Shellfish Aquaculture and Sustainability: The Renewable Seed Source


It’s the first week in June and we are anxiously awaiting a call from the hatchery.  We have been in the shellfish aquaculture business for going on thirty years but there is still that jolt of adrenaline when we get our hard clam (aka quahogs) and oyster seed in Spring.  We scramble to put out the nursery trays on our growing area, a plot on the intertidal flats of Wellfleet where we lease the shellfish rights.  The trays are the summer homes for the clam seed - extruded mesh boxes with a screen liner - they are filled with sand and create a protected micro-habitat for the tiny seed clams.  One hundred and sixty cages are laid out along lines, staked down, filled with sand and closed up tight with slit pvc tube closures along the edges to keep out predators.  Cages out, we are ready for the call from the hatchery.

A renewable seed source is key to the sustainability of shellfish aquaculture.  Every year oyster and clam aquaculturists need to restock their growing areas with seed. There are two sources of seed, either from a hatchery or the wild via natural recruitment. As growers, we are concerned with producing a natural product with a minimum impact on the surrounding environment.  Hatchery produced seed are a more reliable and more sustainable source of seed for growers. We harvest only the shellfish that we stock in our area. Replacing harvested shellfish with a new crop minimizes the impact on wild populations while giving us a consistent supply.  


An individual clam or oyster can produce tens of millions of eggs and sperm. They are called broadcast spawners because out in nature individual males and females “broadcast”  their sperm and eggs into the water where they are wafted by the tides and currents and meet up by chance.  They increase their odds of fertilization by releasing their spawn at the same time.  Tidal waters warm and cool as they move in and out of the shallows in summer.   Many bivalves coordinate their spawning with these temperature pulses.   In the wild, most fertilized eggs perish but in the hatchery, under controlled conditions, we can beat the odds and bring many through the vulnerable juvenile stages.  By pulsing them with warm water hatchery operators can trick the adult stock to spawn early in the Spring.  The resulting larvae are placed in tanks and fed cultivated algae which are microscopic single celled phytoplankton.  After going through a swimming stage that lasts several weeks, the larvae develop shells and drop out of the water column and settle to the bottom.  These miniature shellfish are gathered up and put into upwelling cylinders where phytoplankton rich water is passed through.  They feed and grow for about a month.  Once they reach 3-5 mm in length growers buy the seed and put them out on their beds for the nursery or grow-out phase. Seed production and nursery grow-out is a tough, hair-raising business. The hatchery process is unpredictable, so we don’t really know when to expect seed until we get the call from our hatchery guru.  It’s two parts alchemy to one part art. He is a shaman!    “Do you want seed tomorrow?”  “Absolutely!”  When we get the call we jump.  Talk about all your eggs in one basket!  It’s the single largest expense that you have in a year and you get it in two or three batches.  The survival of these seed determines what kind of crop you will harvest two to three years down the road.  If it’s blowing a gale or its a short tide you are going to have your hands full getting the seed buttoned up in the nursery trays.  Stress levels are high!  We’ll be getting 200,000 clam seed today.  Surprisingly, that’s only two five gallon buckets a bit more than half full!
Although hard clam and oyster seed are produced in a hatchery, a grower can also catch oyster seed in the wild.  Oyster spat collection, as it is called, involves putting some sort of substrate out in the bay in late June or early July for the millions of free floating natural oyster larvae to settle on.  Unlike clams, oysters need a hard substrate to attach to and they prefer other oyster shells or clams shells.  The tiny larvae attach and then grow into little oyster set or spat. In the absence of proper substrates, most of these planktonic larvae would perish.  By providing artificial substrates, growers can increase oyster recruitment into the system.   In our area stacks of mesh plastic “hats” are used.  Before being put out in the bay, the stacks are dipped in a thin mix of Portland cement, lime and sand to give them a calcium coating which attracts the larvae.  After a month, tiny oyster spat can be seen peppering the tops and bottoms of the hats. By the fall, the oysters are the size of quarters and the stacks are taken apart and twisted. Oysters pop off as little singles and are collected and put in mesh bags for growth to harvest size.
I pick up the two buckets of clam seed from our hatchery guru.  He is cheerful but looks exhausted - he has been up most of the night sieving and volumetrically counting clam seed for several growers.  Meanwhile, Barbara and our crew are down on the flats opening up 60 of the nursery trays. The cages are all opened when I get there.  Luckily, there is only a slight breeze and its a good tide.  One person holds the top of the tray open, I measure out a scoop of seed (calibrated to measure out 3300 seed) and sprinkle them into the cage.  The tray top holder evens the seed out and closes the top then two people come behind and fasten the pvc closures onto the edges of the tray. I scoop again and on to the next cage.  We move through the 60 cages, stocking them with seed and by the time we are done the water is returning, inching back only a few feet away from us.  In an hour the cages will be submerged, in six hours at high tide, they will be under ten feet of water.  This huge tidal flushing is what brings the abundant planktonic food source to the clams twice each day but it means our work time on the flats is limited to a two to three hour period around low tide.  We pause and wave our hands over the cages superstitiously paying homage to the clam gods.  Now they just have to survive and grow for three years! Despite all the challenges of getting a crop of farmed clams to market, starting from hatchery raised seed makes it possible to have some control over the supply.  Having a renewable seed source for a fishery is key to it’s sustainability.   Aquaculture techniques help to create sustainable shellfish populations by providing a renewable seed resource for growers and by reducing reliance on wild populations.

1 comment:

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