TL: "Harp Seals and the Northern Cod Stock" (GP) SO: STICHTING GREENPEACE COUNCIL GREENPEACE international Keizersgracht 176 1016 DW Amsterdam, The Netherlands DT: October 20, 1989 Keywords: oceans fisheries greenpeace groups maritimes canada gp / Dr. Leslie Harris, Chairman, Northern Cod Review Panel, Northwest Atlantic Centre, P.O. Box 5667, St. John's, Newfoundland A1C 5X1 Dear sir: Please find enclosed a submission by Greenpeace International to the Northern Cod Review Panel, entitled "Harp Seals and the Northern Cod Stock." The brief reviews the published literature on harp seal diets and food consumption rates, to address the question of whether or not this species of seal could have an impact on the northern cod stock. Both direct interactions, through consumption of cod, and indirect ones, through competition for capelin resources, are examined. It is concluded that the evidence currently available does not provide any support for the suggestion that harp seals have had any impact on the abundance of the 2J3KL cod stock. An examination of the diet of harps reveals that Atlantic cod form an insignificant component. While capelin is important for the seals their consumption of this species is also minor, when viewed in terms of overall predation by other species. Furthermore, capelin stocks have increased in overall abundance in recent years. While there may be demands for a reduction in harp seal numbers to help the northern cod stock, among other reasons, there is no scientific justification at all for such action. I trust that the Panel will consider the evidence contained in this brief when evaluating the various factors which have been suggested as playing a role in the dynamics of the 2J3KL cod stock. Sincerely, Michael Earle Seals/Fishery Interactions Coordinator Greenpeace International HARP SEALS AND THE NORTHERN COD STOCK Brief to the Northern Cod Review Panel Submitted by Michael Earle Seals/Fishery Interactions Coordinator Greenpeace International October 20, 1989 In February of 1989, The Department of Fisheries and Oceans (DFO), announced that the most recent estimate of stock size of northern cod (in NAFO Statistical Areas 2J3KL) that it had received from the Canadian Atlantic Fisheries Scientific Advisory Committee (CAFSAC) was significantly lower than earlier estimates. This implied that, if the DFO's stated goal of FO.1 fishing mortality was to be achieved, the total allowable catch (TAC) should be cut by one half over the earlier level of 266,000 tons. As a result of this finding, the Minister of Fisheries and Oceans, Mr. T. Siddon, imposed an interim TAC of 235,000 tons and created the Northern Cod Review Panel, chaired by L. Dr. Harris. It was given the task of reviewing the scientific advice provided for the management of northern cod since 1977, and asked to provide recommendations for the improvement of stock assessment methods for forecasting the size and growth potential of the stock. Among several specific requests, it was asked to examine the possible role of natural phenomena and other environmental factors in the apparent change in the status of the cod stock. Atlantic cod are often thought to form a predator-prey complex with harp seals and capelin (TGNIF 1987), and certainly harp seals have been publically accused of playing a role in the changed fortunes of the northern cod stock. However, it is worth noting that the earlier Task Group on Newfoundland Inshore Fishery (TGNIF 1987) mentioned seals only as another component of the food web, and did not implicate them as significant predators of northern cod. A subsequent DFO publication (Anon 1988) similarly mentioned that cod is not a significant food of harp seals. Finally, the word "seal" does not appear in the interim report of the Harris Task Force (Harris 1989). Considering the controversial status of the relationship between cod and seals, a review of our present understanding of the role of harp seals in the northwest Atlantic is warranted. This brief will review the published data on harp seal distribution and feeding habits, with a view to evaluating the suggestion that they may have a significant impact on the status of this problematic stock of cod. Harp Seal Distribution The distribution and migration of harp seals (Phoca groenlandica) has recently been summarized by Lavigne and Kovacs (1988). They arrive on the summer feeding grounds in the eastern Canadian Arctic in mid-June, when the fast ice is still extant. Some animals penetrate as far as Southampton Island in Hudson Bay, or as far north as Thule, Greenland, though most remain in an area ranging from western Greenland, around the east coast of Baffin Island, Hudson Strait and Lancaster Sound. There is a tendency for young animals to remain closer to the west coast of Greenland. The ice begins to form in late September, and most harps have left the high Arctic by early October, moving south ahead of the advancing winter ice. They enter the Newfoundland-Gulf of St. Lawrence area in December, and by late February, the seals have congregated on the ice off the northeast coast of Newfoundland (the "Front") and near the Magdalen Islands (the "Gulf"). Here the females give birth and nurse their pup for a period of just under two weeks. After weaning, the females mate with the males which have recently arrived in the area. The population (except for the pups) then disperses, and reappears further north for the annual moult. After moulting, both adults and immatures begin the long migration to the summer feeding grounds. The pups, meanwhile, have undergone their own moult after being weaned, and then straggle north as well. Their voyage is made separately from the older animals. Feeding Habits of Harp Seals Food habits of harp seals in the Arctic were described by Sergeant (1973), who suggested that capelin (Mallotus villosus), arctic cod (Boreoqadus saida), and various crustacea comprised their summer food. Finley and Gibb (undated) observed seal feeding behaviour and examined stomachs from seals killed by Inuit hunters in the eastern Canadian Arctic from August to early October in 1978 and 1979. They found that the dominant food was arctic cod, which made up 70% of the caloric content of the taxa commonly found in the stomachs. Polar cod (Arctogadus glacialis) was also found in many stomachs, though this species was relatively far less important than arctic cod (30% of the caloric content). Other items found included seasnails (Cyclopteridae), eel pouts (Lycodes sp.), Greenland halibut (Reinhardtius hippoglossoides) and sculpins (Cottidae). Small crustaceans mysids and amphipods) occurred in only a few stomachs, which was thought to reflect the small number of young seals in the sample. In a more recent study, Kapel and Angantyr (1989) found that the summer (May to October) diet of harp seals from the west coast of Greenland showed greater diversity. Animals born earlier that year consumed primarily euphausids, but switched to capelin during June. For other age groups, the diet varied, with capelin, arctic cod, polar cod, sand eel (Ammodytes sp.), euphausids and amphipods being dominant, depending on the region and season. A sample taken of over-wintering animals (December to February) from central western Greenland showed capelin, euphausids and squid (Gonatus fabricii) to be important diet items, but several other species were found in some stomachs. These included red fish (Sebastes sp.), Atlantic cod (Gadus morhua), Greenland cod (G. Ogac), Greenland halibut, long rough dab (American plaice, Hippoglossoides platessoides), lanternfish (Myctiphidae) and northern prawns (Pandalus borealis). Southwardly migrating harp seals have been little studied, but indications are that capelin, euphausids and herring (Clupea harengus) are sometimes consumed, though many stomachs are empty at this time (Fisher and Mackenzie 1955, Sergeant 1973). An unpublished report by Foy et al. (1981, cited in Bowen 1985) suggested that capelin and arctic cod were the dominant prey species on the Labrador coast, with other gadoids (Gadidae), sculpins, flat fish (Pleuronectidae) and various invertebrates being eaten less frequently. Wintering animals sampled in the Gulf of St. Lawrence near the confluence of the Saguenay (January to April) contained chiefly capelin, with occasional crustaceans and cephalopods (Sergeant 1973). A later sample, taken pelagic ally from the same area during January and February, showed a more varied diet. Capelin still dominated (86% of gross energy in the stomachs), but was supplemented with ten other fish species, including gaspereau (Alosa pseudoharengus), red fish, smelt (Osmerus mordax) and others. Atlantic cod was found in one stomach, and contributed less than 0.5% of the total gross energy found in the stomachs (Murie 1984). Seals netted near St. Anthony between January and April had been feeding more extensively on decapod crustacea (Pandalus sp.), though other species were present in small quantities, including one with Atlantic cod (less than 1.0% of the stomachs examined). Almost 50% of the 126 stomachs were empty (Sergeant 1973). Spring samples of adults taken between March and May near the Magdalen Islands showed a varied diet, including mostly herring, with some flat fish (Fisher and Mackenzie 1955, Sergeant 1973). The few animals that have been sampled from the ice between March and May have mostly had empty stomachs, but capelin, herring, Atlantic cod, flat fish and various invertebrates have occasionally been noted (Fisher and Mackenzie 1955, Sergeant 1973). Stewart and Murie (1986) examined the stomachs of 199 lactating harps in the Gulf, and concluded that nursing females consumed "trivial amounts of energy, and [could] be considered to fast." Fisher and Mackenzie (1955) also found lactating females from the Gulf to contain mostly empty stomachs, as did the few adult males in the same sample. Several beaters (young of the year) and 1-year-old bedlamers taken off Newfoundland in early April contained primarily northern prawns with a few amphipods (Sergeant 1976). Juvenile seals taken in June during the northward migration near Blanc Sablon contained capelin, and older animals taken had either empty stomachs or, occasionally, some herring (Fisher and Mackenzie 1955, Sergeant 1973). Foy et al. (1981, cited in Bowen 1985) suggested that capelin was the dominant prey species on the Labrador coast, with other species found less frequently. These various observations, taken over a period of some 40 years, still leave gaps in our knowledge of the harp seal diet. Few samples appear to have been taken during the months preceding whelping (December to February) or immediately thereafter, after the moult and during the northward migration (May to June). Since females especially are thought to feed heavily before whelping, to prepare themselves for an energetically demanding lactation, these periods are potentially very important in the harp seal annual energy cycle. However, some tentative conclusions can be reached. According to the samples taken by Finley and Gibb (undated) and Kapel and Angantyr (1989), the summer diet is largely capelin, arctic cod, polar cod or euphausids, depending on location and age. In a compilation of the samples taken by Fisher and Mackenzie (1955) and Sergeant (1973) in southern waters, Northridge (1986) found that, of a total of 1555 stomachs examined, 1018 (66%) were empty. The most common foods (listed according to the number of stomachs found to contain that item) were capelin (occurring in 205 stomachs, or 13% of those examined), herring (196, or 13%) and crustaceans (99, or 7%). As Northridge (1986) noted, many of these samples were highly clumped in both time and space, and may not accurately reflect the overall diet of the population. Atlantic cod was found in only four harp seal stomachs, some 0.3% of the stomachs examined or 0.7% of those which contained some food. Since 1978, DFO personnel have sampled approximately 2000 harp seal stomachs, during the period November to June (Winters pers. comm.). These new data corroborate the earlier results, and show capelin, arctic cod, shrimp and euphausids to be the major food of harp seals in the waters around Newfoundland and Labrador. Atlantic cod made up less than 1% of the total by mass (Winters pers. comm.) Data are scant for harp seals in the northeast Atlantic, but the few that are available are in agreement with the Canadian and Greenland results. Amphipods, euphausids, arctic cod, polar cod, capelin and northern shrimp seem to be the most important prey, with Atlantic cod, squid (Todarodes sagittatus) and salmon (Salmo salar?) also appearing (Bjorge et al. 1981, Northridge 1985). Stomachs examined during the "Norwegian seal invasion" of 1987 contained primarily coal fish (pollock, Pollachius virens), as well as some Norway pout (Trisopterus esmarkii) (Ugland 1987). A pelagic sample collected in the northern Barents Sea near Svalbard was dominated by amphipods (Parathemisto libellula) and arctic cod, with Nybelin's sculpin (Triglops-nybelini), Greenland halibut and northern prawns also being found. Consistently, Atlantic cod did not occur in the sample (Angantyr 1988). Harp Seals and Northern Cod Given the limited number of samples which have been taken of harp seal stomachs, especially in southern waters, the number of reviews of the harp seal diet is somewhat surprising. Many of these have provided assessments of the imagined impact of harp seals on the commercial fish stocks in the northwest Atlantic. There are far too many unknowns and uncertainties to come to any reliable conclusions (see Beverton 1985, Bowen 1985, Murie and Lavigne 1985, Northridge 1986). As mentioned above, serious gaps exist in the published literature of the diet of harp seals. Without samples which span the entire geographic range of the population, from all age categories, during all seasons, it is not possible to evaluate reliably what species harp seals eat, and in what proportion. However, the available data (published and unpublished) do not provide any support for the idea that Atlantic cod form anything other than a trivial component of the harp seal diet. This has been a consistent result of samples taken over 40 years, during which time there have been significant changes in the community of species in the northwest Atlantic. Even with a reliable knowledge of the diet of harp seals, an estimate of total consumption requires good data on population size and individual food consumption rates. The energetic demands of seals have often been grossly over-estimated in the past, due to the erroneous assumption that seals require more energy than terrestrial mammals of similar size. Recent studies indicate population energy requirements for seals which are consistent with other mammals (Innes et al. 1986, Lavigne et al. 1982, 1986). An indirect approach to the problem of estimating food consumption was taken by Lavigne et al. (1985). They used data on harp seal population size, age structure, catch statistics, mean age of maturity, Migration pattern and various energetic parameters to estimate the total energetic requirements of the population. According to this analysis, harp seals consume over half of their annual energy requirements (54%) north of Hamilton Inlet, and only 46% in waters around Newfoundland and the Gulf (see also Northridge 1986). Clearly, with the majority of harp seal food consumption taking place in waters north of the range of 2J3KL cod, and with Atlantic cod such a rare component of their diet, harp seals can not be considered to be significant predators of northern cod. Thus, they could not be viewed as having a direct impact on the population dynamics of this stock. It could be suggested that harp seals may have some indirect impact on northern cod through their consumption or capelin. The following scenario could be envisioned, though it is entirely speculative. Capelin are clearly one of the major foods of the harp seal, as they are of the cod. The recent decline in the catches of harp seals since 1983 may have resulted in an increase in seal abundance, though this has not been demonstrated (see Cooke et al. 1985, Lavigne and Kovacs 1988 for further discussion). It is reasonable to expect that larger population of harp seals would consume more capelin, provided there was sufficient capelin for the seals to eat, and that there was no density dependent change in the rate of harp seal consumption of capelin. Then, if the availability of capelin were a limiting factor for cod (Lilly 1989, Magnusson and Palsson 1989), harp seals could be having a deleterious effect on the cod stock. However, the available data, as well as our current understanding of the trophic relationships in the northwest Atlantic do not support this hypothesis. Harp seals are only one of many predators of capelin in the northwest Atlantic: one recent species account listed ten fish species, five marine mammals and nine sea birds as foraging extensively on capelin (Scott and Scott 1988). It is not possible at this time to quantify all of these interactions, but an earlier work sheds some light on the importance of harp seals relative to three other species. Winters (1975) used a simple deterministic model to estimate changes in the rate of predation on capelin resulting from changes in the abundance of Atlantic cod, harp seals, fin whales (Balaenoptera physalus) and minke whales (B. acutorostrata). Though this approach is of questionable validity, the analysis suggests that harp seal consumption of capelin is between 8.4% and 9.0% of the total predation, depending on whether he used virgin population estimates of the four predator species or their current (1975) population estimates. This range would be much lower if other predators were included as well. The obvious conclusion is that harp seals are not major consumers of capelin. This is in agreement with recent work on the relative importance of various taxa in overall fish mortality (sax 1989, Overholtz et al. 1989). There is no evidence to suggest that changes in harp seal abundance have affected overall capelin abundance. Capelin biomass in 2J3K was drastically reduced during the late 1970's, following the rapid development of an offshore commercial fishery and a series of poor year classes. Following the collapse of the fishery, and aided by a series of strong year classes, capelin stocks began to grow, and biomass estimates during recent years have shown much increased abundance (Carscadden et al. 1988, Lilly 1989). Thus, a possibly increasing stock of harp seals has not had a negative impact on the capelin stock. To the contrary, the capelin stocks have increased during this time. The Royal Commission on Seals and the Sealing Industry (Malouf 1986) suggested that reductions in seal populations in Atlantic Canada could lead to increased fishery yields. However, the model which they used to estimate the impact of seals on fish stocks has been shown to be fundamentally flawed. Holt (1987) and Lavigne (1987) examined the results of the Royal Commission as they related to grey seals. They found that the benefits alleged to accrue to the fishery from culling seals were exaggerated by a factor of at least ten, and that the conclusion that it would be cost-beneficial to cull grey seals was invalid. The model that the Royal Commission used was described as "simplistic and biologically unrealistic", since it was structured in such a way that it could only predict a benefit to culling seals, regardless of what data are input to it" (Holt 1987). Obviously, any conclusions the Royal commission came to about harp seal interactions with fisheries resulting from this model would be equally invalid. Conclusions The evidence currently available does not provide any support for the suggestion that harp seals have had any impact on the abundance of the 2J3KL cod stock. An examination of the diet of harps reveals that Atlantic cod form an insignificant component. While capelin is important for the seals, their consumption of this species is also considerably smaller than that of other predators, such as Atlantic cod. Furthermore, capelin stocks have increased in overall abundance in recent years. While there may be demands for a reduction in harp seal numbers to help the northern cod stock, among other reasons, there is no scientific justification at all for such action. Literature Cited Angantyr, L.A. 1988. Feeding habits of northeast Atlantic harp seals (Phoca groenlandica) in the Barents Sea, August-September 1987. Rapport SPS 8815, Sea Mammal Section, institute of Marine Research, Bergen. Anon. 1988. The appetite of seals. Fo'c'sle 8:25. Sax, N.J. 1989. A comparison of the fish biomass flow to fish, fisheries, and mammals in six marine ecosystems. ICES 1989 MSM Symp./No.33. Beverton, R.J.H. 1985. Analysis of marine mammal-fishery interactions. pp. 3-33. In: J.R. Beddington, R.J.H. Beverton and D.M. Lavigne (eta). Marine Mammals and Fisheries. George Allen and Unwin, London. Bjorge, A., I. Christensen and T. Oritsland. 1981. Current problems and research related to interactions between marine mammals and fisheries in Norwegian coastal and adjacent waters. ICES C.M. 1981/N:18. Bowen, W.D. 1985. Harp seal feeding and interactions with commercial fisheries in the north-west Atlantic. pp. 135-52. In: J.R. Beddington, R.J.H. Beverton and D.M. Lavigne (eta). Marine Mammals and Fisheries. George Allen and Unwin, London. Carscadden, J., B.S. Nakashima, D.S. Miller, R. Harnum and D.B. Atkinson. 1988. Capelin in NAFO SA2 + Div. 3K. CAFSAC Res. Doc. 88/66. Cooke, J.G., A.W. Trites and P.A. Lark in. 1985. A Review of the Population Dynamics of the Northwest Atlantic Harp Seal (Phoca Groenlandica). Tech. Rep. 1, submitted to Royal commission on Seals and the Sealing Industry in Canada. Finley, K.J. and E.J. Gibb. Undated. Summer diet and feeding behaviour of harp seals in the Canadian high Arctic. Unpubl. MS. Fisher, H.D. and B.A. Mackenzie. 1955. Food habits of seals in the Maritimes. Prog. Rep., J. Fish. Res. Board Can. 61:5-9. Harris, L. 1989. Independent Review of the State of the Northern Cod Stock. Report submitted to the Honourable Thomas E. Siddon. Holt, S.J. 1987. Proposed culling of grey seals in eastern Canada. Report to IFAW. Innes, S., D.M. Lavigne, W.M. Earle and K.M. Kovacs. 1986. Feeding rates of seals and whales. J. Anim. Ecol. 56:115-30. Kapel, F.O. and L.A. Angantyr. 1989. Feeding patterns of harp seals (Phoca groenlandica) in coastal waters of west Greenland, with a note on offshore feeding. ICES C.M. 1989/N:6. Lavigne, D.M. 1987. The grey seal in eastern Canada: to cull or not to cull? Report to Greenpeace. Lavigne, D.M., W. Barchard, S. Innes and N.A. Oritsland. 1982. Pinniped bioenergetics. FAO Fish. Ser. No. 5, Vol. IV:191-235. Lavigne, D.M., S. Innes, R.E.A. Stewart and G.A.J. Worthy. 1985. An annual energy budget for northwest Atlantic harp seals. pp. 319-36. In: J.R. Beddington, R.J.H. Beverton and D.M. Lavigne (eta). Marine Mammals and Fisheries. George Allen and Unwin, London. Lavigne, D.M., S. Innes, G.A.J. Worthy, K.M. Kovacs, O.J. Schmitz and J.P Hickie. 1986. Metabolic rates of seals and whales. Can. J. Zool. 64:279-84. Lavigne, D.M. and K.M. Kovacs. 1988. Harps and Hoods. University of Waterloo Press, Waterloo, Ontario. Lilly, G.R. 1989. Inter-annual variability in predation by Atlantic cod (Gadus morhua) on capelin (Mallotus villosus) and other prey off southern Labrador and northeastern Newfoundland in autumn. ICES 1989 MSM Symp./No.17. Magnusson, K.G. and O.K. Pals son. 1989. Predator-prey interactions of cod and capelin in Icelandic waters. ICES 1989 MSM Symp./No.ll. Malouf, A. 1986. Royal Commission. Seals and Sealing in Canada. Report of the Supply and Services Canada, Ottawa. Murie, D.J. 1984. Estimating food consumption of free-living harp seals, Phoca groenlandica (Erxleben 1977). M.Sc Thesis, University of Guelph, Guelph, Ontario. Murie, D.J. and D.M. Lavigne. 1985. Digestion and retention of Atlantic herring otoliths in the stomachs of grey seals. pp. 292-99. In: J.R. Beddington, R.J.H. Beverton and D.M. Lavigne (eta). Marine Mammals and Fisheries. George Allen and Unwin, London. Northridge, S. 1985. Interactions between harp seals and commercial fisheries in the North Atlantic. A review and preliminary analysis. Report prepared for Greenpeace. 1986. Impact on Fish Stocks. Tech. Rep. 2, submitted to Royal Commission on Seals and the Sealing Industry in Canada. Overholtz, W.J., S.A. Murawski and K.L. Foster. 1989. Impact of predatory fish, marine mammals, and sea birds on the pelagic fish ecosystem of the northeastern USA. ICES 1989 MSM Symp./No.10. Scott, W.B. and M.G. Scott. 1988. Atlantic Fishes of Canada. Can. Bull. Fish. Aquat. Sci. 219:731 p. Sergeant, D.E. 1973. Feeding, growth and productivity of northwest Atlantic harp seals (Pagophilus groenlandicus). J. Fish. Res. Board Can. 30:17-29. 1976. The relationship between harp seals and fish populations. ICNAF Res. Doc. 76/X/125. Stewart, R.E.A. and D.J. Murie. 1986. Food habits of lactating harp seals (Phoca groenlandica) in the Gulf of St. Lawrence in March. J. Mamm. 67:186-88. TGNIF. 1987. A Study of Trends of Cod Stocks off Newfoundland and Factors Influencing Their Abundance and Availability to the Inshore Fishery. Report submitted to the Honourable Tom Siddon by the Task Group on Newfoundland Inshore Fisheries. Ugland, K.I. 1987. The 1987-seal invasion. Unpubl. MS. Winters, G.H. 1975. Review of capelin ecology and estimation of surplus yield from predator dynamics. ICNAF Res. Doc. 75/2. Winters, G.H. Personal communication to the author, October 16, 1989.