SARF 054
CONTAINMENT SPECIAL
PREDATOR NETS ARE MORE WIDELY USED IN CHILE, WHERE SEA LIONS POSE A THREAT
predator attack, where appropriate. However, additional research in to the effectiveness of physical measures in deterring predation at freshwater sites is recommended to help inform farmers’ decisions in this regard. Whilst waterline protection may also be used to pro- tect sea water sites against seal attacks most seal attacks occur at greater depths on the net.
Predator nets
THIS PAGE, TOP: NETS IN CHILE ARE REINFORCED TO PREVENT SEA LION ATTACK. ABOVE: THIS SEAL BLIND PROTECTS THE MORT SOCK
OPPOSITE: CHAFING AT A GALLOWS POLE
24
The use of predator nets is fairly rare in Scotland, although they are being used or trialled by several companies. The nets being used are either full box nets, which completely surround the cage net underwater, or a curtain net which does not go under the net but would usually be used in shallower sites where it can reach to the seabed. Predator nets are only in use at sites where seals are a known concern. Predator nets were in more common use in the early days of Scottish salmon farming, but they cause additional complexities for operators, require considerable additional work and can lead to significant by-catch, hence they are no longer favoured. They are usually hung on the outside of the cage and are weighted independently to the cage net. However, this does not always give sufficient separation to ensure that a seal cannot push up against the cage net, which may be one reason why predator nets have not been a proven success. Predator nets are in widespread use in Canada and Chile, where there is the added
risk of sea lion attacks. Such nets may well be positioned at a greater distance from the cage net and surround the entire site. They may also extend above sea level to help prevent sea lion access over the top. While they generate significant additional work and expense, their use in such instances suggests that they can be workable.
Cage nets can also be made to incorporate a false bottom, which is effectively an in-built predator net across the base of the net. It ap- pears that many successful seal attacks are on the base of the net, hence false bottom nets could help provide secure containment. False bottom nets, however, do bring disadvantages in terms of handling, cleaning and reduced oxygen flow, although they are not as difficult to work with as predator nets. They are being trialled by one farming company.
Fish mortalities
Efficient removal of mortalities from cage nets is important to prevent seal attacks. This is thought to be because, should a seal see a mortality at the base of the net, it may encour- age it to attack this area of the net. The CoGP requirement is that, weather permitting, fish mortalities should be removed daily. A few companies use seal blinds in the central area of the base of the net. These are additional panels of thick material which help to reduce seal attacks, as they should keep mortalities out of the sight of seals.
www.fishfarmer-magazine.com
PHOTO: MARINE HARVEST
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100