Limnology in Action: Taking Crayfish for a Walk

Hilts and Caldeira working together to collect crayfish traps.

It is not every day that you get to take a crayfish for a walk on a leash. Well, unless you are part of Alex Latzka’s research team.
“This is going to be the best field day of your life!” promised Emily Hilts, his incredibly enthusiastic undergraduate assistant. And so far, it was true.
I was tagging along with Latzka and his two assistants, Hilts and Yuri Caldeira; spending the day studying how invasive rusty crayfish are affected by fish predation in various habitats. This particular morning started by pulling up crayfish traps (which are modified minnow traps baited with beef liver) all around Sparkling Lake.
Comparing crayfish: virilis (left) can be told apart from a rusty (right) by its yellow-ridged claws.

A rusty crayfish (right) can always be told apart from a native crayfish (left) by its telltale “rust” spot on the carapace.

While pulling up the traps, I got to try my hand at identifying native versus invasive crayfish. Trying not to get pinched by their enormous claws, I carefully inspected the nasty, invasive rusty crayfish alongside the native Orconectes virilis. I found that virilis have beautiful blue claws with pretty yellow spots along the edge while rustys have smoother claws and a tell tale “rust” mark on the sides of their carapace (back). Besides the literal “boat-loads” of crayfish brought up, I also got to see juvenile bass, two beautiful Mud-puppies, and a terrifyingly large leach.
“In this lake, there are three main habitat types,” explained Latzka, “vegetation beds, sand, and cobble. We have found that rustys prefer to be in the cobble.”
“Cobble” is a medium size stone, larger than a pebble, but smaller than a loaf of bread. However, in the last several years a drought has caused the lake level to fall, exposing about half a meter of lakeshore; and a great deal of the cobble habitat. “What we are looking at here is whether this decrease in habitat has had an affect on fish predation on the rustys.”
Reduced shoreline has reduced amount of rusty crayfish habitat which, researchers believe, has aided their decline.

This is where the fun began. In an experiment to study which habitats (cobble or sand) have higher fish predation, the team uses leashed crayfish.
Enormous rusty crayfish goes for a walk after being “tethered.”

With the remaining captured rusty crayfish, I was taught how to attach a “tether” to the crayfish carapace using glue and fishing line. Of course, the first crayfish handed to me was gargantuan; probably the grandfather of all rustys in Sparkling. I held its claws as tight as I could, afraid it would break loose and take off my nose or a finger with its furiously snapping pincers. At last, the glue dried, and Caldeira walked the beast around the bottom of the boat, gently tugging it away from unprotected toes.
Finally, ten crayfish were tethered and ready. They were then placed on a line stretched between two metal poles stuck into the sand at 3 feet water depth. With nowhere to hide, it was an all-you-can-eat crayfish buffet for any hungry bass swimming by.
The whole process was performed again, this time in a cobbled habitat, which offered considerably more cover for the tied-up crayfish. Hilts explained that in the evening the team will come back to see how many crayfish remain on each line, comparing the predation rate of each habitat. Then they will set up a “nighttime tether,” in order to also compare nighttime versus daytime differences.
Research assistants get advice from Latzka on setting up their distance sampling.

But the field day was far from over with this crew. Climbing into wetsuits and snorkeling gear, the undergrads got ready for their “distance sampling,” a technique often used on landscapes to determine the abundance of any given organism a scientist is sampling. For example: in a forest, a scientist would walk down a linear transect and mark down all the oak trees they can see from the transect, then record the distance to the tree, the size, etc.
Hilts swimming to set up the transect.

However, distance sampling is rarely used in the water. But like all projects in limnology, some things just have to be modified to be water-proof. And as I tried my best to help set up a “straight” transect line (battling wind and waves) running perpendicular from the beach into the depths, I saw just how difficult it was to convert a dry-land experimental process to a water-world one. Eventually I got it set up to meet Latzka’s approval, and Hilts began the sampling.
Caldeira and Hilts switch back and forth covering the 5 transects set up over the crayfish tethers. They swim along the transect with a submersible clip board,recording every fish species they see, as well as the depth, distance, abundance and size. From this data, Latzka can estimate the abundance of predators that are near the cobble and sand crayfish lines.
Taking the distance samples along a transect.

Returning to the station well after 5 pm, I was exhausted. I didn’t dare complain however, since I had (for the most part) remained un-pinched, leech-less, dry and glue free compared to the others. Despite being contenders for Discovery Channel’s “Dirty Jobs,” Latzka’s team is entirely enthusiastic and dedicated to their field work, and how it will contribute to lakes management.
For example, one outcome Latzka hopes to see from this research is a management plan dedicated to removing the remaining cobble from Sparkling. If this project does show that rusty crayfish can successfully hide and survive better among the cobble habitat, then Latzka can prove that the most cost effective and energy efficient eradication strategy in Sparkling (and other similar lakes) would be to remove all the stones from the shallows.

(Correction: The usefulness of this study as a management tool would be to identify other candidate lakes that might benefit from intensive crayfish trapping. If water levels are low, and cobble habitat is stranded, the chances of dramatically reducing invasive rusty crayfish increases. Physically removing cobble from a lake would prove time and labor intensive and, perhaps, destructive to other plants and animals that rely on the cobble.)
Latzka is a graduate student in the Vander Zanden Lab at the CFL, studying the patterns of aquatic invasion across various landscapes and lake types in order to better classify lakes vulnerable to aquatic invasive species. This is Latzka’s third summer on Sparkling Lake, in which he and his students are studying the invasive Rusty Crayfish. “What we are looking at is the interaction between fish, crayfish, and habitat,” explains Latzka. “We want to see how invasion is affected by fish predators.”
Alex Latzka shows off an especially large virilis (or native) crayfish.

This research is coming in on the heels of a massive rusty crayfish removal effort by the CFL from 2001-2008. While the removal was extremely successful in significantly reducing the rusty population, there still remains remnant populations. Latzka’s team is now monitoring if the lake can either return to a pre-invasion state, or at least keep the rusty population in check.
As the number of aquatic invasive species grows in Wisconsin Lakes, more management strategies are being devised to either prevent or eradicate invasives. But in order to allocate the most effective and affordable strategies, Latzka is setting out to classify the most vulnerable lakes in order to help the DNR and Lake Associations delegate appropriate resources where needed.

All summer long, Trout Lake Station outreach assistant, Ali Branscombe will be bringing you stories from the field. Join Ali as she follows researchers slogging through wetlands, boarding boats, and wrangling fish – all to bring you Limnology in Action.