Laying Cultch: a Case Study from the Essex Native Oyster Initiative

The Essex Native Oyster Restoration Initiative (ENORI) is a collaboration between oystermen, government, conservationists, and academia, working towards the Essex estuaries having self-sustaining populations of native oysters. The Blackwater, Crouch, Roach and Colne Estuaries in Essex were designated as a 284km2 Marine Conservation Zone (MCZ) in 2013 – the largest inshore protected area in the UK – with the objective of recovering the native oyster and native oyster beds. It is within this MCZ and alongside the historic Blackwater oyster fishery, that the ENORI 2km2 pilot site is located and where physical restoration works began in 2019. The MCZ is both substrate and recruitment limited so cultch was deployed to improve the seabed and mature oysters were translocated from the neighbouring Blackwater oyster fishery. The project benefits from a native population of locally-adapted broodstock and so translocation from further afield was not necessary. However, it was essential to translocate cultch from outside the waterbody to the restoration site which brought with it additional biosecurity risks.

Aggregate (mixed pebbles) was a more-readily available source of cultch, which could be used to provide most of the elevation off the seabed wanted with shell added as a top layer to test settlement success. Of the marine aggregate options available to the project at the time, it was not possible to confirm the source and would likely be a mixture, not of local provenance. To minimise biosecurity risk, a land-source aggregate rather than marine aggregate was therefore chosen. Early dialogue with the supplier confirmed that the gravels were from River Terrace Deposits quarried locally to the restoration site in Essex. A sample of the gravels was also obtained from the supplier to allow for a visual check for oils and other potential contaminants. 

To tap into local sources of shell as much as possible, a recycling initiative was setup to return the shell of Blackwater fishery oysters sold at Borough Market in London and elsewhere to West Mersea. This was considered the lowest biosecurity risk for cultch translocation as the shell was originally removed from the same waterbody as the restoration site and would not have come into contact with shellfish from other waterbodies. 

However, there is a limited supply of Blackwater oyster shell and so other options were considered in order to achieve the volume of cultch required. Scallop, cockle, and blue mussel shell was sourced from national and European suppliers. Again, dialogue with suppliers was critical to confirm (where possible) the exact source of the shell and to understand risk. Where shell is bought from an aggregate company, it should be noted that it is often not possible to know with certainty the geographical source as they are the ‘middle-man’ of the supply chain. Information such as how the shell had already been treated (if by heat for commercial shellfish processing, at what temperature and for how long) and how it was stored (location and duration) was also ascertained from suppliers. 

Although this information was gathered where possible, it was agreed by ENORI that any shell (regardless of source and heat treatment) should be weathered outdoors, exposed to the elements, for a period of 12 months, to ensure as far as possible that hitch-hikers and pathogens would not persist. Samples of the shell were visually inspected for living matter. Locating suitable storage sites was difficult, particularly when seeking a site that:

  1. was close to where the cultch would be loaded for deployment to minimise transport costs, without being directly connected so as to prevent transfer of INNS and pathogens during weathering;
  2. had hardstanding so that large vehicles capable of handling big volumes of shell and stone not suitable for soft-ground can access the drop-off/pick-up location; and
  3. is no-to-low cost for the duration of 12 months (possibly longer).

It is advised that such sites are secured as early as possible in the planning process to avoid any delays to deployment or multiple cultch handling and transport costs. In the end, not all cultch could be stored at one location but was spread across several. Where possible, shell material was stored in small piles to maximise exposure to the elements. When restoring the seabed, even for pilot projects, the volume of cultch required can be substantial and to store it in a relatively thin layer and turn bi-weekly or monthly is a considerable undertaking. The resources (space, contractors, vehicles and potentially volunteers) that are required to deliver this should be considered as early in the planning process as possible and, importantly, built into project budgets. 

Mussel spreading (Photo: Matt Uttley, ENORI)
Scallop shell unloading (Photo: Matt Uttley, ENORI)

Throughout the planning of the pilot restoration works, ENORI sought advice from shellfish health and INNS experts to ensure that the most appropriate risk management approach was adopted. On the advice of Cefas, a project record was setup that listed all the aggregate and shell used, the source, the treatment prior to delivery, the duration and location of storage, rotations and any risks associated with source and the restoration site. This was a valuable exercise for ensuring the appropriate steps were being taken by the project to minimise biosecurity risk and for any audits that may take place in the future. As ENORI look forward to scaling up, a high priority continues to be to develop best practice biosecurity protocol working in partnership with other projects in the Native Oyster Network and NORA. 


Thank you to Thea Cox of the Zoological Society of London and the Essex Native Oyster Restoration Initiative for sharing the ENORI experience in selecting and managing cultch for their in-water restoration efforts in a Bonamia impacted estuary in England. It is hoped that ongoing and future projects may benefit from ENORI sharing this hard-won experience through this blog post.