Zachary Sherker in summer 2017, looking for passive integrated transponders (PIT) tags from salmon in the excrement of great blue herons under their rookery in Stanley Park. Photo: Rhea Storlund.
Salmon, herons and killer whales – what connects these three creatures? That’s what Master’s student Zachary Sherker is studying.
A member of UBC’s Marine Mammal Research Unit (MMRU), and student in UBC’s Institute for the Oceans and Fisheries (IOF) and Institute for Resources, Environment and Sustainability (IRES), he has a special interest in predator-prey interactions.
Under the supervision of Dr. Andrew Trites, Zachary’s research focuses on Pacific great blue herons in British Columbia, which prey on young salmon smolts migrating out of rivers, potentially limiting the amount of adult salmon available for killer whales in the ocean.
Tell us about your research.
My Master’s work focuses on juvenile wild and hatchery-reared Chinook from the Cowichan River, and juvenile hatchery-reared Coho from the Capilano and Big Qualicum Rivers. A lot of existing research has suggested that many juvenile salmon don’t survive the initial journey from the rivers where they were born to the ocean, with mortality being as high as 50 per cent. Drawing from that, I investigated potential freshwater and estuarian predators that may be influencing their survival, like river otters, mink, raccoons, and Pacific great blue herons. Eventually, I was able to identify great blue herons as a major predator.
How did you single out great blue herons, out of all the other animals you looked at initially?
I did it using pill-sized tags called passive integrated transponders, or PIT tags. These tags are implanted near the stomachs or in the body cavity of both hatchery and wild juvenile salmon, and they emit signals that are picked up by PIT tag detectors installed in salmon streams. Since each tag is uniquely numbered, we can track the movement of different salmon individuals, which can give us a lot of insight into their migration and survival.
When a predator eats a tagged fish, the PIT tag will travel through the predator’s digestive system, ultimately ending up in the predator’s scat. As part of my study, I went to various locations in B.C. to retrieve these tags. I wasn’t finding very many salmon tags for river otters, mink and other predators, but when I started scanning heron scat I found lots of tags, which really served to pinpoint them as key predators.
What can you tell us about the tags you’ve found?
To date, I’ve found over 1,000 tags in different great blue heron rookeries around B.C., primarily in the Cowichan River, the Capilano River, and the Big Qualicum River. This accounts for around 1 to 3 per cent of all the salmon tags that are released annually. It should be noted, though, that, since herons do not defecate exclusively in the rookery, and some tags may be broken during ingestion, these tag recoveries represent a minimum predation estimate on outmigrating salmon smolts in our study systems. There’s definitely more work to be done, but this is a first step towards identifying a new predator that was previously unidentified.
What are some of your findings?
So far, I’ve found that smaller smolts are more likely to be eaten by great blue herons, compared to larger smolts. I’ve learned this through PIT tag databases, which store a massive amount of information about tagged salmon, including size upon release.
Basically, once I’ve found a salmon’s tag, I can scan it to find out more information about the fish from a PIT tag database. For example, I could find out when the salmon was tagged, its specific species, its size at the time of tagging, and whether it was detected in a river before it was eaten. All this database information is really useful for telling us which salmon are more likely to be eaten.
What are some reasons why herons might be preying on smaller smolts?
There could be a number of explanations, but one is that smaller juvenile salmon migrate more slowly through rivers than larger salmon, so it could be that they are more likely to be eaten because they are spending more time in the herons’ foraging area . Or it could be that smaller salmon are slower at evading predators due to their size. Larger smolts may also outcompete them for protective refuge habitats, leaving smaller smolts exposed to danger.
How does your research fit into what the rest of the Marine Mammal Research Unit (MMRU) team is doing?
One of things my lab is trying to figure out is why the southern resident killer whales are in poorer health than the northern resident killer whales. Since the southern residents’ major food source is Chinook salmon, it is possible that their poor condition could be explained by a shortage of adult Chinook.
That’s really where my research about juvenile smolt survival and herons comes into play. Maybe many juvenile salmon smolts aren’t making it to adulthood because they’re being preyed on by predators, such as the great blue heron, which results in fewer adult salmon for southern residents to eat and therefore poorer health for the whales. My work aims to provide information that will help us understand the factors that influence survival to adulthood in Pacific salmon.
My work aims to provide information that will help us understand the factors that influence survival to adulthood in Pacific salmon.
What sparked your interest in studying great blue herons and salmon?
I’ve always been really interested in predator-prey interactions. When I was doing my undergraduate degree in Nova Scotia, I studied what’s known as morphological shell plasticity in mussels, or how mussels’ shell growth changes under different environmental conditions. I found that when you introduce a mussel to a predatory drilling snail, it will actually grow a thicker shell and that shell will protect it over time from its predators.
Andrew Trites, my supervisor, studies a lot of predator-prey interactions, especially between southern residents and salmon, so working with him on this project sounded like a good fit for me and my own research interests.
What is your favourite thing about research?
Fieldwork is probably my favourite part. Being outdoors, paying attention to what’s around you and trying to actually understand the system that you’re working in by being part of it — that process is fascinating to me.
I also really enjoy the collaborative aspect of research. It’s great that there are so many people from diverse research areas who have insights into the same questions that I’m working on. I was funded by Mitacs and the Pacific Salmon Foundation — and worked in collaboration with BC Conservation Foundation on this project. I’ve also worked directly with the Cowichan Tribes, on whose territories I first began the study. Everyone really tries to get together and solve these issues to better understand the ecological systems we rely on.
What do you like to do in your leisure time?
I definitely spend a lot of time outdoors! I was living on Canada’s East coast prior to coming out here, where there aren’t mountains like there are in B.C., so I’ve been hiking a lot and trying to slowly introduce myself to mountain biking without incurring any serious injuries. I also joined a Sunday morning hockey league, and play the drums and harmonica in a funk band with my buddies on the weekends (much to the disdain of my neighbours). Vancouver also has a fantastic live music scene that I’ve had the pleasure of enjoying since moving to the city.
We are pleased to announce that Pamela Rosenbaum, Manager of the Marine Mammal Research Unit (MMRU), Institute for the Oceans and Fisheries (IOF), Faculty of Science, has been awarded a President’s Service Award for Excellence. 
