Throughout this report a distinction has been maintained between two categories of interaction. These have been termed ‘operational’ and ‘biological’ interactions respectively, and are defined and discussed in the Report of the IUCN Workshop on Marine Mammal/Fishery Interactions (IUCN 1981). Whereas the distinction between such categories is often blurred, it offers a useful platform on which to base an analysis of interactions between marine mammals and fisheries. In what follows some generalizations are attempted concerning the operational and biological interactions which have been reviewed.
Operational interactions
The records of operational interactions cited in this report suggest that no species of marine mammal can be excluded from the possibility of some conflict with fishermen. Inspection of the table on pages 165–7 indicates that most species, with the exception of the ice seals of Arctic and Antarctic waters, and most of the ziphiid whales, have been involved in operational interactions to some extent. The absence of reported interaction involving these species might be attributed to the scarcity or absence of fisheries in open ocean and polar waters.
Whereas some species appear to interact regularly with particular types of fishery throughout their ranges, the occurence of several highly improbable interactions prevents any overall categorization. The case of humpback whales annoying salmon fishermen in the Soviet Far East, and the reported presence of beaked whales on Japanese tuna longlines are two such examples.
The lack of adequate data concerning most recorded interactions also imposes limits on the extent to which these can be analysed or categorized. It is therefore generally impossible to establish the global extent to which any particular species or fishery is involved in operational interactions. As might be expected, however, coastal species are most often cited in reports of conflict, with gill nets in particular being responsible for a high proportion of all incidental captures of marine mammals. The data are also too sporadic for an analysis of the fish species most often involved in operational interactions. High value fish such as tuna and salmon figure prominently in the reports as these are the species most often fished for with gill nets, and are also the ones which, when damaged in the nets, provoke the most vociferous complaints from fishermen.
The most frequently reported form of interaction is undoubtedly the incidental capture of marine mammals, especially small cetaceans. Depredation of the catch and damage to gear are by no means infrequent; these are more important with regard to pinnipeds. The existence of a third category of operational interaction should not be overlooked. A few species of small cetacean have been reported to assist fishermen in the capture of fish, which may represent the one area where the interests of fishermen and marine mammals coincide.
The available data on operational interactions are in general patchy. Throughout much of the world small scale and artisanal fisheries are, and to some extent at least, probably always have been, in conflict with a number of marine mammal species. For many of these fisheries there is little or no information on the problem, beyond often almost anecdotal references to incidental captures. Exceptions to this are confined largely to areas near to fishery research stations, especially in Japan, North America and Europe. Away from such places, in much of South America, Asia and Africa, scattered coastal communities fish over extensive areas, where data collection may be very difficult. The collection of data may also be made difficult where fishermen are not well disposed towards marine mammals.
In contrast to this situation, some large scale or industrial fisheries have yielded extensive data on the capture of small cetaceans. Notable examples of this are the U.S. tuna purse seine fishery in the Eastern Tropical Pacific and the Japanese drift net fishery for salmon in the North Pacific. Although other fisheries, such as some of the industrial fisheries off the west coast of Africa, as yet provide little data, the collection of data from such enterprises would appear to be much easier, in principle, than from small scale fisheries.
Much of the reported interaction between marine mammals and fisheries does not appear likely to have a particularly significant effect on either the economy of the fishery or the population dynamics of the marine mammal species. Those examples where the conflict may be considered to be significantly affecting either the fishery or the marine mammal population may be characterized in one of two ways. There is either an important loss of fish or of gear suffered by the fishery, or else the number of marine mammals caught in the gear represents a relatively high proportion of the total population. For example the rate of catch may exceed the rate at which the population is able to grow. Examples of the first are provided by the coastal fisheries in Newfoundland, where cod traps damaged by whales may cost thousands of dollars to replace or repair. In Scotland, the Baltic and the North Pacific damage to catches of salmon also represents an appreciable loss to fishermen. Examples of the second case may be provided by the incidental mortality of thousands of Dalls propoises in the North Pacific.
The impact of incidental captures of marine mammals is accentuated where the population is small or localized. For example river dolphins may be confined to a relatively small area and any incidental capture of these species may have a considerable affect on the population, which may not be very large in the first case. Some species of severely depleted baleen whale could also be affected in much the same way by any incidental captures. Dugongs and possibly some other species, being relatively localized in their distribution, may be prone to local depletion through incidents in fishing nets.
In an analogous manner, the impact of marine mammal depredation on commercially valuable fish and damage to fishing gear is accentuated where the fishermen are most dependent on the income from their fishing activities. Where opportunities of employment in other areas are readily available the effect may not be as severe as in a community totally dependent on fishing for its income.
In many areas increased pressure on fish stocks would seem likely to lead inevitably to further conflicts with marine mammals. In some areas offshore or remote fish stocks have yet to be exploited, and these may bring fishermen into conflict with marine mammal species, including those which at present are not reported to be involved in any interactions. Perhaps more significantly an increase in conflict between marine mammals and fishermen might be expected from the increase in the use of synthetic monofilament gill nets, which has been noted by a number of authors (Mitchell 1975, Gaskin 1983, e.g.). Such nets are used to catch a great range of fish species, but their relative invisibility to marine mammals, together with their strength and durability, means that marine mammals are increasingly in danger of entanglement and drowning. The problem is exacerbated where synthetic nets break away from their moorings and whilst no longer producing a useful catch, may drift around the sea for years, entangling and killing both mammals and fish. The problem exacerbated where synthetic nets break away from their moorings and whilst no longer producing a useful catch, may drift around the sea for years, entangling and killing both mammals and fish. It would seem inevitable that as the use of these nets increases, so too the problem of incidental marine mammal mortalities will intensify.
Just as increased fishing effort may lead to further conflicts with marine mammals, so too, expanding marine mammal populations may lead to increased conflict with fisheries. Although examples of this may be rare at present, there are a number of baleen whale and seal stocks which are in the process of expanding, and this cannot be excluded as a potential source of conflict in the future.
Marine mammal populations, however are unlikely to expand at more than a few percent per year, whereas fisheries may expand by hundreds of percent a year. The emphasis on monitoring conflicts between the two might therefore sensibly be concentrated on monitoring changing fishing patterns rather than changes in marine mammal populations.
For most fisheries little or no monitoring of interactions with marine mammals occurs at present. Whereas in some fisheries the scale of the bycatch itself might be enough to indicate a serious affect on the marine mammal population, in many cases it would be necessary to collect records not only of the numbers of animals caught, but also of the age structure of the catch before any assessment of the affect of the fishery on the population would be possible.
The assessment of damage by marine mammals to fisheries has been attempted in a number of fisheries, particularly in areas where fisheries are more developed. Such assessments may either take the form of regular inspection of the catch, or of the gear, for marine mammal damage. Unfortunately it is very difficult to obtain accurate data on the loss of revenue to a particular fishery as many immeasurable quantities are involved when trying to make such an assessment. Hopefully future studies may at least provide some baseline data from which comparisons may be made with damage levels in other fisheries.
As yet there appear to be no studies which have attempted to measure the possible benefit to fishermen from cooperation with marine mammals. This is clearly one form of interaction which will remain very difficult to quantify.
Where they are practicable, the solutions to operational conflicts seem likely to be technological. Monofilament nets have already reduced the amount of damage to some net fisheries (albeit wile increasing incidental catches of marine mammals), and changes in gear design and fishing practice have greatly reduced the numbers of small cetaceans taken in the U.S. Pacific tuna fishery. The reduction of marine mammal population sizes may be unlikely to benefit fishermen with regard to operational interactions, as a few animals can be responsible for most of the damage. An example is seen in the case of the grey seal which is responsible for similar levels of damage to salmon fisheries in Scottish and Baltic waters, despite an order of magnitude difference in the sizes of the two populations.
Technological solutions, where possible, seem likely to include the development of seal
scaring devices and netting which is more visible to marine mammals, without affecting its ability to catch fish. The development of such devices must be considered a priority in the solution of these problems.
Biological interactions
If an increase in fishing effort is likely to produce an increase in the number of operational interactions, the concomitant increase in fishing mortality might be seen as likely to increase the likelihood of biological interaction. Indeed the two may proceed together. It may be that as a fish stock becomes less abundant, marine mammals, particularly seals, may come to regard fishing nets as the most economic source of food.
The biological interaction between marine mammals and fisheries might be seen as competition between two predator species for a common prey, but the extent of the competitive overlap in feeding ‘niches’ is invariably extremely difficult to assess. A species which for example feeds in part on a commercial fish species may not be preying on the same population of that fish as the fishery. Most marine mammals also have extremely varied diets which may change not only from year to year, but also from month to month, area to area, and from one year class to the next. This variability in diet makes the extent of any competition very difficult to assess.
The variability in the diet of most marine mammals may also provide some form of safety valve in the competitive overlap with fisheries. The efficiency of many modern fishing methods may mean that a depleted fish stock becomes an energetically unappealing food source for marine mammals long before it becomes economically unattractive to the fishery, provoking marine mammals to feed on alternative food species.
The exception to this possibility arises where a marine mammal is more or less monophagous, or where it feeds on a group of food items which is affected by a fishery. Such marine mammal species are most vulnerable to competition with fisheries, as any change in prey abundance cannot be met with an appropriate change in diet.
Conversely, specialized fishermen are also vulnerable to marine mammals. In theory a marine mammal population, particularly an expanding one, could reduce either the average size of a prey population, or the number of prey in the population, to such an extent that fishing for that species becomes uneconomic. Multispecies fisheries, such as the increasingly common gill net fisheries, avoid the consequences of such competition as they fish for a wide variety of species.
Viewing marine mammals and fisheries as two predators in an ecological system highlights the fact that not all biological interactions between the groups need necessarily be competitive. Even for those which are, it is very unlikely that the two groups will cover exactly the same ‘niche-space’. In many cases the ramifications of the predator prey complex may serve to reduce the amount of competition between the two groups.
In such circumstances the ecological effect of one group could be beneficial to the other, by for example, reducing the numbers of a predatory fish, which may affect the abundance of another, target, or prey species.
The above considerations suggest that it is very unlikely that there will be a weight for weight equivalence in the prey of marine mammals and the yield of a commercial fishery. This also makes it very difficult to assess the effect of one group on the other, and consequently difficult to define a situation where the reduction of marine mammal stocks may be justified for fishery protection, or where a reduction in fishing effort may be justified for the protection of marine mammal food supplies. Clearly, however, care must be exercised when considering a marine mammal population which is not numerous or which is confined spatially or ecologically. Care must also be exercised in considering a fishery which is ‘confined’ in a similar way, as if for example only one relatively uncommon species is being caught.
As fishing pressure intensifies globally and as marine mammals are more frequently brought into conflict with fisheries, demands for investigation into the competitive overlap between the two groups is likely to grow. The recovery of certain marine mammal stocks may produce the same demands. In such circumstances the neccessity for increased monitoring may become paramount. This may be particularly important in the development of squid fisheries, where a vast potential exists for squid fishing, and for competitive interaction with marine mammals. An early appreciation of the possible problems in such a development would clearly be desirable.
The review presented above suggests that very little analysis of the biological interaction between marine mammals and fisheries has been possible to date. This has largely been because of the complexity of the problem. Fishery interests have frequently complained that marine mammals are responsible for the depletion of fish stocks, particularly evident where marine mammals are involved in gear interactions and are seen to eat commercial species, or are increasing in numbers. It would appear that to date no deleterious effects on fish stocks have been wholly substantiated by subsequent analysis.
The complexity of the interaction is not the only impediment to such studies, and another major feature in this problem is that feeding data for most species of marine mammal are extremely poor. Much information is semi-anecdotal, and may even be heavily biased through a very few observations. It is clear therefore that before any adequate analysis of the problem of biological interaction between marine mammals and fisheries is possible, considerably more data on the diets of marine mammals needs to be collected. In itself, however, such information will not necessarily be of much use. What is perhaps more important for the understanding of this problem is a knowledge of the feeding stategies of marine mammals, and much more about the inter-species population dynamics of the fish and marine mammal species concerned.
