Digging Deep for Crayfish Clues

Surveying for Crayfish Means Cut Hands, Cold Fingers – and Critical Data

Photo collage showing active sampling methods: excavating and suction
Stewart Ray, a U.S. Fish and Wildlife Service volunteer, excavates
a burrow (top), and Zanethia Barnett uses a slurp gun to suction a burrow (bottom). Photo by Susie Adams, USFS.

Kneeling in a wet prairie, arm extended to the armpit in a muddy hole, most biologists would quickly arrive at the thought, “There’s got to be a better way.” So it’s not surprising that, when it comes to sampling for burrowing crayfishes, researchers have devised some creative solutions.

In the southeastern U.S. – the global center of crayfish biodiversity – sampling efficiencies and biases for crayfishes have seldom been quantified.

Quantitative sampling is fundamental to our ability to assess population trends and conservation status, as well as the effects of land and stream management on populations. Reliable data depend on efficient, quantitative sampling methods. That’s why we have been quantifying crayfish sampling methods and biases in diverse habitats since 2013.

The Mississippi Sandhill Crane National Wildlife Refuge was one place where biologists needed some crayfish answers. The refuge was established in 1975 to conserve the last remaining population of the Mississippi sandhill crane (Grus canadensis pulla), an endangered species and one of North America’s rarest birds.

Since the refuge was established, restoring and maintaining crane habitat has been a priority. Managers use fire and mulching to provide open prairie and savanna. As refuge biologists broadened their management goals to include more flora and fauna unique to the sandy, wet pine savanna and prairie, they began wondering how their practices affected at-risk burrowing crayfishes. The questions also applied to nearby lands managed by the USDA Forest Service, The Nature Conservancy, and others.

A photocollage of two crayfish species and four photos - some photos showing color variants of the species
Crayfish species collected from the burrows on the refuge. Top photos of Creaserinus oryktes (two color morphs) by G.A. Schuster; bottom left, a third color morph of C. oryktes by Susie Adams; and bottom right, Procambarus fitzpatricki by C. Lukhaup.

Of all the places to dig for burrowing crayfishes, the refuge offers one of the best experiences. The loose, sandy soils made digging by hand relatively easy. With the wild calls of cranes flying overhead, it was a welcome respite from trash-strewn roadside ditches with vehicles zooming by.

We selected and categorized sites based on whether they had been frequently burned, mechanically treated, or infrequently managed. We coarsely quantified vegetation, counted crayfish burrow entrances, and tested six methods for sampling burrowing crayfishes.

We tested two passive sampling methods and the influence of weather on trap captures:

  • Modified Norrocky traps consist of a PVC pipe with a one-way flap near the bottom and a cap on top. In concept, the crayfish exits the burrow and enters the trap as the one-way flap snaps closed behind it.
  • Mist net traps consist of a square of net material folded numerous times, tied in the middle with a string, and inserted into a burrow. When a crayfish tries to remove the net, it becomes tangled in the netting and awaits extraction by the biologist.

At least that’s how it was supposed to work. For creatures with very small brains, crayfish managed to foil these traps in remarkably diverse ways.

We also assessed four active sampling methods:

  • Excavating was relatively easy: in the refuge’s sandy soils, shovels were not needed, so we used our hands and, if necessary, a Japanese soil knife.
  • Suctioning employed either commercially available slurp guns used by divers to capture fishes or homemade devices. The mouth of the outer tube was placed in water inside the burrow, and the inner tube was pulled to create suction. This sometimes directly extracted crayfish, but more often, pumping it back and forth disturbed the burrow water until the crayfish came to the surface. The method worked well only when the water table was close to the surface and was often used along with excavating.

    The sampling team - six people - stand in prairie with pine trees behind them
    The sampling team at the Mississippi Sandhill Crane National Wildlife Refuge. Photo by Scott Hereford, U.S. Fish and Wildlife Service.
  • Baited line sampling were conducted at night. Crayfishes are most active on warm, wet nights, but we were only able to search on dry nights
  • Visual surveys: on the two  dry nights when we searched for crayfishes, we did not see any at all.

We mostly captured crayfish called flatwoods diggers (Creaserinus oryktes) from burrows. We also caught spinytail crayfish (Procambarus fitzpatricki), a federal priority at-risk species that occurs only in several southern Mississippi counties.

Bad news for the biologists with their arms in muddy holes: excavating produced far more crayfishes than any other method. The hope for passive sampling methods is that they will require less effort than active methods. Indeed, setting and checking traps was relatively easy, but they usually produced few or no crayfishes. A combination of methods captured more crayfish — a result similar to previous assessments of sampling methods in stream and floodplain pool habitats.

We often think of passive methods as being less biased by human factors than active methods, but ours appeared to be biased by weather conditions. Recent rainfall was associated with a jump in captures, perhaps because dropping barometric pressure might trigger crayfishes to leave their burrows. Excavation success depended strongly on the digger. Some people regularly captured multiple crayfishes, while others never caught one.

Future monitoring efforts could benefit – in reduced bias or increased catch per unit effort — from further exploration of the relationships between weather and trapping.

A sandhill crane with a crayfish in its beak
A Mississippi sandhill crane feeds on a crayfish. The image was caught by a remote camera on the refuge on May 30, 2018 and provided the first evidence of the subspecies feeding on crayfish. Photo courtesy of U.S. Fish and Wildlife Service.

In a result welcomed by refuge managers, we discovered that the treatments used to create habitat for cranes also benefit burrowing crayfishes. The density of crayfish burrow entrances was higher in frequently burned and mechanically treated sites than in infrequently managed sites that had more dense, taller woody vegetation.

Shortly after our study, the U.S. Fish and Wildlife Service obtained evidence of what we’d suspected — Mississippi sandhill cranes feed on crayfish. Crayfishes contribute to many trophic interactions on the refuge as both predators and prey. Additionally, the refuge’s unique plant communities co-evolved with burrowing crayfishes, so their burrowing and foraging are likely important to plants.

Muddy biologists kneeling in prairies and roadsides are providing key information about quantitative sampling methods for burrowing crayfishes. These efforts provide a basis for future approaches. They also improve the ability to address ecological and management questions relating to these small — but functionally important — community members.

Read the full text of this article, which was originally published in The Wildlife Professional.  

For more information, email Susie Adams at susan.adams@usda.gov.

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