why killer whales aren’t eating our seals – so far as we know

If ever I fell short of ancient episodes about which to wince in the depths of night I could reach for the time I grossly, incredibly, misidentified a dead whale washed up on the strand below.

It was bad enough to have thought that a sort of roundy head turned a perfectly ordinary pilot whale into a killer whale – this without even the bright white patches and dramatically tall dorsal fin of the orca.

My rash account in Another Life prompted an eminent zoology professor to drive the twisty road from Galway to trudge alone along the strand. Not bothering even to knock on my door, he sent a trenchant letter to the editor.

That was in the very early days, before broadband computers, or the Irish Whale and Dolphin Group, or having the right field guide on my shelf. A few years later I discovered a couple of really rare cetaceans, one of which the professor attended with a little hatchet to retrieve the significant tip of a jaw for the National Museum of Ireland. This earned me better marks.

The reminder was the summer visit to Ireland last month by one of the best-known killer whales around these islands. John Coe, as he is called, with a large notch in his big fin, was first photographed 33 years ago, off the Hebrides. His appearance off Slea Head, Co Kerry, in late June, was the latest of many such in Irish waters over the past 12 years, mostly in June and July.

The individuality of whale and dolphins, bestowed by photographs and nicknames, fits big advances in human knowledge of Orcinus orca. The latest insight has traced the killer's evolution and global dispersal from a common ancestor perhaps less than 250,000 years ago. It helps to explain why, for example, orcas hunt different food in different parts of the world.

Much of what most of us know about the species is from spectacular, rather chilling sequences on television of orcas killing seals, thrusting on to beaches to drag them into the sea on the southern coast of Argentina, or making waves to wash them off ice floes. This left me to wonder why, with the grey seal so abundant off the west of Ireland and Scotland, killer whales were not eating them instead of chasing fish.

An explanation comes in research led by Andrew Foote, a geneticist and ecologist at the University of Bern, in Switzerland, working with a score of international colleagues.

"Genome-culture coevolution promotes rapid divergence of killer whale ecotypes", now published in Nature Communications and available online, has a lot of highly technical reading. It describes the sequencing of entire genomes of 48 killer whales from different groups, including some from the North Atlantic.

It goes on to suggest that orca behaviour, readily adapting to new habitats and ecological niches, and reinforced within small social groups, has created “cultures” that are genetically inherited. Thus some whales hunt sea mammals and others eat fish, seabirds or turtles. (The study also finds parallels with human evolution, as in Inuit colonisation of the Arctic, adapting to a diet uniquely rich in fatty acids.)

Some seal-eating orca groups are nomadic, forming ecotypes known as transients. Others are “residents” that hunt fish off particular coasts. Among hunting techniques of the fish eaters are co-operative herding of herring or mackerel schools into tight balls, ringing them with bubbles and stunning them with slapping tails.

In the north Pacific “transients” and “residents” have been found in the same waters, while keeping each other at a distance. The orcas of the group that commute between west Scotland and Ireland include about 10 adults and are well known as fish eaters, even following salmon and mullet up the River Lee into Cork city while ignoring the harbour’s plentiful seals.

Some in the group, however, have been been filmed attacking harbour porpoises off Scotland, and Ireland’s grey-seal colonies, such as those of the Blaskets, reportedly attract pods of orcas in the breeding season. Is the human competition for fish now forcing a wider adaptation?

Hunting at the top of the ocean food chain, the orca has accumulated toxins spreading from human industry. The chief hazard, shared by other dolphins, is ocean pollution by PCBs, a class of man-made chemicals finally phased out in Europe in the 1980s. In regions from the northeast Atlantic and the Mediterranean to New Zealand (where orcas scoop rays from the seabed) these persistent chemicals have been suppressing reproduction and threatening long-term survival of the species.

For the moment, however, they remain among the cetacea that hundreds of amateur enthusiasts might hope to spot from 20 headlands around the Irish coast in the annual IWDG "whalewatch", on August 27th (iwdg.ie). Inshore sightings of orcas are generally brief sightings of small, travelling pods, and favoured viewpoints are around the Blaskets, Cape Clear, Achill, and Donegal's Arranmore island and Inishowen peninsula.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks