13/06/14: New approach heads off fish deformities

Author: Rose Yoeman

Aquaculture granite-patterned tanks have become the latest 'must have' in hatchery equipment, where the decor is proving critical in addressing fish behaviour that has inhibited production.

Australia's annual production of farmed Yellowtail Kingfish (Seriola lalandi) from sea net pens is on the rise and is expected to soon exceed 1500 tonnes, with a potential value of about $24 million a year.

It is anticipated that further development in South Australia, Western Australia and New South Wales will bring production capacity to about 8000 tonnes a year within the next decade.

Based on South Australia's Eyre Peninsula, Clean Seas is one of Australia's leading producers of farmed Yellowtail Kingfish, but has been challenged by pervasive and persistent fish-health and deformity issues that have hindered viability.

In 2005, Canadian fisheries veterinarian Mark Shepphard, Sakana Veterinary Services, published an FRDC-funded report (2003/216) that focused on the detection and management of health issues in Yellowtail Kingfish. He stressed that the ability to control the natural environment and stressors inherent in raising fish in captivity is critical to success. Among the 'moderate risk' issues were head and jaw deformities and swim bladder deformity.

The cost and inconvenience of these issues has been high in terms of post-handling mortality, lost growth and performance and the elevated labour cost of counting, culling and disposing of dead or compromised fish.

For Clean Seas, 2013 was a landmark year. A research breakthrough that led to altered hatchery practices significantly improved the deformity and survival rates of Yellowtail Kingfish larvae. For the first time in nearly a decade, no additional staff was needed to hand cull deformed Yellowtail Kingfish fingerlings at the company's commercial hatchery at Arno Bay, SA.

Clean Seas CEO Craig Foster describes the new knowledge as a "game changer".

"Prior to 2013, on average 30 per cent of Yellowtail Kingfish were culled by hand sorting at 50 to 60 days post-hatching to eliminate fingerlings with severe jaw malformation," he says.

"Between 2008 and 2013, with more than $2.5 million support from the Australian Seafood Cooperative Research Centre (Seafood CRC) and the FRDC, our hatchery manager Bennan Chen and his R&D team conducted a series of experiments to investigate the causes of the high levels of variability in the survival, growth and malformation rates of Yellowtail Kingfish larvae that caused 'bottleneck' issues in larval rearing."

Survival Of Yellowtail Kingfish Larvae Cultured In Tanks With Different Wall Colour and Pattern Treatments At 23 Days Post Hatching

Critical to this research was the development of a strategic research plan for Yellowtail Kingfish in 2003. This plan integrated results from past research conducted by Wayne Hutchinson from the South Australian Research and Development Institute (who early on determined that deformities could be affected by addressing tank colour) and Jenny Cobcroft from the University of Tasmania, who had worked on the causes of deformity in Striped Trumpeter (Latris lineata) and other species. Research has shown that tank design, management and colour were critical in resolving the jaw-deformity problem.

Jenny Cobcroft worked closely with the research team at Clean Seas to transfer the knowledge from the previous research and to develop the strategic plan, which included the construction of a dedicated laval-rearing research facility funded by the Seafood CRC within the Clean Seas hatchery at Arno Bay.

"So we set out to determine if we could moderate the walling behaviour of Yellowtail Kingfish larvae by changing the colour or pattern on the wall of the larval rearing tank."

– Bennan Chen

Bennan Chen explains that studies of other fish also showed 'walling' behaviour, where larvae congregate around the outer walls of the tank, indicating that it was not a disease or breeding health issue causing deformities but rather a physical issue caused by larvae banging into tank walls.

"So we set out to determine if we could moderate the walling behaviour of Yellowtail Kingfish larvae by changing the colour or pattern on the wall of the larval rearing tank," he says.

In December 2012, preliminary trials were set up to assess a range of different coloured tank walls and tank wall patterns, as well as different light conditions. Walling behaviour was qualitatively assessed by visual observation throughout the day.

Bennan Chen says walling behaviours were more apparent at the beginning and end of each day but the most interesting discovery was that tank colours and patterns that reflected high levels of light reduced the incidence of walling behaviour.

Clean Seas then moved into the next research phase and began a full replicated trial to investigate links between reduced walling and growth, survival and incidence of jaw deformity during Yellowtail Kingfish larval rearing.

Four hatchery tank wall treatments were investigated: glossy green, non-glossy green, yellow and 'granite'– a motif created from an adhesive plastic film or by painting the tank wall.

No significant difference in larval growth rate was observed between any of the tank wall colour or pattern treatments over the 23-day trial, but Bennan Chen says there was a significant difference in survival (Figure 1) and jaw malformation of larvae (Figure 2), with jaw deformity reduced from 30 per cent to about three per cent in granite-patterned tanks.

It has been suggested that the granite pattern, made up of random light and dark brown, black and grey irregular shapes may reflect light of different wavelengths and is thus perceived by the larvae as a structure to be avoided.

Bennan Chen notes that improved larval survival may be the result of larvae spending less time in futile walling behaviours – as they are able to perceive the water and wall boundary – and more time in the water column searching for food and feeding.

The reductions in the incidence and severity of jaw deformity (Figure 2) in granite-patterned tanks compared with the original glossy-green tanks was maintained in different sized tanks, including 8000-litre commercial Yellowtail Kingfish larval rearing tanks.

Proportion of Yellowtail Kingfish Larvae With Jaw Malformation In Tanks With DIfferent Wall Colours And Patterns At 23 Days Post Hatching

Craig Foster explains that larval jaw elements in bony fish remain soft from first feeding until metamorphosis at 15 days post-hatching. "We now think physical damage to oral membranes caused by walling in the two weeks post-hatching is a dominant factor in jaw malformation."

Craig Foster says a jaw deformity rate of less than three per cent can be managed by Clean Seas grow-out production without hand-sorting individual fish, as had previously occurred.

"Manipulating tank wall colour and lighting conditions to reduce walling behaviour has resulted in more high-quality fingerlings being produced more cost-effectively. The key outcome of Clean Seas research has been the doubling of hatchery productivity, which has laid the foundation for a further doubling of overall productivity," he says.

Bennan Chen and Craig Foster presented their research results at the World Aquaculture Adelaide Conference, in June.