The core of all production in the sea is phytoplankton, which multiplies rapidly, subject to presence of mineral nutrients and light. From there on, a short trophic relationship with an organism, which has a good conversion efficiency, would be the most effective and healthiest way to produce anything.

Sessile filter feeders like mussels are good candidates, because they belong to the second marine trophic level, don’t spend any energy to swim and cannot escape. The very short trophic relationship between primary production of phytoplankton and production of mussels assimilating it, combined with the very pure AAIW quality, are the unquestionable guarantee of the lowest level of anthropogenic pollutants at the end of the production process.

Mussel aquaculture has been practiced for centuries, even millennia, because it is rather an easy culture. They multiply profusely, attach themselves on any surface or created device, and feed on any organic matter they find. Several techniques are used nowadays. Using long lines is the most productive technique for mussel aquaculture, because it takes advantage of the volume in the water column, instead of a surface. However, this production method, intended for human consumption, is too expensive, and needs to be improved to fit aquafeed production. Several stage sorting and mussel cleaning can for instance be discarded, sparing a lot of time and money. The long lines themselves are too expensive in their actual form, and need also to be re-thought.

Rope cultures of blue mussel
(Source: The MARICULT Research Programme)

Despite these observations, long lines mussel farming is by far the world’s most productive breeding method, with currently 150 tons, up to 300 tons, per hectare and per year. To put these figures into perspective, beef production is only around 0,340 ton per hectare per year, almost a thousand times less ! With mussels on long lines, we can reasonably forecast a production between 3 to 6 million tons of mussel flesh (75% is shell) in a square of 90.000 ha, like the flat top of Davis Bank is. This figure can also be compared to the 5,5 million tons of Peruvian anchovies caught in 2018, world’s largest fishery dedicated to the production of fishmeal and fish oil. These were their highest landings since 2011 and won't be surpassed, due to government policy to limit the total allowable catches (TACs). Moreover, 2018 has not experienced the famous El Niño event, periodically resulting in a temporary collapse of this reduction fishery.

However, as a consequence of their relative easiness to be cultured, some aspects of mussels’ biology, like their most efficient diet, have only been poorly documented. We just know that mussels ingest preferably plankton and other organic particles from 5 to 15 microns. Diatoms, the main intake, provide DHA (good for our brain) and flagellates provide EPA (good for our heart), both important LC-PUFA omega 3. The control of mussels’ diet, until now neglected, has all its importance here, especially if it is revealed to be adjustable.

Each mussel filters nearly 100-liter water a day. Retaining all the present particles, its food conversion factor can be very fluctuant, between 30 and 80%. That means mussel produce between 20 and 70% faeces (which pass through the digestive system), or pseudofaeces (which are immediately rejected). These sink and pollute the surroundings, because the organic charge is too important for being degraded by aerobic bacteria during the fall, especially when the bottom is not very deep, what is mostly the case in mussel culture areas. Once the faeces are accumulated on bottom, anaerobic bacteria take place, with its inconveniences. That’s why knowledge about mussels’ most efficient diet has its importance. Especially if we are able to sway this food conversion factor and the nature of the metabolized substances, by using the appropriate phytoplankton mix to feed the mussels. In our case, these particular phytoplankton species can be injected in the rising nutrient-rich AAIW passing through the salt fountain pipes, to let them multiply on their way up, thanks to strings of blue LED inside the pipes. That’s a way to control mussels’ diet, in order to minimize faeces and maximize LC-PUFA omega 3 production.

Now we have the framework. But it will not work as it is, because of the environmental footprint of such a huge mussel farm. To avoid contaminations, it still needs to be atomized between several places, or at least separated in some way, by macroalgae for example. Anyhow, it needs also to be associated with other cultures to become a self-sufficient biotope. This technique is called Integrated Multi Trophic Aquaculture (IMTA) where one species' wastes are recycled as feed for another.

Integrated Multi-Trophic Aquaculture concept
(Source: Fisheries and Oceans Canada, drawn by Joyce Hui)

Mussel faeces cause pollution problems in most of today's monoculture areas. To avoid this, the soluble faeces can be assimilated by kelp or another macroalgae, and the solid ones can be assimilated by scavengers on bottom like sea cucumbers. These will also strongly contribute to the project's viability, because of their value for Asiatic people, who appreciate them a lot as food and medicine. You can read more about it on the website of the IMTA Research Laboratory.

However, despite IMTA technics, such a mussel farm will lead to a huge threat for the existing biodiversity of this rhodolith covered seamount, which counts several species of algae, corals, crustaceans, sponges, fishes, among which some are endemic. In short, Davis Bank is a rich reef biotope as shown in this video below entitled "Fish biodiversity of the Vitoria-Trindade Seamount Chain - Pinheiro et al 2015".

Since this filming in 2005, carbonates mining to provide fertilizers for Brazilian sugar-cane plantations has occurred there between 2009 and 2011, together with mining trials of cobalt-rich crusts. Moreover, since several years now, fishing vessels from China, South Korea, Portugal and Spain are commonly on work there, as we can observe on Global Fishing Watch. If this fleets use bottom trawlers, the reef might be in pretty bad shape.

Spanish fishing vessel on Davis Bank April 2018
(Spanish fishing vessel on Davis Bank in April 2018)
Chinese fishing vessel on Davis Bank november 2018
(Chinese fishing vessel on Davis Bank in november 2018)

I don't know how this paradise looks like today, but we may perhaps choose a less space requiring marine organism for our purpose. Copepods, especially calanus can also be good candidates. From 13 000 species of known copepods, 10 000 are marine and 5 000 of them are nonparasitic and free living zooplankter, like calanus. These little 1-2 mm crustaceans represent a considerable marine biomass and are naturally a crucial link between energy-producing phytoplankton and fish. Like mussels, they belong to the second marine trophic level. In spring, they aggregate together in huge swarms near the surface, until seawater becomes like syrup and baleen whales can scoop them in big mouthfuls.

Calanus finmarticus is the most numerous animal species on the planet
(Photo courtesy: Terje van der Meeren, Institute of Marine Research, NO-Bergen)

A single copepod can catch and consume a few hundred thousand phytoplankton cells per day, clearing about a million times their own body volume of water. They feed at the surface at night to avoid visual predators and eventual toxic emanations from phytoplankton. At daytime, they sink several hundred meters down to stay protected. In Polar Regions, they sink until thousand meters depth to hibernate during the cold season and live on their reserves, after fattening up in spring.
This is why their metabolism allows them to build up a very high protein and LC-PUFA content. To be able to sink, they change their oils in more dense fats, wax esters. They accumulate also astaxanthin, a lipid soluble carotenoid, well known for its anti-oxidant properties, which enables them to hibernate with preserved stocks of nutriments. Astaxanthin is also the substance that gives the orange-red color to wild salmon flesh. Fish feed is currently added with synthetic astaxanthin to avoid rancidness and give the right color to farmed salmon flesh.
With a view to calanus breeding for fish feed, some very interesting properties can be highlighted:

  • They are already natural fish feed, a whole feed for aquaculture.
  • They aggregate together naturally and can thus be easily harvested.
  • They have a very high protein and LC-PUFA content, naturally well balanced with all needed micronutriments.
  • Bound as wax esters, the LC-PUFA are much better assimilated as those of fish oil and lead to very pronounced positive effects on important metabolic parameters.
  • Thanks to their astaxanthin content, perhaps they don't even need to be processed into meal and oil to be stored at room temperature.

Hope can be given to be able to breed calanus swarms inside the Perpetual Salt Fountain pipes, harvest them on the top outflow and thus avoid threating the environment.

All these conclusions of mine are written without references to all the sources I have used, because I do not claim to have done a scientific work. My project is all the same a solid base to the challenges of fish farming sustainability and oceans’ biological productivity preservation. As a pragmatic ecologist and amateur oceanologist, I think we have to start by installing perpetual salt foutain pipes on Davis Bank.

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