Abundance of demersal fish resources in the Persian Gulf ... - CiteSeerX

J. Mar. Biol. Ass. U.K. (2006), 86, 1455^1462 Printed in the United Kingdom

doi: 10.1017/S0025315406014512

Abundance of demersal ¢sh resources in the Persian Gulf and Oman Sea T. Valinassab*P, R. Daryanabard*, R. Dehghani* and G.J. PierceO *Department of Resource Management, Iranian Fisheries Research Organization (IFRO), PO Box 14155-6116, Tehran, Iran. O School of Biological Sciences (Zoology), University of Aberdeen, Tillydrone Avenue, Aberdeen, UK. P Corresponding author, e-mail: [email protected]

We report on results of a trawl survey during 2003^2004 to assess the abundance of demersal ¢sh resources in the Persian Gulf and Oman Sea. Samples were taken at a total of 316 trawl stations selected following a strati¢ed random procedure. Catch rates (catch per unit area, CPUA) and total biomass were estimated. Total demersal ¢sh biomass was estimated to be approximately 73,000 tonnes in Persian Gulf waters and approximately 39,000 tonnes in the Oman Sea. The lowest CPUA was recorded in the west of the study area (stratum A, approximately 1700 kg/n.m.2) and the highest in the east (stratum Q, 13943.4 kg/n.m.2), although density of commercially important species was higher in the central area (stratum K). Catch rate and biomass varied signi¢cantly in relation to seabed depth. Commercially important demersal species made up around 60% of the estimated total biomass. The most abundant species groups were rays, cat¢sh, grunts, nemipterids and carangids. Several important species (e.g. silver pomfret, croakers and sharks) appear to have declined since the late 1970s while others, such as rays and cat¢sh, have increased.

INTRODUCTION The waters of the Persian Gulf and Oman Sea are environmentally unique with an unusual faunal assemblage (Carpenter et al., 1997). The Persian Gulf is a semienclosed water body connected to the Oman Sea through the Strait of Hormuz, which is 56 km wide at its narrowest point. The maximum width of the Gulf is 640 km and average depth is 35 m (Reynolds, 1993). The Oman Sea, with an area of 94,000 km2 and a depth reaching 3200 m, connects the Persian Gulf to the Indian Ocean through the Arabian Sea. The Persian Gulf and Oman Sea are in the subtropical zone, lying almost entirely between the latitudes of 248 and 308N and longitudes of 498 to 61825’E (Figure 1). Average annual rainfall is between 3 and 8 cm per year (Reynolds, 1993). The dominant large-scale current is a counter-clockwise movement, whereby less saline (and less dense) water enters at the Strait of Hormuz at the surface and more saline (and more dense) water leaves the area at the bottom (Hunter, 1983; Reynolds, 1993). The entire water column is well mixed. However, primary productivity for the Persian Gulf and Oman Sea is apparently only average, being higher than in most of the Red Sea but lower than in the Arabian Sea (Sheppard et al., 1992). The topography of the bottom is mostly £at and featureless, dominated by soft sediments, with a few rocky areas in the Oman Sea. Next to oil, ¢sheries represent the second most important natural resource, and the most important renewable natural resource (Carpenter et al., 1997). A review of ¢shery statistics shows a trend of increasing ¢shing e¡ort in the Persian Gulf and Oman Sea during the last decade. Thus, the number of ¢shermen increased from 70,729 in 1993 to 109,601 in 2002 (Planning & Development Journal of the Marine Biological Association of the United Kingdom (2006)

Department, 2003). Demersal ¢sh are one of the most important groups for both artisanal and industrial ¢sheries but catch data show a 21% decrease in landings from 110,000 tonnes in 2002 to 87,240 tonnes in 2003 (Planning & Development Department, 2003). Therefore, this ecological group of ¢sh is classi¢ed as over-exploited in the region. The ¢rst studies on demersal ¢sh in this area were carried out between 1976 and 1979 under a United Nations Food and Agriculture Organization regional project covering all southern and northern Persian Gulf and Oman Sea waters and using four research vessels; the total biomass was estimated at more than 120,000 tonnes (Sivasubramaniam, 1981). Further studies in the Persian Gulf waters, to estimate demersal ¢sh biomass, took place between 1994 and 1996 (IFRO Experts Group, 1996). Currently, there is a ¢ve-month open season for ¢sh trawlers in the Oman Sea whereas (from ten years ago) in the Persian Gulf, trawling for ¢sh is banned and only shrimp trawlers are active. In order to provide further advice for the management of demersal resources, a comprehensive research project, covering all Iranian waters of the Persian Gulf and Oman Sea, was designed and carried out in 2002. The main objectives were: (1) to estimate the biomass and catch per unit area (CPUA) of demersal species; (2)to determine the catch composition and distribution pattern of the main species; (3)to quantify variation in abundance across the study area and in di¡erent depth strata; (4)to provide the information needed to underpin future resource management.

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T. Valinassab et al.

Demersal ¢sh resources of Iranian waters

Figure 1. Study area for stock assessment of demersal resources in the Persian Gulf and Oman Sea (the x and y scales show degrees east and north respectively).

MATERIALS AND METHODS Study area and sampling stations

The study area was restricted to Iranian waters of the Persian Gulf and Oman Sea, between longitudes 49800’E and 61825’E (Figure 1) and isobaths of 10 to 50 m in the Persian Gulf and 10 to 100 m in the deeper waters of the Oman Sea. The total area was divided, from west to east, into 17 strata (A to Q), ten in the Persian Gulf (A to J) and seven in the Oman Sea (K to Q). Each stratum was classi¢ed into four depth substrata: 10^20, 20^30, 30^50 and 50^100 m. The total area and area of each substratum was calculated with a plannimeter (Table 1). A total of 316 trawl stations was selected following a strati¢ed random procedure, with the number of hauls in each substratum being proportional to the area of the substratum. There were 199 stations in Persian Gulf waters and 117 stations in the Oman Sea (Table 1). Sampling

Five cruises were carried out in 2003 on board RV ‘Ferdows-1’. This vessel is a stern trawler (673 gross

registered tonnage, 45.4 m length) and was equipped with a Global Positioning System (GPS), ITI net sounder system, two echo sounders and a bottom-trawl net (mesh size of cod end 80 mm and headline 72 m). For each trawl the following data were logged: date, time, duration, bottom depth, GPS position, towing distance, towing speed and distance from the coast. Each trawl lasted 1h, following which the net was hauled and the catch sampled as follows: (a) all large ¢sh (such as sharks, rays, large cat¢sh, etc.) were separated from the catch, counted and weighed; (b) the remaining catch was distributed into equal-sized baskets and one in every ¢ve baskets was selected randomly; (c) for each selected basket, all ¢sh were identi¢ed to species or group (102 categories recognized, see Table 2), and animals in each group were counted and weighed; (d) total number and weight for each category were calculated by multiplying average (total) weight for the randomly selected baskets by the total number of baskets, then adding the number and weight of large ¢sh.

Table 1. The number of trawl stations and area of each stratum in (a) the Persian Gulf and (b) the Oman Sea; see Figure 1 for locations of the substrata. (a) Stratum.

A

B

C

D

E

F

G

H

I

J

Area (n.m.2) Station

621.7 7

1415.6 30

1415.1 40

909.1 27

227.5 6

317.8 14

482.8 17

338.0 12

271.2 20

641.3 26

(b) Stratum.

K

L

M

N

O

P

Q

Area (n.m.2) Station

572.5 12

406.1 13

116.0 15

180.9 17

235.0 20

268.5 20

363.8 20

Journal of the Marine Biological Association of the United Kingdom (2006)

Demersal ¢sh resources of Iranian waters T. Valinassab et al. 1457 Table 2. Checklist of species/species-groups in bottom trawl catches in the Persian Gulf and Oman Sea (2003^2004), their categorization by habitat (D, demersal or P, pelagic) and economic importance (C, commercial, NC, non-commercial) and their percentage contribution to total estimated biomass (%B). Commercial species are those taken by ¢shermen for domestic human consumption or for export. The most abundant species of each family are listed separately while rare and less important species are grouped as ‘others’. % of total biomass

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Species/species-group

Ecologic group

Economic value

Persian Gulf

Oman Sea

Uroteuthis duvauceli URANOSCORPIDAE Sea fans TETRADONTIDAE MULLIDAE Trachinotus spp. CHAETDONTIDAE LEIOGNATHIDAE (others) Leiognathus fasciatus MONACANTHIDAE SCOMBRIDAE PRIACANTHIDAE OSTRACIIDAE Seaweeds GERRIIDAE Saurida tumbil Pampus argenteus Parastromateus niger DIODONTIDAE CHIROCENTRIDAE Crabs (others) Portunus pelagicus Portunus sanguinolentus TRIGLIDAE Platax spp. Sea cucumbers DACTYLOPTERDAE APOGONIDAE POLYNEMIDAE (others) Eluthernema tetradactylum PLATYCEPHALIDAE (others) Grammoplites suppsitus Platycephalus indicus CLUPEIDAE Scomberoides spp. Rays Sea stars Cephalopods (others) Lutjanus malabaricus Lutjanus johni LUTJANIDAE (others) Pomadasys kaakan HAEMULIDE (others) TRICANTHIDAE RHYNOBATIDAE Rachycentron canadum Acanthopagrus bifasciatus Acanthopagrus latus SPARIDAE (others) LETHRINIDAE Ilisha spp. SILAGINIDAE Otolithes rubber SCIANIDAE (others) FISTULARIIDAE Scomberomorus commerson

Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic

Commercial Non-commercial Non-commercial Non-commercial Non-commercial Commercial Non-commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Non-commercial Non-commercial Non-commercial Commercial Commercial Commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Non-commercial Commercial Non-commercial Non-commercial Non-commercial Commercial Commercial Non-commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Non-commercial Non-commercial Commercial Commercial Commercial Commercial Non-commercial Non-commercial Non-commercial Commercial Commercial Commercial Commercial Commercial Non-commercial Non-commercial Commercial Commercial Non-commercial Commercial

0.30 0.00 0.00 0.02 2.27 0.05 0.00 0.72 0.15 0.02 0.04 0.09 0.00 0.00 0.21 6.53 1.31 1.21 ^ 0.17 0.02 0.01 ^ 0.06 0.00 0.00 0.00 0.00 0.22 0.14 0.00 0.33 0.19 1.09 0.97 11.37 0.00 0.00 3.08 0.08 0.31 4.69 0.52 0.01 0.35 1.82 0.07 1.04 0.41 0.50 2.52 ^ 0.65 1.70 0.00 1.56

0.01 0.03 0.00 0.01 0.10 0.13 ^ 0.08 0.90 0.01 0.03 0.01 0.00 0.00 0.72 3.72 0.84 0.46 0.08 0.03 0.19 0.02 0.02 0.89 0.01 0.00 0.00 0.00 1.26 0.00 0.00 0.32 0.01 0.19 0.43 21.92 0.00 0.00 0.06 0.37 0.02 9.81 1.39 0.02 0.41 0.10 0.02 0.80 0.22 0.11 0.24 0.00 1.67 2.39 0.01 0.22 (Continued)


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Table 2. (Continued). % of total biomass

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102

Species/species-group

Ecologic group

Economic value

Persian Gulf

Oman Sea

Ephippus orbis SIGANIDAE Sea shells Rastreliger kanagurta Jelly ¢sh Drepane punctata Drepane longimana SCORPAENIDAE Stomatopods Scomberomorus guttatus CALLIONYMIDAE Megalaspis cordyla Psettodes erumei Pleuronectiformes (others) Argyrops spinifer SPHYRAENIDAE Sharks Arius thalassinus Arius tenuispinis Arius dussumieri PLOTOSIDAE Nemipterus japonicus NEMIPTERIDAE (others) Lactarius lactarius Alectis spp. CARANGIDAE (others) Thenus orientalis HYDROPHIDAE Anguiliformes BALASTIDAE Mene maculate Sepia pharaonis ENGRAULIDAE Protonibea diacanthus Shrimps (others) Penaeus semisulcatus Metapenaeus a⁄nis Penaeus indicus Penaeus merguiensis Epinephelus diacanthus Epinephelus bleekeri Epinephelus coioides SERRANIDAE (others) Trichiurus lepturus TRICHIURIDAE (others) TERAPONIDAE

Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Pelagic Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal Demersal

Non-commercial Commercial Non-commercial Commercial Non-commercial Commercial Commercial Non-commercial Non-commercial Commercial Non-commercial Commercial Commercial Non-commercial Commercial Commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Commercial Commercial Commercial Commercial Non-commercial Non-commercial Non-commercial Non-commercial Commercial Non-commercial Commercial Non-commercial Commercial Commercial Commercial Commercial Commercial Commercial Commercial Commercial Commercial Non-commercial Non-commercial

0.55 0.05 0.01 0.16 0.30 1.17 0.01 0.03 0.00 0.47 ^ 0.04 0.71 0.42 1.36 2.88 2.12 8.82 2.30 2.54 0.00 10.53 0.17 0.01 0.63 8.81 0.02 0.00 0.73 0.00 0.17 1.50 ^ 0.45 0.00 0.61 0.00 0.00 ^ 0.04 0.12 0.51 0.10 2.27 0.00 0.57

0.05 0.00 0.01 0.09 0.26 0.42 4.67 0.11 0.00 0.24 0.01 0.34 0.37 0.91 2.30 2.54 1.10 1.49 2.24 4.81 0.00 4.40 2.24 0.57 1.03 7.14 0.00 0.04 0.78 0.00 0.11 3.28 0.08 0.68 ^ 0.01 0.00 0.00 0.00 0.81 0.05 0.20 0.06 5.78 0.00 0.17

Biomass and CPUA estimation

The biomass and CPUA were estimated based on Sparre & Venema (1992). The towing distance at a given station in a given substratum (dj,k, in nautical miles, n.m.) was measured by GPS or using the formula: dj,k ¼ vj,k tj,k

ð1Þ

where vj,k ¼speed of the vessel during towing (n.m. h71) at Station j in substratum k and tjk ¼towing duration (hours) at Station j in substratum k. Journal of the Marine Biological Association of the United Kingdom (2006)

Swept area (aj,k, n.m.2) at each station was then estimated as: aj,k ¼ dj,k hX

ð2Þ

where dj,k ¼towing distance (n.m.) for Station j in substratum k, h¼headline height and X¼wing spread coe⁄cient¼0.7 (calculated using the ITI net-sounder; thus wing spread is measured in relation to trawl mouth opening). The catch per unit area (CPUAi,j,k, kg n.m.72) for category i at Station j in substratum k is then given by:

Demersal ¢sh resources of Iranian waters T. Valinassab et al. 1459 Table 3. Biomass estimates (tonnes) for ¢sh species in the Oman Sea, 2003^2004. K

L

M

N

O

P

2590.8 1913.7 395.2 4899.7

502.4 390.6 159.6 1052.6

1468.8 1134.8 119.4 2723.0

3341.0 1156.0 191.5 4688.5

2433.8 921.4 63.2 3418.3

Q

Total

(a) In di¡erent spatial strata (K^Q) from west to east. Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

7571.5 3432.6 647.3 11651.4

(b) In di¡erent depth zones.

10^20 m

20^30 m

30^50 m

50^100 m

Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

10157.7 7475.4 724.8 18357.8

5997.8 3012.6 578.8 9589.2

4341.4 1460.4 329.1 6131.0

2694.5 1530.5 275.8 4500.7

CPUAi,j,k ¼ Ci,j;k /aj,k

Bi,k ¼

j

Nk

0:54Ak

23191.4 13479.0 1908.5 38578.7 Total 23191.4 13478.9 1908.5 38578.7

tonnes, with 72,592 (65%) and 38,579 (35%) tonnes in the Persian Gulf and Oman Sea, respectively (Tables 3 & 5).

ð3Þ

where Ci,j,k ¼catch (kg) of category i at Station j in substratum k and aj,k ¼swept area (n.m.2) for Station j in substratum k. The total biomass (Bi,k) for each category in each substratum is then estimated as: X CPUAi,;j,k

5283.1 4529.9 332.2 10145.2

Oman Sea

The total biomass of demersal ¢sh in the Oman Sea was estimated to be 38,579 tonnes. Maximum and minimum values for biomass were found in strata K (11,651 tonnes) and M (1053 tonnes), respectively (Table 3). Biomass was highest in the 10^20 m depth zone, i.e. the shallowest waters (18,358 tonnes, 48% of total biomass of Oman Sea) and lowest in the deepest waters (50^100 m depth, 450 1 tonnes, 12%). The commercial species group was most abundant in the 10^20 m depth zone and in stratum K (west of Oman Sea, Sirik to Jask area). Non-commercial species were most abundant in depths of 10^20 m and stratum Q (east of Oman Sea). The non-demersal (pelagic) species, taken as by-catch, comprised less than 5% of the estimated total biomass (Table 3). Mean CPUA for demersal ¢sh in the Oman Sea was 9002 kg/n.m.2 (Table 4). Maximum and minimum CPUA were found in strata Q (13,943 kg/n.m.2) and M (537.5 kg/n.m.2) respectively. The CPUA was relatively

ð4Þ

where Ak ¼total area of substratum k, Nk ¼number of stations in substratum k and 0.5¼escape coe⁄cient (using the value proposed by Sparre & Venema, 1992, for the multi-species demersal group in tropical and subtropical areas). Catch per unit area and biomass were estimated for di¡erent strata and depth layers for further comparisons. Values were estimated for commercial and noncommercial groups of demersal species, excluding non-demersal species (Table 2).

RESULTS The total biomass of bottom trawl catches (including all demersal and pelagic specimens) was estimated as 11,1171

Table 4. Catch per unit area estimates (kg n.m.72) for ¢sh species in the Oman Sea, 2003^2004. K

L

M

N

O

3189.9 2356.1 486.6 6032.7

2165.7 1683.7 688.1 4537.5

4059.1 3136.0 329.9 7524.9

7108.4 2459.5 407.5 9975.5

4532.1 1715.8 117.7 6365.6

P

Q

(a) In di¡erent spatial strata (K^Q) from west to east. Average Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total (b) In di¡erent depth zones. Average Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

6612.6 2997.9 565.3 10175.9

10^20 m

20^30 m

30^50 m

9637.2 7092.4 687.7 17417.3

10088.5 5067.3 973.5 16129.3

6752.2 2271.4 511.9 9535.5


7261.0 6225.8 456.6 13943.4

50^100 m 1351.2 767.5 3 138.3 2256.9

5411.4 3145.1 445.3 9001.9 Total 5411.4 145.1 445.3 9001.9

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Table 5. Biomass estimates (tonnes) for ¢sh species in the Persian Gulf, 2003^2004. A

B

C

D

E

F

G

H

I

J

Total

(a) In di¡erent spatial strata (A^J) from west to east. Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

1379.1 5519.0 11532.4 10213.9 1616.8 2685.5 3471.9 1907.0 2615.4 655.4 47495.2 515.2 2151.5 4160.0 2687.0 529.6 1406.2 2604.1 840.4 1088.8 2986.8 18969.6 215.8 476.5 1763.2 1280.8 245.1 660.9 835.4 107.9 132.0 409.4 6127.2 2110.1 8147.1 17455.6 14181.7 2391.5 4752.6 6911.4 2855.3 3836.2 9950.6 72592.0

(b) In di¡erent depth zones.

10^20 m

20^30 m

30^50 m

Total

Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

9436.9 4530.1 960.7 14927.6

11430.1 3724.5 1889.2 17043.8

26628.3 10715.0 3277.2 40620.6

47495.2 18969.6 6127.2 72592.0

Table 6. Catch per unit area estimates (kg n.m.72) for ¢sh species in the Persian Gulf, 2003^2004. A

B

C

D

E

(a) In di¡erent spatial strata (A^J) from west to east. Average Commercial demersal ¢sh 1109.1 1949.4 4074.8 5617.6 3553.3 Non-commercial demersal 414.3 759.9 1469.6 1477.8 1164.0 spp. Pelagic species 173.6 168.3 623.0 704.4 538.8 Total 1697.1 2877.6 6167.6 7799.8 5256.1 (b) In di¡erent depth zones. Commercial demersal ¢sh Non-commercial demersal spp. Pelagic species Total

F

G

H

I

J

Total

4225.1 3595.6 2821.0 4821.9 5110.2 3576.4 2212.5 2696.9 1243.2 2007.3 2328.7 1428.4 1039.8 865.2 159.6 243.4 319.2 461.4 7477.3 7157.6 4223.8 7072.6 7758.1 5466.2

10^20 m

20^30 m

30^50 m

Average

2374.1 1139.6 241.7 3755.4

3792.3 1235.8 626.8 5654.9

4232.6 1703.2 520.9 6456.7

3576.4 1428.4 461.4 5466.2

high (416,000 kg/n.m.2) in both 10^20 and 20^30 m depth zones and markedly lower in the deepest zone (50^ 100 m, 2257 kg/n.m.2) (Table 4). The western and eastern parts of the Oman Sea had higher mean CPUA for both commercial and noncommercial ¢sh, while the central strata of M, N, O and P (which are the main ¢shing grounds for industrial bottom trawlers) had the lowest CPUA (Table 4). The CPUA for both commercial and non-commercial species showed similar patterns in relation to water depth, with higher values depths of 10^20 and 20^30 m compared to depths of 50^100 m (Table 4). Amongst all identi¢ed species or species-groups in the bottom trawl catches (Table 2), the ones with highest biomass and CPUA in the Oman Sea were rays (22% of total biomass), grunts (11%; especially Pomadasys kaakan), carangids (9%), cat¢sh (9%, especially Arius dussumieri), nemipterids (7% especially Nemipterus japonicus), Trichiurus lepturus (6%) Drepane longimana (5%), scianids (4%, especially Otolithes ruber), Saurida tumbil (4%), and cuttle¢sh (3%, mainly Sepia pharaonis). Persian Gulf

The total biomass of demersal species in the Persian Gulf was estimated to be 72,592 tonnes. Comparison Journal of the Marine Biological Association of the United Kingdom (2006)

between the ten strata (A to J) showed that the maximum and minimum biomass were found in strata C (414,000 tonnes) and A (north-west of Persian Gulf in border line with Iraq and Kuwait waters; 2110 tonnes), respectively (Table 5). In relation to depth, maximum and minimum biomass were found at 30^50 m depth (40,621 tonnes; 56% of total biomass in the Persian Gulf ) and at 10^20 m depth (14,928 tonnes; 21%), respectively (Table 5). The highest biomass of both commercial and noncommercial demersal ¢sh species groups was found in C stratum, in the Genaveh area, and in the 30^50 m depth layer (Table 5). The estimated biomass of commercial species was 2.5 times higher than that of the noncommercial species group. The biomass of non-demersal or pelagic species was higher than in the Oman Sea, comprising 8.4% of total biomass. Mean CPUA for demersal ¢sh in the Persian Gulf was 5466.2 kg/n.m.2. The highest and lowest CPUA were found in strata D (Motaf region; 7800 kg/n.m.2) and A (borderline with Iraq; 1697.1 kg/n.m.2), respectively (Table 6). In relation to depth, maximum mean CPUA was found at 30^50 m (6457 kg/n.m.2) and the lowest mean CPUA was found at 10^20 m (3755 kg/n.m.2) (Table 6). These results show that spatial patterns in biomass and CPUA in the Persian Gulf are completely di¡erent to those in the Oman Sea.

Demersal ¢sh resources of Iranian waters T. Valinassab et al. 1461 The species and species groups with the highest biomass and CPUA in the Persian Gulf were (Table 2): cat¢sh (13.7% of total biomas, especially Arius thalassinus), rays (11.4%), Nemipterus japonicus (10.5%), carangids (9.5%), Saurida tumbil (6.5%), Pomadasys kaakan (4.7%), lutjanids (3.5%) and Trichiurus lepturus (2.3%).

DISCUSSION Sustainability, in both ecological and socio-economic senses, is now recognized as the essential feature of the exploitation of living marine resources. A rational and long-term approach to management is necessary to achieve sustainable and successful exploitation (Jennings et al., 2001). For this to be achieved it is essential to monitor the status of the resource, including the collection of biological data. Biomass and CPUA estimates are commonly used as stock indices for management of demersal resource species (Sparre & Venema, 1992). The total biomass of demersal ¢sh resources in the study area was estimated at over 110,000 tonnes, of which twothirds are in the Persian Gulf and one-third in the Oman Sea. Density, however, appears to be markedly higher in the Oman Sea (mean CPUA of around 9000 kg/n.m.2 as compared to around 5500 kg/n.m.2). These ¢gures are based on using an escape coe⁄cient from the literature (Sparre & Venema, 1992) so should be treated as approximations. The Persian Gulf has a vast area of shallow water, known to be a preferred habitat for demersal species (Sheppard et al., 1992; Reynolds, 1993), while the Oman Sea has a narrow continental shelf. These di¡erences in bathymetry may account for the higher biomass found in the Persian Gulf but lower CPUA. Also, the Persian Gulf is a semi-enclosed water body and connected to the Oman Sea by a narrow junction of the Strait of Hormuz. The Oman Sea can be assumed to be oceanic in its nature (Reynolds, 1993) as it is connected to the Indian Ocean by the Arabian Sea. In conformity with Iranian ¢sheries policy, bottom trawling for ¢sh has been banned in this region since 1992 (Nia-Meimandi & Khorshidian, 1993) to prevent over-exploitation. This has helped the recovery of demersal resources. The comparison between 17 strata (A to Q) from the north-west of the Persian Gulf (borderline with Kuwait and Iraq countries) up to the north-east of the Oman Sea (borderline with Pakistan) shows the highest ¢sh biomass in the central part of the Persian Gulf (C, D), in the northwest of the Oman Sea near the Strait of Hormuz (K) and at the entrance to the Oman Sea adjacent to the Arabian Sea (Q). These three areas together contain 48% of the entire demersal ¢sh biomass. Similar spatial patterns were found by previous studies in the Oman Sea (Daryanabard et al., 2002; Dehghani et al., 2003). The distribution of biomass and CPUA in relation to depth di¡ers between the Persian Gulf and Oman Sea. In the Persian Gulf, there is an ascending trend for both parameters of biomass and CPUA with increasing depth. In contrast, in the Oman Sea the maximum abundance and CPUA are found in shallow waters of 10^20 m, with a descending trend with increasing depth. In terms of overall catch composition in the bottom trawl surveys, non-demersal (pelagic) species made up Journal of the Marine Biological Association of the United Kingdom (2006)

less than 10% of the catch and commercially important demersal ¢sh accounted for more than 60% of total biomass. This might be indicative of the rehabilitation of demersal stocks due to implementation of closed seasons and area during the last decade. The lower proportion of demersal ¢sh biomass that is composed of commercial species in the Oman Sea, as compared to the Persian Gulf, may re£ect the ¢shing activity of industrial bottom trawlers during the ¢ve months ¢shing season. The low CPUA and biomass values in strata A and B, close to the border with Iraq and Kuwait, re£ect damage to marine habitats and resources resulting from the Iraq ^ Kuwait War (Nikouyan, 2004). The most abundant commercial ¢sh in the Persian Gulf and Oman Sea were Nemipterus japonicus, Saurida tumbil, Pomadasys kaakan, carangids and Trichiurus lepturus. However, the most important commercial species (Pampus argenteus and Otolithes ruber) account for a very small proportion of the total catch. The most abundant noncommercial species were rays and cat¢sh. The decrease of shark abundance over the last two decades (Sivasubramaniam, 1981), along with the decline in silver pomfret and croakers, due to over-exploitation, and the great increase in abundance of groups such as rays and cat¢sh, suggest that ¢shing is a¡ecting community structure. Thus, despite existing restrictions on ¢shing e¡ort (a limited season in the Oman Sea and a ban on trawling for ¢sh in the Persian Gulf ), it appears that there remains a need for additional management measures. The spawning and nursery grounds of some commercial demersal species have been identi¢ed and are suggested to be considered as new protected areas in the near future. This research was funded by the Iranian Fisheries Research Organization. The authors wish to thank the skipper and crew of RV ‘Ferdows-1’ for assistance with sampling. We are also grateful to various colleagues associated with the cruises. So¤nia Mendes from the University of Aberdeen helped with writing the paper.

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Submitted 7 July 2005. Accepted 4 September 2006.