Using tagging data and aerial surveys to incorporate availability bias in the abundance estimation of blue sharks (Prionace glauca)

Milaja Nykänen; Mark Jessopp; Thomas K Doyle; Luke A Harman; Ana Cañadas; Patricia Breen; William Hunt; Mick Mackey; Oliver Ó Cadhla; David Reid; Emer Rogan

doi:10.1371/journal.pone.0203122

Using tagging data and aerial surveys to incorporate availability bias in the abundance estimation of blue sharks (Prionace glauca)

PLoS One. 2018 Sep 11;13(9):e0203122. doi: 10.1371/journal.pone.0203122. eCollection 2018.

Authors

Milaja Nykänen^{1

2}, Mark Jessopp^{1

3}, Thomas K Doyle^{1

2}, Luke A Harman^{1

2}, Ana Cañadas⁴, Patricia Breen^{1

5}, William Hunt^{1

2}, Mick Mackey^{1

2}, Oliver Ó Cadhla⁶, David Reid⁷, Emer Rogan^{1

2}

Affiliations

¹ School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland.
² Aquaculture & Fisheries Development Centre, University College Cork, Cork, Ireland.
³ MaREI Centre, Environmental Research Institute, University College Cork, Cork, Ireland.
⁴ ALNILAM Research and Conservation Ltd, Madrid, Spain.
⁵ School of Geography, National University of Ireland, Galway, Ireland.
⁶ National Parks and Wildlife Service, Department of Culture, Heritage and the Gaeltacht, Galway, Ireland.
⁷ Marine Institute, Oranmore, Galway, Ireland.

Abstract

There is worldwide concern about the status of elasmobranchs, primarily as a result of overfishing and bycatch with subsequent ecosystem effects following the removal of top predators. Whilst abundant and wide-ranging, blue sharks (Prionace glauca) are the most heavily exploited shark species having suffered marked declines over the past decades, and there is a call for robust abundance estimates. In this study, we utilized depth data collected from two blue sharks using pop-up satellite archival tags, and modelled the proportion of time the sharks were swimming in the top 1-meter layer and could therefore be detected by observers conducting aerial surveys. The availability models indicated that the tagged sharks preferred surface waters whilst swimming over the continental shelf and during daytime, with a model-predicted average proportion of time spent at the surface of 0.633 (SD = 0.094) for on-shelf, and 0.136 (SD = 0.075) for off-shelf. These predicted values were then used to account for availability bias in abundance estimates for the species over a large area in the Northeast Atlantic, derived through distance sampling using aerial survey data collected in 2015 and 2016 and modelled with density surface models. Further, we compared abundance estimates corrected with model-predicted availability to uncorrected estimates and to estimates that incorporated the average time the sharks were available for detection. The mean abundance (number of individuals) corrected with modelled availability was 15,320 (CV = 0.28) in 2015 and 11,001 (CV = 0.27) in 2016. Depending on the year, these estimates were ~7 times higher compared to estimates without the bias correction, and ~3 times higher compared to the abundances corrected with average availability. When the survey area contains habitat heterogeneity that may affect surfacing patterns of animals, modelling animals' availability provides a robust alternative to correcting for availability bias and highlights the need for caution when applying "average" correction factors.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Animals
Atlantic Ocean
Behavior, Animal*
Ireland
Models, Biological*
Population Density
Remote Sensing Technology*
Satellite Communications
Sharks*
Swimming
Time Factors

Grants and funding

The funding for the aerial surveys in this project were provided by Ireland’s Department of Communication, Climate Action and Environment and the Department of Culture, Heritage and the Gaeltacht, under Ireland’s ObSERVE Programme established in 2014. Blue shark tagging was supported by UCC Strategic Research Fund 2013. MN was supported by Project FishKosm that is funded by the Department of Agriculture, Food and the Marine’s Competitive Research Funding programmes. ALNILAM Research and Conservation Ltd provided support in the form of salaries for author AC, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section.