Rectangular tanks limit stress of scallop larvae in FT system

Ruby Gonzalez
August 23, 2018
By Ruby Gonzalez
Fifty-liter rectangular tanks were used to study the impact of water flow on larval performances.
Fifty-liter rectangular tanks were used to study the impact of water flow on larval performances. Photos: Dr. Marine Holbach
Hatchery techniques that work excellently for one bivalve specie larvae may not always have similar results on another.

Dr. Marine Holbach, hatchery manager at Fermes marines du Québec, found this out when she used a flow-through (FT) system on scallop larvae that was previously tested on oyster larvae.

The system, which included bubbling, was high-density which had up to 300 ind/ml. Water renewal was also very high at one per hour.

It yielded a 90-percent survival rate on oyster larvae but resulted in great scallop (Pecten maximus) larvae mortality of 70 per cent within a week. This led to a study that looked into the effect of bubbling, tank shape and water renewal on the great scallop larvae.

Holbach presented, Limiting the stress of scallop larvae in flow-through system: the key for a commercial production, at the Aquaculture Canada Conference 2018 in May in Quebec City.

One of the key findings is that larval rearing yield was improved in rectangular tanks.

The experiments initially focused on bubbling. After two days, it was observed that scallop larvae stopped eating and energetic metabolism rose as bubbling increased. “We can say that if larvae don’t feed and the metabolism increases, these lead to exhaustion of the larvae. This was registered with the low growth during the experiment,” she said.

All larvae died within 12 days, showing bubbling was “very deadly” to larvae.

Tank shapes used had lower surface-volume ratio to allow the larvae to rest on the bottom, which is known to be useful for scallop larvae.

Rectangular tanks, which have “huge” bottom surface, were eventually selected for the later stages of the experiment because the shape produced higher percentages of pediveliger survival rate and metamorphosis yield.

Fifty-litre rectangular tanks were used to study the impact of water flow on larval performances. “When you look at the results,” she said while pointing to charts, “you can see here that the addition of bubbling leads to mortality of our larvae. A lot of performances were very similar whatever the intensity of water renewal was.

“We also had a big mortality with 80 per cent, which is not such a good news but the survival of pediveliger percentage and yield of metamorphosis were quite encouraging,” she said.

With regard to bubbling and water flow, she explained that regardless of the intensity of water flow, the larval level of activity remained constant. “There is no impact on the larvae whatever the frequency of water renewal was.” With this finding, she said they could increase water flow while reducing bubbling to allow better water renewal in the tank.

While bubbling has a negative impact on larvae, the experiments indicated that these are impacted more in a small-scale FT system. This is the main parameter she said that needs more research in their system.

“The flow-through system for pectinid larvae clearly needs more development… We have to work on this because it is very promising and very useful for hatchery,” she said.

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