Foraging behaviour indicates marginal marine habitat for New Zealand sea lions: remnant versus recolonising populations

Abstract

The New Zealand sea lion Phocarctos hookeri historically bred on the New Zealand mainland (South and North Islands). Subsistence hunting and later commercial sealing reduced its distribution to 3 breeding areas at the spatial edges of its historical distribution range, in the Auckland Islands (AI) and on Campbell Island. Here, we present foraging areas and foraging trips of female New Zealand sea lions from the Otago Peninsula, where a recolonising population has been found in the core of the historical range of the species. We compare the results with data from the AI in order to assess the theory that the spatial margin of a species' distribution represents the lower end of habitat suitability. Female New Zealand sea lions at Otago had significantly smaller foraging ranges than females at the AI (mean 65% Kernel ranges: 47 ± 25 km 2 versus 687 ± 109 km 2 ), made shorter foraging trips (mean 11.8 ± 2.3 h versus 66.2 ± 4.2 h), and spent 40% less time at sea overall. Juvenile females at Otago from age 2 onwards could access foraging grounds used by adult females nursing pups; this is unlikely to be the case at the AI due to the large distances and associated depths of foraging grounds. Our study illustrates the theory that spatial marginality is related to habitat marginality. Existing management measures to mitigate the impact of bycatch in fisheries on declining remnant colonies around the AI were modelled based on populations exploiting optimal habitat. They should now integrate this new information.

Foraging diversity in lactating New Zealand sea lions: insights from qualitative and quantitative fatty acid analysis

“…Diversity in foraging among individuals of the same species is considered a sign of limited ecosystem productivity (Tinker et al 2008). This strengthens a previous hypothesis found in the literature that the Auckland Islands, historically the southernmost limit of NZ sea lions' range, is a marginal environment for this species (Costa and Gales 2000;Chilvers et al 2005;Augé et al 2011). Any reduction of prey availability at the slope of the Auckland Islands' shelf is likely to be detrimental to the reproductive success of this vulnerable species.…”

Influence of habitat condition and competition on foraging behaviour of parrotfishes

“…Variation in foraging behaviour in response to aspects of habitat condition has been demonstrated in a wide range of mobile organisms, including seabirds (McLeay et al 2010), marine mammals (Augé et al 2011) and fish (Hoey & Bellwood 2011). A more in-depth understanding of foraging behaviour for coral reef fishes is critical in light of existing degradation of reefs (e.g.…”

Preliminary assessment of the foraging behaviour and population dynamics of a cryptic population of the endangered New Zealand sea lion

“…For example, the foraging behaviour of New Zealand sea lions has been hypothesised to be related to the demographics of a population (Augé et al 2011b). Female sea lions at Enderby Island, Auckland Islands (Fig.…”

“…These sea lions have no consistent foraging patterns (i.e. they exhibit both benthic and mesopelagic behaviours; Augé et al 2011a) and remain close to shore while foraging, with short trips over the continental shelf (Augé et al 2011b). This population has a stable pup production, earlier age at first breeding, higher reproductive ability and higher juvenile survival than the Auckland Islands population (Augé 2010).…”