Diving behaviour of Grey Petrels and its relevance for mitigating long-line by-catch -B

Rollinson DP, Dilley BJ, Davies D, Ryan PG (2016) Diving behaviour of Grey Petrels and its relevance for mitigating long-line by-catch -B. EMU 116:340–349. https://doi.org/10.1071/MU15032

The Grey Petrel (Procellaria cinerea) is listed as Near Threatened globally owing to incidental mortality on long-line fishing gear and reduced breeding success on islands caused by the introduction of alien predators. However, there are few studies of its foraging ecology and none of its diving behaviour. We obtained data from temperature–depth recorders (n = 7 birds) and global positioning satellite trackers (n = 15) deployed on Grey Petrels breeding on Gough Island, South Atlantic Ocean. Most birds foraged in the productive oceanic waters west or north-west of South Georgia. Average maximum dive-depth was 3.2 ± 2.2 m with most dives <5 m (85%) and 95% of dives <7 m deep. The maximum dive-depth (22 m) was deeper than previous measurements of dive-depth in Procellaria petrels, and maximum dive-duration also was longer than previously recorded in Procellaria petrels (at least 39 s). Individuals varied greatly in the mean number of dives per day (range 0.4–24.5). Sex did not influence depth or duration of dives but sample sizes were small. The time of day influenced dive-depth, and dives during daylight were, on average, deeper than dives at night, but the effect was weak; the maximum dive-depth at night was 17 m. By providing insights into the diving behaviour of Grey Petrels our findings help to explain their high mortality on fishing long-lines. We suggest that fisheries adopt bird-scaring lines that protect long-lines from scavenging seabirds during the setting process to a depth of at least 10 m, which could be achieved by increasing line-weighting or modifying bird-scaring lines, or both. An understanding of the foraging ecology of commonly recorded by-catch species, such as Grey Petrels, is essential in the design of future devices to mitigate seabird by-catch in long-line fisheries.