The Devil is in the Details
Part 2: Delving into Seal Diet Studies
“These grey seals, Mr. Speaker, don’t need the ice to have their babies on, they just slide on the beach anywhere and have a baby; they can slide up on the street to have a baby. They will be before long… they’ll be up on the streets of Halifax if they keep accumulating. I don’t know how these fish stocks are standing it, I really don’t. —Harold Theriault, MLA, Digby-Annapolis (Hansard, March 29, 2007)
[The following is an excerpt from my 2013 book The Devil and the Deep Blue Sea: An Investigation into the Scapegoating of Canada’s Grey Seal]
The calculation of the importance and the amount of cod in the grey seal diet is a key piece of evidence in the attempt to implicate the species in the cod’s non-recovery. In fact, on both sides of the Atlantic where grey seals have come into conflict with fishers and are believed to be competing with commercial fisheries, diet studies are almost an obsession with some scientists. But grey seal diet studies are fraught with shortcomings and tell us little if anything about how grey seals interact with cod.
Screen shot taken from Fish, Food & Allied Workers (FFAW-Unifor) union’s “awareness campaign” video calling “attention to seal overpopulation in Atlantic Canada and the devastating effects on fish stocks.”
Sara Iverson is a researcher in physiological ecology at Dalhousie University: she has worked with Don Bowen for years studying the diets of grey seals on Sable Island using a technique which was developed to analyze the fatty acids in seal blubber. This technique provides information about what the seal has eaten over a period of weeks or several months. “Fatty acid signature analysis is based roughly on the principle ‘you are what you eat,’” she says. Iverson analyzes a small piece of blubber, taken from a live seal, and calculates both the mixture and amount of prey the seal has eaten.
In one ten-year study of seal foraging behaviour and diet, published in 2006, Iverson and Bowen and three other scientists found that cod are not a staple food for grey seals that breed on Sable Island. “Cod make up a very small proportion of their diet,” she says. Looking at twenty-seven species of fish including invertebrates, the scientists found that between two and five species accounted for more than 80 percent of the diet by weight. “By far the largest diet items for grey seals on the Scotian Shelf are northern sand lance, redfish and other forage species such as capelin and herring.” Iverson says seals prefer eating these fish because they are abundant and have a high fat content — 5 to 14 percent — compared to cod, which is only 1 percent fat.
Bowen says this is because grey seals make decisions on what to eat “based on economics” and on what’s “profitable” for them to eat. “There’s simply a large amount of candy bars on the Scotian Shelf,” he explains, and the candy bars are the redfish, sand lance and herring, which he says are abundant. “Even if there is a cod population, seals may not eat them.” In fact, Iverson’s fatty acid analysis found that cod accounted for only 1.8 percent of a seal’s diet.
However, when seal diets are measured in other ways, by looking at the scat, stomach, or intestine contents, there are often very different results: by these measures, cod appears to be a much more important component. Bowen says that unlike the fatty acid analysis, the other methods are not as representative of a grey seal diet. He also points out that the other methods are notoriously “biased” — a term used in science to mean that the method systematically results in an error inherent to the method itself. Because of this, identifying what a seal may or may not have eaten using any of these techniques is very challenging, to say the least.
“One of the biggest problems is that a very large fraction of the parts of fish completely disappear and we never see them because they get digested,” says Bowen. For instance, one way to know for sure the kind of fish that has been eaten is to find its otoliths, or ear bones, somewhere in the seal’s digestive tract or in its scat. Otoliths normally rest in fluid-filled chambers in the fish’s inner ear and are used for sound detection and for balance, pulled by gravity so their movement helps the fish tell what’s up from down. Depending on the shape of the otolith, it’s possible to tell what species of fish has been eaten. Otoliths also have growth rings of calcium carbonate, protein and various trace elements, so counting the rings, just like you would in the trunk of a tree, can tell you the fish’s age, and from that, scientists can estimate the fish’s size.
Don Bowen, research scientist emeritus at the Bedford Institute of Oceanography and expert in the population dynamics of grey seals on Sable Island, as well as the potential impacts of seal predation on fish stocks.
Cod otoliths tend to be robust and can withstand the process of digestion better than the hard parts of other fish species, such as herring and sand lance, which have small and fragile otoliths that tend to get digested. So, for instance, when scientists look at a seal scat, they may find cod otoliths but no evidence that herring was eaten. But this doesn’t mean herring wasn’t eaten. So, through experiments, scientists have learned that they typically only see evidence of 5 to 10 percent of herring and sand lance otoliths; based on this they have developed crude ways to “correct” for this so that the data are not biased toward species such as cod and haddock. “When you apply the correction factor, for example, to the Sable Island data, the proportion of cod in the diet [based on otoliths] drops by 50 percent to 7 percent,” he says.
The other problem, according to Bowen, is that these correction factors are for fecal or scat samples only — the most widely used technique — and that no one really knows how to correct for stomach or intestine samples. “By the time you get to an animal, unless it’s just eaten that food, you’ve already lost a lot of these otoliths,” he says. The other problem with using stomach samples is that it may contain no information at all: “If the seal ate six or twelve hours ago, the stomach is empty.” Getting information from a stomach that is representative of a seal diet is extremely difficult, he says. Stomach samples may have information on what a seal just ate, and fecal samples, which are collected from haul-out sites, only shed light on the last meal a seal has eaten, typically close to the site. “Meals that we can sample, whether we use stomach or fecal analysis, are vastly less than 1 percent of the meals that are actually eaten,” explains Bowen.
While DFO scientists attempt to correct for the overestimation of cod in a seal diet, the fishing industry has long argued that cod is actually underestimated because they say there’s no accounting for “biting” or “belly-feeding” of the large fish. They argue that seals often prefer eating just the cod’s belly, in which case the otoliths, which occur in the head of the fish, would not be ingested. In this way, they argue, some predation could be overlooked.
Belly-feeding is about as contentious an issue as you can find when it comes to seals and cod. In Newfoundland, the DFO has documented eyewitness accounts, by fishers and divers, of dead cod with holes bitten out of their middles, strewn on the ocean bottom. These reports, however, have more often than not been associated with cod die-off events — where overwintering cod enter icy shallow waters. The largest documented event of this kind took place one morning in early April 2003 when the villagers of Smith Sound, Newfoundland, awoke to a sea brimming with cod. They rushed out in their boats, scooping the fish up in nets and buckets and in three days, they hauled hundreds of thousands of cod aboard their boats, in the end totalling 780 metric tonnes — roughly the combined weight of five hundred average-sized cars. It was reminiscent of the days when the fish were so abundant off their shores that they could be caught in baskets lowered over the sides of boats — except for one glaring difference: these fish were dead. At first, fishers and even some scientists blamed harp seals for chasing cod into water that was too cold for them. A federal investigation eventually showed that the subsurface water in the entire Sound was colder than it had been at any time during the previous decade, causing the cod to freeze to death. So, if seals were eating the bellies out of slow-moving, cold-stricken or dead fish, could we really blame them?
David Lavigne has been the science advisor for the International Fund for Animal Welfare (IFAW) since 1999. Before that he was a zoology professor at the University of Guelph; he’s been studying seals since 1969. He says there is anecdotal evidence that belly-biting does occur, but only around fishing gear: “If you have cod caught in a gillnet, so that the cod is suspended in a water column, the seal will bite away and it could very well bite the belly out,” he says. “But seals don’t have the time or inclination, or the teeth, to bite the bellies out of free-swimming fish. He says seals usually eat their fish whole and head first. “They don’t go swimming at high speeds chasing fish and nipping at their bellies.... I think this is the general consensus of scientists,” he says. But not all scientists.
Doug Swain is DFO’s cod expert in the Gulf of St. Lawrence region. He says belly-feeding isn’t out of the ordinary: “When prey are locally abundant — in areas of dense cod aggregations during migrations and on the over- wintering grounds — the optimal foraging strategy is often to consume only the most energy-rich portions of the prey.” Swain points to other examples of this: bears feeding on spawning Pacific salmon, and sea turtles feeding on jellyfish. He says the most energy-rich portion of the cod is the liver, located in the belly, where the cod store their energy reserves.
Lavigne argues that if belly-feeding was a common practice, we would find — given the thousands of grey seal stomachs that have been examined — evidence of belly-feeding. “To my knowledge none has ever been found,” he says. One recent DFO study of seal diets that used DNA testing to double- check results corroborates Lavigne’s view. It found that when otoliths were not present in the seal stomachs, neither was cod, “suggesting that if seals were feeding on soft parts of cod (i.e., ‘belly-biting), it was not common.”[1]
Instead, there was evidence that the opposite could be the case. Some grey seal stomachs with cod otoliths actually lacked DNA for cod. Scientists say this could be due to the rapid breakdown of DNA or it could mean that otoliths might actually accumulate in a seal’s stomach over time, which could result in an overestimation of the amount of cod eaten in the short term.[2]
Live grey seal pups on Hay Island, 2011. Photo courtesy Rebecca Aldworth (for Humane Society International)
Bowen says that even after all the diet estimates from the various techniques are derived and corrected for, they are still “misleading.” He explains using this analogy: “If we came up with an average diet of Halifax, you might find you don’t eat any of those foods as an individual or might eat them in very different proportions. That’s the same as what we see in seals.” Bowen says the “average” isn’t telling us what we might think it is. He says sometimes averages can be meaningful. For instance, if you took the average height of ten-year-old boys, the average is meaningful because heights are normally distributed with an obvious peak at the centre of the distribution. “So, in fact most of the population of boys will be near the average height.” But when it comes to grey seal diets, averages are “a mythology,” he says. “A ten percent average value [for cod] in the diet could mean that most seals eat about ten percent of cod,” explains Bowen. But it doesn’t actually mean that. It actually means that most seals eat no cod while a small number eat quite a bit of cod. “The average really does not represent the diet of the grey seals sampled, and by inference therefore cannot represent the whole population.”
According to Sara Iverson, pointing a finger at grey seals is also overly simplistic. “Grey seals are only one small cog in a very large wheel,” she says. The “very large wheel” she is referring to is the complex marine food web, which features an overwhelming number of evolving interactions, the majority of which we know precious little about. Food webs also involve feedback loops that are themselves complicated by the fact that fish rarely eat the same foods as they grow and develop. For instance, a juvenile cod dines on a cornucopia of floating life: copepods, small crustaceans, and plankton, so it eats at a lower trophic level than an adult cod, which will eat just about anything, including other cod. The adult fish are so cannibalistic that cod jiggers — a baitless piece of lead used to attract cod by anglers — are often shaped to resemble a young cod. In one stock it was estimated that large cod were the most important predators of small cod and that cannibalism accounted for 44 percent of the juvenile mortality.[3] So when a seal eats an adult cod, for instance, there can be a number of spin-off effects because it’s also eating a predator of small cod and other potential prey such as capelin, herring, sand lance, turbot, crabs, shrimp, and brittle stars, to name a few. In other words, when all the possible interactions are factored in, there’s every possibility that grey seals could be doing more to help cod than to hinder it.
Adult grey seals on Sable Island. Photo courtesy Zoe Lucas.
Debbie MacKenzie is the head of the Grey Seal Conservation Society, a non-profit organization based in Halifax that advocates for a healthy ocean ecosystem and argues that grey seals are integral to that. She says the grey seal is basically taking over the position that the big fish once had: “Fish like hake and cod, the ones that got big enough to be forces to be reckoned with, they were the predators of the small fish.” Mackenzie points to the fact, surprising to some, that the biggest predator of fish is other fish. She says that the collapse of the fish-eating fish — the groundfish — resulted in there being only one predator species left. “There used to be millions of predators. Now we only have one predator left, the grey seal, and there’s more than enough fish for 400,000 of them.” She says that even a large grey seal population has considerably less impact than a large cod population would have had and that overall intensity of predator activity has decreased. “It looks like increased predator activity because they pop their heads up and they crawl out.”
Although Mackenzie is not a trained scientist, her intuitive grasp of fisheries science and her passion for ocean life are remarkable. Her background is in nursing, but fish and the ocean were never too far away. She worked as a public nurse in the fishing community of Shelburne, Nova Scotia; she says she was “married to a family of fishermen.” Her father also worked as a scientist for the DFO. For MacKenzie there is nothing simple about the marine food web or the way the ocean systems work. She seems comfortable with this complexity and has been working tirelessly against the pressure to reduce it to meaningless sound bites. When Mackenzie talks about the fish or the seals she talks about them like they are real, living things. She describes how they behave, and shows a great interest in their life histories and in their true nature. And it’s not just the seals and the fish she’s interested in, it’s the barnacles and the seaweed and the copepods and the plankton. Mackenzie has an encyclopedic knowledge about them all, from reading and researching but also from paying attention — observing them and trying to make sense of patterns over time and of how things fit together.
This way of seeing ocean life — in qualitative descriptive ways — stands in sharp contrast with what Dean Bavington calls “the quantitative abstract ways that involve instruments and experiments but little experience of wild animals.” Bavington grew up in Newfoundland and is a professor at Memorial University in St. John’s. His PhD thesis led to his recent book, Managed Annihilation: An Unnatural History of the Newfoundland Cod Collapse, in which he focuses on the cod fishery and the rise of what he calls “scientific management” — a turning point he links to the rise of industrial capitalism. We’ll explore Bavington’s ideas in greater detail later, but at this juncture the relevant point is that in the past, cod, or seals for that matter, were not seen as something within the realm of control by humans. Bavington says this shifted when “fluctuations, the ebb and flow of codfish themselves, [became] a problem seen as solvable through some form of intervention, as opposed to something that is just given.” Because of a whole range of forces, many of them economic, there was pressure to catch more fish and a demand for fish landings that were at least steady and predictable. Bavington says the agency that was expected to solve this problem was science.
“There’s been a split in fisheries biology,” he says. “There are the natural historians: those looking at the life history of fish, or looking at the acoustic ecology of cod and how they find mates through sound, or how they need old fish to learn the migration routes and what happens when you take the old fish out.” Then there are the “quantified modeler types.” Bavington ex- plains: “It doesn’t matter if they’re talking about fruit flies, cod, or caribou, it’s the same approach, the same models. We’re now in the world of statis- tics.” Bavington says this approach reflects a world-view where animals of all kinds are just seen as resources to be managed. “Fish are just swimming inventory,” he says. “A certain type of science is now required for political and economic reasons.”
[The third and final part of this series will look at the seal diet study that made headlines in 2012 in Atlantic Canada, and is likely the same one that’s informing the FFAW union’s fish consumption calculations.]
[1] DFO, 2011, Impacts of Grey Seals on Fish Populations in Eastern Canada, p. 16.
[2] Based on personal communication with DFO marine mammal specialist, Mike Hammill, July 23 and 24, 2012.
[3] Savenkoff, C. et al. 2007. “Effects of Fishing and Predation in a Heavily Exploited Ecosystem: Comparing Periods Before and After the Collapse of Groundfish in the Southern Gulf of St. Lawrence (Canada).” Ecological Modelling 204: 115-128.