Vocal behaviour and feeding ecology of killer whales Orcinus orca around Shetland, UK

Abstract

Killer whales Orcinus orca are sighted regularly off Shetland, UK, but little is known about their numbers, diet and population identity. We aimed to relate vocal behaviour to diet of killer whales around Shetland in order to investigate population structure and differences in feeding strategies. Fieldwork was conducted in the summers of 2008 and 2009. We located killer whales through a sightings network and shore-based scans and collected photo-ID data, behavioural information, feeding data and acoustic recordings from a small boat. The majority of encounters (n = 14) were of small groups (1 to 15 individuals) travelling close to shore and feeding on marine mammals. Two encounters were with large groups (20+ individuals) feeding on herring Clupea harengus farther offshore. Seal-hunting groups vocalised rarely, producing pulsed calls, echolocation clicks and whistles almost exclusively when surface-active or milling after a kill. Herring-eating groups were largely silent during one encounter, but very vocal during the other. Analysis of pulsed calls identified 6 stereotyped call types for seal-hunting groups and 7 for herring-eating groups. No call types were shared between both kinds of groups. The vocal behaviour of seal-hunting groups showed striking parallels to that of Pacific marine mammal specialists and presumably evolved to decrease detection by acoustically sensitive prey. One call type produced by Shetland herring-eating killer whales matched a vocalisation that a previous study had described from Iceland and identified as a possible herding call that may function to concentrate herring during feeding. These findings point to behavioural and dietary specialisation among Shetland killer whales, which should be taken into account when making management decisions affecting these animals. Aquat Biol 13: 79-88, 2011

Cultural evolution of killer whale calls: background, mechanisms and consequences

“…The increased production of calls, particularly two-voiced calls, during periods of coordinated movement further supports that hypothesis (Shapiro, 2008;Samarra & Miller, 2015a). Deecke et al, (2011) reported differences in vocal behavior of seal-and herring-eating killer whales off Shetland, UK similar to those between N Pacific residents and transients. Vocal rates of herring-eating whales were higher than those of seal eating whales, and seal-hunting groups vocalized almost exclusively when surface-active or milling after a kill.…”

Seasonal movements of killer whales between Iceland and Scotland

“…Sighting frequency varied among whales. One whale (9479/122) was first photo-identified in the Ice landic herring overwintering grounds in 1995 and was next seen in the summer of 2009 feeding upon the North Sea herring stock approximately 5 km offshore of Shetland (see Table 1 in Deecke et al 2011). That whale was seen again in Iceland, in ISS herring overwintering grounds, once in March 2012 and twice in March 2013.…”

“…com/ articles/ suppl/ b024p075_ supp/). Additionally, a group containing 997/19, IF-4/21 and 993/62 was previously observed exhibiting behavior typical of that observed for seal-hunting groups (described in Deecke et al 2011) during a group follow by ADF on 31 May 2008 off the coast of Shetland. Finally, an encounter off St Kilda, Scotland, on 1 July 2015 documented a group containing T-38/12 clearly predating on a seal (Figs.…”

“…Jakobsson & Stefánsson 1999); thus, specializing on a specific stock may not be an effective strategy. However, this would require the ability to shift between specialized feeding strategies adapted to different prey resources (Similä & Ugarte 1993, Deecke et al 2011, Beck et al 2012. Our understanding of the extent of behavioral plasticity in foraging strategies and to what extent killer whales may switch prey types requires future study, particularly combining seasonal behavioral observations and diet assessment in different feeding contexts.…”

North Atlantic killer whale research; past, present and future

“…Killer whales feeding on herring off Iceland and Shetland produce a distinctive low frequency pulsed call just prior to tail slapping the herring (Simon et al, 2006;Deecke et al, 2011). The low frequency of the call may resonate the herring's swim bladder and therefore help herd the herring (Simon et al, 2006).…”