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Indonesian study compares land-based concrete tanks and ocean net pens in tuna spawning trials. ……Net pens win


August 16, 2016
By Ruby Gonzalez

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Spawning captive yellowfin tuna, Thunnus albacares, in offshore net cages may produce better results than spawning in concrete tanks.

A few years after spawning the pelagic fish in a land-based 1,500 m3 –circular concrete tank at the Gondol Research Institute for Mariculture (GRIM) in Bali, Indonesia, additional research was conducted, this time offshore in a circular net cage which was 50 meters in diameter and had a depth of nine meters and capacity of 20,000 m3.

GRIM Project Scientist Jhon Hutapea presented a comparison of the spawning performance of broodstock reared in these two systems at the Asian-Pacific Aquaculture 2016 held last April in Surabaya, Indonesia.

Hutapea noted that while the yellowfin tuna spawned continuously in both systems, spawning in concrete tanks did not seem possible over a long period because of mortality caused by wall strikes and less food intake.

“In the circular concrete tank, first spawning occurred in October 2004 with broodstock size of about 9.1 kg of body weight and 82-cm fork length at two years. While in the circular floating net cage, the first spawning occurred in January 2015 with estimated broodstock size of about 20.0 kg of body weight and 100 cm of fork length and two years,” Hutapea cited in the abstract.

Initiating tuna culture

The research on broodstock reared in concrete tanks was conducted from 2003 to 2010. The other research commenced in 2013.

The projects were backed by a collaborative program between Indonesia, Japan and Australia.

“Yellowfin tuna is an important product in Indonesia and the government deems it necessary to have good resource management and also to develop the technology for propagation of yellowfin tuna to establish a responsible fishery and to initiate tuna culture,” Hutapea said.

Australia’s objective in participating in the project was to have research and development collaborations between the South Australian Research Development Institute, GRIM and commercial companies that have an interest in tuna propagation in Australia.

Japan, the major market of tuna from Indonesia, is also the world’s biggest tuna consumer.

Death and spawning

The drawbacks of having broodstock in concrete tanks are mortality due to wall strikes and less feed intake throughout the rearing period. Death due to wall strikes was considered the biggest problem owing to the high cost incurred from continuous replacements.

There were also indications that the system puts a limited time frame for natural spawning.

 “Based on all spawning data collected between 2004 and 2008, the highest productivity of fish was between 2005 and 2006 and productivity declined sharply in 2008. On this basis, yellowfin tuna broodstock in tanks should only be raised for three to four years, and should then be replaced,” he cited during his 2009 presentation in Japan.

In the Aquaculture 2016 abstract Hutapea pointed out that there is much less mortality of yellowfin tuna broodstock reared in floating net cage, feed intake continuously increases and spawning continues to occur naturally. (See table.)

Parameters

Yellowfin tuna broodstock reared in

Concrete tank

Floating net cage

Volume (m3)

1,500

14,000

Initial size (kg)

3.0 -5.0

0.3 -0.5

Rearing period until first spawning (month)

5

15

Time of spawning

Late afternoon

Midnight

Eggs per spawning (million)

0.5 – 2.0

0.1 – 3.5

Source: Spawning performance of yellowfin tuna (Thunnus albacares) broodstock in concrete tanks and floating net cages.

Broodstock from the wild

Caught from the wild, transferred fish were put in acclimation tanks for five months, when they were projected to grow to five kilos each, before being released in the 1,500 m3-broodstock tank.

Number of fish, sex ratio and water temperature maybe significant factors affecting spawning success for captive yellowfin tuna, according to the final report on spawning of yellow tuna in concrete tanks by the Australian Centre for International Agriculture Research.

Spawning triggers

During the most successful spawning year recorded at GRIM, it was noted that the water temperature was observed to decline to a minimum of 27.3oC preceding spawning.

“This was not the case in 2010 when the minimum water temperature experienced by the recently established broodstock was 28.4oC. It is suggested that the higher than normal water temperatures prevailing during the 2010 season may not have provided the environmental signal required to initiate gonad recrudescence and subsequent maturation and spawning of recently established first maturing broodstock,” notes the report.

It was indicated that, “it may be beneficial to maintain more male fish than female fish in the spawning tank to promote spawning behavior and optimize fertilization of eggs.” The best spawning period had an average sex ratio of less than 1:1.

Ruby Gonzalez


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