Environmental influences on annual migrations of juvenile southern bluefin tuna (Thunnus maccoyii)

Understanding the movement of animals in time and space, and its implications for the abundance and distribution of populations, is a pivotal problem in ecology. Animal migration is often interpreted as a response to environmental heterogeneity, particularly in dynamic ocean environments where prey resources tend to be patchily distributed. In juvenile animals, since migration is not associated with travel to breeding sites, movement is expected to be more tightly coupled to food resources. This study is concerned with the migratory patterns of the juvenile animals of a large, predatory, widely distributed temperate marine species, the southern bluefin tuna (Thunnus maccoyii, hereafter SBT). The long-distance migrations of this predator are investigated directly within the oceanographic context, with environmental influences on movement and behaviour determined through the integration, analysis and interpretation of telemetry-based and oceanographic data. In this thesis I present data and analyses for: (1) Seasonal ocean processes — remotely sensed ocean data were used to identify oceanographic features, and their cycle of development and/or productivity, that may provide important seasonal feeding habitats. (2) Plasticity in vertical behaviour — oceanographic habitats were characterised on the basis of water column structure, using temperature-at-depth data from archival tags, and vertical movements of SBT examined in response to habitat type and other factors. (3) Feeding and foraging ecology — temporal feeding patterns were determined from visceral warming patterns and used to evaluate the relationship between feeding success and time spent in an area. (4) Factors predictive of feeding success — were investigated using an integration of telemetry, environmental data and statistical modelling techniques. General discussion — the integration of biological and oceanographic data provide a significant advancement to our current knowledge on movement, habitat use and foraging ecology in migratory marine animals, and an increased appreciation for the diversity and complexity of biological phenomena. In particular, the ability to detect feeding events provided critical, and sometimes unexpected, insights into the motivations for the observed movements and behaviours, challenging some existing ecological concepts

Stomach content analysis of mesopelagic fish from the southern Kerguelen Axis

Mesopelagic fish represent an important trophic link between zooplankton and higher order predators in Southern Ocean food webs. Information on their feeding habits is still sparse, representing a key area of uncertainty in efforts to understand and model Southern Ocean food web dynamics. We used visual assessment of stomach contents to characterise the diets of three myctophids (Electrona antarctica, Gymnoscopelus braueri, Krefftichthys anderssoni) and one bathylagid (Bathylagus antarcticus) over the southern extension of the Kerguelen Plateau (‘southern Kerguelen Axis’), a highly productive area of both biological and economic importance in the Indian sector of the Southern Ocean. Diets of all four species were dominated by euphausiids, amphipods, copepods and fish. Bathylagus antarcticus also preyed upon a high proportion of soft-bodied organisms. There was strong evidence for dietary variability both within and between species, and this variability was driven by latitudinal variation in zooplankton assemblages. Size-based shifts in diet were apparent, with larger individuals of myctophid and bathylagid species consuming larger prey. Linear mixed effects models also demonstrated that the weight of prey consumed increased in relation to predator weight. Dietary information presented here advances our understanding of the mesopelagic components of Southern Ocean food webs, which will improve the development of food web models for the region

Horizontal‑vertical movement relationships: Adélie penguins forage continuously throughout provisioning trips

BackgroundDiving marine predators forage in a three-dimensional environment, adjusting their horizontal and vertical movement behaviour in response to environmental conditions and the spatial distribution of prey. Expectations regarding horizontal-vertical movements are derived from optimal foraging theories, however, inconsistent empirical findings across a range of taxa suggests these behavioural assumptions are not universally applicable.MethodsHere, we examined how changes in horizontal movement trajectories corresponded with diving behaviour and marine environmental conditions for a ubiquitous Southern Ocean predator, the Adélie penguin. Integrating extensive telemetry-based movement and environmental datasets for chick-rearing Adélie penguins at Béchervaise Island, we tested the relationships between horizontal move persistence (continuous scale indicating low [‘resident’] to high [‘directed’] movement autocorrelation), vertical dive effort and environmental variables.ResultsPenguins dived continuously over the course of their foraging trips and lower horizontal move persistence corresponded with less intense foraging activity, likely indicative of resting behaviour. This challenges the traditional interpretation of horizontal-vertical movement relationships based on optimal foraging models, which assumes increased residency within an area translates to increased foraging activity. Movement was also influenced by different environmental conditions during the two stages of chick-rearing: guard and crèche. These differences highlight the strong seasonality of foraging habitat for chick-rearing Adélie penguins at Béchervaise Island.ConclusionsOur findings advance our understanding of the foraging behaviour for this marine predator and demonstrates the importance of integrating spatial location and behavioural data before inferring habitat use

Environmental drivers of foraging behaviour during long-distance foraging trips of male Antarctic fur seals

Animals may use long-distance foraging trips to capitalize on spatiotemporal variation in food availability, allowing individuals to maximize resource gain from foraging effort. This is particularly important for dimorphic species with polygynous mating where males face strong selection pressures to attain large size and access to reproductive females. We tracked 17 male Antarctic fur seals, Arctocephalus gazella, during their prolonged postbreeding trips and assessed links between their movements and environmental predictors of profitable feeding areas. Males made one of two types of trips: a long trip to the Antarctic ice edge or shorter trips to areas where the southern Antarctic Circumpolar Current fronts generate high biological activity. The trip type was not determined by body size but was related to departure date from the breeding area, suggesting that males must trade off opportunities at the breeding area (reproductive, social interactions) and foraging opportunities between breeding seasons. Regardless of trip structure, males focused search effort far from foraging areas of central-place foraging seabirds and seals including female Antarctic fur seals provisioning offspring. Males showed clear spatiotemporal patterns in dive behaviour, with deep dives in shelf waters during the day and predominantly shallower dives in pelagic waters at night. Diel dive patterns showed monthly changes in photoperiod and lunar phase, consistent with feeding on vertically migrating prey. However, males did not use area-restricted search to focus dive effort, instead performing a mix of foraging and nonforaging behaviour within and between restricted search areas. We discuss the scale and type of inference that can be made from movement models, given the behavioural constraints that govern long-distance trips in vast, heterogeneous environments like the Southern Ocean

From trips to bouts to dives: temporal patterns in the diving behaviour of chick-rearing Adelie penguins, East Antarctica

Breeding Adélie penguins forage at sea and return to land to provision their chicks, adjusting their foraging behaviour in response to environmental fluctuations over time. At Béchervaise Island, a nesting site in an East Antarctic population, Adélie penguin diving behaviour remains undocumented. This represents a key area of uncertainty in efforts to understand and predict foraging success at this colony. Here, we compile a multi-year telemetry dataset from time-depth recorders deployed from 1992–2004 on 64 birds at Béchervaise Island. We examine diving activity at multiple scales: ranging from foraging trips (n = 125) to dive bouts (n = 3461) to individual dives (n = 84521). We characterise the stage- and sex-specific variation in diving behaviour of chick-rearing Adélie penguins using linear mixed effect models. Total foraging trip effort (trip duration, number of dives, vertical distance travelled and number of wiggles) substantially increased as the chick-rearing period progressed (guard through crèche), consistent with increasing chick provisioning and self-maintenance requirements over time. Foraging activity was predominantly structured in periods of sustained diving bouts, indicating sustained foraging effort over the course of the foraging trip. Diving behaviour (dive-level depth, duration, bottom time and ACPUEd) varied in relation to sex and chick-rearing stage. Dives were performed more frequently during high and low levels of solar light which is likely linked to visual predation strategies or prey activity. Our findings advance our understanding of this population’s foraging behaviour, which is ultimately required to underpin the conservation and management of this breeding colony

Modelled mid-trophic pelagic prey fields improve understanding of marine predator foraging behaviour

Biophysical interactions are influential in determining the scale of key ecological processes within marine ecosystems. For oceanic predators, this means foraging behaviour is influenced by processes shaping the distribution of prey. However, oceanic prey is difficult to observe and its abundance and distribution is regionally generalised. We use a spatiotemporally resolved simulation model to describe mid‐trophic prey distribution within the Southern Ocean and demonstrate insights that this modelled prey field provides into the foraging behaviour of a widely distributed marine predator, the southern elephant seal. From a five‐year simulation of prey biomass, we computed climatologies of mean prey biomass (average prey conditions) and prey biomass variability (meso‐scale variability). We also compiled spatially gridded metrics of seal density and diving behaviour from 13 yr of tracking data. We statistically modelled these metrics as non‐linear functions of prey biomass (both mean and variability) and used these to predict seal distribution and behaviour. Our predictions were consistent with observations (R2adj = 0.23), indicating that seals aggregate in regions of high mesoscale activity where eddies concentrate prey. Here, seals dived deeper (R2marg = 0.12, R2cond = 0.51) and spent less time hunting (R2marg = 0.05, R2cond = 0.56), likely targeting deep but profitable prey patches. Seals generally avoided areas of low eddy activity where prey was likely dispersed. Most seals foraged south of the Subantarctic Front, despite north of the front exhibiting consistently high simulated prey biomasses. This likely reflects seal prey or habitat preferences, but also emphasises the importance of mesoscale prey biomass variability relative to regionally high mean biomass. This work demonstrates the value of coupling mechanistic representations of prey biomass with predator observations to provide insight into how biophysical processes combine to shape species distributions. This will be increasingly important for the robust prediction of species’ responses to rapid system change

A quantitative, hierarchical approach for detecting drift dives and tracking buoyancy changes in southern elephant seals

Foraging behaviour of marine predators inferred from the analysis of horizontal or vertical movements commonly lack quantitative information about foraging success. Several marine mammal species are known to perform dives where they passively drift in the water column, termed “drift” dives. The drift rate is determined by the animal’s buoyancy, which can be used to make inference regarding body condition. Long term dive records retrieved via satellite uplink are often summarized before transmission. This loss of resolution hampers identification of drift dives. Here, we develop a flexible, hierarchically structured approach to identify drift dives and estimate the drift rate from the summarized time-depth profiles that are increasingly available to the global research community. Based on high-resolution dive data from southern elephant seals, we classify dives as drift/non-drift and apply a summarization algorithm. We then (i) automatically generate dive groups based on inflection point ordering using a ‘Reverse’ Broken-Stick Algorithm, (ii) develop a set of threshold criteria to apply across groups, ensuring non-drift dives are most efficiently rejected, and (iii) finally implement a custom Kalman filter to retain the remaining dives that are within the seals estimated drifting time series. Validation with independent data sets shows our method retains approximately 3% of all dives, of which 88% are true drift dives. The drift rate estimates are unbiased, with the upper 95% quantile of the mean squared error between the daily averaged summarized profiles using our method (SDDR) and the observed daily averaged drift rate (ODDR) being only 0.0015. The trend of the drifting time-series match expectations for capital breeders, showing the lowest body condition commencing foraging trips and a progressive improvement as they remain at sea. Our method offers sufficient resolution to track small changes in body condition at a fine temporal scale. This approach overcomes a long-term challenge for large existing and ongoing data collections, with potential application across other drift diving species. Enabling robust identification of foraging success at sea offers a rare and valuable opportunity for monitoring marine ecosystem productivity in space and time by tracking the success of a top predator

Iron limitation drives the globally extreme fluorescence/chlorophyll ratios of the Southern Ocean

The ratio between fluorescence (F) and chlorophyll-a (Chl)—where fluorescence is measured with a saturating fluorometer—is variable in the world's oceans, with the highest ratios and highest variability observed in the Southern Ocean. While species composition and Chl packaging per cell are strong drivers for the observed variability, additional factors, including iron limitation, have to this date not specifically been evaluated. Radiometers on biogeochemical (BGC)-Argo floats allow for an independent estimate of Chl concentration that is based on the light attenuation coefficient, Kd. Making use of 4,000 radiometry profiles from BGC-Argo floats in the Southern Ocean, we estimate Chl based on Kd and investigate the variability in F/Chl. Our analysis reveals a positive correlation between F/Chl and a proxy for iron limitation based on non-photochemical quenching dynamics. The strong influence of iron limitation on F/Chl is further corroborated by data from Southern Ocean phytoplankton cultures

Bacterial epibiont communities of panmictic Antarctic krill are spatially structured

Antarctic krill (Euphausia superba) are amongst the most abundant animals on Earth, with a circumpolar distribution in the Southern Ocean. Genetic and genomic studies have failed to detect any population structure for the species, suggesting a single panmictic population. However, the hyper‐abundance of krill slows the rate of genetic differentiation, masking potential underlying structure. Here we use high‐throughput sequencing of bacterial 16S rRNA genes to show that krill bacterial epibiont communities exhibit spatial structuring, driven mainly by distance rather than environmental factors, especially for strongly krill‐associated bacteria. Estimating the ecological processes driving bacterial community turnover indicated this was driven by bacterial dispersal limitation increasing with geographic distance. Furthermore, divergent epibiont communities generated from a single krill swarm split between aquarium tanks under near identical conditions suggests physical isolation in itself can cause krill‐associated bacterial communities to diverge. Our findings show that Antarctic krill‐associated bacterial communities are geographically structured, in direct contrast with the lack of structure observed for krill genetic and genomic data

Ocean circulation and frontal structure near the southern Kerguelen Plateau during the Kerguelen Axis marine ecosystem survey

A relatively large research effort has historicallyfocused on the northern part of the Kerguelen Plateau,providing an improved view of the ocean circulation atregional (McCartney and Donohue, 2007; Park et al.,2008; Roquet et al., 2009) and more local scales (Parket al., 2009; van Wijk et al., 2010; Vivier et al., 2015).Here we focus on the ocean circulation and frontalstructure between the southern section of the KerguelenPlateau and the Antarctic margin (Heywood et al., 1999;McCartney and Donohue, 2007)