Sea Turtles and the Environmental Management of Industrial Activities in North West W.A. - Sea turtle beach usage and beach light exposure

Brief description

Basic information on track size and characterisitics of the local sea turtle nesting populations was gathered so that track counts could be done in the absence of adult animals. From 1998 to 2005 counts of the regional distribution of nesting turtle tracks within the Lowendal, Montebello and Barrow Islands were documented and used as an indicator of nesting effort.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Track identification - During the first summer season tracks were studied for overall species crawl patterns and for characteristics suitable to measure. Track measurements were made of individual flipper widths, overall track width, central belly channel width and individual flipper imprint offset from each other. This was done randomly at three different locations on each track, up and down. A fast and simple method using a metre stick was developed and tested. The species making the track was identified and the presence or absence of the adult noted. Where possible the track was measured on the flat dry sand. The total track width was measured at its widest point (="width"), the belly drag/channel width was measured at its widest point (="belly") and then the rear flipper impressions made in a 1m length of track were counted (="stroke/m"). Measurements were also taken from hatchling tracks to determine if species identification could be done using the track size. The total track width was measured from the outside edge of adjacent rear flipper imprints. Where possible, hatchling track measurements were done on dry sand and on the flat part of the beach. - Physical characteristics and light environment of beaches - Preliminary assessment of nesting habitats were carried out for all beaches following standardised methods (diez and Ottenwalder 1999). Physical features such as beach accessibility, dimensions, wave energy conditions and foreshore topography were recorded. Each beach was assessed for quality of nesting habitat. The amount of available habitat is represented by beach width and sand depth. Beach lengths and widths were measured from aerial photograph of islands. The accessibility of each beach to nesting sea turtles was evaluated based on foreshore and beach characteristics. For further methodology see section 3.3.2 of thesis. - Track count survey beach selection, access and timing - The size and remoteness of the study area precluded detailed on-ground track count survey. Consequently a survey program was designed to provide a mix of aerial and ground truthing surveys. Regular on ground surveys were primarily employed on Barrow and the Lowendal Islands (OG), while aerial surveys (A) were the primary method used for the more remote island group of the Montebello group in conjunction with limited ground truthing transects (T). The survey schedule is listed in Table 3.1 of the thesis. Preliminary surveys were carried out over the 1998/1999 nesting season at all locations. Surveys were timed to coincide with the predicted peak of nesting for the three species (Hawksbill, Flatback and Green turtle), i.e. one in early summer (November or December) and the second in mid summer (January). Where possible selected beaches on Barrow, Lowendal and Montebello islands were surveyed over three consecutive days (a) while all beaches were inspected at least once a season (b) to document evidence of current and past nesting as determined by tracks (current season) and nest craters (current and past seasons). With limited resources this was all that could be achieved. Opportunistic surveys of various beaches were also conducted outside of the regular monitoring schedule where other project work allowed. For further information see section 3.3.3 of the thesis. - Track counts - The method employed to carry out the Barrow, Lowendal and Montebello island complex nesting beach aerial and ground track surveys were based on the recommendations made by Schroeder and Murphy (1999). Attempts were made to assign an age to tracks, however this was abandoned since environmental factors made it difficult to accurately date individual tracks. Factors that influenced the persistence of tracks and therefore confounded estimates of age included wind and rain weathering, beach sediment grain size, beach orientation with respect to the prevailing winds and animal size (heavier green turtles leave deeper track impressions than the lighter hawksbills). Two different overnight track counts were made. These were fresh below high tide (BHT) counts and fresh overnight line cross (x-line) counts. BHT counts - Fresh tracks below last high tide (BHT) counts were made of down tracks traversing the beach after the previous night's high tide. These counts did not include animals nesting prior to the high tide (generally underestimates the number of animals visiting over three consecutive days and for aerial counts. As a minimum these counts were carried out on all beaches surveyed (including census beaches) within the B-L-M complex between 1998 and 2003. x-line counts - During the first summer of preliminary regional track count surveys a number of beaches that were identified as having high-density nesting were selected as representative 'census' beaches for ongoing surveys investigating the impacts of light on sea turtle adults and hatchlings. The number of overnight turtle nesting attempts was enumerated using the x-line track census method over a minimum of three consecutive days. The results provide an indication of the number of animals using individual beaches. This data was then used to quantify the proportion of the nesting population potentially exposed to light. Cross line (x line) counts were made of tracks crossing a line installed along the beach above the high tide line. These counts include all animals nesting during the previous night. This census count was used on beaches that could be visited over three consecutive days and was used on Barrow Island and the Lowendal Islands between 1998 and 2003 and then on Barrow Island only between 2003 and 2005. The application of each of these different count types is presented as: - Beach preferences (BHT) - Length and peak of nesting season (x-line) - Track to nest ratio (x-line) - Distribution of track crawls between and within island groups (BHT) - Beach usage, Barrow and Lowendal islands (x-line) For further information see section 3.3.4 of thesis.

Notes

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ChevronTexaco

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Apache Energy

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Natural Hertiage Trust Grant #9924

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BHP Billiton

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Santos

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Woodside