English - Caribbean Environment Programme

An in-depth study reviewing 22 years of effort-related harbour porpoise data from around the UKfound the coastal waters of west Scotland, including the eastern sectors of The Minches and theSea of the Hebrides (around the Isles of Skye, Mull and the Small Isles), to be one of four sites ofkey importance to harbour porpoises in UK waters 82 . This analysis found that porpoises are presentin every month of the year, with concentrations in several months, including during the April-Septemberperiod when calving takes place. They also found a higher than average proportion of calves toadults in the area which suggests its importance for calving and/or breeding harbour porpoises.The long time series of data showed that this situation has been consistent over many years.Numerous studies in the region over recent years have reported high encounter rates for harbourporpoises. Encounter rates are generally highest in the spring and summer months, particularly inthe Gairloch region 83 , around the Isle of Skye 84 , and the Firth of Lorne and Sound of Jura 85 . Outsideof the summer months data are scarcer, but relatively high encounter rates have been recorded inthe northern part of the Minch in autumn 86 .Data on harbour porpoise calf presence are problematic due to difficulties in sighting such smallanimals and they are likely underreported. However, studies have reported relatively high numbersof groups with calves in The Minches and the Sea of the Hebrides 87 , and an aerial survey of theInner Hebrides found a high calf to adult ratio with calves making up 10% of all animals sighted 88 .The presence of a higher proportion of mother and calf pairs is an indication that the area is usedby harbour porpoises for calving 89 . This is supported by observations of researchers in the Gairlochregion suggesting behaviour typical of porpoises calving 90 .Spatial modelling of harbour porpoise data has been used to look at distribution in the region on afiner scale. One model, created using four years of survey data from the month of August, predictedfour areas of high relative abundance: 1) the region between Ardnamurchan, Coll and the SmallIsles, which the authors highlight is also important sandeel habitat (harbour porpoise prey),2) south east of Barra, an area they speculate may be preferentially used in seasonal movementsto enter the Sea of the Hebrides from offshore areas to the west, where other surveys have madesightings 91 , 3) north east of Skye to Gairloch, and 4) west of Pairc Peninsula (Isle of Lewis) tothe Shiant Islands 92 . The area south of Barra has been found to have relatively high porpoiseabundance by previous studies 93 and as discussed above, Gairloch has been identified as animportant area for porpoise calving, and other studies have noted it to be a highly significantlocation for harbour porpoises 94 .A second habitat modelling study using summer data from further south in the Hebrides predictedhighest densities close to the mainland – in the Sound of Jura, the Firth of Lorne, between Mull andthe Treshnish Islands, in patches around the Small Isles and the Sound of Sleat 95 . Although harbourporpoises clearly use the whole area, the same core, high-use areas were predicted over threeyears. Some inshore areas – the Sound of Mull, upper Firth of Lorne and upper Sound of Jura – hadsightings rates almost as high as the high density areas identified, but could not be included in theanalysis because of a lack of environmental data. The authors suggest that these are also important toharbour porpoises and should be considered alongside the areas identified by the model.Spatial habitat preference modelling such as this allows us to build on the knowledge gained fromsurvey data and observations, and can help to further define areas of critical habitat for cetaceans 96 .West and north of the Outer Hebrides, data are much sparser. Harbour porpoises are generallyonly recorded in low numbers although some studies have reported high relative encounter rateswest of the Uists and north east of Lewis 97 . Higher sea states and poorer weather offshore willinhibit the sighting of harbour porpoises and may mean they are underestimated in these areas.Far West ScotlandThe predominant view of harbour porpoises is of a species largely confined to nearshore watersless than 200m deep but despite this, several studies report their presence in deep offshore waters,albeit in relatively small numbers. The Wyville-Thomson Ridge and surrounding channels is an areanoted for porpoise presence 98 .2482Evans and Wang, 200283MacLeod, 200184Pollock et al, 2000; Goodwin and Speedie, 2008; Embling et al, 2010; Marubini et al, 200985Pollock et al, 2000; Embling et al, 201086MacLeod, 200187Weir, 2008; Evans and Wang, 200288SCANS II, unpublished data (in Embling et al, 2010)89Baines and Earl, 199890Dolman and Hodgins, 200991Macleod et al, 2003; Macleod et al, 200792Marubini et al, 200993MacLeod, 200194Dolman and Hodgins, 2009; Shrimpton and Parsons, 200095Embling et al, 201096Hoyt, 200997Pollock et al, 2000; Lewis et al, 199898Pollock et al, 2000; Evans and Wang, 2002; MacLeod, 2001