Abstract
Research on marine mammal occurrence in ship channels often focuses on large cetaceans in offshore shipping routes, while nearshore research largely addresses small vessel strikes. Marine mammals, such as the West Indian manatee, that reside in or migrate through nearshore areas, have potential to travel through a wide range of channel types, encountering a greater diversity of vessels than previously recognized. We tested the extent and conditions of ship channel use by manatees along the north-central Gulf of Mexico (nGoM) coast by combining data from telemetry-tracked individuals, opportunistic citizen-sourced sightings, and environmental attributes linked to manatee movements. Manatees used both nearshore boat channels (130 and 300m wide) and open water fairways but used nearshore channels much more frequently, consistent with habitat requirements. Satellite-tracked individuals swam faster and moved more directly in all channel types, indicating use of these channels as migratory and travel corridors. Accordingly, generalized additive models revealed that manatees used channels most often during spring/early summer and fall and at temperatures coincidental with entry to and exit from the nGoM during migration. Manatees also occurred in ship channels when freshwater discharges were low, likely because timing of peak manatee occurrence in the nGoM coincides with seasonally low discharge periods. Expanding shipping activity world-wide is likely to increase interactions between marine mammals and a variety of vessel types, and these effects may be particularly impactful to migratory animals like manatees that use nearshore habitats at the interface of recreational boating and commercial shipping. Linking near- and offshore ship channel use to migration and habitat use will better aid risk-assessment for vessel collision and other shipping related activities for migratory marine species globally.
Highlights
Boating and shipping activity are ubiquitous to global oceans and coasts
Tagged manatees occurred in ship channels when north-central Gulf of Mexico (nGoM) water temperature ranged from 22 to 32◦C and local air temperature ranged from 12 to 31◦C (Figures 3B,C), while manatees were sighted in ship channels when nGoM water temperatures ranged from 20 to 32◦C and local air temperature ranged from 7 to 31◦C (Figures 3F,G)
Tagged and sighted manatees used ship channels when freshwater discharges ranged from 9 to 100,733·cfs−1 (Figure 3D) and 164 to 78,329·cfs−1 (Figure 3H), respectively. An example of these patterns can be seen in movements of a tagged manatee (TMA001, “Bama”) during fall 2009, when she used channels, including the Mobile Bay ship channel to access foraging sites in late October, before exiting the region via the Intracoastal Waterway (ICW) between Bon Secour, AL and Pensacola, FL in early November (Supplementary Video S1)
Summary
Boating and shipping activity are ubiquitous to global oceans and coasts. The intensity of shipping has increased drastically during the last several decades to accommodate the expanding global economy (Tournadre, 2014). Most research on vessel interactions with marine megafauna focuses on large cetaceans in offshore shipping routes or recreational boat collisions in nearshore waters (Laist et al, 2001; Laist and Shaw, 2006; van der Hoop et al, 2015; Edwards et al, 2016; Crum et al, 2019). Pressure from shipping, is increasing in nearshore as well as offshore channels (Tournadre, 2014; Zhang et al, 2015) and will likely affect smaller cetaceans, pinnipeds, and sirenians that live in nearshore areas at the interface of recreational and commercial channels. Little research has focused on the use of nearshore channel types by marine mammals
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