Cockles in D&S IFCA’s District

Cockles, Cerastoderma edule, are burrowing filter feeding bivalves which grow up 5cm long.  They are usually found in soft to course muddy sediment to a depth range of around 5cm in the mid to lower intertidal area and can, less frequently, be found subtidally.  They are highly tolerant to fluctuating salinities and as such can be frequently found in estuaries and sheltered bays.  Populations of cockles can be primarily found to the south of the D&S IFCA’s District, within estuaries such as Teign, Exe and Avon.  In two of these estuaries, the Teign and Exe, D&S IFCA carries out its surveys on cockle population density.  The cockle surveys are conducted to monitor population changes, both in density and distribution.  These data can then be used to inform any appropriate management decisions relevant to the estuary in question.

Cockle Stock Survey Methodology

Although there is some minor variance in the survey techniques between the estuaries, the main methodology remains the same.  A grid, comprised of different stations set equal distance apart, is placed over the survey area and co-ordinates generated for each station.  In the Exe Estuary the distance between stations is 115m x 115m whilst in the Teign Estuary it is 75.5m x 75.5m. Officers will then navigate via GPS to the exact location of each station and randomly throw a 0.1 (metre squared) quadrat within a 10m radius.  The contents of the quadrat will then be dug out (to a depth of approximately 6cm), sieved, and then any live cockles found are grouped to station level and collected for later analysis.  Inaccessible sites, and sites with no live cockles are noted.  To ensure that the data collected are comparable the same stations and co-ordinates are used every year.  Whist this works for the Exe Estuary, in the Teign Estuary the station distances were initially different for The Salty and upstream of Shaldon Bridge.  In 2019 this approach was revised to ensure the same resolution of collected data from both sides of the Shaldon Bridge was achieved.  The co-ordinates for both sides of the bridge where subsequently updated to be equal distances apart forming a single continuous survey zone.  The 2019 station co-ordinates will form the new basis for continued annual comparisons within the Teign Estuary. 

When the survey of the beds is complete the cockles collected are taken ashore and measured, using callipers, to the nearest millimetre for length and width.  After measuring, the cockles are then sorted into age classes by determining how many annual growth rings were present on the shell (one per winter) e.g. 0 rings = current year, 1 ring = 1st winter /1 year, 2 rings = second winter/ 2 years and so on.  Each year group from that station is then weighed separately (to the nearest 1g) and recorded.  This is repeated for all station samples and once finished all the cockles are returned to the estuary.  Based on the data collected it is possible to determine how many cockles are present per 1m2 per station across the survey area, as well as providing length frequency, recruitment, and mortality data for the surveyed population.  Whilst it would be preferable to take multiple samples per station, the limits of intertidal surveying mean that D&S IFCA Officers only have a brief four-hour window to work with over the spring low water tide.  Data collection, therefore, needs to be both efficient and effective, the described method is employed widely amongst other IFCAs and is regarded as the appropriate methodology.

Exe Estuary

D&S IFCA began carrying out annual cockle stock assessments in 2010 on the Cockle Sands near Exmouth, in the Exe Estuary, to determine if a sustainable cockle fishery was viable.  Two MSc students, from the University of Plymouth and University of the West of England respectively, looked at the effects of harvesting cockles using an elevator harvester on macrofauna, cockle populations and sediment parameters within the Exe Estuary.  This form of harvesting was found to be low impact and as a result of undertaking this research and the subsequently completed Habitat Regulation Assessment, Natural England advised that the fishery could continue.  However, there was a mass mortality event in 2011 depleting the population of cockles on the Cockle Sands.  The mortality of the cockles was investigated by Cefas, and although no notifiable diseases were found, the parasite profile indicated elevated presence of the haplosporidian Minchinia spp. and digenean Himasthla spp., both of which can cause high mortality in cockle populations.  This led to the newly classified beds to being declassified and plans for a fishery abandoned.  Despite this lack of a commercial fishery, it must be noted that a low level of hand gathering has and still occurs on the cockle Sands.  D&S IFCA continued to conduct cockle surveys annually up until 2018, monitoring the beds for recovery and using the data to inform future management within the Exe Estuary SPA particularly in relation to hand gathering.  The 2010 to 2018 Exe Cockle Survey report can be read here.

Teign Estuary

Cockles are present within the Teign Estuary and have known to be collected at low levels, both historically and up to the present day.  There are currently no marine conservation designations within in the Estuary, although overwintering birds such as oystercatchers are known to frequent the estuary and feed on the mussels and cockles found on the intertidal beds.  The cockle populations within the Teign Estuary are important and concerns about their removal and the potential for over-exploitation by recreational fishers, particularly from The Salty intertidal bank downstream of the Shaldon Bridge, have been documented as far back as 2008.  These concerns remain relevant and have been raised during D&S IFCA’s Call for Information, in 2019, on Hand Working to help inform the potential development of a Hand Working Permit Byelaw.

D&S IFCA understands the local community and ecological importance of cockles within The Salty and has undertaken survey work to establish the population structure, biomass, and distribution of cockles within the areas of the estuary where cockles are known to be present.  To date three cockle surveys have been carried out on the Teign Estuary and the reports can be read following these links: 2011, 2018 and 2019.

Work conducted in 2018 showed that the cockles present in the surveyed areas displayed low biomass and density across all surveyed stations.  These data have helped inform the Authority of the condition of the Teign cockle beds for its current review of Hand Working within the District.  The follow up survey conducted in late 2019, together with previous survey data and future annual surveys, will continue to inform management of cockle stocks within the Teign Estuary.

Avon Estuary

There are known cockle beds in the Avon Estuary at Cockleridge opposite Bantham, in South Devon.  These were mapped in 2010 but have not undergone any annual stock assessments.  Some hand gathering takes place here, but access is a limiting factor.

Page reviewed: 3rd July 2020