Lobster Holding Tanks
Sniff… sniff… something smells good. The lobster crawls a hundred yards across the pristine ocean floor, stomach rumbling, towards the smell of freshly cut mackerel. A brisk climb up a trap wall, a not-so-gracious descent and … dinner is served … life is good. A day later in a buyer’s tank he dies. Why? He was healthy enough to hunt and eat! He didn’t starve … his gut is still full. Given the right water quality, he could have lived to travel the world to your customer’s plate. So, why is there a compost bin full of deads outside many buyers’ facilities?
Aquaculture engineers build tanks to hold as many fish as possible. 2 lbs/gallon? 3lbs/gallon? The bar constantly creeps higher. But, to optimise growth, stress and poor water quality is a no-no. Stressed fish do not eat. Fish that do not eat, do not grow. Fish that do not grow, do not pay the bills.
A lobster holding tank is the simplest of all aquaculture facilities. Ship product in. Keep it alive. Ship product out. The animals are not fed. The product left the facility alive, therefore … success story, right? Ridiculous! The success story is when the product leaves the facility alive and arrives at the customer’s facility lively!
Proper water quality and animal health are still critical. If you are not holding a healthy, dormant animal, your customer will not get a healthy and lively animal when he opens the box half a world away. He may not even get a live animal. And you get zero back for lobster that you invested $7/lb into.
Recirculation flow is designed on three factors:
- Thorough mixing of water in the tank to prevent dead zones
- Minimum water velocity required by each animal to prevent localised variations in water quality
- Flow rate requirements of recirc equipment such as chillers
Many facilities designed for short term holding in floating crates convert to holding tubed lobster and discover that over time the lobster gets coated with stinky, slimy, green film known as biofilm. This biofilm is bacteria and other microorganisms that thrive in the nutrient-rich, slow-moving, water. Most of these organisms don’t directly harm the lobster, but they each consume oxygen and discharge waste as well.
Figure 1 These three 5hp pumps moved over 1700gpm recic flow.
To prevent stressing the lobster, oxygen should be 90% or better at any spot in the tank. When taking measurements, the oxygen probe should be put amongst the product. In many tanks, I have measured 90% in one area and 60% in another. But, you can’t measure oxygen in the middle of 100lb crate where it would be the lowest. If you are measuring below 80% outside the crate, there will be dead lobster in the middle of the crate in short order.
The job of the aeration system is to keep the oxygen level above 90%. It is NOT there to move water around … that is the job of the recirc pumps. Dissolving oxygen into the water happens at the surface of any air-water interface. The more surface area you provide, the more gas is transferred. The longer the air bubble is in the water, the more gas it transfers.
Air that enters the tank from a 1/8th inch hole drilled in a pipe rises very quickly to the surface and is up to 1 inch in diameter when it gets there. By reducing the air bubble size, surface area is exponentially increased and the contact time is also increased. Oxygen can be maintained near 100% saturation throughout the tank and product quality is maintained.
Figure 2 Aeration efficiency. One keeps oxygen above 95%, the other lets it drop below 60%, stresses and kills lobster.
One customer told me he had two problems with his new multi-million dollar facility. His chillers and degassers were not working, he said. After taking a look, it was evident that the degasser worked fine when at the design temperature, but they couldn’t handle the extra load during the summer when the temperature increased. There was really only one problem – a large engineering firm designed the cooling system and decided in-floor cooling in the tank was the way to go. It wasn’t.
Temperature is the corner-stone of any aquatic system. Temperature directly affects the metabolism of the lobster and micro-organisms in the tank. Higher temperature means the lobster breathes and digests faster:
- Oxygen is consumed faster (and warmer water holds much less oxygen)
- CO2 is produced faster
- Solids and ammonia production increases
- Reproduction of bacteria, viruses, algae, is accelerated which brings us back to item 1.
Figure 3 Holding temperature below 1.5°C is the difference between a purging tank and a long-term holding tank.
You can build a facility with all the fancy gadgets in the world. But if you don’t get the temperature to hold at 1.5°C throughout the summer, you have a purging tank NOT a long term holding tank for market lobster.
Picture solids that a lobster would release into the water. The majority of them are large enough to be removed by filtration. If not put through a filter, the solids are pumped in and out of the tank over and over. The single, easily removable solid, becomes hundreds of small bacterial footholds and rotting nutrient bombs. Nutrients and bacteria in a closed system use oxygen and produce ammonia. Without filtration, more new water is required to flush these particles out.
More new water requires a bigger plate exchanger, and a bigger chiller to maintain temperature. You already have to pump water through the chillers. Why not clean it too? Would you rather buy more chillers or some filters. Remember, filters don’t get plugged in!
Figure 4 Filtration on a lobster tank reduces the new water required.
What does it take for bacteria to proliferate? Host surfaces, nutrients, and oxygen … exactly what is found in a tank full of lobster, especially one with trays or tubes. These bacteria compete with the lobster for available oxygen. They add to the ammonia produced (some also remove ammonia). And, many that are naturally present in coastal waters will cause harm to a weak or stressed animal. A UV filter helps to keep bacterial populations under control.
Red tide typically happens 2-3 times per year. This influx of diatoms or algae in the new water can also stress the animals and compete for oxygen. A UV on the new water and/or the recirc water will minimize the impacts on the lobster.
If air enters your pump, the pumping action will supersaturate it and give the lobster ‘the bends’. Nitrogen bubbles will form inside the animals veins and block blood flow. This can be difficult to diagnose because it happens at very low levels and typically in very localised parts of the tank. Degassing inlets as the water enters the tank are the best safeguard against mortality from supersaturation. Proper inlets also offgas CO2 and add oxygen to the water up to 100%. If the recirc water enters the tank at 100%, the aeration system can keep it there.
Improper water inlets are commonly seen that drive the water at high pressure into the tank. This can cause supersaturation as the hard, driving force will pound nitrogen into the tank with the water.
Figure 5 – Degassing inlets remove nitrogen and CO2 and add oxygen as water enters the tank.