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Browsing by Author "Velho, Filomena Vaz"

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  • Day-night differences in Cunene horse mackerel (Trachurus trecae) acoustic relative densities off Angola
    Publication . Velho, Filomena Vaz; Barros, Pedro; Axelsen, Bjorn Erik
    The assessment and the management of the Cunene horse mackerel in Angola rely heavily on abundance estimates from hydroacoustic surveys. Acoustic data collected from 1994 to 1999 were analysed to quantify diurnal variation in relative acoustic densities at 38 kHz. The nautical-area scattering coefficient (s(A), m(2) nautical mile(-2)) was characterized by clear day night differences: s(A) values recorded during the day were significantly higher (mean s(A): 135 m(2) nautical mile(-2)) than the corresponding night-time values (mean s(A) 83 m(2) nautical mile(-2)). This pattern is associated with differences in behaviour and horizontal and vertical distributions between day and night: by day, the fish school near the seabed, and by night, they move into the pelagic zone and disperse into widespread scattering layers. More than 40% of the total backscatter by day originated from the bottom 10 m, but at night this proportion decreased to <10%. The findings demonstrate considerable influences of behaviour and aggregation dynamics on acoustic measurements. Possible implications for the estimates of acoustic abundance are discussed in the light of the differences.
    2010-07Journal article Open access Show more
  • Some aspects of the behaviour of pelagic species species off Angola and its implication on biomass estimation
    Publication . Velho, Filomena Vaz; Barros, Pedro Conte de
    Pelagic fish off Angola constitute about 80% of the total fish landed and 75% of animal protein in the diet of the coastal population. In Angola, the primary means for estimating abundance of small pelagic fish is through acoustic surveys, a method that is sensitive to errors caused by effects of fish behaviour, particularly those that may reduce the acoustic detectability offish. This study describes the interannual variability of the fishery and stocks pelagic off Angola and their behaviour in relation to the abundance estimation. The dynamics of the two main pelagic species, Cunene horse mackerel and Sardinela spp off Angola differ. In recent years the biomass of Sardinela spp is at stable level, whereas the Cunene horse mackerel is at critical level. It was observe that a relatively straightforward analysis of conventional acoustic survey data collected at a single- frequency combined with ancillary spatio-temporal information can provide useful indications on the identification of single- and multi species aggregations of small pelagic fish. With regard to their behaviour, Cunene horse mackerel tend to concentrate in dense schools near the seabed, but at night they move into the pelagic zone, dispersing into widespread scattering layers. The present study found that under some conditions fish tend to avoid the vessel vertically, diving below 12 m depth as vessel was passing over the fish. In general, the results of this study should help improve Angola improve the knowledge of the behaviour of pelagic species off Angola and the quality of the abundance estimations produced.
    2012-07-27Doctoral thesis Open access Show more