Abstract
Abstract Many seabirds dive to forage, and the ability to use this hunting technique varies according to such factors as morphology, physiology, prey availability, and ambient light levels. Proficient divers are more able to seize sinking baits deployed by longline fishing vessels and may return them to the surface, increasing exposure of other species. Hence, diving ability has major implications for mitigating incidental mortality (bycatch) in fisheries. Here, the diving behaviour and activity patterns of the most bycaught seabird species worldwide, the white‐chinned petrel (Procellaria aequinoctialis), tracked from Bird Island (South Georgia), are analysed. Three data sources (dives, spatial movements, and immersion events) are combined to examine diverse aspects of at‐sea foraging behaviour, and their implications for alternative approaches to bycatch mitigation are considered. The tracked white‐chinned petrels (n = 14) mostly performed shallow dives (<3 m deep) of very short duration (<5 s), predominantly during darkness, but only 7 and 10% of landings in daylight and darkness, respectively, involved diving, suggesting that surface‐seizing is the preferred foraging technique. Nonetheless, individuals were able to dive to considerable depth (max = 14.5 m) and at speed (max = 2.0 m·s−1), underlining the importance of using heavy line‐weighting to maximize hook sink rates, and bird‐scaring lines (Tori lines) that extend for long distances behind vessels to protect hooks until beyond diving depths.
Highlights
Seabirds vary widely in the manner in which they exploit marine food resources, with diving providing a means of accessing prey at various depths in the water column (Shealer, 2002; Elliott et al, 2008)
Knowledge of the diving ability of seabirds was revolutionized by the development of electronic time–depth recorders (TDRs) in the 1970s, which use pressure sensors (Kooyman & Campbell, 1971)
Diving is energetically expensive in seabirds, and in practice the frequency and characteristics of dives can differ considerably within and among species according to local prey availability and distribution, ambient light conditions, individual energetic requirements, or the degree of inter- and intra
Summary
Seabirds vary widely in the manner in which they exploit marine food resources, with diving providing a means of accessing prey at various depths in the water column (Shealer, 2002; Elliott et al, 2008). Incidental mortality (bycatch) of seabirds in longline fisheries has severely depleted the population sizes of many species, especially wide-ranging and long-lived albatrosses and petrels (Anderson et al, 2011; Phillips et al, 2016). These birds forage behind fishing vessels, attracted by discards (including offal) and baited hooks available during the deployment of longlines. The aims were to: (1) build a detailed picture of the foraging behaviour of white-chinned petrels during an energetically expensive period of their annual cycle; and (2) consider the implications for the design and performance of seabird bycatch mitigation measures in longline fisheries.
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