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Spawning Secrets

Tracking lake trout on Lake Pend Oreille

Lake trout (Salvelinus namaycush) in Lake Pend Oreille are a common topic of discussion in our bottleneck of land and water. This is a place where talk and walk meet to get things done, currently in the form of a lake trout tracking program.

Relatively little is known about the behavior of the prolific and non-native Lake Pend Oreille lake trout. Generally, lake trout in the West have created fierce competition for native fish species, being the rampant predators that they are. Though lake trout were introduced into Lake Pend Oreille in the 1920s, it wasn’t until Mysis shrimp became established in the 1970s that lake trout survival started to increase, and the 1990s before the population really took off. We are now seeing the aftermath of releasing an army of adaptable trophy trout. The biggest issue we have with lake trout in Lake Pend Oreille is the threat that they pose to the kokanee salmon (Oncorhynchus nerka), who provide most of the diet for the threatened bull trout (Salvelinus confluentus).

Just in case you’re not versed in the jargon of predator removal, let’s walk through the fishery programs currently taking place on the lake. The “Angler Incentive Program” came about in 2006 when Idaho Department of Fish & Game and a fishery Task Force (comprised of stakeholders) recognized the need for immediate action on Lake Pend Oreille to prevent it from being overtaken by lake trout. The mission of the AIP is to save the collapsing kokanee salmon and by doing so, protect the native bull trout. Also in 2006, rod limits were increased to four poles (with a two pole license) to give anglers a greater chance to catch lake trout. There are freezers placed around the lake for anglers to drop off heads of AIP fish. For more information see http://fishandgame.idaho.gov/fish/misc/pendoreille_cash.cfm.

The IDFG Bayview Research Station obtained mitigation funding from the Bonneville Power Administration to take the predator removal program one step further by identifying spawning locations to help guide the placement of gillnets and anglers. To date, not much data has been collected on lake trout spawning sites in Lake Pend Oreille. Research in other areas suggests that lake trout spawning occurs in fall, between September and November, after water temperatures have dropped below 53 degrees F. Lake trout are nocturnal, so spawning occurs at night. They do not construct redds (nests), but rather lay eggs over cobble substrate.  

Because they are such an adaptable species, lake trout are capable of spawning in many different environments. In Ontario, for instance, spawning occurred in just 4.6 feet (average) of water at 243 sites in 90 different lakes. And though there are some documented cases of lake trout spawning deep, such as in the Great Lakes, most of them prefer relatively shallow waters. Also, IDFG wasn’t ruling out the possibility of lake trout moving into tributary rivers or streams for spawning, as was observed in Lake Superior in the 1950s, reports senior research biologist Greg Schoby.

On Lake Pend Oreille, the research team, starring Schoby, utilized trap net catches to tag and track sexually mature lake trout, also known as mackinaw. Schoby reports that team tagged 31 lake trout between 28 and 40 inches in 2007—“all adults since we’re looking for spawners.” These fish weigh between 8 and 21 pounds; and while fish were marked from all parts of the lake, the majority were captured and tagged in the north end. When lake trout of the appropriate size were captured, they were tagged on the trap net boat, a tool also utilized for predator removal on Lake Pend Oreille.

The research team performed a relatively quick surgery on the demo lake trout, which consisted of implanting an acoustic and radio tag combination. The battery life of these tags is approximately nine months. The acoustic tags work better in deeper water, and the radio signal is only good to about 30 feet deep, but is said to work well in shallow water as well as noisy environments, such as rivers, says Schoby. The idea behind using both radio and acoustic tags at first was that at that point, IDFG really didn’t know where lake trout were spawning in Lake Pend Oreille—shallow or deep waters.

“Everything was an option when we started,” says Schoby. “We didn’t want to rule anything out; based on everything we saw [in 2007]; it didn’t look like they spawned in less than 75 feet.”

The surgery consists of sedating the fish so they can’t flop or wriggle during surgery, making an incision between the pelvic and pectoral fins (just along the belly), and placing the transmitter inside the body cavity before stitching the fish back up. The whole process takes between 5 and 15 minutes, depending on which tags are implanted. Schoby reports that the research has had no known surgery-related mortalities on tagged fish.  

After the tagging procedure, the mackinaw are released back into the trap net. The nets are reset, allowing tagged lake trout to recover for 2 or 3 days before the next pull. After the recovery period, when the research team is confident that the tagged fish will survive, they are released back into the wild blue waters of our ear-shaped lake.

Tracking of the tagged fish is done with the aid of a pair of boat-mounted hydrophones, which “listen” for signals emitted by the transmitters. These provide a directional fish location. This system has a detection range up to about three-quarters of a mile, says Schoby. The signals emitted by the transmitters are “heard” by the hydrophones, which in turn send the data to a computer system that tells Schoby which tagged fish are near, which side of the boat they are on, as well as how deep and at what temperature. The team is able to track while moving at boat speeds up to about eight miles per hour, which allows them to track the entire lake in about three days.

“The nice thing about [this system] is you can move while tracking,” says Schoby. “A lot of the older telemetry equipment used a megaphone-like device—you had to stop and listen, remove the gear, go to the next spot and stop and listen all over again.”

While tracking, the team also used a variety of techniques to observe egg deposition and document spawning activity. There were four sets of egg traps deployed in locations where tagged lake trout congregated, in order to verify that the spots the fish were concentrated were, indeed, where they were spawning. Underwater video was used to observe substrate and search for eggs and groups of fish. And finally, divers were used to search this area for deposited eggs. In the end though, the cameras didn’t work much deeper than 100 feet and the egg traps had issues staying upright; no eggs were captured.

So far, the lake trout seem to key in to a couple of spots around the lake, reports Schoby. “They are either not as adaptable as we thought, or the population is not so big that they’ve had to spread out. With the sheer amount of shoreline [on Lake Pend Oreille] we really thought they would be more widespread.”

Though information is still being collected about where the mackinaw congregate the rest of the year, they begin to concentrate in choice areas right before spawning season. By July, tagged lake trout began leaving the sites where they were tagged and start heading towards spawning areas. Even though the tagged fish come from all over the lake, by mid-September, 93 percent of the tagged fish were found in one location! Where? Well, near Windy Point. Between 100 and 150 feet (average 110). Go get ‘em! For interested anglers, IDFG does recommend working the sites over with jigging gear, as the area will be heavily laced with gillnets to maximize the removal effort, reports IDFG Panhandle Supervisor Chip Corsi. “Trollers will be apt to lose gear in the nets.”

To date, information has been collected since the spring of 2007; the research team has been monitoring and tracking the tagged lake trout for nearly a year-and-a-half. They will continue to track these indicator fish throughout the summer and fall. This year, the netting boats will target the areas where the lake trout are congregated.

“The hope is that this year a significant portion of the adult lake trout in Lake Pend Oreille will be harvested before they spawn,” says Corsi. “Interrupting the spawning cycle, while removing large fish, should help buy kokanee some breathing room in the lake, and make a significant contribution to the recovery of the kokanee fishery, and ultimately, the trophy rainbow fishery and bull trout they support.”  

If you catch a tagged lake trout, please gently and carefully release it, as these fish are utilized for research purposes. In the case that a hooked tagged lake trout does not survive the catch, IDFG urges anglers to alert them and report the tag so they can document the mortality and use the tag for another fish.

“A bonus would be if we could look at the fish to determine sex and remove otoliths [ear bones] for aging,” says Principal Research Biologist Andy Dux. “The angler could still keep the fillets and get a reward for the head.”

Anglers are needed to successfully remove predators to conserve the Lake Pend Oreille fishery. Though this can be a daunting task for the weekend angler, there are resources available, such as charter services and free fishing seminars that can aid anglers in learning more about catching lake trout.  For anglers who are interested in participating in the AIP or currently do and would like to know more about the lake trout tracking program or the specific placing of the gillnets, contact IDFG at 208-769-1414.

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Kate Wilson Kate Wilson was a Project Journalist for Avista's Clark Fork Project. She has been interested in environmental issues since she was a youngster.

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