Intraguild predation and partial consumption of blue sharksPrionace glaucaby Cape fur sealsArctocephalus pusillus pusillus

Abstract

The top-down effects of predators on ecosystem structure and dynamics have been studied increasingly. However, the nature and consequence of trophic interactions between upper-trophic-level predators have received considerably less attention. This is especially the case in marine systems due to the inherent challenges of studying highly mobile marine species. Here we describe the first documentation of asymmetrical intraguild predation by a pinniped predator on a mid-sized predatory shark. The report is based on direct observations in South African waters, in which free-swimming blue sharks Prionace glauca were captured and partially consumed by Cape fur seals Arctocephalus pusillus pusillus. These observations are important not just for understanding the interactions between these two species but more broadly for their implications in understanding the trophic ecology of pinnipeds, many populations of which have increased while numerous shark populations have declined.

Prey capture and processing behaviors vary with prey size and shape in Australian and subantarctic fur seals

“…Cape fur seals ( Arctocephalus pusillus pusillus ) have recently been observed killing and processing free swimming blue sharks ( Prionace glauca ) using similar foraging tactics to those described here (Fallows et al . ). After killing the sharks, the fur seals tore open their body cavity to consume the viscera.…”

“…One fur seal was observed killing and consuming the viscera from five sharks over a 2 h observation period (Fallows et al . ). This practice of consuming the viscera while discarding the remainder of the prey item also matched the wild observation reported here of an Australian fur seal eating an angel shark (Squatinidae, Fig.…”

Canine morphometrics as a tool for distinguishing species, sex, and age class in Southern Ocean fur seals

“…Antarctic fur seal populations have a varied diet (Hofmeyr, 2016 ; Jones et al, 2020 ; Walters et al, 2020 ), with those from Bouvet Island feeding primarily on krill (Kirkman et al, 2000 ), and those from Marion Island being generalists (including myctophids, cephalopods, and even penguins; Hofmeyr & Bester, 1993 ; Makhado et al, 2008 ; Reisinger et al, 2018 ). While greater tooth wear (and thus shorter crown length) would be expected for species or populations that feed on sharks for example (Allen & Huveneers, 2005 ; Byron & Morgan, 2016 ; Condit & Le Boeuf, 1984 ; Fallows et al, 2015 ) due to the abrasive nature of shark skin, neither of the tested Antarctic fur seal populations have been recorded to consume sharks and it's unlikely that the observed differences in diet between the two islands would result in change in tooth wear. The observations of larger Antarctic fur seal males at Bouvet Island which translate in bigger, longer, and thicker roots compared to Marion Island specimens as seen in our study, may be the result of ecological or behavioral drivers.…”