Christopher D. Wilson

Discriminant Classification of Fish and Zooplankton Backscattering at 38 and 120 kHz

Abstract Acoustic scattering layers were evaluated for species classification by means of 38- and 120-kHz mean volume backscattering strength () collected during a 1995 acoustic–trawl survey of Pacific hake Merluccius productus off the west coasts of the United States and Canada. Scattering layers selected for analyses were shallower than 150 m and were analyzed with a −79-decibel (dB) integration threshold. Pacific hakes, euphausiids, and Pacific hake–euphausiid mixes dominated the layers. Other scatterers (unidentified, noneuphausiid, or non—Pacific hake sources) were included in the analyses. The overall mean volume backscatter difference (Δ = 120 kHz – 38 kHz) was computed for each species category, and results varied depending on the species composition of the scattering layer (i.e., Pacific hakes = −7.1 dB, euphausiids = 11.9 dB, Pacific hakes–euphausiids = 3.5 dB, and other species = 0.1 dB). Discriminant function analysis of 120 kHz and 38 kHz separated echoes originating from each of the dominant...

Hearing Sensitivity of the Walleye Pollock

Abstract The hearing abilities of three age-groups (which correspond to size-classes) of walleye pollock Theragra chalcogramma were determined from auditory evoked potentials. Walleye pollock had the best hearing sensitivity from 100 to 200 Hz, with thresholds around 75 dB re: 1 μPa. Hearing sensitivity decreased with increasing frequency up to 450 Hz. There was no significant difference in hearing sensitivity between the age-groups, although there was a significant interaction between frequency and age as well as a trend in which the older fish had slightly lower mean thresholds. At the same time, there was a substantial increase in the size of the saccular otolith and associated sensory epithelia of the inner ear, suggesting that a large increase in ear size does not lead to a large change in hearing sensitivity. In addition, there was an effect of water temperature on the hearing thresholds at 350 Hz, whereby each degree of temperature (°C) increase resulted in an 8.3-dB decrease in hearing threshold. ...

Multifrequency species classification of acoustic-trawl survey data using semi-supervised learning with class discovery

Acoustic surveys often use multifrequency backscatter to estimate fish and plankton abundance. Direct samples are used to validate species classification of acoustic backscatter, but samples may be sparse or unavailable. A generalized Gaussian mixture model was developed to classify multifrequency acoustic backscatter when not all species classes are known. The classification, based on semi-supervised learning with class discovery, was applied to data collected in the eastern Bering Sea during summers 2004, 2007, and 2008. Walleye pollock, euphausiids, and two other major classes occurring in the upper water column were identified.

Do silent ships see more fish? Comparison of a noise-reduced and a conventional research vessel in Alaska.

Under some circumstances, fish detect and avoid approaching vessels, often well before the vessel passes over the fish (Mitson 1995). Such vessel-induced avoidance behavior is potentially a major source of error in surveys of fish populations. Sound propagates a long distance in water compared with other stimuli, and fish are generally most sensitive to sound in the frequency range at which the underwater-radiated noise from ships is most intense (Mann et al. 2009). Thus the primary stimulus for this avoidance behavior is thought to be auditory (Mitson 1995). The concern that vessel noise causes fish avoidance has led to the formulation of recommendations for maximum low-frequency (<1-kHz) underwater-radiated noise levels for fisheries research vessels (Mitson 1995). These recommendations, made under the auspices of the International Council for the Exploration of the Seas (ICES), are based on the hearing capabilities of Clupea harengus (Atlantic herring) and Gadus morhua (Atlantic cod), two species with sensitive hearing; the recommendation is therefore expected to minimize noise-induced vessel avoidance for other species as well.

Underwater radiated noise measurements of a noise-reduced research vessel: comparison between a US Navy noise range and a simple hydrophone mooring.

[A feasibility study was undertaken to characterize underwater radiated noise for a new class of noise‐reduced fisheries research vessels using a field‐deployable hydrophone system. Recent studies have demonstrated that vessel‐radiated noise can impact the behavior of fish, and that periodic monitoring of survey‐vessel radiated noise is desirable to characterize potential biases in fish abundance estimates. Vessel radiated noise is traditionally measured at naval ranges, but lower‐cost options are desirable. Beam aspect measurements of a noise‐reduced vessel made at a U.S. Navy noise range are compared to those made using an experimental mooring equipped with commercially available instrumentation. Hydrophone depths and distance‐to‐the‐vessel were comparable for the mooring and those used at the Southeast Alaska Acoustic Measurement Facility (SEAFAC). SEAFAC and mooring measurements were taken within a day of one another. Data processing was consistent with the recent American national standard for measur...

Using acoustic data from fishing vessels to estimate walleye pollock abundance in the eastern Bering Sea.

Eastern Bering Sea walleye pollock (Theragra chalcogramma) supports one of the world’s largest fisheries. Due to pollock’s high recruitment variability and relatively short life span, timely and accurate abundance indices are needed for fisheries management. Annual bottom trawl (BT) surveys track the demersal portion of the pollock population using chartered commercial fishing vessels, while biennial acoustic‐trawl (AT) surveys use NOAA research vessels to track the younger, midwater portion of the population. More frequent, annual indices of walleye pollock stock size may be obtained using acoustic backscatter data collected from the commercial fishing vessels conducting the BT survey at relatively low cost. A retrospective analysis of 38 kHz, AT survey data (1999–2004) identified a suitable index area to track midwater pollock abundance. The BT survey acoustic data (2006–2009) collected in that area tracked both the AT survey pollock abundance and the large‐scale pollock distribution patterns. This stud...

Field trials using an acoustic buoy to measure fish response to vessel and trawl noise

A freely drifting acoustic buoy was constructed to evaluate the response of fish to vessel and trawl noise. The buoy contains an echosounder and split beam transducer operating at 38 kHz. Transducer heading data from the buoy are collected to assess the directivity of the fish response (i.e., movement relative to vessel and trawl). Geographic position of the buoy is monitored via GPS. Data are stored onboard the buoy and telemetered directly to the support vessel. The radio link between the buoy and vessel is used to control the echosounder, receive buoy positions, and remotely generate echograms in real time. GPS data from the buoy are also transmitted via the Argos satellite system to the vessel to locate the buoy in the event that visual, radar, and direct radio contact are lost. Field trials include repeated passes by the NOAA research vessel Miller Freeman past the buoy at different ship speeds while free‐running and trawling to determine the behavioral response of walleye pollock (Theragra chalcogra...

Seabed classification for trawlability determined with a multibeam echo sounder on Snakehead Bank in the Gulf of Alaska

Rockfishes (Sebastes spp.) tend to aggregate near rocky, cobble, or generally rugged areas that are difficult to survey with bottom trawls, and evidence indicates that assemblages of rockfish species may differ between areas accessible to trawling and those areas that are not. Consequently, it is important to determine grounds that are trawlable or untrawlable so that the areas where trawl survey results should be applied are accurately identified. To this end, we used multibeam echosounder data to generate metrics that describe the seafloor: backscatter strength at normal and oblique incidence angles, the variation of the angle-dependent backscatter strength within 10° of normal incidence, the scintillation of the acoustic intensity scattered from the seafloor, and the seafloor rugosity. We used these metrics to develop a binary classification scheme to estimate where the seafloor is expected to be trawlable. The multibeam echosounder data were verified through analyses of video and still images collected with a stereo drop camera and a remotely operated vehicle in a study at Snakehead Bank, ~100 km south of Kodiak Island in the Gulf of Alaska. Comparisons of different combinations of metrics derived from the multibeam data indicated that the oblique-incidence backscatter strength was the most accurate estimator of trawlability at Snakehead Bank and that the addition of other metrics provided only marginal improvements. If successful on a wider scale in the Gulf of Alaska, this acoustic remote-sensing technique, or a similar one, could help improve the accuracy of rockfish stock assessments.

Acoustic observations of the deep scattering layer during the Deepwater Horizon oil spill.

The explosion of the Deepwater Horizon drilling rig on April 20, 2010 resulted in the release of large quantities of oil and gas from the damaged wellhead into the deep waters of the Gulf of Mexico. During the monitoring effort that ensued, a large body of acoustic measurements with scientific echosounders was collected from May to October with the goal of mapping subsurface oil and gas and monitoring the integrity of the well head. These measurements will be used to observe the deep scattering layer (DSL), a ubiquitous community of sound‐scattering mesopelagic organisms in the vicinity of the spill site. Preliminary observations of reduced backscatter in the near‐field of the rising oil indicate that the DSL is perturbed by the rising oil close to the well head. It is unclear whether this is a highly localized effect occurring only near the well, or whether the DSL was also perturbed by a deep oil plume that spreads from the well site. The acoustic measurements of the DSL will be related to fluorometric ...

Acoustic‐trawl surveys to assess walleye pollock in Alaska: challenges faced and progress made.

Large‐scale acoustic‐trawl surveys have been regularly conducted for over 3 decades by researchers at the NOAA, Alaska Fisheries Science Center (AFSC) to assess walleye pollock (Theragra chalcogramma) in Alaska. Research within the AFSC acoustics program (MACE) has addressed several of the challenges associated with both the acoustics and trawling aspects of these surveys. Acoustic issues include species classification of the acoustic data, fish avoidance to survey vessels, and in situ target strength measurements. Large trawls typically form an integral part of acoustic surveys to confirm the species identity of backscatter and to collect other biological information needed to convert the echo integral into numbers and weight of animals per unit area. MACE staff have also made significant progress in evaluating the selectivity of the large midwater trawls used during their acoustic surveys as well as in developing other direct sampling tools to help interpret the acoustic backscatter data. This presentat...