Head-started Agassiz’s desert tortoises Gopherus agassizii achieved high survival, growth, and body condition in natural field enclosures

Ka Nagy,Bt Henen,Ls Hillard

doi:10.3354/esr01067

Abstract

We measured survival, growth, and body condition of 8 hatchling cohorts of desert tortoisesGopherus agassizii(living in predator-resistant outdoor pens in the Mojave Desert, California, USA) over 11 yr to evaluate head-starting methods. At 11 yr of age, 7 times as many of the first cohort had survived than if they had been free-living tortoises. Subsequent improvements in predator control, food and water supplementation, and pen structure increased survival from 7 to 10 times that under wild conditions in younger cohorts. Annual survival averaged 96%. Carapace length (CL) increased 6.95 mm yr-1, similar to that of free-living tortoises. Annual growth rates varied with calendar year (possibly reflecting food and water supply), age, cohort (year hatched), mother, and in 4 dry years, with crowding. Most of the first cohort grew to a releasable size (CL >100 mm) by their 9th year. Body condition indices remained high, indicating little dehydration despite droughts in 8 of the 11 years, because irrigation offered drinking opportunities. Head-started tortoises developed fully hardened shells (≥98% of adult shell hardness) earlier (10.1 vs. 11.6 yr), but at a larger CL (117 vs. 104 mm) than did free-living tortoises. Selective feeding in head-start pens decreased subsequent germination of favored wildflower species, apparently by reducing the natural seedbank. Consequently, we reseeded and irrigated each autumn to promote subsequent spring food supply. We irrigated in early summer to enable drinking and ensuing consumption of dry, dead plants and Bermuda grass hay, a supplement. These procedures can greatly improve juvenile survivorship, and increase numbers of hard-shelled, midsized juveniles to help augment wild populations.

Highlights

Head-starting is one means to augment populations of Agassiz’s desert tortoise Gopherus agassizii, a threatened species (US Fish and Wildlife Service 1994, 2011a)
We demonstrated the ability of head-starting to substantially enhance tortoise nest and egg success and to increase juvenile survival compared to the wild
Head-starting can potentially augment Agassiz’s desert tortoise Gopherus agassizii populations and species recovery (USFWS 2011a) by providing releasable, healthy juveniles that are past their highest mortality stages

Summary

Introduction

Head-starting is one means to augment populations of Agassiz’s desert tortoise Gopherus agassizii, a threatened species (US Fish and Wildlife Service 1994, 2011a). Head-starting should increase survival of tortoise eggs and young by reducing or eliminating death due to predation and the physical environment (e.g. dehydration and starvation during droughts) and by fostering juvenile growth and resistance to these threats (Morafka 1994, Morafka et al 1997, Nagy et al 1997, McGovern 2019). A primary goal is to add older, larger juveniles, with higher survival probabilities, to wild populations to enhance reproduction rates of local females and improve population rates of natural increase. This goal is consistent with specific recovery guidance under the US Endangered Species Act (USFWS 1994, 2011a). Desert tortoises have low egg and juvenile survivorship (Turner et al 1986, 1987a, Karl 1999, Bjurlin & Bissonette 2004), but have high adult survivorship and long lives (Turner et al 1984, 1986, 1987a, Curtin et al 2009), as do many chelonians

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