Development and optimisation of relaxants for use in the Australian abalone industry

Author: Georgia Mercer

Mercer, Georgia, 2020 Development and optimisation of relaxants for use in the Australian abalone industry, Flinders University, College of Science and Engineering

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Handling abalone is often difficult because they adhere strongly to available substrates and are difficult to remove. Relaxants, products that decrease muscle tone and the ability of the abalone to grip the substrate, are used in abalone aquaculture to facilitate handling, transport, grading stock and to minimise injury, immune suppression and mortality associated with handling. The principle aim of this study was to identify and evaluate relaxants for reliable, safe relaxation of greenlip abalone (Haliotis laevigata) in the Australian aquaculture industry. Safe relaxation implies that the relaxant has no acute or chronic detrimental effects on abalone, that it can be applied safely, that meat from treated abalone can be consumed without risks to human health and that there are no unduly hazardous environmental effects of use or release of the product.

The efficacy of 7 potential relaxants: propylene phenoxetol; magnesium sulphate (MgSO4); magnesium chloride (MgCl2); MS-222; clove oil; AQUI-S™ and 2 phenoxyethanol were screened using benzocaine, the only permitted relaxant for abalone in Australia (PER 14638) (APVMA 2016c) as a comparative baseline. Benzocaine has adverse effects when used on farmed abalone, including causing mortality in treated stock. Three relaxants (MgCl2, 2-phenoxyethanol and propylene phenoxetol) met all response, behavioural, growth and water quality criteria for use.

Doses of 2-phenoxyethanol, MgCl2 and propylene phenoxetol were optimised for small (shell length 18-28 mm) and large (70-90 mm) abalone at 14, 18 and 22 °C. Successful relaxation occurred in <5 minutes and small abalone tolerated 10 minutes exposure and large abalone tolerated 20 minutes exposure. All abalone remained relaxed for at least 10 minutes after cessation of treatment but successfully recovered in <20 minutes. Propylene phenoxetol did not, however, successfully relax large abalone. Abalone are subject to frequent handling for grading and density management throughout grow-out and these procedures occur in all seasons, so dose-size-temperature relationships were compiled to show optimised doses under different conditions.

Host safety was assessed for small and large abalone for overdose and overexposure to 2-phenoxyethanol or MgCl2 to assess the lowest doses and shortest durations of exposure at which mortality occur and assess potential negative effects on health or growth. The margins of safety for 2 phenoxyethanol or MgCl2 demonstrate that abalone can be safely relaxed using optimised dose and exposure recommendations with adequate flexibility to avoid acute effects and without long term effects on growth.

Magnesium residues were analysed to determine [Mg2+] in the muscle tissue of large abalone that had been relaxed using 160 g/L MgCl2 for 20 minutes. [Mg2+] depleted to background levels within 3 hours of cessation of treatment and return to seawater. Maximum [Mg2+] in the edible portion of treated tissue is safe for consumption, demonstrating that no withholding period is required and that MgCl2 is a suitable relaxant for use prior to live transport or for rested harvest.

This work successfully identified 2-phenoxyethanol and MgCl2 as suitable for use as relaxants in the Australian abalone industry. Both relaxants meet industry use criteria and are safe for abalone. This work provides a substantial contribution to data required for applications to obtain regulatory authorities for use of these products as veterinary medicines and provides recommendations for work to further refine and optimise practical application in aquaculture.

Keywords: abalaone, anaesthetics, relaxants, residue

Subject: Aquaculture thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Dr. James Harris