Gavin J. Macaulay

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 requiem for the use of 20 log 10 Length for acoustic target strength with special reference to deep-sea fishes

Although it is well known that the slopes of target strength (TS) and length relationships vary widely, it is common in fisheries acoustics to force the TS-length regression through a slope of 20. Is it time to abandon this practice? The theoretical justification was that TS should be proportional to cross-sectional area, and that area should scale as the square of the linear dimension (fish length). There are now many species other than gadoids that are the subject of acoustic surveys, and many of them do not have the same morphology as the gadoid fishes. The slope of the TS-length regressions deviates significantly from 20. The empirical slope should be used wherever it can be shown to be more appropriate than the 20 log 10 L model. Using the data from swimbladder models, it is shown that Macrourids, a merluccid hake and Oreosomatidae have a different relationship should between swimbladder size and fish size compared with that of gadoids. It is demonstrated that the 20 log 10 L model is not appropriate for these deep-water fish and that deviations from the model arise, to a considerable degree, from variation in fish morphotypes. The TS of deep-water Macrourids, a merluccid hake and Oreosomatidae are lower than that of gadoids. This is related to the swimbladder size-fish size relationship in different morphotypes, although not much evidence can be found to support the concept that swimbladder sizes are generally smaller in deep-sea fishes.

Comparisons among ten models of acoustic backscattering used in aquatic ecosystem research

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m(2)) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge.

Identification and target strength of orange roughy ( Hoplostethus atlanticus ) measured in situ

It is often assumed that in situ target strength (TS) measurements from dispersed fish are representative of the surveyed schooling fish. For in situ TS measurements of orange roughy in deep water, it has been difficult to validate the target species, individual lengths, and tilt angles and how representative these are of schooling fish. These problems have been addressed by attaching an acoustic optical system (AOS) to a trawl net. The AOS enables in situ measurements of TS and volume backscattering strength (Sv) at 38 and 120 kHz with optical verification of species and stereo camera measurements of fish length and tilt angle. TS estimates believed representative of the schooling population were derived by (1) weighting the frequency-dependent TS values by the Sv frequency difference distribution of orange roughy schools and (2) weighting the in situ TS measurements with an assumed tilt angle distribution. The 120-kHz TS estimates were less sensitive to variations in frequency difference and tilt angle, suggesting that this frequency may be better for biomass estimates than 38 kHz, the traditional survey frequency. Computations performed with an anatomically detailed scattering model agree with measurements of TS at both frequencies over a range of tilt angles.

Acoustic backscatter and copepod secondary production across the Subtropical Front to the east of New Zealand

The magnitude and spatial distribution of zooplankton biomass and secondaryproduction across the Subtropical Front (STF) to the east of New Zealand is virtuallyunknown. We used conductivity-temperature-depth data to locate the STF and describedthe pattern of mixed layer depth, current shear, and Richardson number across the frontalzone. The meridional distributions of surface chlorophyll, acoustic backscatter, opticalparticle counts, and sizes across the STF were determined from a survey in austral spring1999. Chlorophyll, acoustic backscatter, and optical biomass all were higher in subtropicalwater to the north of the STF. Copepod-sized particles were 50 mm larger in the frontalzone (ESD 400 mm) than to the north and south of the front. Neither acousticbackscatter nor optically estimated copepod biomass was higher in the frontal zone.Particle counts and temperature were used to estimate copepod secondary production.Median potential copepod production was 9.6 mg Carbon m

A least squares method of estimating length to target strength relationships from in situ target strength distributions and length frequencies

A least squares method is presented for estimating length to target strength relationships for a target species and associated species using a series of target strength distributions and associated trawl catches. A by-product of the estimation procedure is an objective determination of the correspondence between modal lengths in the trawl catches and the modal lengths in the associated target strength distributions. The method is illustrated by applying it to a data set collected to determine the length to target strength relationship for hoki (Macruronus novaezelandiae).

Accuracy of the Kirchhoff-approximation and Kirchhoff-ray-mode fish swimbladder acoustic scattering models.

The acoustic backscatter from pressure release prolate spheroids and a three-dimensional representation of a fish swimbladder (Chilean jack mackerel, Trachurus symmetricus murphyi) was calculated using four target strength models (Kirchhoff-approximation, Kirchhoff-ray-mode, finite element solution of the Helmholtz equation, and prolate-spheroid-modal-series). Smoothly varying errors were found in the Kirchhoff-approximation and Kirchhoff-ray-mode model results when compared to the other models, and provide objective criteria for constraining the use of the KA and KRM models. A generic correction technique is also proposed for the prolate spheroid estimates and tentatively tested on a jack mackerel swimbladder, resulting in improvements to the target strength estimates from the Kirchhoff-approximation and Kirchhoff-ray-mode models.

Exploratory measurements using a broadband, split beam echo sounder system.

A prototype broadband echo sounder has been used for measuring zooplankton and fish in a new collaborative project between the Institute of Marine Research and Kongsberg Maritime‐Simrad. The prototype system used is a Simrad broadband system which includes a transceiver operating from 10–500 kHz. We have mainly used the transceiver together with four separate Simrad ESXX‐7CD standard pressure resistant transducers, the ES70‐7CD, ES120‐7CD, ES200‐7CD, and ES333‐7CD, covering the band from 50 to 450 kHz. The research has so far concentrated on data output formats, calibration methods, and measurements from single targets in ex situ and in situ situations. Examples of recorded reflected spectra for selected calibration and biological targets will be shown and the potential for improved aquatic ecosystem assessment discussed.

Behaviours of Atlantic herring and mackerel in a purse-seine net, observed using multibeam sonar

&NA; To ensure efficient and sustainable purse‐seine fisheries, the catch process must be monitored to better understand the reactions of fish to the gear. In this study, we monitored the behaviours of herring (Clupea harengus) and mackerel (Scomber scombrus) schools during purse‐seine capture using a multibeam imaging sonar (Simrad MS70, 75‐112 kHz) mounted on a research vessel. The fish behaviours differed between species and purse‐seine sets. For both species, the acoustic volume backscattering coefficient increased as 0‐80% of the seine was hauled aboard, indicating a corresponding increase in fish spatial density. This increase was significantly greater for herring than mackerel. As 0‐40% of the seine was hauled aboard the fishing vessel, schools changed their spatial distribution and volume independent of seine hauling, while for some schools, depth and height decreased. The acoustic volume backscattering strength was up to 25 dB higher in the centre of the school than in the edges. The average lateral target strength was estimated for individual fish in the captured herring schools, and the effect of incident angle on the backscattering strength is considered.

Comparisons of echo-integration performance from two multiplexed echosounders

A time-series of acoustically derived aquatic biomass estimates relies on the acoustic equipment maintaining the same performance throughout the time-series. This is normally achieved through a regular calibration process. When the acoustic equipment changes it is necessary to verify that the new equipment produces a similar result to the old equipment, otherwise an unknown bias can be introduced into the time-series. The commonly used Simrad EK60 echosounder has been superseded by the Simrad EK80 echosounder and the performance of these two scientific echosounder systems was compared using interleaved pinging through the same transducer. This was repeated for multiple transducer frequencies (18, 38, 70, 120, and 200 kHz) and from two vessels (Norway’s G.O. Sars in the North Sea and The Netherlands’ Tridens in the Northeast Atlantic Ocean). The broadband facility of the EK80 was not used. Regressions of the grid-integrated backscatter from the two systems were highly linear. The difference in area backscattering coefficients in typical survey conditions was less than 0.6 dB (12%) at the main survey frequency of 38 kHz. In most conventional fish acoustic surveys, the observed differences are less than other sources of survey bias and uncertainty.