Abstract
Abstract Vaz Velho, F., Barros, P., and Axelsen, B. E. 2010. Day–night differences in Cunene horse mackerel (Trachurus trecae) acoustic relative densities off Angola. – ICES Journal of Marine Science, 67: 1004–1009. 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 (sA, m2 nautical mile−2) was characterized by clear day–night differences: sA values recorded during the day were significantly higher (mean sA: 135 m2 nautical mile−2) than the corresponding night-time values (mean sA 83 m2 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.
Highlights
Off Angola, horse mackerel are harvested by industrial, semiindustrial, artisanal, and subsistence fleets and are the most important species for human consumption
Higher values of sA were obtained for Cunene horse mackerel by day than by night when comparing consecutive day and night periods, a difference that can be attributed to diurnal changes in the vertical distribution and aggregation patterns
Cunene horse mackerel tended to concentrate in dense schools near the seabed, but at night, they moved into the pelagic zone, dispersing into widespread scattering layers
Summary
Off Angola, horse mackerel are harvested by industrial, semiindustrial, artisanal, and subsistence fleets and are the most important species for human consumption. Two species are found in Angolan waters: Cunene horse mackerel (Trachurus trecae) and Cape horse mackerel Annual acoustic surveys targeting Cunene horse mackerel, Cape horse mackerel, flat sardinella (Sardinella maderensis), round sardinella (Sardinella aurita), and sardine (Sardinops sagax) are carried out in Angolan waters by the RV “Dr Fridtjof Nansen”, and they provide abundance estimates of all principal pelagic fish stocks. The acoustic method enables large volumes of water to be sampled with relatively little effort and is the generally preferred method of making direct estimates of the abundance of schooling pelagic fish (MacLennan and Simmonds, 1992). As for all methods, methodological limitations and sources of error exist (Anon., 2003, 2004), and acoustic estimates may be affected by both target availability and errors related to identification and the conversion from acoustic densities to biomass (Olsen, 1990; MacLennan and Simmonds, 1992; Freon et al, 1993a; Axelsen et al, 2004)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
