First longitudinal study of seal-feeding killer whales (Orcinus orca) in Norwegian coastal waters.

Killer whales (Orcinus orca) have been documented preying on either fish or marine mammals in several regions, suggesting that this odontocete species has the ability to specialize on different types of prey. Off Norway, killer whales have been shown to rely on the Atlantic herring (Clupea harengus) as a main prey resource. Infrequent observations have revealed seals as an additional component of their diet, yet the extent of predation on marine mammals has remained largely unknown. Here, we present the findings of 29 years of photographic and observational data on seal-feeding killer whale groups identified in Norwegian coastal waters. Four groups have been observed preying and feeding on seals over several years, taking both harbor (Phoca vitulina) and grey (Halichoerus grypus) seals. These stable groups are shown to adopt small group sizes, were typically observed in near-shore areas and were not encountered on herring wintering grounds. Behavioral and social traits adopted by these groups are similar to those of pinniped-feeding killer whales from other regions. The potential ecological reasons and the extent of such prey specializations are discussed

Sighting histories of KI and K whales.

<p>Sighting histories of KI and K whales.</p

Photographs of seal-feeding killer whales KI-03, KI-05 and KI-06.

<p>Individuals in upper photographs, observed off central coast of Norway in 1988, match individuals visible on the lower row of photographs, taken off Andenes in June 2015.</p

Important prey areas and observations of seal-feeding killer whale groups.

<p>The left panel (A) shows seasonal herring grounds since 1990 while the middle panel (B) shows areas of abundance of coastal seals since 1990 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180099#pone.0180099.ref032" target="_blank">32</a>]. The right panel (C) shows locations of K and KI whales sightings since 1988. (The map was made in ESRI ArcGIS software [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0180099#pone.0180099.ref033" target="_blank">33</a>] using public domain map data @ <a href="http://naturalearthdata.com" target="_blank">naturalearthdata.com</a>).</p

Sociogram of seal-feeding killer whales observed in at least four sampling periods.

<p>Occurrence and thickness of links show association patterns between pairs of individuals, revealing the two assemblages KI and K.</p

Plots of lagged association rates.

<p>The blue plot represents our real data set while the red plot shows the expected lagged association rates if individuals associated at random.</p

Summary of recorded predation events on seals where involved individual killer whales could be identified.

<p>Summary of recorded predation events on seals where involved individual killer whales could be identified.</p

Photograph of KI-03 preying upon a harbor seal pup in June 2015.

<p>The photograph (A) illustrates the predation observation and the inset (B) confirms effective prey consumption.</p

Data from: First longitudinal study of seal-feeding killer whales (Orcinus orca) in Norwegian coastal waters

The file set contains photographic identification and group size data for seal-feeding killer whales observed in Norwegian coastal waters. All records of observations and/or predation events for these killer whales are available. Additionnal records of predation on seal prey by unidentified killer whale individuals are also reported