Cost of fuelling a white-sided dolphin just went up

There are a lot of Pacific white-sided dolphins along BC’s coast, and they have become more visible in inshore waters in recent years. This raises questions about how much fish they need to thrive—a topic that has been the focus of three recent projects undertaken by M.Sc. student Erin Rechsteiner and her colleagues at UBC’s Marine Mammal Research Unit.

Erin’s first task was to figure out how many calories Pacific white-sided dolphins need to eat to meet their baseline energy requirements. She worked with the dolphin trainers at the Vancouver Aquarium to get three captive dolphins to stay still under a metabolic hood while she measured how much oxygen each animal used to rest calmly at the surface of the pool. Oxygen consumption was translated into the number of calories a dolphin requires to sit still and breath — much in the same way that an athlete wears a respirator to measure their metabolic rate. Oxygen consumption was translated into the number of calories a dolphin requires to sit still and breath. Erin took measurements from the dolphins once every two weeks for a year, and found their metabolisms were remarkably consistent year-round. She also found that they were remarkably high — with each dolphin needing about 34 MJ per day (about 8000 kcal, or 11 big macs) just to rest!

The energetic cost of resting accounted for about 75% of the total calories ingested by the white-sided dolphins. But the amount of food they consumed was not constant during the year, even in an Aquarium environment. Rebecca Piercey, an undergraduate student working in the Marine Mammal Research Unit examined the feeding records for Pacific white-sided dolphins at the Vancouver Aquarium dating back to 1970—and found that food intake in winter was about 15% higher than average food intake, and that food intake in summer was about 15% lower than average food intake. Rebecca hypothesized that increased food consumption was not due to cold winter water, but rather that hormonal changes experienced in the summer mating season limited the dolphins’ appetites and caused them to eat less during the warmer months, and this in turn led them to eat more in subsequent months to make up for any deficit.

Erin combined her Aquarium results with field data in a complex mathematical model that estimated the total number of calories required by a wild dolphin to do everything it needs to do in an average day. Dolphins in the wild are really active, and need about 69 MJ a day (16, 000 calories, or 23 big macs). In terms of fish biomass, this means that dolphins eat about 12-15% of their bodyweight in fish per day (which is remarkably high considering a harbor seal only needs about 3-5% of its body weight per day in fish).

These three studies indicate that dolphins require a lot of energy, and that winter may be a time of year when high energy intake is especially important. Field observations of Pacific white-sided dolphins in British Columbia show increases in numbers nearshore in winter when herring are spawning—which Erin hypothesizes could be a way for dolphins to efficiently meet elevated energy requirements. This has management implications for Pacific white-sided dolphin conservation. Most notably, it points to their need for high-energy fish to meet their high-energy lifestyle.

Erin Rechsteiner is a M.Sc. student at UBC.