Icelandic herring-eating killer whales feed at night

Abstract

Herring-eating killer whales debilitate herring with underwater tail slaps and likely herd herring into tighter schools using a feeding-specific low-frequency pulsed call (‘herding’ call). Feeding on herring may be dependent upon daylight, as the whales use their white underside to help herd herring; however, feeding at night has not been investigated. The production of feeding-specific sounds provides an opportunity to use passive acoustic monitoring to investigate feeding behaviour at different times of day. We compared the acoustic behaviour of killer whales between day and night, using an autonomous recorder deployed in Iceland during winter. Based upon acoustic detection of underwater tail slaps used to feed upon herring we found that killer whales fed both at night and day: they spent 50% of their time at night and 73% of daytime feeding. Interestingly, there was a significant diel variation in acoustic behaviour. Herding calls were significantly associated with underwater tail slap rate and were recorded significantly more often at night, suggesting that in low-light conditions killer whales rely more on acoustics to herd herring. Communicative sounds were also related to underwater tail slap rate and produced at different rates during day and night. The capability to adapt feeding behaviour to different light conditions may be particularly relevant for predator species occurring in high latitudes during winter, when light availability is limited.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-016-3059-8) contains supplementary material, which is available to authorized users.

Declining reproductive success in the Gulf of St. Lawrence’s humpback whales (Megaptera novaeangliae) reflects ecosystem shifts on their feeding grounds

“…There, therefore, appears to be a shift in community structure at these lower trophic levels which have impacts up the food chain resulting in decreases in herring recruitment and abundance (Brosset et al, 2019). Herring are known to be a hugely important prey species for humpback whales in both the North Pacific and the North Atlantic (Gavrilchuk et al, 2014; Jourdain & Vongraven, 2017; Moran et al, 2018; Richard et al, 2017). In fact, forage fish, specifically capelin and herring together, are estimated to make up 44% of their diet in the GSL (Gavrilchuk et al, 2014).…”

Call type repertoire of killer whales (Orcinus orca) in Iceland and its variation across regions

“…It was noticeably more common in Breiðafjörður, despite the overall larger sample of calls from Vestmannaeyjar. Recordings from Vestmannaeyjar were only collected during daytime, while the sample from Breiðafjörður included many hours of recordings collected at night with the EAR when herding calls occurred more frequently (Richard et al, 2017).…”

“…However, the prevalence of I36 was mainly driven by its frequent occurrence in Breiðafjörður. Richard et al (2017) found that the herding call was recorded more often at night. A large proportion of the recordings from Breiðafjörður was collected in the winter months using an EAR and thus included many hours of night‐time recording, while recordings from Vestmannaeyjar were only collected during daytime and in summer.…”

Effects of duty cycles on passive acoustic monitoring of southern resident killer whale (Orcinus orca) occurrence and behavior

“…Cetaceans use sound for communication, foraging, and to obtain information about their environment (Richardson et al, 1995), which allows many species to be detected through passive acoustic monitoring (PAM). Unlike visual survey methods, PAM facilitates long-term observation of cetaceans because it can operate in poor weather conditions, at night, and in study areas that are difficult to reach, while causing little interference in the subject's behavior (Au et al, 2013;Oswald et al, 2016;Richard et al, 2017;Sirovic ´et al, 2009).…”

“…While duty cycles are frequently used in long-term studies of cetaceans, the selection of cycle length or listening proportion is not consistent. For example, in passive acoustic studies of killer whales that use duty cycles, cycle lengths range from 5 to 240 min, and listening proportions from 0.1 to 0.67 (Hannay et al, 2013;Hanson et al, 2013;Rice et al, 2017;Richard et al, 2017;Riera et al, 2019). Additionally, in most published studies using duty-cycled data, the selection of the duty cycle is only justified based on the storage capacity and battery life of the recorder and not related to the acoustic behavior of the focal species (e.g., Diogou et al, 2019;Hanson et al, 2013;Lammers et al, 2017).…”