UVic PhD candidate studying humpback whale feeding behaviour

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Rhonda Reidy is developing a framework that may help with conservation and habitat protection

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A trio of humpback whales breathing together at the surface, one has the suction cup tag on its back. Photo by Rhonda Reidy.

Rhonda Reidy, a PhD candidate in biology at UVic, has been collecting data on humpback whale feeding for three years as part of her PhD research. Her study contrasts the feeding behaviours of humpback whales off the coast of North Eastern and Southern Vancouver Island in an effort to develop a framework for understanding their feeding patterns. 

While humpback whales are one of the most accessible and most studied large whale species, scientists lack important information about the whales’ diet and foraging behaviour in Canadian waters. 

“While surface observations have been informative, they have provided only a partial description of humpback-whale diet in B.C. and they provide no information on the whales’ feeding behaviour at depth on alternate prey sources,” said Reidy. 

After completing fieldwork in September, Reidy is now about to start looking at location data on the whales, which she expects will take a few months to analyze. Then she’ll compare that information to prey location data to get a picture of feeding behaviour under water.

“[Whales on the northeast coast] are consistently surface feeding on herring and possibly other species of small fish,” said Reidy. This feeding behaviour differs from humpback whales off the coast of Southern Vancouver Island that regularly forage in deep waters and out of sight. Her hypothesis is that the areas differ in prey availability. 

Christie McMillan, Cofounder and Director of Humpback Whale Research for the Marine Education and Research Society (MERS) will be paying attention to Reidy’s results.

“With MERS, our work has been largely focused on the Northern Vancouver Island area, not the Southern area so that’s why this comparison is really interesting for us as well,” said McMillan.

Information about humpback whale feeding in deeper waters is limited and difficult to obtain. This is the challenge Reidy seeks to overcome: “see[ing] below the surface of the water” to document humpback whale prey off the Island’s south coast. She has been collecting data over the past three years on humpback whale feeding by using a combination of techniques including systematic prey mapping, concurrent prey mapping, fecal sampling, and whale tagging.

Prey mapping involves a fishery acoustic technique that measures prey distribution and abundance. A scientific echo sounder is used to detect the presence and distribution of fish, plankton and krill, and other potential humpback whale prey. 

Fecal sampling provides important information about what the whale is consuming. Samples are collected during acoustic prey mapping and tagging efforts, whenever whale scat floats to the surface of the water. These samples are studied through microscopic analysis for hard parts like fish bones and DNA analysis for prey species that are too digested to show up in the microscope. 

“Fecal samples provide only temporal snapshots of what the animal has recently eaten but by combining microscope work of hard parts and the DNA analyses of the fecal samples, we can get reliable regional estimates of the whale’s diet,” said Reidy.

Reidy also uses whale tagging, a technique in which a tag is attached to a whale’s back to provide three-dimensional measures of whale diving behaviour. Over three to four hours, Reidy follows the tagged whale and conducts prey mapping to record prey targeted by the whale. She then uses the data related to prey mapping and whale diving behaviour to understand humpback whale feeding patterns.

Her goal with this work is to develop a cost-effective, reliable sampling framework for getting real-time data to study humpback whale feeding and prey. 

“I hope that this becomes a framework used by [Department of Fisheries and Oceans] and non-government collaborators who are annually collecting critical feeding information from humpback whales off Vancouver Island,” said Reidy. 

McMillan also believes Reidy’s study will be useful.

“There’s an increased understanding that fish like herrings, which are so important to so many different species, really need to be managed in terms of their role in the ecosystem,” she said.

Understanding humpback whale feeding behaviour provides information about prey species such as herring that are key for ecosystem-based management. McMillan defines ecosystem-based management as the “concept that better understanding the role of a species in the whole ecosystem is important in our management of that whole species.” Fisheries as well as Reidy’s research can use ecosystem-based management, to ensure that there is enough herring and other species to provide for the needs of whales and the rest of the ecosystem.

“Knowing a whale’s diet is fundamental to understanding predator-prey interactions and food-web structure, to effectively conserve and protect habitat components that are relevant to these animals,” said Reidy.