Assessing Discard Mortality of Commercially Caught Skates (Rajidae) – Validation of Experimental Results
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Assessing discard mortality of commercially caught skates (Rajidae) – validation of experimental results (MB5202) Final Report March 2012 © Crown copyright Project Title: Assessing discard mortality of commercially caught skates (Rajidae) – validation of experimental results Project Code: MB5202 Marine Biodiversity R&D Programme Defra Contract Manager: Carole Kelly Funded by: Department for Environment Food and Rural Affairs (Defra) Marine and Fisheries Science Unit Marine Directorate Nobel House, 17 Smith Square London SW1P 3JR Authorship: J. R. Ellis [email protected] S. R. McCully [email protected] J. F. Silva [email protected] T.L. Catchpole [email protected] D. Goldsmith [email protected] V. Bendall [email protected] G. Burt [email protected] Centre for Environment, Fisheries and Aquaculture Science (Cefas) Pakefield Road Lowestoft. Suffolk NR33 0HT www.cefas.defra.gov.uk Disclaimer: The content of this report does not necessarily reflect the views of Defra, nor is Defra liable for the accuracy of the information provided, nor is Defra responsible for any use of the reports content. Front cover images: Jim Elis (Cefas). Executive Summary Approximately 14 species of skate (Rajidae) occur on the continental shelf of the United Kingdom. These species vary in their commercial importance and also their susceptibilities to over-exploitation. Stocks of some skate species are considered healthy, whereas some of the larger-bodied species have declined or disappeared from parts of their former range. The assessment and management advice for skates has been hampered by a lack of species- specific data from commercial fisheries. A summary of the skate fauna of the British Isles, a historical perspective of their fisheries, long-term changes in skate stocks and in the recent implementation of management measures for skates are summarised in Section 1 of the report. Since 2008 there has been progress in collecting landings data for individual species. The proportion of skate landings reported to species level for UK (English and Welsh) fleets has increased from ca. 42% (2008) to ca. 92% (2010). The quality of these data was appraised by comparing the species composition of landings (by area and gear) with the estimated species composition of retained skates from discard observer data. Although there were some inconsistencies, due to misidentifications, the general quality of official landings data appears to be coherent with other sources and is improving. Information from discard observer programmes indicted that skates <30 cm in length were usually discarded, with about half the skates discarded at a length of ca. 50 cm. Skates larger than 60 cm were typically retained. More detailed information on the species composition of skates captured by UK fleets, discard/ retention patterns and discarding rates are provided in Section 2 of the report. Discarding is an increasingly topical issue for fisheries managers. Quotas for skate may now be restrictive in some fisheries, small skates are often discarded, some skates are not landed and recent regulations have listed some species (e.g. common skate and undulate ray) as prohibited species that must be returned to the sea. Given these scenarios, further studies to better evaluate the potential survivorship of skates in various fisheries were undertaken. About 98% of the skates survived capture in inshore gillnet fisheries with short (24 hour) soak times. This decreased to about 88% for soak times of 43-48 hours. Future analyses of recapture data for the fish tagged in these studies are required to better gauge longer-term survival. About 50% of skates survived capture by beam trawl, and mortality was higher for smaller individuals. The field work undertaken for the project is described in Section 3. The current perceptions of the stock status, as advised by the International Council for the Exploration of the Sea (ICES), is summarised (Section 4), with potential methods for identifying and prioritising skate stocks of concern given. More recent analyses of fishery- independent survey data indicate that catch rates of many of the main commercial skate stocks in English and Welsh waters are stable, though other species are poorly sampled in existing surveys. Options for the improved fisheries management of skates are discussed in Section 5. Existing measures include a total allowable catch (set at the family-level), minimum landing sizes in some areas, and the prohibited species list. Other measures, such as spatial management and gear modifications, are discussed, and the development of more regional management plans could usefully be taken forward. To improve the assessment and management of skates, future research could usefully be targeted at biological investigations (e.g. the life history of data-limited species, stock units, habitat utilisation), commercial information (e.g. on-going quality assurance of catch data, commercial catch per unit effort), bycatch mitigation and options for improving fishery-independent surveys for those species not sampled effectively in existing surveys. ii Contents Executive Summary ..................................................................................................... ii List of Tables .............................................................................................................. vi List of Figures ............................................................................................................. ix 1. Introduction .......................................................................................................... 1 1.1 Project background........................................................................................ 1 1.2 The skate fauna of the British Isles................................................................ 2 1.3 UK skate fisheries: A historical perspective ................................................... 6 1.4 Longer-term changes in skate populations .................................................... 7 1.5 Recent management of skate fisheries ......................................................... 9 1.6 Aims and objectives ..................................................................................... 11 1.7 Soliciting and collating representative views of the fishing industry ............. 11 1.8 Format of the report ..................................................................................... 12 2. The skate species discarded in representative UK fisheries and data gaps in our knowledge of their discard survival ........................................................................... 13 2.1 Introduction .................................................................................................. 13 2.2 Materials and methods ................................................................................ 14 2.2.1 Landings data ....................................................................................... 14 2.2.2 Observer data ....................................................................................... 15 2.2.3 Species identification ............................................................................ 15 2.2.4 Conversion of length to weight .............................................................. 15 2.2.5 Data analysis ........................................................................................ 15 2.3 Appraisal of recent species-specific landings data ...................................... 16 2.3.1 Reported landings from UK-registered vessels ..................................... 16 2.3.2 Irish Sea ................................................................................................ 20 2.3.3 Bristol Channel ...................................................................................... 21 2.3.4 Celtic Sea and Western English Channel ............................................. 21 2.3.5 Southern North Sea and Eastern English Channel ............................... 22 2.3.6 Northern and Central North Sea ........................................................... 22 2.4 Discard-retention patterns ........................................................................... 29 2.5 Discarding rates: A broadscale analysis ...................................................... 33 2.6 Discards rates: Estimating levels of discarding ........................................... 35 2.7 Discussion ................................................................................................... 44 2.7.1 Requirements for species-specific landings .......................................... 44 2.7.2 Comparison of landings data and observer data................................... 45 2.7.3 Data quality ........................................................................................... 45 2.7.4 Prohibited species ................................................................................. 47 iii 2.7.5 Discard-retention patterns ..................................................................... 48 3. Skate discarding and survival: Recent field investigations ................................ 49 3.1 Introduction .................................................................................................. 49 3.2 Inshore gillnet studies .................................................................................. 52 3.3 Offshore trammel and tangle net studies ..................................................... 58 3.4 Longline