Moving Cages Further Offshore: Effects on Southern Bluefin Tuna, T. maccoyii, Parasites, Health and Performance

The effects of offshore aquaculture on SBT health (particularly parasitic infections and haematology) and performance were the main aim of this study. Two cohorts of ranched Southern Bluefin tuna (SBT) (Thunnus maccoyii) were monitored throughout the commercial season, one maintained in the traditional near shore tuna farming zone and one maintained further offshore. SBT maintained offshore had reduced mortality, increased condition index at week 6 post transfer, reduced blood fluke and sealice loads, and haematological variables such as haemoglobin or lysozyme equal to or exceeding near shore maintained fish. The offshore cohort had no Cardicola forsteri and a 5% prevalence of Caligus spp., compared to a prevalence of 85% for Cardicola forsteri and 55% prevalence for Caligus spp. near shore at 6 weeks post transfer. This study is the first of its kind to examine the effects of commercial offshore sites on farmed fish parasites, health and performance

Mean ± SE for length (cm), weight (kg), condition index and blood parameters in ranched SBT at transfer, week 6 and week 23 of ranching in the near shore and off shore cohorts.

<p>Harvest weights are corrected for gg weight collection. Blood parameters include haemoglobin (Hb) (g dL⁻¹), plasma pH, plasma osmolality (mmol kg⁻¹), plasma glucose (mmol L⁻¹), plasma lactate (mmol L⁻¹), lysozyme (ug mL⁻¹), and alternative complement (ACH50) (units mL⁻¹) activity. Different letters denote significant differences over time within each treatment. * denotes statistical differences between the offshore cohort and the near shore control cohort.</p

Comparison chart of the remoteness, above water environment and hydrology of the two farming sites, near shore in the Tuna Farming Zone (TFZ) and offshore.

<p>Distance from shore is represented as distance from port. Wind Speeds were measured using weather stations at Boston Island for the near shore site and an average of stations at Spilsby Island and Thistle Island (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0023705#pone-0023705-g001" target="_blank">Figure 1</a>).</p

Cumulative mortality in ranched SBT in the near shore (denoted in red) and off shore cohorts (denoted in blue).

<p>There was a difference in survival curves over the first twelve weeks of ranching (χ² =  107, df = 1, p<0.001).</p

Map of Boston Bay in the south-west Spencer Gulf indicating commercial lease sites in red.

<p>Also noted are the near shore cage study site (#1) and offshore cage site (#2), and the 2010 Tuna Farming Zone in dotted box. ‘W’ denotes weather stations used for wind speed measurements.</p

Correlation of humoral immune response in southern bluefin tuna, T. maccoyii, with infection stage of the blood fluke, Cardicola forsteri.

The blood fluke, Cardicola forsteri, is a prevalent infection in ranched southern bluefin tuna. This project aimed to define the timing and intensity of the various developmental stages of C. forsteri within southern bluefin tuna as well as to relate infection to host pathology and immune response. Archival samples from several cohorts of T. maccoyii sampled from 2008 to 2010 were used in this study. The prevalence and intensity of C. forsteri infection was described using heart flushes and histological examination. Humoral immune response, i.e. C. forsteri specific antibody, lysozyme activity, and alternative complement activity, was also described. Based on the validated and detailed C. forsteri infection timeline, relationships between infection events, physiological response, and diagnosis were proposed. Immune response developed concurrently with C. forsteri infection, with the majority of physiological response coinciding with commencing egg production. Further research is needed to confirm the origin of C. forsteri antigen which is responsible for immune response development and how T. maccoyii immune response works against infection. To aide this research, further diagnostic methods for confirmation of infection need to be developed

Humoral immune response (mean ± SE or SD) in <i>T. maccoyii</i>.

<p>Significant letters denote statistical differences at p<0.05 within Cohort_2009A, Cohort_2009B and between Cohorts_wild & Cohort₂₀₁₀.</p

Anti<i>Cardicola</i> antibody activity in <i>T. maccoyii</i> serum by prevalence (95% confidence interval) and mean activity ± SE.

<p>Significant letters denote statistical differences at p<0.05 within Cohort_2009A, Cohort_2009B and between Cohorts_wild & Cohort₂₀₁₀.</p