All models found depth to be the most important predictor of sperm whale
occurrence, with more sperm whales detected in deep water off the shelf edge (Figure 3.5), than in shallow waters. Sperm whales are usually found in deep water off the west coast of Scotland (Evans 1997; Macleod et al. 2003; Mendes 2007; Skov et al. 1995; Weir et al. 2001), and elsewhere in the world (Baumgartner et al. 2001; Cañadas et al. 2005; Davis et al. 1998; Davis et al. 2002; Gregr & Trites 2001;
Hamazaki 2002; Kaschner 2004; Kenney & Winn 1986; Whitehead et al. 1992). This preference is strongly influenced by the location of sperm whale prey, predominantly comprising mesopelagic and bathypelagic cephalopods (Evans & Hindell 2004; Martin & Clarke 1986; Santos et al. 1999; Santos et al. 2002; Simon et al. 2003; Smith & Whitehead 2000). Little is known about the ecology of deep water species of cephalopod off the west coast of Scotland, but there is evidence that a wide range of deep water species occur in this area (Collins et al. 2001).
Thermocline strength was the second most important predictor of sperm whale occurrence off the west coast of Scotland, with sperm whales associating with weak
thermoclines (Figures 3.5 & 3.7). Thermocline depth and strength have been used in other studies of sperm whales as an indicator of the presence of eddies or ‘cyclones’ to confirm that sperm whales were associating with these features (Baumgartner et al. 2000; Davis et al. 1998). However, off the west coast of Scotland the weakest
thermoclines were found along the shelf, shelf edges, upper Rockall Trough, and around the Wyville-Thompson Ridge (Appendix Figure A1.1). This is likely to be indicative of mixing that occurs around the steeper slopes of the shelf edges, and around some of the complex topography that dominates the region (Mann & Lazier 2006). Similarly Burrows & Thorpe (1999) found that dispersal of surface NAW water into deeper waters occurs at topographical irregularities such as the Anton Dohrn seamount. This is considered a downwelling area, and hypothesised to be the ideal conditions for the development of deep water fauna due to the increased supply of macrozooplankton and dissolved oxygen supplied from the surface (Berzin 1971). To my knowledge, this is the first study of sperm whales to investigate the effect of halocline depth and strength on sperm whale distribution, but it has been used in the study of other species distributions (Mehlum et al. 1998; Tynan et al. 2005). For example, in a study of thick-billed murres (Uria lomvia) in the Berents Sea, the largest aggregations of birds were found over strong shallow haloclines associated with frontal zones (Mehlum et al. 1998). In my study, both halocline depth and strength were significant in explaining the occurrence of sperm whales in the Faroe- Shetland Channel survey data, but not for sperm whales in the Rockall Trough. Sperm whales were found in areas with strong deep negative haloclines (surface water saltier than the bottom water). This deep strong halocline is characteristic of the deep waters of the Faroe-Shetland Channel, where the salty Gulf Stream NAW water at the surface flows north over the south flowing deep fresh Norwegian Sea Deep Water (NSDW: Turrell et al. 1999). Little is known about the ecology of deep living squid in this area, though it does form the southern part of the range of one of the key sperm whale prey species, Gonatus fabricii (Bjorke 2001; Pierce et al. 2004; Santos et al. 1999).
Surface oceanographic features have been used to identify biological hotspots (Issue 53 Deep Sea Research II 2006; Ballance et al. 2006) and foraging areas of marine mammals (Polovina et al. 2001; Worm et al. 2005). In other studies, sea surface
temperature was correlated with sperm whale occurrence (Cañadas et al. 2005; Davis et al. 1998; Hamazaki 2002; Mendes 2007; Smith & Whitehead 1993; Whitehead et al. 1989). In my study, SST was also significant in explaining the occurrence of sperm whales in the Rockall Trough and, in common with the other studies, sperm whales preferred cooler waters (Figure 3.8). In waters around the Galapagos Islands a preference for colder surface waters was believed to be linked with upwelling events (Smith & Whitehead 1992). However, off the west coast of Scotland there is no known upwelling, the coolest surface waters being associated with the Modified North Atlantic Water (MNAW) on the Faroe Plateau side of the Faroe-Shetland Channel, and in the waters around the Wyville-Thompson Ridge, Faroe Bank and Bill-Baileys Bank. Based on examination of chlorophyll satellite images, this area’s complex topography appears to promote productivity.
Chlorophyll was also a significant predictor of sperm whales in the Rockall Trough being found in areas of high chlorophyll (Figure 3.8). Since primary productivity indicators such as chlorophyll are indirect indicators of productivity up the trophic chain, correlations are often only found at a large scale or grain size (Guinet et al. 2001). This is due to the time lag between a peak in primary productivity and a peak in squid prey of around 4 months and several hundred kilometres (Vinogradov 1955). In areas where oceanography does not aggregate this productivity, areas of high prey concentrations for whales can be displaced by several hundred kilometres (Jaquet 1996). However, in this study a positive relationship between sperm whale occurrence and chlorophyll was found at a relatively small scale, suggesting that oceanographic processes do aggregate this productivity within defined locations at which squid also aggregate in the Rockall Trough.
The final significant variable at explaining distributions of sperm whales in both the Faroe-Shetland Channel and the Rockall Trough was sea surface salinity (SSS). The relationships with SSS were different within the Rockall Trough and the Faroe- Shetland Channel. There was a clear preference for areas with mainly high salinity in the Rockall Trough, but a bimodal preference for both low and high salinity in the Faroe-Shetland Channel (Figures 3.7 & 3.8). This preference can be explained by the different water masses found in the surface waters off the west coast of Scotland. The Rockall Trough surface water is dominated by the warm salty Gulf Stream (North
Atlantic Water NAW) water, whereas the Faroe-Shetland Channel has both the warm salty NAW water mainly on the eastern side of the channel, and the cooler fresher Modified NAW (MNAW) water on the Faroese side of the channel (Turrell et al. 1999). The bimodal preference shows a main preference for fresher waters and a smaller preference for salty waters, perhaps reflecting preference mainly for the fresher MNAW side of the Faroe-Shetland Channel, and to a lesser extent the saltier Gulf Stream NAW side of the channel.
Overall, the environmental variable were able to explain up to 36.6% of the deviance, which is a high proportion in comparison to other studies of cetacean species
(Ferguson et al. 2006a; Ferguson et al. 2006b), and similar to other habitat models of sperm whales (Gregr and Trites 2001; Mendes 2007). This was mainly due to their clear preference for deep water, with depth explaining around 22% of the deviance, and the other environmental variables only explaining 2% - 6% individually
(Appendix Table A2.1-2). This is may be due to the fact that within their deep water habitat, sperm whales are relatively evenly distributed throughout the surveyed area. This is partly due to detecting sperm whales both during travel and foraging activity, the latter more likely to be associated with prey habitat. Identification of foraging habitat through analysis of feeding buzzes or ‘creaks’ (Miller et al. 2004) may allow for increased explained deviance, however in this study too few ‘creaks’ were obtained to allow such an analysis (Mendes 2007).
Overall, these results suggest that sperm whales are using the west coast of Scotland as a foraging ground, rather than solely as a migration route between high latitude feeding and low latitude breeding grounds. There is also some evidence for this hypothesis from the acoustic data itself, because feeding buzzes or ‘creaks’ were detected on several occasions (Mendes 2007), which have been associated with foraging behaviour (Miller et al. 2004). Further evidence from the analysis of sperm whale teeth from around Scotland, suggests that young pre-breeding males tend to stay within more temperate zones to feed before moving to the higher latitude productive feeding grounds shortly before approaching breeding age (Mendes et al. 2007).