John S. Gunn

Classifying movement behaviour in relation to environmental conditions using hidden Markov models

1. Linking the movement and behaviour of animals to their environment is a central problem in ecology. Through the use of electronic tagging and tracking (ETT), collection of in situ data from free-roaming animals is now commonplace, yet statistical approaches enabling direct relation of movement observations to environmental conditions are still in development. 2. In this study, we examine the hidden Markov model (HMM) for behavioural analysis of tracking data. HMMs allow for prediction of latent behavioural states while directly accounting for the serial dependence prevalent in ETT data. Updating the probability of behavioural switches with tag or remote-sensing data provides a statistical method that links environmental data to behaviour in a direct and integrated manner. 3. It is important to assess the reliability of state categorization over the range of time-series lengths typically collected from field instruments and when movement behaviours are similar between movement states. Simulation with varying lengths of times series data and contrast between average movements within each state was used to test the HMMs ability to estimate movement parameters. 4. To demonstrate the methods in a realistic setting, the HMMs were used to categorize resident and migratory phases and the relationship between movement behaviour and ocean temperature using electronic tagging data from southern bluefin tuna (Thunnus maccoyii). Diagnostic tools to evaluate the suitability of different models and inferential methods for investigating differences in behaviour between individuals are also demonstrated.

Feeding ecology of wild migratory tunas revealed by archival tag records of visceral warming

1. Seasonal long-distance migrations are often expected to be related to resource distribution, and foraging theory predicts that animals should spend more time in areas with relatively richer resources. Yet for highly migratory marine species, data on feeding success are difficult to obtain. We analysed the temporal feeding patterns of wild juvenile southern bluefin tuna from visceral warming patterns recorded by archival tags implanted within the body cavity. 2. Data collected during 1998-2000 totalled 6221 days, with individual time series (n = 19) varying from 141 to 496 days. These data span an annual migration circuit including a coastal summer residency within Australian waters and subsequent migration into the temperate south Indian Ocean. 3. Individual fish recommenced feeding between 5 and 38 days after tagging, and feeding events (n = 5194) were subsequently identified on 76.3 +/- 5.8% of days giving a mean estimated daily intake of 0.75 +/- 0.05 kg. 4. The number of feeding events varied significantly with time of day with the greatest number occurring around dawn (58.2 +/- 8.0%). Night feeding, although rare (5.7 +/- 1.3%), was linked to the full moon quarter. Southern bluefin tuna foraged in ambient water temperatures ranging from 4.9 degrees C to 22.9 degrees C and depths ranging from the surface to 672 m, with different targeting strategies evident between seasons. 5. No clear relationship was found between feeding success and time spent within an area. This was primarily due to high individual variability, with both positive and negative relationships observed at all spatial scales examined (grid ranges of 2 x 2 degrees to 10 x 10 degrees ). Assuming feeding success is proportional to forage density, our data do not support the hypothesis that these predators concentrate their activity in areas of higher resource availability. 6. Multiple-day fasting periods were recorded by most individuals. The majority of these (87.8%) occurred during periods of apparent residency within warmer waters (sea surface temperature > 15 degrees C) at the northern edge of the observed migratory range. These previously undocumented nonfeeding periods may indicate alternative motivations for residency. 7. Our results demonstrate the importance of obtaining information on feeding when interpreting habitat utilization from individual animal tracks.

Behaviour and habitat preferences of bigeye tuna (Thunnus obesus) and their influence on longline fishery catches in the western Coral Sea

Data on the depth and temperature preferences of bigeye tuna (Thunnus obesus) derived from archival tags were integrated with data on the spatial and temporal distribution of catches from an eastern Australian longline fishery to investigate the relationship between bigeye tuna behaviour and the fishery. Tagged individuals demonstrated variability in depth and water temperature preferences on diurnal, lunar, and seasonal scales. Deeper, cooler waters were frequented dur- ing the day, and shallower, warmer waters were frequented at night, with nighttime preferences often deeper around the full moon, although this was not consistent between individuals or temporally within individuals. Marked individual varia- bility in depth and water temperature preferences suggest bigeye tuna are flexible in foraging strategies utilized, thereby al- lowing individuals to maximize their ability to successfully forage in a patchy environment. Catches of bigeye tuna corresponded with the spatial and temporal overlap of bigeye tuna distributions within the fishery on similar scales, sug- gesting clear influence of bigeye tuna behaviour on the behaviour of the fishery and catches. However, variability in these relationships suggests that the factors influencing the relative catchability of bigeye tuna are complex, and there are likely to be a range of additional environmental, behavioural, and operational factors that influence bigeye tuna catchability. Resume´ : Afin detudier la relation entre le comportement et la peche commerciale chez le thon ventru (Thunnus obesus), nous avons integredes donnees sur les preferences de profondeur et de temperature des thons provenant detiquettes enre- gistreuses avec des informations sur la repartition spatiale et temporelle des captures dans une peche commerciale ala pal- angre dans lest de lAustralie. Les individus marques montrent une variabilitede leurs preferences de profondeur et de temperature de leau sur des echelles diurnes, lunaires et saisonnieres. Ils frequentent les eaux plus profondes et plus fraoˆches durant le jour et les eaux moins profondes et plus chaudes durant la nuit, avec souvent des preferences pour les eaux plus profondes la nuit au moment de la pleine lune, bien que ce comportement ne soit coherent ni chez les differents individus, ni dans le temps chez un meme individu. La variabiliteindividuelle importante dans les preferences de profon- deur et de temperature de leau laisse croire que les strategies de recherche de nourriture des thons ventrus sont flexibles, ce qui permet aux individus de maximiser leur capaciteachercher leur nourriture dans les environnements parcellaires. Les captures de thons ventrus correspondent au recouvrement des repartitions des thons dans les sites de peche sur les memes echelles, ce qui indique quil y a une nette influence du comportement des thons ventrus sur le deroulement de la peche et les captures. Cependant, la variabilitede ces relations indique que les facteurs qui influencent la capturabilitedes thons sont complexes et quil y a vraisemblablement une gamme de facteurs additionnels environnementaux, comporte- mentaux et operationnels qui influencent la capturabilitedes thons ventrus. (Traduit par la Redaction)

Predicting feeding success in a migratory predator: integrating telemetry, environment, and modeling techniques

Foraging theory predicts that mobile predators should target high profitability areas with plentiful resources and minimize time spent moving between these areas. This has led to a focus in recent literature on the identification of hotspots important for migratory marine predators, i.e., regions where predators spend disproportionate amounts of time ostensibly due to high prey abundance; and determination of the environmental features characteristic of such areas. We investigated factors predicting foraging success in southern bluefin tuna (SBT; Thunnus maccoyii), by integrating telemetry-based feeding and movement data (n = 19 fish, length to caudal fork [LCF] = 99 +/- 3 cm) with environmental data over the scale of their annual oceanic migrations during 1998-2000. We used widely available statistical modeling techniques, generalized linear models, and generalized linear mixed models, formulated to represent feeding as a Markov process. The results showed increased feeding and predictability of feeding occurs in the coastal waters of southern Australia, providing some evidence that this area represents a fixed foraging hotspot for juvenile tuna during the austral summer. However, in oceanic waters southern bluefin tuna did not fit the common model of migration, but rather showed a pattern of relatively high foraging success throughout their migratory range, especially during periods of continuous travel. Interestingly, foraging coldspots (prolonged low-feeding periods) as well as hotspots were apparent across individual tracks, predicted most strongly by warm ocean temperatures. These results provide a new perspective on the ecology of large-scale feeding migrations within the context of the heterogeneous ocean environment, where the continuous and opportunistic feeding of generalist predators may be more common, particularly in predatory large pelagic fishes, than is currently documented.

Electronic Tagging Data Supporting Flexible Spatial Management in an Australian Longline Fishery

Despite widespread claims for the importance of electronic tagging and the resulting data for improved management of fished species, there are few examples where this has occurred. In this contribution we describe how pop-up satellite archival tagging data have been incorporated into a habitat prediction model to support spatial management in an Australian longline fishery, specifically through reduction of unwanted bycatch of a quota-managed species, southern bluefin tuna (SBT, Thunnus maccoyii). This model, and its practical application, has evolved over time (2002-present), due to the successful cooperation between scientists, fishery managers and stakeholders. To illustrate this example of successful use of tag data in management, we review the appropriate biology of SBT and its’ interaction with the fishery, the management challenge that tagging and the habitat model helped resolve, and how this approach has been implemented. Discussion of the management costs associated with this management system shows that fine-scale spatial management is a suitable approach for this bycatch issue. We conclude with some general lessons for the application of flexible spatial management approaches, based on management and science constraints.