Reef effect of offshore structures on the occurrence and foraging activity of harbour porpoises

Abstract

With increasing numbers of offshore structures being installed and decommissioned, a better understanding of their effect on marine predators is timely. There is some evidence that oil and gas platforms may attract marine mammals, acting as artificial reefs. However, it is unclear whether different man-made structure designs have similar effects or whether artificial structures modify the diel patterns of occurrence and foraging of marine mammals. Here, we used passive acoustics to investigate the occurrence and foraging activity of harbour porpoises (Phocoena phocoena) around four artificial structures of different age and complexity. We deployed an array of echolocation click detectors (CPODs) in 2021, along a gradient of distances to these structures and assessed the extent to which porpoises were attracted to them and their effect on porpoises’ diel patterns of occurrence and foraging activity. The probability of porpoise occurrence and foraging activity decreased with distance from offshore structures. A significant increase in porpoise occurrence and foraging was detected during night-time compared to daytime around all four offshore structures (< 200 m). Comparing pre- and post-installation porpoise detections, the daily patterns of occurrence and foraging activity shifted from a weak diel pattern before the structure was installed, to a strong nocturnal pattern when the structure was present. These findings provide evidence that marine mammals are attracted to man-made structures and that porpoises modify their diel patterns of occurrence and foraging activity around them. This research suggests that offshore structures play an important role as foraging areas for some marine mammals and provides key information for decommissioning considerations and the planning of decommissioning activities.

Theory, practice, and design criteria for utilizing artificial reefs to increase production of marine fishes

“…Shallow anthropogenic seafloor structures influence the foraging of marine mammals (Arnould et al, 2015;Todd et al, 2020). The interplay among the potential to redistribute and/or enhance mammal populations as well as attracting these predators to artificial reefs (Fernandez-Betelu et al, 2022) is an open question necessitating further investigation as these predators could increase the natural mortality of fish and invertebrate populations. Determining mechanisms for (or direct evidence of) increased survival or growth of fishes also provided evidence that artificial reef systems are not functioning as ecological traps (Reubens et al, 2014).…”

The Use of eDNA to Monitor Pelagic Fish in Offshore Floating Wind Farms

“…While not a target species in this analysis, a faint eDNA signal from harbor porpoise was detected in the MiFish data from one of the wind-farm stations. Previous reports suggest that porpoises actively seek out offshore wind installations for foraging (Fernandez-Betelu et al, 2022). This shows that optimizing eDNA markers for marine mammals could facilitate inclusion of such species in future eDNA surveys.…”

What the F-POD? Comparing the F-POD and C-POD for monitoring of harbour porpoise (Phocoena phococena).

“…Passive acoustic monitoring of cetaceans has resulted in high-resolution data providing us with insights of population size and abundance (Marques et al , 2013;Amundinet al , 2022), habitat use (Fleming et al , 2018;Palmeret al , 2019), and behaviour (Pirotta et al , 2014;Malinka et al , 2021;Todd et al, 2022) for many species. Such technology is also fundamental for long-term monitoring, particularly with the increase in coastal developments and potential disturbance from construction, marine renewable devices, shipping, and fisheries (e.g., Todd et al, 2020Todd et al, , 2022Omeyer et al , 2020;Ramesh et al , 2021;Fernandez-Betelu, Graham and Thompson, 2022).…”