Study

Experimental attempts to reduce predation by harbour seals on juvenile out-migrating salmonids

  • Published source details Yurk H. & Trites A.W. (2000) Experimental attempts to reduce predation by harbour seals on juvenile out-migrating salmonids. Transactions of the American Fisheries Society, 129, 1360-1366.

Actions

This study is summarised as evidence for the following.

Action Category

Use acoustic devices on moorings

Action Link
Marine and Freshwater Mammal Conservation

Install barriers at wild fisheries

Action Link
Marine and Freshwater Mammal Conservation

Switch off artificial lighting at wild fisheries

Action Link
Marine and Freshwater Mammal Conservation
  1. Use acoustic devices on moorings

    A controlled study in 1996 at one site in the Puntledge River, British Columbia, Canada (Yurk & Trites 2000) found that deploying an acoustic device on a mooring under a bridge reduced the number of harbour seals Phoca vitulina feeding on migrating juvenile salmon Oncorhynchus spp. compared to when no device was used. The average number of seals feeding on salmon was lower with an acoustic device deployed (0.4 seals/night) than without (8 seals/night). In May 1996, an acoustic device (Airmar Seal Scarer with four projectors) was deployed for seven nights at a river below a bridge. The projectors were suspended 40 cm below the water surface attached to ropes and floats. The device emitted 2-second sound bursts at a frequency of 27 kHz. Two observers counted seals using a red-filtered spotlight every 30 minutes from 2100–0300 h during each of seven nights with the acoustic device active and seven randomly selected nights without the device.

    (Summarised by: Anna Berthinussen)

  2. Install barriers at wild fisheries

    A controlled study in 1996 at a site in the Puntledge River, British Columbia, Canada (Yurk & Trites 2000) found that installing a ‘cork line’ barrier did not reduce the number of harbour seals Phoca vitulina feeding on migrating juvenile salmon Oncorhynchus spp. under a bridge. Results are not based on assessments of statistical significance. Average numbers of seals feeding on juvenile salmon under the bridge were similar with a ‘cork line’ barrier installed (2–3 seals/30 minutes) and without (2 seals/30 minutes). Seals were observed ‘playing’ with the barrier. In April 1996, a ‘cork line’ (a 60-m rope with cork floats attached at 1 m intervals) was strung across a river below a bridge for an average of 3 h during each of two nights. Juvenile salmon were released from a hatchery. Two observers counted seals feeding on salmon using a red-filtered spotlight every 30 minutes from 2100–0300 h during each of two nights with the barrier and one randomly selected night without.

    (Summarised by: Anna Berthinussen)

  3. Switch off artificial lighting at wild fisheries

    A controlled study in 1996 at a site in the Puntledge River, British Columbia, Canada (Yurk & Trites 2000) found that switching off artificial lights on a bridge did not reduce the number of harbour seals Phoca vitulina feeding on migrating juvenile salmon Oncorhynchus spp. Average numbers of seals feeding on salmon under the bridge did not differ significantly with artificial lights switched off (1–10 seals/30 minutes) or on (2–15 seals/30 minutes). In April–May 1996, fourteen artificial lights on a bridge over the river were switched off for four nights. Juvenile salmon were released from a hatchery. Two observers counted seals feeding on salmon using a red-filtered spotlight every 30 minutes from 2100–0300 h during each of the four treatment nights and eight randomly selected nights with no treatments.

    (Summarised by: Anna Berthinussen)

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