Canada’s northwest Atlantic waters hold the biggest populations of harp, hooded, and grey seals in the world. While those herds were rising to their highest abundance on record, stocks of cod and other groundfish fell in the early 1990’s to their lowest, and show little sign of recovery.

The Fisheries Resource Conservation Council (FRCC), an advisory group of university scientists and government and fishing-industry representatives, says that predation by the large herds threatens the recovery of some groundfish stocks. Harp and hooded seals are already subject to a centuries-old, quota-controlled commercial hunt. The FRCC has called for further measures, such as seal exclusion zones in some areas, to reduce predation.

But how exactly do seals affect groundfish? Many questions remain about their interactions. To find answers, the Department of Fisheries and Oceans (DFO) in 2003 launched the Atlantic Seal Research Project (ASRP), to provide current information on the extent of predation by harp, hooded and grey seals on Atlantic cod. Principal investigators are scientists Don Bowen of the Bedford Institute of Oceanography (BIO) in Halifax, Mike Hammill of the Maurice Lamontagne Institute in Mont-Joli, Quebec, and Garry Stenson of the Northwest Atlantic Fisheries Centre in St. John’s, Newfoundland.

Findings from the ASRP are expected late in 2005. Meanwhile, research has turned up significant new information through techniques old and new, some of them unprecedented.

Scientists had already developed standard seal-counting techniques. They use aerial photographs of whelping patches to monitor pup production, combining this data with life-history information to estimate total numbers. A new set of surveys, conducted in 2004 and 2005, will provide estimates of current population size.

Harp seals, which breed on winter ice in the Gulf of St. Lawrence and the “Front” area off northeast Newfoundland, rose from about 1.6 million animals in the early 1970’s to more than 5 million by the late 1990s. Hoods, breeding at the Gulf, the Front, and Davis Strait, probably number around 500,000 animals.

Grey seals breed mainly in the southern Gulf and on the sand dunes of Sable Island, roughly 300 kilometres east of Halifax. Numbering less than 10,000 in the 1960’s, greys now likely total more than 300,000. This estimate will be updated on the basis of the 2004 survey, once counts of the photographs have been completed.

An adult grey can reach 400 kilograms in weight and 3 metres in length. Large size means large appetites, for these and other seals. How much do their feeding areas overlap with cod and other groundfish?

New electronic tags, transmitting to satellites when the seals surface, have provided better maps than ever before of seal routes, as in the illustration below. These will enable better modeling of interactions with groundfish.

Portion of a migration plot of hooded seals tagged in Newfoundland. (Courtesy of M. Hammill)

Tagging has brought surprises. Researchers had thought, for example, that grey seals used wide swaths of Scotian Shelf waters where conditions appeared suitable. Instead, tagging showed that greys tend to feed around the offshore banks, shallower plateaus in the ocean. BIO’s Don Bowen says, “It gives us a new understanding of the way they make a living.”

Electronic tagging can also document a seal’s diving behaviour and swimming speed, the water temperature at different depths, and other data useful to biologists and oceanographers. The 100-odd seals (all three species) deployed under the ASRP constitute a new kind of research fleet.

How much do seals eat of cod and other depleted stocks, and how much from more plentiful species? The standard way to document seal diets has been to analyze the stomach contents. But this method shows only the last meal.

Sara Iverson and her colleagues at Dalhousie University in Halifax, in collaboration with Don Bowen at BIO, have developed a new way to investigate a seal’s diet over time, by analyzing samples of its blubber. The method is called Quantitative Fatty Acid Signature Analysis.

In a world first, the researchers showed that the proportion of different fatty acids found in the seal’s blubber reflects the proportion of different fish species in its diet. Developed for seals, the new method also holds promise for determining the diets of seabirds, polar bears and other animals.

As with satellite tagging, the new technique yielded surprises, including less dependence than expected on cod. Fatty acid analysis confirmed that sand lance, a small and unfished species, was a staple food for grey seals. It also showed an unexpected degree of reliance on such species as redfish, skates, and flounders

Even if a particular cod stock makes up only a small fraction of the ordinary seal diet, it would seem obvious that a large population of seals could still threaten that stock’s existence. But researchers point to other possibilities. For example, seals might help cod by eating predators, such as herring, that dine on codfish eggs. In their analyses, the ASRP researchers must take into account such interactions, some of which remain poorly understood and would benefit from more knowledge of fish themselves.

What if it appears that in some instances, saving a local groundfish stock would require reducing or eliminating seal predation in that area? The ASRP in 2004 convened an international workshop on “seal exclusion zones.” Experts discussed use of nets, sound barriers, culls, and other measures, some of which might be practical for small areas, but would become highly expensive and impractical for larger ones.

Meanwhile, back in the early 1990’s, Dalhousie and BIO in another world first developed a seal contraceptive, given by injection. This too proved workable in principle, but expensive and logistically difficult to apply on a large scale.

Complexities abound in the seal-predation puzzle. But DFO researchers and their university colleagues are combining expert knowledge and innovative techniques in the most comprehensive study to date. Their findings will provide a new benchmark for understanding the interactions of seals and fish.

For more information please visit www.dfo-mpo.gc.ca

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