Plaice, Division VIIe (Western Channel), Demersal otter trawls
Landings of plaice in Division VIIe peaked at approximately 3000 tonnes in the late 1980s and early 1990s, but declined thereafter to levels around 1500 tonnes (Figure 1). Spawning stock biomass increased from a record low of 1745 tonnes in 2007 to a record high of 6230 tonnes in 2014. Fishing mortality is estimated to have decreased by 31% between 2014 and 2015, and is now at the lowest level on record, 67% below the long-term average of the time-series. Spawning–stock biomass is estimated have increased by 21% between 2014 and 2015. The increase in spawning stock biomass from 2008 onwards resulted from above average recruitment from 2009 to 2011 and a reduction in fishing mortality. Fishing mortality has decreased substantially since 2007 to reach the lowest levels on record (ICES, 2016ab).
Figure 1. Plaice in Division VIIe. Summary of stock assessment (weights in thousand tonnes). Recruitment, fishing pressure, and SSB are relative in relation to the average of the time-series. The dashed lines in the relative SSB plot indicate the average values of the respective years. Predicted recruitment values are not shaded. Discard estimates are not included (ICES, 2016a).
Stock structure and recruitment
For assessment purposes, the plaice population inhabiting the English Channel has been split into two stocks. These two stocks are: Division VIId (Eastern English Channel) and Division VIIe (Western English Channel). Tagging information indicates that there is a degree of exchange of individuals (i.e. connectivity) between stocks, with both recruits and adults migrating among Division VIId (Eastern English Channel), Division VIIe (Western English Channel) and Sub-area IV (North Sea) (ICES, 2015a). Consequently, ICES reallocates 15% of the mature component of the catches from Division VIId into Division VIIe in the first quarter of the year based on migration estimates from tagging studies (ICES, 2015c).
Recruitment-at-age 2 has been variable throughout the time-series, averaging in the range of 6 – 7 million recruits between 1980 and 2014. A series of good recruitment was evident from 1987–1989 and 1996-1998. Since 2010, recruitment has been above average (ICES, 2016b).
Data gaps and research priorities
More information about the degree of temporal and spatial mixing in the plaice population inhabiting Division VIIe is required. Tagging studies focusing on the migration of plaice in Division VIId (Eastern English Channel), Division VIIe (Western English Channel) and Sub-area IV (North Sea) should be undertaken to better understand temporal and spatial variability in metapopulation (meaning localised) dynamics in the English Channel. Such tagging studies would allow the assumption that 15% of mature individuals migrate from Division VIId into VIIe during the spawning period to be reinvestigated. Additional data is required to determine if the current mixing rate estimates are still valid given the increased population abundance of plaice in the region.
Little is known about the stock dynamics of plaice inhabiting the French coast. Research surveys for plaice are spatially restricted to the same area as the commercial fleets in the United Kingdom. Consequently, additional research surveys on the French coast are required to shed light on the stock dynamics of plaice in this region. In addition, further information on discarding of plaice in Division VIIe should be collected to improve estimates of total mortality. Quantitative estimates of discarding from French fleets targeting plaice in Division VIIe should be provided for inclusion in the assessment (ICES, 2016b).
ICES 2016a Plaice (Pleuronectes platessa) in Division 7.e (western English Channel) ICES Advice on fishing opportunities, catch, and effort. Celtic Seas and Greater North Sea Ecoregions.
ICES. 2016b. Plaice in Division VIIe. Report of the Working Group on Celtic Seas Ecoregion (WGCSE), 4–13 May 2016 Copenhagen, Denmark, 40 pp. [Draft accessed July 2016]
ICES. 2015. Report of the Benchmark Workshop on Plaice (WKPLE), 23-27 February 2015, ICES Headquarters, Copenhagen, Denmark. ICES CM 2015\ACOM:33. 200 pp.