Interactions between Marine Mammals and Fisheries.
1.1 Background to the problem:
In 1972 COFI, FAO's Committee on Fisheries, taking account of the concern over some marine mammal stocks and their exploitation, recommended that FAO's Advisory Committee on Marine Resources Research should carry out a study of this problem. As part of this study ACMRR organised a meeting in Bergen in 1976. Papers presented there included some indicating the importance of the interaction between man and marine mammals in the sphere of fisheries.
At around the same time (1973) the IWC decided to set up a Small-Cetacean Sub-Committee, and in 1974 an IWC meeting was held in Montreal to discuss small Cetacea. Again, a number of the papers presented to that meeting included mention of the specific problem of interactions between marine mammals and fisheries.
In the mid seventies the dolphin culls at Iki Island and the ensuing controversy helped to nudge the IWC Small-Cetacean Sub-Committee into looking directly at this problem and others like it, and in 1976 the first marine mammal-fishery interactions were placed on the agenda of the sub-committee's meeting. Subsequently, the Commission agreed to ask member nations to supply information directly relevant to this problem. In the next few years the increased attention to the problem helped to produced enough concern for the International Union for the Conservation of Nature to organise a meeting at La Jolla in California in 1981 specifically to discuss the interaction between marine mammals and fisheries and to try to assess its nature.
1.2 The La Jolla meeting:
The meeting gathered together a number of papers related to the topic as background papers, and was presented with a total of 22 working papers which were discussed by a group of invited experts. The report of that meeting classified interactions into 5 different types. These were: damage by marine mammals to fishing gear, damage to catch, damage (or death) to marine mammals caused by fishing operations, (these three may be termed ‘operational interactions’), transmission of parasites to commercial fish species from marine mammal intermediate hosts, and predatory interactions. This report surveys these interactions in detail.
1.3 Summary of types of interaction.
Operational interactions.
Operational interactions of whatever type depend on the sort of fishing gear being used and the ecology and behaviour of the marine mammal involved and of the fish being caught. Some types of gear are more prone to being damaged by marine mammals than others, and some inflict more damage on marine mammals than others. Similarly the fisheries for some types of fish are irrelevant to marine mammals, whilst other types of fish are closely bound up with the interaction between marine mammals and fisheries. For example, purseseine nets are more likely to catch marine mammals accidentally than are bottom trawls if marine mammals spend more time near to the surface than the bottom. Similarly, fish such as salmon, which regularly congregate near certain areas are more likely to attract marine mammals, and hence provoke gear conflicts, than are more solitary fish such as sharks.
Types of gear
Gear categories have been described by Nedelec (1982), whose classifications are followed here. ll categories are defined as follows:
Surrounding nets: nets which catch fish by surrounding them from the sides and from below, thereby preventing them from diving below the net. These nets are often very large (e.g. some purse-seine nets), and are often involved in incidental catching of marine mammals which cannot then escape by diving. In tuna purse seines, some dolphins ‘faint’ and, sinking into the bottom of the purse, become entangled and drown. The fact that these nets corral large numbers of fish at high densities and are open at the surface has also led to their being exploited by some marine mammals, especially cape fur seals, which are able to clamber over the float line and into the mass of encircled fish.
Seine nets: operate by surrounding a large area of water with very long nets and then herd the fish toward the point of capture, often entangling them on the way. One of the most common is the beach seine net, operated from land in shallow water and usually set by a boat, the net is dragged in and acts as a barrier to fish inside the enclosed area. This type of net is known to catch coastal marine mammals which feed in shallow waters, such as Tursiops sp., and which become entangled in the net along with shallow water fish.
Trawl nets: towed nets consisting of a cone-shaped net with a codend or bag for collecting the fish. These can be bottom, midwater or surface nets, and are operated from one, or occasionally two, boats. Marine mammals have been known to be caught in bottom trawls, presumably when feeding on fish caught therein, and in surface trawls, which may or may not be coincident with their feeding.
Dredges: gear which is dragged along the bottom usually to collect sessile molluscs. The opening into the collecting net is therefore not usually very large, and the entrapment of a marine mammal would be very unusual.
Lift nets: nets which are first submerged so that they lie horizontally underwater, until fish, which may be attracted by bait or light, are above the net. The net is then lifted by one of a number of means out of the water, taking fish with it. Such nets are normally relatively small and would not usually be involved in operational interactions with marine mammals.
Falling gear: nets which catch fish by falling on top of them. Such gear is usually small and hand operated, i.e. thrown or dropped, and is operated in shallow water. It would be unusual for such nets to catch a marine mammal unintentionally, unless a very small animal was involved, or the operation was in murky waters.
Gill nets and entangling nets: fish are gilled, entangled or enmeshed in such gear, which may be operated as surface, midwater or bottom gear, depending on the ballasting and bouyancy. Gill nets are usually a single sheet of netting, whereas trammel nets have three layers, designed to entangle rather than to gill. These nets are often found in enormous lengths over parts of the oceans, e.g. in the North Pacific, for salmon fishing, and frequently are responsible for the entanglement of marine mammals. The type of gill net used can often be very important in determining the effect on a specific marine mammal. Midwater gill nets for example, may be more likely to ensnare and subsequently drown some species than surface nets, where the animals may still be able to breathe. Also, synthetic nets (not just gill nets though) are more likely to ensnare marine mammals than natural fibres. Such netting is strong and durable, and unlike natural fibre nets is very difficult for an entrapped mammal to break out of; furthermore fragments having broken loose may float around in the sea for a considerable length of time. The relative economy of these nets has also meant that there has been a great proliferation of their use.
Traps: Traps may be large or small, staked or buoyed, wicker or netting, and are usually used only in shallow water. Small wicker or metal traps such as are used on reefs present few problems with regard to marine mammals, but larger pound nets (fixed nets), fyke nets or stow nets may do so. Marine mammals may enter such nets after fish, damage the nets and/or the catch, and may also become trapped themselves.
Hooks and lines: these include handlines, pole-lines, longlines (multiple hooks on long lines) or trolling lines (lines trailed by boat near the surface). These types of line attract marine mammals as they often provide easily taken wounded fish of a medium to large size. Rarely are the marine mammals themselves caught on such lines, but certain species frequently cause annoyance and loss to fishermen in numerous areas of the world, by removing or damaging fish from such gear.
Grappling and wounding: gear for grappling, wounding or killing, including spears, arrows, prongs, tongs, clamps and harpoons. The only involvement such gears have with marine mammals is probably in directed fisheries, where harpoons and spears are used for catching the mammals themselves.
Harvesting gear: includes pumps and dredges, which suck fish or molluscs directly out of the water. This type of gear does not include pumps used in purse seining and other net fisheries, and can only be used in special circumstances and for a few species, where they can either be attracted directly to the pump, or where they can be relied upon to remain in the same location whilst being harvested (e.g. some molluscs and crustacea). There are unlikely to be many instances of operational interaction between such gear and any marine mammals.
The type of gear being used is not the only aspect of the fishing operation which may determine the nature or extent of an operational interaction with marine mammals. The area being fished, whether it is in shallow or deep water may also be important, especially in determining which mammal species are involved. The size of the fishery itself will affect the extent of any interaction, and where the interaction involves a marine mammal feeding on the same species as is being caught, then obviously the nature of the species is also important.
Types of fish - types of marine mammal.
Numerous factors in the behaviour and ecology of marine mammals and their prey are involved in determining the extent of the operational interaction. These factors may broadly be divided into two groups. Firstly the habitat of the marine mammal and the fish may be an important group of considerations. An inshore fish stock is therefore unlikely to involve any interaction between the associated fishery and an offshore marine mammal population. Similarly a demersal fishery may be less likely to involve marine mammals which are primarily pelagic than those which may feed on demersal fish, and a fishery for reef fish is unlikely to involve marine mammals which are associated with open water. Other distinctions may be envisaged whereby the habitat of marine mammals and fish stocks either lessens or increases the likelihood of gear conflicts.
Secondly, the availability of the fish may influence the degree to which operational interactions between fisheries and marine mammals may occur, as this may be important in determining what type of fishery is in operation and how the fishery will affect marine mammals. For example, small pelagic and numerous fish are likely to be caught by purse seines or surface trawls, whereas large demersal solitary and rare fish are more likely to be caught on traps or hooks and lines, and this in turn will influence any gear interaction with marine mammal species.
Factors such as those mentioned above may be important in determining the type of gear employed in a fishery and the type of operational interaction with marine mammals, but they may also be even more important in determining the nature of the inter-species, or biological interaction. This is discussed briefly below.
Inter-species interactions.
The transmission of parasites is one type of interaction between marine mammals and fish which has received relatively less attention than other types of interaction. In brief, certain fish parasites are known to use marine mammals in their cycles; commercial fish are then infected, and their value greatly reduced. The most notable example of this is found in grey seals, which harbour the codworm, Porrocaecum decipiens, which also infects cod.
Predatory interactions between marine mammals may be divided into 5 categories:
depleted mammal stocks may not recover due to heavy fishing of their food supply,
mammal stocks which have not been depleted may be threatened by overfishing of their food supply
increasing predation by recovering mammal stocks may start threatening fish stocks which are already exploited,
abundant mammal stocks may threaten continued expansion of a new fishery, or continued existance of a heavily fished stock,
changes in population size of marine mammal may also indirectly affect the population dynamics of exploited species,
In all of the above, the threat posed by marine mammals to fisheries or vice versa can either be ‘direct’ through the medium of a common prey species, or ‘indirect’ via tertiary species which are prey species common to both mammal and commercial fish.
1.4 Aim of this study.
The aim of this study is to catalogue as many reported interactions as possible and attempt to identify potential ones, with regard to the general impact that such interactions may have on either the fishery or the marine mammal population. To this end a brief summary of the distribution and feeding habits all nominal 114 species of marine mammals is presented. This is followed by an area by area survey of the world's marine areas, listing the mammal species present together with brief notes on the main fisheries and the known and potential interactions between the two. Finally an attempt is made to draw some general conclusions.
1.5 Notes on the text.
i) Species described.
The text includes discussion on all species of extant Cetacea, Sirenia and the pinnipeds, including mention of one phocid species presumed to be extinct. For the sake of wholeness and convenience, those species of these groups which are wholly or partly freshwater have been included, on the basis that they belong, taxonomically, to an order or family that is characteristically marine.
Other species, notably the ‘marine otters’, which are primarily freshwater but which inhabit marine environments have been excluded for the corresponding reason.
ii) Taxonomy.
The species described follow the taxonomy of the following authorities: Cetacea - as described by the IWC, including the list of small cetaceans currently recognised, supplied by the Small-Cetacean Sub-Committee, (IWC 1977) with minor subsequent amendments. Sirenia and the pinnipeds - as described in the FAO/ACMRR reports of the 1976 Bergen meeting on Marine Mammals, and in the volumes “Mammals in the Sea”. Fish names are taken, where appropriate from the FAO Yearbook of Fisheries Statistics - Catches and Landings (FAO 1983).
iii) Distribution.
Each species described in the text is followed by a list of the FAO statistical areas in which it has been recorded. Area numbers bracketed indicate doubtful or unconfirmed presence.
iv) Statistical areas.
The statistical areas covered are the FAO statistical areas for major fishing areas of the world, as described in the FAO Yearbook of Fisheries Statistics. These areas are illustrated in the map on page 191.
