English - Caribbean Environment Programme

SCANS II (in July, 2005) estimated an abundance of 11,141 common dolphins and a density of0.056 animals/km 2 (CV 0.61) in the south west UK region 256 . This was a moderate level of densitycompared to other regions but spatial modelling to reveal finer-scale distribution predicted a highdensity area off south east Ireland and stretching over the southern end of the Celtic Deep 257 .The Celtic Deep − St. George's Channel region is an area particularly noted for high abundance ofcommon dolphins, from May to November 258 . Effort is low during winter and spring months but itappears that this cluster of high abundance disperses into the wider Celtic Sea during the laterautumn and winter. It has been suggested that the high summer abundance is a result of the CelticSea Front that persists in this area over the summer. Fronts are biologically productive areas thatresult in increased prey through enhanced local primary productivity and/or convergence processesthat act to aggregate prey 259 . From November to April, the waters in the Celtic Sea are mixed,but from May to November thermal stratification dominates and a surface front develops 260 thatcan be observed using satellite imagery 261 . As well as high encounter rates in the Celtic Deep −St. George's Channel area, studies have reported a high proportion of sightings of young animals.One study reported that between July and September, 51% of sightings consisted of groupsof adults with calves, often young calves, suggesting they were born recently, in or near thestudy area 262 .Although genetic differences within the population have not been detected, that part of thepopulation remains in on-shelf waters over the summer while another moves offshore, suggestingthat ecological differences exist. This has already been indicated by dietary studies whichinvestigated cadmium exposure in common dolphins and found different levels between animals inoffshore and nearshore waters 263 . These studies suggested the existence of two ecological stockswithin the north east Atlantic, a coastal and a neritic stock, that have accumulated different levelsof this element through the exploitation of different prey species 264 . Further research will benecessary to confirm the existence of at least two stocks of common dolphins, and todetermine if further divisions exist.Abundance of common dolphins increases in the south west England region during the wintermonths 265 , and their distribution mainly seems to be in offshore Celtic Sea waters and the westernEnglish Channel. Data are too sparse to know if there is a movement of animals into this area fromoff-shelf waters, or whether numbers remain high in offshore areas too. There are also insufficientdata to identify locations within the Celtic Sea with particular concentrations of animals. Moreinformation is available for the western English Channel, however, including a density estimate of0.74 individuals/km 2 for a small area south of Cornwall 266 . This is the only density estimate availablefor common dolphins in the winter in UK waters and shows there are high numbers of animals inthe region at this time of year. The survey area was small so it is unknown at present how much ofthe rest of the area has similarly high abundance of common dolphins. Little survey work hasbeen carried out here in the winter but other studies have also shown a high relative abundanceof common dolphins in much of the western Channel area 267 , some reporting a 10-fold increaseover summer numbers 268 . Large aggregations of prey species are in the region at this time of year,such as sprat, horse mackerel and herring, and are likely the reason for the high abundance ofcommon dolphins 269 .38256SCANS II, 2006257Ibid258Reid et al, 2003; Baines and Evans, 2009; Earl et al, 2004; Earl et al, 2005259Bost et al, 2009 ; Hyrenbach et al, 2000260Brown et al, 2003261see for example, Goold et al, 1998 (abstract only seen)262Earl et al, 2004 and 2005263Lahaye et al, 2005264Lahaye et al, 2005265Northridge and Kingston, 2009266De Boer et al, 2008267Reid et al, 2003; MacLeod et al, 2009; De Boer and Simmonds, 2003268Brereton et al, 2005269Northridge and Kingston, 2009