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Red Rock Crab Predator Management Techniques for Intertidal Shellfish Culture

DAVID NIKLEVA
BELOW SEA LEVEL OYSTER CO.
BOX 74
MANSON’S LANDING, BC
V0P 1K0        
250 935 6607 dnikleva@telus.net

Table of contents

EXECUTIVE SUMMARY

Intertidal shellfish culture is under intense predation from the Red Rock Crab. This project has installed four sizes of a subsurface barrier to prevent the movement of Red Rock Crabs onto the oyster beds. The treated area will be contained within a trench boundary, and an adjacent non-treated area will be used as a control in the trials. In the trials, both the treated area and the control area will have 300 oysters laid out in a similar method, bi-daily counts will determine the amount of predation in each area. The trench boundary that is parallel to the LWM will have “T” intersections of the same material that will empty towards deepwater, and allow the crabs to return to the ocean.

  • There has to be non-lethal control of the predator.
  • It has to avoid the Riparian Rights issue.
  • It needs to be cost effective for the oyster farmer.
  • There must be minimal maintenance requirements for the system.

The main objective of the study is to determine if the Crab Trench Trap is effective in reducing the crab predation. We had expected that several crabs could remain in the trench system and either leave, succumb to high temperature, or be preyed upon by birds. Now we were certain that the crabs did exit through the trench system, it promised to be a non-lethal approach to predator management.

When looking at the Mortality Rate As % Per Day table, neither the 6” and 8” trenches have a great difference between the mortality rates of the treatment and control. The 10” trench control has a higher mortality rate than the treatment in all six trials, often by a factor of 3 or 4 times. The 12“ trench control also has a higher mortality rate than the treatment in all six trials, as low a factor as 2 in several trials, but to the extreme of 10 x’s in the October trial.

CONCLUSION

The study as conducted did provide strong support for the objectives of the research.

  • Documented a high degree of crab predation on intertidal oyster culture.
  • The trenches only had minimal siltation over the winter storms.
  • The larger trenches reduce predation on intertidal shellfish.
  • This results in increased production and reduced seed costs.
  • This allows a larger portion of the tenure useable for grow out.
  • This results in increased production per area.
  • This improves the market value of the land for the farmer.

CRAB BARRIER IMPROVEMENTS

Improvements that will be addressed in Phase 2 of the project are:

  • Improve the design integrity that will increase the sidewall strength of the liner that will reduce the compression of the sidewall of the trench liner .
  • Redesign of the liner will also address the visual aesthetics allowing us to omit the concrete tacks used in the first style.
  • Test out the real sizes (uncompressed sidewalls) of trench to prove a statistically significant result in reducing crab predation.

OUTLINE

Intertidal shellfish culture is under intense predation from the Red Rock Crab. This project has installed four sizes of a subsurface barrier to prevent the movement of Red Rock Crabs onto the oyster beds. The treated area will be contained within a trench boundary, and an adjacent non-treated area will be used as a control in the trials. In the trials, both the treated area and the control area will have 300 oysters laid out in a similar method. The trench boundary that is parallel to the LWM will have “T” intersections of the same material that will empty towards deepwater. A crab will come up on to the shore with a flood tide, if it encounters the trench boundary, it will fall into the trench where it is free to move around, but when the tide ebbs, that will be the sign for the crabs to leave the intertidal area and they will utilize the T intersections to do that. This is an innovative solution to sustainable non-lethal predator management.

BACKGROUND

The Red Rock Crab is the most damaging predator for intertidal oyster culture on tenures in the Gorge Harbour, in other areas of Cortes Island, and also in Baynes Sound.

The evidence of this predation is most obvious through visual confirmation (caught in the act) and the amount of shell debris left on the beach that has the telltale damage signature of the Red Rock Crab. Most of this knowledge is by word of mouth among oyster farmers; Cec Robinson, John Shook, Pat McDonnell, Fred Picard, Ian Winter, Helen Radosevic on Cortes Island, and Gordy McLellan, Clark Munro, Keith Reid, and others. These farmers have quoted estimates of 10% to 100% of stock mortality due to the red rock crab. One of the results of the project will be to document the predation rate on the oysters in the four test plots located in the Gorge Harbour.

Various techniques have been used to impede crab predation. Crab fences have been erected, usually a short 12-24” high net of PVC coated galvanized wire.  Oysters are often grown in vexar (polyethylene) pouches, which are completely enclosed. The pouches can be grown either directly on the ground or used in a rack and bag technique. Some farmers have grown their oysters in an intertidal tray system that is secured to the beach. Others have grown their oysters high up the intertidal area, where they survive, but often don’t have good meat quality. Some of these techniques would require written consent from an upland owner before this technique would be approved on a shellfish management plan. This requires an amenable upland owner, but still does not promise any long term security.

An upland owner is not obliged to provide this security to you, and is allowed this power through legislation called The Riparian Rights Act.

One technique that Below Sea Level Oyster Co experimented with in August of 2005 was the Crab Bucket Trap. BSL had a one year scientific permit for this technique.

The bottom must be cut out or at least perforated to allow water to rise and fall with the tide. The bucket trap was effective in trapping red rock crabs, often 6-10 crabs per bucket per day. But the buckets required regular and daily maintenance to remove the crabs and relocate them to deepwater.

It was critically evident that we needed some ability to control the crab predation but it had to meet several criteria:

  • There has to be non-lethal control of the predator to satisfy DFO, the oyster farmer, and the general public.
  • It has to avoid the Riparian Rights issue, than a solution would be transferable to other suitable locations.
  • It needs to be cost effective for the oyster farmer to make the upfront investment of a system.
  • There must be minimal maintenance requirements for the system,

I discontinued using the bucket trap due to the high maintenance of emptying the buckets everyday, but the effectiveness of the simple technique led the way to what was first termed the Crab Trench Trap.

PROJECT OBJECTIVES

The main objective of the study is to determine if the Crab Trench Trap is effective in reducing the crab predation.

A second objective is to identify the correct size, shape and material for the fabrication of the Crab Trench Trap that is most effective in keeping the crabs from climbing out of the trench.

A third objective is to find a cost-effective solution for growers, so the trench was to be made in four different sizes to find the smallest but still effective size.

DESCRIPTION OF WORK

DESIGN

The project required an application for a scientific licence, this was to provide authority to carry out the project, approve any incidental capture, movement or mortality of red rock crabs during this experiment. Appendix 1, Scientific Licence

Through discussions with other oyster farmers, the design of the trench was developed. The CTT was initially fabricated out of fibreglass as it was most easily formed to fit the shape, four test sizes, and trench dimensions for this pilot study.

It was decided to go with a solid fibreglass base and a pattern cut out of the bottom would provide the flow for water to enter and leave the trench. There were several different designs for the perforation of the base of the trenches.

FABRICATION

Fabrication of the main trench channels and the exit channels were done in Campbell River and transported to the site on Cortes Island. There were four sizes that were developed. The cross section shape of the trench was a U / V shape, with a flat bottom.

There were different experiments to see if a 2ply or 3ply lay-up of fibreglass mesh or different reinforcements at the corners made much difference in the final product. The 6” and 12” trenches were treated with a white epoxy gel coat, while the 8” and 10” were left natural fibreglass. It is expected that the gel coat would provide longevity by withstanding the UV rays of the sun.

There was a two inch wide flange that ran along the topside of the trench that provided rigidity to the unit. This flange was predrilled to accept the concrete tacks.

The concrete tacks were ~4” in diameter and 4” in height. A section of PVC pipe was used as a form; it was split lengthwise and held together with a piece of tape. We drilled holes in a board to act as the base for the form; galvanized mesh was inserted into the form along with the rebar stake. One end of the rebar was bent back in a tight j shape, and the nub was left protruding slightly from the concrete in case a hammer was needed during installation on the beach. After the concrete had set, the form was removed and the concrete was allowed to cure before using it in the installation. There were 8-900 tacks used in the project.

INSTALLATION

The installation of the project was subject to approval by Kerra Hoyseth, Habitat Biologist from Campbell River. We had several discussions about the project and the installation procedures. I submitted a description of works for the project, which is included as Appendix 2. along with the letter of approval.

Our beaches are a typical shelf beach at the five foot tide level. From the HWM there is bedrock and very large rock down to the level shelf, then there is a fairly steep drop off to the LWM. The shelf section of beach is really a collection of boulders that provide the fabric of the beach, with a relatively thin layer of shell, gravel and fines covering the boulders.

Overall the installation of the trenches went as planned. Supplies and tools were brought to each site by barge. The 8”, 12’ and 10” trenches utilized the assistance of a mini-excavator to clear a furrow to install the trench.

The actual area where the trench was to be installed was measured out and surveyors tape was tied onto rocks and used as a guide for where the machine would dig. There were numerous small boulders ( 9”-24” in diameter) that lay just under the surface that are disturbed in the excavation. Our excavator operator tried to be sensitive to the environment that he was working in. Often one rock would be removed at a time, with the thumb holding the rock against the bucket, and the machine rotating to lay the rock down towards the ocean, with the other rocks. The fines and gravel were deposited upslope to be used as backfill. The excavator did most of the work digging the main channel and the exit channels every 5 metres. The field crew were removing rock and hand digging to fine tune the excavation. The 6” trench was dug by hand (not recommended with this substrate). The main channel was laid in the furrow, it was then velled to the existing beach substrate and held in place while it was back filled underneath by hand. The tacks were installed and the sides were backfilled. Once one section was done, the next section was lined up and screwed to the previous piece.
The critical part is to be able to keep the trench at the height of the natural beach. As the installation progressed, and an exit channel was to be installed, the main channel was marked and taken to the barge where it was cut out with a jig saw. The exit channel sections were fabricated with the slope taken into account and a flange provided to screw together with the main channel.

The amount of excess material left over after the installation was very minimal. Overall the amount of rock that was taken out was equal to the volume taken up by the trench liner. Therefore there was very little excess material to be spread out over the adjacent area, and very little material brought in where we were short of material. I believe three cubic yards of material were brought from off site for backfill of the entire project. This will be different with other locations that have a different composition of substrate.

SCHEDULING

The schedule of events did not go exactly as proposed in the application. The installation of the four systems was almost completed by the end of the daylight tides, with the last sections installed and some backfilling that occurred in night tides working under headlamp and floodlights. We did run one trial during the fall.

FIRST OBSERVATIONS

During the construction phase, we would float over a section of trench that had been completed to see if there was any crab activity. Again, at low tide we would inspect the trenched areas at low tide for activity. There was almost no residency of crabs in trench at low tide. There were crabs noted in the trench at a high tide, upon inspection at the following low tide there were no crabs evident in the trench. The main trials were to occur during daylight so that there could be proper monitoring of the system. 

OVERWINTERING

One of the questions put forward was whether the installation would even stay in place due to the winter storms. It was satisfying that the installations stayed in place at three out of four sites. At the 6” installation, we found that several sections had been blown out during a storm. A contributing factor may have been that we tried to excavate with man power and not use the excavator, and at times we didn’t go quite as deep, so there was a lip of 1-2” at some locations. This may have provided enough purchase if storm waves were breaking at that beach height and caught the edge of the trench. They were reinstalled and have been stable since.

There were suspicions that the winter storms would fill up the trench area with sediment or silt. On Nov 1, 2008 all four systems were cleaned entirely, and although we inspected the trenches throughout the winter,  we didn’t do another measurement until April 2009, five months later. The results were surprisingly good.

TRIAL DESIGN

LOCATION

The primary initiative was to test the treated plot inside the Crab Barrier against a control plot adjacent to the treated area to see if the Crab Barrier reduces predation by the Red Rock Crab. There were four different sizes of Crab Barrier (CB) that were tested. Each of these different plots had its own control site adjacent to it, therefore four different treatment sites and four different control sites.

As you can see on the location map, Appendix 3, the 6” CB is on the north shore of the Gorge H. just west of the Gorge Harbour Marina. The 8” CB is on the north shore directly across from the entrance to the Gorge Harbour. The 12” CB is adjacent to the 8” CB and has a section of the 8” as a divider between the two test plots. The 10” CB is on the south shore in the first bay to the east of the entrance to the Gorge Harbour. All of the sites are of a very similar geography. The tidal height is ~ the same at the 5-6 foot level. The beaches are all very similar, they are a level intertidal shelf with a fairly steep drop off that shows a foundation of small boulders. The substrate on top of this boulder shelf is a mix of pea gravel, shell particles, and fines.

LAYOUT

Usually the side trench continues past the bottom trench and acts as an exit channel itself (not as shown in schematic). The barrier at the LWM was ~50m at most sites, except the 10” site where it was ~40m. As the area is not entirely enclosed on all four sides, there is opportunity for crabs to come into the treated area by the upper part of the beach.
The control area for each of the four sizes of CB is directly adjacent to the treated plot on the same section of beach.

SETUP

The treatment area and the control area would each have 300 oysters planted in their plot. The test plots were ~ 7 metres along by 3 metres up the beach. The treatment plot was 3-5 metres away from the CB and the control plot was a similar layout on the shelf.
The oysters were laid out in five rows of 60 oysters, the rows are parallel to the LWM.

The oysters were from our deepwater site in the Gorge Harbour and were a small grade, 3-4” in length. The oysters were from string culture, and may have had some fouling on them when put onto the beach for the trials.

The plots were set up on the first day of a low tide series that had a low tide below the 5 foot tide height. The plot was counted ~ every two days during the low tide. Then there would be a period of three to six days where the tide was above the five foot level, too high to count. The next and final count would be on the first day of the next tide series that had a low tide at the five foot tide height. The oysters had their final count ~ fourteen days later and the oysters were gathered up and removed from the site. That is a change over day, so fresh oysters were laid and a new trial was begun. The number of days in a trial did vary as the tide cycles varied.

RESULTS

CRAB COUNTS

The Crab Barrier (CB) is designed to be self-maintaining. The “T” intersections installed every 5 metres are for the crab to continue its exit back to deep water, especially as the tide ebbs. In our design stage, we had expected that several crabs could remain in the trench system and either leave, succumb to high temperature, or be preyed by birds. Now we were certain that the crabs did exit through the trench system, it promised to be a non-lethal approach to predator management.

Crab counts were to be an integral part of the project as one of the goals was to document how many crabs remained in the trench and for how long of a period of time. During the installation phase of the project, we often floated over the sections at high tide and walked over the same sections at low tide and found that there was almost no presence of crabs in the trenches at low tide. This occurred over the 4-5 week installation period.. As this was distinctly not trapping the crabs, we now refer to it as the Crab Barrier!

The monitoring program was revised and the lower channel was not divided into two sections.  Each monitoring period we walked the entire trench system and counted any crabs that were in the trench. Occasionally we would find a crab that was still leaving the trench system. If we arrived early on the low tide, the beach is dry but the trench system still has water running through it and the crab may still be in the process of leaving. We would check back and usually find that the crab has left within that 30 minute period.

OYSTER COUNTS

The first trial in the fall 2008 was meant to test the system as the CB was just installed and did not want to leave the installation untested or unattended all winter. Success was that the treatment area had less oyster mortality than the control area. Another success was that crabs were escaping back to deepwater and not staying in the trenches for an extended time period.

Trial 1, this trial began Oct 23 and Oct 28 and finished on Nov 25, either 28 or 33 days
Trial 2  April 23 – May 6       13 days
Trial 3 May 6 – May 21        15 days
Trial 4 June2 – June 18        16 days
Trial 5 June 18 – July 2        14 days
Trial 6 July 4 – July 29         25 days

The sampling/counting of oysters was carried out as outlined in the methodology section. The data sheets are attached as Appendix 4.

A 2-factor ANOVA was conducted to compare the effects of treatment versus control and trench depth on mortality rate. The presence of a trench had a significant effect on mortality rates (p=0.0110), however there was no significant effect as a result of trench depth (p=0.7138), or interaction of treatment and trench depth.

Further analysis included a series of two-sample t-tests comparing treatment and control at the four different trench depths. These t-tests showed that only the twelve inch trench produced a significant difference between treatment and control (p=0.0163), while the other trials featured a high degree of variability which resulted in low confidences.

A regression of mortality rate versus trench depth, for the treated beaches, was conducted and found no significant trend (p=0.3250).

There are at least two other potentially significant factors that can also be evaluated to determine their contribution to mortality.

First, there seems on visual evaluation, a potential seasonal effect on crab predation. The result of this is most apparent in the 10” trench trial where high mortality rates in the first few trials are followed by low ones, which in turn resulted in extremely high variance in the data. The variance is potentially obscuring other significant trends that may be occurring. However, removing this effect from the data would require a much more sophisticated statistical analysis that was unavailable for this analysis. The 12” trench trial experienced a particularly high mortality in the October trial. These several peak predation events may indicate seasonality by the crabs, but it may be indicating another characteristic of crab behaviour or motility.

A second potential effect that cannot be excluded from the analysis is the effect of different beach locations. Different trial beaches are at different locations within the Gorge Harbour, and there are different tidal current patterns and most likely patchy distributions of crab populations that could affect beaches differently.

Despite the cursory nature of this statistical analysis, a significant effect was found due to the presence of a trench. It is expected that with a more thorough multivariate that a lot more significant trends could be potentially revealed.

When looking at the Mortality Rate As % Per Day table, neither the 6” and 8” trenches have a great difference between the mortality rates of the treatment and control. The 10” trench control has a higher mortality rate than the treatment in all six trials, often by a factor of 3 or 4 times. The 12“ trench control also has a higher mortality rate than the treatment in all six trials, as low a factor as 2 in several trials, but to the extreme of 10 x’s in the October trial.

The 12” trench is really an 8” trench, and the 10” trench  is really a 6.6” trench. So the study did not really examine what effect a real 10” trench and a real 12” trench would have on predation mortality. I suspect that a real 8” trench is just beginning to have an effect on the movement of the crabs. There fore it is worth continuing the investigation that predation rates would be on real sized trenches.

COMMUNITY FEEDBACK

VISUAL AESTHETICS

There was feed back from “The Alliance of Responsible Shellfish Farming”, who did respond to the activity on the beach. Within the first day of having a min-excavator on the intertidal tenure, we had complaints to the Federal Habitat Branch and to the Provincial Fisheries Officers. The next morning these officials were on site to observe the activity and were satisfied that it was conducted as it was outlined in the Description of Works. One of the complaints is about the visual aesthetics of the installation. Many of the photos earlier in the report are of the installation phase and show the disturbance required to install the trench. Those close-up pictures also magnify the cement tacks, albeit they are numerous, there were over 800 tacks used in the four installations.

Visual aesthetics is one area identified by some community members that we are able to respond to. BSL has entered a Phase 2 of the project and we have identified some potential solutions that will be able to get rid of the concrete tacks that were used to keep the installation secured to the substrate.

HABITAT DISRUPTION

There have been concerns about habitat disruption of the foreshore, and we have worked to try and minimize our impact. When large boulders were in the path of the trench, they were removed and placed downwards along with all of the other exposed boulders. Smaller substrate was used as back fill along with the smaller boulders. If there was any excess it was spread over the adjacent area.

STRUCTURAL FAILURE

There was a structural failure with the trench liners. They did not maintain the desired width due to the compression of the beach.

The 12” trench had compressed and the real width of the trench was averaged at just 7.8” wide. This is also demonstrated with the other trenches showing similar reductions in real width. This brings into question the premise we had for studying the four trench sizes. Firstly, if the trench was to be effective, then what would be the smallest size that would be most cost effective and also be the least disruption to the natural environment. The study did not in fact test out the real sizes of 10” and 12” trenches which may have shown more significant results. This will be addressed in Phase 2 of the project where structural integrity to resist compression will be one of the focuses of the study. We will then be able to test out the real sizes of a greater width than this study had the opportunity to.

CRAB BARRIER IMPROVEMENTS

Improvements that will be addressed in Phase 2 of the project are:

  1. Improve design integrity, increase  the sidewall strength to reduce compression.
  2. Visual aesthetics, redesign the trench liner to omit the concrete tacks.
  3. Test out real sizes of trench to prove a statistically significant result in reducing crab predation.

CONCLUSION

The study as conducted did provide strong support for the objectives of the research.

  • Documented a high degree of crab predation on intertidal oyster culture.
  • The larger trenches reduce predation on intertidal shellfish.
  • This results in increased production and reduced seed costs.
  • This allows a larger portion of the tenure useable for grow out.
  • This results in increased production per area.
  • This improves the market value of the land for the farmer.

ACKNOWLEDGEMENTS

Thanks to the DFO staff who assisted in preparation of the project,   Kerra Hoyseth,
Dr. Chris Pearce, and especially Kerry Marcus.

Thanks to the AIMAP staff; Andy, Lisa, and Sean for their assistance.

Thanks to Clint Collins from MAL, who reviewed and supported the project.

Many thanks to the BSL staff, the two project technicians, Wayne Smith and
Matt Swaile. Thanks to Zach Mueller for his assistance in the summer. The field crew for their hard work on the installation. Thanks to Steven Schut for his assistance. 

Appendix 2, Description of Works,
RED ROCK CRAB PREDATOR MANAGEMENT PROPOSAL

DESCRIPTION OF WORKS

The proposal was submitted to AIMAP for the June 20, 2008 deadline, and has been approved for funding under that program. The overall plan is included in the proposal. This description of works provides more detail to the DFO Habitat Branch in order to obtain permission to carry out the project .

The project outlines four different styles of crab trench to be tested on four separate beaches, which will happen on three tenures.

I have reviewed the Pacific Region Operational Statement On And Near-Bottom Intertidal Shellfish Aquaculture which describes the conditions to avoid negative impacts to fish habitat and believe that we can operate within these guidelines.

LOCATION

The intertidal tenures are all in the Gorge Harbour, Cortes Island, Area 13-15.

Site 1 & 2, Land file, 0278667, licence # 105454 will have two treatment areas.
Each treatment will be 50 m parallel to the LWM, and then turn up towards HWM.
Site 1 with the 12” trench
Site 2 with the 8” trench

Site 3, Land file # 1402986, licence # 105498 will have one treatment area.
The treatment will be 50 m parallel to the LWM, and then turn up towards HWM.
Site 3 with the 6” trench

Site 4, Land file, 1402707, licence # 105446  will have one treatment area.
The treatment will be 35 m parallel to the LWM, and then turn up towards HWM.
Site 4 with the 10” trench

SITE 1 & 2
GENERAL DESCRIPTION.

Site 1 & 2, along with the adjacent beach used for access are a shelf type beach
at the 5’ tide height that runs parallel to the shoreline. At the low end of the shelf is a steep drop off with small boulders. The substrate of the shelf beach is typically pea gravel and shell debris. The upper end of the shelf can be a fairly steep incline up to the HWM. There are no eelgrass zones and no salmon spawning streams, in either the work area or the access areas. Both sites 1 & 2 will have a 50 m trench parallel to the LWM, this will have exit channels at 5 m intervals. The 50 m trench will turn up towards the HWM until it reaches bedrock.

SITE 1 &2
ACCESS

This site is relatively close to the Government dock. The land access for this site will be across a roadway that is situated near the HWM on the intertidal zone. This roadway begins from the side of the gravel boat ramp at the Gorge dock. It crosses the tenure of the Cortes Island Harbour Authority, crown land, a neighbours dock tenure, and along our tenure; Lands file 1405036, Licence # 103167. This is a roadway that has been established since the 1950’s, when the area was cleared of boulders for the purpose of driving along to pick up oyster bags from the wild oyster fishery that was prevalent for many decades. At ILMB, I have seen an old letter from DFO giving permission for a bulldozer to proceed with the roadwork and also with the establishment of berms made up of rock and boulders. With a site visit, you would see that this area has had substantial relocation of the larger boulders in the intertidal area, some of it on the current shellfish tenure and some of it on area that may have been tenure at one time. We do on occasion utilize this roadway as part of our general farm work, but do try and minimize our use to when it is required.

It is proposed that the excavator could utilize this roadway to access the tenure where the work will be done. The roadway ends on the tenure adjacent to site 1 & 2. From here it  moves down onto the intertidal area, where a path will be established from this point to the location of the work, ~ 100metre distance. The path would proceed down to the lower end of the intertidal shelf then along the shelf at the similar location as to where the work would occur.

Access for additional supplies, materials, and tools could also come along this same pathway via a pickup truck, although we don’t anticipate using this route. It is more likely that we would alternatively utilize a small barge for bringing these supplies to the work site, as this is our customary mode of transport.

ALL SITES
EXCAVATION

Site excavation will be done with a small landscape style excavator and by hand labour. The excavator will do most of the material removal from the trench area. It will have a ‘thumb’ on the bucket so it is able to grab a single rock , shake the fines off and place it to the ocean side of the trench. Fines will be collected on the land side of the trench until the trench liner is in place. The fines will be back filled on both sides of the liner and tamped into place. If there are additional fines left over, they will be spread out evenly over the adjacent side of the trench at 1-2” thick.

Hand labour will be required to do the fine tuning of the excavation and for most of the filling component.

It is anticipated that the exit channels will be established before the main trench is dug. This pattern will be repeated as each area is completed. The exit channels will go into a more rocky area, and it is anticipated that the channels only have to provide the direction back to the ocean and access out of the trench. This may only be 4-8 feet from the main trench, where they will have some infill sloped into the trench to provide an exit for the crabs.

If there is a requirement for more backfill material than is available from the actual site, it would be brought onsite via the barge. If the amount required is obvious, it could be offloaded each day by the excavator at determined intervals. It is more likely that the amount required will be more discreet and will be unloaded by hand and ramp when required.

The back fill material will be from the pit of Robbie Graham on the south end of Cortes Island. Robbie does some grading of material, and we would likely get a mixture of sand and ¾” minus that would be typical of navy jack that is used in concrete. It is easy to move by hand and would be good for filling and tamping small spaces.

ALL SITES
TRENCH MATERIALS   

The trench liner will be custom fabricated from fibreglass. The profile is an abbreviated V. There will be a 2” flange at the top side which will be ~ level to the beach substrate. There will be four sizes tested. They will be 6”, 8” 10” 12” in the width opening with ~ same size depth to the flat bottom. The sides will be ~ 10 degrees, which will facilitate storage and moving, and will also accommodate any pressure from the surrounding substrate. Each section will be eight feet long and will have a flange at each end so they can be butted to each other and screwed together with stainless steel self tapping screws. Exit channels will be provided at 5m intervals. It is expected that the crabs will utilize the channels to leave the trench area and head back towards the ocean. The exit channels are of the same material and profile. The exit channel will be cut at an angle and the trench will have a piece cut out for an opening. The two pieces will be joined and either connected with a flange and screws or with a series of electrical ties to hold them together.

The base of the trench will have a longitudinal opening which will have a perforated base, this will provide water to move in and out of the trench so it resists floating away.

The flange on the trench will have holes drilled to accept a rebar stake. This stake will be driven through into the substrate, subject to rocks. The stake will have a cement head to it so it will resemble a very large tack. It is estimated that the tack will be ~ 5” in diameter and 3” high, with the rebar stake embedded into it. Our experience with trying to anchor clam netting is that is not always possible to drive the rebar staple in the ground deep enough to be effective in providing an anchor. With the cement tack system, the rebar will still provide some lateral resistance, but the mass of the cement head will assist in keeping the trench stable and resist moving. The cement tacks will be prefabricated off-site and will be cured before moving onto the site. The rebar is estimated to be 12 and 18 inch lengths depending on the installation. It is anticipated up to three tacks per side per length of trench may be required. It would be good to have one part of the installation with the three tacks and one part with two tacks to test what is required for anchoring. Of course, this may have to be adjusted site by site.

ALL SITES
Measures to Protect Fish and Fish Habitat when Conducting an On-or Near-Bottom Intertidal Shellfish Aquaculture Project, point 6 shall be addressed.

A small excavator will be brought to the site;

Site 1 & 2 will use an intertidal road route that has been established.

Site 3 & 4 will have the excavator brought to the site by a small barge.

The balance of the access will be done using the shortest route. Work will be started at the furthest point away and work back towards the exit area. Where there is water access, the machine will be deposited at one spot at the site and picked up where it stops work at the end of the day to minimize additional traffic.

The machine will be washed prior to being brought onto the site, and will be inspected for any maintenance required to keep it free from fluid leaks. There is an oil spill kit available through the Cortes Island Harbour Authority that can be available on a daily basis.

A contingency plan for any breakdowns will consist of having knowledge each working day of where any machines that can assist will be located, and how they can be contacted. There are several other excavators that would be in the area. Dave McCoy has a large boat and barge with capacity for retrieval at a water access site. A list of names and numbers will be kept on site, as will a cell phone, 

ALL SITES
SITE SAFETY

Job site; the pattern or method of the work will be developed as we gain experience with the tasks. I have experience with construction work sites and will organize the job site so we are working in a methodical manner.

Public safety; there is very little public foot traffic on any of the beaches. We will install the bright yellow sign that identify shellfish tenures. We will also install a sign on a short post mounted on the intertidal area that identifies the trench and to use caution when accessing the foreshore. We will send a letter to our upland property owners advising them of the activity and to let any of their guests know of our activity.

We will use bright surveyor tape on a short upright that is positioned regularly along the trench that will attract peoples’ attention.

ALL SITES
MONITORING

To summarize what is in the proposal, each test site will have a control site adjacent to it. Both sites will be seeded and counted on a bi-daily basis. This will determine if the predation from crabs is less in the treated area than the control area.

The trench area with the exit channels will be divided into two equal sections by installing a bulkhead halfway. One side of the trench will be serviced by counting any crabs left in the trench, and removing them to the water nearby. The other side of the trench will be serviced by counting any crabs left in the trench, and leaving them in the trench. This will help determine the effectiveness of the channels in the crabs exit, or the need for people maintaining the trench, to minimize mortality of crabs. After the 10 day trial period, the trench will be cleaned of any crabs and debris. 

SITE 3
GENERAL DESCRIPTION.

Site 3, is a shelf type beach at the 5’ tide height that runs parallel to the shoreline. At the low end of the shelf is a steep drop off with small boulders. The substrate of the shelf beach is typically pea gravel and shell debris. The upper end of the shelf can be a fairly steep incline up to the HWM. There are no eelgrass zones and no salmon spawning streams in the tenure or adjacent areas. Site 3 will have a 50m trench parallel to the LWM, this will have exit channels at 5m intervals. The 50 m trench will turn up towards the HWM until it reaches bedrock.

SITE 3
ACCESS

This is a location that will be accessed by barge. The excavator will be loaded at the Gorge Harbour Government Dock and will be moved over to the tenure site. When the ebbing tide is at the right height, we will move the barge in and unload the excavator. At the end of the tide, the excavator will be removed the same way. We can either dock at the Gorge Harbour Marina which is nearby or utilize a mooring block also nearby.

SITE 4
GENERAL DESCRIPTION

Site 4 is a more protected bay style beach with a protected inner area. The inner beach is from the 5’- 7’ zone with pea gravel, with a little mud. The lower area is a shelf at the 5’ zone, with a rocky drop off, although one part of it is shallower. The substrate of the shelf beach is typically pea gravel and shell debris with an area of cobble. There is a fairly steep incline up to the HWM from the level beach area. There are no eelgrass zones and no salmon spawning streams in the tenure or adjacent areas. Site 4 will start from one corner of the beach against rock, and continue 35 m (half way) across the opening and then will turn up 50 m until it reaches bedrock. 

SITE 4
ACCESS

This is a location that will be accessed by barge. The excavator will be loaded at the Gorge Harbour Government Dock and will be moved over to the tenure site. When the ebbing tide is at the right height, we will move the barge in and unload the excavator. At the end of the tide, the excavator will be removed the same way. We can either dock overnight in the adjacent bay which is quite protected, or at the adjacent deepwater tenure which has good mooring.

Refer to attached map for location of tenures, Map 1
Refer to attached site diagrams for each treatment area, Diagram 1, 2, 3, 4
Refer to attached materials details diagrams, Detail Diagram 1

August 12, 2008

Dave Nikleva
C/O Below Sea Level Oyster Co
Box 74, Manson’s Landing
Cortes Island, BC
V0P 1K0

Dear Mr. Nikleva:

Subject: Red Rock Crab Predator Management Project for Intertidal Shellfish Culture

This letter will serve as advice from the Department of Fisheries & Oceans, Oceans, Habitat and Enhancement Branch (DFO-OHEB) with respect to your red rock crab management research project. This project has been approved for financial support under the Aquaculture Innovation and Market Access Program (AIMAP) in order to try to reduce predation from red rock crabs on shellfish aquaculture in a non-lethal way.

From conversations we’ve had, diagrams and the project description you submitted to me on July 15, 2008, I understand that the project consists of making narrow trenches in three tenured beaches of Gorge Harbour, Cortes Island (tenures: 0278667, 1402986, 1402707), of which the intent is to keep red rock crabs out of shellfish culture areas to reduce predation and loss of product. The trenches will surround four culturing areas, and when crabs approach the shellfish beach at high water, they will be deterred by the trenches or jump into them. If they jump into the trenches, there are outlets they can escape through spaced every 5m at the marine edge. I understand you have applied for a Scientific License from DFO which will give you the authority to have some potential incidental mortality of red rock crabs if they are unable to escape from the trenches, but that the numbers of mortalities will be recorded and submitted to DFO-Resource Management (Kerry Marcus) for review. If mortality of red rock crab or any other species becomes unacceptable during this research project, it may be cancelled by DFO and the trenches removed from the beach by the proponent.

These trenches will be dug by a small landscape style excavator and by hand, a fiberglass form will be placed in the trench to prevent in-filling, and anchored to the beach with rebar stakes. Different beaches will have different trench dimensions, ranging from 6” to 12” wide and deep. Machinery will approach the beach on an established roadway or by barge, and care will be given to prevent impact to the intertidal area. Beach substrate ranges from pea gravel/shell hash to boulders, and you have identified that there are no salmonid bearing streams or eelgrass beds nearby which could be affected by this activity.

Fisheries and Oceans Canada, Oceans, Habitat and Enhancement Branch (DFO-OHEB) has a legal obligation to protect fish and fish habitat. Carrying out any project that could harmfully alter, disrupt or destroy fish habitat by chemical, physical or biological means may constitute an offence under the federal Fisheries Act. DFO-OHEB’s first preference is that the loss of productive fish habitat be completely avoided during all phases of the project.

After reviewing the habitat assessment information you have provided, DFO-OHEB has determined that valuable or sensitive marine habitats will likely not be harmfully affected by your proposal, provided all of the mitigation conditions outlined below are followed.

  • Substrate modification, other than as described above, such as construction of berms, will not occur. If you find you are excavating excess material while making the trenches, you can spread it out in a thin layer (1-2” thick) on the adjacent beach. However care should be given to prevent any kind of smothering, so this material should not be placed on any kind of marine plant/algae, such as eelgrass, saltmarsh grasses or kelp. If you have more material than can be safely spread out, remove excess material from the intertidal area and appropriately dispose of it out of the foreshore and on land.
  • Additional material may be brought on site if additional fine materials (sand-gravel) are needed to keep the fibreglass trench in place. Ensure that it is clean, and use only what is necessary. Do not dispose of any extra material on the intertidal or foreshore area.
  • Select staging areas such that impacts to aquatic vegetation are minimized or avoided and foot traffic through Important Fish Habitat doesn’t occur. Examples of Important Fish Habitat are eelgrass beds, algae beds, saltmarsh, estuaries, kelp beds, lagoons, or salmonid creek, stream or river channel. Maintain a sufficient buffer between activities and these habitats. 
  • Operate machinery in a manner that minimizes disturbance to the intertidal areas and other fish habitat.

    • When operating machinery on the beach is unavoidable, routes should minimize intertidal interference to the maximum extent possible.
    • Single crossings using the shortest route possible should be used, so disturbance of the foreshore is minimized.
    • Prepare a contingency plan for addressing vehicle breakdowns in the intertidal area.
    • Machinery is to arrive on-site in a clean condition and is to be maintained free of fluid leaks.
    • Wash, refuel and service machinery and store fuel and other materials for the machinery away from the water to prevent any deleterious substance from entering the water.
    • Keep an emergency spill kit on site in case of fluid leaks or spills from machinery.
  • Native beach material (i.e. logs, sand, gravel, boulders) are important components of fish habitat and should not be used as mooring structures or taken from the seabed or the intertidal zone. Any boulders that you have to move during excavation should be gently replaced nearby in an appropriate manner.
  • Avoid locating moorings in important fish habitat, if they are required for the barge.  Minimize disturbance to submerged aquatic vegetation when securing mooring structures to the seabed.
  • Moorings (including anchors and floats) are to be made of clean material. If concrete anchors are to be used, they are to be pre-cast and pre-cured away from the water before use, to prevent seepage of potentially toxic substances into the water body.

This letter does not constitute an approval under Section 35(2) of the Fisheries Act for the harmful alteration, disruption or destruction of fish habitat. Independent advice and additional measures may be necessary to ensure the protection of fish and fish habitat during this work. Please be advised that this correspondence addresses only the concerns of DFO-OHEB under the Fisheries Act and does not constitute approval of any other federal, provincial, or municipal, or regional district agencies. It is the obligation of the proponent to meet any other regulatory requirements such as the federal Navigable Waters Protection Act (Transport Canada), the BC Water Act (BC Ministry of Environment) or Regional District or Municipal legislation or regulations.

A copy of this letter must be provided at the request of a Fisheries and Oceans Canada official. If there are any changes to the proposal, or if anything is incorrect or unclear above, please contact the undersigned at (250)-850-5721. Any changes will have to be assessed for potential effects and may require additional mitigation measures or implementation of other management approaches. Please contact us 5 business days before you start any site modifications.

Sincerely yours,

(original signed by:)
Kerra Hoyseth
Senior Habitat Aquaculture Biologist
South Coast Area, DFO-OHEB

Date Modified: 2012-02-17

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