ACOUSTICAL BIOMASS ESTIMATION RESULTS IN MEDITERRANEAN AQUACULTURE SEA CAGES

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1 ACOUSTICAL BIOMASS ESTIMATION RESULTS IN MEDITERRANEAN AQUACULTURE SEA CAGES Ester Soliveres a, Vicent Puig a, Patricia Ordónez a, Isabel Pérez-Arjona a, Miguel Ardid a, Jaime Ramis b, Pedro Poveda b, Vicent D. Estruch a, Mª Dolores López c, Mª del Mar Agraso d, Rocio Robles d, Víctor Espinosa a a Universitat Politècnica de València, C/ Paranimf, 1, Grau de Gandia (Spain) b Universitat d'alacant, Sant Vicent del Raspeig (Spain) c CULMAREX, S.A.U. Don Carnal 13. Pol. El Labradorcico Águilas (Murcia). d CTAQUA, Muelle Comercial s/n, El Puerto de Santa María, Cádiz, Spain Contact author: Víctor Espinosa, Universitat Politècnica de València, C/ Paranimf, 1, Grau de Gandia (Spain), fax: , vespinos@fis.upv.es Abstract: Biomass estimation, both weight distribution and fish density, are clue factors for fish farming management. We present the experimental results of the acoustical estimates for both variables in Mediterranean intensive aquaculture floating cages. Sea bass and gilthead sea bream cages have been studied using scientific split beam echosounders with transducers placed at the sea surface. Dorsal measurements of the acoustical target strenght (TS) of isolated fish tracks in the upper limit of the dense caged schools correlate with strong fish mean size differences. Mean acoustical volume backscattering strength monitoring reveals pen manipulations and fish stock changes, with good agreement in fish density estimations for the studied conditions. Keywords: aquaculture, biomass estimation, echosounder, target strenght 1423

2 1. MOTIVATION In the last decades aquaculture has become a fundamental activity to cover the human needs of food, providing today the 50% of the products with aquatic origin, and having an increasing demand, supposed to reach the 65% in 2030 [1]. Fish farming constitutes almost the half of the aquaculture effort, and several of the most important commercial fish species, like Atlantic salmon (Salmo salar), gilthead sea bream (Sparus aurata) or sea bass (Dicentrarchus labrax) are intensively cultivated in floating sea cages, where juvenile stocks are introduced to be fed until they reach the desired commercial sizes. A critical aspect of the production process, from both the economical and the environmental impact point of view, is the adequate dosage of pelleted food, which is calculated as a function of the fishes biomass, in order to achieve optimal growing rates. Fish size monitoring tools are then crucial to improve the farms management, and different sampling methods have been developed, being the stereoscopic optical image recording the most successful noninvasive one, since it provides fish size measurements (length and height) which are the input for biometric relationships to obtain fish weight [2]. Nevertheless the estimation of total biomass in the cage remains as an open problem and the use of acoustics have been proposed to cover the limitations of optical techniques. Acoustical target strength (TS) measurements from single fish tracks for size monitoring and volume backscattering strength (Sv) scaling for fish density and total biomass estimations are the common proposed approaches in fisheries acoustics stock assessment [3]. TS measurements issues related to its application in aquaculture for growing monitoring were deeply investigated for Atlantic Salmon in [4], and their results were mostly replicated for gilthead sea bream in close range experiments [5], having as main conclusion that TS correlates with fish size only for the ventral aspect measurements, showing dorsal TS values of smallest fish size classes tri- and bi-modal distributions related to fish directivity and remaining mean TS values almost constant with size for measurements taken from the surface. Total biomass estimation following the principle of linear superposition of the backscattered energy was proposed following the ventral measurements scheme (from below the floating cages) and a research project financed by the Norwegian Research Council developed with partially negative results: Sv measurements followed the increasing biomass in a Salmon cage during a production cycle (from 100 g until 2 kg fish growing) until it collapsed when the fishes were around 500g, corresponding with the highest values of biomass density in the cage. Different beam extinction techniques were applied to compensate acoustical shadowing but it resulted in a higher measured biomass than expected, possible due to multiple/forward scattering and school behaviour effects [6]. To our knowledge no other initiatives have been reported in the literature on this subject. However, during the state-of-art vigilance phase (beginning of 2012) of the Spanish technological development project for total biomass estimation in off-shore fish cages (ARM/1790/010), impulsed by the Asociación Empresarial de Productores de Cultivos Marinos de España (APROMAR), led by the Andalusian Aquaculture Technology Centre, CTAQUA, and participated by the UPV as scientific advisor, a communication of the company Biosonics Inc. stated that the American company had developed a system for biomass monitoring in aquaculture, mainly tested in Salmon farming, with very good results in growing monitoring and promising expectations in total biomass estimation, both from an echosounding scheme from the cage surface. Taking into account the previous results and the company proposals 1424

3 it was decided to re-visit the dorsal scheme and the evaluation of the approach with production conditions in Mediterranean fish farms. 2. MEASUREMENTS IN MEDITERRANEAN SEA CAGES PRODUCTION CONDITIONS Gilthead sea bream and sea bass cages with different sizes and densities were measured in September of 2012 in the Grupo Culmarexs plant of Águilas (Murcia, Spain) by Biosonics Inc. engineers in order to evaluate the application of their technology to the estimation of fish growing and total biomass with these Mediterranean species of great commercial interest. The technical assessment was performed by commitment of CTAQUA with the participation of their technicians at the sea campaign. The technical report elaborated by Biosonics Inc. stated that TS measurements were consistent with the size of measured fishes and that Sv scaling showed a correlation with the biomass variations due to the fish farm commercial catches and stock shifts operations. The report concluded that new experiments were needed to obtain proper TS vs. size relationships for the Mediterranean species and to monitor Sv long-time series to identify the periods with minimum variation coefficient in the mean Sv, when the school was supposed to occupy the cage uniformly, and estimated fish density in the insonified volume could be extrapolated to all the cage. In the following, an analysis of the acoustical data obtained in such campaign, but performed by our group in order to validate the mentioned report is detailed, together with the evaluation of an additional measuring campaign performed by us in September 2013 in a commercial cage close to slaughter with decreasing values of gilthead sea bream total biomass. Fig.1: Left: Continuous monitoring tests in floating cages: a Biosonics DT-X echosounder with two floating split beam transducers (123 and 201kHz). Right: 201 khz typical echogram detail of the upper part of dense schools in (433 g) sea bream production cages; isolated traces above the school were used for TS analysis Target strength analysis for size estimation We analysed the acoustical data obtained for three size classes of sea bass (of weight means 60, 480, and 1390 g) and one of gilthead sea bream (433 g) with the help of Sonar- 5 Pro software ( The used echosounder configurations are given in Table 1. n order to avoid shadowing effects we took into account isolated tracks above the dense school, with an average measuring distance between 3 and 5 m; some exemples of fish traces can be seen in the amplitude echogram of Fig. 1. Unimodal TS distributions were found for all sizes, species 1425

4 and frequencies (see Fig 2). The only TS vs. size adjustment possible (only three size classes of sea bass were available) offered excellent correlation coefficients as shown in Fig.3. Note the slope deviation from Love expression [3]. Frequency (khz) Ping interval (ms) Beam aperture (degrees) Source level (re 1 1m) 220,5 222,8 Pulse duration (ms) Table 1: Echosounder configurations used in Águilas campaign at Culmarex group facilities by Biosonics Inc. engineers. I Fig 2: Uni-modal dorsal TS distributions obtained with 123 khz (left) and 201 khz (right) split beam analysis for increasing mean weights ( g) in sea bass cages Sv measurements for fish density and total biomass estimation The time evolution of Sv was studied in order to evaluate the possibility of obtaining fish density estimations consistent with farm data. Unfortunately, most of the biomass data for higher densities (corresponding to growing cages) were just estimations of the producers. These estimations can have deviations up to 25 or 30%. 1426

5 Fig 3: TS vs. log(l) adjustments for both 120 khz (left) and 200kHz (right) frequencies. The only reliable biomass data proceed from cages close to slaughter, known as commercial cages, with reduced densities obtained from partial catches from growing cages. In any case, the observation of the temporal behaviour of mean Sv reveals a strong variability increased when the cage volume was modified by during partial catches. In absence of manual operations, the variation coefficient of the Sv along the water column took minimum values during the night and first hours of the day, and the mean value of this variable has consistent variation with the biomass changes after catches or stock shifts between cages. Even more, Figure 4 shows the good agreement of acoustical density estimation during these time intervals of reduced coefficient variations for both operation frequencies with measurements taken in different days. Fig 4: Estimated biomass density variations and constant-assumed density (horizontal line) from slaughter data for 484 g sea bass in a commercial cage. In order to validate these positive preliminary results for total biomass estimation we realised a second experiment in a commercial cage of gilt-head sea bream in September of 2013 in Piagua (Almeria). The measured mean size was in this case an only class around 541 g. The fishes were initially transferred to the commercial cage from a growing cage up to a biomass density of 6,2 kg/m³. Two consecutive catches in a week time reduced the density to 4,9 and 3,5 kg/m³. The mean Sv was remotely monitored with an equivalent configuration but using a Simrad EK60 200kHz scientific echosounder. Figure 5 plots the time evolution of the variable for recordings corresponding to different reference densities. 3. CONCLUSIONS Dorsal TS measurements in sea bass cages offer a good correlation with strong class size differences in production conditions, where size dispersion in the cage can play a 1427

6 significant role. Additional measurements are necessary to establish weight estimation accuracy. The continuous monitoring of mean Sv allows to follow properly density variations at least for the measured densities, up to the one third of the maximum possible in production cycles in the intensive Mediterranean aquaculture installations. Long time series during several months ina a production cycle could help to establish the proper temporal window criteria for total biomass estimation and the density limits of its validity. Fig 5: Estimated biomass density variations and constant-assumed density (horizontal line) from catches sequence for 541 mean weight sea breams in a commercial cage. 4. ACKNOWLEDGEMENTS We acknowledge the fundamental collaboration of Grupo Culmarex, and the personnel of their aquaculture plants of Águilas (Murcia, Spain) and Piagua (Almeria, Spain). Special thanks to Biosonics Inc. for providing the acoustical data token during their tests in Águilas as well as to the assistance of CTAQUA for the sea campaigns. This work was realised in the frame of the project ARM/1790/010. REFERENCES Asociación Empresarial de Productores de Cultivos Marinos de España (APROMAR) and Asociación Española de Productores de Acuicultura Continental (ESCUA), La Acuicultura en España 2013, Annual Report, B. Zion, The use of computer vision technologies in aquaculture A review, Computers and Electronics in Agriculture, Vol. 88, pp , J. Simmonds and D.N. MacLennan, Fisheries Acoustics-Theory and Practice, Blackwell Publishing 2nd Ed., 2005 F.R. Knudsen, J. E. Fosseidengen, F. Oppedal, O. Karlsen, E. Ona, Hydroacoustic monitoring of fish in sea cages: target strength (TS) measurements on Atlantic salmon (Salmo salar). Fish. Res. Vol, 69, pp E. Soliveres, J.L. Moreno, A. Cebrecos Ruiz, V. Espinosa, Dorsal and ventral measurements on gilt-head sea-bream (Sparus aurata), In X European Conference on Underwater Acoustics (ECUA), Estambul, 2010 K. H. Haugholt, T. Kavli, V. Knappskog, O. Løvhaugen, A. Pedersen, G. Pedersen, EXACTUS Technical Report. T1.1 Technology Survey,