John K. Horne

Acoustic models of fish: The Atlantic cod (Gadus morhua)

Acoustic fish models should represent the fish body form. The Atlantic cod were used to model the acoustic scattering function of teleost fish. The model provides a basis for choices of sonar carrier frequencies. Anesthetized live Atlantic cod ranging from 156 to 380 mm (SL) were ‘‘soft’’ x‐rayed to image inflated swimbladders and skeletal elements. Maximum body heights and widths were 0.18 and 0.13 of fish lengths. Lengths and diameters of swimbladder were approximately 0.25 and 0.05 of the fish lengths. A series of short‐length fluid‐filled cylinders were used to represent body flesh. For carrier frequencies above the breathing mode resonance, swimbladders were modeled as a series of short gas‐filled volume elements of cylinders. A Kirchhoff‐ray approximation was used to compute the high‐frequency acoustic scattering. A low mode solution for a gas‐filled cylinder was used to compute the low‐frequency ‘‘breathing mode resonance.’’ All contributions were added coherently. The scattering lengths L, or targ...

Spatial variance in ecology

Spatial variance of observed measures such as density is no longer viewed as a statistical annoyance. It is now treated as a biologically important quantity that changes value depending on the scale of measurement. Processes that generate spatial variance are often inferred by matching scales of maximum biological spatial variance to dominant physical processes at the same scale. Success in this approach has been limited to patchiness of plant communities along environmental gradients and to patchiness of passive aquatic organisms relative to physical flow structures. Some progress in formalizing spatial variance has been made using empirical models derived from quantitative descriptions of pattern, but further progress requires theoretical models of spatial variance and processes that generate variance as a function of spatial scale.

Mesoscale eddies are oases for higher trophic marine life.

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

A method for evaluating the effects of biological factors on fish target strength

Hazen, E. L., and Horne, J. K. 2003. A method for evaluating the effects of biologicalfactors on fish target strength. – ICES Journal of Marine Science, 60:555–562.Understanding the relationship between fish biology and target strength potentially im-proves 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 relativecontribution of each factor has not been quantified. Dimensionless ratios, whichfacilitate comparison of disparate quantities, were used to evaluate the effects of individualbiological factors on echo intensities. Ratios from 25 adult walleye pollock (Theragrachalcogramma) were calculated using a Kirchhoff-ray-mode, backscatter model parameter-ized for each fish. This comparative approach can be used to identify the influence ofbiological factors on backscatter intensity and is potentially a tool for improving accuracywhen converting acoustic size to fish length.

The influence of ontogeny, physiology, and behaviour on the target strength of walleye pollock (Theragra chalcogramma)

Variability in echo intensities from aquatic organisms is caused by physical factors associated with the transmission of sound through water, and by biological factors associated with the ontogeny, physiology, and behaviour of targets. Acoustic-based density estimates depend on accurately characterizing reflected or backscattered sound from any species of interest. Digitized lateral and dorsal radiographs of walleye pollock (Theragra chalcogramma) were used to characterize intra-specific variability among young-of-theyear, juvenile, and adult life-history stages. Kirchhoff-ray mode (KRM) models were used to quantify variability in backscatter intensities at 38 and 120 kHz. At these geometric scattering frequencies, swimbladder surface areas influence echo intensities. Dorsal swimbladder surface areas were proportionate to fish lengths and decreased after fish were fed. Corresponding changes in backscatter were not proportionate to the reduction in dorsal surface area. The ratio of dorsal to lateral swimbladder surface areas was consistent among gravid and non-gravid fish. Walleye pollock tilt angles were centred at 90 � and did not differ among densities or between light and dark cycles. Target strength–length regressions and KRM-predicted backscatter models closely matched in situ target-strength measurements for walleye pollock in the Bering Sea. Backscatter variability can be minimized through judicious deployment of equipment and equipment-parameter settings, but the relative influence of biological factors on backscatter amplitude has not been determined.

Turbulence, Acoustic Backscatter, and Pelagic Nekton in Monterey Bay

Abstract During August 2006 aggregations of nekton, most likely small fish, intersected microstructure survey lines in Monterey Bay, California, providing an opportunity to examine biologically generated mixing. Some aggregations filled the water column, 80 m deep, and extended 100–200 m along the survey track. Others were half that size, and some were much smaller. Acoustic energy backscattered from the aggregations was measured with a calibrated echosounder and yielded volume backscattering strength Sv values of −80 to −60 dB re 1 m−1. Turbulent dissipation rates ϵ were 10−6–10−5 W kg−1 in the more intense aggregations. Within these, ϵ was much more uniform than turbulence measured outside the aggregations and varied with Sυ. Three similar aggregations contributed half of the average ϵ in 142 profiles taken along a 5-km-long survey line during a 12.5-h tidal cycle. Turbulence within aggregations differed markedly from that outside in the following three ways: (i) Thorpe scales, that is, root-mean-square...

Analysis of scale-dependent processes with dimensionless ratios

Awareness of the role of scale in ecological research has increased over the last decade but few tools have been developed to evaluate biological and physical processes that generate scale-dependent biological patterns. We describe a procedure that combines demographic, growth, and kinematic rates to evaluate the spatial and temporal dynamics of population biomass. Values of dimensionless ratios are plotted and contoured as a function of spatial and temporal scale to summarize knowledge of processes that generate variability in a biological quantity. This summary can be used to indicate variance generating processes at any scale of interest, to identify potential research areas and appropriate sampling scales for field studies, and to limit the range of scales over which conclusions can be generalized. J. K. Home, Ocean Sciences Centre and Dept of Biology, Memorial Univ. of Newfoundland, St. Johns, Newfoundland, Canada, A1C5S7. - D. C. Schneider, Ocean Sciences Centre, Memorial Univ. of Newfoundland, St. Johns, Newfoundland, Canada, A1C5S7.

Characterizing uncertainty in target-strength measurements of a deepwater fish: orange roughy (Hoplostethus atlanticus)

The variability of ensemble 38 kHz, target-strength (TS38) estimates for orange roughy (Hoplostethus atlanticus) (4.9 dB, factor of 3.1) in deep water (>600m) limits the use of echo integration for absolute-biomass estimates. Orange roughy are high in oil content, have a wax ester swimbladder, and show an active-avoidance response to sampling gear. The interpretations of ensemble, in situ target strengths of orange roughy (range hTS38i 1⁄4 52:9 to 51.0 dB for standard fish length hSLi 1⁄4 35 cm) are lower than previous model and surface-based measurements (hTS38i 1⁄4 48 dB, SL 1⁄4 35 cm). In situ TS measurements from individuals on the periphery of dense schools were processed to minimize uncertainties from single-target selection criteria, species composition, and active avoidance. Video and acoustic-tracking data quantified the variability in TS measurements arising from the variability in fish orientation. Multi-frequency acoustics and fish tracking are used to quantify in situ TS variability due to species identification and fish density. The Kirchhoff-ray mode backscatter model was used to illustrate the sensitivity of speciesspecific backscatter to assumptions of tilt-angle and material properties (density and soundspeed contrasts). We conclude that a remaining source of uncertainty for in situ TS measurements is the assumption that dispersed targets are representative of the survey population.

Three-dimensional visualization of fish morphometry and acoustic backscatter

Theoretical acoustic models of fish are used to explain variability in backscatter measurements, improve estimation of target size, and improve target recognition and discrimination among acoustic targets. Acoustic backscatter models that incorporate fish morphology potentially provide more realistic predictions of echo amplitudes than models that approximate morphology using simple geometric shapes. Procedures to obtain digital representations of a fish’s body and swimbladder are presented. These digital images are used in a Kirchhoff ray-mode model to predict backscatter amplitude as a function of fish length, acoustic frequency, and angle of insonification. Backscatter amplitude can be displayed as one-dimensional curves, two-dimensional response surfaces, and a three-dimensional backscattering surface (i.e., ambit).

Classifying multi-frequency fisheries acoustic data using a robust probabilistic classification technique

A robust probabilistic classification technique, using expectation maximization of finite mixture models, is used to analyze multi-frequency fisheries acoustic data. The number of clusters is chosen using the Bayesian Information Criterion. Probabilities of membership to clusters are used to classify each sample. The utility of the technique is demonstrated using two examples: the Gulf of Alaska representing a low-diversity, well-known system; and the Mid-Atlantic Ridge, a species-rich, relatively unknown system.