UNLESS you are a mariner, or a hapless summer bather, it is not immediately apparent that the seas around Britain are ripped by powerful tidal streams. Yet these streams can reach speeds of up to 1.5 metres a second (a brisk walking pace) with average speeds of about 0.5 metres a second. They do not flow continuously in one direction, but first one way as the tide floods and then back the other as it ebbs, with a small residual drift in one direction. These tidal streams are superimposed on the overall circulation produced by currents from the open ocean flowing onto the continental shelf. Fisheries scientists have known for many years that the residual drift plays an important part in the dispersal of fish eggs and larvae, carrying them away from the spawning grounds towards nursery areas near the coast. What was not so obvious was the role of tidal streams. We now know that fish make use of these too, but in an altogether more sophisticated way.
The idea that fish might move around on tidal streams emerged only 45 years ago. This might seem curious to the lightermen of the Port of London, who for centuries let the tides move their cargoes about. But it is extraordinarily difficult to follow the movements of fish in the open sea. So another 15 years passed before the Dutch zoologist Frederick Creutzberg showed that young eels use tidal streams to migrate into the Wadden Sea on their way to fresh water. Even so, the problems of following the movements of wildfish remained.
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A group of scientists at the Ministry of Agriculture鈥檚 Fisheries Laboratory in Lowestoft, on the Suffolk coast, has been trying to solve these problems. We adopted equipment originally developed for the Royal Navy, and the microelectronic technology behind pocket-sized televisions and digital watches. From the Admiralty Research Laboratory at Teddington, we borrowed a high-resolution, sector-scanning sonar that sweeps a sector of 30Degree two to four times a second with an acoustic beam. Objects in the path of the beam reflect some of the acoustic waves, which are converted to a signal on a sonar screen. Although the device was developed to detect underwater objects, such as wrecks or mines, in dark and murky waters, we installed it on the ministry鈥檚 research ship Clione to watch how shoals of fish respond to commercial fishing gear. We also hoped that the sonar would be useful for watching the behaviour of fish. But although it gave excellent pictures of trawls, wrecks and even fish, an individual fish could not be followed for long because when it came close to the sea floor, its echo merged with those from the seabed.
Electronic engineers at the laboratory overcame this problem by designing a small acoustic 鈥榯ransponder鈥�, which could be tied to an ordinary identification tag and attached to the fish. The acoustic tag 鈥榣istens鈥� for the sonar signal and responds by transmitting a pulse of sound at the same frequency. This allows scientists aboard a research ship to identify and follow a single fish and so test the idea that adult fish migrate on tidal streams.
Most of these studies have concentrated on one species of fish, the plaice, Pleuronectes platessa. Plaice spend most of their time on the seabed, and can avoid the pull of the tide by burying themselves in the sand. The population of plaice in the southern North Sea has well-defined feeding and spawning grounds. It seems to migrate regularly between the two, and so provides a relatively simple system in which to unravel the migration story.
Adult plaice spend the summer on rich feeding grounds in the central North Sea, south of the Dogger Bank. In autumn they migrate south to their spawning grounds in the Southern Bight and the eastern English Channel, where they spawn in January and February. The eggs and developing larvae are carried northeast to their nursery grounds on the Dutch coast by the residual current, while the adults return north to their feeding grounds.
For a few weeks each year between 1971 and 1982, we tracked individual adult plaice at different stages of the migration cycle. The fish showed two clear patterns of behaviour. At the summer feeding grounds and winter spawning grounds, the plaice stayed on the bottom during the day, but swam up into midwater at night, presumably to move about under cover of darkness. Because the tidal cycle is out of phase with the cycle of day and night, fish in midwater are not carried far in one direction before they are carried back again. But at the time of the spring and autumn migrations, the fish moved into midwater twice a day. They left the sea floor at about the time of slack water, and stayed in midwater for about 6 hours. The fish returned to the bottom just before the tide turned again. These fish consistently picked either a north-going tide or a south-going tide 鈥� which carried them some 20 kilometres each day.
But was this really the way plaice migrated up and down the North Sea? After all, we had tracked only a handful of fish. And each fish had gone through the process of being caught, taken to the laboratory in Lowestoft, tagged, and returned to the sea: enough to upset any fish鈥檚 natural instincts.
The only way to check that fish really do travel on the tide, a process called 鈥榮elective tidal stream transport鈥�, was to catch wild fish at various points en route. If plaice do take advantage of tidal streams to migrate, then fish caught on south-going tides should be 鈥榬ipening鈥�, getting ready to spawn, while those on north-going tides should be spent. After five years of fishing experiments, the picture is clear: plaice do exactly as expected. Our results suggest that selective tidal stream KK transport may be the basic mechanism of plaice migrationin the southern North Sea. As such, it is of fundamental interest to biologists, but it also has much wider implications.
Marine fisheries are based on 鈥榮tocks鈥� of wild fish living in the open sea. The success of a fishery depends on the health of the stock, while the economics of commercial fishing obeys the law of diminishing returns. When fish are abundant, trawlers with modern fishing gear can be sure of bringing in largehauls reasonably quickly. If they do not exercise restraint, thestock will shrink and fishing will become progressively hardereach year. Eventually it becomes uneconomic to fish at all. Restraint prevents stocks from becoming too small, and prevents prices from fluctuating wildly because of large changes in supply. Unfortunately, one fisherman on his own will not gain by showing restraint; his competitors will take the fish he leaves, making short-term profits but eventually destroying the fishery for all. Ideally, fishing should be managed to maintain stocks year after year.
Effective management of fisheries requires knowledge of the biology of the stock concerned: its distribution and size; the proportions of fish of different ages; and the rate and age at which the fish are caught. With such a 鈥榮tock assessment鈥� to hand, it is possible to calculate how a particularfishing strategy will affect that stock.
Answers to stock questions
In this context, the stock is usually taken to mean the whole fishery in question, for example the North Sea plaice or Irish Sea cod. But we do not know if such a stock is a single biological unit. It may be made up of a number of separate groups of fish that come together and are caught in the same place. Or the stock might be a part of a much larger population. It is difficult to identify separate groups genetically, but we can learn a lot about them by tracking the movements of a population. Only if a whole population always moves in the same way can it truly be considered a single stock for management purposes.
The conventional experiments in which large numbers of fish are tagged with numbered plastic discs and then released at sea are of limited help in studying the movements of populations, because they rely on fishermen to return tags that end up in their catches. Even if all fishermen returned all the tags, the method provides only crude information about the rate and timing of movements. The technique provides no details about what fish have done between the time they were tagged and the time they were recaptured. Understanding how fish migrate will provide a much better idea of the composition of a stock.
At the moment, the European Community regulates fishing through its Common Fisheries Policy, which sets a 鈥榯otal allowable catch鈥� each year for each stock. But this system does not give any control over the ages of the fish in the haul. If fishermen take too many young fish, they will seriously deplete the stock of spawning adults in later years. In mixed fisheries, fishermen must throw back fish whose quota has been reached. This undermines both the purpose of the regulation and the estimates of fish mortality that are used in assessing stocks. Such problems have led to moves to control fishing more by technical measures (setting of closed areas or seasons, increasing the size of mesh, or increasing the minimum landing size) which will protect specific parts of the stocks more effectively. To set these sorts of controls, we need detailed information about the seasonal and geographical distribution of fish stocks.
There are other good reasons for finding out more about fish movements. The great rivers of Europe that flow into the North Sea carry much of Europe鈥檚 waste with them. Yet we know very little about how these wastes affect marine life. Pollution might affect the way fish recognise their feedingand spawning grounds or spawning partners, for example. Or the fish themselves might carry toxic waste far from the outfalls and dumpsites.
If we are to answer any of these questions, we need to know more about the movements of fish. We now know something of the movements of one stock of plaice, but there are many more questions. For example, how do ripening plaice select the south-going tides and spent plaice select the north-going tides? Do they have a sense of direction? And what cues trigger the change in behaviour from moving into midwater only at night, to twice a day on alternate tides? The fish could be responding to the change in the length of the day, the water temperature or perhaps some other environmental factor. There remain questions about how plaice recognise their feeding and spawning grounds. They might respond to pheromones released by other plaice or they might recognise certain landmarks. Is selective tidal stream transport unique to this one population of plaice, or is it a feature of all plaice? We have evidence that cod and eels use the tides, too, so perhaps all bottom-dwelling fish share the same transport system.
One thing we want to know now is why plaice use the tides to migrate. Is it just a way to save energy or is it the fundamental mechanism which determines where the feeding and spawning grounds are? An energetic analysis predicts that in some areas of the sea the tides are too slow for large plaice to save energy by migrating on the tides. If these fish do use the tides, then the behaviour must be a mechanism which 鈥榞ets them there鈥� like some hydrographic conveyor belt, presumably because they do not navigate for themselves. This, in turn, tells us thatthe tidal streams are crucial in determining the distribution of the stock.
There are enough questions here to keep fish biologists busy for life. We can do some experiments in the laboratory 鈥� looking at the factors that control the starting and stopping of migratory behaviour, for example. For these we have built a large tank in which we can simulate the reversing tidal streams. But, ultimately, to understand fully the movements of wild fish, we still need to go to sea.
Today, tracking individually tagged fish with a research ship is becoming less practical: it would take far too long and cost too much to build up a picture of the movement of whole populations. Comparative fishing experiments, too, would be a hit-and-miss affair without knowing in detail the timing of migrations in other areas or of other species. For these experiments we again need the help of microelectronics experts, who are trying to develop a new tag with a depth sensor. The tag will record for many months when the fish moves into midwater, storing the information on a microchip. When the fish is caught and the tag returned, even if it is many years later, the data will show if and when the fish stopped moving into midwater only at night, and began to migrate by moving into midwater on each alternate tide. The biggest advantage of this type of tag is that it allows us to collect data from any number of fish for many months, rather than for a few days from one fish at a time. Data gathered over such long periods will provide a more complete picture of the timing of movements and tell us if all the fish start to migrate at the same time. This, in turn, might give a clue to what triggers the switch to migratory behaviour.
One day, we might not even have to catch the tagged fish. It might be possible to develop a tag that will detach from the fish after a certain time, float to the surface, and transmit its data to the laboratory via satellite.
It may seem slightly crazy to release hundreds of fish, each equipped with an expensive electronic tag, hoping that fishermen will catch those fish and return the tags. But results from earlier experiments suggest that a third or more tags should come back within two years. That so many fish should be caught and returned shows just how heavily the North Sea is fished, and just how important proper management is.
Julian Metcalfe and Geoff Arnold are fish biologists working for the Ministry of Agriculture, Fisheries and Food at the Fisheries Laboratory, Lowestoft, Suffolk. British Crown Copyright 1989. Reproduced with permission of Her Britannic Majesty鈥檚 Stationery Office.
