Sinking death syndrome during finfish larviculture

Sinking death syndrome during finfish larviculture

Masatoshi FUTAGAWA / CORDUNAP

June 30, 2012

Phenomenon

Sinking death syndrome occur during early life stage of finfish larviculture, 3 to 7 days post hatch-out (DPH), that larvae settle on bottom at nighttime mostly and it makes mass mortality due to bacteria contamination and anoxia. Several species has the syndrome and that is one of the bottlenecks of mass production. I had the syndrome during the larviculture of Graus nigra, Pacific beakfish (Oplegnathus insignis), Japanese flounder (Paralichthys olivaceus) and Grater amberjack (Seriola dumerili).

Causes

Increasing body density

Finfish larvae consume yolk and oil droplet few days after hatch-out and change body density. Usually, body density of larvae is heavier than water (1.0236±0.0001 at WT 25 ̊C). According to Teruya et al. 2008, Greater amberjack larva shows it consume yolk and oil droplet until 4 DPH and body water contents reduce from 2 DPH (93.5 %) to 5 DPH (91.1 %) and become stable after that (87 – 88 %).

 

Swim bladder inflation

Swim bladders inflation rate shows (SBI) it starts from 4 DPH and increase until 10 DPH. Though body density shows similar to sea water at 0 DPH (1.024 g/cm³) and become heaver until 10 DPH (1.04) after start taking feeds. Comparison between swim bladder inflated and no swim bladder, inflated lighter than non-inflated, even inflated larvae heavier in daytime than nighttime both are similar to sea water density, however non-inflated larvae in nighttime is heavier than daytime and both are heavier than water. Thus, one of reason of sinking death syndrome is non-inflated swim bladder.

Further study by Hirata et al. 2009 about the relation among the photoperiod, WT, growth, survival, feeding and SBI says, 18 L (18 hours lighting per day) shows the best growth and survival, totally dead under 0L until 8 DPH, and WT 22 ̊C shows good survival. SBI at 12 to 24 L shows same rate under different WT. Thus, the larvae swallow air at surface during daytime.

Late study by Imai et al. 2011 says the larvae not shown swim bladder which surface covered by oil film and growth slower than inflated larvae. Based on organs observation, the swim bladder develops on 3 to 4 DPH at TL 4 mm. So, extension of photoperiod gives more chance to inflating the bladder and minimizing the syndrome. Even so, SBL rate increase later due to improvement of the bladder function. Its needs more study.

Flow field

According to Sakakura et al. 2006, seven-band grouper (Epinephelus septem fasciatus) larvae are cultured under variable air flow (1,000, 200, 50, 0 ml/min) with cylinder tank (1.4 m dia., 70 cm depth) and shows the highest growth and survival with 200 ml/min air flow at 10 DPH. They measured flow field by three dimension flow meter and found the flow velocity 8 cm/s at center/middle of tank, 6 cm/s at surface. Flow velocity rate at middle/middle (larvae abundant here), surface/center and center/middle are 1 : 10 : 100. Thus, flow field at center/middle gives any negative impact for sensitive grouper larvae possibly.

The results scale up to large tank (concrete cylinder, 8 m dia., 2 m depth and 100 kl volume) for mass larviculture which has flow velocity is 8 cm/s or flow volume 630 ml/min at center/middle. The tank equipped water supply and aerator at center/bottom, and four aerators diagonally.  The flow velocity distribution shows well flow field at bottom and survival rate at 10 DPH increased from 21 to 61 % in 2000 to 2001.

   

According to Tanaka et al. 2010, survival rate and TL of Pacific bluefin tuna larvae (7 DPH) 63.3±12.9 % and 4.98±0.09 mm under air flow 1.0 l/min at night (19:00-06:00) with 25 ̊C compare to 39.9±21.8 % and 5.10±0.18 mm under 1.0 l/min at night with 28 ̊C, 19.6±4.0 % and 4.73±0.28 mm under 0.1 l/min with 25 ̊C and 8.3±1.3 % and 5.03±0.28 mm under 0.1 l/min with 28 ̊C which culture in 200 l cylinder tanks (0.7 m depth). Thus, air flow 1.0 l/min and WT 25 ̊C show better results and that suggest strong water current at bottom during the night minimize sinking death syndrome.

 

Countermeasure

According to Takebe et al. 2010, they proved the theory that tank bottom flow field minimize sinking death syndrome with Leopard coral grouper (Plectropomus leopardus) under mass production scale and the method reduced the syndrome dramatically.

The experiment conducted with octagonal tanks (concrete, bottom area 25 m², 50 kl), stocked 203 x10³ hatch-out larvae each tank, applied concentrated Nannochloropsis oculata at density 500 x10³cells/ml from 2 DPH, fed enriched rotifer (Brachionus plicatilis sp. complex Type-SS Thai strain and Type-S) as maintain 20 R/ml density from 2 to 25 DPH, fed Artemia sp. from 15 to 45 DPH, fed artificial feed from 25 DPH. Other treatments are follows.

• (a)     HFS (horizontal water flow at side) consist two air stones at center, water supply (50 %/day) at side from 1 DPH at bottom horizontally by PVC pipe (40 m/m, 12 m/m holes, 5 cm distance) that installed at the  side of tank. Photoperiod (1,000 lx) 24 hours/day until 7 DPH.
• (b)      SFD (slanted water flow diagonal, tuna larviculture method) consist one air stone at center, water supply (50 %/day) at side from 6 DPH. Rearing water recirculated by submersible pump and agitate bottom, water flow (1.5 kl/H) from four PVC pipes (13 m/m, 2 m/m holes, 10 cm distance, flow 45̊ upper) that installed diagonally. No photoperiod control.
• (c)       VHFD (vertical and horizontal water flow diagonal) consist two air stones (0.5 l/min, air and oxygen) at center, water supply (50 % /day) by two pipes at center from 1 DPH. Rearing water recirculated by submersible pump and agitate bottom, water flow (1.5 kl/H) from four PVC pipes (13 m/m, 2 m/m holes, 10 cm distance, vertical flow by two pipes and horizontal by two pipes) that installed diagonally. . Photoperiod (1,000 lx) 24 hours/day until 7 DPH and increase recirculation water to 2.2 kl/H and air to 2.2 l/min after closing the light. Additionally, four air block (13 m/m, 50 mm long, 0.5 l/min) installed at corners. Air and oxygen supply by air stones increased to 1.0 l/min and 2.0 l/min by air blocks from 7 DPH.

Survival rate of HFS, SFD and VHFD were 20.6, 6.4 and 3.1 % in 10 DPH and growth (TL) of VHFD was significantly higher than others due to taking much rotifer.

New batch larviculture, 600 to 900 x10³ larvae (1 DPH) are stocked in each three tanks, run under VHFD, oxygen and water supply was controlled depending on water quality from 11 DPH. Survival shows 41.4, 45.0 and 46.5 % on 10 DPH, and 18.0 % (TL 30.1 mm) on 55 DPH at Tank 1, 30.0 % (30.0 mm) on 56 DPH at Tank 2 and 13.0 % (31.2 mm) on 60 DPH at Tank, harvested 340 x 10³ fries totally.

 

Summary

Sinking death syndrome occurred by decreasing body density due to consumption of yolk and oil droplet during 2 to 5 DPH, swim bladder deformation due to unable to swallow air from surface because of oil film, strong current or/and wavy condition at surface from 2 to 4 DPH and slow current at bottom.

Countermeasures are removing oil film at surface, extend photoperiod to 18 hours and give more chance to swallowing air, making current by aeration, ideally 8 cm/sec in case of grouper larvae, and making current by water flow at bottom, ideally set outlet diagonally.

Author thinks how much the symptom happening in natural condition, mortality occur more than a half in artificial (hatchery) condition. In the nature, it is much wavy condition compared to the surface at larviculture tank and it’s more difficult to swallow air at sea surface. It is not acceptable to a half of sibling larvae die in natural condition even though some of larvae inflate swim bladder at fry stage.

Author observed that the larva takes green thing, looks like microalgae, with water while opening the anus and mouth before taking rotifer at 1 or 2 DPH.  The hypothesis is larva drinks water before taking zooplankton and the gas in the water, usually sea surface is supersaturated, makes swim bladder inflation or the microalgae releases gas inside larva body. It is necessary to confirm that rearing larva under supersaturated condition.