Behavior of herring Clupea harengus stimulated by two
ocean-going research vessels; respectively designed with and without regard
to radiated-noise-standards. Both vessels generate a reaction pattern, but,
contrary to expectations, the reaction initiated by the silent vessel is stronger
and more prolonged than the one initiated by the conventional vessel. The
recommendations from the scientific community on noise-reduced designs
were motivated by the expectation of minimizing bias on survey results
caused by vessel-induced fish behavior. In conclusion, the candidate stimuli
for vessel avoidance remain obscure. Noise reduction might be necessary but
is insufficient to obtain stealth vessel assets during surveys

Egil Ona

Geir Pedersen

Mitson R. B.

Nils Olav Handegard

Olav Rune Godø

Olsen K.

Ona E.

Ruben Patel

Vidar Hjellvik

Journal home page: http://scitation.aip.org/jasa/Behavior of herring Clupea harengus stimulated by two
ocean-going research vessels; respectively designed with and without regard
to radiated-noise-standards. Both vessels generate a reaction pattern, but,
contrary to expectations, the reaction initiated by the silent vessel is stronger
and more prolonged than the one initiated by the conventional vessel. The
recommendations from the scientific community on noise-reduced designs
were motivated by the expectation of minimizing bias on survey results
caused by vessel-induced fish behavior. In conclusion, the candidate stimuli
for vessel avoidance remain obscure. Noise reduction might be necessary but
is insufficient to obtain stealth vessel assets during surveys

Ona, Egil

Godø, Olav Rune

Handegard, Nils Olav

Hjellvik, Vidar

Patel, Ruben

Pedersen, Geir

Brage IMR

Ona et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007J. Acoust. SocSilent research vessels are not quietEgil Ona, Olav Rune Godø, Nils Olav Handegard, Vidar Hjellvik,Ruben Patel, and Geir PedersenInstitute of Marine Research, P.O. Box 1870 Nordnes, 5817 Bergen, Norwayegil.ona@imr.no, olavrune@imr.no, nilsolav@imr.no, vidar.hjellvik@imr.no,ruben.patel@imr.no, geir.pedersen@imr.noAbstract: Behavior of herring Clupea harengus is stimulated by twoocean-going research vessels; respectively designed with and without regardto radiated-noise-standards. Both vessels generate a reaction pattern, but,contrary to expectations, the reaction initiated by the silent vessel is strongerand more prolonged than the one initiated by the conventional vessel. Therecommendations from the scientific community on noise-reduced designswere motivated by the expectation of minimizing bias on survey resultscaused by vessel-induced fish behavior. In conclusion, the candidate stimulifor vessel avoidance remain obscure. Noise reduction might be necessary butis insufficient to obtain stealth vessel assets during surveys.© 2007 Acoustical Society of AmericaPACS numbers: 43.80.Nd, 43.80.Ev, 43.30.Sf, 43.40.Rj CMDate Received: December 12, 2006 DateAccepted: January 11, 20071. IntroductionVessel-induced fish behavior during acoustic density estimation1,2 and trawl sampling3,4 maybias survey estimates of stock abundance. As noise has been considered a major stimulus, fish-eries research institutions worldwide are investing in new silent research vessels5 in accordancewith recommendations from the International Council for the Exploration of the Sea ICES.6The lack of fish avoidance observed from a stealth vessel7 has been considered a result of re-ducing vessel noise,7,8 but no direct comparison with a traditional research vessel has demon-strated this. Nevertheless, the scientific community has tacitly accepted that the major avoid-ance stimulus originates from the sound characteristics of the vessels, which is also the basis forthe ICES recommendations.1,6,9In 2003 the new Norwegian diesel-electric propulsioned research vessel “G. O. Sars”GS Gross Registered Tonnage GRT 4067 tons, Length Overall LOA 77,5 m, fulfillingthe ICES demands for a silent vessel, was put into operation. Several earlier reports have docu-mented vessel avoidance of Norwegian spring spawning herring Clupea harengus, showingthat this stock is underestimated acoustically when distributed in the upper 100 m of the watercolumn.1,2 Currently the acoustic observations are used to establish an abundance index. As-suming similar or randomly varying conditions for observation among years, the time series ofindices gives a relative change in abundance from one year to the next that is utilized in thestock evaluation. A vessel comparison was therefore an absolute necessity before the new GScould be used in data collection for the official annual stock assessments. Vessel comparisonexperiments were carried out in December 2004 in the Ofotfjord in northern Norway betweenGS and the previous standard vessel “Johan Hjort” JH GRT 1828 tons, LOA 64,4 m. JH isa traditional research vessel with sound emission above the ICES standard6 and thus far noisierthan GS.2. MethodsThe two vessels followed each other at standard cruising speed and maximum distance alongthe exact same triangular cruise track Fig. 1a. Both vessels collected acoustic data accordingto the standard protocol for acoustic surveys, and, along the pursue track, an upward-lookingechosounder and an acoustic Doppler current profiler ADCP were placed. This allowed us torecord herring density by depth as well as the mean swimming velocity of the fish layer during. Am. 121 4, April 2007 © 2007 Acoustical Society of America EL145Ona et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007EL146 J. Acouand between passages of the two vessels. The mooring was passed four times by JH and threetimes by GS at moderately dense recordings of herring at 40–80-m depth during the night. Thetwo vessels were completely darkened during the experiments. First, we present the method andprotocol for collecting and analyzing the data from the vessel-mounted echosounders. Then, themethod to collect and analyze the data from the bottom-mounted platforms is presented, includ-ing noise level confirmation for the vessels.Raw echosounder data were recorded using the Simrad EK500 Kongsberg Gruppen,Kongsberg, Norway, 18 kHz on JH, and the Simrad EK60, 18 kHz on GS. The calibrated rawdata were directly transferred to the format of the postprocessing system BEI, and scrutinizedby standard procedures for herring surveys by the same two operators. In fjord surveys withhigh densities, this basically involves the removal of bottom detection errors and isolating theherring layers by integration boundaries. The processed herring density data were stored in adatabase with 0.1 nmi10-m depth bins in absolute, linear units for the “nautical area scatter-ing coefficient,” sA m2/nmi2, a standard unit in fisheries acoustics.10The sA values from the two vessels were compared using a standard method,11 slightlymodified since the method is designed for two vessels following parallel transects, whereas inour experiment each part of the survey transect was traversed 3–4 times by each vessel.Also, weFig. 1. Results from the pursuit experiment and mooring passages. a The numerical-density vessel-transectsintegrated over intervals of 1 nmi, shown as numbered light and dark sequences of points. Each point indicates thestart of a 0.1 nmi sampling distance. Gray points indicate bottom depth lightest: about 60 m, darkest: about 520 m.The cross at sampling distance no. 11 indicates the position of the moored platforms. Arrows indicate cruisingdirection. b, upper panel. Numerical density along the transects. Each point represents the average of the log-transformed numerical densities for one vessel at one interval. The error bars show ±2 standard errors of theaverages. b lower panel: The corresponding vessel differences in numerical density, and the number of passages foreach vessel. Dashed vertical lines indicate the corners of the triangle in a. c Vertical swimming velocity compo-nent when GS red curves or JH blue curves passed the moored ADCP. Thin lines denote single passages, andthick lines denote the averages over all passages. Vertical dotted lines are drawn 2 min before and after the point ofpassage. d Vertical fish distribution median depth when GS red curves or JH blue curves passed the mooredechosounder. Thin lines denote single passages and thick lines denote the averages over all passages. Vertical dashedlines are drawn 2 min before and after the point of passage. e Average sA when GS red curves or JH blue curvespassed the moored echosounder. Thin lines denote single passages and thick lines denote the averages over allpassages. Vertical dotted lines are drawn 2 min before and after the point of passage.have chosen to replace lnx in the published method with 10 logx. The modified method is asst. Soc. Am. 121 4, April 2007 Ona et al.: Silent research vessels are not quietOna et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007J. Acoust. Socfollows: Let i be the true fish density at elementary sampling distance unit i, and xij=ji10ij the density measured at sampling distance i by vessel j, j= 1,2, averaged over allpassages there is no visible trend in the measured densities over the time period used in theanalysis. Here, j is a vessel-dependent bias, 2 is the variance, and ij is random noise. De-fining di=10 logxi1−10 logxi2, we have di=+i, where =10 log1 /2 and i=10i1−i2. Testing the null hypothesis that H0 :1=2 is equivalent with testing H0 :=0,and if the di are independent, this can be done using a two-sided t-test. The ratio 1 /2 isestimated by 10d¯/10, where d¯=idi.The horizontal resolution of the data is 0.1 nmi, but using this resolution in the analy-sis would yield autocorrelated di, and the t-test could not be used.We have therefore aggregatedthe data so that each sampling distance is 1.0 nmi Fig. 1a. This has another advantage aswell: the sampling distances for the two vessels do not match exactly, but the mismatch issmaller relative to the sampling length when this is increased or resolution is decreased.At a selected position in both surveys Fig. 1a, the vessels passed directly above abottom-moored platform, carrying a calibrated, upward-looking EK60, 38 kHz echosounder,and a calibrated underwater hydrophone Naxys A/S, Bergen, Norway with a computer fordigital sound recordings. The hydrophone was used to verify the noise levels from the twovessels Fig. 2. An upward-looking bottom-moored 75-kHz acoustic Doppler current profilerTeledyne RD Instruments, San Diego 50 m perpendicular to the track line was used to mea-sure the swimming speed of the herring layer. The raw backscattering data from all of the fourADCP beams were used to create a mask to isolate the herring layers from the surroundingwater, and to calculate the resulting vertical and horizontal velocity components.Avoidance reactions were analyzed using data from seven vessel passages over themoored platforms, four by JH and three by GS, including data from 10 min before each passageto 10 min after. The acoustic backscatter from the moored echosounder was integrated over thefish layer and is presented in sA units. Herring depth distribution and sA were available with aresolution of about 3 pings per second, and ADCP data with one recording every fifth second.To remove some of the random noise, the depth and sA data were smoothed using a movingaverage with a window of 10 s. The ADCP data were smoothed using a window of 15 s for thevertical component and 25 s for the horizontal component. Vessel differences and effects ofFig. 2. Noise signatures of JH blue and GS red, respectively, measured in the keel aspect of both ships using theNaxys hydrophone. The maximum recommended levels of noise from a free-running survey vessel at any speed upto and including 11 knots according to ICES CRR 2096 is also included in the figure black curve. Due to a possibleinaccuracy in the hydrophone positioning, the absolute levels may be 3 dB off. This does not affect the relativedifference between the vessels.vessel passages were tested for using two-sided t-tests with n=3 JH, 4 GS, or 3+4 both. Am. 121 4, April 2007 Ona et al.: Silent research vessels are not quiet EL147Ona et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007EL148 J. Acouvessels. For example, when testing for a vessel difference in vertical displacement, the differ-ence DJH,i= q¯50,i,before− q¯50,i,after between the median depths averaged over the time periods5–2 min before and 2–5 min after passage i , i= 1, . . . ,4, was calculated for JH, and similarly,DGS,j , j= 1, . . . ,3was calculated for GS.A difference betweenDJH andDGS was then tested forusing a standard t-test with n=3+4, not assuming equal variance.3. Results and discussionFirst, we show that the herring numerical densities measured from the two vessels were similar.The average numerical density recorded by GS during the pursuit experiment was 97% of theaverage recorded by JH, which is a nonsignificant difference p=0.75 under the null hypothesisof no vessel differences.Next, we show that GS initiated a more intense and prolonged avoidance reaction thanJH by analyzing the results from the moored instrumentation. The vertical mean swimmingvelocity as estimated by the ADCP during the period of passage from 2 min before to 2 minafter was significantly higher for GS −70.5 cm/s than for JH −42.7 cm/s t-test, pFig. 3. The echogram for a single passage over the moored echosounder when passed by GS a and JH b,respectively. The black line is the median depth distribution for this passage, and the blue and red lines are the meansof the median depth distributions for all passages for JH and GS, respectively.=0.009, n=3+4, Fig. 1c. For GS the mean vertical velocity of the fish layer corresponds tost. Soc. Am. 121 4, April 2007 Ona et al.: Silent research vessels are not quietOna et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007J. Acoust. Soc2–3 bl s−1 body lengths per second, while the maximum recorded corresponds to about4 bl s−1. Since the horizontal velocity component is not taken into account, and as variation isexpected for individuals over the depth range of the layer, this is a strong reaction compared tothe maximum individual swimming speed for herring of 6–7 bl s−1.12 The horizontal swim-ming speed in the same period was similar for the two vessels.From the moored echosounder Fig. 3, the diving is observed as a vertical displace-ment of the herring layer Fig. 1d. From about 2 min before passage and to 2 min after pas-sage a median displacement of about 20 and 40 m is seen for JH and GS, respectively Fig. 1dand Fig. 3. The difference between the vessels in terms of change in median depth distributionfrom 5–2 min before passage to 2–5 min after passage is significant on a 1% level t-test, n=3+4, p=0.003. The fish also needed more time to return to its original distribution after beingdisturbed by GS Fig. 1d.The numerical density when passing the mooring was on average similar to that beforevessel passage Fig. 1e. The variability in numerical density was large during the experiment,and it is difficult to separate vessel-induced effects from natural variations on a detailed levelbased on only seven passages. Nevertheless, there was a clear decrease in numerical densityafter the vessels had passed. Comparing the intervals 2–5 min before passage and 2–5 minafter passage, the decrease is significant on a 5% level both for GS t-test, p=0.022, n=3 andJH t-test, p=0.045, n=4.4. ConclusionAlthough the moored echosounder did not record any significant difference in numerical den-sity before and during passage for any vessel, the fish reaction pattern is strong and clearlyvessel dependent. The differences in behavioral response and the magnitude of the responsedemonstrate the potential to cause severe bias, particularly seen in the perspective of earlierexperience.1,2 This illustrates the complexity of the vessel avoidance behavior, but more impor-tant, the results show that a stimulus other than noise, as defined by ICES, must be responsiblefor the reaction. Silent vessels have many advantages for reliable acoustic surveys, e.g., improv-ing signal-to-noise ratio. Reducing the vessel noise may be necessary but is not a sufficientmeasure, and as long as influential candidate stimuli for fish avoidance remain obscure, theICES goal of a establishing a stealth vessel design appears unrealistic.AcknowledgmentsThis work was partially supported by grants from the Norwegian Research Council. We aregrateful to the engineers Ronald Pedersen, Ingvald Svellingen,AtleTotland, andTerjeTorkelsenwho established and operated the instrumentation. Captains and crews on G. O. Sars and JohanHjort are thanked for their patience and cooperation during the experiments.References and links1K. Olsen, J. Angell, F. Pettersen, and A. Løvik, “Observed fish reactions to a surveying vessel with specialreference to herring, cod, capelin and polar cod,” FAO Fish. Rep. 300, 131–138 1983.2R. Vabø, K. Olsen, and I. Huse, “The effect of vessel avoidance of wintering Norwegian spring spawningherring,” Fish. Res. 58, 59–77 2002.3E. Ona and O. R. Godø, “Fish reaction to trawling noise: The significance for trawl sampling,” Rapp. P.-V. Reun.-Cons. Int. Explor. Mer 189, 159–166 1990.4N. O. Handegard and D. Tjøstheim, “When fish meets a trawling vessel: Examining the behaviour of gadoidsusing a free floating buoy and acoustic split-beam tracking,” Can. J. Fish. Aquat. Sci. 62, 2409–24222005.5J. Moore, “Stealth ship sets sail for a quiet life fishing for data,” Nature London 423, 7 2003.6R. B. Mitson, “Research vessel standards: Underwater noise of research vessels, Review andRecommendations,” ICES Co-op. Res. Rep. 209, 1–61 1995.7P. G. Fernandes, A. S. Brierley, E. J. Simmonds, N. W. Millard, S. D. McPhail, F. Armstrong, P. Stevenson, andM. Squires, “Fish do not avoid survey vessels,” Nature London 404, 35–36 2000.8P. G. Fernandes, A. S. Brierley, E. J. Simmonds, N. W. Millard, S. D. McPhail, F. Armstrong, P. Stevenson, andM. Squires, “Fish do not avoid survey vessels addendum,” Nature London 407, 152 2000.9R. B. Mitson and H. P. Knudsen, “Causes and effects of underwater noise on fish abundance estimation,”Aquat. Liv. Res. 16, 255–263 2003.10D. N. Mac Lennan, P. G. Fernandes, and J. Dalen, “A consistent approach to definitions and symbols in. Am. 121 4, April 2007 Ona et al.: Silent research vessels are not quiet EL149Ona et al.: JASA Express Letters DOI: 10.1121/1.2710741 Published Online 15 March 2007EL150 J. Acoufisheries acoustics,” ICES J. Mar. Sci. 59, 365–369 2002.11R. Kieser, T. J. Mulligan, N. J. Williamson, and M. O. Nelson, “Intercalibration of two echo integrationsystems based on acoustic backscattering measurements,” Can. J. Fish. Aquat. Sci. 44, 562–572 1987.12J. H. S. Blaxter and W. Dickson, “Observations of the swimming speeds of fish,” J. Cons., Cons. Int. Explor.Mer 24, 472–479 1959.st. Soc. Am. 121 4, April 2007 Ona et al.: Silent research vessels are not quiet

Silent research vessels are not quiet

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