5.1. Trawl surveys (Nikolaj A. Ivanin, Galina M. Kogomatchenko and Evgeny V. Romanov)

5.1.1. Gear

Trawl fishing operations at the seamounts of the Southern Indian Ocean were undertaken using large trawlers of the classes BMRT, RTMA, and RTMS type (Table 4.1). Trawling, unless otherwise stated, was done using a 1625 design bottom trawl that had a horizontal opening of 36.6 m. Figures 4a and 4b illustrate the dimensions configuration of this net. Midwater trawling was undertaken using a 110-600 trawl (see Figures 4c and 4d). These trawls have a head rope length of 110 m and a stretched-mesh circumference at the opening into the trawl of 600m.

5.1.2. Cruises

1972

The First Cruise of the RTMA R.V. Fiolent, February - August 1972

This vessel generally operated in the Antarctic waters of the Indian Ocean south from 45°S. During this cruise records show that one seamount, 770 at the southern boundary of the Southwest Indian Ridge was located. No trawling operations at this seamount were reported.

1973

The Sixth Cruise of the BMRT R.V. Skif, July 1973 - January 1974

A hydrographic and hydrological research cruise was undertaken in the area 40°00' - 47°30'S, 38°00' - 45°00'E. Four geo-hydrological zones were described along the Southwest Indian Ridge. The third zone included the crest of the Southwest Indian Ridge, which consists of five large parallel ridges with a relative height in the range 2 000 - 2 800 m. Seamounts that were earlier unknown were found; these were at relatively shallow depths and several had elevations suitable for deepwater bottom trawling. No trawling operations in the area of the underwater ridges of the Southern Indian Ocean were carried out during this cruise.

1974

The Fifth Cruise of the RTMA R.V. Fiolent, August 1974 - January 1975

Survey operations along the Southwest Indian Ridge were carried out from 20 November until 6 December 1975. Two areas were studied during the cruise by acoustic means: 44º - 47°S, 35º - 39°E and 30º - 40°S, 44º - 48°E. Several large seamounts were found that had rough surfaces; they located in the axial part of the ridge. Some of the seamount peaks were suitable for the trawling, however, the complex seabed and rocky ground of the mounts of the ridge did not allow bottom trawling with ordinary fishing gears. Three sites were found with favourable bottom topography for fishing with a bottom trawl. Attempts to fish off bottom were undertaken by means of a 1 625 design bottom trawl that had a horizontal opening of 36.6 m. Five trawl sets were undertaken for 4.05 hours of trawling, three of these sets malfunctioned, including one that resulted in a complete loss of the trawl. The principal species caught were Macrourus carinatus, Notothenia squamifrons, Alepocephalus sp., Antimora rostrata and Dissostichus eleginoides.

1976

The Sixth Cruise of the RTMA R.V. Zvezda Kryma, May - October 1976

This cruise was the first expedition, whose principal goal was the search for and study of fish resources at great depths. The main research areas were as follows: Madagascar Ridge, Southwest Indian Ridge and Ninety East Ridge. The search operations were made at the Mozambique, Mascarene and Broken Ridges, there were examined areas north-eastwards the Broken Ridge and Equator Seamount. In all the areas except Southwest Indian Ridge (due to the absence of spots suitable for bottom trawling and stormy weather), exploratory trawl hauls were made.

In the search for fish aggregations, suitable stations for trawling were identified using a KhAG-432 echo sounder. A Furuno netsounder was used to control the trawl. The main gear was 31 m 326 design bottom trawl equipped with a rigid foot rope and 500 mm diameter bobbins. Its minimum mesh size was 30 mm and it had a nylon (kapron) codend 25 m in length. A midwater trawl was also used that was 99.4 m in length with a minimum mesh size of 30 mm and a codend 27.2 m in length. The principle results are presented in Table 5.1.1. All the ridges depths down to 1 500 m were examined and at the Ninety Eastern Ridge, fishing extended to a depth of 1 840 m.

Figure 4: Design diagrams of trawls used by Soviet vessels in the Southern Indian Ocean Bottom trawl design

4a Side view of trawl opening

4b Plan diagrams of trawl

4c Midwater trawl 110-600

4d Midwater trawl 110-600

4e Midwater trawl 110-620

Table 5.1.1

Results of trawl surveys at the Madagascar, Mozambique and Broken Ridges during the 6^th Cruise of the R.V. Zvezda Kryma

The Seventh Cruise of the RTMA R.V. Zvezda Kryma, December 1976 - May 1977

The objective of the cruise is to search for deepwater fish resources in the region of the underwater seamounts in the Southern Indian Ocean (Mozambique, Madagascar and Southwest Indian Ridge) at the depths down to 1 500 - 1 800 m. A 326 design bottom trawl of 31.2 m with minimum mesh size 30 mm, a nylon (kapron) codend 25 m in size and a 99.4 m pelagic trawl with a minimum mesh size of 30 mm and nylon (kapron) codend of 27.2 m size were used.

Forty-six deepwater trawl tows were made at depths of 1 200 - 1 570 m at the Mozambique Ridge. Aggregations of deepwater dory (Allocyttus verrucosus) were found at depths of 1 260 - 1 450 m, which were located within a water stratum with temperatures of 4.6 - 3.8 °C. Fifty-seven bottom trawl hauls were made on the Madagascar Ridge for a total of 109 hours fishing and in depths ranging from 600 - 1 500 m. Operations to study the bottom topography and charting of the seabed were carried out. The principle results are presented in Table 5.1.2.

Table 5.1.2

Results of trawl surveys at the Madagascar and Mozambique Ridges during the 7th Cruise of the R.V. Zvezda Kryma

1977

The Seventh Cruise of the RTMA R.V. Fiolent, May - October 1977

The purpose of the expedition was deep-water research in the area of the Ninety East Ridge and Broken Ridge to locate commercial aggregations of fishes at greater depths. The exploratory trawl hauls were made with Khek-M, Khek-4M bottom trawls, a 326 design 31 m trawl and a 1 625 design 37.2 m trawl and a 99.4 m deepwater trawl. The choice of the fishing gears was related to the principal goal of the expedition - exploration of deepwater fish resources. The vertical open of the nets depended on the trawling speed and varied from 5.5 - 4.0 m for the Khek-Ì trawl and from 2.5 - 3.5 m for the 1625 design trawl.

The Ninety East Ridge Area

An echo sounder survey of this area for sites suitable for the trawling operations was undertaken. This area is peculiar for its bottom topography and unfavorable hydrometeorological conditions (heavy currents, high winds and rough sea conditions), which impeded trawling even at the suitable sites. from 13 to 25 June six trawl hauls were carried out for a total duration of 10.5 hours at the depths of 1 400 - 2 000 m. The majority of catches were marine eels, smoothheads (Alepocephalidae) and grenadiers (Macrouridae).

The Broken Ridge

Trawling operations were carried out from 22 July until 4 August 1977. The permanent strong winds, currents and significant cloudiness limited the ability of the vessel to determine its position using astronomical methods (once or twice a day) and did not allow precise locating of the stations at sites indicated on the charts as favourable for trawling. Nine deepwater trawl hauls were undertaken at depth ranging from 1 050 to 1 350 m. The total duration of trawling was 18 hours. The catch consisted of smoothheads, dories (Allocyttus spp.), grenadiers and sharks. The catch levels were not of commercial interest. However, comparing the trawling results with the catches of the RTMA Zvezda Kryma which had operated in the area in 1976, it should be noted that the catches of the Fiolent were slightly higher. It was concluded that the area had potential for commercial fisheries but required more in-depth study. Data on the principal operations are shown in Table 5.1.3.

The Eighth Cruise of the RTMA R.V. Zvezda Kryma, June - December 1977

The purpose of the expedition was to support operations of the fishing fleet through studies of the Agulhas Shoal (1^st stage) and studies of deepwater fish resources of Mozambique, Madagascar, and the Southwest Indian Ridges (2^nd stage). During the search operations from 14 September until 5 November, though interrupted by breaks, of the Southwest Indian Ridge, areas suitable for bottom trawling were not found on all the seamounts examined, at least in depths to 2000 m. No fish aggregation were observed in the pelagic zone.

No commercial fish aggregations were found during 37 days of operations on the Madagascar Ridge, however two potentially productive spots, based on fish distribution and catches, located. The first was in the region of 32°40' - 32°50'S and 44°44'- 44°56'E in depths of 900 - 1 250 m. The catches consisted of moras, sharks, grenadiers and smoothheads. The second areas was in the region 33°06' - 33°22'S and 43°56'- 44°16'E in depths of 720 - 760 m, south-eastwards from Walters shoals. The dominant species taken was armourhead (Pseudopentaceros richardsoni) (from 30 to 80% of the catch), sharks, stargazers (Uranoscopidae) and rockfishes (Scorpaenidae). The main results are presented in Table 5.1.4.

Goblin shark (Mitsukurina owstoni), is found on outer continental shelves and upper slopes. It has highly specialized jaws for rapid projection from the head to snap up its prey of pelagic octopus and crabs. Slow-moving and neutrally buoyant it may be used dried and salted.

Table 5.1.3

Results of trawl surveys at the Ninety East and Broken Ridges during the 7th Cruise of the R.V. Fiolent

Table 5.1.4

Results of trawl surveys at the Madagascar and Mozambique Ridges during the 7th Cruise of the R.V. Zvezda Kryma

1979

The Eleventh Cruise of the RTMA R.V. Fiolent, February - July 1979

The R.V. Fiolent undertook surveys of the Madagascar and Southwest Indian Ridges during two short periods enroute to and from its principal area of research, which was the Ob and Lena Banks and the Agulhas Bank. In this connection the fishing operations were experimental in nature. The fishing gears used during the surveys were a 1625-À design bottom trawl, a 55 m bottom (lobster) trawl, midwater trawls of 130.6 m and 99.4 m and trawl for fishing capelin of 81.6 m. HAG-331, HAG-432 and Simrad EQ-120 acoustic equipment were used during the fishing operations.

Eight bottom trawl hauls (13.09 hours of trawling) were undertaken in the area of the Madagascar Ridge during 13 - 15 July in depths of 720 - 1 050 m. The catches obtained were small and consisted of dogfishes, moras and other not specified species. A total of 6 trawl hauls of one hour duration each were undertaken with a bottom trawl from 31 March until 2 April and from 4 to 8 May in the area of the Southwest Indian Ridge. The catches taken were small. Three trawl sets (also of one hour duration each) were undertaken with a midwater trawl in depths of 18-60 m and 280-300 m but were not successful and took only small catches of Myctophidae, Chauliodontidae and Gonostomatidae.

The Twelfth Cruise of the RTMA R.V. Fiolent, August 1979 - January 1980

The R.V. Fiolent surveyed the Madagascar Ridge area during a short period at the start of its cruise en route to its principal area of research, the Agulhas Bank. Due to a trawl winch failure fishing operations were stopped. A total of three bottom trawl hauls and five pelagic trawl hauls were undertaken from 22 August to 9 September. The catches taken were small and deepwater dories and dogfishes predominated in the catch. The fishing gears used during the expeditions were a 1625-À design bottom trawl, a 55 m bottom (lobster) trawl, midwater trawls 130.6 m and 99.4 m and a capelin trawl of 81.6 m. HAG-331, HAG-432 and Simrad EQ-120 acoustic systems were used during fishing operations.

1980

The First Cruise of the RTMS R.V. Geroevka, March - August 1980

The goal of the first expedition of the R.V. Geroevka was to search for prospective fishing areas in the Antarctic part of the Indian Ocean, Southern Indian Ocean and in waters adjacent to the EEZs of Mozambique and South Africa. The vessels used 110/560 pelagic trawls (vertical opening of 35 m) at the trawling speed six knots with a vertical opening of 45 - 47 m at a trawling speed of 4.5 knots and a 110/468 trawl with a vertical opening of 45 - 47 m at a trawling speed of six knots, and an opening of 60 m at a trawling speed of 4.5 knots. Bottom trawling was undertaken with a 1125-design 50.8 m trawl. The southern part of the Madagascar Ridge was surveyed from 6 June until 2 July. In this time 74 trawl hauls (36 bottom and 38 midwater) were completed. The main catch of the midwater trawl catches were jack mackerel (Trachurus longimanus). No positive results were achieved from the bottom stations. Searching operations at the Southwest Indian Ridge were carried out from 4 - 5 June and 27 July - 13 August 1980. Seamounts 422, 360, 250 and 150 were surveyed. The results from the operations on the most important seamounts are given in Table 5.1.5.

Table 5.1.5

Results of trawl surveys at the Southwest Indian Ridge during the 1st Cruise of the R.V. Geroevka

The Second Cruise of the RTMS R.V. Novoukrainka, July - December 1980

Operations were carried out from 14 September until 23 November 1980 of seamounts of the Southwest Indian and Mid-Indian Ridges. For the first time commercial aggregations of fish were found and explored at Seamounts 251, 260 and 336. Search and fishing operations were carried out at the aggregations found earlier at Seamounts 102 and 150. A midwater 110/468 trawl was used. Towing speed was in the range 4.8 - 5.3 knots; the vertical opening of the trawl was generally 50 m. The trawl ground rope was usually located at depths between 110 - 170 m, and best catches were observed if the ground rope was located 20 - 30 m deeper than the fish schools as indicated by the echo sounder records, i.e. in the close vicinity to the bottom. Daily catches were only limited by the vessel's fish processing capacity. The catch consisted of rubyfishes (77.6%), E. nitidus (29.2%) and P. rubiginosus (48.4%). The percentage of butterfishes varied from 5 to 25% in several hauls, and was 12.9% on average. The catch of jack mackerel varied in the range 5 - 20%; and was on average 9.6% (Table 5.1.6).

Table 5.1.6

Results of trawl surveys at the Southwest Indian and Mid-Indian Ridge Seamounts during the 2nd Cruise of the R.V. Novoukrainka

The Fifteenth Cruise of the RTMA R.V. Chatyr-Dag, August 1980 - February 1981

Surveying of Seamounts 150 and 102 of the Southwest Indian Ridge were undertaken from 27 August 1980 until 20 January 1981. A 74/360 m midwater rope trawl with a vertical opening of 35 m was used together with warps pf 250 - 300 m and a towing speed of 3.0 - 3.8 knots. KhAG-432 and KhAG-331 echo sounders were used to locate fish concentrations. The results of operations over the most important seamounts are given in Table 5.1.7.

Table 5.1.7

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 15th Cruise of the R.V. Chatyr-Dag

The Second Cruise of the RTMS R.V. Geroevka, September 1980 - March 1981

Exploratory fishing operations were undertaken in the area of the Southwest Indian Ridge from 20 October 1980 until 10 March 1981 during which time Seamounts 102, 251, 150, 358, 440, 475, 415, 422 and 360 were surveyed involving 260 trawl hauls (Table 5.1.8). Fishing over seamounts was done using 110/468 m midwater trawls that had a vertical opening of 38 - 45 m and were towed at a speed of 4.3 - 4.8 knots, a 110/600 m trawl with a vertical opening of 32 - 40 m, which was towed at a speed of 4.5 - 5.6 knots and a 81.8 m trawl with a vertical opening of 30 - 48 m,which was towed at a speed of 2.5 - 3.5 knots. While fishing at Seamount 150 and for other bottom trawling, a 1625À design 31.2 m bottom trawl was used. The pelagic trawls of types 110/468 m and 110/600 m were equipped with hexagonal 28 mm mesh codends. A 81.8/241 - 6 m trawl was equipped with a 46.0 m kill codend cover of 7 and 20 mm mesh.

Table 5.1.8

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 2nd Cruise of the R.V. Geroevka

Fish aggregations were associated with the tops and slopes of the seamounts (excluding Seamounts 360 and 422). Seamounts 358 and 415 were discovered during this cruise. Based on the results of these commercial fishing operations detailed fishing assessments of the seamounts were prepared. Seamounts 102, 251 and 150 were the most productive, fish aggregations were recorded here and fishing was successful. There was no record of fish at Seamounts 422 and 360. Fishing over Seamounts 102, 251 and 150 were possible at night only, when fish schools rose off the bottom. The majority of the catch consisted of two species of rubyfishes and butterfishes. Jack mackerel was found only at Seamount 150.

During the period November - February at Seamounts 358, 440, 475 and 415 within the depth range 300 - 600 m, alfonsino (Beryx splendens) aggregations were found that were distributed in the near-bottom layer over the slopes and peaks of seamounts. Some schools reached 110 m height and 600 m length. The fish often escaped the trawl by diving under the ground rope. Development of new variations of trawls gears, suitable for such fish behaviour were suggested if successful exploitation of resources in these areas was going to be possible.

1981

The First Cruise of the RTMS R.V. Zvezda Chernomorya, February - July 1981

Exploratory fishing operations at the Southwest Indian Ridge area were undertaken 5-7 April 1981. These operations surveyed Seamount 150 of Madagascar Ridge. In June and July operations were carried out at the Ninety-East Ridge (Seamounts 927, 870 and 631) at the Mid-Indian Ridge (Seamount 336) and at the area of the Broken Ridge. More than 150 trawl hauls in total were undertaken over the seamounts and in the pelagic zone.

The following trawls were used: a 110/468 trawl with a vertical opening of 50-60m; a 110/600 trawl made by Klaipeda, Lithuanian trawl factory, which had a vertical opening of 35-40 m; and a 110/600 trawl made by the Kerch, Ukrainian trawl factory, which had a vertical opening of 35-40 m. Midwater trawls without fine-mesh covers were used for fishing at Seamounts 150 and 336, which is believed to be the reason why small-sized species of the sound scattering layer were absent from the catches in that area. Towing speeds were rather high, 3 - 5.3 knots. The results of the operations at seamounts where commercial catch rates were encounter are given in Table 5.1.9. According to the estimates of the survey scientists, fish biomass at Seamount 631 were not high and during the period of operations biomass present was unlikely to exceed 600t.

Table 5.1.9

Results of trawl surveys at the Southwest Indian Ridge, Mid-Indian and Ninety-East Ridge Seamounts during the 1st Cruise of the R.V. Zvezda Chernomorya

The First Cruise of the RTMS R.V. Zvezda Azova, March - August 1981

This vessel undertook exploratory fishing operations in the eastern part of the Southwest Indian Ridge in the are of 24-30 °S, 60-70 °E from 5 - 30 May and over the seamounts of the Mid-Indian Ridge and Ninety East Ridge from 24 April - 15 July 1981. Surveys were made of Seamounts 260 and 336 in the area of operations of the RTMS Novoukrainka in late October - early November 1980. For the first time the deep seamounts 631 and 549 were surveyed and the deep seamounts 400 and 620 were discovered. Acoustic studies of bottom topography were also carried out. Two underwater elevations in the area of Seamount 336 were discovered. A total of 143 trawl hauls were done in the pelagic zone and over seamounts with duration of 205 hours of fishing. The results of the operations over the seamounts are given in Table 5.1.10.

Table 5.1.10

Results of trawl surveys at the Mid-Indian and Ninety-East Ridge Seamounts during the 1st Cruise of the R.V. Zvezda Azova

Fishing was undertaken using 110/600 and 123/640 pelagic trawls and the vertical opening of trawls varied from 55 to 58 m. Acoustic surveying was done using Priboj-10 and Sargan-E systems and trawling was controlled with a Igla echosounder.

The First Cruise of the RTMS Zvezda Sevastopolya, March - September 1981

Operations were undertaken from 4 July to 22 August. A total of five seamounts (102, 430, 251, 150, and 358) were studied during this period. Fish aggregations, which may present particular interest for potential commercial fisheries was found, these dense fish concentrations formed under conditions of the winter cooling of waters. The results of the fishing operations are presented in Table 5.1.11. A 110/600 midwater trawl was used. Attempts to use a bottom trawl resulted in trawl damage.

Table 5.1.11

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 1st Cruise of the R.V. Zvezda Sevastopolya

The Sixteenth Cruise of the RTMA R.V. Chatyr-Dag, March - September 1981

The main goal of the cruise was the exploration of commercial fish concentrations at seamounts of the Southwest Indian Ridge. Seamounts 150, 251, 430 and 360 were surveyed. A 72 m trawl and fine-meshed 86 m trawl were used for pelagic fishing. A 1625-design bottom trawl was used for demersal fishing. The principal results of fishing operations are presented in Table 5.1.12.

Table 5.1.12

Results of trawl surveys at the Southwest Indian Ridge and Madagascar Ridge Seamounts during the 16th Cruise of the R.V. Chatyr-Dag

¹ Bottom trawling.
² Pelagic trawling.

Exploratory fishing operations were carried out of Seamount 150 during three periods: in April, May and May - June. Trawl operations using a 72 m trawl and a fine-meshed trawl were used. An unsuccessful attempt was made to fish with a 1625-design bottom trawl at depths of 190 - 195 m. At Seamount 251, two trawl hauls were undertaken on 24 June. Acoustic equipment did not detect any fish aggregations and no catch was taken. At Seamount 102 fish aggregations were not recorded from March until July. In August the fishing situation improved greatly Alfonsino aggregations were distributed at the depths from 90 to 200 m. At Seamount 360, which is located in the zone of the Sub Antarctic front, acoustic equipment did not record any fish aggregations. An area suitable for bottom trawling was found at depths of 290 - 340 m. Experimental hauls by trawl showed positive results.

Vertical "drop" line fishing stations were undertaken at Seamounts 150 and 102 using the vessel's hydrological winches. Wreckfishes (Polyprion americanus and Polyprion oxygeneios) were principal species in the catch.

The Third Cruise of the RTMS Geroevka, April - August 1981

Fishing search operations were carried out at the Southwest Indian Ridge Seamounts 150, 358, 251 and 102 and at the Walters shoals of the Madagascar Ridge. Midwater trawls of the designs 110/468 and 110/600 were used. The largest catch was recorded using a 110/468 trawl, probably as a result of its high vertical opening (55 m); the speed of trawling varied from 3.9 to 5.2 knots with an average of 4.7 knots. Trawling was carried out along 1 - 3 transect lines. At the Walters Shoals trawling operations were generally conducted with an upgraded 110/600 trawl; only 2 trawl hauls were made with the 110/468 m trawl. The upgraded 110/600 trawl, which was lighter and its trawling speed was higher (up to 6 knots); it was used for target fishing. Its vertical opening was 50 m. During the research on the seamounts, fish aggregations were generally located in zones that were unsuitable for fishing. The principal results of operations are given in Table 5.1.13.

Table 5.1.13

Results of trawl surveys at the Southwest Indian and Madagascar Ridge Seamounts during the 3rd Cruise of the R.V. Geroevka

The Fourth Cruise of the RTMS Geroevka, December 1981 - May 1982

The Geroevka operated from 2 February until 2 May 1982 at the seamounts of the Southwest Indian and Madagascar Ridges as well as in the area of the ridges in more open waters. Fishing operations were carried out over Seamounts 358, 150 and 630 and over a newly discovered seamount, numbered 690. A total of 64 trawl hauls were undertaken. The principal results of studies are given in Table 5.1.14. During this cruise, potentially productive zones were examined that had been insufficiently studied earlier. Fish searching was carried out using Priboj, Sargan, Simrad and a Koden colour acoustic fish detection equipment. Fish was caught only at night and during the evening and morning twilight period.

Midwater trawls of the 110/600 and 110/468 designs were used. Vessel towing speed varied from 2.8 to 4 knots and fishing was undertaken in the 50 - 350 m lay. The codend had a mesh size of 40 - 60 mm; the trawls were equipped with a fine-mesh cover. The speed of the vessel varied from 2.8 - 4.2 knots when fishing in the 600 - 800 m layer and increased to 5.5 - 6.3 knots when trawling in the 0 - 60 m layer.

The Second Cruise of the RTMS Zvezda Chernomorya, August 1981 - January 1982

Survey operations were undertaken at Seamounts 150, 251, 102, 700 and 420 of the Southwest Indian Ridge from 1 until 25 September. Sixty-five trawl sets were made for a total duration of 144 hours. At Seamounts 260, 336 and 631 of the Mid-Indian Ridge the vessel operated during 5 - 9 October 1981 and 12 - 14 January 1982. A total of eight bottom (15 hours) and twelve surface trawls (for 12.2 hours) were undertaken. The results of the operations are given in Table 5.1.15. Midwater 110/468 (42 hauls) and 110/600 (23 hauls) trawls used for fishing. Trawls of the 110/468 design have a vertical opening of 54 m at a trawling speed of 5.2 - 5.5 knots and the 110/600 trawls had an opening of 54 - 48 m at a trawling speed of 5.0 - 5.5 knots.

Table 5.1.14

Results of trawl surveys at the Southwest Indian Ridge and Madagascar Ridge Seamounts during the 4th Cruise of the R.V. Geroevka

As a result of the fishing operations, fish aggregations and schools were observed with specific features. Interaction of circulating currents with bottom elevations in the Southwest Indian Ridge resulted in upwelling of deep Antarctic waters rich in biogenic elements. The consequence was the development of all levels of the trophic chain from phytoplankton and zooplankton to fish. Formation and the stability of alfonsino aggregations was in direct relation to the synoptic gyres. Favourable fishing situations were recorded in anti-cyclone gyres where alfonsino aggregated densely and were foraging actively. The optimal temperature of alfonsino aggregation was 8 - 12 °C in depths of 400 - 200 m.

The Third Cruise of the RTMS Vozrozhdenie, June - December 1981

Fish scouting operations in the area of the Southwest Indian Ridge were undertaken from 6 September until 31 October 1981. Seamounts 360, 680, 422, 415, 940, 102, 900, 251, 150 and 358 were examined. At the Mid-Indian and Ninety East Ridges, the RTMS Vozrozhdenie surveyed from 6 November until 8 December 1981. Seamounts 260, 336, 400, 620, 631 and 549, which are in the area 24 - 35°S, 80 - 88°E, were examined. During this cruise, Seamounts 462, 433, 800, 760, 741 and 510 were discovered. A 110/468 m trawl was used. The results of the fishing operations are given in Table 5.1.16.

Table 5.1.15

Results of trawl surveys at the Southwest Indian, Mid-Indian and Ninety-East Ridge Seamounts during the 2nd Cruise of the R.V. Zvezda Chernomorya

The Fourteenth Cruise of the RTMA R.V.Fiolent, August 1981 - January 1982

Fishing survey operations were undertaken 13 August 1981 until 14 January 1982 at the seamounts 358, 150, 251, 102, 415 and 720 of the Southwest Indian Ridge. The results of operations are given in Table 5.1.17. Midwater trawls of the type 85.2/400, length 86 m and with a maximum codend mesh size 20 mm. The vertical opening of the trawls was 30 - 35 m (as measured with a netsounder) and with a horizontal opening of 45 - 50 m. Fishing started using a 86 m trawl which was lost in the beginning of fishing operations and it was replaced by trawl of a 85.2/400 m design.

During the fourteenth cruise of the RTMA Fiolent an acoustic survey of the Southwest Indian Ridge in the area of Seamounts 358, 150 and 251 was undertaken. According to provisional calculations, the total fish biomass at the seamount 150 was about 3 700t during the period of the assessment cruise. During the acoustic survey at the Southwest Indian Ridge seamounts suitable for the occurrence of fish resources were found with minimum depths 540, 640 and 680 m.

1982

The Eighteenth Cruise of the BMRT R.V. Skif, March - September 1982

The main area of this expedition was the Antarctic waters of the Indian Ocean. Search operations in the southern part of the Southwest Indian Ridge were carried out periodically on 29 July and during 2 - 5 and 30 - 31 August. A total of twelve one hour trawl hauls were undertaken over the peaks of the seamounts as well as in depths from the surface down to 340 m. The catches were small and varied from 1 to 70 kg; fish of the family Myctophidae predominated in the catch. Some seamounts not indicated on the chart were discovered.

The Fifth Cruise of the RTMS R.V. Geroevka, June - November 1982

Exploratory fishing of the seamounts of the Southwest Indian Ridge was done during the period June - November 1982 to determine the state of fish resources. Twelve seamounts were surveyed involving 219 trawl hauls. The results of operations at the most important seamounts are given in Table 5.1.18. Three trawl designs were used: a 110/600 m trawl that had a minimum inner mesh size of 60 mm; a 110/600 trawl with a krill cover and codend with a minimum inner mesh size of 13 mm and an experimental finemesh 120/230 m trawl with a minimum mesh size 13 mm. Surface trawl hauls were made at speeds of 5.8 - 6.0 knots and trawl hauls with fine-meshes were made at speeds of 3.5 - 4.8 knots.

Table 5.1.16

Results of trawl surveys at the Southwest Indian and Madagascar Ridge Seamounts during the 3rd Cruise of the R.V. Vozrozhdenie

The Third Cruise of the RTMS R.V. Zvezda Sevastopolya, March - August 1982

Survey and fishing operations were undertaken at eight seamounts of the Southwest Indian Ridge and at some seamounts of the Madagascar Ridge. Research was undertaken in two stages: from 18 April to 13 May and from 1 - 13 June 1982. Seamount 690 was examined most completely and the rest of the seamounts were surveyed intermittently. Trawling was done with a 110/600 midwater trawl whose vertical trawl opening depended on the speed of towing; it reached 60 - 70 m during trawling at depths of 50 m. The results of the fishing at seamount 690 are given in Table 5.1.19.

The Fifteenth Cruise of the RTMA R.V. Fiolent, June - November 1982

Research activities in the Southwest Indian Ridge region were carried out from 21 June till 7 July 1982 at Seamounts 690, 150 and 251. Fifty-three trawl stations were undertaken with a 78/520 rope trawl that had a 28 m codend. The chafer was made of 20 mm net. Trawling operations at the Broken Ridge were conducted from 18 - 20 July. Eight bottom trawl sets were undertaken over the top surface of the Ridge in depth ranges of 890 - 1 000 m for a total duration of 12.9 hours. Smoothheads, orange roughy and dogfish were the predominant species in the catches. Trawl sets were made with a 1625 design bottom trawl with a 20 mm mesh size in the codend and standard rigging. The results of operations at the most important seamounts are given in Tables 5.1.20 and 5.1.21.

Table 5.1.17

Results of trawl surveys at the Southwest Indian and Madagascar Ridge Seamounts during the 14th Cruise of the R.V. Fiolent

The Sixth Cruise of the RTMS R.V. Geroevka, December 1982 - June 1983

The main objective of this cruise was to obtain estimates of the potential commercial fish production of the most productive seamounts of the Southwest Indian Ridge. The seamounts 690, 980, 358, 610, 150, 251, 102, 700, 360 and 422 were surveyed. A new seamount, 940, was also located. Fish aggregations were not found over tops of seamounts with the exception of Seamounts 150 and 251. According to echo sounder records fish aggregations migrated into deeper waters and were distributed near the slopes of seamounts, thus being inaccessible for fishing with a midwater trawl. Seamount 260 of the Mid-Indian Ridge was also surveyed. The results of operations at the most important seamounts are given in Table 5.1.22.

Orange roughy (Hoplostethus atlanticus) photographed from a submersible deployed from a Soviet research vessel. Although orange roughy occur through out the southern Indian Ocean, they were never targeted by Soviet fishing operations in this area (because of the purgative effect when fillets are improperly skinned).

Table 5.1.18

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 5th Cruise of the R.V. Geroevka

Table 5.1.19

Results of trawl surveys at the Southwest Indian Ridge during the 3rd Cruise of the R.V. Zvezda Sevastopolya

All trawl hauls were made with 110/600 midwater trawls. A 110/600 trawl with a krill or sprat codend was used when fishing myctophidae aggregations in the sound scattering layer. The speed of towing when targeting rubyfishes, alfonsino and jack mackerel varied from 4 to 5 knots; for Myctophidae fishing and fishing targeting the sound scattering layer, a fine-mesh trawl was used towed at a speed of 2.2 - 3.5 knots. The depth of fishing varied from 0 to 920 m.

1983

The Seventh Cruise of the RTMS R.V. Geroevka, July - November 1983

The objective of this cruise was to continue the assessment of the fishing potential of the seamounts of the Southwest Indian Ridge. The search for fish concentrations in the region of the seamounts was undertaken using first preliminary acoustic surveys and then controlled trawl hauls. Two rope pelagic trawls of 110/600 and 91.4/468 designs and Priboj-101G, Sargan-G, Priboj-101E and Koden (with colour display) echo sounders were used. A Sargan-E navigational echo sounder was also used as a search devices. A Simrad acoustic system was used to assess the fish biomass. The cruise examined fish aggregations of the following seamounts: 150, 710, 480, 690, 415, 251, 102, 358, 610, 630 and 422. The results of operations at the most important seamounts are given in Table 5.1.23.

Table 5.1.20

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 15th Cruise of the R.V. Fiolent

Table 5.1.21

Results of trawl surveys at the Broken Ridge Seamounts during the 15th Cruise of the R.V. Fiolent

The Nineteenth Cruise of the RTMA Chatyr-Dag, August - November 1983

Operations at the seamounts of the Southwest Indian Ridge were carried out during 8 - 19 September (Seamount 490) while en route to the principal area of research, the Antarctic Ocean. Thirty midwater trawl hauls, for a total duration of 44.2 hours, were undertaken using a 78/520 trawl. The catch consisted of alfonsino (92%) and butterfish (8%). The length of alfonsino in the catches varied from 19 to 39cm and the modal fish size was 30 - 32 cm; average weight of individuals was 658 g. Alfonsino aggregations were observed at night over peaks of seamounts in a small area.

The Ninth Cruise of the BMRT R.V. Mys Ostrovskogo, October 1983 - April 1984

The principal area of operations were the Antarctic waters of the Indian Ocean. The search operations at the Southwest Indian Ocean were carried out at the end of the cruise, from 31 March to 9 April over Seamounts 150 and 690. A 1498 design midwater trawl was used in near-bottom mode, mainly at night. Alfonsino, rubyfishes, and armourhead predominated in the catch. The principal results of operations are presented in Table 5.1.24.

Table 5.1.22

Results of trawl surveys at the Southwest Indian Ridge and Mid-Indian Ridge Seamounts during the 6th Cruise of the R.V. Geroevka

Table 5.1.23

Results of trawl surveys at the Southwest Indian Ridge during the 7th Cruise of the R.V. Geroevka

Table 5.1.24

Results of trawl surveys at the Southwest Indian Ridge during the 9th Cruise of the R.V. Mys Ostrovskogo

1984

The Sixth Cruise of the RTMS Zvezda Chernomor'ya, December 1983 - May 1984

The principal area of the operations for this expedition was the south-eastern Pacific and the Antarctic waters of the Indian Ocean. The survey operations in the area of the Southwest Indian Ridge were carried out at the end of the cruise, from 5 - 10 April over Seamounts 150, 690 and 358. The area of the Mozambique and Madagascar Ridges was examined as well. Trawl hauls were made with a 110/600 design pelagic trawl. The catches were small and mainly consisted of alfonsino, rubyfishes and armourhead.

The Seventh Cruise of the RTMS Zvezda Chernomor'ya, June - November 1984

The goal of the operations of this expedition was to search for commercial aggregations of pelagic and bottom fish species and squids in the high seas of the southern Indian Ocean. The main areas of research were the Mid-Indian and Ninety East Ridges. Search operations were carried out on the Southwest Indian Ridge, pelagic waters of the southern Indian Ocean southwards and south-westwards from Australia, the Saya-de-Malha Bank and equatorial seamounts. The cruise examined Seamounts 260, 330, 400, 620 of Mid-Indian Ridge and 631, 510 of the Ninety-East Ridge. Acoustic search was undertaken using Priboj 101 Eh, Priboj 101 G, Sargan G, Sargan Eh echosounders and a Igla netsounder was also used. Trawl hauls were carried out using a 110/600 m pelagic trawl. When fishing for pelagic species the trawling speed was kept in the range of 3.5 - 5.5 knots. When fishing for small and bottom fishes the trawling speed was in the range 2.8 - 3.5 knots. The principal results of operations are presented in Table 5.1.25.

The BMRT fishery research vessel, F.R.V. Akademik Knipovich, used in Southern Indian Ocean surveys. This vessel 's design was based on the Mayakoskiy type factory trawler. The Nicolaev and Baltiya shipyards built 109 class fishing vessels between 1958 and 1969. They have dimensions of 1519/1300 dwt, 3162/2691 GRT) (Greenway 1980).

1985

The Eight Cruise of the RTMS Zvezda Chernomor'ya, January - June 1985

The main area of operations for this expedition was Antarctic waters of the Indian Ocean. The search operations in the area of the Southwest Indian Ridge were carried out at the beginning of the cruise from 24 - 31 January on Seamount 150 (Geroevka) and at the end of the cruise from 27 April to 9 May on Seamounts 150, 480, 710 and 102. Trawl sets were undertaken using a 110/600 pelagic trawl; towing speed was 5.5 knots. Significant fish concentrations were only registered at Seamount 150, but they quickly disaggregated under the impact of fishing, which lasted for two to three days. The principal results of the operations are presented in Table 5.1.26.

Table 5.1.25

Results of trawl surveys at the Mid-Indian, Ninety-East and Southwest Indian Ridges during the 7th Cruise of the RTMS Zvezda Chernomor'ya

Table 5.1.26

Results of trawl surveys at the Southwest Indian Ridge during the 8th Cruise of the RTMS Zvezda Chernomor'ya

1987

The Fourteenth Cruise of the BMRT R.V. Mys Ostrovskogo, June - August 1987

The main goal of this expedition was to explore for commercial aggregations of mesopelagic fish (generally Myctophidae family) and determine the prospects for profitable fishing in the zone of the Sub-antarctic front of the Indian Ocean. The area of the Madagascar Ridge (Walters Shoals, Seamount 516) and the Southwest Indian Ridge (Seamounts 102, 415, 480 and 150) were examined as additional areas for research. A 78.7/416 trawl was used for pelagic fishing, which had a vertical opening of 30 - 45 m. The trawling speed was 4.5 - 5.6 knots. When targeting mesopelagic fish, a 98/640 trawl with a vertical opening of 45 - 60 m was used; the trawling speed was 2.7 - 3.7 knots. The principal results of operations are presented in Table 5.1.27.

Table 5.1.27

Results of trawl surveys at the Madagascar and Southwest Indian Ridges during the 14th Cruise of the BMRT R.V. Mys Ostrovskogo

The Fifteenth Cruise of the BMRT R.V. Mys Ostrovskogo, October 1987 - March 1988

The main goal of this expedition was to search for commercial aggregations of mesopelagic fish (generally Myctophidae family) and to determine the prospects for profitable fishing of mesopelagic fish in the zone of the Sub-antarctic front and the Antarctic waters of the Indian Ocean. Six tows were made in the southern part of the Southwest Ridge but there were no significant catches.

1988

Cruise of the BMRT Mys Ostrovskogo, 17 September 1988 - February 1989

Seamounts 690 and 358 were examined along the Southwest Indian Ridge. Three and two tows were of 3.1 and 1.5 hours, respectively. No catch was taken.

1992

The Twenty-ninth Cruise of the RTMS R.V. Zvezda Kryma, April 1992 - April 1993

This vessel examined the seamounts of the Southwest Indian Ridge, Broken Ridge and Ninety East Ridge from June 1992 until February 1993.

Broken Ridge: Here, twelve pelagic tows over Seamount 590 and one bottom tow on the plateau south-east of the seamount were undertaken (Table 5.1.28).

Ninety East Ridge:Over this seamount, one trawl haul was undertaken at Seamount 631 (Table 5.1.28).

Southwest Indian Ridge: The main operations were carried out over seamounts of the Southwest Indian Ridge (Table 5.1.29).

Table 5.1.28

Results of trawl surveys at the Broken Ridge Seamounts and Ninety East Ridge Seamounts during the 29th Cruise of the R.V. Zvezda Kryma

¹ Pelagic trawling.

Surveys were undertaken from 27 July 1992 until 4 February 1993 in three stages (27 July - 5 September 1992, 6 October - 23 November 1992 and 13 December 1992 - 4 February 1993). In totally there were 365 pelagic tows for a total of 450 hours fishing. The results of operations are given in Table 5.1.29. The main species in catches over the Broken Ridge were pelagic armourhead (37 - 57 cm fork length, but generally 40 - 46 cm; alfonsino 19 - 48 cm fork length, but generally of 32 - 41 cm, and orange roughy. At the Ninety East Ridge seamounts the principal species taken were alfonsino, and pelagic aromourhead of 38 - 56 cm fork length,but generally of 43 - 49 cm.

At the Southwest Indian Ridge the species catch composition varied greatly by seamounts (Table 5.1.29). Alfonsino, rubyfishes, butterfishes, armourhead and jack mackerel were the main commercial species. Over Seamounts 710, 690, 480 and 415, alfonsino characterized the distribution layers. Large, pre-spawning and foraging individuals as a rule stayed in the deeper layers over the slopes, usually at a depth lower than the top of the seamount. At the same time medium-sized and small-sized alfonsino formed aggregations over the seamount tops in the lower part of the sound scattering layer and mesopelagic zone. Cape bonnetmouth were found in the catches at three seamounts - 150, 251 and 102. Large-sized fishes were recorded at Seamount 251 with individuals of minimum size found at Seamount 102. This species spawned in July - September; by December all of the Cape bonnetmouth were foraging. Armourhead were also recorded at two seamounts - 690 and 710, but only at 690 in commercial quantities. Aggregations of this species reached the largest density during the middle of the night.

Bluenose warehou were recorded in the catches from most of the seamounts but did not form dense aggregations. Jack mackerel were caught in small amounts at Seamount 150 where they formed small but dense schools in the lower part of the sound scattering layer at night during January - February. Cardinalfish, dories, Roudi escolar (Promethichthys prometheus), Myctophidae, Gonostomatidae, and viperfishes (Chauliodontidae) were the principal bycatch species from the Southwest Indian Ridge seamounts.

7. SURVEY RESULTS

Acoustic Estimates

Two acoustic surveys of Seamount 150 were undertaken in 1983: on 18 - 19 August and 4 - 5 September. The respective estimates of biomass of the fish aggregations at this shallow seamount were 3 200t and 4 500t. These were considered to correspond with the general pattern of fish aggregations observed during surveys. Detailed biomass estimates of the species were as follows:

Estimate of Fish Biomass by the Trawl-acoustic Method

Surveys with QM-MK II echo integrator of the deepwater seamounts was impossible due to the limited technical capabilities of the equipment. The maximum range of integration of the echointegrator QM-MK II was 499 m, which made it impossible to estimate the fish biomass at the Seamount 710. The maximum interval of the echo integrator (199 m) also limited the capability of stock assessment because the depth range of schools over the Seamount 480 reached 390 m. In this case, the aggregations were only partially recorded. For these reasons at Seamounts 710 and 480 the fish aggregations biomass was estimated by the trawl-acoustic method.

The trawl-acoustic method was based on the determination of the volume of aggregations by means of the echo sounder and the netsounder and comparison with the trawl catches. The mean density of aggregations was calculated using the proportion of the catch to the volume of the aggregation recorded in the trawl mouth by means of the netsounder. It was assumed that the schools could be approximated by ellipsoid of revolution and their volumes were recorded by the EQ-38 echo sounder using the following formula:

Y = l/G× p[h(30×l/u×V - 2H×tg2ag/2)²] (m³)

For schools recorded by the NS-36 netsounder the following formula was used:

Y = l/G× p[h(30×l/u×V)²] (m³)

where,

h = height of the school fished (metres)
l = length of the echo record on the register tape (mm)
u = speed of paper tape in the register (mm/min)
V = vessel speed (knots)

The expression 2H_ltg2ag/2 provides a correction in determination of the horizontal length of the school where,

H_l = depth of the shoal location (m)
2ag = width of the echo sounder r beam (degrees),

which for the EQ-38 echosounder was 7°.

The errors in the fish aggregations' vertical dimensions were not calculated. The difficulty of the determination of density of the fish aggregations should be noted because of the variable catchability of the pelagic trawls, especially when fishing schools where their height exceeded the vertical opening of the trawl. Further there were no data describing the compression of the fish aggregation at the mouth of the trawl as a result of the hearding effect of the trawl warps and doors. For these surveys a gear vulnerability of 0.32 was adopted for alfonsino.

The mean fish biomass density estimated by trawl-acoustic method at Seamounts 710 and 480 was 39.4 kg/10³ m³ and 19.9 kg/10³ m³ respectively. The total biomass of fish aggregations at the seamounts was assumed to be equal to the maximum biomass recorded during the trawl operations at the seamount. The fish biomass estimated at Seamounts 710 and 480 were 6 200 and 1 500t respectively (Table 7.1).

Table 7.1

Results of trawl-acoustic surveys during the 17th Cruise of the R.V. Fiolent

Nineteenth Cruise of the R.V. Fiolent, May - October 1984

The assessment surveys of fish biomass of seamounts in the Southern Indian Ocean continued in 1984 with the 19^th cruise of the R.V. Fiolent. The area of operations covered by this cruise were banks off the Madagascar, the Southwest Indian Ridge (30 - 45°S, 40 - 55°E) - Walters Shoal and Seamounts 710, 480, 251, 150 and 102. Trawling was done using a 78/520 midwater trawl. The vertical opening of the trawl was 40 m and its horizontal opening was ~ 60 m. Bottom tows were made with a KhEK-4M trawl. Totally At Walters Shoal 29 trawl hauls were done 52 on the seamounts of the Southwest Indian Ridge. The numbers of trawl hauls and the species catch composition are presented in Tables 7.2 and 7.3.

Table 7.2

Results of trawl surveys at the Walters shoal during the 19th Cruise of the R.V. Fiolent

¹ Bottom tows.

Trawl-acoustic and echo sounding surveys

This cruise undertook three surveys of the seamounts of the Southwest Indian Ridge - one of Seamount 710 and two of Seamount 150 (Table 7.4). The equipment used and the survey methodology were the same as those used during the seventeenth cruise of the R.V. Fiolent.

A biomass assessment was made at Seamount 710 using the trawl-acoustic method with a result of 1 120±90 tonnes. At Seamount 150, the estimate of fish aggregation biomass using the acoustic method from the first survey was 3170t and 2500t from the second (Table 7.4). Fish aggregations that were accessible for acoustic assessment were absent on the other seamounts.

Table 7.3

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 19th Cruise of the R.V. Fiolent

Table 7.4

Results of trawl-acoustic surveys during the 19th Cruise of the R.V. Fiolent

The Madagascar Ridge

In the area of the Madagascar Ridge exploratory surveys were carried out at Walter Bank centered around coordinates 33°15'S and 43°47'E. Jack mackerel (Trachurus picturatus (longimanus?)) in a spawning condition predominated in the catches. A trawl survey at Walters Shoal was also undertaken. The total fish was determined using formula:

Q = (q / {V₁ j}) V₀

where,

q/V_l× j = density of a aggregation in kg/m³
j = vulnerability of fish to capture by the gear
V₁ = volume of water strained by the gear
q = average catch per hour of trawling (kg);
V₀ = volume of the sonified aggregation (m³) calculated from the height of schools (m) and area of aggregations (m²).

The estimated biomass of jack mackerel on the Walters Shoal at the period of research cruise was 1 500t.

After 1984 direct research expeditions at the seamounts of the Southwest Indian Ridge were not undertaken until 1987.

The Twenty first cruise of the RTMA R.V. Fiolent, May-September 1987

In 1987 - 1988 two expeditions by the R.V. Fiolent were undertaken.

The 21^st was in May - September 1987 when trawl surveys were undertaken of seamounts in the area 35° - 44°S, 40° - 60°E (Seamounts 358, 630, 150, 251, 710, 102, 415, 480, 360, 422, 430, 350, 800) of the Southwest Indian Ridge. Fish aggregations were recorded at Seamounts 358, 630, 150, 251, 710 and 102. Trawl hauls were made using a 70/370 midwater trawl, which had a vertical opening of 32 - 40 m and a horizontal opening of 28 m. The mesh size in the cod end was 60 mm. The number of trawl hauls and species catch composition are listed in Table 7.5.

Table 7.5

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 21st Cruise of the R.V. Fiolent

Acoustic fish biomass assessments were made using Sargan-Eh, a Simrad EQ-38 and SIORS echo integrators. To control depth of trawling, trawl opening and to record the passage of fish into the trawl, a IGEhK netsounder was connected to the head rope. The QM-MK II Simrad echo integrator was replaced with a SIORS echo because of the poor speed compensation by the QM-MK II echo integrator due its highly variable error of log and the large drift from zero of the operational amplifiers during survey conditions.

The target strength (TS) for fish was based on the relation of Love (Urikh, 1978) using equation:

TS = 18.4 log m + 1.6 log f - 61.6 (dB)

where,

m = an average weight of individual fish, (g)
f = the operational frequency of the EQ-38echosounder, i.e, 38 kHz.

Love's formula was used because of the absence of published data on the target strength of those fish species inhabiting the seamounts and the impracticality of estimating their back scattering cross-sections during cruise (e.g. by means of the control devices such as IGEK) due to the difficult fishing and weather conditions.

As the fish aggregations were of mixed species composition, the integrator constant was determined for each species separately and a scale value for the integrator was determined taking into account the percentage of the different species in the aggregation:

where,

C_j = value of the integrator scale point applied to the different species (t/naut. mile²)
K_j = % weight of a given species in the aggregation as determined by the fishing results
K = the number of species.

The total fish biomass estimates were:

At Seamount 550, which is located in the southern part of the Southwest Indian Ridge, an acoustic survey of subantarctic lanternfish electron (Electrona carlsbergi) resulted in a biomass estimate of 800t. Other details of the acoustic surveys are listed in Table 7.6.

The Twenty-third Cruise of the R.V. Fiolent, June - November 1988

In 1988 studies of fish stocks of the seamounts of the Southern Indian Ocean continued on the Southwest Indian and Ninety East Ridges by the R.V. Fiolent. There were three stages of studies during this cruise along the seamounts of the Southwest Indian Ridge: 10 - 15 June, 23 - 26 July and 14 October - 6 November, 1988. The following seamounts were surveyed: 150, 251 and 358 (during the first and second stages), 690, 358, 150, 251, 480, 415 and 102.

At the Mid-Indian Ridge and Ninety East Ridge research was carried out on 4 - 28 August 1988. Seamount 260 (Mid-Indian Ridge), and Seamounts 470, 462, 510, 462, 400, 336, 631, 620 and 549 (Ninety East Ridge) were surveyed. Trawl was undertaken using a 70/370 midwater trawl. This trawl had a vertical opening of 32 - 40 m and a horizontal opening of 28 m. The mesh size in the codend was 60 mm. From time to time a 41.7/39.6 bottom trawl was used. The results of the trawl survey and species catch composition are listed in Tables 7.7 and 7.8.

The hydroacoustic devices used in this cruise for the stock assessment was the same as those used in the 21^st cruise of R/V Fiolent (1987). Before steaming the acoustic transects the fish behavior was observed over different times of the day and the school positions were noted in relation to the seamount. School depth, location and size were recorded and the direction of drift over seamount was also determined. All of these data were then used in designing the location of the track positions for the acoustic survey. The integration interval was varied so as to cover the areas with fish records completely and to avoid area that had shown no indication of fish presence. The interval usually varied from 0.2 to 0.6 miles with a distance between survey transects of up to 1.5 miles. The biomass calculations were carried out by the methods used in the previous cruises. The target strength was calculated by the formula:

TS = 19.4 × log L + 0.6 × log l - 24.9 (dB)

where,

L = mean length of fish (m)
l = wave length (m)

Table 7.6

Results of trawl-acoustic surveys during the 21st Cruise of the R.V. Fiolent

Table 7.7

Results of trawl surveys at the Southwest Indian Ridge Seamounts during the 23rd Cruise of the R.V. Fiolent

Table 7.8

Results of trawl surveys at the Mid-Indian Ridge and Ninety East Ridge Seamounts during the 23rd Cruise of the R.V. Fiolent

The Southwest Indian Ridge Seamounts

The acoustic assessments of fish biomass were carried out only after fish had been detected. Fish biomasses were calculated at Seamounts 150, 480, 415 and 102 (see Table 7.9 and 7.10). The results were as follows.

Table 7.9
Estimates of fish biomasses

Seamounts of the Mid-Indian Ridge and Ninety Degree East Ridge

Research at the seamounts of the Mid-Indian Ridge and Ninety Degree East Ridge was undertaken from 4 to 28 August 1988. One seamount in the Mid-Indian Ridge (260) and eight seamounts in the Ninety East Ridge (336, 462, 510, 400, 620, 549, 470 and 631) were studied. Acoustic assessments of biomass were only made at Seamounts 260 and 336 (Table 7.11). At the other seamounts no fish aggregations were apparent or the fish had a scattered distribution (e.g. Seamounts 470 and 631). The results of the surveys are listed in Table 7.11.

The Ninth Cruise of the R.V. Ignat Pavlyuchenkov, December 1989 - April 1990

The R.V. Ignat Pavluchenkov started studies of fish stocks at the Southwest Indian Ridge seamounts in 1990. Research was undertaken from 18 February 1990 until 15 March 1990. Surveys were undertaken of Seamounts 690, 150, 251, 630, 740, 480, 415, 102, 358, 360, 422 and 350. Acoustic surveys were conducted only at night when fish aggregations were detected at Seamounts 690, 150, 251, 630, 740, 480 and 415. Trawl hauls were made with the midwater 43/176 trawl. ÅK-400 and ÅS-400 Simrad acoustic systems were used and a SIORS echo integrator. EK-400 and ÅS-400 echo sounders used a single transducer. An ÅS-400 was used as an additional echo sounder to map fish aggregations and to provide a preliminary estimate of target strength of the individuals within the fish aggregation. A CI-30 was used to control the vessel speed during the survey and monitor the direction and drift of the vessel over the transects of the acoustic survey as a current indicator.

The EK-400 echo sounder was calibrated during calm weather using a standard 60 mm diameter copper sphere (target strength = -33.7 dB). Simultaneously with the calibration of the EK400 and calibration of the ÅS400 sounder was done to determine the error of the target strength calculation.

Before each acoustic survey a scouting survey was undertaken to determine the location of the schools and their species composition. On the basis of the scouting survey the acoustic survey transects were plotted and stations for the control trawlings planned. The transects were zigzag shaped. The integration interval usually did not exceed a quarter of the distance between two parallel track lines. During the processing of the survey integrator readings those readings that were not consistent with the echogram records were not considered or were adjusted in their values. The reason of such errors was lost data due to the vessel pitching and rolling, acoustical inference due to sea roughness and the integrator incorporating part of the bottom echo. The vessel speed was kept constant at 8 knots during the survey.

Before the start of the acoustic surveys a scouting survey and control trawlings were undertaken. During those operations the school location and its species composition were determined. The maximum accumulation of the integrator data for each transmission was estimated. During the detailed acoustic survey the size and location of aggregations were made more precise and the contribution of the interference to the integrator readings was measured. The target strength was calculated using Love's empirical equation.

Table 7.10

Results of trawl-acoustic surveys at the Ninety East Ridge Seamounts during the 23rd Cruise of the R.V. Fiolent

Table 7.11

Results of trawl-acoustic surveys at the Southwest Indian Ridge Seamounts during the 23rd Cruise of the R.V. Fiolent

Table 7.12

Results of trawl surveys at the Mid-Indian Ridge and Ninety East Ridge Seamounts during the 23rd Cruise of the R.V. Fiolent

The total estimated fish biomass was 11 200 ± 1 400 t and included:

The biomass estimates for the different seamounts are listed in Table 7.13.