During the past 30 years there has been so great an expansion of world fisheries, and world landings of sea fish have risen so steeply, that virtually all easily accessible fish stocks must now be regarded as fully or over-exploited. This is backed up by the fact that between 1970 and 1980 world landings hardly rose at all, and since then landings have risen only slowly despite greatly increased fishing efforts and now appear to have fallen since peaking in 1989 (Figure 9.25).
The need to relieve pressure on local fish stocks has led to searches further afield. Several nations now have long-distance fleets operating far from home waters. British fishing vessels have travelled to Australian and Antarctic waters in hopes of finding new supplies, though with disappointing results. The pelagic long line fishery by Japanese and Korean ships, mainly for tuna, now extends over all the warmer parts of the major oceans. Fisheries for hake and a variety of pelagic species have been rapidly developed since the mid-1960s, by fleets from Europe, the USSR, Japan and Korea in the productive areas of upwelling along the west coast of Africa. There are already grounds for fearing that some of these relatively newly exploited stocks are being overfished. If there are any remaining underfished stocks, they must be mainly in remote areas, possibly off the southern part of South America or around the East Indies. There may also be some unfamiliar demersal species living well down the continental slope that might be worth fishing if conservative habits of diet are overcome. Alternative methods of presentation, such as 'fish fingers', have certainly helped to extend the market to a wider variety of fish. But prospects of finding major new sources of fish, at present unexploited, do not seem very promising. On the contrary, in the immediate future there may possibly be some contraction of world fishing as coastal states seek to protect their resources by extending their territorial fishing limits, thereby excluding the fishing boats of other nations except by special agreements. As fishing within the 200-mile fishery zones becomes more subject to restrictions, some decline in world landings is to be expected before the benefits of conservation measures become apparent.
In some underdeveloped parts of the world, the productivity of local fisheries is severely limited by their primitive methods of fishing, preserving and marketing. Small craft can only make short voyages, with the result that the close inshore grounds are intensively overfished. Much better landings might be obtained if the fishermen could extend their operations over wider areas. Furthermore, the consumption of sea fish is often restricted to coastal regions because little is done to preserve the catch for distribution inland. The expansion of these primitive fisheries requires large capital investments for the mechanization of fishing, for the training of fishermen in new methods, and for the provision of proper facilities for preservation and distribution.
The difficulty of finding the expertise and large sums of money needed for equipping, crewing and maintaining modern fishing vessels has led some coastal states to negotiate with foreign companies to allow them to fish within their fishing zones. These concessions do not necessarily bring much benefit to coastal states because, although there may be some increase in local fish landings, the greater part of the catch is liable to be removed for marketing in more affluent areas where fish that the local population would use for human consumption are instead converted to fishmeal.
The classic history of the Peruvian fishery for anchoveta (Engraulis ringens) has been a remarkable example of the rapid development of a primitive fishery to become the world's major source of fish, unfortunately followed by a dramatic decline. During the period 1948 to 1968 this fishery increased over a hundred-fold to 10 million tonnes per year (more than one-sixth the entire world catch), providing Peru with a larger catch than any other nation. In 1970 the fishery took 12.5 million tonnes, but by 1973 landings had fallen to little more than 2 million tonnes. This failure exemplifies some of the problems of distinguishing the effects of overfishing from those of concurrent environmental changes. The collapse of the fishery occurred at a time when changes of the circulatory pattern of the area reduced the extent of upwelling of deep water along the continental slope, upon which the area's prolific production of marine life depends. There had previously been signs of impending threat to adult stocks because fewer young fish were entering the shoals, probably the result of over-exploitation. Although there has subsequently been some recovery of the fishery, the 1987 catch was only 2.1 million tonnes and stocks remain low to date.
Various suggestions have been made of ways in which the productivity of the more intensively fished areas might be maintained or increased. For example, yields might be increased by transplanting young fish to areas of more abundant food.
Since the early years of the twentieth century Professor Garstang repeatedly advocated the transplantation of young plaice, and demonstrated a three- to fourfold increase in growth rate of young North Sea plaice moved from their crowded coastal nurseries to the Dogger Bank, where bivalves suitable as plaice food are plentiful. Calculations indicated that, on a large scale, this could prove a profitable enterprise, but such a project would require close international cooperation to ensure that the transplanted fish were left long enough in the sea to benefit, and that costs and profits were equitably shared.
There have also been hopes that it might become possible to increase the stocks of certain species of marine fish by artificial rearing during the early stages of life. The first few weeks are a period of special danger when extremely heavy fish mortality always occurs. The number of fish surviving in each year-brood seldom bears any close relationship to the number of eggs spawned; generally, the more eggs, the greater the number of casualties. Survival is mainly determined by environmental factors, for example, temperature, salinity, food supply, currents and predation (see Section 9.4.4 Recruitment). It might, however, be possible to increase the stocks of fish if eggs and larvae could be raised in very large numbers in protected conditions, and supplied with ample food until the danger period is passed before setting them free in the sea. The unusually heavy catches made on some of the north-east Atlantic fishing grounds immediately following the two World Wars during which very little fishing took place, indicate that these areas are able to support many more fish than they ordinarily do when fishing is proceeding at peacetime rates.
In experiments during the 1950s, J.E. Shelbourne (1964) and his colleagues of the Lowestoft Fisheries Laboratory (UK) developed successful methods of rearing large numbers of young plaice by stocking the open circulation seawater tanks at the Marine Biological Station at Port Erin, Isle of Man, with plaice eggs spawned in captivity. Methods were devised for bulk preparation of suitable planktonic food for the developing larvae, mainly nauplii of Artemia salina. A measure of bacterial control was achieved by treating the water with antibiotics and ultraviolet light. In these conditions many thousands of young plaice have been reared to the completion of metamorphosis, and survival rates of over 30 per cent of the original egg stock have been achieved. However it became apparent in due course that the number of artificially reared fry that could conceivably be released into the sea each year is infinitesimal compared with the number produced naturally, and that no significant contribution to north-east Atlantic plaice stocks was feasible by this means. Release into the sea of hatchery fry in this area therefore ceased and the experiments were reorientated towards the raising of hatchery fish to market sizes in captivity, i.e. fish farming (see below).
Despite the discouraging conclusion of work on the release of plaice larvae into coastal waters around the UK, restocking the seas may be a more realistic proposition with other species in particular areas. In the Sea of Japan artificially cultivated young of red bream (Sparus major) and prawns (Penaeus japonicus) are regularly released with the objective of improving fishery landings. In the Caspian Sea, sturgeon fisheries have suffered badly from over-exploitation and the damming of vital spawning rivers. Many have been successfully revived by the yearly release of hatchery-reared fry. In Prince William Sound in Alaska, hundreds of millions of pink salmon fry (Oncorhynchus gorbushca) are released from hatcheries every year. This species has a relatively simple two-year life cycle and such releases have been quite successful in boosting harvests. In the North Atlantic there are some prospects that, if netting at sea could be adequately controlled and costs of rearing equitably shared, release of hatchery-reared smolts of Atlantic salmon (Salmo salar) to feed naturally at sea might prove more profitable than ongrowing them in fish farms on expensive food which might be better used in other ways.
Although we must conclude that there is no immediate prospect of obtaining much larger quantities of food from more intensive exploitation of the natural fish stocks of the oceans, we may reasonably hope that measures to conserve these resources by scientific management of fisheries may eventually lead to some further increase in world landings. Beyond this, there are suggestions that the productivity of fisheries might be enhanced by modification of the ecosystem. For instance, we have previously referred to the competition for food that exists between bottom-feeding fish and the large numbers of predatory invertebrates, and to Thorson's calculations that 'only 1-2% of the fish food is actually eaten by fish, the rest is taken by invertebrates' (see Section 6.4.5). An area could presumably support a greater number of food fish if competitors could be eliminated, or at least considerably reduced. Hardy (1959) in his classic text, suggested that it might eventually become possible to weed the sea-bed of 'unwanted' creatures so that a larger proportion of the food becomes available for fish. He writes:
If Thorson's calculations are correct, and if man could eliminate just a quarter of the pests and so allow the fish to have some 20%, instead of 2%, of the potential food supply, then he could make a given area support ten times the quantity of fish. How are such pests as star fish to be eliminated? I believe that just as we harrow and roll the land in addition to reaping our crops, we shall in time systematically drag some combing or other devices over the sea floor at intervals to weed out the creatures that take food from the more valuable fish; and the pests themselves may well be ground into meal to feed poultry ashore. It will require much more ecological research to determine just which of the animals we can do with in smaller numbers.
In the present climate, with increased knowledge of the complexity of ecosystems and the interrelationships between species, this approach seems unlikely to gain acceptance even if it became feasible.
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