SPATIAL DISTRIBUTION AND SHOALING BEHAVIOUR OF FISHERY RESOURCES lN THE WATERS OFF WESTERN COAST OF ACEH: PREL|MINARY RESULTS FROM THE POST TSUNAMI EXPEDITION 2OO5

Acoustic investigation, which is one of the programmes of the Post Tsunami Expedition, was done in Aceh waters during 24 July to 14 August 2005. Research vessel Bawal Putih I and Baruna Jaya Vlll were used for the survey. The objeclive of this survey is to obtain pelagic fish resources distribution, afler the area hit by tsunami at the end of 2004. Two different models and frequencie6 were used in acquisition of the daiai SIMRAD EK-500 Scientific Echosounder with 38 kHz splitbeam transducer (Baruna Jaya Vltl) and Simrad EK€o scientific echosounder with 120 kHz splitbeam transducer (Bawai Putih t). Resutti show that fish aggregations appeated in high density layers in deep waters all over the study area. Fish shoals o@uned in relation to topographic features such as shelf edges and deep valleys. On the shelf, fish schoals tend to stay close to the bottom during daytime, while i[ raised, wittL more fish shoals were observed, during night time. More abundance prevails in areas of rough bottom conditions where trawl fishing was not possible, specifically in north areas between Lhok Nga and Catang. Abundances of tish in the coastal ar6a off Sibolga seem to be relativoly lower comparsd with thoss in its north€m part.

are required in estimating fish distribution and abundance by the statistical analysis on the historical data.
Following the advance development of hydro acoustic lechnology, sonar and echosounder have I major impact on fisheries, especially their applicalion in fishing sysiem, such as saarching for protitable concentration of fish.In tisharies research, il has become increasingly importance 9y91.tle years for studying and mon odng orsrnDurton, congregation, and migration of fish (Engas & Ona, 1997; Misund, 1987; Thome et al.   1987).
Hydro acoustic method can be considered as on of the suitable methods for estimating fish abundance direc-tly.The procedure involves echo sounding along a transect area and measuring the 1, Rslearch Institul€ for Marine Fbheri6, Muara garu-Jakada ' hslitute of Marina Resaarch, Bergen, Norway return echo intensity.A calibrated hydro-acoustic system then provides an estimate of the fish biomass for specified depth and distance intervals along the transect area (Burczynski, 1982), A programme of the investigation of tsunami effecls on marine life in Aceh and North Sumatera was carried out during July to August 2005.
Research vessel Bawal Putih I and Baruna Java Vlll were commissiohed to survey the fis'heries re€ources, mainly those of the walers off west and north coast of Aceh that was severely hit by tsunami.
Acoustic and exploratory fishing survey were done to meet the objeclive.Acoustic system observed fish , biomass, depih, and bottom topography, while fishing system observed catch, species, fish size and composition, and fish biological data.
Oceanographic data were also collected duiing the survey.This paper discuss specifically on the preliminary results of the acouslic survey, which are rolated to the spatial distribution and shoaling behaviour of pelagic fish resources.

Description of Acoustic Equipment
Acoustic data acquisition was conducted simultaneously with bathymetric transect.Two different models and frequencies were used in acquisition of the data; SIMRAD EK-sOO Scientific tnd.Fish Res.J. Vol.12 No.1 June-2006: 15-25   Echosounder with 38 kHz splitbeam transducer was used for data acquisition onboard of Rny' Baruna Jaya Vlll, while Simrad EK-60 scientific echosounder with 120 kHz solitbeam transducer was usad for data acouisition onboard of R / Bawal Putih l.Before the survey the equipment was calibrated using 60 mm copper standard target according standard procedures from Foole et al. (1987).The ecoustic parameters setting for both of the EK-500 and 8l-60 is showed in Tablel .Acoustic data were collected within the enlire water column.However. in the deep walers the data were recorded only from the surface down to maximum of 750 m.Echo integration was performed by 25 m depth layers Acoustic samples (echo integrations, echograms) were collecled iontinuously 24 hours a day, and acquired within the oath of the vessel along the designed transect lines with the vessel's speed at 6 to 7 knot' The ESDU (elementary sampling distance unit tlle distance over which the echo integral is accumulated lo give one sample) was '1 nautical mile (nmi).Acoustic data stored on the hard disk and dedicated softwares, Bl-500 (Baruna Jaya Vlll) and 8l-60 (Bawal Putih l) were used for scrulinizing echo signals.
Basically, the inlegrator performs integration in the vertical direclion within the layers ano averaging in the horizonlal direction along the path traveled by the vessel The integration process ls based on the quantity 10 log (Sv) and is defined by the equation as follows: oo=u"^nfi parameter from the integrator eacn averaglng interval oa represents the mean back scattering area per unit horizontal area and is obtained by averaging the individual aol6r'S over one interval in" qi.n-titv so (m2 nm-2) is related to oa 1m2 nm-'?) asl SA=(1852 m/nm)2 oa . .... ... ( 4The algorithm implemented by the sounder is obtained by combining the four equations: The first equation converls volume back scattering strenglh to back scattering area per unit of volume.The corresponding back scattering area per unit of horizontal area is obtained by inlegrating over the layer vertically, rl to 12 The output 16 f2 Sa=4rro2[lsvdr] (1852 mhm)/ . .fl Scrutinizing Given the facl that sampling of the acoustlc reoistrations (through trawl fishing operation) was im-oossible, especially in the offshore areas' it was also imoossibie to scrutinize data jnto species' Howevei, based on the characteristics of layers and taroets strenoth distributions, an attempt was made td scrutinize the recorded data into 5 different groups; 1) plankton,2) presumed fish schools,3) ir"sumed fish and plankton mixed in specific iavers.4) Dresumed plankton and fish mixed oisoersed in the water column, and 5) presumed olahkton and fish in a dispersed mix in the water column including acoustic noisd.The recorded area echo abundance, i.e. the nautical area backscattering coefficient (NASC), SA (lvlacLennan' 1990), was influenced by acoustic noise from the vessel or from other equipment.As far as possible this noise was manually removed.However, in large parts of the survey the noise was simply too much to remove manually, and hence these data were put into group 5.During very detailed multi- beam acoustic mapping, only parts of the survey were scrutinized.

RESULTS AND DISCUSSION
Investigations of FyV Bawal putih I were mosuy limited to the narrow continental shelf, /.e. the area 0f 20 to 200 m bottom depth, in the waters of north and west coast of Nangroe Aceh, whereas Baruna Jaya Vlll went to a larger degree covered the deeper parts along and off the shelf.The acoustic density vaned geographically in the study area, being higher along and off the shelf than on the shelf itself.The hlghest values were recorded off Banda Aceh (Figure 1).The acoustic density tended to increase at nighttime, with up to 4 times higher values (Figure 2).At the same time there was a shift in the depth of highest density from o to 25 m to 75 to 100 m (Figure 3).Target strength date indicated that this increase was due to fish migrating towards the surface from deeper parts mixing with plankton at a layer just above the thermocline at ca. 1 00 depths (Figure 4).Fish was observed in deep layers at 300 to 450 m depth all over the studied area.Nole that in areas with depth less than 400 m the layer appeared to undertake vertical migrations towards the plankton layer closer to surface (0 to 1OO m depth), whereas in deeper areas (depth>400 m) the layer appeared to stay at . the same depth regardless of the diurnal cycle (Figure 5).This may imply that we are talking about lwo different species with different behaviour, or it may be a difference between young and adult fish we are seeing.In temperate waters it is common for the young fish of pelagic resources (blue whiting, herring, mesopelagic fish) to undertake migrations towards the surface at nighttime, whereas the adults maintain their position in the deep (Slotte et a/., 2004).lf these presumed fish layers and thejr behaviour have not been previously studied, it is certainly an interesting subject for future investigations. t.
Fish shoals were observed in all the three main use of sonar equipment for shoal tracking may be areas.Most of the shoals were observed off Banda very valuable.The really big schoals in the deeper Aceh, and typically they were observed in part deserve some special attention.
connection with the shelf break (Figure 6).They appeared to be two different type ofschools:1)the Also off the Lhok Nga barrier reef at the really big shoals with high densities peaking at presumed flsh schools typically appeared along the more than 200.000 in 1 nmi NASC values, staying shelf break (Figure 7), two different kinds of close to the bottom at around 125 to 150 m depth presumed flsh shoals were observed: 1) quite loose just below the shelf break; and 2) The more loose shoals almost appearing like a layer at some shoals of less density appearing from lhe surface distance from the shelf, which seemed to under and down to 75 m depth, appearing along the shelf take some kind of vertical migration relaied to the break, but also to some exlenl on the shelf itself.lf diurnal cycle, staying close to the bottom at 200 to these presumed fish shoals and their behaviour 250 m dept during daytime and mixing with the have not been previously studied, it also an plankton layer at 0 to 100 m depth during darkness, interesting subject for future projects in which the The acoustic response from these shoals was Average target strength by depth belween day and night, higher at thB 120 kHz, whlch is not common for fish wilh swim bladders, wheraes it is common for fish without swim bladder, as in mackerel.Denser shoals typically appeared at 100 to 250 m depth along the shelf, and in the deep channel entering the shelf.
Off Simeuleue less time was used to cover the area, and only a few relatively loose shoals were observed along the shelf (Figure  8a and gb), at 150 to 200 m depths.However, note that in this area.opposite to areas 1 to 2, several shoals of large fish were observed hunting al the surface.
The acoustic data from Bawal putih I demonstrato that coral reefs and rocks dominated most of sea bottom of northern area off the west coast, and made up a hindrance for bottom trawl operation (Figure 9).pelagic fish schools on the .shelf were appeared more oflen along the acouslic transects between Lhok Nga and south eastern off Calang, mainly during the night, but continuing south eastward to Sibolga they tended to be less frequent.Typical pelagic fish schooling in this shallow area is showed in Figure 10.The fishes on the shelf eppeared to scatter in the daytime and tend to stay close to the bottom.This was in accordance with the fact that pelagic fishes were frequently caught in the daytime fishing operalions of bottom trawl.The highest traces is obtained in the water layer between 20 m and 40 m. the strength of targets traced was mosfly less than -45 dB, giving indication that the targets were small pelagic tishes.
along the survey track of Baruna Jaya; comparison Several investigations of the fishery resources in lhe west coast of Sumatera have been done before the tsunami evenl.Among them, expedition of RAy' Dr. Fidtjof Nansen (Aglen el a/., '1981) was the comprehensive one.Results of the acoustic observation from RA/ Dr. Fridtjof Nansen expedition showed the increasing quantities of the fish north southeasternward (from Lhok Nga up to southern Sibolga) along the shelf, of which this was different oppositely with the results of the cunent acoustic obseNation.All means that th6re is an indication of decreasing fish stock, which may be caused by the rapid increase of fish net fishery, in areas of th6 eastern and southern of Sibolga.
Although fish were observed acoustically all over the sludy area, no attempt was made to calculate abundance and biomass.There are lwo reasons lo this decision.First, the acoustic survey of Baruna Jaya Vlll was not designed wlth the specific objective to estimale distribulion and abundance of pelagic fish resources in the affected areas.Due to time constraints and all the other tasks of the survey, the lrack was more designed for multibeam acoustic mapping of sea bottom, ROV studies, geological and environmental studies.
Secondly, Baruna Jaya Vlll was equipped with a very small pelagic trawl (5x5 m opening), not fit for the sampling of pelagic fish species.During the survey four altempts to sample proposed fish layers were unsuccessful.Bawal Putih I only had a bottom trawl, and could nol be used to sample pelagic species.Consequently, there was no confirmation of species and size composition of the observed Echograms (15 nmi) showing fish layers observed off Meulaboh with Baruna Jaya Vlll; comparison between day and night.In a) and b) the same type of layer is shown to migrate towards the surface, whereas in c) is a deeper fish layer is shown, with no signs of diurnal changes in vertical distribution, and which were found all over the deepest part of the study area.a) Distribution and density of lish shoals from obseNed with Baruna Jaya Vlll off Lhok Nga barrier reef related to the bottom topography coming from the multi beam acoustic survey.In b) it is shown an example echogram of the observed fish shoals, and in c) it is shown example echogram of fish layer ofl the shelf in the Benguela strait, which is migrating towards the surface in the afternoon and mixing with plankton in the night.The acoustic method used for fish abundance estimation may be sub.jected to some errors.The source of efrors of an acoustic estimate may be classified in lwo main categories.The firsl concerns error of the estimate, which includes spatial sarnpling error (lhe way we conduct our sampling).
The second category includes the more technical aspects concerning the equipment, transmission of the sound and the target strength.The sources of errors may again be classified by their way of affecting the estimate, either randomly or systematically.One major problem pertaining to the pelagic species is related to the ensuring that the whole distribution area has been covered.The waters off Aceh west coast encloses a large area and it is impossible to cover all potential areas of distribution in a single survey.When estimaling the abundance of populations of fish with schooling behaviour, the shadowing effect must always be considered as a Dotential source of error.When fish schools, the SA-values are no longer proportional lo fish density, but rather too low.This problem increases when the schools become larger.The present estimates are not corrected for lhe shadowing effect, but it is not very likely that this will have seriously affected the abundance estimates, since schools of pelagic fishes in the west coasl rarely form schools dense or large enough for this to be a major source of error.fypicat of bottom leatures on lhe shelf areas in (A) norlhern (B) the southern part of the survey area.-51 CONCLUSION Fish aggregations appeared in high density layers in deep waters all over the study aTea.One species migrates vertically with the diurnal cycle (300 to 0 m depth).Another stays in the deep (300   to 500 m) regardless of the diurnal cycle.In all three areas that were mapped in detail with multibeam echosounder, fish schools occurred in relation to topographic features such as shelf edges and deep valleys.
The results of the investigation indicate that the western Aceh waters have considerable fish resources, but the potential for exploitation is still not colculated yet.The biomass of pelagic resources should be estimated with echosounder and sonars in combination with biological sampling.To find the best season for stock assessment one has to study the stock dynamics related to seasonal changes.
information is necessary for estimating abundance.For future studies a larger trawl effective is clearly needed.
Figure 10.Distributions of traced targets along the survey tracks of R/V Bawal Putih l.

Table 1 ,
Parameter selting from EK 60 and EK 500 during the survey