Image courtesy of The Fish Site.
The Dublin Bay prawn is one of Europe’s key Atlantic and Mediterranean fisheries with landings of almost 60,000 tonnes, worth some €300 million a year
When the second World War finally ended, we could get to the sea again. Landmines were dug out carefully from the pebbles of Brighton’s beaches and the anti-tank spikes of girders hauled away. At our end of town, where the white cliffs began, the last barbed wire was unwound from access to the shore.
I was 12 years old and keen to fish for the prawns the family talked about. With hand nets baited with bits of crab, I waded the chalky pools below the cliffs. After years left in peace the prawns were abundant, some as long as your finger. Their spiky snouts could prick your finger, too.
All this to show that I know what prawns are and that Dublin Bay prawns are not in that family at all but skinny, brightly orange and rather elegant little lobsters, 18-20cm long. Linnaeus knew them first from Norway, hence Nephrops norvegicus or Norwegian lobster, nephrops being the kidney shape of their eyes.
The Dublin Bay label comes from their bycatch by fishermen who brought them ashore for private sale in the city. This was before the Irish Sea cod stocks collapsed and nephrops, with fewer fish predators, were left to become a mainstay of the nation’s trawler industry. The catch from muddy seabeds round the island is now worth some €60 million, or more than that from all whitefish combined.
The little lobster is, indeed, one of Europe’s key Atlantic and Mediterranean fisheries with landings of almost 60,000 tonnes, worth some €300 million a year. There are, inevitably, signs of decline, hastened by threats from climate change and plastic pollution.
The most urgent threat, from overfishing, has brought even greater need to know how many nephrops there are. Some 30 European scientists, including a couple from Ireland’s Marine Institute, have just produced a remarkable paper for the journal Frontiers in Marine Science that explores assessment of nephrops stocks, with new monitoring technologies. These include mobile seabed robots, telemetry, environmental DNA and artificial intelligence.
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Dublin Bay prawns live in individual burrows, some almost as complex as badger setts. In shallower depths at sunset they emerge to hunt shrimps and molluscs and seabed worms.
Since their holes in the muddy seabed are perfectly visible and nephrops will vigorously defend its home, traditional counting of holes had assumed that one burrow equalled one lobster. Occasional occupants perched at the entrance — “doorkeepers” as surveyors called them — encouraged that view.
Traditional estimates of nephrops numbers were often based on the catch in trawler hauls. But modern figures derive from hours of footage of the holes from underwater, sled-borne televisions, widely employed by the Maríne Institiute and other agencies.
These can suggest phenomenal populations. At the big nephrops ground on the Porcupine Bank, off Ireland’s southwest, a 2019 survey at 57 television sites estimated 1,010 million burrows across some 7,100 square kilometres. That was nearly 10 per cent fewer than in 2018, with trawl marks at many of the stations.
This left the problem of knowing how many burrows had hidden occupants. MI scientists Jennifer Doyle and Colm Lordan joined colleagues from Italy and New Zealand in a first mass observation of nephrops at times of maximum emergence. These can vary between day and night and important sunsets and dawns, according to the light at different depths of seabed.
Using MI research vessels, the team made 3,055 video transects at nephrops grounds around Ireland. These averaged only one visible animal per 10 tunnel systems.
But this may not fit all nephrops fisheries and the research team was expanded to 30 to study “new autonomous robotic technologies” for monitoring these valuable but vulnerable stocks.
Among many explorations of nephrops comings and goings, captured animals have been fitted with acoustic tags, hydrophones tracking their travels. The team propose fixed and mobile robots to count and track everything.
The review of possibilities is wide-ranging. But even far more accurate estimates of stocks seem unlikely to change the new and common practice of pair-trawling to scoop up the catch.
A less damaging way of fishing is with baited pots, a traditional mode still used in Strangford Lough and some lochs in western Scotland.
In 2007, when Northern Ireland trawlers were stopped from fishing out the last Irish Sea cod, a group of skippers, with EU encouragement, tried catching nephrops off the Co Down coast, setting 240 pots at a time.
They caught larger animals fetching higher restaurant prices, but the catches were poor. They decided that only the wider spread of 1,000 pots would be economic, and this could lead to too many rows between skippers as to whose pots were whose.
Full story courtesy of Michael Viney writing in the Irish Times.