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Adaptations to deepwater living make the fishes of the deep sea particularly vulnerable to overfishing.
The main reason for their vulnerability is their slow growth rate, a fact of life at depth. Light does not penetrate to the continental slope, so no photosynthesis takes place there, and there is no in situ food production. Virtually all the energy for the slope’s food chain must be imported. Most comes in as debris raining from above—from dead phytoplankton to dead whales. Even so, there isn’t much. Throughout the deep sea the available food declines quickly with depth, increasing somewhat on the seafloor itself.
As a result, the abundance, biomass, and metabolic rates of organisms also decline. With a low metabolic rate, a fish takes longer to grow to maturity: a cod on the shallow continental shelf matures in four to seven years, whereas a grenadier on the continental slope may take eighteen years. By the time many deepwater fishes are big enough to be worth catching, they are older than your grandmother: the fillet of orange roughy you order at a restaurant could easily come from a fish born before the invention of the automobile.
Thus populations turn over quite slowly. A fishery naturally targets the largest individuals, and so can quickly wipe out most of a population’s mature, reproductive fish, which can take decades to replenish. Add to that the possibility that some slope fishes might only spawn once at the end of their long lives, and you have a recipe for extinction.
Except for a few special cases, deep-sea fisheries are all of recent vintage. In the early 1970s Soviet fishermen became the first to systematically locate and exploit continental-slope fishes. But despite glowing accounts of the tastiness of the fishes—blue hake, deepwater sharks, grenadiers, and slickheads, among others—a strong international market never developed. The fishery instead supplied immense quantities of low-quality product to the former Eastern Bloc, where cheap protein was in demand. State-sponsored fleets from the Soviet Union—and later, from its former republics—aggressively fished the northwest Atlantic and various other regions into the 1990s; some are still active today.
Worldwide, the expansion to the deep sea stems directly from the severe depletion of shallow-water fisheries. In the northwest Atlantic, for instance, the collapse of cod in the early 1990s became one of the most dramatic fish-stock crashes of all time. But the industry found a substitute in the deepwater Greenland halibut. Predictably, that species is now in trouble throughout the Atlantic. Elsewhere, deep-sea fisheries have developed for numerous species, including icefish in the waters off Antarctica, Patagonian toothfish off Argentina and Chile, hoki off Australia and New Zealand, black oreos off New Zealand, thornyheads near Pacific seamounts, and giant rattails in the northwest Pacific.
The deep-sea orange-roughy fishery near Australia is a prime example. It was developed to satisfy the middle-American market for a bland, white fish. Even the name was picked through careful supermarket research (its original moniker, the slimehead, sounded far less appealing). And when that fishery began its inevitable decline, the industry moved on to another deep-sea species, the Patagonian toothfish. It was renamed the Chilean sea bass, even though it is neither a bass nor exclusively Chilean. It, too, has now suffered sharp declines.
The bottom-trawling gear for deepwater fishing is basically the same as the gear deployed in shallow waters, but on an enormous scale. Factory ships longer than 300 feet can hold 1,000 tons of fish and stay at sea for 300 days a year. Massive winches and cables are needed to reach the slope bottom, ten times deeper than the continental shelf. The net itself is woven from heavy polypropylene line and can span the area of several football fields [see illustration left]. A pair of steel plates called doors, weighing as much as five tons each and connected to the net with heavy cables, spread the net open under water. The doors and cables scrape along the ocean floor and herd fish into the net—as much as twenty tons in each haul. The entire rig can weigh fifteen tons, and is dragged across the bottom for several hours at a clip of four miles an hour or more. Little more than finely ground rubble remains on the seafloor in its wake.
In theory, a fishery should be managed sustainably, taking no more each year than what reproduction replaces. Thus the size of the take is determined by numerical models that can estimate future populations. Accurate estimates depend on accurate information about a species’ age at maturity and its growth rate. Those numbers depend, in turn, on survey data on abundance and body size—a proxy for age.
Yet none of those data existed reliably for deep-sea fishes as their fisheries developed. Managers set catch quotas essentially by guesswork, relying on their knowledge of shallow-water species. They took no account of the far slower turnover rates in a typical population of deep-sea fishes. And even when fisheries could not catch enough fish to meet their quotas, as was the case with the roundnose grenadier for more than twenty years, the quotas were not decreased.
Unfortunately, circumstances today have not much changed. Most deep-sea fisheries, as in decades past, are little more than mining operations that run until they exhaust their target populations and collapse. Then they move on to another species.