Another Life: The sea in a summer heatwave looks emptier for being so calm: a silken sheet of aquamarine stretched out to a hazy horizon without so much as a white tick of a sail to animate it, writes Michael Viney
Below it, however, in a melee of the minuscule, the ocean has been linking up its food chain. Since the spring flush of plant plankton was grazed by infant animals, most of these have been wolfed in turn by only slightly more visible creatures. Among them are the fish, or what we've left of them.
Tiny herring, spawned inshore over the winter, have long absorbed the blob of oil that lifted them from the seabed and fed them for a week. They have started on the copepods, minute planktonic crustacea they'll be hunting for the rest of their lives. As larvae, the herring drifted helplessly in currents that run north up the west of Ireland. From gravelly spawning grounds along our Atlantic coasts, they were swept into Galway Bay and Donegal Bay, while Donegal's own larvae drifted on to the west coast of Scotland. Most of those born at our south-eastern corner have ended up in the Irish Sea.
Just now, at some three months old, four centimetres long, and complete with swim bladders, scales and fins, they are beginning to gather into little shoals in what I'm foolishly surprised to find are really well-known places. "For instance," says John Molloy, "the area at the back of St John Point in Donegal Bay is a very important nursery ground." Such specifics abound in his new book, The Herring Fisheries of Ireland 1900-2005 (Marine Institute, €25). Unlike his previous chronicle, on the mackerel industry, the new one has more than a hint of elegy, devoted to a species notoriously fickle and abused, and perhaps finally to be drawn northwards, away from Ireland, by climate change.
Molloy is a marine biologist, recently retired after more than 40 years of assessing our stocks of commercial fish. Keeping tabs on herring has meant judging the reproduction and growth of at least three separate stocks that migrate between their Irish spawning grounds and summer feeding areas perhaps 600km out to sea. Even the otoliths - the "ear-stones" of fish that let biologists count annual rings of life - vary between the separate Irish tribes of herring.
Historically, the fish have "disappeared" from time to time, and the supply and distribution of copepod plankton in the ocean, infuenced by wind and weather, can produce enormous differences in the recruitment of young from year to year. Such natural fluctuations, combined with swings in markets and competing fleets of trawlers, have driven the often turbulent story of Irish herring fisheries.
The boom years of the later decades of the last century led to the "industrial" fishing of juvenile herring for fishmeal that helped to wreck the adult fisheries of the North Sea and Irish Sea. Today, a minimum landing size of 20cm keeps the young fish relatively safe for the two years they spend in Irish bays and shallow waters.
Heading out on migration, they swim fast, fuelled with oil from vast intakes of copepod plankton. By late summer, this oil can account for a quarter of their body-weight, stored as fat reserves for the winter.
Fishermen find herring shoals not only by sophisticated instruments but by watching for plunging gannets and an oily slick on the sea.
Fish shoal to find safety in numbers, and herring tend to stick to the bottom during daylight hours, rising at dusk to follow the zooplankton that lifts to the surface at night. But even the wariness that makes them scatter away from engines or vessel lights overhead can't protect them from nets with a mouth as big as a football field. Norwegian and Scottish scientists have measured shoals eight kilometres long and containing as much as 2,000 tonnes of herring.
They pack themselves together most densely when spawning - and also when their numbers are declining. This has led to bitter arguments between fishermen, who see a dense shoal as a sign of healthy stocks, and scientists who may read a different meaning into the marks on the echo-sounder.
A female Irish herring 27cm long produces 80,000 to 100,000 eggs, helping to carpet the seabed beneath clouds of fertilising milt. This laying of eggs on the bottom, on gravel or shingle, often at the mouth of rivers where fresh and salt water mingle and tidal currents are strong, is unique among sea fish. The Irish spawning grounds are well known and fished intensively each year - not least to help satisfy the Japanese gorging of herring roe.
If they survive enormous predation and natural mortality and grow into fish that spawn again, just two eggs from each female herring will sustain the stock. It seems amazing that Molloy and his fellow scientists are still having to make the case for a wide protection of the spawning grounds.