Rearing captive eastern hellbenders (Cryptobranchus a. alleganiensis) with moving water improves swim performance

Abstract

Translocations often use captive-reared animals to help bolster or re-establish wild populations. However, captive environments are highly dissimilar from wild conditions and may deprive animals of experiences that promote normal development. Captive-rearing and translocation efforts are underway for eastern hellbenders (Cryptobranchus alleganiensis alleganiensis). Yet, hellbenders reared in aquaria that lack stimuli often make long-distance downstream movements following release, perhaps because of their naïveté to riverine environments. We altered standard captive techniques and reared juvenile hellbenders with (treatment) and without (control) water current for 18 months. We quantified morphological plasticity and swim performance as a function of rearing environment to assess the value of environmental enrichment in hellbender head-start programs. We compared broad-scale growth rates for mass, snout-vent length, and total length, but found no difference between treatment and control hellbenders (mass difference = 0.1 g/month, P = 0.596; snout-vent length difference = 0.01 cm/month, P = 0.360; total length difference = 0.01 cm/month, P = 0.533). We also examined fine-scale tail morphology measurements and found treatment individuals developed more shallow tails that grew 49% slower than control individuals during the rearing period (mean difference = 0.86 mm/month, P = 0.017). We interpret this as evidence of either energy expenditure or phenotypic plasticity as more streamlined tail forms are found in lotic systems. Moreover, we found water current to be positively associated with hellbenders’ swimming ability. After three swim trials, treatment hellbenders were 46% quicker in their swim time (P = 0.033), required 29% fewer upstream attempts (P = 0.012), and were 60% less likely to need manual motivation to make it to an upstream tile hide (P = 0.010). Moreover, treatment hellbenders tended to improve these responses linearly through time (P = 0.016) compared to control individuals that showed no improvement across the three trials (P = 0.075). Together, our data suggest that the addition of water current to hellbender rearing environments does not have any detrimental impact on hellbender body morphology, but rather, acclimates hellbenders to moving water and improved their ability to reach upstream refugia. We advocate incorporating water velocities, representative of natural conditions, into hellbender captive-rearing programs. Rearing animals with semi-natural conditions in captivity may better prepare animals for and potentially improve the success of future translocations. This advancement to standard rearing techniques may positively influence the preservation of wild hellbender populations throughout the nation.