Pilot Assessment on Abundance and Distribution of Killer Whales

D1 - Biological Diversity

D1.1 - Species distribution

D1.2 - Population size

D4.3 - Abundance/distribution of key trophic groups/species

Killer whales are long-lived, slow reproducing, top predators. Several populations exist, but data on abundance are scarce. Thus only a pilot assessment can be made. Killer whales are vulnerable to the effects of persistent organic pollutants accumulated through their diet, with high pollutant levels potentially impacting reproduction.

Area assessed

Printable Summary

Background

OSPAR’s strategic objective with respect to biodiversity and ecosystems is to halt and prevent further loss in biodiversity, protect and conserve ecosystems and to restore, where practicable, ecosystems, which have been adversely impacted by human activities. This pilot assessment considers changes in abundance and distribution of populations of killer whales in the North-East Atlantic.

Some killer whales may seasonally reside in relatively small areas, close to shore. As a result, they have the potential to be exposed to a greater level of human activity than populations further offshore due to their proximity to human activities.

Although killer whales are widespread, they are not particularly numerous. Much of the information on population structure comes from photographic identification studies focused on particular groups occurring in coastal waters.

Killer whales are long-lived top predators and are susceptible to changes in their environment. Changes in abundance and distribution provide important information on the state of the population.

Killer whales are vulnerable to the accumulation of pollutants through the food chain, a key pressure identified for this species. Underwater noise can have long and short-term effects on cetaceans (for example, hearing loss or displacement from an area), but it is not known to what extent killer whales are affected.

Killer Whales (Orcinus orca) (courtesy of Christopher Michael)

Killer whales (Orcinus orca), are long-lived top predators, and are among the most iconic and recognisable of the cetacean species.

Killer whales are one of the most widely distributed of all cetacean species globally and occur throughout the North Atlantic. Killer whales are most numerous in Arctic Waters. In the remaining OSPAR Regions, killer whales are most commonly sighted along the coastal shelf edge west of the United Kingdom and Ireland, especially off Shetland and the Hebrides (United Kingdom) (Evans, 1988; Reid et al., 2003; Bolt et al., 2009; Foote et al., 2009; Beck et al., 2014). They are also seen near the Strait of Gibraltar, in Spain (Esteban et al., 2013). In these southern areas there are small, genetically distinct groups of killer whale.

Human Pressures

ICES (2015) reviewed and prioritised the key human pressures on killer whales in the OSPAR Maritime Area. The pressures listed in Table a represent those considered to have most relevance for killer whales (OSPAR, 2012; ICES, 2015). Pressures from human activities may affect killer whales directly or indirectly (such as by changing the food web structure and thus prey availability). For killer whales, the main pressures were identified as pollutants and military activities generating underwater noise (predominantly sonar but also including the potential of explosions occurring) (Table a).

Killer whales, along with other toothed whales, are top predators and so are vulnerable to bioaccumulation of pollutants. A recent study has shown that blubber samples from stranded killer whales within the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast regions contain very high levels of polychlorinated biphenyls (PCBs) (Jepson et al., 2016). PCBs are known to cause pathological changes, and high levels of PCBs may be inhibiting reproduction and thus adversely affecting group size in small coastal cetaceans. PCB concentrations within the OSPAR Maritime Area are assessed in PCB Concentrations in Biota and PCB Concentrations in Sediment.

The pressures listed in Table a represent those considered to have most relevance for marine mammals. The risks associated with the identified pressures are classified as high, medium or low using the following criteria:

  • High: evidence or strong likelihood of negative population effects mediated through effects on individual mortality, health and / or reproduction;
  • Medium: evidence or strong likelihood of impact at individual level on survival, health or reproduction. Effect at population level is not clear; and
  • Low: potential negative impact on individuals but evidence is weak and / or occurrences are infrequent.
Table a: Pressure matrix for killer whales in the OSPAR Maritime Area
Pressure Category Pressure Risk
Pollution & Other Chemical Changes Contaminants High
Nutrient enrichment Low
Physical Loss Habitat loss Low
Physical Damage Habitat degradation Low
Other Physical Pressures Litter (including microplastics and discarded fishing gear) Low
Barrier to species movement (offshore windfarms, wave or tidal device arrays) Low
Underwater Noise Changes Military Sonar Medium
Seismic surveys Low
Pile-driving Low
Shipping Low
Death by Injury or Collision Death or injury by collision (with ships) Low
Death or injury by collision (with tidal devices) Low
Biological Pressures Introduction of microbial pathogens Low
Removal of target and non-target species (prey depletion) Low
Removal of non-target species (bycatch) Low
Disturbance (e.g. wildlife watching) Low

Quality Status Report 2010

No comparable quantitative assessment of the distribution and abundance of killer whale was included in the OSPAR Quality Status Report (QSR) 2010. The report highlighted human pressures on marine mammals more generally, such as through bycatch, increased shipping and bioaccumulation of persistent hazardous substances.

The conservation status of killer whales has been assessed as unknown under the European Union Habitats Directive (Council Directive 92/43/EEC), Article 17 reporting in 2013 (EU, 2013).

No assessment units (AUs) have been agreed for killer whale (Orcinus orca). However, there are distinctions between some groups based on photo-ID surveys, studies of well-known populations, genetic studies (biopsies and investigation of stranded animals) and on prey specialisation. Due to the lack of suitable data it is not possible to assess trends in abundance.

Results

Within the OSPAR Maritime Area, several thousand killer whales occur in Arctic Waters and the northern parts of the Greater North Sea, Celtic Seas, and Wider Atlantic. These animals are mobile and feed predominantly on pelagic shoaling fish such as mackerel and herring. Some also feed on seals and birds. Further south, killer whales are most frequently sighted near the Strait of Gibraltar and along the Atlantic shelf edge, especially around the United Kingdom and Ireland, including the Northern North Sea. The abundance of these more southerly killer whales may not exceed 100 individuals.

There is a very small distinct group (10 to 12 individuals) occurring around west Scotland and north-west Ireland. The killer whales that live near the Strait of Gibraltar are seasonally resident in the Gulf of Cadiz (spring) and Strait of Gibraltar (summer), following the Atlantic bluefin tuna, on which they feed extensively. They are considered genetically distinct from other killer whale populations.

High persistent organic pollutant loads in individuals leading to health problems and reproductive failure may have contributed to a decline in killer whale numbers and distribution, especially in industrialised areas.

The conservation status of killer whales has been assessed by the European Environment Agency as ‘favourable’ in terms of range and ‘unknown’ in terms of population under the European Union Habitats Directive (Council Directive 92/43/EEC).

Killer Whales (Orcinus orca) (courtesy of Robert Pitman NOAA)

There is no overall estimate for the size of the killer whale (Orcinus orca) population within the OSPAR Maritime Area. There is some information available for some localities.

The Strait of Gibraltar

The Strait of Gibraltar killer whales have been shown to be a genetically distinct population compared to those of the northern North Atlantic (Foote et al., 2011). The Strait of Gibraltar animals are seasonally resident in the Gulf of Cadiz (spring) and Strait of Gibraltar (summer) and comprise two types of killer whale: those that interact with the drop-line Atlantic bluefin tuna (Thunnus thynnus) fishery and those that do not (Esteban et al., 2016a). The population is estimated at 39 individuals using data collected between 1999 and 2011 (Esteban et al., 2016b). It comprises five pods, of which two interact directly with the tuna fishery through predating fish from drop-lines (Esteban et al., 2016a).

Survival parameters estimated for these animals have shown a difference, with adult survival rates being slightly higher in tuna fishing interacting pods (0.991, standard error (SE): 0.011) than non-interacting pods (0.901, SE: 0.050), suggesting access to the line-caught tuna improves the success of individuals by reducing energy requirements compared to active hunting, and providing access to larger prey items (Esteban et al., 2016b). As these two types of killer whale are not regularly seen intermixing, it is possible that this specialisation may lead to fragmentation of an already small population (Esteban et al., 2016a). Although calving rates are also higher for the interacting pods (0.22, SE: 0.02) than the non-interacting pods (0.02, SE: 0.01), calf survival is potentially lower. No calves survived in the interacting pods between 2005 and 2011, a period that coincided with a drop in tuna catches (Esteban et al., 2016b). Only one juvenile and one calf were observed among the non-interactive pods during the assessment period (1999–2011) (Esteban et al., 2016b).

Greater North Sea and Celtic Seas

In the Greater North Sea and Celtic Seas, killer whales are most commonly seen off Atlantic coasts and in the northern North Sea, while sightings are rare in the English Channel and in most of the North Sea (Reid et al., 2003). These killer whales have not been as extensively studied and little information is available on survival and breeding.

Greater North Sea, Celtic Seas and Wider Atlantic

Killer whales in the northern areas of the Greater North Sea, Celtic Seas and Wider Atlantic are known to form part of a wider population of animals inhabiting Arctic Waters (Foote et al., 2010, 2011; Samarra and Foote, 2015). There is no overall population estimate available, but in 2001 a survey found 15,041 (CV: 0.42) individuals in part of this area. Prey in this wide area mainly comprises pelagic shoaling fish, with some individuals also feeding on seals and birds at some times of the year (Luque et al., 2006; Foote et al., 2010, 2011; Samarra and Foote, 2015).

Celtic Seas

Within the Celtic Seas, photo identification studies have identified ten individuals regularly seen around the Hebrides and in Irish waters (Foote et al., 2010; Beck et al., 2014). This group is genetically distinctive. One of the killer whales of the Hebrides population died in 2016, and two adult males are nearing the expected life-span for the species, indicating that the population is likely to decline. No calves have been recorded with any of the individuals in this group while two adult females identified in 1992 have never calved (Foote et al., 2010; Beck et al., 2014).

Conclusion

Killer whales in the OSPAR Maritime Area are found predominantly in Arctic Waters and northern parts of the North Sea, Celtic Seas and Wider Atlantic. Further south there are distinct smaller groups occurring in coastal waters off west Scotland / north-west Ireland and in the Strait of Gibraltar.

Owing to a lack of data only a pilot assessment can be made at present.

Killer whales are vulnerable to the effects of accumulating persistent organic pollutants, with high pollutant levels potentially inhibiting reproduction. Mid-frequency military sonar may also negatively affect killer whales.

The abundance and distribution of killer whales may be indicative of specific aspects of the status of the marine environment, such as food web integrity and pollutant load.

Killer Whales (Orcinus orca) (courtesy of Christopher Michael)

The number and distribution of killer whales (Orcinus orca) in the OSPAR Maritime Area is not known, partly due to the difficulty of surveying a rare species. In northern waters (Arctic Waters and the northern parts of the Greater North Sea, Celtic Seas and Wider Atlantic), killer whales range widely and feed on pelagic schooling fish such as mackerel (Scomber scombrus) and herring (Clupea harengus). In all waters, including those further south, killer whales occur in distinct groups of relatively few individuals in coastal waters. These whales feed on fish, with some individuals feeding on seals and birds at some times of the year. Two areas are known to hold these groups: west Scotland / north-west Ireland and south-west Iberia / Strait of Gibraltar. Some individuals in the Strait of Gibraltar population interact with a drop-line fishery for tuna. These have been shown to have higher adult survival rates and higher calving rates than those animals that do not interact, although calf survival may be lower. As these two sets of individuals are not regularly seen intermixing, it is possible that this specialisation can lead to the fragmentation of an already small population.

Killer whales, as long-lived top predators, are vulnerable to the effects of persistent organic pollutants, with high pollutant loads possibly inhibiting reproduction. Military activities using mid-frequency SONAR may negatively affect cetaceans, including killer whales.

Knowledge Gaps

Historical information on abundance and distribution of killer whales is either scarce or lacking. Due to this lack of information, no assessment of abundance can be undertaken, so only a pilot assessment of distribution is possible. There is no estimate for the size of killer whale populations within the Greater North Sea, Celtic Seas, and Bay of Biscay and Iberian Coast.

Human activities can affect killer whales. However, the relationship between human activities (e.g. disturbance, pollution, fishing, habitat alteration) and the impact of these activities on killer whale populations needs further study.

Assessments for the groups of killer whales (Orcinus orca) in the OSPAR Maritime Area are not currently possible because trends in population size are unknown. Monitoring is required for at least ten years, with a minimum of four data points during that period, before an assessment is possible.

Information on historical distribution and, in particular, abundance is scarce or lacking. There are some published accounts, but much of the information is based on anecdotal accounts.

Beck, S., Foote, A.D., Kötter, S., Harries, O., Mandleberg, L., Stevick, P.T., Whooley, P. and Durban, J.W. (2014) Using opportunistic photo-identifications to detect a population decline of killer whales (Orcinus orca) in British and Irish waters. Journal of the Marine Biological Association of the United Kingdom, 94(6): 1327–133

Bolt, H. E., Harvey, P. V., Mandleberg, L. and Foote, A. D., 2009. Occurrence of killer whales in Scottish inshore waters: temporal and spatial patterns relative to the distribution of declining harbour seal populations. Aquatic Conservation: Marine and Freshwater Ecosystems, 19, pp.671–675.

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Esteban, R., Verborgh, P., Gauffier, P., Giménez, J., Foote, A. D. and de Stephanis, R. 2016a. Maternal kinship and fisheries interaction influence killer whale social structure. Behavioral Ecology and Sociobiology, 70, 111–122.

Esteban, R., Verborgh, P., Gauffier, P., Giménez, J., Guinet, C. and de Stephanis, R. 2016b. Dynamics of killer whale, bluefin tuna and human fisheries in the Strait of Gibraltar. Biological Conservation, 194, 31–38.

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