Female New Zealand Sea Lions Research Articles

Identifying female foraging ecotype and fisheries impacts through pup whisker stable isotopes

Abstract Individual specialization in the foraging behaviour of marine predators can affect the ecology, evolution, and ability of a population to respond to environmental variability and human impacts. Here, blood serum and whisker stable isotope values from New Zealand (NZ) sea lion pups (Phocarctos hookeri) are compared with their mother’s values to identify whether they can be used as proxies to identify their mother’s isotopic niche and foraging strategy. Female NZ sea lions have been identified, both through telemetry and stable isotope research of blood serum and whiskers, to have two distinct foraging ecotypes (mesopelagic or benthic), consistent across their adult life. Females who are mesopelagic foragers have higher overlap and a greater risk of harmful interactions with fisheries. Stable isotope analysis of adult females can be used to determine the proportion of the female population exposed to these detrimental interactions. However, the capture, restraint, and removal of a whisker or blood from an adult female NZ sea lion is an expensive, time‐consuming, and invasive undertaking. Instead, by comparing the blood serum and whisker δ13C and δ15N values of 12 NZ sea lion mother‐and‐pup pairs, for whom the foraging behaviour of the female is known, the question as to whether a pup’s blood serum or whisker isotope value can be used as a proxy to identify female foraging ecotype was investigated. The δ13C and δ15N values for the blood serum and whiskers of pups and their mothers were correlated and differed significantly between foraging ecotypes. This research validates that pup blood serum and whisker stable isotope values from 1‐month‐old pups can be used as indicators of female isotopic niche and therefore foraging ecotype. This tool can be used across all NZ sea lion colonies to indicate female NZ sea lion foraging ecotypes. For the Auckland Islands, it could determine the proportion of breeding females exposed to negative interactions with fisheries, leading to a better understanding of the level of these effects and helping to implement appropriate management to mitigate impacts.

Winter foraging behaviour of lactating New Zealand sea lions (Phocarctos hookeri)

The overlap and conflict between marine mammals and fisheries is widespread and increasing globally. This study used satellite telemetry to examine the foraging patterns of lactating female New Zealand (NZ) sea lions Phocarctos hookeri from the Auckland Islands (50°50′S, 166°28′E), over two consecutive austral winters, 1996 and 1997. Overall, all individuals tagged were characterised as benthic foragers and were found to be diving beyond their calculated aerobic dive limits on 65% of all dives. They exhibited preferential use of the continental shelf and its edge, as they do in summer (59% overlap between winter and summer foraging areas). This study confirms that NZ sea lion foraging behaviours and areas are similar between seasons, and within and between years. Female winter foraging areas spatially overlapped by 38% with the operation areas of sub-Antarctic trawl fisheries. These findings are important because: female NZ sea lions show extreme foraging behaviour year round; and overlap with trawl fisheries is likely to occur throughout the entire lactation period of NZ sea lions. These results need to be accounted for in the management of this nationally critical, declining species.

Foraging Behaviour of Juvenile Female New Zealand Sea Lions (Phocarctos hookeri) in Contrasting Environments

Foragers can show adaptive responses to changes within their environment through morphological and behavioural plasticity. We investigated the plasticity in body size, at sea movements and diving behaviour of juvenile female New Zealand (NZ) sea lions (Phocarctos hookeri) in two contrasting environments. The NZ sea lion is one of the rarest pinnipeds in the world. Most of the species is based at the subantarctic Auckland Islands (AI; considered to be marginal foraging habitat), with a recolonizing population on the Otago Peninsula, NZ mainland (considered to be more optimal habitat). We investigated how juvenile NZ sea lions adjust their foraging behaviour in contrasting environments by deploying satellite-linked platform transmitting terminals (PTTs) and time-depth recorders (TDRs) on 2–3 year-old females at AI (2007–2010) and Otago (2009–2010). Juvenile female NZ sea lions exhibited plasticity in body size and behaviour. Otago juveniles were significantly heavier than AI juveniles. Linear mixed effects models showed that study site had the most important effect on foraging behaviour, while mass and age had little influence. AI juveniles spent more time at sea, foraged over larger areas, and dove deeper and longer than Otago juveniles. It is difficult to attribute a specific cause to the observed contrasts in foraging behaviour because these differences may be driven by disparities in habitat/prey characteristics, conspecific density levels or interseasonal variation. Nevertheless, the smaller size and increased foraging effort of AI juveniles, combined with the lower productivity in this region, support the hypothesis that AI are less optimal habitat than Otago. It is more difficult for juveniles to forage in suboptimal habitats given their restricted foraging ability and lower tolerance for food limitation compared to adults. Thus, effective management measures should consider the impacts of low resource environments, along with changes that can alter food availability such as potential resource competition with fisheries.

Autumn diet of recolonising female New Zealand sea lions based at Otago Peninsula, South Island, New Zealand

New Zealand (NZ) sea lions (Phocarctos hookeri) are slowly recolonising the Otago coast, South Island, New Zealand. The increase in their numbers may lead to resource competition with other marine predators and fisheries. We determined the diet of female NZ sea lions at Otago during autumn. In total, 571 scats and 110 regurgitations were collected on Otago Peninsula during 2008 and 2009. Barracouta (Thyrsites atun) and jack mackerel (Trachurus sp.) were the two main prey species and accounted for 26% and 31% of the reconstituted biomass, respectively. This was consistent between two years. Only five other species contributed > 5% of the diet by biomass in either year. Prey species are all found on the narrow continental shelf surrounding Otago Peninsula. The main prey species of Otago NZ sea lions may be of higher energy content than prey in the Auckland Islands (remnant breeding area). Resource overlap with other marine predators and fisheries appears to occur around Otago Peninsula. A marine trophic model of the area off Otago Peninsula would help understanding potential competition between marine predators and fisheries in this area.

Behavioural responses and attraction of New Zealand sea lions to on-land female decoys

Using decoys to attract gregarious animals is a common management practice, but rarely used for pinnipeds. We investigated the behavioural responses of New Zealand (NZ) sea lions, Phocarctos hookeri, at Sandy Bay, Enderby Island, to determine whether decoys can attract female sea lions and so could be useful for the establishment of new colonies. We deployed decoys near existing breeding aggregations for 4 hours at a time. One or a group of three artificial decoys, made of white fabric, attracted up to 54% of females coming ashore, of which 73% came close enough to sniff the decoys. Up to 62% of males coming ashore within 15 m either side of these decoys moved towards them. A single taxidermied female NZ sea lion did not attract females significantly more often than a single fabric one, but was investigated as if alive by males. It appears that female NZ sea lions are attracted to decoys by their colour, at least when located close to an existing group of females. A technique using decoys could be trialled to direct recolonising females to suitable pupping sites.

Predicting Life-History Traits for Female New Zealand Sea Lions, Phocarctos hookeri: Integrating Short-Term Mark-Recapture Data and Population Modeling

The trade-off between survival and reproduction by individuals is central to understanding life-history parameters of a species. Few mammal species have life-history information from long-term research. Instead, demographic models are commonly utilized to investigate an individual’s life-history strategy, species dynamics, and population trends. This research investigates age-related survival and reproductive performance of adult female New Zealand (NZ) sea lions (Phocarctos hookeri), using multi-state mark-recapture data from known-age branded individuals over five years. The mark-recapture analysis was integrated with a population model to predict the lifetime reproductive output of female NZ sea lions. The integration of an analysis of short-term datasets with population modeling allows for the prediction of life-history parameters of long lived animals when long-term information is not available. While such approaches involve some caveats, it provides a framework for investigating population dynamics and is preferential to unsubstantiated assumptions. This technique can lead to better design and implementation of conservation management for long lived species.

Diverse foraging strategies in lactating New Zealand sea lions

Diving and foraging behaviours of marine predators are thought to be coupled with species demographics, with benthic foraging species found to have small population sizes and low growth rates. This was thought to be the case for New Zealand (NZ) sea lions Phocarctos hookeri, previously identified as solely benthic foragers with a small declining population. Female NZ sea lions dive deeper and for longer than any other otariids (eared seals). They exceed their calculated aerobic dive limits (cADL) during 68% of all dives, indicating that they are operating near their max- imum physiological capacity. Recently, however, individual NZ sea lions have been found to have 2 distinct dive profile types or foraging patterns: a benthic diving profile and a deeper, more varied meso-pelagic diving profile. The ecological significance and implications of individual foraging specialisation in otariids have had limited attention in the past. Here, we compared the foraging behaviour of female NZ sea lions that use these 2 distinct dive profiles and discuss how the 2 forag- ing patterns may indicate differing prey choice, as well as their possible implications for individual reproductive ability and interactions with fisheries and, therefore, species demographics. Our ulti- mate goal is to understand the causes of this individual foraging specialisation and to incorporate such variation when determining management options for this threatened, declining species.

Diving to extremes: are New Zealand sea lions (Phocarctos hookeri) pushing their limits in a marginal habitat?

AbstractWhen studying diving behaviour, it is important to know whether a species is operating at or close to its maximum physiological capacity, because if it is, it will be less capable of compensating for normal environmental or human‐induced fluctuations in its environment. New Zealand (NZ) sea lions Phocarctos hookeri are among the world's rarest pinnipeds with a restricted distribution and abundance to the most southerly limit of their known range, NZ's sub‐Antarctic. Female NZ sea lions are the deepest and longest diving of the otariids, foraging further from their breeding rookeries than any other sea lion. In this study, the diving behaviours of 18 female NZ sea lions from Enderby Island, Auckland Islands, were recorded during early lactation over two austral summers, 2003 and 2004. While at sea, sea lions dived almost continuously, spending on average 52.7% of their time submerged (>6 m). The mean dive depth (±se) for all dives was 129.5±5.3 m (range 94.6±1.1 to 178.9±1.6 m). The mean duration of dives was 4.0±0.1 min with an average of 40±2.9% of all dive times spent in the deepest 85% of the dive. Although there was high variation in diving behaviour among individuals, overall, animals were found to be diving beyond their estimated aerobic dive limits on 68% of all dives. Given that female NZ sea lions have a high percentage of dives that are beyond their theoretical aerobic limits, we ask whether this represents a miscalculation in aerobic limits, is it a species highly adapted to carry an anaerobic load or a species that is physically stretched to their limits? A species that is presumably under physiological stress just to maintain its current low static population numbers is also likely to be more susceptible to external impacts and this needs consideration for their management.