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Fisheries acoustics surveys are effective tools in marine resource assessment and marine ecology. Significant advances have occurred in recent years with the application of multiple and broadband frequencies to enable remote species identification. There is, however, still the need to obtain additional evidence for identification, and the estimation of the size and tilt angle distribution of fish, which influences their acoustic target strength. The former two requirements are usually met by obtaining simultaneous net samples: there are limited, if any, recognized successful techniques for the latter. Here, two alternative tools for obtaining evidence for all three requirements are examined: angling gear and small video cameras. These tools were deployed during surveys of Atlantic mackerel (Scomber scombrus). In 2014, angling was actually more efficient than pelagic trawling (the standard technique) and over two survey periods (2012 and 2014) provided length frequency distributions that were not significantly different. A small video camera was deployed into mackerel schools, providing species identification and fish orientation. Image analysis was then applied, producing tilt-angle distributions of free swimming wild mackerel for the first time. Mean tilt angles from three deployments were very variable with 95% of observations falling between −70° and 39° with evidence of a multinomial frequency distribution. A video equipped lander was also deployed onto the type of rocky seabed where deployment of a trawl would be impossible: this confirmed the presence of Norway pout and suggested it was the dominant scatterer on this type of seabed. These techniques are complementary to traditional trawling methods, but provide additional insights into fish behaviour whilst satisfying standard requirements of identification and supplying biological samples. Crucially, the small cameras deployed approximate the size of the animals under observation and allow for measurement of behaviour (specifically tilt) that are more likely to represent those conditions encountered during surveying. [ABSTRACT FROM AUTHOR]

Kubilius, R., and Ona, E. 2012. Target strength and tilt-angle distribution of lesser sandeel (Ammodytes marinus) – ICES Journal of Marine Science, 69: 1099–1107.North Sea stocks of lesser sandeel have recently become depleted, and improved methods for abundance estimation are sought. This paper focuses on the acoustic target strength (TS) and orientation of sandeel, measured simultaneously in several field experiments. A specially designed cubic cage, fitted with an echosounder and a video camera, was lowered onto the sea bottom, trapping wild sandeel inside. Methods for manually selecting valid echotraces from individual sandeel are described. Scattered mean TS values from several experiments are reported. These are, in spite of the observed variability, summarized in a TS–fish length (cm) relationship as TS = 20logL – 93.1 (dB) at 200 kHz. We believe that the accuracy of the relationship may still be debated; incorporating larger uncertainty in the overall mean TS will increase the total uncertainty of the stock biomass estimate from sandeel acoustic surveys. This uncertainty is now, using standard narrow-beam echosounders, dominated by the fish patchiness relative to the survey coverage. Results from pilot investigations of the sandeel swimming orientation using video cameras are also presented, showing that sandeel usually has an anguilliform swimming pattern with substantial positive (head-up) tilt. The spread of the tilt-angle distribution is also larger than for more neutrally buoyant fishes. [ABSTRACT FROM PUBLISHER]

Alignment of projection images in tomographic reconstruction is a critical process that governs the quality of the reconstructed three-dimensional (3D) image. The most popular alignment method is the marker-based alignment, which typically uses colloidal gold particles added to the specimen (called fiducial markers) to calculate the coordinates of each projection image in the tilt series. This method, however, is not effective when each image contains only a small number of fiducial markers. Therefore, of all the parameters required for alignment, we focussed on the tilt angle and attempted to gage it directly in order to examine whether the acquired angle is accurate enough to perform tomographic reconstruction. We showed that the tilt angle measured using a commercially available capacitive liquid-based inclinometer is more precise than the reading from the monitor of the electron microscope and that it can lead to 3D reconstructions of quality similar to those obtained by the marker-based alignment method. [ABSTRACT FROM AUTHOR]

To measure the influence of changes in tilt angle on the acoustic target strength (TS) of the Japanese common squid (Todarodes pacificus), we conducted a series of experiments to estimate TS in relation to tilt angle and swimming angle. Swimming angle was measured in a seawater tank using two infrared, underwater cameras under dark conditions. Ex situ measurements of TS in relation to tilt angle on live specimens using a fishhook and cage method were then conducted at 38 and 120kHz; mantle length (ML) ranged from 21 to 27cm (mean 24.75cm). For the more precise TS measurement with tilt angle, another set of ex situ TS measurements relative to tilt angle was made at 38 and 120kHz on tethered, anesthetized specimens in seawater. The mean swimming angle was −17.7° (±12.7° s.d.). The mean TS varied from −48.6 to −44.6dB and was relatively higher at 120kHz than at 38kHz, in the order of 0.7 and 2.5dB. The empirical relationship between TS (dB) and ML (cm) is given by TS=20log10(ML)−75.4 (r=0.81) at 38kHz or TS=20log10(ML)−73.5 (r=0.64) at 120kHz. Based on the tethered method for the anesthetized squid, the mean standardized TS values (b20) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b20 were expressed as: b20 =−73.3+0.48×θ+0.0122×θ2 +0.00016×θ3 for 38kHz and b20 =−72.6+0.53×θ+0.0134×θ2 +0.00014×θ3 for 120kHz, where θ is the negative tilt angle in degrees. The mean TS based on the measurements using live squid was higher than that of tethered measurements, i.e., 2.6dB at 38kHz and 4.0dB at 120kHz. The higher mean TS in the ex situ measurements for the live squid can be explained by the influence of the low tilt angle on the overall TS data. The results can be used to understand the influence of tilt angle on the TS of Todarodes pacificus and thus improve the accuracy of biomass estimates. [Copyright &y& Elsevier]

Depth data from archival tagging studies of cod (Gadus morhua) were used in three different analyses with the aim of testing basic assumptions of cod behaviour. Examination of post-release depth profiles from cod tagged in the Barents, North, and Irish Seas revealed that some cod underwent a post-release period of adaptation to increasing depth as they readjusted their buoyancy to its pre-tagging level. This depth adaptation behaviour was characterized by gradually increasing mean depth, and enabled the calculation of neutral-buoyancy compliant descent rates, which were less than 1mh-1. Estimated rates of vertical movement were shown to be highly dependent upon the frequency at which depth was sampled. Maximum estimated rates of ascent and descent from sampling intervals of 10 or 15min were inconsistent with the maintenance of neutral buoyancy, but estimates from sampling intervals greater than 1h were not. Calculation of tilt angles using depth data sampled at 10-s intervals showed that cod were often tilted more than 5° relative to the horizontal, and that this effect was more pronounced at night. These findings suggest that basic assumptions regarding cod physiology and behaviour require revision if the accuracy and precision of acoustic methods are to be improved. [Copyright &y& Elsevier]

Pre- and post-spawning capelin from the Barents Sea stock were observed in a net pen with a calibrated 38 kHz scientific split-beam echosounder. The transducer was positioned vertically or tilted. The acoustic target strength (TS) of capelin depended on swimbladder length. In female capelin within 15.5–18.0 cm length the logarithm of weight was significantly negatively related to TS. The negative effect of weight on TS could be due to a higher condition factor (assumed higher fat content), which gave the fish additional buoyancy and less need for swimbladder volume. The effect of gonad weight on TS was not significant. Comparisons of measurements made with vertical and tilted transducers demonstrated a small but significant effect of tilt angle on TS. [Copyright &y& Elsevier]

Vertical orientation (tilt angle) is known to affect the target strength (TS) of ensonified fish and is a large component of the variability inherent in acoustic-biomass estimates. To measure the effects of changes in tilt angle on TS during diel vertical migrations, a concentration of migrating Atlantic cod (Gadus morhua) was observed acoustically from a research vessel over several days. Single-target data were collected from a split-beam echosounder and were subsequently tracked, corrected for vessel orientation and movement, and analysed for 3-dimensional displacement (speed and direction). The results revealed a large variability in TS and several patterns of swimming behaviour from random to directed orientation and movement, with changes in both vertical and horizontal displacements and inferred orientation. These behavioural patterns and their affects on TS were analysed as a function of “time-since-sunset”. Regular diel orientation patterns were observed as cod rose from the ocean bottom in the evening, increasing their tilt angle, and descended at sunrise to regain the ocean floor. Standardized TS (B20) was found to be highly correlated with tilt angle. This relationship can be used to correct for the diel changes in the TS of these migrating cod as a function of the in situ-measured tilt angle and thus to improve the accuracy of acoustic-biomass estimation. [Copyright &y& Elsevier]

Understanding the relationship between fish biology and target strength potentially improves the accuracy of acoustic assessments. The effects of individual biological factors (e.g., length, tilt, and depth) on backscatter amplitude have been examined, but the relative contribution of each factor has not been quantified. Dimensionless ratios, which facilitate comparison of disparate quantities, were used to evaluate the effects of individual biological factors on echo intensities. Ratios from 25 adult walleye pollock (Theragra chalcogramma) were calculated using a Kirchhoff-ray-mode, backscatter model parameterized for each fish. This comparative approach can be used to identify the influence of biological factors on backscatter intensity and is potentially a tool for improving accuracy when converting acoustic size to fish length. [Copyright &y& Elsevier]

A research programme was carried out in French Polynesia to study tuna behaviour using acoustics and fishing experiments. Acoustics are of great importance for the study of tuna behaviour and estimation of abundance but estimates of individual target strength that are available are particularly inaccurate. In this study, four yellowfin tuna (Thunnus albacares) and two bigeye tuna (Thunnus obesus) of weight 4–50 kg were individually caught, identified, and equipped with ultrasonic tags for telemetry experiments. While tracking the fish, simultaneous underwater acoustic data were recorded with a split-beam echosounder in order to estimate their in situ acoustic target strength. It was observed that target strength was stronger when fish were diving than when they were ascending toward the surface. This can be explained by the tilt angle orientation of the swimbladder. A target strength bias according to depth was also observed. [ABSTRACT FROM PUBLISHER]

When plankton production in the feeding areas decreases in the fall, adult Norwegian spring-spawning herring migrate into two fjords in northern Norway. In these wintering areas the herring occupy deeper water. Lacking the ability to refill the swimbladder they are constantly negatively buoyant. This leads to different adaptive behaviour during the day and at night, behaviour which is reflected in swimming angle. Split-beam tracking methods and still-frame photography have been used to study the herring behaviour inside the dense wintering schools. Negative buoyancy seems to be controlled through constant swimming at speeds between 0.25–0.42 ms−1 because these are sufficiently high to generate lift when the pectoral fins are used as spoilers. During the day, when the layers aggregate, the average swimming angle is close to horizontal while positive average swimming angles of up to 40° were recorded at night. A bimodal distribution of tilt angles, with one positive and one negative component, indicating a “rise and glide” swimming strategy was also observed at night. Vertically undulating split beam tracks confirmed this particular type of swimming behaviour. As adult herring are directional targets at the echo-sounder frequency used for acoustic assessment of the stock, the possible impact of the observed tilt angles on average acoustic target strength is discussed. [ABSTRACT FROM PUBLISHER]

This method has been devised for easy staining for stereoscopic observation of thick sections under a high voltage transmission electron microscope (HVTEM). It uses carbohydrazide as an osmium bridging agent and both osmium tetroxide and uranyl acetate as electron staining agents. Osmium tetroxide-fixed and -stained tissue blocks are embedded in a Quetol 651 resin mixture. Thick sections (2–3 μm thick) without double staining are observed at an accelerating potential of 300 kV and a tilt angle of ± 10° by an H-9000 TEM with a side-entry goniometer. Stereoscopic electron micrographs can be obtained. [ABSTRACT FROM PUBLISHER]