Evaluating artificial shelter arrays as a minimally invasive monitoring tool for the hellbender Cryptobranchus alleganiensis

Abstract

Hellbenders Cryptobranchus alleganiensis are critically imperiled amphibians throughout the eastern USA. Rock-lifting is widely used to monitor hellbenders but can severely disturb habitat. We asked whether artificial shelter occupancy (the proportion of occupied shelters in an array) would function as a proxy for hellbender abundance and thereby serve as a viable alternative to rock-lifting. We hypothesized that shelter occupancy would vary spatially in response to hellbender density, natural shelter density, or both, and would vary temporally with hellbender seasonal activity patterns and time since shelter deployment. We established shelter arrays (n = 30 shelters each) in 6 stream reaches and monitored them monthly for up to 2 yr. We used Bayesian mixed logistic regression and model ranking criteria to assess support for hypotheses concerning drivers of shelter occupancy. In all reaches, shelter occupancy was highest from June-August each year and was higher in Year 2 relative to Year 1. Our best-supported model indicated that the extent of boulder and bedrock (hereafter, natural shelter) in a reach mediated the relationship between hellbender abundance and shelter occupancy. More explicitly, shelter occupancy was positively correlated with abundance when natural shelter covered <20% of a reach, but uncorrelated with abundance when natural shelter was more abundant. While shelter occupancy should not be used to infer variation in hellbender relative abundance when substrate composition varies among reaches, we showed that artificial shelters can function as valuable monitoring tools when reaches meet certain criteria, though regular shelter maintenance is critical.

Weathering the storm: Improving the availability and stability of artificial shelters for hellbender salamanders

“…Boot design shelters are made from concrete and consist of a rectangular‐shaped chamber that hellbenders can access through a single tunnel entrance. Assuming artificial shelters remain consistently secured in place, stay unblocked by sediment, and are used by hellbenders, they have the potential to serve a variety of functions critical for hellbender conservation, including providing eggs for captive rearing, serving as population monitoring tools, improving knowledge about hellbender reproductive biology, and augmenting existing hellbender habitat (Bodinof Jachowski et al, In Press; Bodinof Jachowski, Ross, & Hopkins, 2020).…”

Factors influencing the availability of artificial hellbender (Cryptobranchus alleganiensis) shelters in North Carolina, USA

“…In streams, numerous fish habitat restoration projects have included instream flow refugia in the form of boulders and large woody debris (Louhi et al 2016), but improvement of habitat availability for cavity–dwelling vertebrates has received much less attention. Artificial cavities as habitat augmentation and monitoring tools have been used successfully for Carolina madtoms and, more recently, hellbender salamanders ( Cryptobranchus alleganiensis ; Briggler and Ackerson 2012, Cope et al 2019, Bodinof Jachowski et al 2020).…”

“…The original shelter design is boot shaped and composed of a rectangular concrete chamber with a single narrow entrance that mimics natural cavities used by hellbenders under boulders (Briggler and Ackerson 2012). Since initial development, artificial shelters for hellbenders have been used in at least 8 states to augment and improve habitat, soft–release captive bred individuals, collect eggs for head–starting and captive propagation, and facilitate minimally invasive monitoring (Messerman 2014, Bodinof Jachowski et al 2020). Most studies evaluating artificial shelter use have occurred in southwestern Virginia where, in the most successful stream reaches, over 80% of shelters may be occupied by adults on a single occasion (Bodinof Jachowski et al 2020) and approximately 24% of shelters are used for nesting each year (Button et al 2020 b ).…”

“…Since initial development, artificial shelters for hellbenders have been used in at least 8 states to augment and improve habitat, soft–release captive bred individuals, collect eggs for head–starting and captive propagation, and facilitate minimally invasive monitoring (Messerman 2014, Bodinof Jachowski et al 2020). Most studies evaluating artificial shelter use have occurred in southwestern Virginia where, in the most successful stream reaches, over 80% of shelters may be occupied by adults on a single occasion (Bodinof Jachowski et al 2020) and approximately 24% of shelters are used for nesting each year (Button et al 2020 b ). Bodinof Jachowski et al (2020) and Button et al (2020 b ) found that adult hellbender density positively influenced rates of artificial shelter use in Virginia, especially when per capita natural shelter (i.e., large rock) availability is low and shelters function to supplemental an otherwise limited resource.…”

“…Most studies evaluating artificial shelter use have occurred in southwestern Virginia where, in the most successful stream reaches, over 80% of shelters may be occupied by adults on a single occasion (Bodinof Jachowski et al 2020) and approximately 24% of shelters are used for nesting each year (Button et al 2020 b ). Bodinof Jachowski et al (2020) and Button et al (2020 b ) found that adult hellbender density positively influenced rates of artificial shelter use in Virginia, especially when per capita natural shelter (i.e., large rock) availability is low and shelters function to supplemental an otherwise limited resource. However, work in Virginia has also shown that artificial shelters are vulnerable to destruction, dislodgement, and obstruction by sediment, making them intermittently unavailable to hellbenders, especially following seasonal peak discharge and high flows from major storm events (Bodinof Jachowski et al 2020).…”