Catch me if you can: study reveals the best ways to mark and recapture sea lampreys

Sea lampreys (Petromyzon marinus) are primitive, eel-like predators that can grow to be 0.75 meters (2.5 feet) long. Originating from the Atlantic coast of North America, they have invaded all five of the Great Lakes. These nocturnal fish have a sucker-type mouth they use to feed off many types of sport and commercial fish, often leaving a large wound that can become infected and ultimately lead to a prey fish’s death.

One sea lamprey can damage an average of 18 kilograms of fish over its lifetime, and the species has caused extensive harm to the commercial, Indigenous and sport fishing stocks throughout the Great Lakes.

“While eradication is not an option for this destructive species, both Canada and the United States have spent decades working jointly to develop and improve control programs to keep the Great Lakes lamprey population in check,” says Jessi Nelson, a graduate student in the Department of Integrative Biology who conducted research on sea lampreys for their undergraduate thesis.

One area where extensive sea lamprey management efforts have been undertaken is the St. Mary’s River, a 120 km-long waterway that serves as part of the international border between Ontario and Michigan and links Lake Superior to Lake Huron.

Managing any sea lamprey population requires the ability to reliably estimate their abundance. One popular method is what’s known as “capture-mark-recapture”. This requires trapping and marking (tagging) individual lampreys during their annual spawning run. Lampreys are then re-captured and counted at a different location, and the proportion of tagged individuals that get recaptured helps scientists and managers estimate the total population in an area.

There are different tagging methods available, and researchers and managers will use different types of tags based on their specific need or research question. But this can also lead to differences in recapture rates.  To help shed more light on this issue, Nelson teamed up with Dr. Rob McLaughlin, a professor of animal biology in the same department.

“We wondered if differences in tag size, or how much physical handling of the lamprey each tagging method requires, could explain different rates of recapture,” explains Nelson.  

Nelson and McLaughlin compared the trapping efficiency of three different commonly used tagging methods for sea lampreys. The first method – which is also the most common – is fin clipping, where a lamprey’s fin is clipped with scissors in an easily recognized pattern.  The second method involves the implantation of a small passive integrative transponder (PIT).  This approach requires a 5 mm incision in the lamprey, but no sutures.  The final method employs a larger acoustic transponder which sends an active radio signal and thus offers better tracking capabilities, but it requires a 10mm incision and sutures to close the incision.

Tagging sea lampreys for research or monitoring presents unique challenges. “Lampreys have a really long slender eel-like body and they undulate a lot as they move, which is different compared to many other fish species,” notes Nelson.

For their research, the team tagged over 800 sea lampreys using one of the three methods, then released and re-captured a portion of these tagged individuals in large underwater traps located upstream. The lampreys were then sent through a scanning apparatus to detect the transponders. Unfortunately, a malfunction in the scanner occurred, causing some of the PIT-tagged sea lamprey to go through undetected. Luckily, the problem was identified, and Nelson and colleagues were able to replicate the dataset in lab and statistically correct the error before the results were fully analysed.

The team ultimately found that recapture rates of sea lampreys implanted with PIT tags were similar to those of externally fin clipped individuals. However, sea lampreys with the larger internal acoustic tags were less likely to be re-caught compared to those that were PIT tagged individuals.   

The results suggest that larger tags might interfere with the lamprey’s ability to swim normally, making it less likely to end up in one of the traps used for recapture. The findings help explain differences in trapping success observed when using different tags.

Nelson says that the results will be helpful to biologists who study both sea lamprey management and behaviour.

“This research is helpful because it contributes to our understanding of tagging methods and their influence on lamprey trapping behaviour. For any mark or tag to work, animals with tags must behave the same as those without tags. In this case, we did see differences in trapping across the tagging methods, suggesting that larger tags may not be suitable for sea lamprey.”

For sea lamprey managers, who use trapping success data to gauge the success of the control program, resolving the differences helps allay concerns about the reliability of estimates from the capture-mark-recapture method the control program has used for decades.

This study was funded by the Great Lakes Fishery Commission’s Sea Lamprey Research Program, Canada Foundation for Innovation, and Ontario Innovation Trust.

Read the full study in the Journal of Great Lakes Research.

Read about other CBS Research Highlights.