Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys / David A. Demer

Public ISBD Faire une suggestion d'achat Titre : Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys Type de document : texte imprimé Auteurs : David A. Demer, Auteur Importance : pp. 1664-1674 Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : Multibeam echosounders can improve the efficiency and the precision of acoustic-survey estimates by providing greater sampling volumes than single-beam echosounders. For a multibeam echosounder, the target strength of fish can vary with its pitch, roll, and yaw. Here, normalized, acoustic backscatter patterns from fish schools are modelled by beam-incidence angle, 0-180, considering the scattering-directivity patterns of each fish. Variation of pitch angle causes a decrease in the mean and an increase in the variance of the backscatter in the vertical beam, but has no effect on the backscatter in the outer beams. Conversely, variation of the yaw causes a decrease in the mean and an increase in the variance of the backscatter in only the outer beams. Because the fish-scattering model predicts different backscatter at dorsal- vs. lateral-incidence angles, backscatter did vary with roll angle. In the hypothetical case of fish avoiding a vessel, the backscatter decreases strongly the outer beams. The results of the model were compared with multibeam measurements of fish schools. In general, the measured mean backscatter vs. beam-incidence angle was nearly uniform. The methods described here provide an approach to accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys. Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys [texte imprimé] / David A. Demer, Auteur . - [s.d.] . - pp. 1664-1674. Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : Multibeam echosounders can improve the efficiency and the precision of acoustic-survey estimates by providing greater sampling volumes than single-beam echosounders. For a multibeam echosounder, the target strength of fish can vary with its pitch, roll, and yaw. Here, normalized, acoustic backscatter patterns from fish schools are modelled by beam-incidence angle, 0-180, considering the scattering-directivity patterns of each fish. Variation of pitch angle causes a decrease in the mean and an increase in the variance of the backscatter in the vertical beam, but has no effect on the backscatter in the outer beams. Conversely, variation of the yaw causes a decrease in the mean and an increase in the variance of the backscatter in only the outer beams. Because the fish-scattering model predicts different backscatter at dorsal- vs. lateral-incidence angles, backscatter did vary with roll angle. In the hypothetical case of fish avoiding a vessel, the backscatter decreases strongly the outer beams. The results of the model were compared with multibeam measurements of fish schools. In general, the measured mean backscatter vs. beam-incidence angle was nearly uniform. The methods described here provide an approach to accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys.

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Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996-2006 / David A. Demer

Public ISBD Faire une suggestion d'achat Titre : Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996-2006 Type de document : texte imprimé Auteurs : David A. Demer, Auteur Importance : pp. 497-508 Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : The time-series of acoustically surveyed Antarctic krill (Euphausia superba) biomass near the South Shetland Islands (SSI) between 1996 and 2006 is re-estimated using a validated physics-based model of target strength (TS), and a species-discrimination algorithm based on the length-range of krili in plankton samples to identify krill acoustically, derived from TS-model predictions. The SSI area is surveyed each austral summer by the US Antarctic Marine Living Resources Program, and the acoustic data are used to examine trends in krill biomass and to assess the potential impact of fishing to the reproductive success of land-based predators (seals and penguins). The time-series of recomputed biomass densities varies greatly from that computed using an empirical log-linear TS-model and fixed-ranges of differences in volume-backscattering strengths (A Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996-2006 [texte imprimé] / David A. Demer, Auteur . - [s.d.] . - pp. 497-508. Langues : Anglais (eng) Index. décimale : 508 Voyages et relevés scientifiques. Histoire naturelle Résumé : The time-series of acoustically surveyed Antarctic krill (Euphausia superba) biomass near the South Shetland Islands (SSI) between 1996 and 2006 is re-estimated using a validated physics-based model of target strength (TS), and a species-discrimination algorithm based on the length-range of krili in plankton samples to identify krill acoustically, derived from TS-model predictions. The SSI area is surveyed each austral summer by the US Antarctic Marine Living Resources Program, and the acoustic data are used to examine trends in krill biomass and to assess the potential impact of fishing to the reproductive success of land-based predators (seals and penguins). The time-series of recomputed biomass densities varies greatly from that computed using an empirical log-linear TS-model and fixed-ranges of differences in volume-backscattering strengths (A

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