Spatial ecology of true sea snakes (Hydrophiinae) in coastal waters of North Queensland

Udyawer, Vinay (2015) Spatial ecology of true sea snakes (Hydrophiinae) in coastal waters of North Queensland. PhD thesis, James Cook University.

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Abstract

Aquatic snakes are a diverse group that represent multiple evolutionary transitions from a terrestrial to an aquatic mode of life. Current systematics of aquatic snakes identifies four independent lineages (file snakes, Acrochordidae; homalopsid snakes, Homalopsidae; sea kraits: Laticaudinae and 'true' sea snakes, Hydrophiinae), species of which are represented on almost every continent. Despite their widespread distributions, this group of snakes is under−represented in the scientific literature with many fundamental questions about their ecology and biology still unanswered. In Chapter 2 of this thesis, I review the current scientific literature on the spatial ecology of aquatic and semi−aquatic snakes and assess both the horizontal (i.e. geographic movements) and vertical (i.e. dive patterns) patterns in their movement. I also assess what is currently known about the intrinsic (e.g. food, predator avoidance, reproductive state, ontogenetic shifts, philopatry and homing) and extrinsic (e.g. temperature, salinity, lunar and tidal cycles) factors that drive movement and space use in this group of snakes and identify key knowledge gaps. Chapter 2 also reviews the current knowledge on natural and anthropogenic threats these animals face and how movement affects their susceptibility to these threats. Incidental trawl capture represents a major threat to sea snake populations throughout their global distribution where they often represent a large proportion of bycatch in artisanal and commercial trawl fisheries. Recent global assessments have highlighted the need for data regarding the distribution patterns and spatial ecology of sea snakes to better understand their interactions with trawl fisheries throughout their range.

This dissertation focuses on 'true' sea snakes, which are found in tropical waters of South East Asia, Australia and the Pacific Islands. Data obtained using multiple techniques were used to define the distribution patterns, spatial ecology and physiology of true sea snakes within the Great Barrier Reef Marine Park (GBRMP), Australia. These data were used to explore and better understand how sea snakes are distributed and utilise space throughout the GBRMP over multiple spatial (i.e., geographic to regional) and temporal (i.e., diel to seasonal) scales. In Chapter 3, data from baited remote underwater video stations (BRUVS) were used to estimate geographic−scale distribution patterns of three species of sea snake (Aipysurus laevis, Hydrophis curtus and H. ocellatus) over 14˚ of latitude within the GBRMP. A total of 2471 deployments of BRUVS were made in a range of locations, in sites open and closed to trawl fishing. Sightings of sea snakes were analysed alongside six spatial factors [depth, relative distance across (longitude) and along (latitude) the GBRMP, proximity to land, proximity to the nearest reef and habitat complexity] to determine the factors that most strongly influenced the distribution and abundance of sea snakes. The results showed a strong latitudinal effect on the distribution of all three species, with the highest densities and diversities occurring in central and southern GBRMP locations, while the northern Great Barrier Reef (GBR) was relatively depauperate in terms of both occurrence and diversity. Shallow inshore areas were identified as key habitats for A. laevis and H. curtus, whereas deeper offshore habitats were most important for H. ocellatus. No significant difference was found in the mean number of snakes sighted per hour between sites open and closed to trawling. Overall, sea snakes displayed 'patchy' geographic distribution patterns in the GBRMP. Inshore waters of the central GBR were one area that all three species had high abundances, indicating that this area is particularly favourable for sea snake populations on the GBR.

In Chapter 4, the movement patterns and three−dimensional home ranges of two species of sea snake (Hydrophis curtus and H. elegans) were examined at multiple temporal scales using passive acoustic telemetry. Over a diel period, monitored snakes exhibited a clear diel pattern in their use of space, with individuals displaying restricted movements at greater depths during the day, and larger movements on the surface at night. Hydrophis curtus generally occupied space in deep water within the bay, while H. elegans were restricted to mud flats in inundated inter−tidal habitats. The overlap in space used between day and night showed that individuals used different core areas; however, the extent of areas used was similar. The space use patterns of monitored sea snakes were also evaluated alongside environmental parameters to determine what factors influenced the spatial ecology of sea snakes in nearshore habitats. Presence, movement and three−dimensional home range metrics calculated from monitoring data were tested against environmental (water temperature, atmospheric pressure, wind speed, accumulated rainfall and tidal range) and biological (snout−vent length) factors on daily and monthly temporal scales to identify key environmental drivers of movement and the use of space. A generalised linear mixed model (GLMM) framework using Akaike information criterion (AIC) indicated that tidal reach and atmospheric pressure strongly influenced the daily presence and movements of tagged individuals, respectively. Accumulated rainfall significantly influenced the volume of space used on a monthly timescale.

In Chapter 5, the data obtained from passive acoustic telemetry was used to determine how sea snakes select habitats based on habitat type, depth and proximity to sources of freshwater within a nearshore environment. A hierarchical Bayesian model was used to estimate if individuals were selecting habitats significantly more or less than random on a population− and individual−level. Composition of diet was also assessed using regurgitate from captured individuals. Selection of habitats by the two species differed with H. elegans displaying an affinity for mudflat and seagrass habitats less than 4 km from sources of freshwater and depths less than 3 m. Hydrophis curtus selected for slightly deeper seagrass habitats (1 – 4 m) further from freshwater sources (2 – 5 km). Data from regurgitate showed H. curtus displayed some level of intraspecific predation. Both species prominently selected seagrass areas indicating these habitats provide key resources for sea snakes within nearshore environments. Any degradation or loss of these habitats may have significant consequences for local sea snake populations. Understanding the habitat requirements of sea snakes is essential to defining how natural and anthropogenic disturbances may affect populations and is necessary to inform targeted management and conservation practices.

This thesis also explored the physiological basis of movement patterns in sea snakes and examined how environmental factors may affect their susceptibility to trawl fishing. In Chapter 6, laboratory observations showed that sea snakes displayed shorter dive durations and surfaced more frequently as water temperature increased. Animal−borne accelerometers were used to provide the first estimates of movement−associated energy expenditure in free−roaming sea snakes and explore diel and seasonal patterns in metabolic rates. The energy requirements of sea snakes estimated in the field showed a doubling of metabolic rate from the cooler dry season to the warmer wet season, which potentially increases their susceptibility to fishing activities that occur in summer months. In bimodally respiring animals like sea snakes, the up−regulation in cutaneous respiration is an important mechanism that can potentially prolong dive durations during periods of stress. This mechanism is important and can potentially allow sea snakes to prolong their dive durations when caught in trawl nets and increase their chances of survival. Results of this thesis showed that sea snakes may not have much control over the amount of oxygen they uptake cutaneously, which may impede their chances of survival once caught in fishing gear.

The use of spatial closures (e.g. Marine Protected Areas; MPAs) is effective in reducing the exposure of bycatch species to fishing activities in the GBRMP, and may be useful in managing fishing−related mortality in sea snakes. However, identifying important habitats for sea snakes is critical to ensure that MPAs function effectively. In Chapter 7, I examined the importance of protected, shallow coastal habitats as possible refuge sites for sea snakes in the GBRMP. Extensive boat−based surveys were conducted to investigate the assemblage and abundance of sea snakes within a protected, shallow coastal bay adjacent to trawl fishing grounds. Hydrophis curtus and H. elegans were the most commonly encountered species within the bay. Based on the age structure of these two species the bay was primarily used by juveniles. Temporal trends in age structure showed that H. curtus may use Cleveland Bay as a nursery ground with gravid females entering the bay in summer months to give birth. In contrast, H. elegans appears to use the bay more consistently through the year with approximately 30% of individuals being adult. This chapter also showed that shallow tidal habitats, which are too risky to undertake trawl fishing, are regularly used by sea snakes and may provide refugia for vulnerable life stages of sea snakes. The identification and protection of such habitats may further mitigate risks to sea snake populations from trawl fishing.

Item ID: 46245
Item Type: Thesis (PhD)
Keywords: 3D, acoustic monitoring, boosted regression tree, BRUVS, bycatch, Cleveland Bay, coastal habitat, diel movements, dive duration, entanglement, Great Barrier Reef, home range, Hydrophiinae, Hydrophis (Lapemis) curtus, Hydrophis elegans, kernel utilisation distribution (KUD), marine debris, multivariate regression tree, nearshore habitats, oxygen use, sea snakes, spatial distribution, zero-inflated model
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3:Udyawer, Vinay, Cappo, Mike, Simpfendorfer, Colin A., Heupel, Michelle R., and Lukoschek, Vimoksalehi (2014) Distribution of sea snakes in the Great Barrier Reef Marine Park: observations from 10 yrs of baited remote underwater video station (BRUVS) sampling. Coral Reefs, 33 (3). pp. 777-791.

Chapter 4:Udyawer, Vinay, Read, Mark, Hamann, Mark, Simpfendorfer, Colin A., and Heupel, Michelle R. (2015) Effects of environmental variables on the movement and space use of coastal sea snakes over multiple temporal scales. Journal of Experimental Marine Biology and Ecology, 473. pp. 26-34.

Chapter 4:Udyawer, Vinay, Simpfendorfer, Colin A., and Heupel, Michelle R. (2015) Diel patterns in three-dimensional use of space by sea snakes. Animal Biotelemetry, 3. pp. 1-9.

Appendix 8.1: Udyawer, Vinay, Read, Mark A., Hamann, Mark, Simpfendorfer, Colin A., and Heupel, Michelle R. (2013) First record of sea snake (Hydrophis elegans, Hydrophiinae) entrapped in marine debris. Marine Pollution Bulletin, 73 (1). pp. 336-338.

Date Deposited: 23 Nov 2016 00:20
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 34%
06 BIOLOGICAL SCIENCES > 0606 Physiology > 060601 Animal Physiology - Biophysics @ 33%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060809 Vertebrate Biology @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 34%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 33%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 33%
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