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The performance of midwater trawls used during acoustic surveys for walleye pollock ( Gadus chalcogrammus ) was estimated using a novel method of combining acoustic and catch information. Direct comparisons of acoustic and catch derived density showed a poor correlation between the two ( r 2 = 0.07), suggesting this discrepancy might be in part due to trawl performance. Trawl efficiency and selectivity were determined by comparing acoustically derived fish density within the trawling volume with catch-based density in a modeling framework. Additional factors thought to potentially influence trawl performance, consisting of environmental conditions including water temperature, bottom and fishing depth, variables relating to survey methodology including survey vessel, codend liner, and the time of day when trawling occured, and fish population characteristics including spawning state and condition factor were evaluated in the models. Incorporating efficiency and selectivity parameters resulted in improved model fits, as did the addition of explanatory variables. Efficiency was found to be associated with the proportion of spawning fish in the catch, water temperature, and the time of day, while selectivity appeared to be influenced by condition factor and the survey vessel used. The acoustic-catch modeling approach confers advantages of making available large historic datasets where acoustic data was collected during trawling and providing sufficient contrasts in parameters of interest not easily achieved with direct trawl performance experiments. [ABSTRACT FROM AUTHOR]

Abstract: Trawls are standard tools for surveying fisheries resources, yet they are selective in what they retain, and thus provide potentially misleading information about fish populations. In order to evaluate the potential for selective retention in a midwater survey trawl used in conjunction with acoustic surveys of walleye pollock, fish behavior was examined using an integrated approach of optical, acoustic and recapture net methods. A stereo-camera system was used to provide length, position and orientation information, and a dual-frequency identification sonar was used to track fish targets in the trawl. Fish escaping the trawl were sampled using recapture, or pocket, nets mounted to the outside of the trawl. Most fish were found to be oriented along the main trawl axis, facing the forward trawl opening. Nearest distance to the trawl panel did not appear to be length-dependent, however, at night when ambient light levels were lower, fish maintained less distance to the trawl panel compared to daytime observations. Consequently, significantly more fish escapes occurred at lower light levels. Trajectories of fish escaping the trawl were highly variable compared with fish that herded into the net, or those whose retention state was unknown. Greatest escapement into pocket nets was observed from the bottom panel of the trawl at night. These findings suggest that survey trawl samples will be less biased due to selectivity when trawls are conducted during the day. [Copyright &y& Elsevier]

Somerton, D. A., Williams, K., von Szalay, P. G., and Rose, C. S. 2011. Using acoustics to estimate the fish-length selectivity of trawl mesh. – ICES Journal of Marine Science, 68: 1558–1565. Estimation of the retention probability of a trawlnet traditionally involves conducting experiments during which the fish escaping through the meshes are recaptured using either small-mesh pocketnets attached to the outside of the net or by enclosing the entire trawlnet in a small-mesh net. A new method of estimating the length selectivity of trawl mesh is demonstrated; it does not require the recapture of escaping fish but instead uses standard acoustic methods to estimate the abundance of fish entering the net before mesh selection. The method was applied to the 83–112 eastern otter trawl used by the Alaska Fisheries Science Center (AFSC) to conduct bottom-trawl surveys in the eastern Bering Sea (EBS), and the Aleutian wing trawl used by the AFSC to collect midwater biological samples of walleye pollock (Theragra chalcogramma) during fishery acoustic surveys of the EBS and Gulf of Alaska. The length selectivities of both trawls were also estimated using standard recapture experiments. For both, the estimated lengths at 50% selection (L50) from the acoustic method were similar to the estimates from the recapture experiments, but the estimated selection ranges were narrower. The advantages of the acoustic method are that it is simpler to use than traditional fish-recapture methods and it does not alter normal trawl performance. [ABSTRACT FROM PUBLISHER]

Williams, K., Punt, A. E., Wilson, C. D., and Horne, J. K. 2011. Length-selective retention of walleye pollock, Theragra chalcogramma, by midwater trawls. – ICES Journal of Marine Science, 68: 119–129. Midwater trawls are commonly used during acoustic surveys of fish abundance to determine species and length compositions of acoustically sampled fish aggregations. As trawls are selective samplers, catches can be unrepresentative of sampled populations and lead to biased abundance estimates. Length-dependent retention of walleye pollock was estimated using small recapture nets, so-called pocket nets, attached to the outside of the trawl. Experimental haul sets comprising eight hauls each were conducted in the Gulf of Alaska in 2007 and 2008 and in the eastern Bering Sea (EBS) in 2007. Pocket-net catches were then modelled by fitting parameters for selectivity and escapement location along the trawl. Within- and between-haul variability was jointly estimated using hierarchical Bayesian methods. There was significant undersampling of juvenile (<25 cm) pollock, with the length-at-50%-retention (L50) estimated between 13.5 and 26.1 cm among haul sets. In the EBS set, L50 values were correlated with light level, escapement being greater at night. Trawl selectivity may be a significant source of error in acoustic-survey estimates of the abundance of pollock. [ABSTRACT FROM PUBLISHER]

We describe the application of two types of stereo camera systems in fisheries research, including the design, calibration, analysis techniques, and precision of the data obtained with these systems. The first is a stereo video system deployed by using a quick-responding winch with a live feed to provide species and size composition data adequate to produce acoustically based biomass estimates of rockfish. This system was tested on the eastern Bering Sea slope where rockfish were measured. Rockfish sizes were similar to those sampled with a bottom trawl and the relative error in multiple measurements of the same rockfish in multiple still-frame images was small. Measurement errors of up to 5.5% were found on a calibration target of known size. The second system consisted of a pair of still-image digital cameras mounted inside a midwater trawl. Processing of the stereo images allowed fish length, fish orientation in relation to the camera platform, and relative distance of the fish to the trawl netting to be determined. The video system was useful for surveying fish in Alaska, but it could also be used broadly in other situations where it is difficult to obtain species-composition or size-composition information. Likewise, the still-image system could be used for fisheries research to obtain data on size, position, and orientation of fish. [ABSTRACT FROM AUTHOR]

Handegard, N. O., and Williams, K. 2008. Automated tracking of fish in trawls using the DIDSON (Dual frequency IDentification SONar). – ICES Journal of Marine Science. 65: 636–644.An application for the automated tracking of dual-frequency, identification sonar (DIDSON) data was developed and tested on fish observations taken in midwater trawls. The process incorporates target detection, multiple target tracking, and the extraction of behaviour information such as target speed and direction from the track data. The automatic tracker was evaluated using three test datasets with different target sizes, observation ranges, and densities. The targets in the datasets were tracked manually and with the automated tracker, using the manual-tracking results as the standard for estimating the performance of the automated tracking process. In the first and third dataset, where the targets were smaller and less dense, the automated tracking performed well, correctly identifying 74% and 57% of targets, respectively, and associating targets into tracks with <10% error compared with the manually tracked data. In the second dataset, where targets were dense and appeared large owing to the shorter observation range, 45% of targets were correctly identified, and the track error rate was 21%. Target speed and direction, derived from the tracking data, agreed well between the manual and automatic methods for all three test cases. Automated tracking represents a useful technique for processing DIDSON data, and a valuable alternative to time-consuming, manual data-processing, when used in appropriate conditions. [ABSTRACT FROM PUBLISHER]

Acoustic-trawl (AT) survey methods are widely used to estimate the abundance and distribution of pelagic organisms. This technique relies on estimates of size and species composition from trawl catches along with estimates of the acoustic properties of these animals to convert measurements of acoustic backscatter into animal abundance. However, trawls are selective samplers, and if the catch does not represent the size and species composition of the animals in the acoustic beam the resulting abundance estimates will be biased. We conducted an experiment to quantify trawl selectivity for species encountered during an AT survey of the Alaska Arctic. The pelagic assemblage in this environment was dominated by small young-of-the-year (age-0) fishes and jellyfish, which may be poorly retained in trawls. A large midwater trawl (Cantrawl) and a smaller midwater trawl (modified Marinovich) were used during the survey. The Marinovich was equipped with 8 small-mesh recapture nets which were used to estimate the probability that an individual that enters the trawl is retained. In addition, paired hauls were made with the Cantrawl and Marinovich to estimate the difference in selectivity between the two trawls. A statistical model was developed to combine the catches of the recapture nets and the paired hauls to estimate the length-dependent selectivity of the trawls for the most abundant species (e.g., age-0 fishes and jellyfish). The analysis indicated that there was substantial size and species selectivity: although the modified Marinovich generally had a higher catch per unit effort, many of the animals encountered in this environment were poorly retained by both trawls. The observed size and species selectivity of the trawls can be used to select appropriate nets for sampling pelagic fishes, and correct survey estimates for the biases introduced in the trawl capture process. [ABSTRACT FROM AUTHOR]

Pelagic trawls are one of the primary methods of sampling midwater fishes. However, these trawls are species- and size-selective, and small fish can escape through trawl meshes. This can introduce uncertainty and bias into survey abundance estimates if not accounted for. The small, abundant pelagic fishes of the Alaska Arctic are challenging to sample with trawls as they are sufficiently motile to avoid small fine-mesh trawls but are also small enough to escape through the meshes of trawls designed to capture larger fishes. A pelagic herring trawl equipped with a fine-mesh codend liner was used to quantify the size and species composition of pelagic fishes during a baseline acoustic-trawl survey of the Chukchi Shelf. Subsequent experiments with recapture nets attached to the outside of the trawl netting suggested that escapement of small fishes was substantial, particularly in the aft net section. Thus, the trawl was further modified by reducing the taper in the aft net section and adding a small-mesh section in front of the codend to potentially reduce escapement. Further use of recapture nets during two subsequent acoustic-trawl surveys confirmed that this trawl modification substantially increased retention of small fishes and resulted in less size selectivity. These improvements will reduce biases in estimates of abundance, size, and species composition of pelagic Arctic fishes. This work highlights the importance of quantifying escapement from survey trawls and demonstrates that escapement estimates can guide successful trawl modifications. [ABSTRACT FROM AUTHOR]