Crayfish stocks are heavily depleted in Irish waters. This depletion has been driven by over exploitation since the introduction of tangle netting in the 1970s. The restoration potential relies on acquiring better data on crayfish essential habitat, information on migration and stock structure and in managing fishing interactions. The specific project objectives and tasks are:
- Characterisation of crayfish habitat at selected sites to improve knowledge for spatial management including the value of reserves
Crayfish habitat will be characterised and mapped using a combination of acoustic data from the national seabed survey and SCUBA and vessel based surveys at selected sites. The extent of reef habitat is available from the national seabed survey data. The extent of reef suitable for crayfish is a subset of this. Crayfish live in physically complex elevated and structured reef habitat. Analysis of slope and elevation of existing shaded relief data for reef habitat can be used to further define the extent of reef that may definable physically as crayfish reef. The physical and biological structure of crayfish reef can be described and ‘groundtruthed’ for a number of areas using SCUBA to further inform the classification of reef habitat. SCUBA will also be used in a limited number of areas to classify crayfish reef to Eunis level 4 or 5 based on characterisation of species diversity in these areas. This will be extended using vessel based camera and video surveys. The output will be a crayfish essential habitat map which has been physically and biologically characterised.
Modelling larval dispersal and stock structure.
Crayfish have a prolonged larval life of 6-9 months. Larval dispersal is the main determinant of population structure and recruitment. The connectivity across crayfish reef habitat in Irish waters will be investigated using the Ichthyop particle dispersal model interfacing with archived modelled oceanographic files in the Marine Institute.
Assessment of crayfish stocks off the north Kerry coast using a mark recapture programme.
The fleet will be contracted to catch, tag, release and record tag re-captures of crayfish in the area to provide information on migration, movement and possibly estimates of population size.
- Assessment of catch and by-catch species composition in the commercial tangle net fleet.
This will form part of the work of the by-catch project (MB 2017, 14).
- Management plans for stock recovery and reduce the effects of the fishery on biodiversity.
The outputs of the project will inform future management plans for crayfish including the need to minimise by-catch of endangered species.
The inshore fishing sector and in particular vessels using tangle nets off the coast of Kerry.
The project has a 2 year duration and runs between 2017 and 2018.
- Data sets: size composition, catch and effort, tag releases, tag recaptures, georeferenced data, acoustic map data on seabeds, biological data on reef fauna, distribution of crayfish reef habitat, modelled larval dispersal data and maps. These will be incorporated into existing Marine Institute data bases where appropriate.
- Reports: 1. Report on fine scale acoustic mapping of crayfish reefs and the distribution of crayfish reef habitat. 2. Report on larval dispersal modelling. 3. Report on catch and by-catch in the crayfish fishery. 4. Report on tag release and recapture of crayfish
- Incorporation of data and new information into stock assessment and scientific advice and publications/dissemination thereof.
Information from the project will be collated and used to design management measures for the fishery with the objective of re-building crayfish stocks and reducing the environmental effects of the current set net fishery. Particular emphasis will be placed in areas where by-catch is significant and where the data shows potential for re-building of stocks. Measures might include the increased use of spatial reserves for crayfish, changing from set nets to trap fisheries and temporary closures to enable stock recovery or inclusion of additional technical conservation measures. Improved scientific knowledge on the essential habitat, stock structure and life cycle/connectivity of crayfish stocks in SW Ireland.
Overall expected benefits are:
1. Improved scientific knowledge on the fishery and its impact on target stock and biodiversity
2. Proposed management measures to restore crayfish stocks and protect biodiversity impacted by the fishery taking account of the above
3. Dissemination of results and related management consideration at regional/national inshore fisheries management fora