My study objects: pilot whale (Photo: Lise Doksæter), common dolphin (Photo: Erik Olsen), white-sided dolphin (Photo: Wikipedia/NOAA) and striped dolphin (Photo: Wikipedia/NOAA) Benthosema glaciale, the most abundant mesopelagic fish Marine mammals were spotted by observers standing on the roof of R/V G.O. Sars-bridge. When they discorvered an animal, the time, GPS position and the animals distance and direction from the ship was noted, so that the mammals exact position could be calculated. The knowledge of this exact position of a dolphin observation, can be linked to several different variables collected along the ridge, such as temperature, salinity, bottom topography, acoustics and biological samples. This enables me to study physical and biological characteristics of the dolphins habitat.
along the mid-Atlanic ridge, and probably an important prey item for the dolphins. (Photo: Tracey Sutton)
Among the physical factors examined, water temperature had largest influence of dolphin distribution. The dolphins could be divided into two distinct groups; common- and striped dolphins were inhabiting warm, subtropical waters, while pilot whales and white-sided dolphins were found in colder, more artic waters.
An important question in my study was how the dolphins were associated with the bottom topography of the mid-Atlantic ridge. The actual bottom depth in the area of dolphin observations were highly variable, and seemed to be of minor importance. Bottom slope, on the other hand, appeared to largely influence their distribution, with dolphins tending to aggregate in areas of steep slope. This was probably caused by underwater hills forcing cold waters up from the deep. Such deep waters are rich in nutrition, and may lead to higher primary production, which means more food for planktivorus animals. As a chain effect, production increases on all trophic levels, with dolphins on top, seeking out such sites of high food abundance.
Studying feeding ecology for those dolphins is not an easy affair. We cannot simply take samples of dolphins and study their stomach contents as we do for fish. Our visual observations are limited to what can be seen when they come the surface to breath, while most of their activity, including feeding, take place under water. Therefore, I had to use some indirect methods when trying to figure out what those dolphins eat.
The ship's echosounder emitted small "pings" of sound at regular intervals during the entire cruise, which was backscattered by fish and other animals in the water. Because every fish species has its unique backscattering profile, acoustic data can be used to study what sort of fishes are found at different depths along the ridge.
I have used these acoustic data together with trawl catches to identify the fish species in the depths dolphins are able to dive to, in order to study what animals are available along the ridge as dolphin food. Both in trawl catches and acoustics, small mesopelagic fishes were most abundant, and probably constitute the bulk of a typical dolphin meal. These fishes swim up to the surface to feed on zooplankton at night, and down again to depths of 400 – 1000 m during the day to avoid being seen by predators. If feeding during the day, dolphins will have to dive to depths of at least 400 m to feed on the same fishes that are available at the surface during night. Since diving is energetically costly for dolphins, nighttime dining are probably the most common for those mid-Atlantic dolphins.