Pursuitplunging by northern gannets (Sula bassana) feeding on capelin

• Main Authors: Stefan Garthe, Silvano Benvenuti, William A. Montevecchi
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2000
• Subjects: diving; foraging; swimming speed; predator^prey interaction; pelagic ¢shes; seabirds; Funk Island; North West Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.485.1210; http://play.psych.mun.ca/~mont/pubs/pursuit.pdf

Northern gannets (Sula bassana) are considered to obtain prey usually by rapid, vertical, shallow plunge dives. In order to test this contention and investigate underwater foraging behaviour, we attached two types of data-logging systems to 11 parental northern gannets at Funk Island in the North-West Atlantic. We documented, for the ¢rst time to the authors ’ knowledge, gannets performing long, £at-bottomed, U-shaped dives that involved underwater wing propulsion as well as rapid, shallow, V-shaped dives. The median and maximum dive depths and durations were 4.6 and 22.0m and 8 and 38 s, respectively. Short, shallow dives were usually V-shaped and dives deeper than 8m and longer than 10 s were usually U-shaped, including a period at constant depth (varying between 4 and 28 s with median 8 s). Diving occurred throughout the daylight period and deepest dives were performed during late morning. On the basis of motion sensors in the loggers and food collections from telemetered birds, we concluded that extended, deep dives were directed at deep schools of capelin, a small pelagic ¢sh, and we hypothesized that V-shaped dives were aimed at larger, pelagic ¢shes and squids. Furthermore, these V-shaped dives allowed the birds to surprise their pelagic prey and this may be critical because the maximum swimming speeds of the prey species may exceed the maximum dive speeds of the birds.