How big is a seal's appetite?

A visiting graduate student completes energetics study at Memorial's Ocean Sciences Centre

By Ivan Muzychka

The interaction of seals and cod at the forefront of fisheries research in Newfoundland and Labrador. With the cod population in crisis and the seal hunt under attack, scientists are redoubling their efforts to understand more about the diet of seals.

Not long ago Memorial University and the Canadian Centre of Fisheries Innovation (CCFI) -- together with the Department of Fisheries and Oceans (DFO) -- teamed up on a study aimed at understanding what seals eat.

Over the past year CCFI has been funding the work of visiting graduate student Hugo Ochoa-Acuņa, who is working with the harp seals at Memorial's Ocean Sciences Centre in Logy Bay. Mr. Ochoa-Acuņa is originally from Chile, and is completing his PhD at the University of Florida. His research specialty -- energetics -- caught the eye of researchers in Newfoundland who thought that his work would complement their ongoing efforts to understand seals' diets.

"Essentially, energetics is the study of how animals can obtain the energy that they require to survive and reproduce," Mr. Ochoa-Acuņa told the Gazette. "In a sense it is the study of how much fuel an animal needs for its basic activity."

Diet is key

Knowing how much a seal eats would shed light on how environmental factors -- the food web of the sea and the seasons, for example -- affect the basic biology of the seal. Knowing the amount of fuel the animal needs to stay alive helps scientists know how seals interact with other species in the ocean. In this way, Mr. Ochoa-Acuņa's energetics research might ultimately help researchers at the Ocean Sciences Centre, CCFI, and DFO understand more about the impact seals are having on cod.

Mr. Ochoa-Acuņa noted that there are many unknowns in the field of energetics. He said that people have always made assumptions about how much food different species need to consume in order to stay alive.

"It was thought that seals had a higher energy expenditure than terrestrial animals, and that was related to the fact that they were living in the sea," the biologist explained. "However, in terrestrial animals -- for example, carnivores like cats and bears -- you have a higher energy expenditure than in terrestrial herbivores like cows. Energetics research tries to understand the reasons for the different energy requirements."

How is energy used?

The objective of Mr. Ochoa-Acuņa's project was to estimate how much a seal needs to eat in order to stay alive, concentrating on the expenditure energy of the animal. In a related study, Dr. Jack Lawson of DFO was studying how much of the seal diet is cod. Mr. Ochoa-Acuņa wanted to see how much energy the seal expends, and how the expenditure of energy changes throughout the year. His year-long study, which began in May 1996, measured seals' food intake and physical activity levels during the four seasons.

"We try to compare the amount of foods they are eating with the body mass changes on the animals," he explained. "That will give you an idea how much the animals are using for activity, their basal metabolism and energy storage. We are also looking at oxygen consumption; by measuring the oxygen consumption we can calculate how much energy the animal is consuming under standard conditions."

Weight gain, loss

Besides looking at diet amount, activity levels, and body mass, his research is examining the type of weight the seal gains and loses.

"In addition to knowing how many kilograms the animal is gaining or losing, we have to know the composition of the body -- it could be water or fat," he said.

In order to establish how just much water or fat there is in the seal's body, Mr. Ochoa-Acuņa gives the animal an amount of deuterium oxide (heavy water).

"Biologically the deuterium oxide acts like water and dilutes throughout the body," he said. "Later, we take a blood sample and analyse it for the proportion of deuterium oxide to water. The proportions of proteins and other substances are fixed in the body so from this analysis we can tell how much fat and water the animal has gained."

His research has also involved attaching small electronic monitors to the seals.

"The amount of food the animal consumes can be stored as body mass, or it can be used during muscular activity," he said. "With the monitors we can see how much food is used for different tasks. With statistical analysis we can work out the percentage spent on maintenance, activity, or both. The next step is to use the same activity recorders we are using in the tank in the wild, and make activity comparisons."

Seals can consume large amounts of food for several months at a time, and then eat nothing for several more. They get fat quickly and lose the weight just as quickly. Therefore, in addition to shedding light on the seal/cod controversy, the study may also reveal information about human obesity problems.


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