Anthropogenic activities which may affect marine mammals generally and pilot whales specifically can be divided into two main categories: habitat degradation and oceanographic changes. Knowledge about pilot whale habitat use is limited and it is therefore difficult to assess the relative impact that such changes may represent for the species.
A range of human activities has the potential to degrade habitat important to pilot whale survival. The species occurs in coastal waters, but is essentially oceanic. Main threats include:
• changing water quality and pollution (e.g. runoff from agriculture, oil spills)
• entanglement (e.g. in marine debris, fishing gear, etc.)
• prey depletion due to over-harvesting
• acoustic pollution with disturbance from low-frequency noise (vessels, seismic surveys, military activities)
In addition to their direct toxicity, anthropogenic contaminants may affect the resilience and increase susceptibility to disease in marine mammals (Reijnders and de Ruiter-Dijkman 1995). The accumulation of contaminants in pilot whales has been studied on both sides of the Atlantic (e.g. Aguillar et al. 1993, Borrell 1993; Borrell and Aguilar 1993, Borrell et al. 1995, Caurant et al. 1993, 1994, 1996, Caurant and Amiard-Triquet 1995, Tilbury et al. 1999, Dam and Bloch 2000, Weisbrod et al. 2000, 2001, Sonne et al. 2010, Méndez-Fernandez et al. 2014ab). Studies dating back to 1977 have shown an overall increase in contamination of the meat, blubber, liver and kidneys of pilot whales. Together with the killer whale (Orcinus orca), the long-finned pilot whale is one of the most polluted species in the world (AMAP 2004, 2005; Dam and Bloch 2000).
Faroese pilot whales have concentrations of organic contaminants in their blubber which are roughly in the mid-range of such concentrations in other species of toothed whales in the North Atlantic (Borrell and Aguilar 1993, Mendez-Fernandez et al. 2014b). It is not known if these contaminants are affecting the health of pilot whales in the North Atlantic.
The concentration of trace metals (other than copper) in the liver of pilot whales from the Faroe Islands is on the higher end of the range of what is found in other marine mammal species of the North Atlantic (Caurant et al. 1993) and pilot whales from other area of the North Atlantic (Méndez-Fernandez et al. 2014a). Cadmium levels, in particular, are very high and even higher than in species from highly industrialized areas (Caurant et al. 1993). This is probably related to their heavy consumption of squids (Bustamante et al. 1998).
The heavy metals mercury (Hg) and cadmium (Cd) produce adverse effects in various mammals, with risks of health impacts on internal organs, such as the liver and kidneys. Possible direct and indirect effects (e.g. immunodeficiency) of trace metals on pilot whales have been little studied. However, a recent study showed a high prevalence of histopathological changes in liver and renal tissues of Faroe Island pilot whales and suggested that age, heavy metal, and organic contaminants may be important factors in pathology development (Sonne et al. 2010).
The Faroe Islanders rely on pilot whales as an aboriginal and important wildlife food resource. However, the consumption of pilot whale meat is thought to representing a hazard to the health of consumers (e.g., Weihe et al. 1996, Steuerwald et al. 2000, Weihe and Joensen 2008, 2012, Choi et al. 2009, Grandjean et al. 2011). Pilot whales today contain contaminants (both organochlorines or metals) in concentrations such that neither meat nor blubber would comply with current limits for acceptable concentrations of toxic contaminants, and it has been recommended that pilot whales should no longer be used for human consumption (latest: Weihe and Joensen, 2012). As more developing nations industrialize, marine pollution is likely to increase. As the meat and blubber become more contaminated, their risk may increasingly outweigh their benefits (Fielding 2010).
|Faroese stamps, 1986|
Pilot whales are taken as bycatch in some fisheries on both sides of the Atlantic. They were taken in significant numbers in the Mediterranean driftnet fishery for swordfish (Xiphias gladius) in the 1980’s, however limitations on net size imposed in 1990 have probably ameliorated this problem (Notarbartolo-di-Sciara 1990, Natale and Notarboartolo-di-Sciara 1994). Currently, bycatch of pilot whales in the eastern Atlantic appears to be insignificant. For the period 2008–2012, the ICES Working Group on Bycatch of Protected Species reports only a few observed pilot whale bycatch in European Fisheries, all in the Atlantic: four in 2008 by a French midwater pair trawl; five in 2011 by a German midwater otter trawl; and one in in 2012 by a Dutch midwater otter trawl (ICES 2010, 2011, 2012, 2013, 2014).
In the western Atlantic, pilot whales have been taken as bycatch in several types of fisheries, including squid traps (Lien 1994), pelagic drift gillnets, trawls, purse seines and longlines (Waring et al. 1999). Bycatches were highest in the pelagic drift gillnet fishery for swordfish, ranging from 9 to 135 whales per year between 1989 and 1996. This fishery was closed in 1997. Bycatch of pilot whales in the western Atlantic was not considered to be causing stock decline or significantly impeding stock recovery (Nelson and Lien 1996, Waring et al. 1999), although there were probably more pilot whales taken incidentally than were documented (Bernard and Reilly 1999). The average annual estimated fishery-related mortality during 2000–2004 in the Northeast mid-water trawl fishery was 8.9 (CV=0.35) pilot whales and in the Atlantic herring mid-water trawl fishery 11 animals (Waring et al. 2007ab). The average annual mortality in the pelagic long-line fishery was 114 (CV=0.20) pilot whales during 2005–2009 (Waring et al. 2012). The 2005–2009 average mortality attributed to the mid-Atlantic bottom trawl was 30 animals (CV=0.16), to the northeast bottom trawl 12 animals (CV=0.14) and to the to the Mid-Atlantic mid-water trawl fishery was 2.4 (CV=0.99) (Waring et al. 2012). The total U.S. fishery-related mortality and serious injury for long-finned pilot whales is unknown, since it is not possible to partition mortality estimates between the long-finned and short-finned pilot whales, but it cannot be considered insignificant (Waring et al. 2012).
Solve the bycatch problem: fish wireless
Pilot whales prey on some commercially important invertebrate and fish species such as squid, shrimp, mackerel, blue whiting and herring. The availability of these for whales might therefore be reduced by commercial fisheries. Fisheries for squid are widespread on both side of the North Atlantic, targeting species which are the main prey eaten by pilot whales in the area: short-finned squid (Illex illecebrosus) off Newfoundland, long-finned squid (Loligo pealeii) off the US coast, European flying squid (Todarodes sagittatus) off the Faroe Islands, Norway and Portugal, curled octopus (Eledone cirrhosa) off Spain and Portugal. This raises the possibility of prey depletion for pilot whales.
The abundance of squid available to fisheries is known to exhibit dramatic yearly fluctuations, with no obvious pattern. Periods of either high or low abundance may last for several years. These changes would be determined by the interaction of fluctuating year-class strength and varying hydrographic conditions influencing natural mortality and the distribution of the early life stages (Arnold 1979, Fisheries and Oceans Canada 2013).
Long-finned pilot whales feed on a wide variety of prey, and appear to be able to adjust their diet in response to changes in prey abundance (Desportes and Mouritsen 1993; Santos et al. 2013). They are therefore likely less vulnerable to prey depletion than more specialised species. However, pilot whale diet includes several high quality species, the quality of food being dictated by the cost of living for each species; shifting to food of lower-quality species (species with lower energy densities per mass unit) may negatively affect the population dynamics of a species (Spitz et al. 2011, 2012). The intensified pelagic fishery targeting blue whiting (Micromesistius poutassou) west of the British Isles is likely one of the factors reinforcing the apparent decrease in abundance of pilot whales since the 1990s (Hátun and Gaard 2010). The blue whiting is preyed upon both by the flying squid and the pilot whale.
This species, like many beaked whales, is likely to be vulnerable to loud anthropogenic sounds, such as those generated by navy sonar and seismic exploration (Cox et al. 2006), and almost all behavioural responses are expected to result in a change in dive pattern (Sivle et al 2012). Indeed intense military sonar signals should be audible to cetaceans over large distances given the efficient propatation of sound and the sensitive hearing capabilities of cetaceans.
Exposure to military sonar pulses is associated with changes in behaviour in pilot whales, suggesting acoustic disturbance. Changes in vocalizations, traveling, surfacing and diving/foraging behaviours have been observed, the latter potentially reducing the foraging efficiency of the affected animals (Rendell and Gordon 1999, Miller et al. 2011, 2012, Sivle et al. 2012).
Pilot whales may, however, be less sensitive to sonar exposure, compared to species such as killer whales and beaked whales. They exhibit higher response thresholds to sonar exposure than these species, and an avoidance response restricted to the duration of sonar exposure, while the responses reported for killer whales and beaked whales may last longer than the sound exposure. There are however individual differences and the behavioral responses to sound stimuli are strongly affected by the context of the exposure (Antunes et al. 2014).
Predicted impacts of global climate change on the marine environment may affect long-finned pilot whales, and may induce changes in the species' range, abundance and/or migration patterns (Learmont et al. 2006). In particular, pilot whales may be affected indirectly, through their prey, as the migration and survival of squid is very temperature-dependent.
MacLeod et al. (2005) suggest that the late 1980s warming of local waters has, indeed, led to changes in the cetacean community of north-west Scotland, with a decline in occurrence of cold water species, including pilot whales, an increase in the occurrence of existing warm water species and the addition of new warm water species to the community. They predicted that if the temperature rise continues, colder water species like the pilot whale might be entirely lost from the north-west Scottish cetacean community.
A clear link has been identified between the abundance of long-finned pilot whales and the marine climate in the northeastern Atlantic throughout the last three centuries (Hoydal and Lastein 1993, Hátun et al. 2009). During warm periods the whales are observed in high abundances while they can be completely absent from the region during cold periods. The linkage between the marine climate and the abundance of pilot whales probably involves their main prey items, the flying squid and the large but highly variable blue whiting stock (Hátun and Gaard 2010). This link, however, was broken post-1980s, when warming and an increase in the blue whiting stock were not followed by an increase in the abundance of flying squids and pilot whales. Hátun and Gaard (2010) identified potential causes rooted in global warming and an intensified pelagic fishery, which collectively might explain this apparent disruption.
Pilot whales have a tendency to mass-strand throughout their range in the North Atlantic (e.g. Sergeant 1962, Bloch et al. 1993a, Sigurjónsson et al. 1993, Waring et al. 1999, 2007ab, 2012). It is not known whether human activity influences these occurrences, and at present, such strandings must be considered a component of the natural mortality of the species. However Culik (2011) report on three mass stranding events in Tasmania, just prior to and after the arrival of naval vessels using high frequency (5–200 Kz) sonar, where a behavioural reaction to the sonar facilitating the second and third events could not be ruled out.