In order for the clam lab to be cost effective, it is important that our clam population has a high survival rate as well as a typical growth rate. The good news is, most of our two-month-old clams have survived up to this point. The bad news is, their growth rate is significantly slower than we expected. We quickly began looking for ways to identify what was preventing our clams from growing at a faster rate. At first, many variables came into question – was the water temperature too cold? Was there not enough water flow? Was there not enough microalgae and other nutrients in the harbor? To answer these questions and find the source of our problem, we got help from Kyle Pepperman, the hatchery and production manager at Downeast Institute.
Kyle seemed confident that the tidal changes were cycling plenty of plankton and nutrients into Bunker’s Harbor. He also did not think that our water temperature in the upwellers (ranging from 56ºF to 61ºF) was causing the clams to feed at a slower rate. However, Kyle did seem very concerned that we were not supplying enough water flow in our upwellers. It’s important that we cycle a sufficient amount of fresh, saltwater from the harbor into our upwellers so that the clams have a readily available food source at all times. It turned out that our single ½ horsepower water pump was not nearly powerful enough to supply both of our upwellers with a flow rate fast enough to keep up with our hungry clams.
Our pump sits inside a plastic pipe which is attached to a dock pole. It must push water from the harbor over 25 feet high against gravity until it eventually reaches the clam lab on shore. Once the water gets to the clam lab, it must be redirected through PVC pipes and corner pieces until it flows into our upweller tanks. The combination of gravity and 90º turns increases the total dynamic head and therefore, slows down our water flow even more. A few days ago, Bill Zoellick and I tested out our new water flow monitor to get a more concrete and comparable measurement of our water flow. He attached the monitor to the end of one of the PVC valves. With the other flow valves shut off, we got a reading of around 7.9 gallons per minute. When he opened up the flow valve to the other upweller, the water flow was halved (about 3.9 gallons per minute). For reference, Bill’s garden hose produces over 6 gallons of water per minute. With both tanks full of clams, Mr. Pepperman suggested that we should have a water flow rate of at least 20 gallons per minute. It’s safe to say that a lack of water flow was our main problem.
To solve this issue by increasing our water flow, we need to find a way to either reduce total dynamic head or increase the horsepower of our pump. We considered redoing the plumbing with a larger pipe diameter and less 90º PVC elbows which both reduce friction head; however, our plumber didn’t think that option would be possible with our current setup. The other option involved buying a new pump with more horsepower. This option would be costly as new pumps are expensive and a pump with more horsepower would increase our monthly electric bill. However, if this option increases our clam growth rate substantially, our clams might only need one summer to grow to a size where they are large enough to be placed in the mud. This option becomes more reasonable in the long term because it would save us from running the pump throughout the entire winter as we would have no clams to overwinter in our upwellers.
Last week, we were finally able to put all of our one-year-old clams in the mud. We covered them with predator netting and wished them the best of luck in their new home. Putting around 10,000 clams in the mud left one of our upwellers entirely empty. We quickly saw this as an opportunity to provide more water flow for our tiny seed clams. By transporting all of our two-month-old clams into this empty upweller, we were able to turn off the water flow to the other upweller. With only one upweller running and only half of the slots being used in that upweller, the 7.9 gallons of water flowing into our upweller per minute is plenty for our small clams to grow at a more satisfying rate. Until we get more clams or until we need to distribute our clams among more buckets, there is no point in replacing our pump immediately. Just in the past few days, we have observed our clams growing at a faster rate.
Our next step is to put a sample of these clams in nursery trays and float them in the old lobster pound. Our goal in doing this is to compare the growth rate and mortality of our clams in the upweller with the growth rate and mortality of our clams in the nursery trays. If we get better results from growing our clams in nursery trays, then there is no point in continuing to spend the time, money, and energy growing our clams in upwellers.