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Echograms are used to visualize fisheries acoustic data, but choice of colour map has a significant effect on appearance. Quantitative echograms should use colour maps, which are colourful (have a perceived variety and intensity of colours), sequential (have monotonic lightness), and perceptually uniform (have consistency of perceived colour contrast over their range). We measure whether colour maps are colourful (⁠ M ^ (3) > 0 ⁠), sequential (⁠ r s = ± 1 ⁠), and perceptually uniform (ρ  = 1) using an approximately perceptually uniform colour space (CIELAB). Whilst all the fisheries acoustic colour maps tested are colourful, none is sequential or perceptually uniform. The widely used EK500 colour map is extremely colourful (⁠ M ^ (3) = 186 ⁠), not sequential (⁠ r s = 0.06 ⁠), and has highly uneven perceptual contrast over its range (⁠ ρ = 0.26 ⁠). Of the fisheries acoustic colour maps tested, the Large Scale Survey System default colour map is least colourful (⁠ M ^ (3) = 79 ⁠), but comes closest to being sequential (⁠ r s = − 0.94 ⁠), and perceptually uniform (⁠ ρ = 0.95 ⁠). Modern colour maps have been specifically designed for colour contrast consistency, accessibility for viewers with red-green colour-blindness, and legibility when printed in monochrome, and may be better suited to the presentation and interpretation of quantitative fisheries acoustic echograms. [ABSTRACT FROM AUTHOR]

Seismic surveys use airguns that emit low frequency high magnitude sound to detect subsea resources and to map seabed geology. The effect of seismic blasts on Calanus spp. a key food source for commercially important fish, was assessed in field experiments. Immediate mortality of copepods was significantly different from controls at distances of 5 m or less from the airguns. Mortality 1 week after the airgun blast was significantly higher—by 9% relative to controls—in the copepods placed 10 m from the airgun blast but was not significantly different from the controls at a distance of 20 m from the airgun blast. The increase in mortality—relative to controls—did not exceed 30% at any distance from the airgun blast. Only two genes changed in response to the airgun blast; however, their function is unknown. There were no sublethal effects of the seismic blasts on the escape performance or the sensory threshold needed to initiate an escape response at any of the distances from the airgun blast that were tested. Results from these experiments suggest that seismic blasts have limited effects on the mortality or escape response of Calanus sp. within 10 m of the blast and no measurable impact at greater distances. [ABSTRACT FROM AUTHOR]

Estimating the biomass of gas-bladdered organisms in the mesopelagic ocean is a simple first step to understanding ecosystem structure. An existing two-frequency (38 and 120 kHz) acoustic and optical probe was lowered to 950 m to estimate the number and size of gas-bladders. In situ target strengths from 38 and 120 kHz and their difference were compared with those of a gas-bladder resonance-scattering model. Predicted mean equivalent spherical radius gas-bladder size varied with depth, ranging from 2.1 mm (shallow) to 0.6 mm (deep). Density of night-time organisms varied throughout the water column and were highest (0.019 m−3) in the 200–300 m depth range. Predictions of 38 kHz volume-backscattering strength (Sv) from the density of gas-bladdered organisms could explain 88% of the vessel's 38 kHz Sv at this location (S 40.9, E 166.7). Catch retained by trawls highlighted the presence of gas-bladdered fish of a similar size range but different densities while optical measurements highlighted the depth distribution and biomass of gas-inclusion siphonophores. Organism behaviour and gear selectivity limits the validation of acoustic estimates. Simultaneous optical verification of multifrequency or broadband acoustic targets at depth are required to verify the species, their size and biomass. [ABSTRACT FROM AUTHOR]

There are currently no dedicated recruitment survey data available in support of the assessment of the abundance and distribution of Northeast Atlantic (NEA) mackerel (Scomber scombrus), one of the most widespread and commercially important fish stocks in the North Atlantic. This is despite the fact that an estimate of recruitment is an important requirement for the provision of advice to fishery managers. The work here addresses this by compiling catch rates of juvenile mackerel from bottom-trawl surveys conducted between October and March during 1998-2012 and applying a log Gaussian Cox (LGC) process geostatistical model incorporating spatio-temporal correlations. A statistically significant correlation between the modelled catch rates in adjacent quarters 4 and 1 (Q4 and Q1 ) demonstrates that bottom-trawl surveys in winter are an appropriate platform for sampling juvenile mackerel, and that the LCG model is successful in extracting a population abundance signal from the data. In this regard, the model performed appreciably better than a more commonly used raising algorithm based on survey swept-area estimates. Therefore, the LCG model was expanded to include data from the entire survey time-series, and a recruitment index was developed for use in the annual ICES stock assessment. We hypothesize that catchability is positively density-dependant and provides supporting evidence from acoustic observations. Various density-dependant transformations of the modelled catch rates were furthermore found to improve the correlation between the derived annual recruitment index and recruitment estimated by back-calculation of adult mackerel data. Square root transformation led to the strongest correlation, so this is recommended for further analysis of mackerel abundance. Finally, we provide maps of spatial distributions, showing that the most important nursery areas are around Ireland, north and west of Scotland, in the northern North Sea north of 59°N and, to some extent, also in the Bay of Biscay. [ABSTRACT FROM AUTHOR]

Odontocete bycatch on and depredation from tropical pelagic longlines is globally widespread, having negative impacts on the economic viability of affected fisheries and on the conservation of affected odontocete populations. Reports by fishers that depredating odontocetes avoid gear tangles has underpinned the development of simulated structures to physically deter depredating odontocetes. This study assessed the efficacy of two such devices developed to mitigate odontocete depredation and associated bycatch. Of particular interest was their impact on (i) soak depth and (ii) sink rate using truncated trials, before determining their impact under full operational conditions on rates of (iii) catch of the five most economically important fish, and (iv) odontocete depredation and bycatch, on changes in (v) fish survival and size, and (vi) setting and hauling speed. The results indicated that the inclusion of devices on longlines had negligible impact on soak depth, thus were unlikely to impact on the suite of fish specifically targeted and caught. The sink rate was slowed, perhaps by drag, trapped air, or propeller wash, although the addition of weight might remedy this if the devices were to be used in areas where seabird bycatch could occur. Most importantly, trials conducted in Australian and in Fijian waters indicated that pooled fish catch rates (i.e. albacore, yellowfin, bigeye, mahi mahi, and wahoo) increased in the presence of the devices, possibly because more fish were attracted by them or because more depredators were deterred. Catch rates on control gear next to gear with devices attached were higher than more distant control gear, suggesting the influences of the devices may have extended to adjacent branchlines. The size of caught fish was mostly unaffected, although the survival of yellowfin and bigeye increased significantly in the presence of the devices. Hauling was slowed by the use of the devices and the need for an extra crewmember during setting and hauling, which could be cost prohibitive in some fisheries, especially if economic benefits from their use are not obvious. Despite the small sample size, odontocete bycatch only occurred on unprotected fishing gear and all individuals were released alive, although their fate was uncertain; there was evidence of injuries sustained from the event. The outcomes are positive and should motivate stakeholders to view such devices as a potentially effective tool for mitigating odontocete bycatch and depredation in this and similar longline fisheries. Future efforts should focus on improving operational integration and reducing implementation costs to encourage voluntary uptake and thus avoid non-compliance and the need for costly monitoring. The use of this technology could bring about marked improvements to the conservation situation for affected odontocete populations and to the economic situation for affected longline fisheries. [ABSTRACT FROM AUTHOR]

In the pelagic foodweb, micronekton at the mid-trophic level (MTL) are one of the lesser known components of the ocean ecosystem despite being a major driver of the spatial dynamics of their predators, of which many are exploited species (e.g. tunas). The Spatial Ecosystem and Population Dynamics Model is one modelling approach that includes a representation of the spatial dynamics of several epi- and mesopelagic MTL functional groups. The dynamics of these groups are driven by physical (temperature and currents) and biogeochemical (primary production, euphotic depth) variables. A key issue to address is the parameterization of the energy transfer from the primary production to these functional groups. We present a method using in situ acoustic data to estimate the parameters with a maximum likelihood estimation approach. A series of twin experiments conducted to test the behaviour of the model suggested that in the ideal case, that is, with an environmental forcing perfectly simulated and biomass estimates directly correlated with the acoustic signal, a minimum of 200 observations over several time steps at the resolution of the model is needed to estimate the parameter values with a minimum error. A transect of acoustic backscatter at 38 kHz collected during scientific cruises north of Hawaii allowed a first illustration of the approach with actual data. A discussion followed regarding the various sources of uncertainties associated with the use of acoustic data in micronekton biomass. [ABSTRACT FROM AUTHOR]

Atlantic mackerel do not have a swimbladder, which is the dominating scatterer in most fish. Schools of Atlantic mackerel can be identified by the level of backscatter at 38, 70, 120 and 200 kHz, but these are too few points to describe the continuous (broadband) frequency spectrum of the school or to identify which components of the fish are responsible for the different characteristics of the spectrum. In this study, backscatter from different mackerel body parts was investigated in a laboratory tank in the frequency range 68–450 kHz, and tilt angles ±30° (defined as the angle between the sound axis and the normal to the fish axis). Measurements were averaged over tilt angles to simulate the results from a fish school. Our results show that scatter from the backbone is significant around 0° tilt angle, but less important when the fish is tilted. The head, including gills, appears to be an important scatterer when the fish is tilted. Segments of the mackerel body, including both flesh and backbone, give strong backscatter with wide directivity, and show an increase in the backscatter with frequency. [ABSTRACT FROM PUBLISHER]

Demer, D. A., Zwolinski, J. P., Cutter, G. R. Jr, Byers, K. A., Macewicz, B. J., and Hill, K. T. Sampling selectivity in acoustic-trawl surveys of Pacific sardine (Sardinops sagax) biomass and length distribution. – ICES Journal of Marine Science, 70: .To annually assess the northern stock of Pacific sardine (Sardinops sagax) in the California Current and set harvest quotas for the US fishery, managers have used an age-structured stock synthesis model fitted with results from acoustic-trawl (ATM), daily-egg-production, and aerial-photogrammetric survey methods, fishery landing and individual-length data, and many assumed or empirically derived parameters. In these assessments, sardine landed at ports spanning from Ensenada, México to Vancouver Island, Canada were assumed to be solely from the northern stock. It was also assumed that the ATM estimates of sardine biomass were negligibly biased for the sizes of fish sampled by the survey trawls (i.e., catchability q = 1 for sardine standard length (SL) values greater than ∼17 cm). Due to these catchability and length-selectivity assumptions, the ATM- and assessment-estimated abundances are mostly similar for larger sardine. However, the assessment estimates include large abundances of small sardine (SL values less than ∼15 cm) that are not represented in either the ATM-survey results or the fishery landings, and generally did not recruit to the migrating northern stock sampled by the ATM surveys. We considered four explanations for this disparity: (i) the ATM length-selectivity assumption is correct; (ii) the non-recruiting small fish may comprise a smaller portion of the stock than indicated by the assessments; (iii) during years of low recruitment success, those size classes may be virtually completely fished by the Ensenada and San Pedro fisheries; or (iv) they may belong to the southern sardine stock. This investigation emphasizes the previously identified importance of differentiating samples from the northern and southern stocks and surveying their entire domains. [ABSTRACT FROM PUBLISHER]

Macaulay, G. J., Kloser, R. J., and Ryan, T. E. 2013. In situ target strength estimates of visually verified orange roughy. – ICES Journal of Marine Science, 70:215–222.The first estimates of orange roughy (Hoplostethus atlanticus) target strength at 38 and 120 kHz with visual verification were obtained from a self-contained echosounder and video camera system affixed to a demersal trawl towed through dense aggregations of spawning orange roughy. Mean target strength estimates were obtained from 24 tracks of orange roughy containing 83 echoes. The mean target strength at 38 kHz was –52.0 dB with a 95% confidence interval of –53.3 to –50.9 dB for fish with a mean length of 33.9 cm. At 120 kHz the mean target strength was –47.9 dB (confidence interval of –48.8 to –46.4 dB). This work makes two significant advances: in situ TS measurements have been made that can be confidently attributed to orange roughy, and using a trawl to herd orange roughy past the system resolved the previously intractable problem of fish avoidance. [ABSTRACT FROM PUBLISHER]

Kang, M., Nakamura, T., and Hamano, A. 2011. A methodology for acoustic and geospatial analysis of diverse artificial-reef datasets. – ICES Journal of Marine Science, 68: 2210–2221.A methodology is introduced for understanding fish-school characteristics around artificial reefs and for obtaining the quantitative relationship between geospatial datasets related to artificial-reef environments using a new geographic information system application. To describe the characteristics of fish schools (energetic, positional, morphological characteristics and dB difference range), acoustic data from two artificial reefs located off the coast of Shimonoseki, Yamaguchi prefecture, Japan, were used. To demonstrate the methodology of the geospatial analysis, diverse datasets on artificial reefs, such as fish-school characteristics, marine-environmental information from a conductivity, temperature, and depth sensor, information on artificial reefs, seabed geographic information, and sediment information around the reefs, were utilized. The habitat preference of fish schools was demonstrated quantitatively. The acoustic density of fish schools is described with respect to the closest distance from reefs and the preferred reef depths, the relationship between fish schools and environmental information was visualized in three dimensions, and the current condition of the reefs and their connection to seabed type is represented. This geospatial method of analysis can provide a better way of comprehensively understanding the circumstances around artificial-reef environments. [ABSTRACT FROM PUBLISHER]

Kloser, R. J., Ryan, T. E., Macaulay, G. J., and Lewis, M. E. 2011. In situ measurements of target strength with optical and model verification: a case study for blue grenadier, Macruronus novaezelandiae. – ICES Journal of Marine Science, 68: 1986–1995.In situ measurements of target strength (TS) of isolated fish surrounding dense schools need to be representative of the schooling fish to calculate their echo-integrated biomass. Using synchronous optical and acoustic measurements from a pelagic fishing net, the standard length (81 cm, n = 128), tilt-angle (−9°), and net-disturbed in situ TS (−34.4 dB) of Australian Macruronus novaezelandiae (blue grenadier) were confirmed at depth. In situ drift experiments of assumed undisturbed but dispersed blue grenadier recorded a mean TS of −31.8 dB (CI −33.1 to −30.9 dB) with attributed fish standard lengths of 83 cm (s.d. 7.5 cm) and weight 2.5 kg. Modelling the gasbladder showed that uncertainties in fish length, orientation, and gasbladder size could explain the differences observed. Blue grenadiers have negative buoyancy because the cavity size of their gasbladder is smaller than the volume of gas required for neutral buoyancy at depth. For the same species and length, New Zealand hoki weigh less and have smaller gasbladders than Australian blue grenadier, suggesting a conversion factor of 1.10 in length for comparative measurements. Net-attached acoustic and optical measurements indicate that model and drift in situ measurements are biased high by 2.9 and 1.0 dB, respectively. Net-attached acoustic and optical measurements are a cost-effective method of monitoring TS routinely at depth for changes in species length and weight. [ABSTRACT FROM PUBLISHER]

Cox, M. J., Watkins, J. L., Reid, K., and Brierley, A. S. 2011. Spatial and temporal variability in the structure of aggregations of Antarctic krill (Euphausia superba) around South Georgia, 1997–1999. – ICES Journal of Marine Science, 68: .Antarctic krill are important in the South Georgia (54°S 35°W) marine ecosystem. They form aggregations that vary widely in packing density (<1 to 1000 s of individuals m−3), length (tens to thousands of metres), and height (tens of metres). Acoustic surveys are often used to estimate krill biomass and provide data that give insight into aggregation structure. Using dual-frequency (38 and 120 kHz) acoustic data collected during six surveys conducted around South Georgia during the 1997, 1998, and 1999 austral summers, we isolated 2990 aggregations by applying the Shoal Analysis and Patch Estimation System algorithm in Echoview and a krill-length-dependent acoustic identifier (ΔSv120–38). Multivariate cluster (partition) analysis was applied to metrics from each of the aggregations, resulting in three aggregation types with an overall proportional split of 0.28:0.28:0.44. Types 1 and 3 had low mean densities (<2 g m−3), whereas Type 2 had a mean density of 94 g m−3. Intersurvey differences were found between the effort-corrected numbers of aggregation types (p = 2.5e−6), and between on- and off-continental shelf areas (p = 1.5e−7), with a greater number of Type 2 aggregations being found on-shelf. The findings suggest intersurvey variation in krill catchability, with krill being more likely to be caught on-shelf. [ABSTRACT FROM PUBLISHER]

Demer, D. A., Cutter, G. R., Renfree, J. S., and Butler, J. L. 2009. A statistical-spectral method for echo classification. – ICES Journal of Marine Science, 66: 1081–1090.The frequency dependence of sound-scatter intensity is commonly exploited to classify fish, zooplankton, and the seabed observed in acoustic surveys. Although less utilized, techniques based on the statistics of echo amplitudes can also be used to extract information. For example, single-frequency echo statistics have been used to determine whether backscatter originates from single or multiple fish or from rough or smooth seabeds, and to estimate scatterer sizes and densities. The efficacies of the amplitude-based techniques are challenged, however, by the usual requirement to group echo measurements to facilitate meaningful comparisons with model predictions. Groupings of data over space, time, or both, can combine scatter from multiple taxa or species, confounding the comparisons. These methods are improved with a hybrid, statistical-spectral method for target identification (SSID), which incorporates information contained in both the signal amplitudes and phases. The SSID uses multifrequency echo statistics from individual time-space intensities (pixels) to identify general scattering types, before applying model-based identification schemes for target identifications. The effectiveness of the SSID is demonstrated for fine-scale separation of scatter from demersal fish and the seabed and estimating seabed depth, within-beam slope, hardness and roughness, and the height of the dynamic acoustic dead zone. [ABSTRACT FROM PUBLISHER]

Anderson, J. T., Holliday, D. V., Kloser, R., Reid, D. G., and Simard, Y. 2008. Acoustic seabed classification: current practice and future directions. – ICES Journal of Marine Science, 65: 1004–1011.Acoustic remote sensing of the seabed using single-beam echosounders, multibeam echosounders, and sidescan sonars combined and individually are providing technological solutions to marine-habitat mapping initiatives. We believe the science of acoustic seabed classification (ASC) is at its nascence. A comprehensive review of ASC science was undertaken by an international group of scientists under the auspices of ICES. The review was prompted by the growing need to classify and map marine ecosystems across a range of spatial scales in support of ecosystem-based science for ocean management. A review of the theory of sound-scattering from seabeds emphasizes the variety of theoretical models currently in use and the ongoing evolution of our understanding. Acoustic-signal conditioning and data quality assurance before classification using objective, repeatable procedures are important technical considerations where standardization of methods is only just beginning. The issue of temporal and spatial scales is reviewed, with emphasis on matching observational scales to those of the natural world. It is emphasized throughout that the seabed is not static but changes over multiple time-scales as a consequence of natural physical and biological processes. A summary of existing commercial ASC systems provides an introduction to existing capabilities. Verification (ground-truthing) methods are reviewed, emphasizing the difficulties of matching observational scales with acoustic-backscatter data. Survey designs for ASC explore methods that extend beyond traditional oceanographic and fisheries survey techniques. Finally, future directions for acoustic seabed classification science were identified in the key areas requiring immediate attention by the international scientific community. [ABSTRACT FROM PUBLISHER]

The potential for fish to avoid survey vessels is a major source of uncertainty in stock-assessment surveys. Although walleye pollock (Theragra chalcogramma) are the subject of a substantial commercial fishery in the North Pacific, their behavioural responses to approaching survey vessels remain poorly understood. As a first step in an effort to determine if walleye pollock avoid survey vessels engaged in trawling operations, we made pairwise comparisons of acoustic backscatter recorded by survey vessels while free-running and while trawling. Results are presented of acoustic backscatter recorded from NOAA''s RV “Miller Freeman”, which used a midwater trawl during the 1996–2002 eastern Bering Sea surveys, and a chartered commercial fishing vessel, which used a bottom trawl during a survey in 2003 in the Gulf of Alaska. In both cases, average backscatter from a vessel-mounted echosounder was significantly higher when free-running than when trawling. These decreases in backscatter are consistent with increased vessel avoidance while trawling. There were no differences in the vertical distribution of backscatter when free-running and trawling, indicating that pollock do not exhibit elevated diving responses when approached by a trawling vessel rather than by a free-running vessel. Although the study indicates that pollock respond to trawling vessels, the nature of the behavioural response cannot be determined with the methods used in this study. Future work should evaluate potential stimuli produced by trawlers to which pollock may react, and should document changes in behaviour that may occur in their presence. [Copyright &y& Elsevier]

This paper describes the results of a short-term experiment using measurements of sv from the diel, spatial, clupeoid distribution in the southern Baltic. The aim of the experiment was to verify fish-behaviour characteristics measured over the period from 1995 to 2001. It was also intended to estimate the dynamics of fish behaviour over one separate diel cycle. The studies were based on a 24h continuous integration of fish echoes using an EY500 echosounder at 38kHz. Measurements were carried out by RV “Baltica” travelling along the sides of a square of 4 nautical miles at a constant speed of 8 knots. The South Gotland Deep was chosen for the experiment because of the greatest amplitude there of fish vertical, diel migrations within the Polish EEZ. The duration of the experiment was limited by weather conditions, but nearly 300 EDSU samples were collected. 3-D distributions of echoes were correlated to the values of coincident environmental factors of time, depth, water temperature, salinity, and oxygen level. Fish distribution compared with environmental factors is described by different macrosounding visualizations, statistical, and mathematical models. Measurements are compared with the average characteristics of fish behaviour based on data from the autumn acoustic studies between 1995 and 2001 in a wider environment of the Gotland Deep. The results strongly confirmed the instability of the diel acoustic response of fish echoes in both situations. More precise measurements during the experiment indicated the biggest column-scattering strength (Svc) increase during the sunrise period, appearing simultaneously in the whole area. A similar increase was detected by analysis of 1995–2001 data from the South Gotland Deep environment. Major emphasis is given to the explanation of the diel irregularities. Diel instability of fish acoustic response can significantly effect the results of target-strength measurements (up to 400% during the sunrise) and, as a consequence, the calibration of acoustic fish stock-assessment models. [Copyright &y& Elsevier]

A series of experiments comparing acoustic and bottom-trawl surveys was conducted on Atlantic redfish (Sebastes spp.) on the edge of the Green and Grand Banks of Newfoundland, Canada. Redfish were on or near bottom by day and migrated vertically in the water column at night. In an attempt to account for biases attributable to the presence of fish in the near-bottom dead zone (DZ), a correction factor was applied based on density values measured within the first few metres above the detected bottom. Acoustic densities within increasing range increments above the bottom were compared with densities estimated from the trawl catch. Swept area was calculated using both the trawl's wing spread and door spread as proxies for the minimum and maximum fishing widths. Uncorrected acoustic densities were significantly higher during the night than during the day. No significant day/night differences for the entire water column were observed after DZ corrections. Close agreement between acoustic and trawl densities was obtained by integrating within the first 10 to 20 m off the bottom, with or without the DZ corrections, for both day and night experiments, but regression slopes differed. Trawl catchability appeared to be density-dependent at night, being higher at lower fish densities. Daytime acoustic estimates were more variable than those made at night, as indicated by consecutive passes of several transects and CVs of density (means of 131% during day, 35% at night). We conclude that acoustic measurements made at night provide the most reliable and least variable density estimates, and make recommendations for surveys. [ABSTRACT FROM PUBLISHER]

Acoustic target-strength (TS) measurements are presented for tethered and free-swimming individual Chrysaora hysoscella (Scyphozoa) and Aequorea aequorea (Hydrozoa) medusae in Namibian waters. Tethered individual C. hysoscella (17–54 cm total umbrella diameter) and A. aequorea (19–28 cm total umbrella diameter) were ensonified at 38 kHz using a portable echosounder. Mean TS values for individual medusae at this frequency ranged from -67.3 to -52.8 dB for C. hysoscella and from -65.4 to -50.1 dB for A. aequorea. There was a positive relationship between medusa diameter and TS for both species. TS of individual medusae varied cyclically over time by about 15 dB, probably because of the periodic contraction of the medusae whilst swimming. C. hysoscella was parasitized by hyperid amphipods (maximum infestation >1800 parasites per medusa). A fluid-cylinder scattering model was used to determine the expected backscatter from the parasites, and it suggested that even at the highest observed level of infestation the jellyfish itself remained the major contributor to total backscatter at 38 kHz. Single-target echoes from targets identified by trawling as medusae were obtained from vessel-mounted echosounders at 18, 38, 120, and 200 kHz. Triangulation between echosounder beams to identify targets detected simultaneously at all four frequencies increased confidence that echoes were in fact from single targets. The 38-kHz TS values from free-swimming medusae corresponded with values obtained from tethered animals at the same frequency, providing strong evidence that the TS estimates were robust. TS values at all four frequencies (Chrysaora hysoscella mean umbrella diameter 41 cm, TS at 18 kHz=-60.0 dB, 38 kHz=-65.5 dB, 120 kHz=-68.0 dB, and 200 kHz=-70.5 dB. Aequorea aequorea mean inner-umbrella diameter 6.5 cm, TS at 18 kHz=-66.0 dB, 38 kHz=-66.5 dB, 120 kHz=-71.5 dB, and 200 kHz=-73 dB) were consistent with previously published data. Given these robust TS estimates, the possibility may now exist for multi-frequency identification and evaluation of these jellyfish species in some circumstances, and for the use of acoustic-survey techniques to estimate jellyfish abundance. [Copyright &y& Elsevier]

Calibrated, digitized data from multi-frequency echo sounders have been used to generate new, synthetic echograms. The relative frequency response measured at four acoustic frequencies (18, 38, 120, and 200 kHz) is the main acoustic feature used to characterize the acoustic targets. Synthetic echograms are used to enhance and colour-code sample volumes with similar acoustic properties. The method is invaluable during scrutinizing, particularly in areas with many types of target. Several broad acoustic categories can be clearly distinguished, and detailed examples based on different fish species and zooplankton are demonstrated. The limitations of the method are described and discussed. [Copyright &y& Elsevier]

Interpretation of the behavioural dynamics and distribution of herring requires explicit consideration of spatial and temporal scales since no single mechanism can explain the pattern on all scales. High resolution sonar and echosounders were used as our observation tools. The cluster ratio, mean Nearest Neighbour Distance (mean ISD): mean of the average Inter-school Distance (mean ISD) provides a descriptive index linking scales of distribution pattern within and between clusters of schools. It can be used to compare school clustering patterns for surveys made at different places and seasons. Two mesoscale patterns of school clustering were observed, as indicated by the distribution of nearest neighbour distances; one at 0.05–0.3 km revealed by sonar and another at 0.8–2.5 km revealed by echosounder. In general, schools were tightly aggregated, whilst school clusters were patchily distributed. Heightened feeding motivation explains the predominance of relatively small schools and their tendency to be found closer together and more clustered than large schools. Differences in distribution and density of food patches and predators may account to some degree for the observations (1) schools were distributed according to their size; both small and large schools had nearest neighbours of similar size more often than expected, (2) splitting and joining of schools were the most frequently recorded behavioural events. A considerable amount of variation in distribution and dynamics can be accounted for by changes in behaviour associated with diurnal vertical migrations. Some schools apparently did not perform diurnal vertical migrations. We suggest that when profitable, some herring may “dip in to the fridge” (feed in deep, cold water) during the day rather than pursue food on potentially high energy migrations. [ABSTRACT FROM PUBLISHER]

Traditional scientific echosounders operate at discrete frequencies ranging from 38 to 420 kHz. We investigated the applicability and accuracy of length-based population estimates using commercially available acoustic frequencies and the inverse approach under ideal conditions. The inverse approach combines modelled and measured backscatter values to estimate the abundance of organisms in specified length classes. Reference backscatter values of individual fish were calculated using a Kirchhoff-ray mode backscatter model. Single and multi-cohort fish populations were simulated based on length-frequency samples from purse seine catches of threadfin shad (Dorosoma petenense) and used to calculate the total frequency-dependent volume backscatter of each population. A non-negative least squares (NNLS) algorithm was used to estimate total abundance and numbers of fish in each length class. Total abundance estimates were within <1–38% of population numbers. Within length-class estimates were inconsistent among frequency combinations and across length-class criteria. Increasing the number of frequencies does not guarantee improved accuracy of within length-class abundance estimates. Predictability of inverse simulations is non-linear when rather high frequencies are combined with non-monotonic scattering models. Accuracy of length-based abundance estimates is optimized by maximizing the amplitude range of reference backscatter measures and the number of features identified by reference scattering points. [ABSTRACT FROM PUBLISHER]

Biases in the measurements of spatial distribution of fish schools and their consequences for school biomass estimates during conventional acoustic surveys are mainly due to vertical and lateral avoidance of the vessel. In this paper, we quantify school avoidance during an acoustic survey carried out from 13 to 29 May 1994 in the Catalan Sea. From a lateral multi-beam sonar the geometric characteristics (depth, length, width, height, surface, and volume) of 1268 schools were obtained. The 60 beams (1.5°×15°) of the sonar scanned a vertical plane from 0° to 90°, perpendicular to the vessel path within a range of 100 m. Within this plane, the projected area ensonified by the echo-sounder used aboard for acoustic evaluation was evaluated to simulate a comparison between the sonar and the echo-sounder. The results have enabled us to improve our knowledge on the vertical and lateral avoidance patterns of schools in relation to their size, external structure, and their position in the water column, and to quantify the vessel influence on biomass estimated by echo-sounder. [ABSTRACT FROM PUBLISHER]

Geographic Information System techniques were used in the analysis of the distribution of three pelagic resources: anchovy, sardine, and jack mackerel. The data were derived from hydroacoustic estimations obtained during four seasonal cruises in 1993–1994 in the region off northern Chile. The spatial distribution of these resources was analysed relative to the surface temperature and salinity distributions, the latter two variables being measured concurrently during these cruises. Results indicated the occurrence of a stratified distribution of the three species together with a high variability in the distribution pattern between cruises. The distribution of the three species was associated with the occurrence and intensity of thermal and haline fronts. [ABSTRACT FROM PUBLISHER]

Two acoustic surveys were carried out in the Bay of Biscay, in 1993 and 1994, to study the distribution and abundance of pelagic stocks. Acoustic data were systematically stored using the INES/MOVIES acquisition and processing system along isobath perpendicular transects, from 25 m to 200 m bottom depth. Species identifications were obtained from midwater trawl catches. From the acoustic data, sections of cruise track in the vicinity of the trawl catches were selected for analysis. Three species, anchovy (Engraulis encrasicolusL.), sprat (Sprattus sprattusL.), and horse mackerel (Trachurussp.) were investigated. The objectives of this study were to analyse the echo traces for estimation of species-specific preferences and to examine the influence of species assemblages on the vertical position and shape of the schools. Using MOVIES-B software, 4789 fish schools were detected and isolated. Characteristics investigated were size (height, vertical cross-sectional area), elongation (length/height), back-scattered energy (density), and vertical distribution (bottom depth, school altitude). Comparative analysis of the vertical distribution of echo traces and catch compositions revealed some species-specific preferences. Horse mackerel were found close to the bottom and anchovy at a mean of 16 m above the bottom. No variation in vertical distribution was detected where sprats were mixed with anchovies. In the presence of horse mackerel in the bottom layer, anchovy appeared to be displaced upwards. A modification of the shape of schools was also detected when sprats were mixed with anchovies. In anchovy–horse mackerel mixtures, a suggested modification of behaviour could explain the observed decrease in the density of anchovy schools. [ABSTRACT FROM PUBLISHER]