The making of our mackerel industry

Another Life: Slowly we're mapping the sea - not just the canyons, mountains and plains of the seabed, but all the invisible forces that shape the ocean's interior and the intricate migrations of its inhabitants.

It's a maze in three dimensions, a world of glass walls, ceilings and corridors set by differences in temperature, salinity and the layered flow of currents - even by the wind. Above the rocky contours of the sea floor, it takes all these to fix the prime sources of food and thus to shape the seasonal movements of everything from whales to seabirds.

Just now, for example, looking out past Inishbofin, I can picture shoals of mackerel moving in to their spawning grounds out on the continental shelf, well into an incandescent disc of setting sun. It was at Inishbofin, just over a century ago, that biologists moored a research boat, a floating laboratory, to take plankton samples and investigate the summer mackerel brought in by the local fleet of glothogues, pookawns and currachs.

Today, the island's curing station is an abandoned shell on the East End quay, and researchers steam hundreds of miles across deep ocean to sample eggs released by the fish in a great arc from the north of Spain, up the western shelf of Ireland, to Scotland.

All this is made vivid by the maps and text of a book by the Marine Institute of Ireland's John Molloy, a fisheries biologist, who has been assessing the stocks of mackerel and other fish of the open sea for more than 40 years.

The Irish Mackerel Fishery and the Making of an Industry is the work of a scientist who has lived and worked among practical men, and while it still needs to be a bit technical here and there, it is written with ease and warmth; no wonder the fishermen hold him in such evident regard.

The book fits many things into place. Take the oiliness of mackerel, for example, and the fact that the fresher they're cooked the better they taste.

As some of the fastest, most streamlined fish in the sea (miniature tuna, in fact, and of the same family), they are fierce feeders, gorging themselves on any food within reach. Their spawning migrations take them to the richest zones of plankton production in spring and early summer. As plant plankton is succeeded by the animal kind (zooplankton), they feed mainly on copepods, the same kind of planktonic crustaceans that feed the basking sharks (whose stomachs swell with what looks like tomato ketchup). But instead of engulfing the copepods, as the sharks do, the mackerel dart at them one by one, like birds catching flies.

Most of this food goes into immediate energy - mackerel have to keep swimming, like tuna - but any left over is stored as oil in the muscles, liver and belly cavity, and by the time late-summer shore anglers are tempting mackerel to snatch at feathers, it amounts to one-third of body-weight. A big trawl of mackerel in autumn comes near to floating on the surface, so buoyant is the oil, whereas the same catch of small mackerel in spring, in bad weather, can burst a net with its dead weight.

The oil is intended to help the mackerel through the winter, but it turns rancid quickly after death, and the need to freeze fat mackerel straight away has made problems for Irish trawlers steaming long distances to northern feeding grounds.

The biggest mackerel are the oiliest. Kevin McHugh has one of more than 2 kilos, caught in April, on the wall of the Atlantic Dawn office in Killybegs, to echo the inordinate size of his trawler.

John Molloy relates the phenomenal growth of the Killybegs mackerel fleet after the herring closures of 1977 and creation of the EU fishery zone, and the rise of exports to eastern Europe and Africa. The gape of nets towed by faster, more manoeuvrable trawlers with ever more powerful sonars has grown from 240 metres to around 1,600 metres (a mile!) and the new pelagic vessels at Killybegs (pictured in the book) are sleek indeed.

Europe's huge catch from what is now seen as a single stock of North-Eastern Atlantic mackerel has to be kept in some sustainable balance, even with the mackerel's prodigious reproductive output: a single fish can spawn up to 700,000 eggs during the spawning season and live, with luck, for 20 years. The great difficulty and complication of stock assessment, and the long tension between the scientists' and fishermen's convictions do nothing to mar the positive tone of the book, which is published jointly by the Marine Institute and the Killybegs Fishermen's Organisation (a cheque for 25.99 will obtain it from either, with proceeds going to Killybegs charities).