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Scientific Tool Uses Otolith Geochemistry To Identify Source Of Illegally Introduced Invasive Fish
Posted on Friday, September 28, 2018 (PST)

An innovative scientific tool was used in Montana to identify when an invasive fish was introduced to a water body and where that fish came from.


Two researchers used forensic geochemistry using otoliths to identify the source of the first walleye found in Montana’s Swan Lake, finding that it was illegally introduced in the same year and that it came from Lake Helena, nearly 200 road miles away.


With additional forensic evidence – fishing license sales, webcams and boat registration – investigators could even identify the culprit who plucked the fish from one lake and took it to another, according to researcher Samuel Bourret, fisheries biologist with Montana Fish, Wildlife and Parks.


“By identifying the source water body where the illegal walleye were born it can help lead investigators in finding the culprit by pinpointing a place where the illegal actions were carried out,” Bourret said. The more information about the crime that is gathered the more likely a case can be formed. “For instance, our findings now allow investigators to look at fishing license sales, webcams, and boat registrations around the Lake Helena area for the time period when the walleye were illegally introduced (spring 2015).”


“Using Forensic Geochemistry via Fish Otoliths to Investigate an Illegal Fish Introduction” was published on the web August 26, 2018 ( in the Canadian Journal of Fisheries and Aquatic Sciences. Bourett’s co-author is Naill Clancy, graduate student at Utah State University.


The study says that illegal introductions of fish “create some of the most challenging problems for resource managers because of their potential to harm existing recreational fisheries and their impact on species of conservation concern.  However, if the origin of the fish introduction can be determined, then it could help fisheries managers in “preventing the colonization and subsequent ecosystem impacts of introduced species.”


Swan Lake, a relatively deep lake near Bigfork, Montana, is critical habitat for threatened bull trout and native westslope cutthroat, but it is also a lake that has been illegally planted with lake trout and northern pike. The lake is isolated above Big Fork Dam and there are no connected sources of walleye, making it impossible for walleye to naturally colonize Swan Lake, the study says, yet two walleye and three lake trout were caught in the lake in October 2015. Lake Helena is located in southwestern Montana. Both lakes are in the Columbia River basin.


In 2017, MFWP took walleye otolith samples from each of 13 lakes and reservoirs that had the highest pressure from walleye anglers (measured in angler days) and located near Swan Lake, thus creating a database. These waterbodies were sampled to build a geochemical database used to assess the potential sources of the illegally introduced Walleye, the study says.


Furthermore, MFWP is constructing a database for all geochemical samples collected in lotic (flowing water) and lentic (still water) waterbodies that would allow quick response to the detection of future illegal fish introductions, the study says.


The location and time identification uses the “chemical chronology of a fish’s life history deposited in the inert concentric layers of CaC03 in otoliths (Kennedy et al. 2000; Barnett Johnson et al. 2008),” the report says. “Naturally occurring strontium (Sr) substitutes for calcium (Ca) in otoliths and its relative concentration provides a temporal and spatial record of changes in Sr concentration encountered over a fish’s lifetime (Campana and Thorrold 2001; Wells et al. 2003). Sr can be found in differing concentrations among waterbodies due to landscape scale variation in geology, land use, and weathering patterns.”


Using otolith geochemistry, the researchers also found that a walleye population may not have been established in Swan Lake (the caught walleye was the introduced fish), but still the lake should be monitored for their presence.


“If the fish were found to have been naturalized in Swan Lake, the outcome could have included widespread suppression efforts targeting walleye habitat and figuring out ways to suppress the population before it established a base,” Bourret said.


“Our results also highlight the utility of otolith geochemistry as a forensic tool, which has led to another study of a suspected illegal northern pike introduction in Lake Mary Ronan (near Flathead Lake),” he added. “We hope that by understanding more about illegal introductions and getting closer to making an arrest, our study will help curb future illegal fish introductions by notifying the public that MFWP is serious about stopping the problem.”


The study also showed the first ever use of otolith geochemistry as a forensic tool on a large scale (the state of Montana), he said.


“Management implications for our findings reach far and wide with invasive species management and AIS (aquatic invasive species) in Montana and the Columbia Basin,” Bourret said.


The study “provides a tool to fisheries professionals that may improve the problem of illegal fish introductions by providing law enforcement information about where the crime was initiated. Such awareness can aid investigators by determining a place to start looking for additional evidence such as a personal witness, webcams, boat registrations, or fishing license sales,” it concludes. In this case, a $32,500 reward has been offered for information that would lead to a conviction.


Bourret had previously presented this study’s findings to the Northwest Power and Conservation Council in 2016 at its meeting in Missoula. While data at the time indicated the walleye had not been born in Swan Lake, it was prior to the second phase of the project, which is when MFWP began to form its geochemical database throughout Montana walleye waters and before Bourret had identified the source water body.


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