Acoustic Estimate Of Antarctic Krill In The South Orkney Islands

Antarctic krill（Euphausia superba）is a key species of the Antarctic marine ecosystem and plays an important role in the process of energy flow in the ecosystem.The conservation and sustainable use of krill resources has received increasing attention from the international community.China became a full member of CCAMLR in 2007 and began to send fishing vessels to conduct exploratory fishing for krill in Antarctic in 2009²010 fishing season.In 2016,China caught 6.50×10⁴ t of krill and become one of the main Antarctic krill fishing countries.The assessment of Antarctic krill resources was conducted early by Chinese scientists,which mainly used the data collectec by net sampling.The estimate results were not accurate enough for resources management.Using the echo integral technique to estimate the biomass of the Antarctic krill would be a development direction of the krill resources study in China.There were few acoustic studies of krill in China,and all of them are scientific surveys of the acoustic data collected by the ship“Xue Long”,and the estimate cost was relatively high.In 2012,the Subgroup on Acoustic Survey and Analysis Methods of CCAMLR turned its research focus to the analysis and utilization of acoustic data which collected by the krill fishing vessel.The scientific ecosounder in the fishing vessel was calibrated and the collected acoustic data could also be used for krill biomass estimate.China has not yet conducted a study on krill biomass evaluation that based on acoustic data from fishing vessels.This study was estimate the krill biomass through a series of data processing methods to estimate the krill biomass that based on the acoustic ecogram data collected by the China’s krill fishing vessels“Long Teng”.The estimate results can not only fill the gaps in the assessment of Antarctic krill biomass which conducted by the China Fisheries Working Group,but also provide some suggestions for the promotion and application of the new biomass estimate method that based on krill swarms echo.The main results are as follows:（a）The background noise and SNR estimate results were:The minimum background noise of the 38 kHz acoustic ecogram was-107.25 dB,the background noise threshold was set to-105 dB;the minimum background noise of the 120 kHz acoustic ecogram was-138.57 dB,and the threshold was set to-135 dB.The SNR were both 12.After the non-biological signal removed,the NASC of transect 6 was reduced from 6893.52 m²·nmi^-2to 2352.33 m²·nmi^-2.（b）The average length of krill in the Orkney Islands was 33.01±4.06 mm,with a maximum length of 49.21 mm and a minimum length of 25.50 mm.The 99%frequency distribution of the krill length was 26.35-43.85 mm.（c）According to the CCAMLR specification,the length ofδS_{v（120-38）}was 20-50 mm.At 20 mm,δS_{v（120-38）}=14.26 dB.At 50 mm,δS_{v（120-38）}=5.54 dB,then theδS_{v（120-38）}range of the krill identification was 5.54-14.26 dB.The larger the frequency difference range,the more the echo retained after the echo determination.When the frequency difference range was 2¹6 d B,the NASC value of transect 6 was 5702.48m²·nmi^-2,and the frequency difference range was 5.54¹4.26 dB,the NASC value was 4754.77 m²·nmi^-2.（d）School parameter sensitivity analysis has determined the most suitable school detection parameters of fishing vessel acoustic ecogram in China.The minimum total length was 15 m,the minimum total height was 2 m,the minimum candidate length was 10 m,and the minimum candidate height was 1 m,the maximum horizontal linking distance was 15 m,the maximum vertical linking distance was 3 m,and the data threshold was-70 dB.Compared with other school detection parameters,this study detected the largest number of krill swarms and the largest swarm NASC value.（e）A total of 2539 krill swarms were detected in the subarea of South Orkney Islands,of which 1389 were daytime swarms and 1150 were night swarms.The size of the krill swarms（swarm length,thickness,and area）in the daytime was smaller than that of the night swarms.Compared with the night swarms,the mean swarm depth was deeper,the mean swarm density was smaller,and the mean distance between the swarms was closer during the daytime.There were significant differences in all the characteristic values of diurnal krill swarms.The krill swarms in this subarea could be divided into three clusters.Cluster A contains the largest number of swarms but the least abundance of krill,while cluster C has the lowest number of swarms but the largest krill abundance.The cluster A are mainly distributed in deep waters in the northern and northwestern of the islands,with a depth of>1000 m.The cluster B and cluster C are distributed throughout the entire surveyed area.A total of 1,410 krill swarms are detected in the Antarctic Cooperation Sea subarea,of which 577 are daytime swarms and 833 are swarms at night.Compared with the nighttime,the depth of krill swarms are shallow during the day,and the density is larger.Swarms in this area can be divided into three clusters with similar swarm numbers.The krill abundance of cluster C is the smallest;and cluster A contains the largest number of swarms and the largest krill abundance.The swarms of cluster A and cluster B are distributed throughout the surveyed area.The cluster C swarms are mainly distributed in the western part of the survey area（west of 125°E）.A total of 4490 krill swarms were detected in the Antarctic Mawson Sea subarea,of which 1947 are daytime swarms and 2,543 at night.The krill swarms in this subarea can also be divided into three clusters.Cluster A has the largest number of swarms and the largest krill abundance,and cluster B has the lowest number of swarms with the smallest krill abundance.There are significant differences in all the characteristic values of the krill swarms in the three subareas.（f）The 9 transects acoustic ecogram of the surveyed area can be divided into1490 integration units,of which 804 integration units in daytime,123 units has NASC values,and the largest NASC values is 667.32 m² nmi^-2,the minimum NASC values is 38.21 m² nmi^-2;There were 686 integration units at night,80 has NASC values,the maximum NASC value is 591.49 m²·nmi^-2,and the minimum NASC value was 22.71m²·nmi^-2;the krill was patch distributed,with 87%of the integral units has no krill biomass.There is no significant difference in diurnal NASC and Sv values of the 9transects（P=0.12>0.05）,which the krill diurnal vertical movement has no effect on the krill density estimate.The average density of Antarctic krill assessed by the swarm-based method is 17.30 g·m^-2,the CV value is 82.7%,the total biomass was4.31×10⁵ t.The average density of Antarctic krill assessed by the grid-based method is 19.22 g·m^-2,the CV value was 97.4%,and the total biomass was 4.79×10⁵t.The krill biomass assessed by the swarm-based method is approximately 90.01%of the grid-based method,and the CV value is slightly smaller than the grid-based method.The time of estimate the krill biomass using swarm-based method is 48.35 hours,which is approximately 56%of the grid-based method（83.36 hours）.Compared with the grid-based method,the swarm-based method improves the processing efficiency of the fishing vessel acoustic ecogram data.It is becoming a new method of biomass estimate based on fishing vessel ecogram which recommended by CCAMLR.