Author: Jenny

Figure 2. A picture taken by an underwater
camera showing a grey seal taking a bait.

What does the result show?
From the results it can be concluded that grey seals use their vision when locating buoys (fishing gears) and that buoy size matters because larger buoys are more often discovered by seals. I suggest that as small buoys as possible should be used when marking fishing gears, to decrease the damages caused by seals.

Contact:
Jenny Kleiner
j.kleiner@chello.se
Applied Biology
International Masters Program,
Dep. Of Biology, IFM,
Linköping University, Sweden

Grey seals (Halichoerus grypus)
use their vision to find buoys,
representing fishing gears!
Jenny Kleiner

Applied Biology
International Masters Program
Linköping University, Sweden

The aim
The aim of the study was to investigate if grey seals use their vision to locate buoys, which are representing fishing gears, and if the size of the buoys matter.
Why?
Grey seals cause economic losses for professional fishermen when taking their catch and destroying their fishing gears1. There is also a risk that seals get stuck in the nets and die by drowning. We need more knowledge about how seals find fishing gears, to be able to decrease the damages.
Where and when?
The study was conducted in two areas in the Baltic Sea; in Skärså, 29/6 — 25/8, 2004, and in Nävekvarn, 25/9 – 1/11, 2004.

How?
I used red buoys in four different sizes which were baited with herring. One third of the buoys were equipped with an
underwater camera to verify that it actually was seals that took the baits. When the bait was taken the camera was triggered and a picture of the “thief” was taken. Buoys were dispersed in the areas, checked once a day and moved if the baits were taken. I changed the baits every day to always have fresh fish under the buoys.

What were the results?
In total 382 buoys were put out and from these 74 baits were taken. The number of taken baits increased with buoy size (Figure 1).
The pictures taken by the underwater cameras all showed grey seals taking the
baits (Figure 2).

Figure 1. The relative frequency of taken baits vs. the buoy diameter

1Westerberg H, Fjälling A & Martinsson A (2000) Sälskador i det svenska fisket. (seal damage in the Swedish fishery) Fiskeriverket rapport 3, 4 – 38 (in Swedish with English summary)
2Harding KC & Härkönen TJ (1999) Developments in the Baltic grey seal (Halichoerus grypus) and ringed seal (Phoca hispida) populations during the 20th century. Ambio 28; 7, 619 – 627.

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