Yasha Hetzel

Artificial light on water attracts turtle hatchlings during their near shore transit

We examined the effect of artificial light on the near shore trajectories of turtle hatchlings dispersing from natal beaches. Green turtle (Chelonia mydas) hatchlings were tagged with miniature acoustic transmitters and their movements tracked within an underwater array of 36 acoustic receivers placed in the near shore zone. A total of 40 hatchlings were tracked, 20 of which were subjected to artificial light during their transit of the array. At the same time, we measured current speed and direction, which were highly variable within and between experimental nights and treatments. Artificial lighting affected hatchling behaviour, with 88% of individual trajectories oriented towards the light and spending, on average, 23% more time in the 2.25 ha tracking array (19.5 ± 5 min) than under ambient light conditions (15.8 ± 5 min). Current speed had little to no effect on the bearing (angular direction) of the hatchling tracks when artificial light was present, but under ambient conditions it influenced the bearing of the tracks when current direction was offshore and above speeds of approximately 32.5 cm s−1. This is the first experimental evidence that wild turtle hatchlings are attracted to artificial light after entering the ocean, a behaviour that is likely to subject them to greater risk of predation. The experimental protocol described in this study can be used to assess the effect of anthropogenic (light pollution, noise, etc.) and natural (wave action, current, wind, moonlight) influences on the in-water movements of sea turtle hatchlings during the early phase of dispersal.

Wind and tidal mixing controls on stratification and dense water outflows in a large hypersaline bay

In Shark Bay, a large inverse estuary in Western Australia, longitudinal density gradients establish a gravitational circulation that is important for Bay-ocean exchange and transport of biological material such as larvae. The relative contributions of energy from wind and tidal mixing on the control of vertical stratification and gravitational circulation were investigated using the three-dimensional baroclinic ocean circulation model (General Estuarine Transport Model, GETM). In this large inverse estuary, the effects of the winds and tides on vertical mixing were found to be of similar magnitude. A critical depth of ∼15 m was identified that determined whether winds or tides or a combination of the two was required to create vertically mixed conditions. Where it was shallower than the critical depth, either the wind or tide could fully mix the water column. In contrast, a combination of both winds and tides was required to mix the deeper channels. Density-driven circulation peaked 0–3 days after periods of maximum stratification, resulting in a fortnightly modulation of dense water outflows along the seabed following the tidal stage. Salt flux calculations provided new evidence for the predominance of outflow through the deeper northern entrance channel where outflows persisted through all stages of the tide. In contrast, outflows through the western channel were more intermittent with a stronger tidal component. Wind driven lateral circulation between the entrances was also important and could temporarily reverse the circulation during northerly wind events.

The response of the southwest Western Australian wave climate to Indian Ocean climate variability

Knowledge of regional wave climates is critical for coastal planning, management, and protection. In order to develop a regional wave climate, it is important to understand the atmospheric systems responsible for wave generation. This study examines the variability of the southwest Western Australian (SWWA) shelf and nearshore wind wave climate and its relationship to southern hemisphere climate variability represented by various atmospheric indices: the southern oscillation index (SOI), the Southern Annular Mode (SAM), the Indian Ocean Dipole Mode Index (DMI), the Indian Ocean Subtropical Dipole (IOSD), the latitudinal position of the subtropical high-pressure ridge (STRP), and the corresponding intensity of the subtropical ridge (STRI). A 21-year wave hindcast (1994–2014) of the SWWA continental shelf was created using the third generation wave model Simulating WAves Nearshore (SWAN), to analyse the seasonal and inter-annual wave climate variability and its relationship to the atmospheric regime. Strong relationships between wave heights and the STRP and the STRI, a moderate correlation between the wave climate and the SAM, and no significant correlation between SOI, DMI, and IOSD and the wave climate were found. Strong spatial, seasonal, and inter-annual variability, as well as seasonal longer-term trends in the mean wave climate were studied and linked to the latitudinal changes in the subtropical high-pressure ridge and the Southern Ocean storm belt. As the Southern Ocean storm belt and the subtropical high-pressure ridge shifted southward (northward) wave heights on the SWWA shelf region decreased (increased). The wave height anomalies appear to be driven by the same atmospheric conditions that influence rainfall variability in SWWA.

The Influence of the Subtropical High-Pressure Ridge on the Western Australian Wave Climate

ABSTRACT Wandres, M., Pattiaratchi, C., Wijeratne, E.M.S., Hetzel, Y., 2016. The influence of the subtropical high-pressure ridge on the Western Australian wave climate. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 567–571. Coconut Creek (Florida), ISSN 0749-0208. Understanding the wave climate of a region is critical for its coastal zone management. The southwest Western Australian (SWWA) wave climate is dominated by waves generated in the energetic and variable Southern Ocean (SO) storm belt. The latitudinal variability of the SO storm belt can be described by the position (latitude) of the subtropical high-pressure ridge (STRP). To gain an understanding in how the SO storm belt influences the SWWA wave climate, the relationship between the STRP and the waves was examined. Wave data from three directional wave buoys along the SWWA shelf were compared to the STRP. The seasonal and interannual variability of the STRP and the wave climate indicated a significant correlation between the STRP and the SWWA wave heights with the strongest relationship during winter. A northward shift of the STRP resulted in a northward shift of the SO storm belt which led to increased wave heights in SWWA whereas a southward shift of the STRP resulted in decreased wave heights. The close relationship between the STRP and the local wave climate, suggests it could be used to estimate future wave climates in SWWA.

Factors influencing the occurrence of Dense Shelf Water Cascades in Australia

ABSTRACT Mahjabin, T.; Pattiaratchi, C., and Hetzel, Y., 2016. Factors influencing the occurrence of Dense Shelf Water Cascades in Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 527–531. Coconut Creek (Florida), ISSN 0749-0208. Transport of inshore waters and suspended material off the continental shelf by Dense Shelf Water Cascades (DSWC) has important ecological and biogeochemical implications in Australian waters. Because of high rates of evaporation, denser saline water occurs in the shallow coastal regions around Australia, setting up horizontal density gradients that can drive DSWC. Ocean glider data available from the Integrated Marine Observing System (IMOS), which is operated by the Australian National Facility for Ocean Gliders (ANFOG) located at the University of Western Australia, were used to measure cross-shelf density profiles under varying wind and tide conditions for seven contrasting regions around the entire continent. Overall 97 sets of spatial and temporal resolution data from year 2008 to 2015 collected by the ocean gliders and analysed with a subset presented here. Data from 19 transects covering the years 2012 to 2015 for the Pilbara region of Western Australia, indicated that cascades occur during the autumn and winter due to cooling of the coastal waters which already have higher salinity due to evaporation during the summer months. The cross-shelf density gradient in this continental shelf was found to be maximum in July with a value of 14.23×10−6 kg m−4.

Use of particle tracking to determine optimal release dates and locations for rehabilitated neonate sea turtles

Sea turtles found stranded on beaches are often rehabilitated before being released back into the wild. The location and date of release is largely selected on an informal basis, which may not maximise the chance of survival. As oceanic conditions have a large influence on the movements of neonate sea turtles, this study aimed to identify the best locations and months to release rehabilitated sea turtles that would assist in their transport by ocean currents to the habitat and thermal conditions required for their survival. A particle tracking model, forced by ocean surface velocity fields were used to simulate the dispersal pathways of millions of passively drifting particles released from different locations in Western Australia. The particles represented rehabilitated, neonate turtles requiring oceanic habitats (green (Chelonia mydas), hawksbill (Eretmochelys imbricata) and loggerheads (Caretta caretta)) and flatback turtles (Natator depressus) which require neritic habitats. The results clearly identified regions and months where ocean currents were more favourable for transport to suitable habitats. Tantabiddi, near Exmouth on the north-west coast, was consistently the best location for release for the oceanic species, with dominant offshore-directed currents and a very narrow continental shelf reducing the time taken for particles to be transported into deep water. In contrast, release locations with more enclosed geography, wide continental shelves, and/or proximity to cooler ocean temperatures were less successful. Our results produced a decision support system for the release of neonate marine turtles in Western Australia and our particle tracking approach has global transferability.