It may be eight hours before the catch is hauled aboard, when at least a third of it will be tossed back into the water. Only prime specimens are wanted, not the "by-catch" of juveniles, less marketable fish, and even porpoises, swept up by the nets that rake in all that lies in their path. The porpoises' lungs may be punctured before they are cast adrift, ensuring that they sink.
For years, the unpalatable details of the industry that delivers fish to supermarket shelves have been a well-kept secret, along with the scientific data that shows that some fish are being fished to near-extinction. But not any longer.
The ranks of enlightened fish consumers grow by the day, and proof that some of the supermarkets are waking up to the issue arrived when Greenpeace scaled the Asda headquarters in Leeds in January. No sooner was a banner unfurled from the roof depicting some of the dead and mutilated by-catch, than the campaigners were ushered inside for a chat about the sustainability of the supermarket's fish-buying policies.
Asda, which was rock bottom of a Greenpeace table of supermarkets ranked on their fish-buying policies several months ago, and in the bottom half of a similar Marine Conservation Society (MCS) table published earlier this month, announced immediately that it was removing skate wings, Dover sole, ling and dogfish from its stores. Since Asda's move, Sainsbury's has announced that it will no longer sell skate and huss, both endangered, while in the US, the discount giant Wal-Mart (which owns Asda) has declared that it will buy all of its fresh fish from sustainable sources within three to five years.
The MCS's research found that fish sold at those supermarkets that compete most ferociously on price (Tesco, Morrisons, Asda and, worst of all, Somerfield, Iceland and Lidl) are generally far less sustainable than that available in smaller, upmarket chains (Waitrose and M&S).
So, we now have a better picture of how the oceans are being plundered by a global fishing industry whose catch has risen from 18 million tonnes to 95 million tonnes over the past half-century, and, according to the UN's Food and Agriculture Organisation, has left 17 per cent of commercial species overexploited, and 8 per cent depleted. But knowing which products are legitimately sourced still requires every ounce of the brain function that eating fish is supposed to assist.
Cod presents all manner of dilemmas. Icelandic, Faroese and Norwegian Arctic varieties have all appeared in the MCS's Good Fish Guide as well-managed stocks that may be eaten. But recent concerns about their diminishing populations registered by the International Council for the Exploration of the Sea now leads the MCS to place them on its "fish to avoid" list, alongside cod from the North Sea, Irish Sea and Baltic. And yet there is some good news for the conscientious customers for whom chips without cod just isn't the same. Line-caught cod from Iceland, the Faroes and the north-east Arctic do qualify for consumption and are "the better choice to make", according to the MCS.
Dover sole is another fish that many consumers consider unsustainable, and it was a recently "delisted" Asda species. Consumers should chose only from stocks caught in the Eastern Channel; avoid eating immature sole (less than 28cm); fish caught during the breeding season; and preferably chose fish caught by gill or fixed-net fisheries that have adopted measures to reduce dolphin by-catch. If there's any time left after taking all that into consideration, you can take your sole home and cook it.
Not all fish are so problematic. Lemon sole, red mullet, red gurnard and most salmon are generally acceptable, while Atlantic halibut, skate, swordfish and marlin are not. If a blue certification logo (a fish and a tick) is stamped on a product, it is one of the 50 products accredited by the Marine Stewardship Council, and is sustainable.
But many supermarkets don't provide labelling, particularly on fresh produce, which might detail method of capture and stock from which fish is caught. In the absence of information, consumers have resorted to printing out details from the MCS website (www.fishonline.org), which offer guidelines on 120 fish species and has been developed from its Good Fish Guide.
Purchases are less complicated in M&S and Waitrose - the supermarkets that, according to the MCS, have specialist fish buyers to ensure that their produce is bought from sustainable farmed or wild sources. In last week's report, M&S was found to sell 15 species from the MCS's "fish to eat" list, and one from its "fish to avoid". Waitrose sold 20 "fish to eat'" and three to avoid, while Sainsbury's sold the greatest number of MCS-approved products but also stocked five species from the endangered list.
Producers of fish products are also waking up to consumer concern. For example, a method of maintaining the sustainability of supply is being developed for the langoustine boats of Scotland by the seafood company Young's. It has worked with Glasgow and London universities to develop satellite-tracking technology that enables data of every catch's location, maturity and quality to be transmitted from boat to shore the minute it lands. The data is analysed and, if the catch is found to contain too many juvenile or pregnant fish, lobstermen will be asked to release it and move to different waters. The technology will be on 200 vessels by the end of the year.
The supermarkets whose policies have been questioned insist that they are taking action. Asda plans to introduce a "sustainable fish of the month", and Morrisons, to which, as the poorest performer, Greenpeace has now turned its attention, says that it may label fish with an MCS rating to allow customers to make more informed decisions about what they buy.
Where's the catch? How to buy with a clear conscience
Easily the supermarket with the best reputation for stocking sustainable fish. It sells 15 species that are safe, and only one on the MCS endangered list. M&S pioneered the use of nets that catch haddock but exclude cod.
Sells more fish than any other supermarket - £300m worth of chilled fish a year. Comes a comfortable third on Greenpeace and MSC league tables. Recently agreed to stop offering skate and huss, both endangered. Its skate sales alone were worth £650,000 a year.
Makes some effort on sustainability, although its stocks include four that should be avoided, according to the MCS, including North Sea cod. One of its fish adverts reads, "One moment it's in the North Sea. The next it's somewhere off the South Circular Road".
To the delight of Greenpeace, which listed it bottom for sustainability last October, Asda has improved its stock, now selling 10 safe species, but six to avoid. Stopped selling shark and introducing "sustainable fish of the week".
Morrisons and discount chains
Morrisons is currently the focus of Greenpeace protests. It has no clear policies on fish procurement. Somerfield, Iceland and Lidl have the poorest record on sustainable stocks.
Sits alongside M&S as pacesetter for sustainable fish retailing, and was named most compassionate supermarket by the Compassion in World Farming Trust. Sells 20 species from MCS "safe" list, three from "avoid" list.