Spatial patterns and trends in abundance of larval sandeels in the North Sea: 1950–2005

ICES Journal of Marine Science ICES Journal of Marine Science (2013), 70(3), 540 –553. doi:10.1093/icesjms/fst006 Spatial patterns and trends in abundance of larval sandeels in the North Sea: 1950 – 2005 Christopher P. Lynam 1*, Nicholas C. Halliday2, Hannes Höffle3, Peter J. Wright 4, Cindy J. G. van Damme 5, Martin Edwards 6, and Sophie G. Pitois 1 1 Centre for Environment, Fisheries & Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK The Marine Biological Association of the United Kingdom, The Laboratory Citadel Hill, Plymouth PL1 2PB, UK 3 DTU Aqua, National Institute of Aquatic Resources, Technical University of Denmark, Kavalergården 6, DK-2920 Charlottenlund, Denmark 4 Marine Scotland Science, Marine Laboratory, 375 Victoria Road, Aberdeen AB11 9DB, UK 5 Institute for Marine Resources and Ecosystem Studies (IMARES), Haringkade 1, 1976 CP IJmuiden, The Netherlands 6 Sir Alister Hardy Foundation for Ocean Science (SAHFOS), The Laboratory Citadel Hill, Plymouth PL1 2PB, UK 2 *Corresponding Author: tel: +44 1502 524514; fax: +44 1502 513865; e-mail: Lynam, C. P., Halliday, N. C., Höffle, H., Wright, P. J., van Damme, C. J. G., Edwards, M., and Pitois, S. 2013. Spatial patterns and trends in abundance of larval sandeels in the North Sea: 1950 –2005 – ICES Journal of Marine Science, 70: 540– 553. Received 1 June 2012; accepted 10 January 2013; advance access publication 18 February 2013. Early recruitment indices based on larval fish data from the Continuous Plankton Recorder (CPR) have the potential to inform stock assessments of Ammodytes marinus in the North Sea. We evaluate whether the CPR data are reliable for sandeel larvae. Spatially, CPR larval data were comparable with catches by dedicated larval samplers (Gulf and bongo nets) during ICES coordinated surveys in 2004 and 2009. ICES data are also used to explore environmental influences on sandeel distributions. Temporally, CPR data correlate with larval data from plankton surveys off Stonehaven (1999 – 2005), with sandeel 0-group trawl data at the east Fair Isle ground (since 1984), and with recruitment data (since 1983) for the Dogger Banks stock assessment area. Therefore, CPR data may provide an early recruit index of relative abundance for the Dogger Banks assessment area, where the majority of the commercial catch of A. marinus is taken, and the Wee Bankie area that is particularly important for seabird foraging. While warm conditions may stimulate the production of sandeel larvae, their natural mortality is typically greater, in the Dogger Banks and Wadden Sea areas, when the larvae are hatched in warm years and/or with abundant 1-year-old sandeel that are likely to be cannibalistic. Keywords: environment, keystone, management, modelling, statistical. Introduction Sandeels (Ammodytidae) support a large fishery in the North Sea. However, the stock is difficult to assess and manage owing to the short life cycle of sandeels and the reliance of the fishery on new recruits. Sandeels are a key prey species and are consumed by fish (e.g. gadoids; Greenstreet et al., 1998), seabirds (Daunt et al., 2008), and marine mammals (Thompson et al., 2007). Sandeels are also an important commercial species and have been exploited in the North Sea since 1953 by small-meshed industrial fisheries, for oil and meal. Landings of sandeels increased to  600 – 800 kt year21 during the late 1970s and 1980s before peaking in 1997 at 1150 kt (ICES, 2011). Owing to concern over the potential detrimental effects of the exploitation of sandeel on seabirds and mammals (Daunt et al., 2008), the sandeel fishery Crown copyright # 2013 was closed off the northeastern UK coast in 2000. A combination of low recruitment since 2002 and restrictions on effort (ICES, 2010a) have reduced subsequent landings for the entire North Sea to between 178 and 356 kt year21. ICES ceased to treat lesser sandeels (Ammodytes marinus) in the North Sea as a single stock in 2011, following a review of evidence on habitat, larval drift, and regional growth differences that indicated that there were seven subpopulation regions that differed in their vulnerability to exploitation (ICES, 2010a). Although there is some spatial dispersal of larvae between banks up to 300 km apart, most dispersal is ,100 km (Proctor et al., 1998; Christensen et al., 2008) and the assessment areas chosen by ICES (2010a) were determined in order to relate local recruitment with spawning-stock biomass (SSB). The scientific and fishery Spatial patterns and abundance trends of North Sea larval sandeels 541 Figure 1. Sampling areas and sandeel abundance. (a) CPR sample locations (light grey points) and presence of Ammodytidae larvae (dark grey circles) from all transects 1950 – 2005 showing North Sea sandeel assessment regions (solid black lines), the subregion “South of Shetland”, and 542 information available to inform the assessment differs by region, and as a result analytical assessments are only available for the most commercially important of these (ICES, 2010a, 2011). In sandeel assessment areas 1 [Dogger Banks (DB), Figure 1a] and 2 [Wadden Sea (WS)], a dredge survey index from 2004 (collected in area 1 only) is used as a tuning index for an analytical assessment. In sandeel region 3 [Fisher and Klondyke (FK)], the assessment is based on commercial indices only. A dredge survey from 2008 informs the trends-only assessment in area 4 [Wee Bankie (WB)]. No assessment is made in areas 5 (Viking and Bergen Banks, northeastern North Sea) or 6 (Kattegat, Baltic Sea) where the landings are too low or sporadic for an assessment. A recruitment index is available from 1984 to 2007 for area 7 [Shetland (S)], although this region was always treated as a separate stock region and managed under national regulations. No assessments are made in the Irish Sea (ICES division VIIa) or Celtic Sea (ICES divisions VIIfg) because of the lack of large-scale sandeel fisheries there. Given that the sandeel stocks are highly dependent on the incoming year classes (ICES, 2011), short-term forecasts of yearclass strength may be improved by early indices of year-class strength. The Continuous Plankton Recorder (CPR) survey samples throughout the year and the data are considered to be consistent at a monthly resolution. In addition to zooplankton and phytoplankton, fish larvae are retained in the samples. Prior to 1979, identification and enumeration of fish larvae was carried out as part of routine sample analysis. Due to economic restraints and a lack of available expertise, larval fish analysis ceased in 1980. In 2010, retrospective analysis of archived CPR samples for larvae were carried out and the data obtained were combined with the archived pre-1980 data and added to the CPR database. The two most abundant families are the Clupeidae (herring/ sprat/pilchard) and Ammodytidae (sandeels; Edwards et al., 2011). A limitation of the current analy (...truncated)