Revealing the Secret Lives of Seals:
Are Seal Herds Damaging Atlantic Fish Stocks?

Canada's northwest Atlantic holds the world's biggest populations of harp, hooded, and grey seals. Harp and hooded seals are subject to a centuries-old, now quota-controlled commercial hunt. After declines during the 18 and 19th centuries, herds have recovered to their highest abundance on record, while stocks of cod and other groundfish fell to their lowest, and still show little sign of recovery.

The Fisheries Resource Conservation Council (FRCC), an advisory group of university scientists, government and fishing-industry representatives, says that seal predation threatens the recovery of some groundfish stocks.

But how exactly do seals affect groundfish? Fisheries and Oceans Canada's Atlantic Seal Research Project has turned up significant new information through techniques old and new, some of it unprecedented and surprising, concerning populations, their habits and the extent of predation by harp, hooded and grey seals on Atlantic cod.

The ASRP began in 2003 and will conclude in March 2006. Principal investigators include scientists from several Fisheries and Oceans Canada research institutes: 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.

Standard seal-counting uses aerial photographs of whelping patches to monitor pup production. This data is combined with life-history information to estimate total numbers. New surveys, conducted in 2004 and 2005, estimated current population sizes of harp, grey and hooded seals.

Harp seals, which breed on 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 5.5 million by the mid-1990s, where they remain stable today. A 1990 survey of hoods, breeding in the Gulf, the Front, and Davis Strait put their number at around 450,000 animals. A new survey was carried out in 2005 and the results will be available in late spring 2006.

There are two main components to the Canadian Grey seal population - the Sable Island and Gulf groups. Numbering less than 10,000 in the 1960's, this population is now estimated to be around 250,000 seals, based on the latest survey.

Adult harp seals weigh up to 135 kilograms with adult grey and hooded seals much larger, reaching 400 kilograms and 3 metres in length. Large size means large appetites. How much do their feeding areas overlap with cod and other groundfish?

Electronic tags on about 100 seals, transmitting to satellites when they surface, have provided better information than ever before of seal routes. These enable better modeling of interactions with groundfish.

The tagged seals gathered important data, some of it surprising. 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.

How much do seals eat of cod and other depleted stocks, and how much from more plentiful species? The amount of prey consumed is estimated using a bioenergetics model. In order to use this model you need information on the energy requirements of seals of different ages and sexes, numbers of seals in each age/sex category, how much time they spend in the areas of interest and what they eat in each area.

The standard method of documenting seal diet is to analyze the hard parts in stomach contents or scat (faeces). But the method is limited, as it depends on analysis of hard parts remaining in scat, such as squid beaks and fish bones, and identifies only the last meal.

Sara Iverson and colleagues at Dalhousie University in Halifax, in collaboration with Don Bowen at BIO, developed a way to investigate diet over time by analyzing blubber samples, 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.

When used to examine the diet of grey seals on the Scotian Shelf, 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 scientists 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? In 2004, the ASRP 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.

A two-year pilot Seal Exclusion Zone project was established in Smith Sound, Trinity Bay, during the winters of 2003/04 and 2004/05. However, preliminary results indicate that a Seal Exclusion Zone is not practical in this area and may be unrealistic in larger areas.

An injectable seal contraceptive was also developed in the 1990s, but this proved too expensive and logistically difficult to apply on a large scale.

Complexities abound in the seal-predation puzzle. But researchers are combining expert knowledge and innovative techniques to understand the interactions of seals and fish.