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Marine Ecology Progress Series

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MEPS 429:209-218 (2011)  -  DOI:

Acoustic observations of dusky dolphins Lagenorhynchus obscurus hunting Cape horse mackerel Trachurus capensis off Namibia

Matteo Bernasconi1,2,*, Leif Nøttestad1, Bjørn Erik Axelsen1, Jens-Otto Krakstad1

1Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway
2Pelagic Ecology Research Group, Scottish Oceans Institute, University of St Andrews, St Andrews KY16 8LB, UK

ABSTRACT: Predator–prey interactions of mammals and their fish prey in marine ecosystems are rarely identified and recorded. We document for the first time the underwater behaviour of hunting dusky dolphins Lagenorhynchus obscurus and the responses of their prey, Cape horse mackerel Trachurus capensis, observed in open ocean waters off northern Namibia. Predator–prey interactions were monitored acoustically during a continuous period of 2 h with a split-beam scientific echo sounder, and the surface behaviour of the dolphins was observed from the ship’s deck. In total 54 predator–prey events were observed (mean, 0.45 events min–1). The maximum burst speed during attack was 9.9 m s–1, while the average attack speed was 3.4 m s–1 (mean swimming speed: 1.4 m s–1). Dolphin traces were predominantly located underneath the fish aggregation (63% of the time), as the dolphins attacked the schools from underneath (mean depth: 120 m; maximum depth: 156 m). The dolphin target strength at 38 kHz was, on average, –31.5 dB (95% CI: –33.4 to –30.2 dB). The attacks caused immediate reactions in the Cape horse mackerel aggregations: subschools were forced towards the surface where they were herded into dense aggregations by the dolphins. Observed predator response patterns included ‘Vacuole’ (n = 21), ‘Split’ (n = 19), ‘Bend’ (n = 10) and ‘Hourglass’ (n = 4). Packing densities changed significantly before (0.4 fish m–3), during (2.0 fish m–3) and after (0.3 fish m–3) the dolphin attacks, and when predators burst into the fish aggregations, mean (±SD) intraschool packing densities of horse mackerel were significantly lower in front of the predators (0.33 ± 0.17 fish m–3) than behind (6.65 ± 1.76 fish m–3).

KEY WORDS: Dusky dolphin · Lagenorhynchus obscurus · Cape horse mackerel · Trachurus capensis · Feeding behaviour · Ecology · Predator–prey interactions · Acoustics · Target strength

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Cite this article as: Bernasconi M, Nøttestad L, Axelsen BE, Krakstad JO (2011) Acoustic observations of dusky dolphins Lagenorhynchus obscurus hunting Cape horse mackerel Trachurus capensis off Namibia. Mar Ecol Prog Ser 429:209-218.

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