56 Trawling-acoustic and acoustic surveys Equipment Acoustic surveys to obtain fish biomass assessments used the following Simrad equipment: 1. EQ-50 fish searching echo sounder 11. EQ-38, EQ-120 scientific echosounders 111. QM-MK 11 echointegrator IV. CM H C-I electronic recorders and v. SQ Hydrolocator. A NS-3B acoustic netsounder was used for controlling trawl movement. The acoustic systems were used as follows. On transects during the acoustic searches the SQ echo sounder was operated in horizontal mode (i.e. with the transmitter tilt angle from 8 to 20 0 depending on the time of a day, and a search sector of 60 0 to starboard and port sides) using the transducer and electronic register CM, as well as the EQ-50 fish searching echo sounder operated in association with the electronic recorder (mode pelagic - 90 m). The echosounder HAG-432 was used when operating over the seamounts instead of the SQ echo sounder and the EQ-38 scientific sounder and the QM­ MK 11 echo integrator were also used. Methodology Used to Estimate Fish Aggregations Biomass Acoustic surveys for fish biomass assessment were undertaken using a scientific echo sounder EQ-38 and echo integrator QM-MK-II whose operating modes were as follows: Echo sounder EQ-38: range 500 m, bandwidth 1 kHz, pulse duration 3 x 1O-3s amplification 8 echo integrator QM-MKII integration range 100- 200 m amplification 30 dB. Echo integrator QM-MK-II: The echo integrator was calibrated using a graphical method according to the trawl catches. The basis for the calibration was the catches from positive trawl hauls at Seamounts 251 and 150 of the Southwest Indian Ridge. To determine the surface density of the fish aggregation the following formula was used: ps 1852 P L1r / L Sy tonnes/nautical mile where, p catch in tonnes L1 r integration range in metres L distance covered by a trawl through the fish aggregation (nautical miles) 2 Sy area of the cross-section of the trawl mouth (m ). 57 The diagram was plotted on the axis of M and ps where M deflection of the integrator recorder in millimeters for number of miles covered, i.e. - M M = - (mm/mile) (6.1 ) K and M deflection of the integrator recorder (mm) K nautical miles covered. Survey operations were undertaken at night when fish aggregations moved into the pelagic zone. This enables more comprehensive estimates to be made, i.e. less bias would occur from unavailability of the fish to the trawl. The average vessel speed during the first survey was nine knots and five knots during the second survey. The speed values were entered into the integrator in the manual mode in accordance with the data ofthe log IEhL-2M. Integrator readings were made every 0.5 mile during the first acoustic survey and every 1.0 mile during the second survey. Acoustic survey data were processed as follows. The actual survey tracks were drawn at the chart of the seamount where the integrator data were recorded. Then biomass contour lines were drawn in accordance with the chosen intervals and the mean deviations of the integrator were determined for each outlined region. With the estimated M values (in latter case they were equal to M; in former case the values of M) that were obtained using (6.1) and the calibration diagram the areal density of fish aggregations, ps, was determined. Taking into account the scale, the each area of equal aggregation, the surface density region was calculated and thus fish biomass was estimated according to the following formula: B (tonnes) where, ~ area of the /h region with the surface area Ps fish density in tonnes per square nautical mile. The biomass estimates that were obtained from these calculations were underestimates as the integrator calibration were based on catch data. Analyses of echograms of the echosounder and netsounder, as well as echograms of the echo integrator QM - MK 11 showed that not all the fish recorded by the netsounder entered the trawl codend. A considerable amount of fish passed through the wide mesh part of a trawl or escaped from the trawl when its speed dropped during hauling of the warps. To adjust for this, a gear vulnerability factor of 0.15 was used. 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 3 200 t and 4 500 1. These were considered to correspond with the general pattern of fish aggregations observed during surveys. Detailed biomass estimates ofthe species were as follows : First survey Second survey (t) (t) Rubyfishes 2560 3600 Jack mackerel 320 450 Butterfishes 320 450 58 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: 3 Y = IIG x7r[h(30 xliv x V - 2Hxtg2agl2/] (m ) For schools recorded by the NS-36 netsounder the following formula was used: where, h height of the school fished (metres) I length of the echo record on the register tape (mm) v speed of paper tape in the register (mm/m in) V vessel speed (knots) The expression 2Hltg2agl2 provides a correction in determination of the horizontal length of the school where, HI depth of the shoal location (m) 2ag = width of the echo sounder r beam (degrees), which for the EQ-38 echosounder was i. 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 3 39.4 kgll03 m and 19.9 kgll03 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 I 500 t respectively (Table 7.1). 59 Table 7.1 Results of trawl-acoustic surveys during the 17th Cruise of the R. V. Fio/ent Seamount Date of study Density Cumulative volume Total biomass 3 3 (Kr/ 10 M ) of aggregations of aggregations 3 (m ) (CC was used) (tonnes) 27 May 1983 39.4±10.36 4535 559 ± 47 710 21 Jun 1983 39.4±10.36 4808 590 ± 50 24 Jun 1983 39.4±10.36 11914 1466 ± 123 25 Aug 1983 19.9±4.73 97000 6031 ± 459 26 Aug 1983 19.9±4.73 65000 4044 ± 307 480 26 Aug 1983 19.9±4.73 99000 6156±468 27 Aug 1983 19.9±4.73 69000 4261 ± 326 Nineteenth Cruise of the R. V. Fio/ent, May - October 1984 The assessment surveys of fish biomass of seamounts in the Southern Indian Ocean continued in 1984 with the 19th 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 oS, 40-55 °E) - Waiters 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 Waiters 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 Waiters shoal during the 19th Cruise ofthe R. V. Fio/ent Catch of principal species % ....... ~ No . ;:::l Q) Hours Cl) I-< 0 Date of of <l) <l) of ..s::: ~ Cl) Cl) S Cl) I-< ro survey trawl <..> ~ <l) <l) trawling t;:: ro I-<ro ..s::: r/) hauls >. S ..s::: ....... .0 r/) ~ 0 <..> ~ ,....,ro Waiters July 1984 27 25.8 0.2 98.9 0.8 0.1 shoal July 1984 2 2.2 83 .9 16.1 I Bottom tows.