Gulf of Oman
The upper layer at night was found to have an average vertical extension of 74 m and was found all over the Oman Gulf at night. The deeper night layer with a vertical extension of 60 m was found in about 75% of the area at night. The trawlcatches from these two layers have been used to calculate the abundance of mesopelagic fish in the Gulf of Oman.
|
|
Vertical extension, |
Average fish catch per n. mile2, kg
|
Total quantity per n. mile2,
tonnes |
|
Upper layer |
74 |
87 |
48 |
|
Lower |
61 |
12 |
6 |
|
|
|
|
54 |
The total sea surface of the Gulf of Oman (excluding the shelf area) is estimated to be 2.66 x 104 n. mile2. This gives a total abundance of myctophid fish of 1.4 million tonnes.
Gulf of Aden
In the Gulf of Aden the upper night layer had a mean vertical extension of 65 m and the lower layer 155 m. Both layers were found all over the Gulf. The day layer had a vertical extension of 161 m.
If the trawl catches from these layers are used, the following abundance estimates can be made.
|
|
Vertical extension, m |
Average fish catch per n. mile2 kg |
Total quantity per n. mile2 tonnes |
|
|
Night upper layer |
66 |
18 |
9 |
26 |
|
Night lower layer |
156 |
15 |
17 |
|
|
Day |
161 |
9 |
10 |
|
No trawl catches 100% of the fish in the filtered water. Estimates of abundance based on trawl catches therefore always tend to be too low. In this case the estimate for the Gulf of Oman was only 23% of that based on acoustical methods. For the Gulf of Aden the estimates based on trawl catches gave about 50% and 20% for night and day catches respectively.
No estimates based on day catches was made for the Gulf of Oman as the catches in the upper day layer gave a very high variance.
Similar comparisons of acoustic abundance estimates and trawl catches were made by Myrseth (in prep.) off Pakistan and by Gjøsæter and Blindheim (1980) off North-west Africa. They got ratios between 10 and 50%.
