Compensating for geographic variation in detection probability with water depth improves abundance estimates of coastal marine megafauna

PLoS One. 2018 Jan 25;13(1):e0191476. doi: 10.1371/journal.pone.0191476. eCollection 2018.

Abstract

The probability of an aquatic animal being available for detection is typically <1. Accounting for covariates that reduce the probability of detection is important for obtaining robust estimates of the population abundance and determining its status and trends. The dugong (Dugong dugon) is a bottom-feeding marine mammal and a seagrass community specialist. We hypothesized that the probability of a dugong being available for detection is dependent on water depth and that dugongs spend more time underwater in deep-water seagrass habitats than in shallow-water seagrass habitats. We tested this hypothesis by quantifying the depth use of 28 wild dugongs fitted with GPS satellite transmitters and time-depth recorders (TDRs) at three sites with distinct seagrass depth distributions: 1) open waters supporting extensive seagrass meadows to 40 m deep (Torres Strait, 6 dugongs, 2015); 2) a protected bay (average water depth 6.8 m) with extensive shallow seagrass beds (Moreton Bay, 13 dugongs, 2011 and 2012); and 3) a mixture of lagoon, coral and seagrass habitats to 60 m deep (New Caledonia, 9 dugongs, 2013). The fitted instruments were used to measure the times the dugongs spent in the experimentally determined detection zones under various environmental conditions. The estimated probability of detection was applied to aerial survey data previously collected at each location. In general, dugongs were least available for detection in Torres Strait, and the population estimates increased 6-7 fold using depth-specific availability correction factors compared with earlier estimates that assumed homogeneous detection probability across water depth and location. Detection probabilities were higher in Moreton Bay and New Caledonia than Torres Strait because the water transparency in these two locations was much greater than in Torres Strait and the effect of correcting for depth-specific detection probability much less. The methodology has application to visual survey of coastal megafauna including surveys using Unmanned Aerial Vehicles.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Australia
  • Conservation of Natural Resources / statistics & numerical data
  • Demography / methods*
  • Demography / statistics & numerical data
  • Dugong
  • Ecosystem
  • New Caledonia
  • Oceans and Seas
  • Papua New Guinea
  • Population Density*
  • Probability

Grants and funding

This work was supported by: 1) James Cook University - PhD scholarship for Rie Hagihara; 2) The Australian Marine Mammal Centre, 2011 (11/8 Improving the accuracy of dugong aerial surveys by better correcting for availability bias) - funding for field works in Moreton Bay 2011 and 2012; 3) The National Environmental Science Program Tropical Water Quality Hub - funding for field works in Torres Strait; 4) The Torres Strait Regional Authority - funding for field works in Torres Strait and logistical support from the ranger staff; 5) ZoNeCo - funding for field works in New Caledonia and data analysis; 6) The Agence des Aires Marine Protegees - funding for field works in New Caledonia and data analysis; 7) the New Caledonian Dugong Technical Committee under the 2012-2015 Dugong Action Plan - funding for field works in New Caledonia and data analysis; and 8) anonymous donor - funding for field works in Moreton Bay 2011 and 2012.