The relationship between marine mammals and fishermen is one of conflict. Both parties are after the fish. Often the same fish. Marine mammals get themselves caught in the fishermen's nets and are drowned. Marine mammals are the host of parasites that end up contaminating the fish, causing substantial losses for the fishing industry. Fishermen demand a reduction in the size of marine mammal stocks, or compensation for their losses, while «green» organisations demand the regulation of the fisheries in order to ensure more food for the marine mammals, or to prevent them being killed by the fishermen's tackle.
It is the task of management authorities to decide between these conflicting demands. The first problem they are met with is a lack of information.
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Research efforts have only recently begun to come up with good answers about what, and how much, marine mammals eat. These answers vary considerably between the various areas of the oceans. On the continental shelf off the west coast of the US, marine mammals take approximately twice as much fish as the fishermen, whereas in the North Sea they take only a few percent of what the fishermen haul up. However, in the whole of the northern part of the North Atlantic, the marine mammal consumption of fish is, in general, of a similar magnitude to that of the fishermen. It remains for the scientists to analyse what impact the marine mammal consumption of fish has on the fisheries and, conversely, what impact the fisheries have on the development of marine mammal stocks. This can be a difficult sum to add up, with many different and uncertain factors to take into consideration. (See below: The Seals help themselves, the fishermen are driven ashore.)
In certain countries such as the US, considerable progress has been made towards registering the number of marine mammals taken as unwanted by-catch in fisheries. Other countries have only just begun such research. Intense research efforts are also being made to devise measures to avoid such by-catches. Knowledge of the size of the various marine mammal stocks and their reproductive capabilities is necessary in order to ascertain whether such by-catches pose a threat to stocks.
However, the fishermen's tackle is not merely a potential source of danger to marine mammals. It can also make it very easy for them to get hold of food. Seals and whales help themselves to fish from the fishermen's tackle. In some places, this is such a major problem that the fisheries are unprofitable.
Every year, the fishing industry loses vast sums of money due to parasites, such as cod worms, growing in the flesh of fish. A stay in the digestive system of marine mammals is an essential part of the cod worm's life cycle. In the filleting factories, those parts of the fish infected with parasites are cut away. In certain coastal areas where seals abound, there are so many parasites that the fish halls refuse to buy the fish.
How administrators will relate to the information that is gradually being made available about the interaction between fisheries and marine mammals will depend on the political objectives that have been drawn up for the management of these resources. American legislation favours marine mammals. Here, the nutritional requirements of marine mammals are taken into account first, before the size of the fishermen's quotas is established. In Norway, however, the seal hunt is maintained with the help of subsidies in order to retain the possibility of controlling the size of seal stocks for the sake of the fisheries.
At the heart of every country's management system is the obligation to maintain healthy and abundant stocks of all species in the marine ecosystems. This obligation, however, does not prevent the Canadian government from deciding whether there should be 5 million or 3 million harp seals off the coast of Newfoundland.
THE SEALS HELP THEMSELVES, THE FISHERMEN ARE DRIVEN ASHORE
In Newfoundland, the ban on cod fishing is strictly enforced. A catch of 15 cod has proven enough to have fishermen's boats confiscated and have them severely fined. At the same time, 4.5 million harp seals helped themselves to about 140,000 tonnes of the same cod stock in 1994. That is the equivalent of 2 billion codfish.45
The seals, however, can in no way be blamed for the decimation of the cod stocks off Newfoundland. That is the responsibility of the fishermen and the authorities in both Canada and Europe.
Those suffering most from this, however, are the people of Newfoundland, where a soaring unemployment rate and increasing social hardship have followed in the wake of the collapse of the cod stocks.
In spite of protection measures, a growth in the cod stock is long in coming. The question is whether the harp seals - in such an extreme situation as this, where the cod stock is at a minimum - are hindering the recovery of the cod.
The harp seal stock has more than doubled over a period of 15 years. Since cod only comprise a small part of their diet, the seals can live comfortably on a main diet of other species and still eat cod for dessert. Organisations opposed to sealing raise doubts as to whether the seals really do have a negative impact on the cod fisheries. Some suggest that the seals eat fish that normally feed on cod, and thus have a positive effect on the cod stock. There is, however, no indication that this hypothesis is correct. So far, it has not been possible to identify a cod predator that is eaten in such large quantities by the seals that it can be said to compensate for the seals' own consumption of cod. It has also been suggested that the cod that manage to avoid the jaws of the seals do not necessarily end up in the fishermen's nets, but may end up in the stomachs of other predatory fish. This is hardly likely though, since the seals help themselves to larger cod than other cod predators, i.e. cod that have reached an age of 1-2 years and are 10-20 cm in length.
Research is now under way that will hopefully provide a basis for conclusions with regard to the harp seals' impact on the growth of the cod stock. First and foremost, we lack information on the distribution and abundance of cod up to the age of two years, and the predators that prey on cod of this size. Pending the results of this research, the Canadian authorities have stepped up their sealing efforts to a level that will limit the growth of the seal stock.
There is a similar conflict going on in the Bering Sea, but this time in reverse. Here the stock of Steller sea lions has been drastically reduced over the past few years. Greenpeace claims that this is a result of the commercial fisheries depriving them of their food. On the other hand, the American fisheries authorities claim that there is no scientific proof that fishing is the reason for Steller declines.
The attitudes of certain «green» organisations towards fisheries and marine mammals might appear inconsistent. They focus on the rivalry between the fisheries and marine mammals to eat the fish, while at the same time denying that marine mammals are in competition with fisheries.
A SIMPLER WAY OF SEEING THINGS
The various species in marine ecosystems eat one another, one another’s prey and one another’s predators - inter-changeably. It is therefore difficult to quantify the role of marine mammals in this complex system, and to establish the effects on fish - and fisheries - should stocks increase or decrease in number.
There is, however, a simpler way of seeing things. Primary production, through photosynthesis, in the form of plankton, is a limiting factor on the size of the biomass further up the food chain. For each link in the food chain, approximately 90% of the energy is lost. In view of this, it is possible to calculate how great a share of primary production is occupied by marine mammals on the various levels of the food chain. This share will, of course, vary with the size of the stocks. Should we decide to maintain large stocks of marine mammals, while at the same time deciding not to harvest these stocks, then an entire segment of the flow of energy through the food chain will be inaccessible to the fisheries. Such a management system will, in other words, reduce the possible sustainable yield of the marine ecosystems.
Norwegian scientists weigh cod found
in the stomach of a minke whale.
Notes:
45. The figures stems from the Report on the Status of Harp Seals in the Northwest Atlantic, Science Branch, Northwest Atlantic Fisheries Centre (St. Johns, Newfoundland, 1995).
