With the announcements in the past few months from several Japanese companies that their operations are near to commercial viability, closed-cycle Pacific bluefin tuna farming appears to be closer than ever to reality. However, concerns remain over survival rates of farmed bluefin tuna. Hatchery operation have reported that out of 200,000 tuna fry, as few as 6,000 fish survive to reach a harvestable size. Aquaculture researchers continue to aim for higher survival rates, and have also successfully identified the leading causes of mortality for bluefin tuna in each cycle of their development.
The causes of poor Pacific bluefin tuna survival in fingerling production include surface and sinking death of early larvae during the first 10 days-post-hatch (dph); aggressive behavior and cannibalism after approximately 10 dph; trauma due to contact or collision with rearing tank walls or sea cage netting after approximately 30 dph; heavy damage due to skin injuries during transport from hatchery to sea net cages at approximately 30 dph; and iridoviral infection and blood fluke infection in sea cages.
In Michio Kurata’s 2015 “Study on prevention of sinking death in the fingerling production of Pacific bluefin tuna” (in Bulletin of the Fisheries Laboratory of Kinki University) he writes:
“The survival in mass fingerling production was 2.35 percent during hatchery phase, 21.3 percent during intervening culture, and consequently 0.5 percent from egg to the shipment in the 2009 year group, whereas the survival from eggs to juveniles of other species with well-developed technologies has the efficacy of several tens of percent (Kumai 2012; Ishibashi 2012).”
Sinking death is when the larvae sink to the bottom of the tank and become trapped there and die. This occurs because their bodies are heavier than water, and because they become inactive at night. Older bluefin make use of a swim bladder, but in newly hatched larvae, the swim bladder is not yet inflated. To prevent the larvae from sinking, the researchers used aerators to create an upward water current and found that it helped. They further found that a stronger current was more effective, and some reports suggest that a deeper tank in relation to the radius of the tank (called “aspect ratio”) may create a better upward current.
Surface death is when bluefin come to the surface of the water in a tank, but become trapped there by surface tension. In order to prevent surface tension, a thin layer of liquid paraffin or oil is used. However, this layer can also prevent the larvae from reaching the surface to gulp air and inflate their swim bladder – which in turn leads to sinking death. The surface of the water can be skimmed with a skimming tool to remove the oil or paraffin, but this then allows more surface death. It seems to be a trade-off.
The research investigated the day post-hatch that bluefin larvae attempt to gulp air and found that most do so at three dph, and further that they are stimulated to do so by light conditions, preferring to do so at twilight. So, the best survival was achieved by using oil or liquid paraffin, then skimming the water for one hour between 6 and 7 p.m. Most larvae will gulp air to inflate their swim bladders during that narrow window. After that, the layer of oil of liquid paraffin can be replaced.
To reduce cannibalism and aggressive behavior, a guide by Kinki University “Aquaculture Science and Technology – Bluefin Tuna and Other Cultivated Fish” recommends supplying live food early in the morning, so that the fish will not become hungry. Large differences in size among fish also leads to the death of smaller fish.
In ship transport, when bluefin juveniles are carried in tanks to other areas for grow-out, they often ram themselves into tank walls when they are exposed to daylight, often resulting in death. Research reported in 2013 in the article “Effect of tank wall colour and pattern on the survival rate of juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel) during ship transportation” (Okada et al., in Aquaculture Research) found that a wall with a lattice pattern of red and blue allowed the fish to recognize and avoid ramming the wall, while with a solid-colored wall, the fish suffered more frequent death from wall collisions.
Ishibashi et al. found in “Effects of tank wall pattern on survival, bone injury rate, and stress response of juvenile Pacific bluefin tuna, Thunnus orientalis” (2013, Aquaculture Engineering) that juvenile Pacifc bluefin tuna aged 30 dph or more are at high risk of mortality through contact or collision with tank walls, but that a polka-dot wall pattern also works in preventing wall collisions.
Skin injuries from handling during transport has resulted in up to 50 percent mortality in PBT. Kinki University researchers found that when the fish had a total length around four centimeters, they were less prone to skin injury in transport than larger fish.
When the juveniles are moved to sea cages, as many as 40 to 70 percent can perish on the first night due to ramming and collisions. The researchers found that adding a light to the cages allowed the fish to see better and helped prevent mass mortality.
In short, though survival rates of bluefin tuna are low, experimentation and many small improvements are leading to improvement in yields.
