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    Comparing surface and planimetric area across multiple scales and assessing the impact of different data sources on seabed area estimation in research blocks in the CAMLR Convention Area

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    Document Number:
    WG-SAM-15/01
    Author(s):
    CCAMLR Secretariat
    Submitted By:
    Ms Doro Forck (CCAMLR Secretariat)
    Agenda Item(s)
    Abstract

    Having accurate surface area estimates of toothfish (Dissostichus spp) habitats is important for CCAMLR in estimating the fishable biomass of these species in data-poor areas, developing scientific survey designs and models of habitat use.  Planimetric seabed areas (i.e. a two-dimensional surface) within the Convention area are commonly calculated, but surface area (i.e. a three-dimensional surface) is rarely estimated. The aims of this paper are to examine differences in: (1) planimetric seabed area based on different datasets (GEBCO 20014 and GEBCO 2008) and; (2) planimetric and surface area estimates for the same areas using the most up-to-date global bathymetry dataset (GEBCO 2014). Comparisons were performed at four different scales. At the coarsest scale areas were calculated within research blocks (all depths included), then estimates were limited to the fishable depth range of each research block (600-1800m included), the region were scaled down further to look at differences in eight depth classes and at the finest resolution areas were compared in individual (~500 m x 500m) grid cells within research blocks. Differences between the datasets varied between 0-62% depending on the research block and we considered the GEBCO 2014 dataset to be the most accurate so we used this in comparing different area metrics.  Results from the comparison of total surface and planimetric areas across all depths and the fishable depth range within research blocks showed differences of less than 2%. However, at the smaller grid cell scale, larger differences, of up to 137%, between the area metrics emerged. Given differences between surface and planimetric areas within fishable depth ranges at the scale of a research block were no greater than 2% this is not sufficiently different to impact toothfish biomass estimates based on the current CPUE-analogy calculations. However, in many of the research blocks a higher proportion of the grid cells within the fishable depth range (i.e. -600 - -800, -800 - -1000 etc.) had a greater surface to planimetric area difference than those cells that were outside of the fishable depth ranges. In those research blocks that showed larger difference at the grid cell level, it may be worth examining whether fishing occurred more frequently in cells with a large differences and if so whether deriving a more specific measure of surface area in fished locations (as opposed to the fishable depth range) impacts estimates of biomass. Spatial habitat modeling of toothfish species’ could also benefit from the use of surface area measurement at the grid cell level.