Reference levels for corticosterone and immune function in farmed saltwater crocodiles (Crocodylus porosus) hatchlings using current Code of Practice guidelines

• We studied corticosterone and immune function in hatchling saltwater crocodiles.
• The lowest corticosterone levels for farmed saltwater crocodiles are reported.
• Farmed saltwater crocodiles are not inherently stressed.
• Immune function was not related to corticosterone using simple techniques conducted.
• Results provided support management practices of the Australian Code of Practice.

To determine reference levels for on-farm stressors on immune responsiveness and growth rate, 253 hatchling crocodiles from 11 known breeding pairs were repeatedly measured and blood sampled during their first year. Plasma corticosterone (CORT) was used to quantify baseline stress levels in captive animals and were found to be lower (mean 1.83 ± SE 0.16 ng/mL) than previously reported in saltwater crocodile hatchlings. Two tests of immune function were also conducted. Innate constitutive immunity was assessed using bacterial killing assays (BKA) against two bacterial species: Escherichia coli and Providencia rettgeri, whereby the latter causes considerable economic loss to industry from septicaemic mortalities. Although the bactericidal capabilities were different at approximately 4 months old (32 ± 3% for E. coli and 16 ± 4% for P. rettgeri), the differences had disappeared by approximately 9 months old (58 ± 2% and 68 ± 6%, respectively). To assess immune responsiveness to a novel antigen, the inflammatory swelling response caused by phytohaemagglutinin (PHA) injection was assessed but was only significantly different between Samplings 1 and 3 (5% LSD). There were no significant clutch effects for CORT or PHA but there were for both BKA traits. CORT was not significantly associated with growth (head length) or the immune parameters except for P. rettgeri BKA where higher CORT levels were associated with better bactericidal capability. As such, these results suggest that the crocodiles in this study are not stressed, therefore endorsing the management strategies adopted within the Australian industry Code of Practice.