TL: SEAL-FISHERY INTERACTIONS A paper produced by Greenpeace International to describe the relationship between seals and fish (GP) SO: STICHTING GREENPEACE COUNCIL GREENPEACE international Keizersgracht 176 Telephone (31) 20 5236555 1016 DW Amsterdam Fax (31) 20 5236500 The Netherlands Telex 18775 GPINT NL DT: August, 1987 Keywords: oceans fisheries greenpeace groups gp reports seals marine mammals / SEAL-FISHERY INTERACTIONS: BACKGROUND TO THE ISSUES Introduction Predatory Interactions I Food Consumption of Seals ii Impact of Seals on Fish Stocks III Will Removing Seals Increase Fishery Yields? Parasite Transmission Operational Interactions Conclusions SEAL-FISHERY INTERACTIONS: BACKGROUND TO THE ISSUES INTRODUCTION Pinnipeds (including sea lions, fur seals, true seals and walrus) are large mammals that must come to the surface to breathe,, and gather on land or ice in large numbers to breed. They are thus highly visible to humans. Since many species include fish in their diet, and have been known to interfere with fishing operations, they are commonly assumed to be in conflict with commercial fisheries. This perceived competition is often used as justification for such "control programmes" as commercial hunts or culls. By blaming seals for declining yields, the fishing industry is able to avoid dealing with such problems as overfishing, poor management and the effects of pollution on fish reproduction and survival. Seals thus become a readily available scapegoat for the many serious problems which affect modern commercial fisheries. This issue has become very emotional lately, and much of the basic background information has gotten lost in the shuffle. There are three types of interaction between pinnipeds and fisheries. One is competition for fishery resources, or predatory interactions. This argument depicts seals as "vacuum cleaners of the sea and accuses them of eating fish which rightfully belong in the nets of fishermen. Another involves parasites: since pinnipeds are necessary for the completion of the life cycle of certain fish parasites, it is frequently assumed that fewer seals would result in fewer parasites in the fish. The third is often termed operational interactions, and includes damage to fishing gear and catches and the incidental killing of pinnipeds. In most areas of the world where seals are found, one or more of these arguments has been used as the basis for the fishing industry's demands for the reduction of seal populations. Each of these aspects will be discussed in turn. It should be noted that, though this brief deals exclusively with pinnipeds, similar situations may exist for other marine mammals (cetaceans, sea otters) and other animals (sea turtles, sea birds). PREDATORY INTERACTIONS Many seal species consume fish, often in a highly visible manner. It is thus easy to see why fishermen assume that they are adversely affecting the yield of commercial fisheries. However, as the IUCN Workshop on Marine Mammal/Fishery Interactions expressed it, "It is one thing for fishermen or scientists to observe that the marine-mammal in question can and does eat commercially species; it is another to establish whether the marine mammal population is having an appreciable effect on the abundance of its prey species, and yet another to convince commercial fishing interests that the marine mammal is not reducing the commercial productivity of the fish stock. It is even more difficult to reliably predict the long-term benefit that would accrue to the fishery if the marine mammal population were to be reduced as a control measure, and to convince other interests of the validity of such assessment." The issue of the impact of seal populations on commercially important fish stocks may be considered in a series of questions: how many fish do they eat, of what species, and in what location? What impact does the seal population have upon fish stocks, and how much of these fish would become available to the commercial fisheries in the absence of seals? I Food Consumption of Seals One of the most frequent complaints against seals is that they have huge appetites. This claim has been based on the assumption that, since seals frequently live in cold water, they must consume large quantities of food in order to keep warm. Many scientists and fishermen assume that seals eat vast quantities - often up to 10% (or even 27% in one case) of the animal's body mass per day. However, a series of papers by David Lavigne, Stuart Innes and others at the University of Guelph has shown that food consumption rates of marine mammals are far lower than previously thought, typically in the order of 1-3% of their body mass per day for adult animals. In other words, statements that seals possess "voracious" appetites are without foundation in reality. Another misconception is that seals consume only commercial fish species such as salmon or cod. In fact, pinnipeds are usually opportunistic feeders, feeding on whatever kinds of food are readily available. Studies of stomach contents of seals show that they consume a wide variety of fish and invertebrate species, many of which are of no commercial importance. For example, the only published study of the food habits of harbour seals in British Columbia identified a total of 29 different food items. There are many difficulties in determining what seals eat. Most estimates are based on analyses of stomach contents or faeces. However, some soft-bodied prey types, such as squid and octopus, are digested more rapidly than bony fish. Similarly, the remains of small fish such as herring and capelin will disappear more quickly than those of larger cod and salmon. These differing rates of digestion mean that commercial species, which are usually large fish, will be over-represented in stomach contents while small fish and invertebrates may appear in samples less frequently than they are actually eaten. The location of the food consumed must be known. In order to properly reflect the overall diet of the seals, samples must be taken throughout the year and from all parts of the species' range. In the harbour seal study referred to above, a large percentage of the stomachs contained salmon. This is due to the fact that many of the samples were taken near river mouths during the salmon spawning season. It does not reflect a heavy dependence on salmon throughout the year. Since many seal species either migrate seasonally or disperse over wide distances to forage, much of their prey may come from areas in which there is no commercial fishery. For instance, a simulation model constructed for harp seals in the northwest Atlantic suggested that over 50% of the total food consumed by the population came from waters in northern Canada, where the seals migrate for summer feeding, This area is outside the range of most traditional fisheries. Obviously, there are many factors to consider in making comprehensive statements about food consumption based on the limited information which usually exists. Others include changes related to the age and sex of the seal, energy content of the food consumed and extensive periods of fasting during the breeding and moulting seasons. Few studies make allowances for them all; indeed, for many of the factors the answers are unknown at present, and investigators must settle for "educated guesses". Yet accurate and reliable estimates of the diet of seal populations require detailed knowledge of all of these parameters. II Impact of Seals on Fish Stocks All of the information discussed in the previous section only describes the seal's basic diet. Reliable estimates of the impact of the seal population on fish stocks also require detailed knowledge of the composition of the seal population. Most important, obviously, is an accurate and precise estimate of the total number of seals in the area. Since seals of different ages and sex consume different amounts of food, and frequently different types of food, the sex and age structure of the population (how many are of what sex and at what age) will affect the calculations. This is particularly true for sexually dimorphic species (in which males and females are of different size), for the two sexes may disperse into different areas to feed after the breeding season. Food consumption also depends upon the reproductive status of the seal, for pregnant and/or lactating seals will consume relatively more food than non-reproducing animals. Many of these parameters are notoriously difficult to estimate, and the scientific literature on marine mammals contains much discussion of the theoretical and practical problems involved. Few species have good data for all of these' factors. Yet without such data, calculations of energy requirements and estimates of the total food consumption of the population will be very unreliable-and biased. For instance, the harp seal model mentioned above suggests that results are particularly sensitive to variations in estimates of population size and activity budgets. III Will Removing Seals Increase Fishery Yields? The assumption behind all arguments that seals eat "too many fish" is that much or all of the fish eaten by seals would become available to the commercial fishery if pinniped populations were reduced or exterminated. The usual tactic is to perform a series of calculations of the food consumption of the population, frequently inflating the appetites of the animals and shifting their diet towards commercial species. The assumption is sometimes made that the entire diet is of commercial importance. Few of the parameters discussed in this paper are considered. Then, with no supporting evidence, it is stated that, in the absence of some or all of those seals, all of the fish they eat would miraculously end up in the nets of fishermen. Such a blanket assumption is not valid. Currently, there is no evidence that the reduction of any seal population will result in increased fishery yields. The question of the results of reducing seal populations is usually approached through mathematical models. Details of these are beyond the scope of this review, but a few points should be noted. (Much of the following comes from Beverton's 1985 paper, and the reader is referred there for more information). Many patterns of interaction can be imagined. The simplest, in which a single prey species is exploited by one commercial fishery and by one seal species (Fig 1.1 from Beverton 1985), is usually too unrealistic to be applicable. Unfortunately, this is the model most often used. However, as components are added to the system to allow consideration of other predators and a broadened food base for the seals, the analysis becomes more complicated. Even the most realistic models depend on detailed and accurate input data, so it quickly becomes impossible to reliably predict the effect of removing seals. An example will illustrate the difficulties. In some areas of the western US, sea lions feed on both lamprey and salmon, while the lamprey are themselves predators of the salmon (Beverton, Fig 1.4). Very detailed understanding of the interactions among all three species is required in order to predict the results of any type of management interference. The removal of sea lions as an attempt to increase salmon abundance could result in more lamprey. Decreasing the fishing pressure on salmon could increase both the lamprey and the sea lions. It is very difficult to know in this case whether the removal of sea lions would benefit the salmon fishery. Beverton describes several other situations, in which the results of interfering in the system would be equally unpredictable. PARASITE TRANSMISSION Pinnipeds are the final host (in which the larvae mature and reproduce) to several parasites which also pass part of their life cycle in fish. This brief will review only codworm (Pseudoterranova decipiens, also known as sealworm). Two other common seal parasites (Anisakis simplex, commonly known as herringworm, and Contracaecum osculatum) will not be discussed, for these species primarily depend on cetaceans for the completion of their life cycle. In addition, they reside mostly in the liver and other intestinal organs of fish, and so are generally removed during normal processing. The codworm problem arises because large numbers of parasites are often found in the fillets of such groundfish species as cod and flatfish. Consumer reluctance to purchase fish that they perceive to be "wormy" could cause marketing problems. In an attempt to avoid this problem the processing industry "candles" the fish: fillets are passed over a light source and visible worms removed. Candling is costly work, and inefficient in eliminating worms, resulting in complaints from the processors about expenses. One important consideration is that codworm is not a medical problem, since worms are killed by freezing and normal cooking procedures. Most of those people who have consumed viable codworm have shown no symptoms. Anisakis, however, can be a medically serious parasite, causing severe stomach pains, diarrhoea and fever in humans. The life cycle of the codworm is complex. Eggs are passed in the faeces of the seal and fall to the ocean floor, where they hatch. They are then consumed by a series of invertebrate hosts (whose number and identity are not yet fully understood). These hosts are in turn eaten by fish. Here, in the fish fillets, the codworm larvae encyst and await consumption by seals, the final host in which the cycle is completed. In many areas of the north Atlantic it has been suggested that the incidence of codworm has increased in recent years. The situation is not that simple, since modern procedures used to detect worms are from two to five times as efficient as methods used in the 1950's and 1960's. Recent investigations also examine a larger portion of the fish body, including the digestive organs, whereas earlier studies examined only the fillets. It is difficult to account for these differences when comparing results between studies. There are many difficulties in interpreting the data which have been collected. In order to demonstrate that parasites are increasing, a well-designed sampling programme must consider many factors which may affect the numbers of worms found. Longterm studies have often found significant variations from year to year. Codworm tend to accumulate in larger fish, so the samples must consist of individuals of similar sizes to be comparable. They must be collected from the same area at the same time of year. These are typical problems with any biological sampling, but very few studies have considered all of these factors in the analysis. In many cases the data are not good enough to support the conclusion that codworm infestation rates are significantly higher now than they were 30 years ago. All seals are not of equal importance in parasite transmission. Grey seals have been,found to harbour from five to ten times as many codworm as harbour seals, and both have more than harp seals. Codworm also grow bigger (80 mm), produce more eggs (over 25,000 per day) and live longer (5-10 weeks) in greys than harbours. Since pinnipeds are the most important final host of codworm, the most frequently proposed solution to the problem is to reduce the numbers of seals. However, the results of such action are difficult to predict. A relatively small number of seals is quite capable of maintaining a high level of codworm infestation, since some species have high numbers of worms (grey seals may have an average of 1-3,000 worms in some areas, and individuals with 10,000 have been reported!) and range over wide areas. Migratory stocks of fish could acquire loads of worms in one area, even after local eradication of seals in other areas. The length of the life cycle further complicates the situation. It is quite conceivable that individual worms could persist in a fish for a number of years, so that even if all seals were eliminated from an area, and no more eggs were produced, it would be a very long time before the incidence of worms in fish began to noticeably decline. A complete new crop of fish, consisting of young, uninfected individuals, would have to replace the older generations. This could take a decade or more. Uncertainty in so many aspects of codworm biology make it difficult to predict the results of culling numbers of seals. It is not clear, then, that a limited reduction in seal numbers in an area would result in a reduction in codworm infestation levels within a reasonably short period of time. As the recent Malouf Report on the Royal Commission on Seals and the Sealing Industry in Canada stated, "The dynamics of a parasite with several intermediate hosts are complicated, and it is unlikely that the frequency of parasites in one host (e.g. cod) will be related in any simple way to the abundance of one of the other hosts (e.g. seals)." In other words, reducing the seal population by, say, 20% will not lead to a 20% reduction in codworm infestation rates. Alternative solutions exist. Removal of the flaps (heavily infested part of the fillet that surrounds the body cavity) may reduce numbers of codworm in some species of fish. Improvements could be made in the candling procedure (optimizing lighting and schedules to avoid worker fatigue). New techniques such as sophisticated ultra-sound detection and laser removal systems are currently being explored by Europeans. Finally, the economic results of parasite infestation are not all negative. The candling procedure often provides employment to many people in areas which are economically depressed. Nor should all of the costs of candling be attributed to codworm. Many fish would have to be examined for such things as bones, blood clots and other parasites, even in the absence of codworm. Estimates from economic analyses of the fishing industry in Atlantic Canada suggest that the total cost of candling is approximately 2-3% of the total value of the fishery. Such a figure is too small to justify drastic reductions of seal populations. OPERATIONAL INTERACTIONS Reliable information on the extent of damage caused by seals is difficult to get. Most estimates are based on data supplied by fishermen responding to a survey or in dock-side interviews. A few comparisons are available between fishermen's figures and information collected by independent observers placed aboard fishing vessels. In some cases agreement was good, but in many instances the fishermen's figures exaggerated the amount and extent of the damage. It has also been suggested that fishermen from areas with heavy damage respond more readily to surveys than those who experience few problems. This results in an increase in the overall estimates of damage. Much more data are needed, but most surveys suggest that damage to gear is in the order of a few hundred dollars per year per fisherman, and loss of fish ranges from 1-8% of the total catch. However, these figures have a high variance, so while some fishermen may experience negligible losses, the damage may be substantial to others. In many instances, the costs of the damaged gear is much less than the loss of fishing time while the nets are being repaired or replaced, The type and extent of damage by pinnipeds varies according to the fishery: factors involved include gear type, season, fish species and location. Static gear (gill nets, long lines, fish traps) are often more vulnerable than mobile gear (trolled hooks, trawls, purse seines). For some of these gears, when they are located in certain areas, the impact of seal predation can be very severe. For instance, gill nets spread across the mouth of a salmon spawning river often experience very high losses to seals. Research into non-lethal alternatives such as modifications of fishing techniques and gear could be very effective in minimizing the amount of interaction. other possibilities include acoustic devices to scare seals away, or, alternatively, to attract them to other areas. As with the other two types of interaction, there probably is not a direct relationship between the number of seals and the amount of damage they cause. In some areas an increase in the number of seals has not resulted in an increase in the amount of damage to fish. One method of dealing with the losses which do occur is government compensation to the fishermen. Alternatively, the marketplace could absorb them through increases in the price of fish. Damage to gear and catches would become another cost of conducting a fishery. Wholesale reduction of pinniped populations is not necessarily the most effective method of dealing with the problem. It has been suggested that much of the damage is wrought by a relatively small number of marauding "rogue" seals. Unless the cull happened to include these individuals, the damage could continue despite large numbers of animals being killed. CONCLUSIONS Pinnipeds all over the world are plagued by a wide variety of problems. Organochlorines are affecting the reproduction of harbour seals in the Netherlands. South African fur seals are commercially hunted. Northern fur seals are declining in numbers, possibly due to entanglement in fishing gear. one species of monk seal is extinct, and the remaining two are almost gone. Some populations of southern elephant seals are declining, for unknown reasons. In addition to these specific threats to the health of the world's seals, there are such insidious ones as marine pollution and loss of prey due to overfishing. While seals may contribute to some of the woes of fishermen, there is no justification for killing them in large numbers as scapegoats for problems with fisheries. Pinnipeds are an integral part of a healthy, functioning marine ecosystem. As such, they deserve our respect and protection. The same factors which cause problems for seals also cause problems for all other components of the marine ecosystem. Those who are concerned with the health of the world's fisheries should concern themselves with all of the truly serious threats to the marine environment.