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

  • Author(s): Nagy, Kenneth;
  • Henen, Brian;
  • Hillard, L Scott
  • et al.

Published Web Location

https://doi.org/10.3354/esr01067
Abstract

We measured survival, growth and body condition of eight hatchling cohorts of desert tortoises in captivity over 11 years to evaluate head-starting methods. At 11 years of age, seven times as many of the first cohort had survived than if they were free-living tortoises. Improvements in predator control, food and water supplementation and pen structure increased survival from seven to 10 times that under wild conditions. Annual survival averaged 96%. Carapace length (CL) increased 6.95 mm per year, 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 four dry years, with crowding. Most of the first cohort grew to a releasable size (CL >100 mm) by their ninth year. Body Condition Indices (CI) remained high, indicating little dehydration despite droughts in eight 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 years), 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.

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