Protecting Manila Clams from Sea Otter Predation

Final Report
Nootka Sound Shellfish
AIMAP 2012-P13

Executive Summary

Nootka Shellfish are a limited company farming manila clams and Pacific oysters in the Nootka Sound area, which is in the central portion of the west coast Vancouver Island. We believe that we are one of the first active intertidal clam farms that is being impacted by sea otters (Enhydra lutris), with a significant impact on our standing stock of manila clams. Pacific oysters do not seem to be affected.

Sea otters are a fairly large member of the weasel family, weighing from 14 to 45 kg. Their range currently includes California, Washington, British Columbia, Alaska, Russia and Japan. Otters were re-introduced to the coast of Vancouver Island between 1969 and 1972 and their population is expanding at close to twenty percent per year. They are protected under the species at risk act (SARA), their status was down listed to threatened in 1996. Otters rely on very thick fur rather than blubber to counter the effects of cold water. Therefore, their metabolic rate is two to three times as high as would be expected in a similar sized terrestrial mammal. To fuel this high metabolic rate, otters eat twenty five to forty percent of their body weight daily, mainly shellfish. They are capable of digging at least one foot deep, and will dig through rock or thick oyster cover. Sea otters arrived in this area in 2007, and have been increasing in numbers since. 

In order to protect our farmed manila clam stocks from this predation, our company placed two different types of webbing on one of our tenures. This tenure is at the most risk of predation; due to its location in Kendrick Inlet, Nootka Sound; license #AQSF 1798, 2012. Otters seem to be moving up the intertidal from consuming larger species of clams in the subtidal portions of the beach, and we hope that a 30 meter wide barrier between the stocks of larger clams and the standing stock of manila clams may break the trend. Manila clams above the barrier would be smaller in size and hopefully less attractive to sea otters. We have therefore covered 1.5 ha of our tenure with these two types of netting. The first net type is a 7.5 meter by 15 meter rectangular net, using two inch nylon web, which is not buoyant. This net has minimal stress points as it is not attached to the perimeter line, making it very difficult to tear, and does not float, reducing the possibility of entanglement. A finished net weighs approximately 25 kilos. The second net is a 4 meter by 15 meter panel of extruded bi-oriented polypropylene mesh. This netting has mesh openings of 28 mm by 38 mm, tensile strength of 1200 to 2150 foot/pounds, and a net weight of 23 kilos per panel. Both nets types are pinned to the beach with steel staples and the corners are also held with vexar sacs containing approximately 10 kilos of rock.

The site is approved for up to 3.77 ha of protective netting; however we propose to cover only 1.5 ha. There are many shellfish farms and / or fisheries on the west coast of Vancouver Island, and the BC Central coast, most of which are utilized by local First Nation groups, and all of which are not financially viable without protection from these otters. This predation also affects all the larger species of clams, geoducks, abalone, sea urchins, sea cucumbers and scallops. There is no doubt that sea otter predation will deplete a shellfish farm to the point that it is no longer commercially viable. There will be no sustainable production of any of these species in any of these regions if sea otter predation cannot be contained. In keeping with improved environmental performance, it is critical that the safety of the otters be maintained while their predation of commercially viable species at specific locations is curtailed.

Introduction

We are a limited company farming manila clams and Pacific oysters in the Nootka Sound area, which is in the central portion of the west coast of Vancouver Island. We believe that we are one of the first active intertidal clam farms that is impacted by predation from the sea otters (Enhydra lutris), which is having a significant impact on our standing stock of manila clams. Pacific oysters do not seem to be affected.

Sea otters are a fairly large member of the weasel family, weighing from 14 to 45 kg. Their range currently includes California, Washington, British Columbia, Alaska, Russia and Japan. Otters were re-introduced to the coast of Vancouver Island between 1969 and 1972. I believe that their population is expanding at close to twenty percent per year. Their status was down listed to threatened in 1996. They are diurnal, tending to forage early in the morning or in the evening. They tend to congregate in single-sex rafts, often up to one hundred animals in each raft. Otter can swim at speeds up to 9 km/hr, and rely on very thick fur rather than blubber to counter the effects of cold water. Therefore, their metabolic rate is two to three times as high as would be expected in a similar sized terrestrial mammal. To fuel this high metabolic rate, otters eat twenty five to forty percent of their body weight daily, mainly shellfish. They dive to the seabed to forage, and can stay submerged for up to five minutes. They are capable of digging at least one foot deep, and will dig through rock or thick oyster cover. Sea otters arrived in this area in 2007, and have been increasing in numbers since. Otters tend to be in our area in high numbers from early October until mid-June annually.

Please be aware that river otters are not an animal of concern on manila clam farms, as they do not consume clams. They may even be considered beneficial as they prey upon flatfish, which are a manila clam predator. Currently there is very little data on methods to reduce the impact of sea otter predation, as this is a new issue in most aquaculture farm areas on the west coast of Canada. Without protection, stock losses would likely be 80 to 85 percent of standing stock on a manila clam farm. Obviously, this would not be viable on a commercial basis. Other species that are being severely impacted include scallops, sea cucumbers, sea urchins, abalone, mussels and geoducks. As well, First Nation access for food will be significantly impacted. With a rapidly expanding range and population growing at close to 20 per cent per year, this issue will have a huge impact on much of the west coast of Canada.

It should be noted that intertidal ground which naturally grows manila clams can be extremely productive per square meter. Seeded and netted ground can be projected to produce five kilograms per square meter per year, for an annual value of approximately $20.00 per square meter. The Kendrick Creek site has excellent growth rates due to the tidal level of the substrate, where clams are very low on the intertidal and can grow very quickly. The most likely source of diggers in remote areas is often local First Nation communities, who are likely to welcome employment opportunities. Work is restricted to part-time due to the limits of tidal access to the farm ground.

Methodology

The site is a large, highly productive intertidal beach in Kendrick Inlet that; because of its location, on the western side of Nootka Sound, is currently being targeted by large rafts of otters, often containing 200 to 400 sea otters. This license, #AQSF 1798, is a 5.5 Ha, fairly flat beach with a seven nautical mile fetch to the South-east. The beach is gravel and shell substrate, and has a solid standing stock of manila clams and seed, making it attractive to the otters. On the Eastern side, the site is impacted by Northerly outflow winds during winter cold spells. There is a mid-sized creek that flows down the western side of the tenure, with the tenure boundary being set back by five meters from the creek bed. This creek floods annually during major winter storms, but the only clam ground close to the creek has been netted with the plastic mesh, with no significant impact on the netting. The fetch to the SE results in both surge and chop waves, which can be significant during major winter storms. Otters seem to be moving up the intertidal, as they have been consuming larger species of clams and geoducks in the subtidal portions of the beach. While this site is approved for up to 3.77 ha of protective netting; we expect that a 30 meter wide barrier covering 1.5 Ha of beach between the stocks of larger clams and the standing stock of manila clams may break this trend. Manila clams above the barrier would be smaller in size and hopefully less attractive to sea otters. Due to a strong annual set of bay mussels, which tend to foul netting, we have used a larger mesh size and a smaller diameter of twine.

The challenge is to create a net that will hold in place, will not bury in the substrate, will not become overly fouled with mussels, will protect the standing stock, can be easily removed to access the working ground and will not endanger sea otters.

The nylon nets were very easy to place on the beach, but the 15 meter by 15 meter size proved to be too large, which resulted in our reducing the net size to 15 meters by 7.5 meters. This size was much easier to manage. We used a slightly larger size of polyester rope than projected, as 11/32 rope was readily available. The rope threaded through the mesh quite easily, the mesh was lashed to the ropes at the corners and 18 inch ends were left protruding. This allowed us to tie all the net panels together at the corners, and then pin the nets down. Steel pins worked well in this section of the beach. All sixty one nets were placed on the outer portions of the site, on the active ground where there is a lot of wave and chop action. The mesh proved to be cheaper than expected as the quote was done by length of mesh but sold to us by weight. This savings was countered by increased costs in rope and labour for building the nets. The mesh tends to snag sticks and debris easily, but has not torn except in encounters with propellers. There was quite a bit of substrate moved by winter storms, which tended to collect at the top edge of the nets. However the nets pulled out of the berms very easily due to the thin size of the twine. One significant benefit to the lack of buoyancy of the mesh is that rock sacs are only required along the perimeter lines, and not used at all on the center of the nets. This makes opening a panel very easy and quick. We did use plastic cable ties every six feet along the perimeter lines to ensure continuity of the panels, but are seeking an alternate solution. We simply don't like the requirement to cut the cable ties every time we open the net, resulting in plastic garbage and loss of time during the tide.

The fact that the mesh remained loose on the perimeter lines is critical to the flexibility of these nets. They are very easy to spread out on the beach and are very difficult to tear as the mesh simply moves along the rope in the direction of the force being exerted on the mesh. We are very impressed with these nets but have not had time to observe the effects of fouling on this mesh. They have proved to remain it place even in areas of breaking waves of at least one meter in height. There have been absolutely no issues with either fish of marine mammals becoming entangled in this mesh.

The second type of net is 4 meter wide panels of extruded bi-oriented polypropylene mesh, which is a rigid material. This mesh comes in rolls that are 75 meters long. We did deploy half of this material at full length of seventy five meters, attached together with cable ties and sharing rock sacs on the seams. This proved disastrous as the first winter storm blew most of this material up the beach, while a current we were not aware of moved the balance of the sheet seaward. We immediately changed the orientation of the nets ninety degrees so they were perpendicular to the water's edge, and reduced the length to 15 meters, which proved to be much more manageable. An issue of serious concern was that algae growth on the beach became trapped under this plastic mesh to the degree that air pockets were being held by the algae, causing the nets to lift and spill the rock sacs through the seams. 

So lesson number one was: have enough rock on hand to hold down the plastic mesh before attempting to deploy the nets. We stopped deploying these nets until we could get a better handle on how to manage them. After several months of struggle, we changed our thinking of what the final product should look like. We stopped trying to work with a continuous sheet of plastic and considered each panel to be a stand-alone unit. We stopped sharing rock sacs and increased the number of rock sacs on each panel to twenty one. To ensure that we would not lose the material, we tied two rock sacs directly to the mesh with rope. This is the first thing that we do when placing a net on the beach, and these rock sacs are then never removed. The nets are dragged sideways with those two sacs still attached, which is not difficult. The balance of the rock sacs are then placed on the net about fifteen inches in from the edge, and have not spilled since we made this change. We have stopped using cable ties on the plastic mesh; we found them ineffective and simply didn't want the plastic pollution on our tenure. As well, we found that steel pins were not holding in the softer substrate, despite our lengthening the pins to 30 inches. While we still use steel pins on the seaward corners of the plastic mesh, they are of limited value with mesh that is buoyant. We continue to use shorelines and anchor lines to keep the nets in the general area that we wish to cover, but run a header rope across the nets and fasten to each individual net independent of each other.

While at first we were quite intimated by the task of managing this material, we are now very comfortable with it and are beginning to favour these nets in areas where there is not too much surge and chop. However this material is not suitable for very active areas of the beach. We are finding that this is the preferred net in areas of current where there may be a lot of debris moved along by the water, i.e. near the river or in wallows where waterlogged wood would accumulate. We cannot stress enough that this material requires a lot of rock!!  We have reduced the size of rock sacs to about eight kilos and increased the number of sacs to 21 to 24 per panel. This means that instead of requiring 1550 rock sacs, we actually used closer to 4000 rock sacs. One interesting fact was that we did not run short of rock on the tenure, we still have areas where there is rock that could be picked. So the total weight of rock currently in use is approximately 40,000 kg.

In terms of timing for this project, absolutely nothing occurred as projected. Our company had difficulties with the site's license, which delayed the approval of the required funding. Then the plastic mesh was back-ordered at the factory, causing a further delay. In the meantime we built and deployed the nylon nets, but rather than shipping twice on the Uchuck III, we transported those nets and steel staples into Nootka with our own truck and skiff. By the time the plastic mesh arrived with the Uchuck III, on July 31t, our Vietnamese contract crew had left to honour other prior obligations. We did manage to complete the site preparation before they left, and we used our regular crew to deploy seventy percent of the plastic mesh. The plastic mesh proved problematic, so we ended up lifting and storing some of the panels and storing the balance of the mesh. We did not continue deploying the plastic mesh until mid-November, following both the first major winter storm and our change in thinking of what we wished to accomplish with the plastic mesh. At this moment all of the plastic mesh is on the beach and working well. So while we were not exactly on time, we can only consider the final result an unqualified success.

There is a portion of this beach which is not included within our tenure due to it being closed to direct harvest of bi-valve shellfish. We use this portion as a control area, to monitor predation on an unprotected area of the intertidal zone. Our nets have been monitored bi-weekly since August 4, 2012. Nootka Sound and the surrounding area are very difficult to make use of predation nets successfully, due to a large annual bay mussel set which will attach to the webbing. As the nets are laid out flat on the intertidal ground, these mussels then attach to pebbles and shell pieces on the beach. This can result in a very heavy web which is difficult to move and which will smother the standing crop if the fouling becomes too thick, reducing water flow and starving the animals. We therefore have designed the nylon predation nets to have a larger mesh size and a thinner twine, reducing opportunity for the mussels to attach. The plastic mesh has quite large mesh openings and a flat profile, so small mussels can be crushed by a weighted roller, keeping the fouling population in check. Keeping with green technology, it is extremely important that our experiment does not endanger the sea otters. We will be working with Mr. Paul Cottrell, Pacific Marine Mammal Coordinator, Fisheries and Oceans Canada, to ensure that the safety of sea otters is maintained. 

This final report is somewhat premature, as the most likely season for major sea otter predation will be during the spring and early summer months. As well, the nets are now leached out and will be attractive to various fouling organisms during the spring and fall spawning seasons. Our company is therefore committed to continue monitoring these nets and will issue a supplementary report, following the fall mussel set.

Going Forward

There are many shellfish farms and / or fisheries on the west coast of Vancouver Island, and the BC Central coast, most of which are utilized by local First Nation groups. All of these sites will be at risk of sea otter predation in the next several years. As well, First Nation access for food will be significantly impacted. This predation also affects all the larger species of clams, geoducks, abalone, sea urchins, sea cucumbers and scallops. There is no doubt that sea otter predation can deplete a shellfish farm to the point that it is no longer commercially viable. There will be no sustainable production of any of these species in any of these regions if sea otter predation is not contained. In keeping with improved environmental performance, it is critical that the safety of the otters be maintained while their predation of commercially viable species at specific locations is curtailed.

We have developed a respect for the appetites of these sea otters, especially when they show up in large numbers at your site. At one site, we put down nets on half the beach, planning to finish the job on the next tide. At least a hundred otters showed up in the evening and the next day we were greeted with dozens of buckets of broken and empty clam shells on top of the nets we had placed the day before!   When they show up, your crop is either protected or gone. Individual otters also have a huge impact simply because they eat several times a day, and will stay in an area for months. That said, in the last three years we have come into frequent contact with otters and have found them to be quite timid and sensitive to human contact. It is interesting that there is absolutely no evidence that the sea otters made any serious effort to breach or displace the nets, while impacting ground adjacent to the net panels. This is true for both of these net styles as well as a polyester net that we have used at another site. Perhaps we are being optimistic but it seems that sea otters will not be difficult to fence out of cultured stock or specific areas. It is important to diligently maintain the nets. We are confident that we can continue farming manila clams despite large populations of sea otters in the area.

Conclusion

We had stated that we would consider this project to be very successful if the otters did not breach either type of netting, if the netting neither is blown off the beach or buried in the substrate, and if the control area shows evidence ( holes and broken shell ) of sea otter predation. Despite several set-backs when the nets moved or blew up the beach, or substrate buried the top edge of the nylon nets, we feel that we have met the criteria to declare this demonstration project successful.

There can be no doubt that without this netting we would have lost this clam crop during this winter and into the future. It would have been unlikely that our company would have undertaken this work without the funding made available through Fisheries and Oceans Canada's Aquaculture Innovation and Market Access Program (AIMAP). Since the standing crop is hopefully now secure, it will act as brood stock for the surrounding area, encouraging recruitment in the wild Area F clam fishery. Both the farm and the wild fishery will provide employment for local First Nations, who hold licenses and harvest for our company. We anticipate that we will provide opportunity for at least eight people, jobs that would have been lost without this investment. As well, other companies facing this dilemma will be encouraged by the positive results and may choose to attempt to protect their crops with predation barriers. While we are only dealing with manila clams, many other species are also impacted, over a large and growing portion of the British Columbia coast. This work will have a huge impact on the aquaculture industry's willingness to confront and limit sea otter predation.

Sea otters are beautiful animals!  We wish them no harm. We simply wish to learn to co-exist with sea otters as their population stabilizes, while maintaining a sustainable business.