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Making Baby Shellfish

Aquaculture is all about underwater agriculture; growing seafood rather than relying on Nature to provide it.  Aquaculture is not new.  Asians have been doing it for thousands of years.  What IS new about it is that an increasing number of species are coming under cultivation… and none too soon.  Even in the vastness of the world oceans, the harvest pressure put on fish and shellfish stocks have brought many species to commercial extinction within the last twenty years.  But technology is starting to help fill the void.  Eventually, even the giants of the sea, the tunas and billfish, will be spawned and grown like so many pelagic cattle.  Hopefully, that will leave the wild stocks to recover and survive into posterity.

Ironically… growing shellfish starts at exactly the opposite end of the calendar than Nature would start on her own.  It is during the shortest days of the year that algae culture begins in the hatchery.  Around the winter solstice, the lights come on and the temperatures inside start to rise.  Oysters, clams and scallops are vegetarians.  They eat algae… lots of it.  This is why a healthy shellfish community is so beneficial for the estuary environment.  Unchecked, algae blooms can quickly create a stifling turbidity, which can eventually lead to a low oxygen condition known as hypoxia.  So every shellfish hatchery also functions as a greenhouse.

In the Noank hatchery, we have two big rooms upstairs devoted to algae culture.  We grow 4 different species along with one species of diatom.  Diatoms are plants, similar to algae, which have a siliceous outer shell.  Algae rooms are usually painted white and are lined with full-spectrum lighting.  In our hatchery, the rooms are large and square so we also have lighting, which hangs from the ceiling too.  The idea is simply to create a very bright environment conducive to plant growth.  Also lining the room but inside of the lights, are clear plastic tubes called cawall tubes.  They are filled with water and are where most of the algae is grown.  Air and CO2 are bubbled in.  Most of the tubes sit directly in front of the lighting while larger tanks sit directly under the hanging lighting. Algae culture is tricky business.

The original cultures from which the entire seasons worth algae crop are to be grown, must be kept separate, in a climate controlled, medically sterile room.  No one but the hatchery manager is allowed in when live cultures are there.  Thousands of water-borne organisms like to eat algae.  Some are even present in our drinking water.  There are probably some on your dry hands right now!  So ANY introduced contaminant into either the original cultures, or to the inoculated tubes, will quickly spread, causing a bloom of contaminant organisms, that can quickly devour a desired algae population.  This can be disastrous if it occurs when the hatchery is full of millions of hungry baby shellfish.  While the hatchery occasionally supplements the diets of adult and juvenile shellfish with commercially available algae paste, live food is always preferred and the paste is expensive.

Under ideal conditions, the individual tubes and tanks are inoculated with a small quantity of the desired algae, which blooms to a high concentration.  More water is added and the bloom continues.  After a couple of days of adding water, the entire tube is filled with water that can look like anything from coffee to pea soup, depending on which species is being grown.  Left unchecked, such population growth would eventually crash on its own, but at the maximum density, the volume starts to get drained off as food to the animals living downstairs.  All the tubes have valves that allow them to be gravity fed to the main floor.  The tube is eventually emptied, cleaned, refilled, and the whole process starts again.

Meanwhile, downstairs, the animals are getting ready to spawn.  For the past month, adult oysters clams and scallops have been living in shallow tanks, being fed, and having their ambient water temperatures slowly raised.  Winter storms might howl outside but inside the brood stock room it is 80 degrees and the sound of water is bubbling.  With the algae cultures up and running, the hatchery manager can go the final step with his brood stock, which have been building gonad tissue for the past couple of weeks on a heavy diet and increasingly summer-like water temperatures.  On spawning day, the hatchery manager transfers the brood stock into a shallow black tray.  He or she will now spike the temperature the last two degrees and hope Nature does the rest.

Shellfish are broadcast spawners.  They time their reproduction so that millions of animals will release their eggs and sperm all with the same proximate time frame.  Marine fecundity is an amazing thing and it also provides the most abundant source of food on the planet.  A single female oyster can release up to 15 million eggs in a single spawn.  It is the quintessential bottom of the food chain.  In the wild, the billions of eggs and sperm float along in the water column, meeting by chance and quickly developing into larvae.  Only one in a million will survive to adulthood, most are eaten by other organisms.  But in the hatchery, we aim to capture that fecundity.  As the animals start to spawn, they “flap” their shells and eject a visible milky (sperm) or grainy (eggs) puff.  The hatchery manager quickly transfers the animal to a separate bucket, labeled by sex, where it will continue to eject it’s spawn.  As all “willing” animals do their thing, the manager ends up with a small but viable genetic pool, which will eventually comprise the entire current spawning run.  Animals, which did not spawn, are returned to their original tank to try again at another date.

Now the manager has buckets of eggs and sperm, which need to be put together.  So the process moves along to the main hatchery room.  And the first vessels to contain the new life are called conicals.  Conicals are round fiberglass tanks, which, as their name suggests, are cone shaped at the bottom.  There is hole in the bottom into which air can be pumped, or from which the tank can be drained.  The conical shape is to encourage “upwelling”, a term you will hear increasingly used.  Upwelling is simply the process of providing water turnover in a closed or contained system.  Creating artificial current if you will.  The manager will put a specific volumetric quantity of eggs and sperm into each conical which by now are full of eighty degree water and have air bubbling up from the bottom.  The eggs and sperm must stay in suspension just as they would if they were floating along in the currents of the estuary.  If they attach to the sides of the tank they will adhere and eventually die.  By the time the day is over, as many as twenty bubbling conicals line the walls of the hatchery, each containing half a million clumps of cells which are busy dividing and becoming larvae.  In a mere day, the cells contained will form themselves into shell-less miniature oysters, swimming around by means of cilia, eating every cell of algae their little “mouths” come in contact with.

After 10 days or so of daily feeding, and alternate day water changes, the developing larvae are ready to start on their next stage.  You might be wondering how one changes the water in a conical when it is full of larvae of nearly microscopic size.  Well… there are meshes and fabrics used in medical and other applications so small, as to allow the filtering out the larval shellfish and letting the water pass.  Every hatchery has a battery of these “sieves” with the capacity to strain objects as small as 20 microns, up to screens that can separate 3mm oysters from 5mm oysters.  Separating animals by size in the hatchery is one of the most tedious, time-consuming chores, but it must be done.  Growth rates and thus size variations can be enormous, and the larger animals tend to out-compete the smaller ones.  So even as early as four or five days post-spawn, the hatchery manager will start to “screen” the animals for size.  The larger, more developed animals will move on, the smaller will go back into the same place in the system and try to catch up.  But they better hurry.  The hatchery manager is already planning another spawn for next week and if these “runts” are not onto the next stage by the time that next spawn goes off, they’re going to end up out in the Sound swimming in 38 degree water!  The animals that are caught in the larger mesh sieves have reach the stage known as “eyed” larvae.  That is simply a term for the stage of development where the gut is visible under the microscope.  A day or two more and they’ll be ready for the set trays.

In nature, as the surviving larvae continue to grow, they get heavier and heavier.  They begin to settle toward the bottom.  They are still considerer zooplankton and have no shell to protect them.  When the larval oyster reaches the bottom, it will attach itself to a fixed object.  Hopefully (for the young oyster) it will be another piece of shell, or a rock, or a piling, somewhere below the low tide mark.  In the hatchery, once again, we control the conditions to optimize the ability of the young oyster to survive and form a shell.  The “eyed” larvae are really getting heavy.  So the hatchery manager transfers them to a tank in which float, a set of screened bottom trays.  The trays have a dusting of “cultch” in the bottom.  Cultch is crushed clam shell, and is the perfect medium for the larval oyster to attach to.  Within a couple of days of being introduced to the set trays, and with constant feeding, most of the larval oysters have attached themselves to a single piece of cultch and have formed their own shell around themselves.

Another few days of feeding and growing and the next stage of hatchery life will begin.  Another week has gone by.  It’s the end of January and the first oysters of the hatchery season already have their shells.  Now the hatchery manager “sieves” them out, separating them from the cultch, and is ready to put them into their “silos”.  A silo is a round tube about 16 inches around with one end open and the other end screened.  Every hatchery has a bunch of silos, all with different screen sizes.  When the baby oysters come off the set trays, they will sit directly on the screens of the smallest mesh silos.  The silos are themselves lined up in long tanks containing water.  Each tank can hold about fifteen silos so the entire unit can once again hold half a million animals.  In the tank, the silos can become either “downwellers” or “upwellers”.  Downwellers have water pumped into them from the top, so the flow of water is down through the screen.  Upwellers have an exit port, so water is simple pumped into the tank, where is percolates up through the screen and out the exit port.  In either case, as in the entire hatchery system, the idea is to create artificial current, within a closed (recirculating) system, which brings food and oxygen to sedentary, filter feeding organisms.  Pumps move water around in the tank, and food is constantly provided.  This is where you can really see how much those little rascals eat.  I have left in the evening with the tank water pea-green, only to come back in the morning with it perfectly clear.

The upweller systems will be the basis for animal growth for the rest of their time in the hatchery as well as through the outdoor nursery process.  The screen sizes will be increased.  They will be eventually moved to tanks outside, to both help them acclimate to cooler temps, and to make room for successive spawning batches.  Eventually, as spring begins to show, the animals will be moved to “in-water” upweller systems.  They now eat so much, the hatchery can no longer provide enough food to meet their needs.  The outdoor systems rely on pumps to move water through the upweller units where the animals feed on natural spring algae blooms.  An early spring is a welcome event for the hatchery and nursery staff.

Raising shellfish in a hatchery setting is an odd combination of science and cookery.  No hatchery operates exactly the same as another.  Systemic peculiarities and counter-intuitive situations can make the endeavor frustrating and challenging.  But as the vagaries are one by one overcome, and the survival rates and spawning success goes up, it becomes a feat almost impossible to duplicate by another.  It is no surprise that where the number of insurance salespeople can fill pages in a phone book, the number of shellfish hatcheries in a 300 mile radius, on the New England coast, can be counted on a single hand.