The method of stock assessment of mesopelagic resources (myctophids) was relied on both echo integration data and trial fishing. This method is shown by Equation:
Biomass
B = ò .w = ò o / j .w (1)
ò o = Qo/F.v.t.
Q = Qo/Q.100
Q = Qo + q
Density
ò = Q/F.v.t (2)
Where
B = biomass (kg)
ò = distribution density (kg/m³)
ò o = catch density (kg/m³)
W = distribution volume (m³)
F= opening mouth area (m²)
V = tow velocity (m/s)
T = towing time (s)
Qo = amount of catch (kg)
Q = amount of fish in towing region (kg)
q = amount of fish escaped from net (kg)
j = catch coefficient (%)
The amount of fish that escaped through the net meshes in different parts of nets was estimated by installing small meshed nets to various parts of the main net. The amount of escaped fishes were calculated using formula 3:
q = q1 F1 + q2 F2 +....qnFn (3)
q1, q2, qn : amount of fish entered in each sac (kg)
F1, F2, Fn : the area of the opening of the small mesh sac (m²)
Also, there were seven experimental hauls to find out and estimate the
amount of fish escaped from main net (Table 1).
The results showed that, for each 10t catch, about 2.5t fish escaped. In addition, some other fishes escaped from the codend and as other fishes (e.g., Nesepinnula sp.) damaged the net and therefore some Myctophids would have escaped from the net as a consequence.
Amount of escaped fish from the trawl
No. |
Area (m²) |
Fish proportion in small mesh sacs (tonnes) |
Amount of escaped fish (t) |
|||
Min |
Max |
Mean |
||||
1 |
640 |
0.4 |
0.9 |
0.65 |
0.416 |
|
2 |
432 |
0.4 |
1.0 |
0.70 |
0.302 |
|
3 |
473 |
0.5 |
1.2 |
0.85 |
0.402 |
|
4 |
453 |
0.5 |
1.3 |
0.90 |
0.408 |
|
5 |
374 |
0.6 |
1.6 |
1.10 |
0.411 |
|
6 |
227 |
1.0 |
2.5 |
1.75 |
0.397 |
|
7 |
178 |
0.8 |
1.5 |
1.15 |
0.205 |
|
T O T A L |
2.541 |
|||||
The biomass estimates were made for the four different cruises (see Table 2). As the table shows, the biomass estimates ranged from 1 118 000 to 1 330 000t within a 10 400 nm2 area (limited by the 50nm bound offshore from the Iranian coastline). If the total area of the Gulf of Oman (Iranian side) is taken as 20 000nm2 and if it is assumed that the fish density was 9/m3, the total biomass estimate is then 2 100 000 - 2 300 000t.
