Aquatic Ecosystems are Complex
But that’s no reason to avoid the subject. Read about it, and learn everything you can.
Harp seals and cod
In 2010, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) recommended listing four of the cod stocks (“designatable units”) as endangered (listing under the Canadian Species At Risk Act). The DFO was thus tasked with assessing whether the stocks would recover if they received this special protection. It rejected this designation, deciding that no additional measures were needed. (See Rebuilding of marine fisheries, Part 2: Case studies, chapter: “Northern (Newfoundland) cod collapse and rebuilding” p. 144-181 by Jack Rice)
Harp seals have been hunted for their fur for centuries in Canada. In the 1950’s and 1960’s, in most years, over a quarter million seals were killed. This led to a collapse of the harp seal population to about a million seals and prompted a quota system to be instituted. The population has recovered, thanks primarily to bans on seal product imports around the world, which reduced the demand for seal pelts and the level of hunting. Canada’s DFO scientists have estimated the herd size as between 6.55 million and 8.82 million, with a mean of 7.6 million. Because of climate change, these seals, who depend on strong, thick sea ice for pupping, suffer high rates of pup mortality in some years, when the ice cover is poor. If climate change is not controlled, this is likely to worsen.
Harp seals eat a variety of fish, crustaceans, and other marine species. Atlantic cod is just one small part of their diet. Nevertheless, after the cod collapse in 1992, the Canadian Department of Fisheries and Oceans (DFO) blamed harp seals for causing the collapse and spread the propaganda around Newfoundland, where most sealers live. Though the DFO stopped blaming seals for the collapse after many years, they then blamed harp seals for the failure of the cod to recover.
But according to DFO’s own scientists, the cod’s recovery is being hindered by the availability of their most important prey, capelin. Capelin were and still are overfished. Their population collapsed along with the cod population, in the early 90’s, and has not recovered. DFO’s scientists say that capelin are also adversely affected by environmental conditions, yet the DFO fishery managers still allow fishermen to catch over 20,000 tonnes each year, and in some recent years, over 35,000 tonnes. What’s more, egg-bearing females are being targeted in the capelin fishery. As Prof. William A. Montevecchi of Memorial University, Newfoundland and Labrador said in his criticism of the capelin fishery, “You’re not only fishing what you catch, you’re essentially fishing the next generation.” He warned that easing fishing pressure on capelin could be crucial to the recovery of cod stocks.
Canadian scientists have also implicated continued cod fishing for inhibiting the recovery of the northern cod stock. Fishing pressure comes from multiple sources, including the directed commercial fishery, recreational fishing, by-catch, and the so-called “Stewardship fishery.” The Atlantic cod catch in 2016 and 2018 was over 16,000 tonnes, and in 2017, it exceeded 20,000 tonnes.
Is recovery of northern cod limited by poor feeding? The capelin hypothesis revisited
Darrell R. J. Mullowney(1,2) and George A. Rose(1)
1Centre for Fisheries Ecosystems Research, Fisheries and Marine Institute of Memorial University of Newfoundland, PO Box 4920, St John’s, Newfoundland and Labrador, A1C 5R3, Canada
2Science Branch, Department of Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, PO Box 5667, St John’s, Newfoundland and Labrador, A1C 5X1, Canada
ICES Journal of Marine Science, vol. 71 issue 4, pp. 784-793, 2014
The slow recovery of the “northern” Atlantic cod (Gadusmorhua) stock off Newfoundland and Labrador has been ascribed to many factors. One hypothesis is poor feeding and condition as a consequence of a decline in capelin (Mallotus villosus), their former main prey. We compared the growth and condition of cod from known inshore (Smith Sound) and offshore (Bonavista Corridor) centres of rebuilding in wild subjects versus captive subjects fed an unlimited diet of oily rich fish. Wild fish in these areas have had different diets and population performance trends since stock declines in the early 1990s. Captive cod from both areas grew at the same rates and achieved equivalent prime condition, while their wild counterparts differed, with smaller sizes, lower condition in small fish, and elevated mortality levels in the offshore centre. Environmental temperature conditions did not account for the differences in performance of wild fish. Our results suggest that fish growth and condition, and hence rebuilding in the formerly large offshore spawning components of the northern cod, have been limited by a lack of capelin in their diet. Furthermore, we suggest that these groups are unlikely to rebuild until a recovery in capelin occurs.
“Before the closures, it was not uncommon in some stocks for cod harvesting rates to exceed a level at which 60% of the older adult biomass was removed annually, as opposed to the 20% rate recommended to sustain such fisheries.” (See “Age at Maturity as a Stress Indicator in Fisheries: Biological processes related to reproduction in northwest Atlantic groundfish populations that have undergone declines” by Edward A. Trippel, in BioScience Vol. 45, No. 11, 1995, pp. 759-771)
Grey seals and cod
Grey seals have historically existed along the Atlantic coasts of the northeastern U.S. and Canada (along the Scotian shelf and in the Gulf of St. Lawrence). Grey seals were abundant in the Gulf of St. Lawrence in the 16th century, when European explorers arrived, despite having been hunted by natives, such as the Mi’kmaq and Beothuks. However, after hunting walrus to extinction in the Gulf of St. Lawrence, European settlers and explorers increased hunting pressure on grey seals, reducing their number severely by the mid- to late-1800’s. In the 20th century, bounties were placed on the heads of grey seals, and they became rare in eastern Canada and the northeastern U.S. In 1966, their population was estimated at only 5,600 animals. Once hunting was stopped and the seals gained protection, such as the Marine Mammal Protection Act in the U.S., passed in 1972, the population began to recover. (See “Distribution and seasonal movements of grey seals, Halichoerus grypus, born in the Gulf of St. Lawrence and eastern Nova Scotia shore,” Lucie Lavigueur and Mike O. Hammill, The Canadian Field Naturalist, v. 107 pp 330-340, 1993 and “Genomic signatures of population bottleneck and recovery in Northwest Atlantic pinnipeds,” Cammen et al, Ecology and Evolution, V. 8 pp. 6599-6614, 2018)
DFO scientists created a model to estimate the current grey seal population size and trends. According to this research, the 2016 total grey seal population estimate was 424,300. The range within 95% confidence limits was 263,600 to 578,300.
After grey seals were nearly extirpated in Canada, the population rebounded once hunting stopped. Their population growth was high as they were recovering. Now, the population growth of grey seals is decreasing. It has been estimated at about 4% in Atlantic Canada by DFO scientists.
“Estimated pup production for the Sable Island colony was 83,600 (rounded to nearest hundred) with 95% confidence limits of 60,600 and 103,500 (based on log-normal distribution). The current estimate of pup production is below the exponential growth curve fit to the data from 1976 to 1997. This is the fourth consecutive survey in which the residuals of the estimated rate of increase are negative, providing additional evidence for a decrease in the rate of growth in pup production.”
(See “Grey seal population trends in Canadian waters, 1960-2016 and harvest advice,” Hammill, et al, Canadian Science Advisory Secretariat Research Document 2017/052 Quebec region. Note: CSAS documents are not peer-reviewed articles but may refer to peer-reviewed research.)
On the other hand, there is an increase in the number of grey seals that are killed or seriously injured by humans. For the period 2012–2016, the average annual estimated human-caused mortality and serious injury to grey seals in the U.S. and Canada was 5,688 (878 U.S./4,809 Canada) per year. (https://www.fisheries.noaa.gov/webdam/download/93630506)
Grey Seal Diet
Grey seals are generalist feeders. Factors that play a role in determining the species on which they prey include prey availability (which can vary seasonally) and behavior (e.g., aptitude for avoidance of being preyed on), the age and sex of the seal, the nutritional content of the prey, and the time of year. Prey availability depends not only on natural migration patterns; it also depends on fishing practices.
Scientists have studied the diet of grey seals over the years. The primary methods used are:
1. Analyzing hard parts of fishes left in seal feces or in the stomach or intestines of killed seals
2. Analyzing the fatty acid signatures in the blubber of seals (where the blubber is taken with biopsies or from killed seals).
“However, all of these approaches have different biases associated with them, complicating attempts to understand true diet composition…This is problematic because in the absence of an accurate diet for seals, trophic modelling approaches aimed at understanding the predator’s influence on fish populations, such as mass balance models, extended single-species assessment models, or minimum realistic models, could provide misleading results…
A challenge to understanding diet composition is obtaining samples throughout the animal’s range and throughout the year. Grey seals are large and difficult to handle, adding to the logistical challenges of sampling in more offshore areas, particularly during the more inclement autumn–winter period. Overall, the species compositions of male and female diets are similar, but the contribution of demersal species is more important among males, while pelagic species are generally more important for females. This study has shown that in areas where Atlantic cod and hake are aggregated, they can comprise a very significant component of the grey seal diet.”
“Diet composition may vary considerably between seals. Here sample means are presented, but because of small sample sizes and large variation among individuals, 95% confidence intervals were estimated using percentiles from bootstrapped distributions”
“In total, 27 prey species were identified (Table 1). Atlantic cod (cod), Atlantic herring (herring), sandlance, mackerel, various species of flatfish (winter flounder, American plaice, yellow-tailed flounder, windowpane and Greenland halibut) and white hake accounted for over 90% of the diet in the Cabot Strait area and off the west coast of Cape Breton Island.”
Although it is known that grey seals also feed on cephalopods, such as squid, these researchers did not analyze the stomach or intestinal contents for squid beaks. These researchers also did not account for (known) grey seal consumption of cartilaginous species that do not have otoliths, such as skates.
(See M.O. Hammill, et al, “Feeding by grey seals on endangered stocks of Atlantic cod and white hake,” ICES Journal of Marine Science, Vol 71, #6, 1332-1341, 2014.)
Another grey seal diet study was conducted by Beck, et al in 2007. The Beck study showed the importance of redfish in the diet of both males and females as well as juveniles, year-round. This species represented over 30% of the diet for all time frames and each sex except for juveniles in spring when it represented over 22% of the diet, and females in summer, when it represented over 23% of the diet. (See Carrie A. Beck, et al, in Journal of Animal Ecology, V. 76, 2007, pp. 490-502.)
But overfishing and destructive fishing practices have decimated this population:
“Fishing and bycatch in other fisheries such as the Northern Shrimp fishery are the main threats to the survival and recovery of this population. It is estimated that the Atlantic population of Acadian Redfish has declined 99% in abundance since the late 1970s, except on the Scotian Shelf where it seems to have remained stable. The commercial Redfish fishery is prohibited in some areas of this population’s distribution, but is allowed in some sectors.” https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/acadia-redfish-sebaste-acadie-eng.html The latest data from the DFO (from 2018) shows that redfish were still caught in Nova Scotia (6,284 metric tonnes) and Newfoundland and Labrador (5,893 metric tonnes).
Cod stock analyses
DFO scientists have studied the status of the cod stock in the southern Gulf of St. Lawrence over the years, but have not conducted studies of diseases and parasite infestations in cod for over a decade.
Based on stock surveys and models using this data, they have concluded that grey seals are the main cause of the higher than normal mortality of adult cod.
While this may be the case, the DFO should conduct studies on alternative possibilities on an ongoing basis. According to the research conducted from 2008 to 2009, “Analyses of worm count frequency distributions of larval anisakine nematodes have, however, provided evidence of parasite-induced mortality in cod from the Cape Breton and Central Scotian shelves, and in American plaice throughout the southern Gulf and Scotia-Fundy regions. Parasite infection may contribute to the elevated natural mortality of southern Gulf cod by increasing the susceptibility of heavily infected fish to predators.” (See McClelland, G., Swain, D.P., and Aubry, É. 2011. Recent trends in abundance of larval anisakine parasites in southern Gulf of St. Lawrence cod (Gadus morhua), and possible effects of the parasites on cod condition and mortality. DFO Can. Sci. Advis. Sec. Res. Doc. 2011/038)
Scientists in the Baltic states continue to study disease and parasite infestations in Baltic cod. (See for example, “First evidence of the presence of Anisakis simplex in Crangon crangon and Contracaecum osculatum in Gammarus sp. by in situ examination of the stomach contents of cod (Gadus morhua) from the southern Baltic Sea,” Pawlak, Joanna, et al. Parasitology; Cambridge Vol. 146, Iss. 13, (Nov 2019): 1699-1706. DOI:10.1017/S0031182019001124 and “Increasing occurrence of anisakid nematodes in the liver of cod (Gadus morhua) from the Baltic Sea: Does infection affect the condition and mortality of fish?” Jan Horbowy, et al, Fisheries Research v 179 (July 2016) pp 98-103)
DFO Grey Seal Cull Proposal
The DFO considered proposals to increase cod numbers in the southern Gulf of St. Lawrence. The focal year for the analysis was 2009. Based on the distribution of satellite-tagged seals, it was estimated that 36,000 Sable Island seals, 5,000 eastern Shore seals and 63,000 Gulf seals forage in these areas at some time in the year (or in winter in the case of NAFO area 4Vn), for a total of 104,000 seals in 2009. Given these estimates of seal abundance and estimates of 5 year old or older cod consumption, the number of seals that would need to be removed to reduce their natural mortality (death by predation) to 0.4 was calculated. Given this level of mortality and current levels for other components of productivity, the cod population would be expected to increase in the absence of fishing.
Due to spatial and seasonal gaps in the diet sampling, it is difficult to quantify the consumption of NAFO area 4T cod by grey seals. Two different approaches were used to fill these data gaps. One approach yielded a consumption estimate of 2500 tonnes of 4T cod 38 cm or more in length (corresponding to ages 5 years and older). Consumption at this level accounts for 11% of mortality for this group. Given this level of consumption, it is not possible to reduce their mortality to 0.4 by removal of grey seals. Under the assumptions of this approach, the mortality due to factors other than predation by grey seal amounts to 0.56, a level that is too high to allow population recovery under current productivity conditions.
A second approach yielded a consumption estimate of 11,000 tonnes of 4T cod in the 38 cm and larger length range, accounting for 49% of mortality. This second approach is more consistent with the conclusion, based on weight of evidence, that predation by grey seals is a major component of the current high natural mortality of this cod. Given this level of consumption, mortality would decline to 0.4 if the number of seals foraging in the areas occupied by 4T cod were reduced to 31,000 animals. If grey seals show diet specialization, with a fraction of seals specializing in predation on large cod, and it is possible to target these seals for removal, then fewer seals would need to be removed to promote cod recovery. For example, if all the consumption of this cod was due to half the seals foraging in the areas occupied by the stock, and it was possible to target those seals for removal, then the required removals would be half as large, i.e. the number of seals preying on 4T cod would need to be reduced by 36,000 animals.” (See “Impacts of grey seals on fish populations in eastern Canada,” Canadian Science Advisory Secretariat, Science Advisory Report 2010/071, DFO 2011)
This assumes that other seals would not replace those killed.
Note the mortality referred to here is instantaneous natural mortality (i.e., death due to factors other than fishing and other human activities). Mortality = 0.4 is equivalent to a 33% annual mortality rate. 4T is the NAFO division in the Gulf of St. Lawrence. 4Vn is the NAFO division on the northeast side of Cape Breton, on the south side of the Laurentian Channel and going up the west coast of Newfoundland.
One problem with this culling proposal is that the Canadian government’s stated objective with respect to grey seals, who are recovering from being nearly extirpated by hunting, is to keep the seal population above the Precautionary Reference Level with 80% certainty (meaning that there is a 20% chance that if the government follows this scientific quota advice, the seal population could fall below that reference level). Given this objective, the scientific guidance is that the following number of seals could be killed and still meet this objective.
4,500 seals in the Gulf of St. Lawrence, where 95% are the young of the year
Or 2,400 seals in the Gulf of St. Lawrence, where 70% are the young of the year
For the Scotian Shelf, where the Sable Island (main breeding grounds) and coastal Nova Scotia herds are, in order to meet this objective, there could be a kill quota of
30,000 seals, where 95% are young of the year
Or 17,000 seals, where 70% are young of the year
(See “Grey seal population trends in Canadian waters, 1960-2016 and harvest advice,” Hammill, et al, Canadian Science Advisory Secretariat Research Document 2017/052 Quebec region)
The DFO did not consider other solutions to the problem of cod’s continued decline in the southern Gulf of St. Lawrence. Whether grey seals are causing the cod to continue to decline by preying upon their overwintering aggregations or whether multiple factors are at play, increasing the abundance of other prey of seals – and of cod – by reducing fishing quotas would likely help cod in the long run. Both cod and seals eat Atlantic mackerel, but this fish population has been decimated by overfishing.
As this CBC article explains, “The latest stock assessment for Atlantic mackerel contains grim news for one of the region’s most iconic fish.
Scientists say the spawning population is down 86 per cent from pre-2000 levels, and the number of fish surviving to breed is at all-time lows.
An assessment by the Department of Fisheries and Oceans says mackerel are in the “critical zone” where serious harm is occurring and recovery is threatened by overfishing.”
“Mackerel winter off New Jersey and swim up the Atlantic coast arriving first off Nova Scotia, then they move around Cape Breton to spawn in the southern Gulf of St Lawrence.
After spawning, they disperse.
The assessment points the finger at overfishing in Canada and the United States.
Mackerel is caught as bait fish. It is the primary bait used in the lobster fishery, making it a key input in a billion-dollar industry.
Commercial catch limits
The total allowable commercial catch in Canada was 10,000 tonnes in 2018 — down from nearly 55,000 tonnes in 2005. The American quota this year is 9,100 tonnes.
Atlantic herring is also an important prey for both cod and grey seals. But it, too, is being overfished. There is also a great deal of uncertainty in the size of the population. Using the DFO’s low estimate for the spring spawner stock, fishermen killed almost 19% of the stock in 2017. According to DFO scientists, this herring stock has been in the “critical zone of the Precautionary Approach framework since 2004, and the probabilities that the spawning stock biomass remained in the critical zone at the beginning of 2017 and 2018 were over 90%,” yet commercial fishing continued. And the spring catch of herring is primarily sold as bait.
The fall spawner stock is also overfished. The DFO scientists have not been able to assess the population with a great deal of certainty: “Current retrospective patterns indicate that the assessment model may overestimate the exploitable biomass. Consequently, harvest options presented may be optimistic relative to attainment of management objectives.” Herring roe is also being targeted in the fall fishery, jeopardizing the survival of the species for extravagant caviar. (See “Assessment of the southern Gulf of St. Lawrence (NAFO Div. 4T) spring and fall spawning components of Atlantic herring with advice for the 2018 and 2019 fisheries,” CSAS Science Advisory Report 2018. https://waves-vagues.dfo-mpo.gc.ca/Library/4071309x.pdf)
Pacific Seals and Sea Lions – Predation on Salmon
We should note that there are three “types” of salmon: wild, hatchery, and farmed salmon. The fighting is about restoring populations of wild salmon. Hatcheries are used to repopulate salmon in the wild but, though they look alike, they are less ‘fit’; that is, they are less likely to survive in the ocean and, if they do reproduce, they produce fewer offspring. They are bred to be caught by fishermen. Yet they compete with wild salmon and contribute to the decline of wild populations. Farmed salmon are bred to be eaten, not released. There is ample controversy about raising them in ocean pens, where their diseases and parasites can be transmitted to wild salmon and from which they can escape.
Though salmon lead short lives, there anadromous nature makes them very interesting and unusual species. Their lives begin in fresh water. From there, they migrate to the ocean, where they spend their adult lives, which can be as short as 6 months or as long as 7 years, depending on the species of salmon. Then they return to freshwater to spawn and die. Salmon don’t feed when they leave the ocean, so some become too weak to make the trip back to their spawning grounds and die along the way.
These fish are not just prized by many humans; they are also sought by otters, eagles, bears, whales, seals, sea lions, and many other animals. For some killer whales, they are their main prey.
What’s causing their decline, and what can be done about it?
Anywhere that humans have added impediments or restrictions to waterways, wildlife come to take advantage of the easier salmon catch. But how important is this predation – by seal lions, bears, otters, eagles, other fish, etc. – to the survival of salmon? As Prof. Lackey says in the film, “It’s not among the major effects on the salmon runs, but it is a measurable effect.”
How important is predation by seals and sea lions on salmon in general? According to Dr. Peter Ross, “seals in BC are largely eating middle of the food chain fish that are not the ones that we’re typically going after, so hake and herring and other species.”
Dr. David Costalago says, “There have been some studies with sea lions feeding on salmon because they believe there is no herring available. And herring and anchovy are what we call forage fish, these fish are nutritionally richer than salmon for most pinnipeds because of their content in lipids and some particular fatty acids. So whenever herring and anchovy are available for the sea lions to eat, they will prefer to eat these fish rather than the salmon.”
However, there is a herring fishery in B.C., and it’s a controversial one. Herring are important prey for seals and sea lions, whales, salmon, various sea birds, and many other species. The DFO’s herring quotas have resulted in estimates of herring biomass falling from 130,000 metric tons in 2016, to about 54,000 in 2020.
“I think we’re getting to the point where I can say that restoration of our forage fish and other preferred prey species would help steelhead and coho salmon smolts survive their migration to the ocean. It is probably not the case that we have too many harbor seals, but the lack of their preferred prey items may cause them to switch to eating salmonid smolts.” according to Dr. Barry Berejikian, Supervisory Research Fish Biologist, NOAA Northwest Fisheries Science Center, Manchester Research Station – Station Chief (sent in an email).
Are predator-prey issues the main issues that have caused the decline in salmon? As Prof. Lackey explains, changes to salmon habitat are far and away the most significant factor in the decline of salmon. “The biggest single thing, I think, that causes problems for salmon is not predation by humans or anything else, it’s the fact that the habitat that they live in is just so different than it was pre-1850. If you looked at the 1850 habitat and looked at the habitat now, you’d say, “wow, how can we even have any salmon here? The habitat’s just so different, and not only that, is your on a trajectory, in the 4 states up here, including British Columbia, you’re on a trajectory to go from 14 or 15 million people to 65 million people by the end of the century.”
“We have 4 populations of salmon worldwide, and they have followed the same general pattern. And those populations of salmon are on the west coast of North America, the ones we’re talking about, the east coast of North America, Europe and the Asian Far East. They’ve all followed the same general decline pattern. And that is, as the human population grew in those areas, economic development took place, land use changed. There was all kinds of human activities. The salmon population declined. In those areas where there’s been relatively little human action, the Asian Far East in the Northern half, which is Russia and that area, populations of salmon are doing fabulous,” Prof. Lackey said.
Though humans, not seals and sea lions, caused the wild salmon population to decline, many fishermen say that seals and sea lions are overpopulated and are harming the salmon population. They say that a cull (or “harvest”) would bring back the salmon.
“”You know, overpopulation is an interesting conversation for us as humans, because we’re often observers in the grand scheme of the planet that we’re, we’re living on. And you know, we have a serious overpopulation problem in the form of us. I mean there are a lot of people, over 7 billion people on this small planet right now. And in the case of the most abundant seal in the world, we’re talking maybe 8 or 10 million. In the case of most seal populations, we’re talking about numbers in the area of a few hundred thousand or a few million,” asserted Dr. Peter Ross.
“You know it’s funny, at the end of the day, you know, culling seals because we think there are too many of them and we expect that that will lead to a beneficial outcome on our fisheries, is ethically, morally and scientifically indefensible. There’s really no grounds to actually harvest seals and expect our own access to be improved,” he said.
“There’s a lot of effort on the part of fisheries management agencies to predict how many fish or how many kilos or tons of fish are eaten by seal populations or species. The problem with this. I mean it’s a useful exercise. But there’s one fundamental problem with this. And it ignores the beneficial role that a predator plays in terms of creating ultimately a stronger prey population and stronger individuals. Mother Nature prides itself in creating a mix of all sorts of species and they don’t get stronger by having no competition. They get stronger by having predators picking off the weak and the infirm and the genes that are not going to contribute to the long life expectancy of this population,” he added.
Prof. Trites put the seal population in perspective: “Is there an overpopulation of seals and sea lions? And I think if a seal or sea lion could talk for themselves, they would actually point their flipper and say, “No, the trouble is you. If there’s overpopulation, it’s the people that are here. If there’s competition occurring, it’s because of what you are taking out.” And I think it points to the need for us to have a conversation about this ecosystem that we share at the end of the day.”
Dr. Costalago cautioned about a cull of pinnipeds: “When we try to remove a top predator from the ecosystem, we risk impacting an entire food web. For example, if we open a cull for pinnipeds in the strait of Georgia, we will probably reduce the available food for the transient killer whales. At the same time, it is not only going to be a problem for a species that is a predator of these seals or sea lions, it is also that we don’t know if the fish are actually being controlled by the predators or by the food production in the system. We don’t know if eliminating the predators is actually the solution for the fish to recover.”