Razor clam biology, ecology, stock assessment, and exploitation: a review of Ensis spp. in Wales

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Abstract

Razor clams of the genus Ensis are the subject of valuable fisheries and are exploited by hand gathering in the intertidal and by vessels using various gear types in the subtidal. Concerns about the sustainability of razor clam harvesting in Welsh waters have recently resulted in the closure of an intertidal area and the need to determine effective measures to assess stocks. The aim of this report is to provide a review of the biology of razor clams in Welsh waters; to map known, and potential, areas of exploitation; and to review and recommend possible stock assessment methodologies for both intertidal and subtidal fisheries.

Of the species of razor clam present in Welsh waters, three are of commercial significance and the fishery has mainly targeted Ensis siliqua, with other species of much less significance. There are very few records of the invasive Ensis directus which has recently become abundant to fishable levels in areas of continental Europe and on the east coast of England.

Key aspects of the biology of razor clams relating to their assessment and exploitation are that: they have a very patchy distribution; the density of individuals within fished beds is highly variable; there are inconsistent recruitment patterns; populations can extend from the intertidal into the subtidal and are age structured with larger individuals found lower down the shore.

Mapping of known razor clam harvesting areas has been carried out to identify recent fishing activity. Predictive modelling in GIS was undertaken using available data, to identify potential distributions for each of the species of interest.

There are a wide range of potential methods for the assessment of Ensis spp. stocks. For intertidal surveys, both hydraulic and manual survey methods were considered with the latter deemed to be most cost effective. A combination of techniques would be recommended in order to ensure that a wide size range of individuals was captured in intertidal sampling. For subtidal sampling, hydraulic dredging is considered to be a reliable and well-studied methodology; however, it was recommended that this be supplemented with a new electrofishing technique involving cameras. This may enable sampling within areas where dredging might not be appropriate, such as marine protected areas. For all suggested assessment methods it is important to determine any potential source of bias in the data collection early in the process so that measures can put in place to reduce it or to incorporate it into the assessment process (e.g., appropriate training for intertidal sampling or quantification of gear efficiency for subtidal sampling).

A range of data gaps and potential methods for collecting additional data were considered and outlined. Consideration of the chosen assessment methods and proposed management structures will influence the data required. It is recommended that surveys for stock assessment are supported by an appropriate fisheries data collection programme and that both fisheries and assessment data are effectively integrated within a clearly defined management plan. The early establishment of management goals will help to inform appropriate data collection and assessment methodologies.
Original languageEnglish
PublisherWelsh Government
Commissioning bodyWelsh Government
Number of pages62
Publication statusPublished - 16 May 2018

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Keywords

  • Ensis
  • razor clam
  • Wales

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