Investigations of Demersal Mackerel Icefish (C.Gunnari) Spatial Distribution in Relation to Improvements in Stock Estimates by Trawl-Acoustic Survey

Not to be cited without prior reference to the author ICES CM 2003/Q:05

INVESTIGATIONS OF DEMERSAL MACKEREL ICEFISH (C.GUNNARI) SPATIAL DISTRIBUTION IN RELATION TO IMPROVEMENTS IN STOCK ESTIMATES BY TRAWL-ACOUSTIC SURVEY

Kasatkina S.M. and Frolkina Zn.A

Atlantic Scientific Research Institute of Marine Fisheries and Oceanography (AtlantNIRO). 5, Dm.Donskoy Str., Kaliningrad, 236000 Russia . Fax: 007 0112 21999, e-mail: [email protected]

ABSTRACT

Conventionally, quantitative assessment of mackerel icefish C. gunnari, as a demersal fish species, has been made based on the results of inventory bottom trawling surveys in Subarea 48.3. The survey technique assumes that mature and large fish stay at the bottom in the daytime and may undertake vertical migrations at night, dispersing in the water column. The scientific observations made during the acoustic and bottom trawling surveys in 2000 and 2002 by the Russian vessel evidence that although the mackerel icefish occur close to the seabed the significant amounts distribute in the water column above the sampling range of the bottom trawls currently used in the standing stock estimations. This means that estimates of the standing stock based solely on bottom trawl results will be biased (underestimated) depending on the proportion of the mackerel icefish population present in the mid-water layer at the daytime. Estimates of pelagic and bottom components of icefish biomass and abundance by length classes based on the trawl and acoustic surveys results are discussed. It is shown that particularities and a non-uniformity of icefish horizontal and vertical distribution within the near-bottom layer affects the efficiency of the fishing gear. It is also shown that the acoustic observations provide valuable information on temporal and spatial structure of fish aggregations that can be incorporated into the overall estimate of the standing stock. The relationship of mackerel icefish spatial distribution and abundance heterogeneity with krill distribution, as the preferred food item, has been revealed. The importance of krill and mackerel icefish interaction study in understanding the latter regularities as well as in the mackerel icefish assessment technique within the ecosystem approach is shown. In conclusion the approaches to the integration of up-to-day acoustic surveys methodology and the problem of semi-pelagic resources quantitative assessment by the example of icefish are considered. Keywords: acoustic and bottom trawling survey, density and biomass estimates, fishing gear efficiency, interaction between fish and krill.

INTRODUCTION

The mackerel icefish (Champsocephalus gunnari) has been subject to commercial fishing over the past 30 years, in some regions feeds extensively on krill and is itself subject to predation by seals and birds. In terms of the requirements of Article II of CCAMLR it is therefore both a ‘harvested’ and a ‘dependent’ species. The management regime needs 2 therefore to ensure the sustainability of the fishery in the long term whilst at the same time ensuring the long term viability of the dependent species populations. Assessments of icefish need to satisfy two primary requirements, firstly to satisfy the requirements of a single species assessment and secondly to quantify their interactions within the ecosystem (Everson et al, 2001).

Quantitative assessment of C. gunnari is traditionally made from the results of bottom trawl surveys. Methodology of a bottom survey conduction suggests that in the daytime, mature and large fish stably keep to the bottom while at night they may perform vertical migrations and disperse in the water column. The example of Subarea 48.3 shows that conduction of bottom surveys over a long time period posed more questions on stock state and regularities of icefish distribution than gave answers to these questions. The CCAMLR Scientific Committee has more than once indicated remaining uncertainty in the stock state of this biological object (SC-CAMLR, 2000). In this respect, a known phenomenon is worth mentioning when fish biomass sharply increased dozens of times from the results of the surveys carried out from 1988/89 to 1989/90, having incredibly dropped from 1989/90 to 1990/91.

Successful fishery of icefish in pelagic zone as well as the occasions recorded often enough during the years of C. gunnari fishing on the S. Georgia shelf when the catch of commercial vessels exceeded biomass estimates from the results of bottom trawl surveys (WG-FSA, 1996) shows that, C. gunnari may be regarded as a pelagic object (fig.1).

Available results of investigations of the icefish habitats (South Georgia Island and Herd Island) confirm it and indicate that C. gunnari are a typical example of semi-pelagic distribution (Shust, 1998, Gerasimchuk et.al,. 1987). Both at South Georgia and Herd areas, the fish of two first age groups and length classes of 15-24 cm permanently occur in pelagic zone. Distribution of mature fish may be inconsistent with traditional conception: near- bottom in the daytime and pelagic at night. Catches per unit time yielded during the daytime with midwater and bottom trawls appeared to be comparable, but diverse size groups form pelagic and near-bottom components of the stock (Gerasimchuk, 1987).

Thus, as is evident from available results of fishing and investigations of icefish habitats, their distribution pattern is at variance with the methodology of bottom surveys. A pelagic part of the stock permanently occurring during the daytime will not be taken into account during bottom trawl surveys and, therefore, will be disregarded when computing TAC. Composition of catches taken by bottom trawls will not reflect the stock structure in terms of young and immature fish. The study of icefish population structure will facilitate consideration of pelagic component, enabling estimation of recruitment, which will enter the fishery next year, etc

The above-stated is indicative of the necessity of changing the system of survey data collection and processing, which would have allowed estimating bottom and pelagic components of icefish stock. For this purpose, the use of up-to-date acoustic method is considered to be a prospective line. Conduction of trawl and acoustic survey will allow making quantitative assessment of near-bottom (bottom trawl survey) and pelagic (acoustic survey) components of icefish stock.

A pilot trawl - acoustic survey of icefish on the South Georgia shelf was performed by Russian RV Atlantida in February-March 2002. Results of this survey are discussed in this paper. As the methodic aspects of bottom trawl survey are known (Frolkina and Gasyukov, 3

2000), we put attention to methodic problems of acoustic survey. The attempt to estimate the uncertainty in biomass estimates from this survey was made.

MATERIALS AND METHOD

The trawl- acoustic survey was conducted from January 30 to March 10, 2002. In accordance with the developed design (Kasatkina et al., 2001), it was a stage-by-stage procedure with alternating bottom trawl and acoustic surveys. Both trawl and acoustic survey were carried out during the daytime, from 6 a.m. to 6 p.m. Time mode of the survey was in line with the rhythm of vertical daily icefish migrations observed in the South Georgia area during daily stations made earlier, during the fishery and trawl surveys (Frolkina and Shlibanov, 1991; Frolkina and Gasyukov, 2000; Frolkina, 2002).

Bottom survey

The bottom trawling survey, accompanied with the acoustic observations, was carried out in the first stage. The area covered by trawl survey has been outlined within depth range of 100-500 m. This survey was based on the stratified design of trawl station positions and concluded 81 trawlings by bottom trawl Hek-4m with 8- m vertical opening. Haul duration was 0.5 hour.

Acoustic survey

The area covered by acoustic survey was in general the same as bottom survey. But, moving of the vessel to depths less than 100 m on the side of shoreline owing to acoustic transaction limit is permissible by bottom areography and the CCALMR Regulations.

Survey design

The acoustic survey assumed a stratified design on randomized parallel transects as a basis. Three strata (northern, eastern and southern) have been singled out proceeding from the condition that transects are orthogonal relative to regional bathymetry and from expediency of covering with transects of traditional fishing grounds, the areas favourable for icefish concentration and their migration The northern and eastern strata of South Georgia shelf encompass traditional fishing grounds and the areas where the fish concentrate during feeding period and at the beginning of pre-spawning period. Transect scheme was developed basing on two-stage randomization (fig. 2).

Acoustic sampling

Acoustic data (volume backscattering and individual target strength data) were collected using a Simrad EK-500 echosounder, 38 kHz, 120 kHz and 200 kHz hull-mounted transducers, and SonarData’s EchoLog_Ek data logging software. Ping intervals were 2 sec and pulse duration were 1 msec for all frequencies. A Simrad EK-500 echosounder was calibrated at all three frequencies immediately before the survey in the Stromness Bay using copper spheres as standard acoustic targets.

Acoustic data were analyzed by SonarData’s EchoView post-processing software using “dB-difference method” on the base of virtual echogram cascade developed by Bo workshop (Anon, 2000). Backscattering was attributed to fish fraction when the difference (Sv120kHz – Sv38kHz ) was less than 2 dB and attributed to krill when (Sv120kHz – Sv38kHz ) was 4 between 2 dB and 16 dB (Watkins and Brierley, 2001; Hewitt et al, 2002). Icefish density was estimated at 38 kHz frequency. Separation of backscattering attributed to icefish from fish fraction backscattering, mixed in terms of species composition, was made based on the echo trace analysis and length-species composition of trawl samples using the methodology for assessment of mixed biomass reported in Hagstrum et al, 1991; Mamylov et al, 1989; Kasatkina et al, 2002. The method of Jolly and Hampton (Anon, 2000) was used to calculate the icefish biomass.

Net sampling

Hauls were made with a midwater trawl RT/TM 70/300 (43±3m vertical trawl opening) during the daytime. Haul duration was 0.5 hour. The near-bottom 8-m layer covered with trawl survey (a bottom trawl Hake-4M with 8m vertical opening) was excluded from the fished depth range. The horizon of the RT/TM 70/300 trawl travel and its parameters (distance between boards, trawl mouth shape and its horizontal and vertical opening) were controlled by the trawl system Simrad FS 900/025 MK II

TS in situ measurements

TS in situ measurements were made on 38 kHz frequency by comparing an observed TS histogram with the size L composition of the ensonified fish, obtained by trawling. Collection and processing of acoustic data were performed using a SonarData EchoViev software. Only targets detected within 1° of the beam axis and within fished depth range were processed. Target strength and length distributions were only compared when the studied fish species amounted to at least 85% of the catch (in abundance), giving preference to one-modal histograms of TS and fish size.

The parameter b20 in equation TS = 20 logL – b20 (Foot, 1987) was calculated using a least- square fitting procedure (Maclennan and Minz, 1996). Statistical characteristics of unknown parameters were obtained using delta-method (Bard, 1979), (Ratcovsky, 1983). The values of b20 derived by using this method may be considered to be normally distributed, which, given mean value and dispersion known from computations, allow simulating this parameter as a random value.

Uncertainty in acoustic biomass estimates

In the paper by Rose et al., (2000) various sources of uncertainties in the acoustic survey are investigated using the simulation technique. As is evident from the results produced by these authors and our results (Kasatkina et al, 2003), acoustic sampling (index Sa) variability , target strength and length-species identification are primary sources of uncertainties. The influence of main factors was simulated by procedure of bootstrap type. The influence of b20 parameter in the target strength equation was simulated in accordance with the normal low of distribution.

Since the acoustic data processing assumes performance of consecutive stages, the simulation technique is supposed to be used for studying both the influence of each factor separately and combined influence of these factors on the resultant estimate of abundance and biomass of the object under investigation. All calculation was carried out for each stratum using separate factors and combining all the factors together. It gives the possibility to obtain the resulted distribution of the total biomass estimates and their statistical characteristics: mean and median values, standard deviation, coefficient of variation, 95% 5 confidence intervals . The simulation algorithm was described in the paper (Kasatkina et al, 2003).

RESULTS AND DISCUSSION

Target strength measurements

Frequency distributions of in situ target strength measurements at 38 kHz were related to length of icefish sampled with pelagic trawl (fig.3). Volume of acoustic samples amounted to 430 targets. Statistical characteristic (mean, standard deviation SD) of parameter b20 were calculated as: Mean = 82.74 dB; SD = 0.74 dB

The icefish TS values appeared to be higher than forecasted basing on comparison with the same for the other fish species, the North Atlantic mackerel (Scomber scomber), also devoid of swim bladder and compatible with the icefish in terms of basic morphometric features (Kasatkina, 2000). A parameter b20scomber = 86.5dB derived for Scomber scomber (Misund and Beltestad, 1996; Edwards et al., 1984) proved to be by 3.76 dB higher as against our estimate (b=82.74 dB).

Biomass and mackerel icefish distribution

The analysis of species composition of trawl catches showed that the bulk of pelagic ichthyofauna on the South Georgia shelf was represented by C. gunnari, Ph. georgianus, Myctophidae (G. niccholsi, E. carlsbergi, E. antarctica, P. Tension) and young fish, with Ch. gunnary 4-6 cm in length predominating. Pelagic aggregations of C. gunnari and Ph. georgianus were mainly represented by size-classes of 12 to 29 cm.

As is evident from trawl samples, the young fish prevailed within the near-surface 50-m layer. Their aggregations were recorded as the swarms or “layers” and were positively identified based on a “dB difference method”. An example of echogram of young fish aggregations observed during the trawling (young fish making up 100% of the catch) is shown in fig.4. A masked-free noise 38 kHz echogram is depicted in the same figure.

C. gunnari made the bulk of the catches taken when fishing the near-bottom 50-m layer above the bottom 8m layer for the bottom depths less than 300m. The bottom 8m layer was fished by bottom trawl. Icefish formed sparse aggregations or were recorded as indistinct “patches” (fig. 5). The schools were mainly detected above the slopes of narrow canyons with the bottom depths of 250-280. Mackerel icefish was not detected for the bottom depths more than 300-350m. Myctophidae were dominated in the catches above the near-bottom 50m layer. The trawlings also showed the presence of icefish in the pelagic zone outside the near-bottom 50-m layer. Unfortunately, the number of trawl samples taken outside the said 50-m layer appeared to be insufficient for estimation of length-species composition of mixed aggregations in this depth range within the entire shelf.

Considering that nearly 80% of trawl samples were obtained when fishing the pelagic 50-m layer above the bottom 8m layer which provided positive identification of size-species composition of fish aggregations within that depth range as well as confinement of a major part of pelagic icefish biomass to that depth range, it was decided to estimate the icefish biomass and abundance in the said 50-m pelagic layer. The estimated biomass may evidently 6 be considered as minimum value of the pelagic component distributed beyond the investigated depth range.

Acoustic survey reveals the presence of the pelagic component of C.gunnari inaccessible for a bottom trawl survey. The size composition revealed from midwater trawl catches is presented in fig.6. The biomass of pelagic component amounted to 26.0 thous. tons (Table 1). ) and abundance distribution by size class is depicted in fig.7. The bulk of pelagic biomass was represented by the fish of 1+ and 2+ years of age, which indicates that a significant recruitment and successful fishery may be expected next year.

In addition to the above-stated estimates of the icefish abundance and biomass in the pelagic zone, we attempted to assess the icefish biomass in the near-bottom 8-m layer using data of acoustic survey. To escape the “dead zone”, the integration was made within the layer of 1 m to 8 m above the bottom. The procedure of distinguishing the backscattering of icefish was similar to that used for the analysis of pelagic mixed biomasses. In this case, however, the biological data obtained from the bottom survey results were used (fig.8). C. gunnari and Pseudochaenicthys georgianus made the bulk of the bottom trawl catches. The TS-length relationship for icefish was used for the closely related Ps.georgianus also devoid a swim bladder. Acoustic assessment of the icefish biomass in the near-bottom 8-m layer resulted in the estimate of 62,328 tons (Table1). Estimation of abundance by size classes shows that the fish older than two years of age predominated (fig.9).

The sum acoustic estimate of the icefish biomass in the subarea of South Georgia amounted to 88.3 thous. tons. Nearly 30% of this biomass was concentrated in the pelagic zone. From the results of bottom survey the biomass was estimated at 46,600 tons. Our acoustic estimate of the pelagic biomass constitutes about 60% of that biomass. Fishing with midwater and bottom trawl reveals variation of size composition in the stock pelagic and bottom components in different shelf areas (fig. 6,8).

The confidence intervals of biomass estimates can be found in the Table1.

Sources of uncertanty in C.gunnari biomass estimate from results acoustic survey.

Results of the simulation of the biomass estimates as a function of variations in the density distribution are presented in the figure 10. Results of the simulation of the biomass estimates as a function of species identification and length composition of the catches are presented in the figure 11. Results of the simulations of biomass estimates as a function of uncertainty in target strength are presented in Figures 12. Statistical characteristics of the biomass estimates are shown in Table 2. In the figure 13 and in the table 2 there are presented the simulation results as a function of combined impact of all the sources of uncertainties.

The main uncertainty in the biomass estimates are formed by uncertainty in density distribution and in target strength. The influence of uncertainty in length composition of C. gunnari in catches is less. Combined impact of the all sources resulted in coefficient of variation 0.24.

Interaction between mackerel icefish and krill distributions

Results of acoustic survey and acoustic observation during the bottom survey are indicative of non-uniformity of vertical and horizontal distribution of pelagic and bottom icefish biomass components and timing of this distribution to krill distribution. The distribution of icefish density in the near-bottom 8-m layer replicates the krill density distribution within the 58-m layer above the bottom (fig.14). Correlation factor for the two density samples was estimated at r = 0.82. The fish density distribution within the pelagic 50-m layer (above the 8-m near-bottom layer) can be also matched with krill distribution (fig.14). During the survey, over 30% of krill biomass was distributed near the bottom (Kasatkina et al, 2002; Kasatkina and Malyshko, 2003). Sometimes, the krill density at the ground exceeded that in the pelagic zone. Most likely, the observed krill distribution was responsible for keeping the fish at the ground during the daytime. Given a different vertical krill distribution, a change in vertical distribution of the icefish biomass during the daytime can be expected.

Particularities of icefish distribution within the near-bottom layer

The density estimated by bottom trawl were compared with those from acoustics. Within the sampling range of net (8-m near bottom layer) acoustic estimates of density were higher than those from the nets (fig.15). This indicates that bottom trawl catchability is much less than a unity. Acoustic observations show that mackerel icefish can extend upwards above the seabed higher than the headline height of research trawls (8m- for trawl Hek-4m, 6m -for UK trawl).This is particularly evident in dense aggregations. It was concluded that differences between results of surveys could be in general explained by vertical distribution of icefish near-bottom aggregations on the efficiency of the gear with different vertical opining (fig.15,16).

It was revealed high “patchiness” of distribution detected by observations with coefficient of variation of acoustic density between trawl stations more than 150% (fig.17). The scale of this aggregations are lesser than a standard hauling track.

Particularities of icefish distribution within the near-bottom layer reveal the sources of uncertainty in biomass estimates by bottom trawl survey. The above-stated is indicative of the fact that the result of bottom survey cannot be regarded as the characteristic of icefish stock state in the investigated area.

CONCLUSION

The first trawl -acoustic survey carried out in the South Georgia area demonstrated advantages of such surveys and highlighted the problems related with stock assessment of the semi-pelagic fish.

Advantages of trawl - acoustic surveys: • Trawl - acoustic survey enables performance of a wide variety of investigations complying with two primary requirements: firstly to satisfy the requirements of a single species assessment and secondly to quantify their interactions within the ecosystem ; • assess biomass and abundance by length classes of bottom and pelagic components of icefish stock and use derived data for TAC computations; 8

• assess juvenile icefish abundance which was previously made using fry survey data. It should be noted that no such specialized surveys were carried out in the recent years; • study vertical and horizontal distribution of icefish pelagic component including interaction with krill and zooplankton using multi-frequency methods of data collection and processing; • provide acoustic observation during bottom surveys to produce important information on the spatial distribution of near-bottom aggregations that can be incorporated into the overall estimate of standing stock.

Integration of results from trawl - acoustic survey will include the estimate of total biomass and total length-age structure of icefish population in the investigated area. Solution of this task needs to know catchability and differential catchability of the bottom and midwater trawls. This investigations can be carried out applying the models of probabilistic- statistical theory of fishing trawls developed in the AtlantNIRO (Kadilnikov, 2001).

REFERENCES

Anon, 2000.Report of the Bo workshop (La Jolla, USA, 30 May to 9 June 2000).

Bard Y. 1974. Non-linear parameter estimation. Academic Press, New York, pp.349. Ratkowsky D.A. 1983. Non-linear regression modelling. Marcel Dekker. New York. 276 pp.

Brierley, A.S., Ward, P., Watkins, J.L., and Goss, C. 1998. Acoustic discrimination of Southern Ocean zooplankton. Deep-Sea Research II, 45: 1155-1173.

Edwards J., Armstrong F., Magurran A. And Pitcher T., 1984. Herring, mackerel and sprat target strength experiments with behavioural observations. ICES, CM 1984/B:34, 21pp Everson I., Kasatkina S., Goss C and M. Belchier, 2001. Assessment of mackerel icefish. Document WAMI-01/14. CCAMLR, Hobart, Australia.

Gherasimchook V.V. 1987. Brief report of the joint Soviet-Australian expedition of the USSR FRV Professor Mesuatsev to the Australian fishing zone around the territory of Heard and Mcdonald Islands, May-August, 1987. In: Selested Scientific Papers,1991(SC- CAMLR-SSP/4).CCAMLR,Hobart,Australia:49-74.

Frolkina Zh. And Shlibanov V. 1991. Vertical migrations of mackerel icefish (C.gunnari) on the South Georgia Shelf. In: Selested Scientific Papers,1991(SC-CAMLR- SSP/8).CCAMLR,Hobart,Australia.

Frolkina Zn and P.S. Gasiokov, 2000. Distribution, biological characteristics and biomass of icefish based on the results of the trawling survey, carried out at RV Atlantida in February 2000. Document of the WG-FSA-00/* CCAMLR,Hobart, Australia

Frolkina Zn.A. 2002. Distribution of mackerel icefish (Champsocephalus gunnari) (Channichthyidae) around South Georgia at Various Stages of its life Cycle. CCAMLR Science, vol. 9 (2002):49-69.

Foote K., 1987. Fish target strength for use in echointegrator survey. Journal JASA,82: 981- 987.

Hagstrцm, O.; Palmen, L.-E., Hakansson, N.; Kдstner, D.; Rothbart, H. Gцtze, E.; Grygiel, W.; Wyszynski, M. 1991. Acoustic estimates of the herring and sprat stocks in the Baltic proper October 1990. ICES CM 1991/J: 34.

Hewitt R.P., Watkins J., Naganobu M., Sushin V., Brierley A., Damer D., Kasatkina S., Takao Y., Goss C., Malyshko A., Brandon M., Siegel V., Trathan P., Emery J., Everson I. and D. Miller, 2002. Biomass of Antarctic Krill in Scotia Sea. Oceanolgraphy, vol.15, No.3: 25-23.

Kadilnikov Yu.V., 2001. Probability-statistical theory of fisheries system and of technical accessibility of aquatic biological resources to those system. Kaliningrad, Russia:274.

Kasatkina S.M., Frolkina Zh.A. and P.S.Gasioukov , 2001. Proposals for improvement of census surveys formackerel icefish quantitative assessment. design of acoustic trawling survey in subarea 48.3. Document WAMI-01/06. СCAMLR, Hobart, Australia 10

Kasatkina S.M., V.Yu.Sunkovich, Malyshko A.P. and A.P Frolkina Zh.A., 2002. Mackerel icefish biomass and distribution on the results of acoustic survey carried out in February- March 2002. Document of FSA-02/44, СCAMLR, Hobart, Australia.

Kasatkina S.M., Gasyokov P.S. and Frolkina Zn.A. 2003. Methodical problems of trawl – acoustic surveys in mackerel icefish stock assessment. Document of SFA -03/06, СCAMLR, Hobart, Australia.

MacLennan D. And A.Menz, 1996. Interpretation of in situ target strength data. ICES Journal od Marine Science, 53 :233-236.

Mamylov et al, 1989. Acoustic estimates biomass of multi-species aggregations. PINRO, Myrmansk, Russia, 119p.

Misund o. And A.Beltestad, 1996. Target-strength estimates of schooling herring and mackerel using the compatison method. ICES Journal od Marine Science, 53 :281-284.

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Rose G., Gauthier and G.Lawson.2000. Acoustic surveys in the full monte: simulating uncertainty. Aquat. Living Resours,13 (2000): 367-372.

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Shust K.V. 1998. Fishes and fish resources of the Antarctic. Moscow, VNIRO: 163pp.

Slosarczuk W. 1985. Contribution on to the early life history of Channichthyidae from the Bransfild Strait and South Georgia (Antraction). In: S.O. Kullander and B.Fernhoin(eds). Proc.5-th congress of European Ichthyologysts, Stockho;m, Swed.Mus.Nat.Hist.: 472pp.

Table 1. Biomass estimates of C.gunnari in the South Georgia area from results of trawl- acoustic survey during February-March 2002.

Survey Biomass (in tons) Mean Low CL Upper CL Acoustic survey in the 26037 12704 39371 pelagic zone(8-58 m above the seabed) Acoustic survey in the 62328 39182 85474 bottom 8m survey Bottom trawl survey 46647 12297 96382

Table 2. Statistical summary of bootstrap simulation: biomass from acoustic survey (in tones) using different sources of uncertainties. SD-standard deviation, CV-coefficient of variation; Lower CL-lower 95% confidence limits; Upper CL- upper 95% confidence limits; one sided 95% Low bound.

Sources of uncertainties Index Sa Target Strength Length Frequency Combined Mean 23659 23304 23131 23659 Median 22993 23001 23149 22993 SD 4122 3821 1040 5696 CV 0.177 0.163 0.045 0.241 Lower CL 15929 16752 21003 14227 Upper CL 31832 31477 25106 36773 OneSided95LowBound 16872 17743 21350 15674

Fig.1. Vertical distribution of mackerel icefish. January 2000. Daytime 10.00-12.00.. Catch 26.1 tonn

53.0° S

53.5° S W1 54 57 W8 49 55 E1 53 50 56 54.0° S 51 58 69 70 .

67 68 о 59 рг 66 ия 54.5° S S10 61 65 60

55.0° S 64 E7

S1 62 63 55.5° S

40° W 39° W 38° W 37° W 36° W 35° W 34° W Fig.2. Design of acoustic survey in the South Georgia area. Arrows are shown the trawl station position. W1-W8 - transects of West stratum; E1-E7 - transects of East stratum; S1-S10 - transects of South stratum.

25 a 20

10 frequency,% 5

0 -70 -67 -64 -61 -58 -55 -52 -49 -46 -43

TS, dB

35 b 30

frequency,% 10

0 13 15 17 19 21 23 25 27 29

length, cm

Fig. 3. Frequency distributions of in situ target strength measurements at 38 kHz were related to length of icefish sampled with pelagic trawl

Fig 4. Example of young fish echogram obtained during trawl station. The masked -free-noise 38 kHz echogram based on “dB difference method”is presented in the upper figure

Fig.5. Example echogram of icefish aggregation within 50 m depth range (without 8m bottom layer).

-52

% 15 N-W % 20 Haul.57 N-E

10 15 10 5 5 -53 0 0 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 L,cm L,cm

-54

% 25 S-W 20 15 % -55 40 S-E 10 5 20 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 L,cm L,cm -56 -40-39-38-37-36-35-34

Fig.6. Length composition of icefish from pelagic trawl catches obtained during acoustic survey.

45000 40000 35000 30000 25000 20000 15000 number of fish 10000 5000 0 4 7 10 13 16 19 22 25 28 31 34 37 40 length,cm

Fig.7. Mackerel icefish abundance for different length group in the pelagic zone obtained from results of acoustic survey

-52

N-W % 40

-52.5 30 % 40 N-E 20 30 10 -53 20 0 0 5 10 15 20 25 30 35 40 45 50 55 60 10 L,cm -53.5 0 0 5 10 15 20 25 30 35 40 45 50 55 L,cm -54

% 40 S-W 30 -54.5 20 10 0 % 40 S-E -55 0 5 10 15 20 25 30 35 40 45 50 55 60 30 L,cm 20 10 -55.5 0 0 5 10 15 20 25 30 35 40 45 50 55 60 L,cm -56 -40-39.5-39-38.5-38-37.5-37-36.5-36-35.5-35-34.5-34

Fig. 8. Icefish length composition from bottom trawl catches during trawl-acoustic survey.

160000 140000 120000 100000 80000 60000

Number of fish 40000 20000 0 8 121620242832364044485256 length,cm

Fig.9. Bottom component abundance of icefish for different length classes from results of acoustic survey.

Fig. 10. Simulation results of the biomass estimates from acoustic survey as the function of uncertainty in Sa values.(Parameter of TS equation was determined by MacLennon and Minz’s method).

Fig. 11. Simulation results of the biomass estimates from acoustic survey as the function of uncertainty in parameter b20 of the TS equation.

Fig. 12. Simulation results of the biomass estimates from acoustic survey as the function of uncertainty in length composition.

Fig. 13. Simulation results of the biomass estimates from acoustic survey as the function of uncertainty in Sa, TS, length composition.

2 300.00 250.00 200.00 150.00 100.00 50.00 0.00 mean transect density,g/m^ W1 W4 W7 e2 e5 S1 S4 S8 S11 Number of transect

16 14 12 10 8 6 4 2 0 mean transect density,g/m^2 W1 W3 W5 W7 e1 e3 e5 e7 S2 S4 S7 S9 S11 number of transect 2 10 8 6 4 2 0 mena transect density,g/m^ W1 W4 W7 e2 e5 S1 S4 S8 S11 transect number

Fig.14.Comparison of mean transect density of krill and mackerel icefish. Data obtained from results of acoustic survey. a- mean krill density in the near bottom 58 –m layer b- mean fish density in the near bottom 8-m layer c- mean fish density in the near bottom 50 –m layer (above bottom 8-m layer).

70000

60000 acoustic density in 8m-bottom layer

50000

40000 acoustic density in 6m-bottom layer

30000

density, kg/km^2 20000 net density in 6m bottom layer

10000

0 6 2119293745 trawl station number

Fig.15. Comparison acoustic and trawl density estimates.

Fig. 16. Influence of vertical distribution of icefish near-bottom aggregations on their availability to fishing gear with different vertical opening. Green lines indicate 6m and 8m vertical opining of bottom trawls.

50000

45000

40000

35000

30000

25000

20000 density, kg/km^2 density, 15000

10000

2226.3 5000 159.1 0 Tr.36 3 6 9 12 15 18 21 24 Tr.37trawl 37 1.75mile segment number of transect between trawl station

Fig.17. Acoustic estimates of icefish density in the near-bottom 8m layer between two bottom trawl stations. Densities obtained at the station were 2226.3 kg/km2 and 159.1 kg/km2 . The distance of 1.75 mile is the distance covered by bottom trawl during the standard station.