May 19, 2020

Tracking Cutthroat Movement & Mortality in Cowichan Lake

In a large waterbody like Cowichan Lake, ever wonder just how widely fish move around? As part of a study to better understand mortality rates in Vancouver Island’s biggest wild cutthroat trout fishery, provincial biologists have teamed up with Nanaimo-based Kintama Research Services (a consulting company with expertise in fish tracking) to attach acoustic tags onto catchable-sized cutthroat trout and monitor their movements in the lake.

Cowichan Lake supports a popular, limited-harvest fishery for cutthroat trout, and has done so for years. However, recent surveys suggest that angler effort has declined to an all-time low, and catch data suggests that fewer and smaller fish are available for the fishery. Without some indication of what’s going on in this cutthroat population, it’s difficult to determine if fishing regulations need to be updated. 

Holding pen for tagged fish. 

A five-year study was started in 2019 to improve biologists’ understanding of cutthroat mortality due to natural causes versus being captured in the fishery. Also of interest is the piscivorous (fish-eating) wild cutthroat population’s co-existence with kokanee. Most of the large-lake cutthroat trout fisheries on Vancouver Island have this same species-pairing, but research into these kinds of fish communities – and our understanding of cutthroat trout movements and behaviour in the presence of kokanee – is limited. What is clear is that kokanee provide an excellent food source for these cutthroat, which allows them to grow to impressive sizes at maturity (with some specimens reaching lengths of more than 60 centimetres).

Tagged cutthroat trout. 

The cutthroat trout in Cowichan Lake are adfluvial, meaning they spawn in the inlet streams around the lake, and then migrate as juveniles into the lake’s more productive habitats, where they generally stay until they reach maturity. In a sizeable lake like Cowichan, which has three sub-basins, there are plenty of options to either hunker down in one area, or explore farther to seek out prey. Biologists have wondered to what extent individual fish use different areas: are there sub-populations that restrict themselves to just one section, or do the fish move around the whole lake? Addressing these types of questions helps to identify vulnerabilities of fish at different stages of their lives, and provides context when interpreting (or designing) fisheries surveys.

During October 2019, 83 catchable-sized fish (greater than 20 centimetres in length) were each tagged externally with a $100 high-reward tag; 42 of these fish were also implanted with an internal acoustic transmitter. Fish were then released in approximately the same location where they were captured. To date, anglers have already captured five fish with high-reward tags and reported them to provincial fisheries. Such high-reward tags have been successfully used in other lakes on Vancouver Island, including Horne and Comox, to encourage anglers to report the date and location of their catch.

Performing surgery to insert acoustic tag. 

The acoustic transmitters will send out signals for two to three years, and are tracked by an array of semi-permanent acoustic receivers that have been placed in the lake. Six receivers are also being seasonally deployed at key spawning streams to track spawner migrations during the spring. All receivers are downloaded periodically, and transferred to a geospatial mapping program that tracks fish movement over time. As well, more fish will be added to the study in each of the next three years, which means that biologists will continue to track fish movement and behaviour for the duration of this five-year project.

Biologists Mike McCulloch and Jennifer Sibbald on the tagging platform. 

Biologists hope to gain information on cutthroat survival (based on estimating fishing-related versus natural mortality) using the movement information and reporting of high-reward tags by anglers. They also hope to improve the understanding around cutthroat spatial ecology by asking questions like:

  • Does in-lake movement change with season or depend on fish size?
  • Do individual fish use the full extent of the lake or just portions of it?
  • What types of movements do mature fish make before and after spawning?
This multifaceted investigation into Cowichan Lake cutthroat trout is still in its early stages, but is already producing some interesting insights into fish behaviour and angler use. The data suggest that during this time, most fish tend to stay within the sub-basin where they were originally tagged, but some fish do move around significantly. Over the next five years, the results of this study will help to best manage this special wild cutthroat trout population, and ensure that anglers continue to enjoy the quality fishery.

This initiative is a joint project between provincial biologists and Kintama Research Services. The project is funded in partnership with the Habitat Conservation Trust Foundation and the Freshwater Fisheries Society of BC.

For more information, please contact Brendan Anderson (Fisheries Biologist, Nanaimo, Ministry of Forests, Lands, Natural Resource Operations and Rural Development).

Watch tagged fish movement on an interactive animator

The movements of acoustically tagged fish between November 2019 and March 2020 can be viewed via Kintama’s open access, interactive animator. The animator can be panned, zoomed, and queried. Users can customize the animation by changing many options using the left-hand menu, including choices to increase or decrease the speed of the animation (Timing tab) or view fish by zone (Query tab). By pausing the animation and right- or left-clicking on a fish or a receiver, additional information can be displayed.

Try this! Under the ‘Query’ menu, type 8882 in the Tag ID field, click Go, and then the play button at the top. The animation shows a single cutthroat trout that stayed in the western section of the lake for more than two months before briefly foraying into the eastern section. If you pause the animation and right-click on the fish dot, you can select Show Info to see how big this fish was, and when it was tagged. Right-click again and you can select Show Depth Plot. Reload the web browser page to reset the animation.

This blog series has been established as a way to inform freshwater anglers in B.C. about projects, such as this one, that their angling licence dollars support. In 2015, the Freshwater Fisheries Society of BC began receiving 100% (up from 70%) of all freshwater angling licence fees. With this additional funding, the Society committed to broadening the scope of its activities to include joint initiatives with the Provincial Government to support projects that benefit freshwater recreational fishing around B.C.

Author: Sue Pollard, Freshwater Fisheries Society of BC.
Images: Sue Pollard; Kintama Research Services.