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Influence of shelf oceanographic variability on alternate foraging strategies in Long-nosed fur seals
Abstract
Central place foragers often change their foraging behaviour in response to changes in prey availability in the environment. Lactating Long-nosed fur seals (LNFS; Arctocephalus forsteri) at Cape Gantheaume in South Australia have been observed to display alternate foraging strategies where they forage on the shelf in summer and switch to oceanic foraging in winter. We investigated the relationship between changes in shelf summertime upwelling and the timing and variability when females switch from predominantly shelf to oceanic foraging. Geolocation tags were deployed on females from summer to winter in 2016 and 2017, giving us longitudinal tracks over the transition period. The timing of switching from shelf to oceanic foraging was primarily driven by seasonal oceanographic changes on the shelf – specifically when the strength of the seasonal localised upwelling began to decline. The individual variability in the timing of the switch was driven by the strength of the coastal upwelling with variability being greater in years when upwelling strength was weaker. By comparing our results to that of previous studies on the same colony, we found qualitative evidence that inter-annual environmental variability likely influences whether individuals display a single or multiple foraging strategies. This further highlights the flexibility in foraging strategies used by LNFS in response to environmental changes. The effect of inter-annual differences in foraging strategies on overall reproductive success warrants further investigation.
... Mothers nurse their pups for 8 -11 months while in gestation (Goldsworthy 2006). During the austral summer, the majority of the lactating females from Kangaroo Island (South Australia) forage on the Australian continental shelf and shelf break associated with seasonal upwelling (Foo et al, 2019); whereas during the austral winter, the majority of the lactating females forage in oceanic waters associated with the Subtropical Front (Baylis et al. 2008a). The marked change from coastal to oceanic foraging as lactation progresses is referred to as an alternate or bimodal foraging strategy (Foo et al, 2019). ...
... During the austral summer, the majority of the lactating females from Kangaroo Island (South Australia) forage on the Australian continental shelf and shelf break associated with seasonal upwelling (Foo et al, 2019); whereas during the austral winter, the majority of the lactating females forage in oceanic waters associated with the Subtropical Front (Baylis et al. 2008a). The marked change from coastal to oceanic foraging as lactation progresses is referred to as an alternate or bimodal foraging strategy (Foo et al, 2019). There is nonetheless variability in their oceanic foraging trip distances and not all females travel as far as the Subtropical Front. ...
... Locations were estimated from the raw light data by first using the BAStag (v0.1-3) (Sumner et al. 2009, Wotherspoon et al. 2016a (Sumner et al. 2009, Lisovski et al. 2012. The accuracy of location estimates using this method was 45 ± 29 km (mean ± SD) for a lactating LNFS carrying GLS and GPS tags simultaneously (Foo et al. 2019). Females usually begin to move from shelf to oceanic foraging as upwelling activity on the shelf declines in April, hence we classified locations into early autumn (February -March), late autumn -winter (April -August) or spring (September). ...
Understanding the drivers to foraging strategies in space and time is an important aspect of ecology that is necessary for management and conservation. Within the highly dynamic marine environment, prey availability changes spatially and temporally over seasons and years. Consequently, marine predators may have to employ different foraging strategies in response to the changing environment to ensure they acquire sufficient resources for their survival and reproductive success. This in turn ultimately affects population abundance and distribution. This study is concerned with identifying the drivers to alternate foraging strategies observed in lactating Long-nosed fur seals (LNFS) from one of the primary breeding colonies at Cape Gantheaume, Kangaroo Island, South Australia. Early in lactation (December – April), females breeding at Cape Gantheaume undertake short foraging trips (~ 5 days) to near shelf waters (70-90 km from the colony), in regions associated with localised seasonal upwelling, which occurs from the austral summer to autumn. However, around late autumn (April – May) most females switch to foraging in distant oceanic waters associated with the Subtropical Front, 700-1,000 km to the south of the breeding colony and may continue to forage in these waters up until the weaning of their pups in September/October when they are about 10 months old. These winter foraging trips can last more than two weeks. The at-sea distribution of LNFS the austral summer to winter was quantified using archival global location sensing (GLS) loggers. Whisker regrowths produced over the study period were also collected. Specifically, this thesis aims to (1) examine the spatial-temporal variability in the oceanography of the study region, (2) identify the timing of switch from shelf to oceanic foraging in relation to oceanographic changes on the shelf, (3) ascertain the degree of individual foraging site fidelity within the oceanic region and (4) examine the inter-annual spatial and temporal variability in prey trophic level of adult females.
... Mothers nurse their pups for 8-11 months while simultaneously gestating a foetus (Goldsworthy 2006). During the austral summer, the majority of the lactating females from Kangaroo Island (South Australia) forage on the Australian continental shelf and shelf-break associated with seasonal upwelling (Foo et al. 2019); whereas during the austral winter, the majority of the lactating females forage in oceanic waters associated with the Subtropical Front (Baylis et al. 2008). The marked change from the coastal to the oceanic foraging as lactation progresses is referred to as an alternating foraging strategy (Foo et al. 2019). ...
... During the austral summer, the majority of the lactating females from Kangaroo Island (South Australia) forage on the Australian continental shelf and shelf-break associated with seasonal upwelling (Foo et al. 2019); whereas during the austral winter, the majority of the lactating females forage in oceanic waters associated with the Subtropical Front (Baylis et al. 2008). The marked change from the coastal to the oceanic foraging as lactation progresses is referred to as an alternating foraging strategy (Foo et al. 2019). Foraging site fidelity is generally high in the continental shelf habitats (Baylis et al. 2012;Sommerfeld et al. 2015) and the continental shelf in our study region is narrow, thereby concentrating productivity into a small predictable area. ...
... Nonetheless, the greater spatial variability in oceanic regions potentially allows for more intra-and inter-individual variability in foraging strategies to develop and, thus, lead to different levels of individual reproductive success. Indeed, the variability in individual oceanic foraging trip distances and direction has been observed in lactating LNFS (Foo et al. 2019); however, the underlying environmental drivers to this variability are still unclear. ...
Finding food is crucial to the survival and reproductive success of individuals. Fidelity to previous proftable foraging sites
may bring benefts to individuals as they can allocate more time to foraging rather than searching for prey. We studied how
environmental conditions infuence when lactating long-nosed fur seals (Arctocephalus forsteri) adopt a risky (low fdelity)
or conservative (high fdelity) foraging strategy at two intra-annual temporal scales when foraging in a highly variable oceanic environment. Core foraging areas (CFAs; n=534; 30×30 km cells) of consecutive foraging trips were obtained from
geolocation tracks of 12 females from summer to winter in 2016 (n=5) and 2017 (n=7). We used the spatial variability
(standard deviation) of CFAs between or among oceanic foraging trips as a proxy for individual foraging site fdelity (IFSF).
Over the entire oceanic foraging period (n=12), IFSF in the latitudinal axis increased with stronger sea-surface temperature
gradient (SSTgrad), but decreased with greater SSTgrad and sea-surface height gradient variability. Over a period of two
consecutive oceanic foraging trips (n=66), IFSF decreased with greater SSTgrad variability in the earlier foraging trip. LNFS
show evidence that they use IFSF as a strategy to potentially optimise food acquisition, and that this behaviour is infuenced
by mesoscale oceanographic parameters.
... During the breeding season and subsequent lactation period, adult females provision a single pup onshore. These females forage from this central place, typically feeding on the continental shelf during the austral summer coastal upwelling period (January−April), switching to feeding in oceanic waters in late autumn to winter (mid-April−September), when the upwelling activity ceases (Baylis et al. 2008, Foo et al. 2019. While the seasonal changes in foraging habitat of lactating LNFS are fairly well documented (Foo et al. 2019), our understanding of individual inter-annual variability in their foraging behaviour is still limited. ...
... These females forage from this central place, typically feeding on the continental shelf during the austral summer coastal upwelling period (January−April), switching to feeding in oceanic waters in late autumn to winter (mid-April−September), when the upwelling activity ceases (Baylis et al. 2008, Foo et al. 2019. While the seasonal changes in foraging habitat of lactating LNFS are fairly well documented (Foo et al. 2019), our understanding of individual inter-annual variability in their foraging behaviour is still limited. Tracking and measuring the foraging behaviours of the same individuals over multiple years using biologgers can be challenging; however, the use of vibrissa stable isotopes may be a way of solving this problem. ...
... The study was undertaken between February 2016 and September 2017 at Cape Gantheaume (36° 04' S, 137° 27' E), Kangaroo Island, South Australia (Foo et al. 2019). In late summer (February−March), 18 lactating female LNFS (2016 n = 5, 2017 n = 13) were randomly selected and captured using a hoop net. ...
We investigated how foraging ecotypes of female long-nosed fur seals (Arctocephalus forsteri) could be identified from vibrissa stable isotopes. We collected regrowths of vibrissae from adult females (n = 18) from Cape Gantheaume, Kangaroo Island, South Australia from two breeding seasons (2016, 2017). The period represented by the regrowth was known and 8 individuals were administered with 15N-enriched glycine as a biomarker to mark the start date of the regrowth. Non-glycine marked and glycine marked vibrissae were used to estimate the rate of the individual vibrissa regrowth. Using individual growth rates (0.18 ± 0.04 mm d-1), we reconstructed a stable isotope (δ13C and δ15N) time series for each regrowth and allocated them to corresponding at-sea locations either based on geolocation tracks (n = 14) or foraging habitat type (shelf or oceanic) based on diving data (n = 2) of the sampled seals. Mean δ15N from vibrissa segments was higher when females foraged on the continental shelf region (16.1 ± 0.7, n = 29) compared to oceanic waters (15.1 ± 0.7, n = 106) in 2017, whereas it was similar in both regions in 2016 (shelf: 15.3 ± 0.4, n = 13; oceanic: 15.4 ± 0.4, n = 15). Based on the stable isotope signatures of vibrissa segments, model-based clustering analysis correctly classified 79.8% of them into shelf or oceanic foraging habitats. This demonstrates the potential of using vibrissa stable isotopes for studying the foraging ecology of an important top marine predator.
... In addition, life cycles of prey species (recruitment, spawning) further impact on their availability at different spatiotemporal scales (Croxall et al., 1985). Marine predators are expected to alter their foraging in response to such heterogeneity and consequently, temporal differences in foraging effort (Angel et al., 2015;Harding et al., 2007), behavior (Botha & Pistorius, 2018;Foo et al., 2019) and diet (Chambellant et al., 2013;Reisinger et al., 2018) have been well documented. ...
Information on resource use and trophic dynamics of marine predators is important for understanding their role in ecosystem functioning and predicting population-level responses to environmental change. Where separate populations experience different local environmental conditions, geographic variability in their foraging ecology is often expected. Within populations, individuals also vary in morphology, physiology, and experience, resulting in specialization in resource use. In this context, isotopic compositions of incrementally grown tissues such as keratinous hairs offer a valuable opportunity to study long-term variation in resource and habitat use. We investigated the trophic ecology of female Cape fur seals (Arctocephalus pusillus pusillus) using carbon and nitrogen isotopic compositions of serially sampled whiskers collected at four breeding sites along the coast of South Africa. Drawing on over 900 isotopic measurements, we assessed geographic variability in isotopic niche width between colonies and the degree of individual specialization. We found slight, but clear geographic differences in isotopic ratios and isotopic niche widths, seemingly related to ecological setting, with niche widths being proportional to the area of available shelf and shelf-slope habitat surrounding the colony. We further identified periodic oscillations in isotopic ratios, which likely reflect temporal patterns in foraging distribution and prey type, linked to shifts in the availability of prey resources and their interaction with constraints on individual females throughout their breeding cycle. Finally, individual specialization indices revealed that each of the study populations contain specialist individuals that utilize only a small subset of the total population niche width. The degree of individual specialization was, however, not consistent across colonies and may reflect an interactive influence between density-dependent effects and habitat heterogeneity. Overall, this study provides important information on the trophic ecology of Cape fur seals breeding in South Africa and highlights the need to consider geographic and individual variability when assessing the foraging ecology of marine predators.
... The intraspecific strategies which females use to acquire food can present several potential trade-offs that affect energetic investment and allocation (Costa 2007;DeRango et al. 2021). For instance, higher trophic level prey is usually larger and more energy dense but are sometimes located farther from the reproductive colonies than immediately available yet calorie deficient prey (Villegas-Amtmann et al. 2008;Kernaléguen et al. 2015;Páez-Rosas et al. 2017;Foo et al. 2019b). Additionally, individuals can expend greater energy to dive deeper, thus accessing prey rarely exploited by competing species or conspecifics, expanding available niches (McHuron et al. 2016(McHuron et al. , 2018. ...
The success of maternal foraging strategies during the rearing period can greatly impact the physiology and survival of dependent offspring. Surprisingly though, little is known on the fitness consequences of foraging strategies during the foetal period. In this study, we characterized variation in maternal foraging strategy throughout pregnancy in a marine top predator (South American fur seal, Arctocephalus australis), and asked if these shifts predicted neonatal health and postnatal survival. We found that during early pregnancy all pregnant females belonged to a single, homogenized foraging niche without evident clusters. Intriguingly though, during late pregnancy, individual fur seal mothers diverged into two distinct foraging niches characterized by a benthic-nearshore and a pelagic-offshore strategy. Females that shifted towards the benthic-nearshore strategy gave birth to pups with greater body mass, higher plasmatic levels of glucose and lower levels of blood urea nitrogen. The pups born to these benthic females were eight times more likely to survive compared to females using the pelagic-offshore foraging strategy during late pregnancy. These survival effects were mediated primarily by the impact of foraging strategies on neonatal glucose independent of protein metabolic profile and body mass. Benthic-nearshore foraging strategies during late pregnancy potentially allow for the greater maternal transfer of glucose to the foetus, leading to higher chances of neonatal survival. These results call for a deeper understanding of the balance between resource acquisition and allocation provided by distinct foraging polymorphisms during critical life-history periods, and how this trade-off may be adaptive under certain environmental conditions.
... Both, long-nosed fur seals and penguins are highly mobile marine predators demonstrating vastly different foraging strategies which are broadly shaped by system-specific cycles of productivity and life history constraints (Page et al., 2005;Sidhu et al., 2012;Pelletier et al., 2014;Foo et al., 2019). Feathers originating from St. Kilda penguins were undetected from scats, likely explained by the specialised foraging that occurs from this colony throughout the year within the confines of Port Phillip Bay (Preston et al., 2008;Chiaradia et al., 2012). ...
Marine predators recovering from historic, commercial, over-harvesting can create conservation challenges when they prey on vulnerable species. Pinniped predation of seabirds presents one such challenge and identifying the source colonies experiencing seal predation are needed to inform conservation management and decision planning. Here, we present a novel application of stable isotope and trace element techniques to identify the source colony of little penguins ( Eudyptula minor ) predated by long-nosed fur seals ( Arctocephalus forsteri ). We created baseline biochemical ‘feather-prints’ from feathers for six major breeding colonies across south-east Australia to compare with feathers from predated penguins recovered from seal scats. Feeding trials of captive seals confirmed that digestion of penguin feathers did not compromise stable isotope (δ ¹³ C and δ ¹⁵ N) or trace element (Al, Ti, Sr and Mg) signatures. The resulting biochemical ‘feather-prints’ were found to be robust in being correctly classified to local sites (78%) and broader regions (85%). The distinguishing ‘feather-prints’ appeared to be driven by industrial inputs from land, colony-specific foraging patterns and potentially proximity to oceanographic systems (i.e. upwelling). Here, we show that 46-70% of predated feathers were assigned to ‘local’ penguin colonies. We consider that the regional penguin abundances and the proximity of their colonies to seal sites, as well as demographic-specific foraging patterns may shape their contribution to seal diet at local, regional and inter-regional scales. This diagnostic tool is powerful, having broad applications identifying seabird colonies at greatest risk to pinniped predation and informing targeted, site-specific, conservation effort.
... They have delivered oceanographic profiles along both coasts of South America, the Galapagos Islands, Southwest Africa, the West Coast of North America across the Aleutian Islands to the northern reaches of Hokkaido Japan (Weise et al., 2010). They are also well suited for sampling the coastal zone of the South Island of New Zealand, the sub-Antarctic Islands and the southern coasts of Australia (Lowther et al., 2013;Foo et al., 2019;Wege et al., 2019) and the Sea of Okhotsk (Nakanowatari et al., 2017). ...
Marine animals equipped with biological and physical electronic sensors have produced long-term data streams on key marine environmental variables, hydrography, animal behavior and ecology. These data are an essential component of the Global Ocean Observing System (GOOS). The Animal Borne Ocean Sensors (AniBOS) network aims to coordinate the long-term collection and delivery of marine data streams, providing a complementary capability to other GOOS networks that monitor Essential Ocean Variables (EOVs), essential climate variables (ECVs) and essential biodiversity variables (EBVs). AniBOS augments observations of temperature and salinity within the upper ocean, in areas that are under-sampled, providing information that is urgently needed for an improved understanding of climate and ocean variability and for forecasting. Additionally, measurements of chlorophyll fluorescence and dissolved oxygen concentrations are emerging. The observations AniBOS provides are used widely across the research, modeling and operational oceanographic communities. High latitude, shallow coastal shelves and tropical seas have historically been sampled poorly with traditional observing platforms for many reasons including sea ice presence, limited satellite coverage and logistical costs. Animal-borne sensors are helping to fill that gap by collecting and transmitting in near real time an average of 500 temperature-salinity-depth profiles per animal annually and, when instruments are recovered (∼30% of instruments deployed annually, n = 103 ± 34), up to 1,000 profiles per month in these regions. Increased observations from under-sampled regions greatly improve the accuracy and confidence in estimates of ocean state and improve studies of climate variability by delivering data that refine climate prediction estimates at regional and global scales. The GOOS Observations Coordination Group (OCG) reviews, advises on and coordinates activities across the global ocean observing networks to strengthen the effective implementation of the system. AniBOS was formally recognized in 2020 as a GOOS network. This improves our ability to observe the ocean’s structure and animals that live in them more comprehensively, concomitantly improving our understanding of global ocean and climate processes for societal benefit consistent with the UN Sustainability Goals 13 and 14: Climate and Life below Water. Working within the GOOS OCG framework ensures that AniBOS is an essential component of an integrated Global Ocean Observing System.
... Foraging trip metrics and movement patterns may also be related to other oceanographic influences such as current speeds, eddies and upwelling cells, which vary substantially along the South African coast (Roberson et al. 2017). Although the spatial distribution and foraging strategies of pinnipeds are often related to environmental influences (Foo et al. 2019(Foo et al. , 2020, other factors such as competition may also be im plicated, particularly at smaller spatial scales (Wege et al. 2019). Thus, the longer foraging trips observed for animals from Kleinsee and False Bay could be linked to higher levels of intra-specific competition at these larger colonies and may reflect strategies of resource partitioning (e.g. ...
Knowledge of animal foraging behaviour has implications for management and conservation. While Cape fur seals Arctocephalus pusillus pusillus comprise a major proportion of the southern African marine predator biomass, little is known about their at-sea movements. We investigated foraging distribution, habitat use and diving behaviour for 35 adult female Cape fur seals from 3 breeding colonies experiencing contrasting oceanographic regimes. Animals from Black Rocks, the smallest and eastern-most colony, undertook shorter foraging trips and utilised shallower waters over the shelf. In comparison, animals from the larger west coast colonies, at Kleinsee and False Bay, travelled further and utilised deeper shelf and shelf-slope waters. However, across colonies, females typically preferred depths of <500 m and slopes of <5°. Kleinsee and False Bay seals selected sea surface temperatures within the range typically preferred by pelagic prey species such as round herring, sardine and anchovy (14-19°C). Black Rocks individuals showed bimodal preferences for colder (16°C) and warmer waters (>22°C). Dive behaviour was similar between Kleinsee and False Bay individuals (unavailable from Black Rocks), with both pelagic and benthic foraging evident. Diel patterns were apparent at both sites, as dive depth and benthic diving increased significantly during daylight hours, likely reflecting vertical movements of prey species. We provide the first assessment of Cape fur seal movement behaviour for the South African component of the population. Observed geographic differences likely reflect the availability of suitable habitat but may also indicate differences in foraging strategies and density-dependent effects throughout the range of this species.
... Movements of parts of the long-nosed fur seal population in SA are known from satellite telemetry studies at Cape Gantheaume (Page et al. 2006;Baylis et al. 2008;Foo et al. 2019Foo et al. , 2020. In brief, adult males forage at the edge of the continental shelf, adult females typically use continental shelf or near-shelf waters in summer and autumn, and pelagic waters associated with the subtropical front much further south during winter, the same region that is favoured by juveniles. ...
Long-nosed fur seals (Arctocephalus forsteri) were tagged as pups in colonies on Kangaroo Island, South Australia in eight consecutive pupping seasons from 1988–89 to 1995–96. Thirty-nine tagged animals were sighted on the southern Australian coast, being 0.89% of those tagged. They were aged from 9 months to 14 years 6 months, with half in their second and third years. Most records (88%) were of animals that moved eastwards. The most distant records were from Sydney in the east (1700 km), south of Tasmania in the south (1240 km) and Head of Bight in the west (700 km). One animal was seen twice, both times on the north coast of Kangaroo Island, once underwater and two years later ashore. Satellite telemetry studies of juvenile A. forsteri from Kangaroo Island showed that they typically forage in pelagic waters ~1000 km further south in association with the subtropical front. The study reported here shows that some animals tagged as pups disperse widely as juveniles around the southern Australian coast. The possibility of genetic interchange between breeding colonies is suggested by sightings of three tagged females aged 4 years and older at non-natal colonies.
Establishing patterns of movements of free-ranging animals in marine ecosystems is crucial for a better understanding of their feeding ecology, life history traits and conservation. As central place foragers, the habitat use of nesting seabirds is heavily influenced by the resources available within their foraging range. We tested the prediction that during years with lower resource availability, short-tailed shearwaters (Puffinus tenuirostris) provisioning chicks should increase their foraging effort, by extending their foraging range and/or duration, both when foraging in neritic (short trips) and distant oceanic waters (long trips). Using both GPS and geolocation data-loggers, at-sea movements and habitat use were investigated over three breeding seasons (2012-14) at two colonies in southeastern Australia.
Most individuals performed daily short foraging trips over the study period and inter-annual variations observed in foraging parameters where mainly due to few individuals from Griffith Island, performing 2-day trips in 2014. When performing long foraging trips, this study showed that individuals from both colonies exploited similar zones in the Southern Ocean. The results of this study suggest that individuals could increase their foraging range while exploiting distant feeding zones, which could indicate that short-tailed shearwaters forage in Antarctic waters not only to maintain their body condition but may also do so to buffer against local environmental stochasticity. Lower breeding performances were associated with longer foraging trips to distant oceanic waters in 2013 and 2014 indicating they could mediate reductions in food availability around the breeding colonies by extending their foraging range in the Southern Ocean.
This study highlights the importance of foraging flexibility as a fundamental aspect of life history in coastal/pelagic marine central place foragers living in highly variable environments and how these foraging strategies are use to buffer this variability.
Breeding seabirds are extreme central-place foragers, Commuting long distances between colonies and feeding areas. Central-place foraging theory predicts that prey items close to the colony will be preferred over prey items distant from the colony, which can lead to prey depletion near the colony ("Storer-Ashniole's halo"). To investigate the relevance of these ideas to a single-prey loader, we equipped chick-rearing Thick-billed Murres (Uria lomvia) with time-depth recorders (1999-2007) and monitored prey deliveries (19932008). Because feeding rates were constant for chicks 3-15 days old, we restricted analyses to that age group. Between-date relationships were examined within individuals to avoid confounding effects of specialization and parental quality. The mass of prey items increased with foraging distance(0.5), which suggests that large prey items were depleted by foraging pressure. Foraging distance for pelagic species increased through the season in years without spawning. After accounting for bathymetry, foraging effort decreased with distance from the colony for benthic fish and, in years without spawning, for pelagic fish. Within each season, Thick-billed Murres "fished down the food web"; they began by feeding on large fish, progressed to medium-sized prey (small fish), and finished feeding on small prey (invertebrates). We concluded that pelagic species responded to seabird foraging pressure by moving away from the colony, creating a three-dimensional halo. Benthic species, meanwhile, were depleted from a nearby shelf but remained abundant at a more distant offshore bench. We Suggest that populations of central-place foragers are partially regulated by prey depletion. Received 5 December 2008, accepted 18 March 2009.
The long-nosed (or New Zealand) fur seal (Arctocephalus forsteri) breeds in southern Australia and NewZealand. Most of the Australian population is in South Australia, between Kangaroo Island and Eyre Peninsula. Fur seal populations in southern Australia were heavily exploited by colonial sealers between 1801 and 1830, resulting in major reductions. Numbers remained low for 150 years, then slowly built up and new colonies established across their presumed former range. Here we present estimates of pup abundance at South Australia colonies, mostly during the 2013–14 breeding season.
Long-nosed fur seals bred from Baudin Rocks in the south-east to Fenelon Island in the north-west. In total, 29 breeding colonies produced 20 431 pups, 3.6 times greater than the 1989–90 estimate; the increase is attributed to recovery from 19th century overharvesting. The 2013–14 pup estimate leads to an estimate of abundance of long-nosed fur seals in South Australia of 97 200. Most pups were on Kangaroo Island (49.6%) and the Neptune Islands (38.6%). New breeding colonies were identified on Williams Island and at two small sites on Kangaroo Island. The increasing trend in South Australia is likely to continue over the coming decade, primarily by expansion in colonies on Kangaroo Island and by establishment of new colonies.
The optimal allocation of time and energy between one's own survival and offspring survival is critical for iteroparous animals, but creates a conflict between what maximises the parent's fitness and what maximises fitness of the offspring. For central-place foragers, provisioning strategies may reflect this allocation, while the distance between central-places and foraging areas may influence the decision. Nevertheless, few studies have explored the link between life history and foraging in the context of resource allocation. Studying foraging behaviour alongside food load rates to chicks provides a useful system for understanding the foraging decisions made during parent-offspring conflict. Using simultaneously deployed GPS and time-depth recorders, we examined the provisioning strategies in free-living Manx shearwaters Puffinus puffinus, which were caring for young. Our results showed a bimodal pattern, where birds alternate short and long trips. Short trips were associated with higher feeding frequency and larger meals than long trips, suggesting that long trips were performed for self-feeding. Furthermore, most foraging was carried out within 100 km of sea fronts. A simple model based on patch quality and travel time shows that for Manx shearwaters combining chick feeding and self-maintenance, bimodal foraging trip durations optimise feeding rates.
Marine animals forage in areas that aggregate prey to maximize their energy intake. However, these foraging 'hot spots' experience environmental variability, which can substantially alter prey availability. To survive and reproduce animals need to modify their foraging in response to these prey shifts. By monitoring their inter-annual foraging behaviours, we can understand which environmental variables affect their foraging efficiency, and can assess how they respond to environmental variability. Here, we monitored the foraging behaviour and isotopic niche of little penguins (Eudyptula minor), over 3 years (2008, 2011, and 2012) of climatic and prey variability within Port Phillip Bay, Australia. During drought (2008), penguins foraged in close proximity to the Yarra River outlet on a predominantly anchovy-based diet. In periods of heavy rainfall, when water depth in the largest tributary into the bay (Yarra River) was high, the total distance travelled, maximum distance travelled, distance to core-range, and size of core- and home-ranges of penguins increased significantly. This larger foraging range was associated with broad dietary diversity and high reproductive success. These results suggest the increased foraging range and dietary diversity of penguins were a means to maximize resource acquisition rather than a strategy to overcome local depletions in prey. Our results demonstrate the significance of the Yarra River in structuring predator-prey interactions in this enclosed bay, as well as the flexible foraging strategies of penguins in response to environmental variability. This plasticity is central to the survival of this small-ranging, resident seabird species.
Within a heterogeneous environment, animals must efficiently locate and utilise foraging patches. One way animals can achieve this is by increasing residency times in areas where foraging success is highest (area-restricted search). For air-breathing diving predators, increased patch residency times can be achieved by altering both surface movements and diving patterns. The current study aimed to spatially identify the areas where female Australian fur seals allocated the most foraging effort, while simultaneously determining the behavioural changes that occur when they increase their foraging intensity. To achieve this, foraging behaviour was successfully recorded with a FastLoc GPS logger and dive behaviour recorder from 29 individual females provisioning pups. Females travelled an average of 118 ± 50 km from their colony during foraging trips that lasted 7.3 ± 3.4 days. Comparison of two methods for calculating foraging intensity (first-passage time and first-passage time modified to include diving behaviour) determined that, due to extended surface intervals where individuals did not travel, inclusion of diving behaviour into foraging analyses was important for this species. Foraging intensity 'hot spots' were found to exist in a mosaic of patches within the Bass Basin, primarily to the south-west of the colony. However, the composition of benthic habitat being targeted remains unclear. When increasing their foraging intensity, individuals tended to perform dives around 148 s or greater, with descent/ascent rates of approximately 1.9 m•s-1 or greater and reduced postdive durations. This suggests individuals were maximising their time within the benthic foraging zone. Furthermore, individuals increased tortuosity and decreased travel speeds while at the surface to maximise their time within a foraging location. These results suggest Australian fur seals will modify both surface movements and diving behaviour to maximise their time within a foraging patch.
Foraging site fidelity has profound consequences for individual fitness, population processes and the effectiveness of species conservation measures. Accordingly, quantifying site fidelity has become increasingly important in animal movement and habitat selection studies. To assess foraging site fidelity in king penguins (Aptenodytes patagonicus) breeding at the Falkland Islands (51.48°S, 57.83°W), we measured overlap in time spent in foraging areas (at a 0.1° × 0.1° grid resolution) between successive foraging trips and foraging route consistency during the crèche period. In total, 30 complete foraging trips from seven king penguins were recorded between April and October 2010. King penguins predominantly foraged on the highly productive Patago-nian slope, to the north of the Falkland Islands [median foraging trip distance 213 km (SD = 215 km) and dura-tion 12.8 days (SD = 14.7 days)]. Overlap in time spent in an area on consecutive foraging trips ranged between 2 and 73 % (mean 27 %, SD = 22 %). Bearing during the outbound portion of foraging trips was typically highly repeatable for individual birds, but foraging trip duration and distance were not. Travel during the outbound phase of foraging trips was consistent with the direction of the northward-flowing Falkland Current that may act as a directional cue or facilitate rapid transit to foraging areas. Flexibility in foraging trip distances and durations may be a response to changes in resource availability and changes in the energetic requirements of adults and chicks over an extended breeding cycle.
At-sea behaviour of central-place foraging fur seals and penguins in the Southern Ocean is understudied during the latter stages of parental care and the subsequent pre-moulting period. This biologically important period is costly to investigate due to the risk (or certainty) of losing tracking instruments when the animals moult. Early in this period, parents must meet the increasing demands of larger, more mobile offspring that are still nutritionally dependent and then the parents must recover lost body condition prior to the onset of their annual moult. This study reports late-season, at-sea movement patterns of macaroni penguins, chinstrap penguins and adult female Antarctic fur seals from the subantarctic island Bouvetøya, in relation to remotely-sensed oceanographic features. Foraging trips differing significantly in direction and distance travelled compared to those performed earlier in the breeding season, coincide with the time when offspring would be expected to become independent. On these trips, macaroni penguins moved towards the Polar Front while chinstrap penguins and Antarctic fur seals moved southward. Individuals from all three species appeared to target submesoscale ocean features once they were presumed to have been released from the constraints of feeding their young and were able to travel greater distances from the colony.
A study of climatological and conductivity-temperature-depth (CTD) data for 2004 is made to provide a conceptual model of upwelling for the eastern region of the Great Australian Bight. In particular, the data and other studies provide strong evidence that shelf break upwelling is confined to the southwest Kangaroo Island region and does not occur farther to the west off the Eyre Peninsula. Rather, the upwelled water is likely to remain in a Kangaroo Island "pool" until subsequent upwelling events draw the water to the shallower and surface coastal regions of the eastern Bight. In this manner the surface upwelling apparent off the Bonney Coast, Kangaroo Island, and the eastern Great Australian Bight (GAB) can appear to be simultaneous. Moreover, it appears likely that the water within the Kangaroo Island pool remains nutrient rich. Support for this model comes from CTD sections collected in 2004 that show that the upwelled signal (cool, <17°C; fresher, <35.6; dense, στ > 26 kg m-3) diminishes in width and intensity with increasing distance from Kangaroo Island. The pattern of fluorescence is similar to that for temperature in the upwelled plume and indicates that the Kangaroo Island pool remains nutrient rich. Relatively low oxygen concentrations may indicate a previous bloom. The warmest water is found near the shelf break along with very low values of fluorescence and relatively higher levels of oxygen suggesting nutrient-limited growth of phytoplankton. These data also support the notion that the upwelled nutrient-rich water is supplied from the Kangaroo Island pool and not by shelf break upwelling in the eastern GAB. Anomalously salty and fresh water is identified as resulting from evaporation in coastal bays and groundwater aquifer discharge.
1. Determining global position by light measurements (‘geolocation’) has revolutionised the methods used to
trackmigratory birds throughout their annual cycle.
2. To date, there is no standard way of analysing geolocator data, making communication of analyses cumbersome
and hampering the reproducibility of results.
3. We have, therefore, developed the R package GeoLight, which provides basic functions for all steps of determining global positioning and a new approach in analysingmovement pattern.
4. Here, we briefly introduce and discuss the major functions of this package using example movement data of European hoopoe (Upupa epops).
The southern coastline of Australia forms part of the worlds' only northern boundary current system. The Bonney Upwelling occurs every austral summer along the south-eastern South Australian coastline, a region that hosts over 80% of the worlds population of an endangered endemic otariid, the Australian sea lion. We present the first data on the movement characteristics and foraging behaviour of adult male Australian sea lions across their South Australian range. Synthesizing telemetric, oceanographic and isotopic datasets collected from seven individuals enabled us to characterise individual foraging behaviour over an approximate two year time period. Data suggested seasonal variability in stable carbon and nitrogen isotopes that could not be otherwise explained by changes in animal movement patterns. Similarly, animals did not change their foraging patterns despite fine-scale spatial and temporal variability of the upwelling event. Individual males tended to return to the same colony at which they were tagged and utilized the same at-sea regions for foraging irrespective of oceanographic conditions or time of year. Our study contrasts current general assumptions that male otariid life history strategies should result in greater dispersal, with adult male Australian sea lions displaying central place foraging behaviour similar to males of other otariid species in the region.
As central-place foragers, seabirds from colonies located close to multiple and/or productive marine habitats might experience increased foraging opportunities and enhanced resilience to food shortages. We tested whether this hypothesis might explain divergent trends in 3 populations of black-legged kittiwakes Rissa tridactyla, a surface-feeding piscivore, in the eastern Bering Sea. We simultaneously studied the foraging behavior, diet, nutritional stress, and breeding performance of chick-rearing kittiwakes from 2 continental shelf colonies (St. Paul and St. George) and an oceanic colony (Bogoslof). Although shelf-based forage fishes were rare or absent in bird diets during the cold study year, not all kittiwakes from the 3 colonies concentrated foraging along the productive shelf break habitats. Compared to the oceanic colony, birds from both shelf-located colonies had lower chick provisioning rates, higher levels of nutritional stress, and lower breeding performance. Although birds from both shelf-based colonies foraged in nearby neritic habitats during daytime, birds from St. George, a stable population located closest to the continental shelf break, also conducted long overnight trips to the ocean basin to feed on lipid-rich myctophids. In contrast, birds from St. Paul, a declining population located farthest from shelf break/oceanic habitats, fed exclusively over the shelf and obtained less high-energy food. Birds from Bogoslof, an increasing population, foraged mainly on myctophids close to the colony in the oceanic basin and Aleutian coast habitats. Our study suggests that proximity to multiple foraging habitats may explain divergent population trends among colonies of kittiwakes in the southeastern Bering Sea.
We investigate the impact of the Southern Annular Mode (SAM) on surface wind, sea surface temperature (SST), and surface chlorophyll concentration on intraseasonal to interannual timescales in the Southern Ocean using 8-day average satellite observations. Positive phases of the SAM are associated with enhanced westerly winds over the Antarctic Zone (AZ) and Polar Frontal Zone, driving increased equatorward Ekman transport and cold SST anomalies in these regions. Positive SAM is also associated with easterly wind and warm SST anomalies in the Subtropical Zone. South of the Antarctic Polar Front (APF), chlorophyll concentration anomalies are positively correlated with the SAM, however this correlation is negative north of the APF. We suggest that the positive correlation in the AZ is due to the increased supply of iron by upwelling, while the negative correlation north of the APF is caused by stronger light limitation as a consequence of deeper mixed layers.
Foraging movements of Adelie penguins are constrained both by environmental conditions (e.g. sea ice cover) and life history factors (e.g. regular offspring provisioning), We describe within season changes in foraging range, trip duration and body condition of Adelie penguins nesting at Bechervaise Island, East Antarctica, in the context of these constraints. Penguins were satellite tracked over multiple seasons during the incubation, guard, creche and pre-moult phases of their annual cycle. They ranged farthest during incubation when sea ice was extensive and shortest during the guard stage when chicks were small and sea ice limited in extent. Prior to their annual moult the birds foraged hundreds of kilometres to the west and east of their breeding sites. A recurrent polynya facilitated access to the sea early in the season when ice cover was extensive. Kernel analyses showed that penguins foraged most intensively close to the colony, along submarine canyons and at the continental shelf break. Increases in foraging range, as the chick rearing period progressed, were consistent with changing energy requirements of adults and chicks and likely intraspecific competition. Whilst provisioning their offspring, penguins adopted a variable combination of time minimising and food maximising foraging behaviour in which choice of foraging rule was determined largely by adult body condition.
The study examined the distribution of critical habitat for foraging by female Antarctic fur seals breeding at the island of South Georgia. Bathymetric features of the continental shelf around the island of South Georgia were an important indicator for the localisation of foraging. This pattern was consistent among years of different prey availability. Lactating females were constrained to forage mainly within 100 km of the location at which the offspring was being raised. When this constraint was removed at the end of lactation, females foraged to much greater ranges and dispersed to specific regions of the continental shelf east of Patagonia (>1000 km) and to the northern edge of the Antarctic pack ice (500 km). The empirical distribution of foraging during the breeding season was used to develop a function that described the foraging distribution for the whole breeding population of females. The result was consistent with past observations from ship-based surveys and it allowed estimation of the spatial impact of breeding female fur seals on krill at South Georgia. This suggested that, in extreme cases and assuming that krill influx is limited, female fur seals could eat most of the krill present in some regions where they forage intensively. However, mean consumption was about one-tenth of the mean density of krill.
This study investigates the foraging behaviour of lactating Subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island, Indian Ocean, in relation to sea-surface temperatures (SST) of the surrounding waters over 3 consecutive breeding years (1995 to 1997). Foraging habitat and diving activity were investigated using time depth temperature recorders (TDRs), deployed on 29 individuals during the first trip after parturition in December (n = 7), later in the summer (n = 13), and in winter (n = 9). Argos satellite transmitters (n = 4) and a direction recorder (n = 1) were also used in the 1995 and 1996 austral summers, respectively. Sea temperatures recorded by TDRs in conjunction with the IGOSS SST database were used as a locational cue to estimate the foraging range. The foraging habitat of lactating females was associated with the northern part of the Subtropical Front (STF) (axial SST = 14.2 degrees C), where their main prey, myctophid fish, are known to be abundant. The organisation of the foraging trips, in terms of diving activity, showed seasonal changes but remained similar among years. During the first trip after parturition, females foraged within the STF 60 to 130 km from the colony, and exhibited a diving activity that did not vary significantly throughout the trip. Later in summer, when STF was south of Amsterdam Island, most females travelled in a straight south-east direction without diving (suggesting that they travelled regularly to reach a known area). They then concentrated their diving activity in the middle of the STF during 50% of the foraging trip duration, and continued diving on the return trip to the colony. In winter, the 14 degrees C surface isotherm was 250 km north of Amsterdam Island and SST gradient was very low. Females increased their foraging range up to 530 km, and there was no evidence for females concentrating their diving activity within a given area, suggesting that they did not encounter dense patches of prey. In winter, seals also increased their diving effort probably in response to a decrease in food availability. Annual changes in SST surrounding Amsterdam Island, and in the surface temperature gradient of the STF appear to affect the time spent at sea, and the relative diving activity throughout the trips in summer. These results suggest that Subantarctic fur seals adjust their foraging behaviour according to both seasonal and annual changes in oceanographic conditions, and thus probably, food availability.
Measured foraging strategies often cluster around values that maximize the ratio of energy gained over energy spent while foraging (efficiency), rather than values that would maximize the long-term net rate of energy gain (rate). The reasons for this are not understood. This paper focuses on time and energy constraints while foraging to illustrate the relationship between efficiency and rate-maximizing strategies and develops models that provide a simple framework to analyze foraging strategies in two distinct foraging contexts. We assume that while capturing and ingesting food for their own use (which we term feeding), foragers behave so as to maximize the total net daily energetic gain. When gathering food for others or for storage (which we term provisioning), we assume that foragers behave so as to maximize the total daily delivery, subject to meeting their own energetic requirements. In feeding contexts, the behavior maximizing total net daily gain also maximizes efficiency when daily intake is limited by the assimilation capacity. In contrast, when time available to forage sets the limit to gross intake, the behavior maximizing total net daily gain also maximizes rate. In provisioning contexts, when daily delivery is constrained by the energy needed to power self-feeding, maximizing efficiency ensures the highest total daily delivery. When time needed to recoup energetic expenditure limits total delivery, a low self-feeding rate relative,to the rate of energy expenditure favors efficient strategies. However, as the rate of self-feeding increases, foraging behavior deviates from efficiency maximization in the direction predicted by rate maximization. Experimental manipulations of the rate of self-feeding in provisioning contexts could be a powerful tool to explore the relationship between rate and efficiency-maximizing behavior.
The distribution and availability of marine resources are directly affected by seasonal spatial changes in physical and oceanographic features. They are thus likely to influence maternal foraging provisioning patterns, efficiency, and subsequent pup growth rate of central place foragers such as otariid seals. While previous studies have documented foraging locations and diving activity of female otariids in relation to oceanographic features, few have focused on species characterised by a long pup-rearing period. The present study investigated seasonal changes in foraging parameters in relation to the following oceanographic features and maternal characteristics: foraging grounds (using satellite tags, geolocation, GIS and kernel estimation techniques), at-sea activity budget (using time-depth recorders), and the foraging success and diet of female subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island (southern Indian Ocean). The seals mainly exploited the subtropical front, but also exhibited large differences in seasonal distribution, from short trips in restricted foraging areas during summer to widely distributed foraging grounds during the winter. This is consistent with a seasonal shift in diet and an increasing proportion of time being dedicated to diving and resting. This increase in foraging trip duration throughout the season paralleled decreasing rates in maternal mass gain and pup growth, suggesting a decrease in food availability. During the summer, maternal mass gain rate and pup growth were related to foraging tactics (percent of time spent diving and in the core foraging area), whereas maternal characteristics such as body length were more important during the latter months of the pup-rearing period. However, no influence of age and thus no apparent advantage of female experience was detected in this study.
Maternal attendance, nursing behaviour, pup growth and weaning age were investigated in the New Zealand fur seal (Arctocephalus forsteri) at Kangaroo Island, South Australia. Foraging trips to sea increased throughout lactation (5.9-9.8 days), while the duration of shore attendance bouts (1.7 +/- 0.7 days) remained unchanged, resulting in time ashore declining from 24% to 14% throughout lactation. Mothers with sons made longer foraging trips than mothers with daughters throughout lactation and, as a consequence, maternal time ashore was less for sons (21%) than for daughters (28%). Pups maintained an overall sucking-rate of 1.4 h day(-1) throughout their mothers' lactation, by sucking more frequently and for longer periods as their mother became less available. Because mothers with daughters were ashore more frequently, the overall sucking-rate of females (1.7 h day(-1)) was greater than that of male pups (1.1 h day(-1)). The growth patterns of male and female pups from two cohorts were similar, but males grew faster and weaned heavier in one of the years. Age at weaning was similar to 285 days (9.4 months). This study indicates some significant differences in the nature of maternal expenditure in male and female pups, and the growth strategies used by pups both within and between years.
We studied the foraging behaviour of lactating female, adult male and juvenile New Zealand (NZ) fur seals to compare and contrast their foraging strategies and assess the degree of spa- tial separation of their foraging habitats. Adult male fur seals are longer and heavier than lactating females, which are longer and heavier than juveniles. Trip duration was positively correlated with the distance travelled by all age/sex groups. Juveniles conducted longer trips and travelled further from the colony than males. Both juveniles and males conducted longer trips and travelled further than females, which made brief trips because they were provisioning pups. There were no seasonal differ- ences in the behaviour of males, but females and juveniles foraged closer to the colony in summer when they were moulting and females had younger pups. Behavioural differences were recorded between lactating female, male and juvenile seals in the directional bearing from the colony, the dis- tance travelled, the minimum size of the area that was potentially visited and the horizontal swim speed. Intra-specific foraging competition among these age/sex groups was minimal because lactat- ing females typically used continental shelf waters and males utilised deeper waters over the shelf break, adjacent to female foraging grounds. Furthermore, juveniles used pelagic waters, up to 1000 km south of the habitats used by adults. Differences in the habitats used by females, males and juveniles were also apparent in the seafloor gradient, the SST and the surface chl a concentration, with females using regions with the highest chl a concentrations. Results from this study suggest that smaller seals cannot efficiently utilise prey in the same habitats as larger seals because smaller seals do not have the capacity to spend enough time underwater at the greater depths.
Maternal attendance patterns in free-ranging wildlife can provide insight into basic biology, foraging behavior, and population dynamics. We collected detailed visual observations of attendance patterns by adult lactating Steller sea lions (Eumetopias jubatus) from 2005 to 2007 on 6 major rookeries in the Russian Far East, including those with an increasing population trend (Sea of Okhotsk), severely depleted populations that were recovering (Kuril Islands), or those that were stable (Commander Islands). Individually identifiable females were observed during the postpartum period, with special attention paid to presence and absence during the day and to departure and arrival times. Within Russia females on several Kuril Islands rookeries exhibited extremely short foraging trip durations (median 6.5–8.0 h) and spent higher proportions of time on the rookery (75–82%), whereas females in the Sea of Okhotsk population had the longest trips (median 19.8 h) and spent the least amount of time on the rookery (60%). Most indices of attendance pattern were more favorable (longer peripartum period, higher proportion of time spent on rookery, shorter trips, and longer visits) than those reported in Alaska and much more so than those in California during El Niño years, where the proportion of time spent on the rookery was nearly half that in Russia. Females >6 years of age had shorter trips and longer visit durations than the youngest females (4–5 years), and older mothers exhibited significantly longer periods of nursing before taking their 1st trip, suggesting greater physical condition for older females. Although trip durations remained constant throughout the season, visit durations shortened significantly as the pups aged, suggesting that increasing nutritional demands of pups are met by more frequent, rather than longer, trips. No apparent relationships between attendance patterns and population status were observed; however, many of the differences in attendance patterns throughout the range of the Steller sea lion could be related to local variation in bathymetry and diet patterns.
During the last 30 years, at-sea studies of seabirds and marine mammals in the oceans south of the Subtropical Front have described an association with major frontal areas. More recently, the advancement in microtechnology has allowed the tracking of individuals and investigations into how these marine predators actually use the frontal zones. In this review, we examine 1) the relative importance to apex predators of the different frontal zones in terms of spatial distribution and carbon flux; 2) the processes that determine their preferential use; and 3) how the mesoscale dynamics of frontal structures drive at-sea foraging strategies of these predators. We review published results from southern waters and place them in a broader context with respect to what has been learned about the importance of fronts in oceans farther north.
The ability of an animal to acquire energy will affect its allocation to offspring and will ultimately influence fitness. This study investigated the relative influence of maternal traits, seasonal demands of pup-rearing, and individual effects on the foraging behavior of 27 female Antarctic fur seals Arctocephalus gazella at Cape Shirreff, Livingston Island, Antarctica during 2 austral summers (2005 and 2006), using time depth recorders. The relationship between foraging and allocation was investigated using pup growth as an indicator of parental investment. While female diving behavior differed between years, trip duration was below the 10 yr mean and did not differ between the 2 years of the study, indicating favorable feeding conditions in both years. Study year and maternal age accounted for a significant amount of the variation in day and night dive characteristics, while maternal mass influenced only parameters related to night dive effort. As the season progressed, females increased their dive effort at night, made shorter daytime dives, with more bout diving. Individual variability accounted for a significant amount of the variation in all foraging parameters. Females could be assigned to one of 4 behavioral dive groups (high effort, low effort, intermediate effort with high dive rate, and intermediate effort), based on 13 dive parameters although year instrumented and age appeared to be important in determining group affiliation. Age, mass, year, and trip number influenced diving behavior; however, there was no relationship between foraging behavior and pup growth rate, except in young females.
A detailed high resolution survey of a small region (68
68 km) of the Subtropical Front south of Australia over a period of 14 days is
used to study the interaction between the mixed layer and the permanent frontal
structure underneath during summer conditions. The front extends through the
mixed layer as a salinity front, while its temperature structure is modified by
seasonal warming. Wind-driven movement of the mixed layer combines with the
short-time development of indentations and filaments in the front to produce
some degree of decoupling between the mixed layer and the underlying structure,
and the front is not always found at the same location in and below the mixed
layer. Intrusions and parcels of distinct water properties are found just below
the mixed layer, produced as a result of the relative movement of the front in
and below the mixed layer. These parcels are typically 10 km in width and 10–50
m in depth. Successive surveys of the front with a time separation of 2 days
showed that these features persist over at least 1 week. Large scale surveys of
the front show that parcels are ubiquitous along the Subtropical Front over a
distance of several hundred kilometres. The results suggest that any study
aimed at understanding the intricate interaction between the mixed layer and
the layers below in oceanic fronts will have to address wind-driven dynamics
and frontal dynamics together.
Adlie (Pygoscelis adeliae) and chinstrap penguins (P. antarctica) are morphologically and ecologically very similar, have very similar diet and breed sympatrically in the Scotia Arc from the South Sandwich Islands to the Antarctic Peninsula. To investigate how these two species co-exist, their foraging distribution and diet were studied during the chick-rearing period at Signy Island, South Orkney Islands, during the breeding seasons of 2000 and 2001. Satellite tracking data from of 19 Adlie penguins and 24 chinstrap penguins were used to compare foraging distributions. In both years the diet of both species was exclusively Antarctic krill (Euphausia superba) of the same size range. In a year of low prey availability (2000), there was a statistically significant segregation of foraging areas between the two species, however, in a year of normal resource availability (2001) there was no such segregation. There was a significant difference in the foraging areas used by Adlie penguins between years but not for chinstrap penguins. Adlie penguins foraged significantly farther (mean 100 km) from the colony than chinstrap penguins (mean 58 km) in 2000 but not in 2001 (mean 58 km and 35 km respectively). In 2000, the breeding success of Adlie penguins was 51% lower than the long-term mean compared to 15% lower in chinstrap penguins. Both species achieved above average breeding success in 2001. The changes in foraging distribution and breeding success suggest that in years of low resource availability, chinstrap penguins may be able to competitively exclude Adlie penguins from potential inshore foraging areas. Current trends in climatic change and possible effects on ice distribution and krill abundance suggest that conditions could become less favourable for Adlie penguins than chinstrap penguins in areas where both species occur.
Dive duration generally decreases with dive depth in air-breathing vertebrates. In most penguin species, this occurs due to increasing transit (descent and ascent) durations. However, in U-shaped dives, this is also because the duration of the bottom phase of the dive increases with increasing depth. We considered the time-based efficiency with which birds can use different diving depths by using depth recorders on 9 foraging emperor penguins Aptenodytes forsteri, studied during the early and late chick-rearing period in Adelie Land, Antarctica. Dive and post-dive duration positively correlated with diving depth, but particularly long recovery durations were apparent for dives longer than 456 s. This inflection point (i.e. 456 s) corresponded to a theorized maximum rate of gain of prey per dive cycle. By using the number of undulations in the bottom phase of the dive as a proxy for prey capture success, we conclude that the most lucrative dive depths for the birds studied were between 50 and 225 m. Since these depths were also those most often visited, we think that foraging emperor penguins focus on depths where profitability is highest.
Maternal allocation to growth of the pup was measured in Antarctic fur seals (Arctocephalus gazella )a t the Kerguèlen Islands during the 1997 austral summer. Absolute mass gain of pups following a maternal foraging trip was independent of the sex of the pup but was positively related to foraging-trip duration and maternal length. However, daily mass gain (i.e., absolute mass gain of the pup divided by foraging-trip duration) decreased with increasing forag - ing-trip duration but increased with maternal length. While the pup were fasting, their daily mass loss was related to their sex and initial body mass: both heavier pups and female pups lost more mass per day than lighter pups and male pups. The mass-specific rate of mass loss was significantly higher in female than in male pups. Over the study period, the mean growth rate was zero, with no difference between female and male pups. The growth rate in mass of the pup was positively related to maternal length but not to maternal condition, and negatively related to the foraging-trip dura - tion of the mother and the initial mass of the pup. This indicates that during the study period, heavier pups grew more slowly because of their higher rate of daily mass loss during periods of fasting. Interestingly, for a given maternal length, the mean mass of the pup during the study period was higher for male than for female pups, even though the rate of daily mass gain was the same. Such differences are likely to result from sex differences in the mass-specific rate of mass loss. As female pups lose a greater proportion of their mass per day, a zero growth rate (i.e., mass gain only compensating for mass loss) is reached at a lower mass in female pups than in male pups. Our results indicate that maternal allocation does not differ according to the sex of the pup, but suggest that the two sexes follow different growth strategies. 483
The large temporal and spatial variability in marine productivity encountered by marine predators may negatively influence breeding success. The Antarctic fur seal Arctocephalus gazella (AFS), a marine predator in the Southern Ocean (SO) ecosystem with a circumpolar distribution, exhibits a short, 4 mo lactation coinciding with increased summer marine productivity. The diet of AFS, and the distance to significant and productive oceanographic features, such as the Antarctic Polar Frontal Zone (PFZ), varies considerably between populations. We studied the foraging activity, foraging efficiency and the pup provisioning strategies of lactating AFS at a key breeding site in the southern Indian Ocean, the Kerguelen Archipelago. Foraging parameters were examined in relation to interannual variability in oceanographic conditions and prey availability in the PFZ over 3 consecutive breeding seasons (1998 to 2000). The location of foraging zones, diving activity, diet and foraging efficiency varied significantly between years, concurrently with annual changes in sea- surface temperature (SST) and prey availability. The strongest recorded El Niño Southern Oscillation event in 1997-1998 coincided with anomalously warm waters in the vicinity of the Archipelago. Deeper diving by females, reduced maternal and pup body condition, and minimal pup growth rates and low catch per unit effort of the primary prey species, lanternfishes (Myctophidae) were all recorded in this period. Maternal size was positively related to the growth performance of pups only in this period, indicating the importance of age/size and/or experience in mediating environmental fluctuations. Foraging efficiency over a foraging cycle and variability in mean provisioning rates (trip duration), were identified as proxies of prey availability within the foraging range of seals, emphasis- ing the effectiveness of the use of AFS foraging behaviour as an indicator of both food and oceano- graphic variability and climatic anomalies. The increasing frequency of anomalously warm SST events in sectors of the SO, however, may elicit specific behavioural responses from 'central place foragers' (i.e. species that return to breeding sites to feed their young) to avoid sustained poor body condition of females and their weaned offspring.
Mixing and primary productivity was examined in upwelling influenced nearshore waters off south western Eyre Peninsula (SWEP) in the eastern Great Australian Bight (EGAB), the economically and ecologically important shelf region off southern Australia that forms part of the Southern and Indian oceans. Mixing/stratification in the region was highly temporally variable with a unique upwelling circulation in summer/autumn (November–April), and downwelling through winter/spring (May–September). Highest productivity was associated with upwelled/stratified water (up to 2958 mg C m−2 d−1), with low productivity during periods of downwelling and mixing (∼300–550 mg C m−2 d−1), yet no major variations in macro-nutrient concentrations were detected between upwelling and downwelling events (silica > 1 μmol L−1, nitrate/nitrite > 0.4 μmol L−1, phosphate > 0.1 μmol L−1). We hypothesise that upwelling enriches the region with micro-nutrients. High productivity off SWEP appears to be driven by a shallowing of mixed layer depth due to the injection of upwelled waters above Zcr. Low productivity follows the suppression of enrichment during downwelling/mixing events, and is exacerbated in winter/spring by low irradiances and short daylengths.
Long-lived organisms maximise lifetime reproductive output by shifting the negative effects of temporal environmental variability onto their offspring while maintaining adult survival. Amongst seabirds, chick growth and survival could be influenced by a range of factors, including prey delivery and parent behaviour as well as predation and exposure to adverse weather conditions. In this study, an automated method of recording adult Cape gannet Morus capensis time-activity budgets by means of new generation VHF technology was used to relate their behaviour to chick growth and survival at Bird Island, Algoa Bay, South Africa, across 2 breeding seasons (2011−2012 and 2012−2013). Using one of the most comprehensive datasets of breeding seabird foraging effort, we showed that, although chick growth was predominantly associated with age, parents which made shorter foraging trips clearly raised chicks which grew faster. Chick survival varied greatly between the 2 seasons (40 versus 97%), which was explained by different levels of nest non-attendance by parents. Chicks in the 2011−2012 season which were exposed to longer periods of non-attendance were particularly vulnerable to unseasonal storms and they were also more vulnerable to kelp gull Larus dominicanus predation while young. In contrast to Cape gannets in the declining west coast populations, chick survival at Bird Island was primarily related to parental protection rather than food provisioning rate, which probably reflects favourable foraging conditions along the southern coast of South Africa. This study highlights the importance of understanding the interplay between factors affecting chick growth and survival for the effective conservation of threatened seabird populations.
[1] Nonlinear mesoscale eddies can influence biogeochemical cycles in the upper ocean through vertical and horizontal advection of nutrients and marine organisms. The relative importance of these two processes depends on the polarity of an eddy (cyclones versus anticyclones) and the initial biological conditions of the fluid trapped in the core of the eddy at the time of formation. Eddies originating in the eastern South Indian Ocean are unique in that anticyclones, typically associated with downwelling, contain elevated levels of chlorophyll-a, enhanced primary production and phytoplankton communities generally associated with nutrient-replete environments. From analysis of 9 years of concurrent satellite measurements of sea surface height, chlorophyll, phytoplankton carbon, and surface stress, we present observations that suggest eddy-induced Ekman upwelling as a mechanism that is at least partly responsible for sustaining positive phytoplankton anomalies in anticyclones of the South Indian Ocean. The biological response to this eddy-induced Ekman upwelling is evident only during the Austral winter. During the Austral summer, the biological response to eddy-induced Ekman pumping occurs deep in the euphotic zone, beyond the reach of satellite observations of ocean color.
1. Foraging behaviour of Antarctic fur seals rearing pups at Bird Island, South Georgia, was assessed using at-sea activity patterns measured by electronic time-depth recorders. Information was obtained for a total of 75 individuals and 191 foraging trips to sea over five reproductive seasons from 1988/89 to 1992/93; this included one season (1990/91) of low prey abundance. A method was developed to divide the diving record up into logical units or bouts which differed from past methods used for defining bouts of behaviour.
2. Foraging trips were significantly longer in 1990/91 than in the other years. There were significant differences between years in the proportion of time spent foraging when at sea and in the distribution of foraging through the day and night. These differences probably represent behavioural responses to changes in prey distribution and abundance and were reflected in the frequency of occurrence of different types of foraging behaviour.
3. Four types of foraging bout were recognized using a cluster analysis. Type I (short) bouts were of short duration (17 min) and occurred mainly during daytime and at dusk. They probably represented exploratory behaviour. Type II (long) bouts occurred mainly at night and were of long duration (80 min). They increased in frequency in 1990/91 when food was scarce and 61-73% of time spent foraging was in these bouts. Type III (shallow) bouts occurred mainly at night, were of short duration (12 min) and represented feeding close to the surface, possibly in association with other, surface-feeding krill predators. Shallow bouts accounted for 8-14% of time spent foraging. Type IV (deep) bouts were of medium duration (19 min) and represented feeding at greater depth (40-50 m) than other bout types. They were most abundant around dawn.
4. Mean dive duration during bouts exceeded the theoretical aerobic dive limit on > 30% of occasions for short, long and deep bouts. There were positive correlations between mean dive duration and surface interval duration for most of these bout types in most years. This suggested that long dives incurred a cost in terms of the amount of time spent at the surface between dives.
5. The study demonstrated that female fur seals invest a significantly greater effort in foraging during periods of low prey abundance by both increasing the time spent foraging and by increasing activity during foraging. This could represent a 30-50% increase in the costs of foraging during years of low food abundance.
The composition, distribution, abundance, biomass and size-structure of macroplankton/micronekton communities at the Subtropical Convergence (STC) and the Antarctic Polar Front (APF) regions were investigated during the South African Antarctic Marine Ecosystem Study (SAAMES II, January-February 1993; SAAMES III, June-July 1993). A total of 115 and 32 macroplankton/micronekton species were found in the epipelagic zones of the STC and APF, respectively. Cluster analysis based on species composition indicated the occurrence of 3 different plankton communities: one in the STC region, another to the north of the APF and the third to the south of the APF. Although the APF and the STC were investigated in different seasons, average abundance and biomass were similar in both regions. Tunicates, euphausiids, decapods and myctophiid fishes dominated the total stock of the 2 frontal regions, in terms of both abundance and biomass. Both fronts exhibited considerable fluctuations in the abundance and biomass levels which appeared to covary with the spatial distribution of the phytoplankton stock in the area. The dominant size-classes were composed of a small group of 5 to 30 mm siphonophores, tunicates and euphausiids and a larger group of 40 to 80 mm euphausiids, chaetognaths, vertically-migrating decapods and myctophiid fishes.
We present the first data on attendance patterns, at-sea movements and diving behaviour of Antarctic fur seals breeding at Bouvetøya (Bouvet Island), Southern Ocean. While other colonies have been extensively studied, this remote and second largest global population remains relatively unknown. Time depth recorders and satellite relay data loggers were deployed on breeding females during the 2000–2001 and 2001–2002 summers. Attendance and foraging patterns were similar to those observed at colonies in the Scotia Sea region where Antarctic krill Euphausia superba is the predominant prey. Early to mid-lactation trips ranged within ~100 km of the island, usually towards the west; the dominant direction shifted later in the season and the range also increased markedly to a peak between early February and early March. Solar elevation influenced arrivals and departures from the island, with most departures occurring around sunset. Diurnal variations in diving behaviour were consistent with the vertical migration of krill. Diving frequency was higher at night and diving effort peaked around morning twilight. Afternoon deep diving was common, suggesting that females might target dense daytime krill aggregations between the photic zone and the thermocline. Trip
durations increased throughout early to mid-lactation, peaking in late January to early March, before again decreasing towards the end of lactation. Our results illustrate the substantial variability, both between individuals and within individuals over time, that is likely to reflect variations in prey distribution and in the growth requirements of pups. Such variations need to be taken into account when estimating habitat use and resource utilisation in marine top predators.
A bimodal foraging strategy has previously been described for procellariiform seabird species and is thought to have evolved in response to local resource availability being too low for adult birds to meet chick requirements and simultaneously maintain their own body condition. Here, we examine the dual foraging trip pattern of an alcid, the little auk Alle alle, at five colonies with contrasting oceanographic conditions. In spite of large variation in local conditions, little auks at all colonies showed the same general pattern of alternating a single long-trip with several consecutive short-trips. However, we found that the foraging pattern was flexible and could be adjusted at three levels: (1) the length of long-trips, (2) the frequency of short-trips, and (3) the total time spent foraging. Birds facing unfavorable conditions increased the duration of long-trips and reduced the number of short-trips. These adjustments resulted in reduced provisioning rates of chicks despite the fact that birds also increased the time allocated to foraging. Travel times during foraging trips were positively correlated to the total duration of the trip suggesting that differences in trip length among colonies were partly driven by variation in the distance to foraging areas. Most birds spent substantially more time traveling during long compared to short-trips, indicating that they accessed distant foraging areas during long-trips but remained close to the colony during short-trips. However, the difference in travel times was small at the site with the most favorable conditions suggesting that bimodal foraging in the little auk may be independent of the existence of high-quality areas at distance from the breeding ground.
This study investigated how female Antarctic fur seals adapt their foraging behavior, over time scales of days, to spatial unpredictability in the distribution of their food. Lactating Antarctic fur seals are central-place foragers that feed on highly patchy but spatially and temporally dynamic food. We measured the foraging distribution of 28 fur seals to test whether variation in foraging trip durations was reflected in variation in the location of foraging and the diving behavior of seals at sea. Based on the maximum distance travelled from the breeding beach, three categories of foraging trips were denned: those to the continental shelf area (n= 12, median = 71 km), to oceanic water (n= 11, median =164 km), and to farther offshore oceanic waters (n= 5, median = 260 km). Trip duration and mean surface speed were positively correlated with the maximum distance travelled from the breeding beach. Seals on longer trips spent proportionally less of their time submerged, but there was no significant difference in the total number of dives or the total time spent foraging by seals in relation to trip duration. Evidence from this study and previous work investigating energy gain suggests that an animal on a longer foraging trip could potentially have a higher mean energy return per dive than a similar animal on a shorter foraging trip. Evidence presented suggests that the type of foraging trip (near or far) is not predetermined by the animal but may be a simple response to the stochastic distribution of the resources available.
We studied the influence of sex of pup, maternal age, birth date of pup, number of foraging trips, and the mean duration of foraging trips at sea and nursing visits ashore on the growth and mass at weaning of pups of Antarctic fur seals (Arctocephalus gazella) during austral summers of 1988-1990. Although growth and mass at weaning were highly correlated, they were not related to maternal characteristics in 1988 or 1989. However, in 1990 there was a negative relationship between growth of pups and mean duration of foraging trips. Growth rates of males and females varied considerably between 1972 and 1991 and appeared to decline from 1984 through 1990. Methods used to collect and weigh the pups influenced the nature and magnitude of sex differences in estimated growth rates. Growth rates of male and female pups did not differ when weighed serially (same individuals weighed throughout lactation), but males grew faster than females when weighed cross-sectionally (different individuals weighed throughout lactation). Based on our results of pairs of mothers and pups followed over the lactation period, maternal investment was greater in males than females because males were heavier at birth and older at weaning than females and not because of any differential growth between the sexes. Mothers appear to have to work longer, but not harder, to wean males than females. Under the favorable feeding conditions that usually exist, individual differences in the growth of pups are most likely influenced by variation in foraging efficiency of mothers.
The onshore and at-sea cycles of females, suckling behavior of pups and their milk intake were studied in Steller sea lions (Eumetopias jubatus) during 1983 at Año Nuevo Island, California. Females averaged approximately 21 h ashore and 36 h at sea. The trips to sea lengthened as pups aged, resulting in an overall decline in female time ashore to 30% by the sixth week following parturition. Activity budgets of pups showed no significant differences among suckling time, age and sex. Milk intake, estimated using labeled water studies, revealed that heavier pups consumed more milk than lighter ones (milk ingestion in ml/d = 4.26 + 0.0687 [Pup Mass in kg]). Mean milk intake was 1.78 ± 0.33 liters/d. Mean pup growth rate was 0.38 ± 0.1 kg/d. The results suggest that female attendance patterns are shaped by the increasing nutritional demands of growing pups and their increasing efficiency at suckling.
We tested the prediction that lactating fur seals (Arctocephalus gazella) at South Georgia will take prey of greater energy density with increasing distance of foraging from the colony. The study
investigated the differences in diet of fur seals foraging within two regions, one near the breeding colony and the other
at greater distance. Diet varied significantly in relation to foraging location. Dietary items of low quality were eaten in
both regions but more food items with a high-energy content appeared in the diet of seals travelling to distant oceanic waters.
We conclude that there is likely to be a trade-off between energy gain and distance travelled which enables female fur seals
to maintain a relatively constant rate of energy delivery to their offspring irrespective of the distance travelled to find
food.
A review is presented of the ocean circulation along Australia’s southern shelves and slope. Uniquely, the long, zonal shelf is subject to an equatorward Sverdrup transport that gives rise to the Flinders Current – a small sister to the world’s major Western Boundary Currents. The Flinders Current is strongest near the 600 m isobath where the current speeds can reach 20 cm/s and the bottom boundary layer is upwelling favourable. It is larger in the west but likely intermittent in both space and time due to possibly opposing winds, thermohaline circulation and mesoscale eddies. The Flinders Current may be important to deep upwelling within the ubiquitous canyons of the region.