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.

The Start

The ice finally cleared off of our shellfish bed yesterday, some chunks bobbing in the channel ominously.   Shifts in the winds could bring them back in or shoot them out of Wellfleet bay for good.  The sandy stretch of beach where I worked looked surprisingly familiar compared with the chaotic jumble of huge icebergs of just two days ago.  Somehow you expect the winter ice to transform the flats but there are only subtle changes and the impression of resilience.  The oyster cages that I left out were still there and most of the ropes that marked the sides of the clam rows were in place or angled offshore, dragged by the ice still anchored by one stake on the end.  When I pulled them straight and staked them down they recreated the orderly outlines of the clam rows.  The deepest parts of the grant, where there is considerable soft muddy sediment, was pock marked from where clumps of the sediment had been frozen to the rising and falling icebergs and floated offshore.  There were curious piles of clean coarse sand in places like over sized ant hills.  Sand that had been  stuck to ice from the upper beach and dropped as the icebergs melted and moved off.  It had rained all day which created a kind of ground fog of melting snow and ice and I could see the retreating storm clouds moving off on the horizon.  This is the very beginning.  How many of these clams survived?  It seems so improbable at the very start of a season.  For the next few months I’ll go through the various year classes of clams and see how big they are, how many survived and figure out when they will be ready for harvest.  I looked down at a row with new seed from the previous summer.  There were a few little clams on the surface and I could see that their siphons were out.  I was surprised that they would be active this early.

They were reaching up, ready to draw water into their bodies to feed.  Four days until Spring.  The clams seem to know.  A net was twisted and pulled off of a clam row.  It was saturated with fine sediment and water and almost too heavy to lift.  I wrestled with it for about twenty minutes unraveling and stretching it back out and staking it in place over the clams.  I’m way too old for this, I'm thinking.  I get back in the truck and head out.  Got the radio on, the heat on and I feel exhilarated as I drive home.

Check the Ice

I can’t remember a winter where we have had this much snow this late in the year.  I have to walk down a path like a corridor through the snow to get to my truck.  Going to take a ride to check the ice.  Our shellfish beds have been under sea ice for the last six weeks but it has been a smooth sheet covering the whole inner bay.  The sheets float up and down with the tide and may even insulate the shellfish from extreme temperatures.   But just past the jetty, which borders our beds, there is a huge flow of broken up icebergs, many of them the size of SUV’s that extend all the way down the bay.  Those are the ones that leave big ruts in your clam beds as the bergs move in and out with the tide.   I pull into the pier parking lot for a closer look across the bay and I see three other growers, their trucks pulled up close to each other, windows down.  Probably speculating “If it rains like it is supposed to on Wednesday and the wind comes around to the  Northeast... might just blow it out.”  Hope so.  The wind has been out of the south all week packing the ice from the inside of the Cape up into Wellfleet bay.   I pull around to the Mayo beach parking lot for another view.  My friend Bob, also a grower, pulls up. “How’s your bed lookin?” I ask. “It’s all locked up, we need a noreaster.”  As we talked a Boston TV crew started to unload setting up a tripod on the beach.  “I guess the ice in Wellfleet is big news!” Not exactly a fast breaking story, but a media event none the less.  
Barb and I drive down Indian Neck, the following day, towards our shellfish bed.  The assistant shellfish warden is driving in the opposite direction.  I wave and he shakes his head no, meaning that our beds haven’t cleared of ice yet.  When we get down to the parking lot at the end of Indian Neck a couple is getting out of their car.  “Where are the icebergs?” the woman asks. I’m a bit taken aback, I’ve never been asked that before.“Out there.” I say pointing past the dune to the bay which is covered in ice as far as the eye can see.      

Clam Sorter Design

The drive system of our experimental sorter
Aside from Notes I am writing a blog that chronicles our process in developing a field sorting machine for clam aquaculturists to use to sort and grade clams on their shellfish beds.  In the past year we have launched the design/research branch of our business to focus on research and development in shellfish aquaculture. Since aquaculture is a relatively new kind of fishing there aren't always established techniques for handling the various challenges and problems that arise.  As a consequence we are in R and D mode constantly, trying to evaluate and improve methods, techniques and equipment.  But now we are trying to do more specific concrete research to quantify the efficacy of our new developments.  Check out the field sorter blog and the design research section of our website.

Fall Planting

Rinsing seed clams and removing crabs
The clam seed have spent the whole summer in nursery trays on our growing area.  They are pushing up out of the sand and need to be "planted out".  We'll shake them from the nursery trays to remove the sand and then spread the seed in rows in the sand.  The rows are covered with nets to exclude predators like fish, birds, snails and crabs.
Deniz hoeing a trench along the edge of the row.
See the seed clams spritzed in the row?
This is sustainable shellfish aquaculture, so the seeding process is essential to replenish the clams that we harvest.  The cycle takes about 3 years from seed to harvest, so each year we are planting and tending to clams of different sizes and year classes.  Clams spend about one to two years under the protective netting.  As clams get larger they tend to be less vulnerable to predation and the nets more prone to fouling by macroalgae so we remove the nets.  When the nets are covered with algae they trap silt and inhibit flow to the clams which can result in smothering.  While the clams are covered we spend a lot of time sweeping and clearing the nets.

Laying out the netting and fastening it in the trench