Combining multiple visual surveys to model the habitat of deep-diving cetaceans at the basin scale
Résumé
Aim: Deep‐diving cetaceans are oceanic species exposed to multiple anthropogenic
pressures including high intensity underwater noise, and knowledge of their
distribution is crucial to manage their conservation. Due to intrinsic low densities,
wide distribution ranges and limited presence at the sea surface, these species are
rarely sighted. Pooling data from multiple visual surveys sharing a common linetransect
methodology can increase sightings but requires accounting for heterogeneity
in protocols and platforms.
Location: North Atlantic Ocean and Mediterranean Sea.
Time period: 1998 to 2015.
Major taxa: Ziphiidae; Physeteriidae; Kogiidae.
Methods: About 1,240,000 km of pooled effort provided 630 sightings of ziphiids,
836 of physeteriids and 106 of kogiids. For each taxon, we built a hierarchical
model to estimate the effective strip width depending on observation conditions
and survey types. We then modelled relative densities in a generalized additive
modelling framework. Geographical predictions were limited to interpolations
identified with a gap analysis of environmental space coverage.
Results: Deeper areas of the North Atlantic gyre were mostly environmental extrapolation
in the predictions, thereby highlighting gaps in sampling across the different
surveys. For the three species groups, the highest relative densities were
predicted along continental slopes, particularly in the western North Atlantic
Ocean where the Gulf Stream creates dynamic frontal zones and eddies.
Main conclusions: Pooling a large number of surveys provided the first basin‐wide
models of distribution for deep‐diving cetaceans, including several data‐deficient
taxa, across the North Atlantic Ocean and the Mediterranean Sea. These models can
help the conservation of elusive and poorly known marine megafauna.