Marine turtles and dugongs of the Torres Strait - Spatial models of dugong and turtle distribution and relative density of aerial surveys from 1987 - 2013 (NERP TE 2.1, JCU)

This dataset shows (1) a raster spatial model of the distribution and relative density of dugongs (Dugong dugong) in the Torres Strait region based on an aggregate of 27 years (1987 - 2013) of systematic aerial surveys; and (2) a raster spatial model of the distribution and relative density of marine turtles (green turtles, Chelonia mydas) in the Torres Strait based on an aerial survey conducted in 2013.

Aerial surveys were conducted using the strip transect method described by Marsh and Sinclair (1989). The survey region was divided into blocks containing systematic transects of varying length, which were typically perpendicular to the coast across the depth gradient. Tandem teams with two observers on each side of the aircraft independently recorded sightings of dugongs and marine turtles, including information on group size and calf numbers. Transects were 200 m wide at the water’s surface on either side of the aircraft.

For the dugong spatial model, spatial data from all the aerial surveys in the region (1987, 1991, 1996, 2001, 2006, 2011, and 2013) were corrected for differences in sampling intensity and area sampled between surveys. For details on sampling intensities and areas, please refer to the reports and publications for each individual survey (list below). For the turtle model, spatial data were used from the survey conducted in 2013. The corrected data were then interpolated using universal kriging over the spatial extent of the aerial surveys.

Planning units were 2km * 2km and were classified as low (1), medium (2), high (3) and very high (4) dugong/turtle density on the basis of the relative density of dugongs and turtles estimated from the models and a frequency analysis. Low density areas: 0 animals per square km; medium density areas 0.0015 - 0.25 animals per square km; high density areas 0.25 - 0.5 animals per square km; very high density areas > 0.5 animals per square km.

Note: A project is currently under way (funded by AMMC) to create an integrated database for all aerial survey data which will host raw data.

Limitations:

The modelled abundance and distribution in this dataset shows the relative density of dugongs and turtles (areas where there are more or less animals). It does NOT show the absolute dugong and turtle density. The observations only count a small percentage of the total number of dugongs. Some dugongs will be missed by the observers in the survey plane (perception bias) and some dugongs will be diving and can't be seen due to water turbidity (availability bias). Corrections for these under estimates can only be estimated at the spatial scale of entire surveys (thousands of square kilometres) and so when they are allied to the dataset they are assumed to be constant over the entire survey area. However these correction factors would vary spatially and the absolute numbers of the dugongs at the fine scale of this dataset (in a small area) are unlikely to be accurate. Nonetheless, the relative densities among regions should be approximately comparable (H. Marsh, personal communication).

Format:

The spatial model is 794x680 pixels with a spatial reference of WGS84 UTM Zone 54S. The original dataset is stored in ESRI GRID format (60 KB), which was converted to a GeoTiff for use in the eAtlas (22 kB). The GeoTiff was then converted to a shapefile.

Publications for aerial surveys in the Torres Strait:

- Marsh, H, Harris, ANM, & Lawler, IR. (1997) The sustainability of the Indigenous dugong fishery in Torres Strait, Australia/Papua New Guinea. Conservation Biology 11:1375-1386.
- Marsh, H, Lawler, IR, Kwan, D, Delean, S, Pollock, K, & Alldredge, M. (2004). Aerial surveys and the potential biological removal technique indicate that the Torres Strait dugong fishery is unsustainable. Animal Conservation 7: 435-443.
- Marsh, H.D., Hodgson, A., Lawler, I., Grech, A., & Delean, S. (2007) Condition, status, and trends and projected futures of the dugong in the Northern Great Barrier Reef and Torres Strait; including identification and evaluation of the key threats and evaluation of available management options to improve its stautus. Marine and Tropical Sciences Research Facility Reports Series. Reef and Rainforest Research Centre Limited, Cairns (77pp.)
- Marsh, H., Grech, A., & Hagihara, R. (2011) Aerial survey of Torres Strait to evaluate the efficacy of an enforced and possibly extended Dugong Sanctuary as one of the tools for managing the dugong fishery. Report to the Australian Marine Mammal Centre and the Torres Strait Regional Authority (33pp.)

本数据集包含两类栅格空间模型（raster spatial model）：其一为基于1987-2013年共计27年的系统航空调查汇总数据构建的托雷斯海峡（Torres Strait）海域儒艮（Dugong dugon）分布与相对密度栅格空间模型；其二为基于2013年航空调查数据构建的托雷斯海峡海域海龟（绿海龟，Chelonia mydas）分布与相对密度栅格空间模型。

本次航空调查采用Marsh与Sinclair（1989）提出的条带样线法（strip transect method）开展。调查区域被划分为多个区块，每个区块内设不同长度的系统样线，这些样线通常垂直于海岸并跨越深度梯度。调查采用双机组配置：飞机两侧各配备2名观察员，独立记录儒艮与海龟的目击信息，包括种群数量及幼崽数量。样线在飞机两侧的水面宽度均为200米。

针对儒艮空间模型，本研究对该区域1987、1991、1996、2001、2006、2011及2013年所有航空调查获取的空间数据进行校正，以消除不同调查间采样强度与采样面积的差异。关于采样强度与采样面积的详细信息，请参阅各单项调查的报告与出版物（详见下文列表）。海龟模型则仅采用2013年航空调查的空间数据。经校正的数据随后通过通用克里金插值法（universal kriging）在航空调查的空间范围内进行插值处理。

本次研究的规划单元为2km×2km，基于模型估算的儒艮与海龟相对密度及频率分析结果，将规划单元划分为低（1）、中（2）、高（3）、极高（4）四个密度等级。各等级对应的密度范围如下：低密度区：每平方公里0只；中密度区：每平方公里0.0015~0.25只；高密度区：每平方公里0.25~0.5只；极高密度区：每平方公里>0.5只。

备注：目前一项由AMMC资助的项目正在推进中，该项目将构建包含所有航空调查原始数据的整合数据库。

### 局限性
本数据集的模型丰度与分布结果仅反映儒艮与海龟的相对密度（即种群数量多寡的区域差异），并未体现二者的绝对种群密度。本次调查仅统计了儒艮总种群数量的极小一部分：观测人员可能会遗漏飞行平面内的部分儒艮（感知偏差），同时部分儒艮会因潜水及水体浑浊而无法被观测到（可获得性偏差）。针对这些低估情况的校正仅能在整个调查区域（数千平方公里）的空间尺度上进行估算，因此在应用于本数据集时，校正因子被假设为在整个调查区域内恒定不变。但实际上这些校正因子在空间上存在差异，因此本数据集精细尺度（小区域）下的儒艮绝对数量并不准确。不过，区域间的相对密度仍具备大致的可比性（H. Marsh，私人通信）。

### 数据格式
空间模型的像素尺寸为794×680，空间参考系为WGS84 UTM 54S带。原始数据集以ESRI GRID格式存储（60 KB），为适配eAtlas平台已转换为GeoTIFF格式（22 KB），后续又进一步转换为Shapefile格式。

### 托雷斯海峡航空调查相关出版物
- Marsh H、Harris ANM与Lawler IR（1997）《澳大利亚/巴布亚新几内亚托雷斯海峡本土儒艮渔业的可持续性》，《保护生物学（Conservation Biology）》11卷：1375-1386。
- Marsh H、Lawler IR、Kwan D、Delean S、Pollock K与Alldredge M（2004）《航空调查与潜在生物移除技术表明托雷斯海峡儒艮渔业不可持续》，《动物保护（Animal Conservation）》7卷：435-443。
- Marsh HD、Hodgson A、Lawler I、Grech A与Delean S（2007）《北大堡礁与托雷斯海峡儒艮的种群状况、现状、趋势及未来预测：包括关键威胁识别与评估，以及改善其种群现状的可行管理方案评估》，《海洋与热带科学研究设施报告系列（Marine and Tropical Sciences Research Facility Reports Series）》，凯恩斯：大堡礁与雨林研究中心有限公司（Reef and Rainforest Research Centre Limited），共77页。
- Marsh H、Grech A与Hagihara R（2011）《托雷斯海峡航空调查：评估强制及可能扩展的儒艮保护区作为儒艮渔业管理工具的有效性》，提交给澳大利亚海洋哺乳动物中心与托雷斯海峡地区管理局的报告，共33页。