The effect of prey abundance and fisheries on the survival, reproduction, and social structure of killer whales (Orcinus orca) at subantarctic Marion Island

Abstract

Most marine apex predators are keystone species that fundamentally influence their ecosystems through cascading top-down processes. Reductions in worldwide predator abundances, attributed to environmental-and anthropogenic-induced changes to prey availability and negative interactions with fisheries, can have far-reaching ecosystem impacts. We tested whether the survival of killer whales (Orcinus orca) observed at Marion Island in the Southern Indian Ocean correlated with social structure and prey variables (direct measures of prey abundance, Patagonian toothfish fishery effort, and environmental proxies) using multistate models of capture-recapture data spanning 12 years (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). We also tested the effect of these same variables on killer whale social structure and reproduction measured over the same period. Indices of social structure had the strongest correlation with survival, with higher sociality associated with increased survival probability. Survival was also positively correlated with Patagonian toothfish fishing effort during the previous year, suggesting that fishery-linked resource availability is an important determinant of survival. No correlation between survival and environmental proxies of prey abundance was found. Atisland prey availability influenced the social structure of Marion Island killer whales, but none of the variables explained variability in reproduction. Future increases in legal fishing activity may benefit this population of killer whales through the artificial provisioning of resources they provide.

Killer whale acoustic patterns respond to prey abundance and environmental variability around the Prince Edward Islands, Southern Ocean

“…RF model results indicated that the occurrence of echolocation clicks fluctuated with the increase in number of killer whales sighted around Marion Island, supporting the notion that the PEIs killer whales have complex foraging strategies driven by their adaptive social structure [10,22,28]. The number of killer whales (when eight or more animals were in the vicinity of Marion Island) resulted in an increase in the occurrence of social calls, which validates the dependence of large groups on acoustics for communication [35].…”

“…Diel vocalizing behaviour of killer whales indicated a strong pattern for most seasons, a suggestion of adaptive and diverse vocalizing patterns probably linked to their dynamic foraging behaviour, social structure and activities [ 1 , 7 , 10 , 27 , 32 , 33 ]. These killer whales produced equally high rates of echolocation clicks during daytime and night-time in autumn and spring, but produced higher click rates at night in summer, which indicates that they might have been hunting around the PEIs when the SES abundance was high [ 1 , 5 ].…”

“…These killer whales produced equally high rates of echolocation clicks during daytime and night-time in autumn and spring, but produced higher click rates at night in summer, which indicates that they might have been hunting around the PEIs when the SES abundance was high [ 1 , 5 ]. A higher echolocation click rate was observed at night in winter when SES abundance was relatively low, another adaptive behaviour of this species in response to prey abundance [ 10 , 28 ]. Summer and winter were characterized by high social call rates before and after sunset respectively, perhaps reflecting that important social activity could be linked to group movement and/or hunting strategies of pods at that time of day [ 1 , 33 , 65 ].…”

“…Several killer whale ecotypes (A, B, C and D) differing in morphology, genetics, diet, movement, foraging behaviour, acoustic repertoire and social structure exist within the Southern Ocean [2]. The distribution and occurrence of these killer whales are influenced by the occurrence of prey, suitable environmental conditions and sexual maturity [3,7,9,10]. Overall, there is a lack of information about their whereabouts in the open ocean [4,5] and the environmental factors responsible for their spatial variation are not well understood [9,11].…”

“…The PEIs killer whales, which are more genetically related to ecotype B [ 26 ], prey on southern elephant seals ( Mirounga leonina ), sub-Antarctic fur seals ( Arctocephalus tropicalis ), king penguins ( Aptenodytes patagonicus ), gentoo penguins ( Pygoscelis papua) , macaroni penguins ( Eudyptes chrysolophus) and Patagonian toothfish ( Dissostichus eleginoides ) [ 1 , 6 , 8 , 10 ]. Shore-based sightings determined that killer whales frequenting the inshore region of Marion Island rely on the high prey abundance [ 23 , 27 , 28 ], and nearby Patagonian toothfish fisheries (through depredation) for reproduction, social structure regulation and survival [ 10 ]. While the PEIs killer whale population has been observed here year-round, they have been reported to be most common between September and December [ 1 , 6 , 28 ].…”