Sascha M.M. Fässler

Depth-dependent swimbladder compression in herring Clupea harengus observed using magnetic resonance imaging

Changes in swimbladder morphology in an Atlantic herring Clupea harengus with pressure were examined by magnetic resonance imaging of a dead fish in a purpose-built pressure chamber. Swimbladder volume changed with pressure according to Boyles Law, but compression in the lateral aspect was greater than in the dorsal aspect. This uneven compression has a reduced effect on acoustic backscattering than symmetrical compression and would elicit less pronounced effects of depth on acoustic biomass estimates of C. harengus.

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.

Effects of target strength variability on estimates of abundance: The case of Atlantic mackerel (Scomber scombrus)

Atlantic mackerel is a small pelagic, migratory fish which supports valuable commercial fisheries. Despite the fact that these fish school in massive numbers, and are readily detected using echosounders, fishery-independent estimates of the abundance of mackerel in the Northeast Atlantic do not yet consider acoustic data. Echo-integration surveys could provide annual estimates of abundance, with additional scope for studying mackerel distributions throughout the year. However, as in all acoustic surveys, this requires accurate estimates of target strength (TS). The present study provides in situ TS estimates for mackerel from measurements made at sea with a multi-frequency split-beam echosounder. Empirical results suggest mean TS of -51.22 dB at 18 kHz, -59.83 dB at 38 kHz, -55.51 dB at 120 kHz, and -53.43 dB at 200 kHz, for a mean fish length of 33.3 cm. These differ significantly from the values currently used in acoustic surveys. The sensitivity of mackerel abundance estimates to variations in TS estim...

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.

Classification of pelagic fish using wideband echosounders

The landing obligation for pelagic fisheries was introduced in January 2015 by the European Commission and will gradually be extended to all fisheries in 2019. This regulation follows the trend for sustainable fisheries in Europe and states that all catches must be landed and counted against the quotas. Pre-catch acoustic fish classification can remotely provide information about fish school composition in terms of species and size distribution. This technique has been tested using three commercial wideband transceivers mounted on a commercial freezer trawler. Together, the three SIMRAD EK80 transceivers cover a frequency range of 45 kHz to 260 kHz. Acoustical data was collected for four months during fishing operations in the North Sea and the English Channel. Custom, through-the-sensor image processing algorithms have been developed to perform automatic school detection under severe noise, interference and ship motion conditions. Dynamic Factor Analysis was applied to exploit common trends in the spectr...

Modeling target strength of individual herring (Clupea harengus) at any aspect as a function of pressure and frequency

Fish target strength is primarily dependent on the physical dimensions of the fish, the acoustic frequency, and the orientation of the fish. In traditional vertically observing echosounder surveys, fish are insonified in the dorsal aspect with fairly limited tilt angle variation. In oblique-angled sonar surveys, however, fish may be insonified at other aspects. For herring, target strength is also depth dependent as they cannot refill the swimbladder at depth. Understanding the depth dependent target strength from several insonification angles is thus required for quantitative measurements with sonar. The dataset used in this study consists of seven herring, imaged using magnetic resonance imaging (MRI). The herring were placed in a pressure chamber inside the MRI, and subjected to different pressures corresponding to water depths of 0, 20, 40, and 60 m. Images were acquired of each specimen at each pressure. The swimbladders were segmented and 3D models of the swimbladders for each fish and pressure were...

Investigation of the target strength-to-length relationship of Baltic herring (Clupea harengus) for use in biomass estimation

EU environmental protection policy, based on a sustainable development approach, requires fast and non-expensive methods of marine ecosystem monitoring. Acoustic techniques, which satisfy these requirements, are widely used as a reliable method of fish stock assessment. Systematic acoustic surveys are conducted in the Baltic Sea in order to estimate the abundance of ecologically and commercially important fish populations such as herring (Clupea harengus) and sprat (Sprattus sprattus). An important requirement for acoustic abundance estimation is the species specific target strength (TS)-to-fish length relationship to convert acoustic energy into numbers of fish. TS is a measure of a fishpsilas potential to reflect sound and one of the major challenges in the Baltic Sea is the development of a reliable TS-length relationship for Baltic herring. Currently, a TS relationship originally determined for North Sea herring is used for the assessment of Baltic herring. In situ TS measurements of Baltic herring conducted in different parts of the Baltic Sea during different seasons demonstrated that Baltic herring have a higher TS than herring living in the north-east Atlantic such as Norwegian spring-spawning or North Sea herring. Additionally, there is a strong variability in Baltic herring TS in different regions and seasons (up to 8 dB). To obtain an accurate TS-length relationship the impact of the biological difference between different herring stocks on the variability of the measured Baltic herring TS should be understood. The presented paper deals with these two important issues. A detailed comparison between the acoustic backscattering properties of Baltic and Norwegian spring-spawning herring was made. The effect of different physical, environmental and biological (depth, acoustic frequency, salinity, fish orientation, fat content and condition) factors on the backscattering of the Baltic herring stock is shown. It is suggested that a different TS-to-fish length relationship should be used for Baltic herring than the one that is currently applied.