An effective way to catch salmon commercially is by gill netting. Traditionally, gill netters hang long monofilament nets vertically over the fishing grounds. As the salmon encounter these nets they become entangled in the nets by their gill covers (operculum) and die. Many other species also become entangled in the nets and die. These animals are euphemistically called " by-catch". As part of a conservation project on by-catch reduction, three modified techniques are used on boats fishing for salmon with gill nets.
I. W1. White-topped gill nets (the top 7 feet of the net is white mesh that is considerably easier to see in the water column).
II. W2. White-topped pingers that emit an annoying beep similar to those made by watches with alarms.
III. U3. Unmodified gill nets with pingers.
IV. U4. Unmodified gill nets (standard method).
Due to constraints of the design, the experiment was run over several days. Are there differences amongst the types of nets? These data meet the assumptions of parametric statistics. Do post hoc tests to determine which type of net(s) best reduce by-catch.
|
Catch of salmon
(100 kg/boat-day)
|
|
Day
|
I
|
II
|
III
|
IV
|
|
1
|
23
|
21
|
24
|
25
|
|
2
|
16
|
14
|
19
|
17
|
|
3
|
19
|
17
|
23
|
21
|
|
4
|
11
|
12
|
15
|
16
|
|
5
|
41
|
30
|
40
|
42
|
|
6
|
32
|
20
|
37
|
35
|
|
7
|
9
|
11
|
19
|
17
|
|
8
|
10
|
14
|
19
|
16
|