Variable spatial structure of schooling pelagic fish off Namibia: implications for acoustic surveys

  • JC Coetzee Marine and Coastal Management, Private Bag X2, Rogge Bay, 8012 Cape Town, South Africa
  • OA Misund Institute of Marine Research, P.O. Box 1870 Nordnes N-5024 Bergen, Norway
  • G Oechslin Formerly National Marine Information and Research Centre, Ministry of Fisheries and Marine Resources, P. O. Box 912, Swakopmund, Namibia
Keywords: abundance estimates, acoustics, geostatistics, spatial distribution


Schools of horse mackerel Trachurus trachurus capensis, sardine Sardinops sagax and round herring Etrumeus whiteheadi were surveyed by conventional echo-integration along systematic transects. The transects were surveyed twice, in opposite directions. Clupeoid biomass was concentrated in a few dense aggregations, whereas horse mackerel were less dense and distributed over larger areas. The influence of aggregation patterns of the different species on the precision of the acoustic estimates was analysed with respect to spatial variability and diurnal effects. Isotropic variograms computed from values of acoustic back-scattering strength showed little or no structure for all three species. Indicator variograms showed relatively large structures (>15 miles) for all three species at the lowest cut-off level, confirming the influence of high values on the variograms. The autocorrelation range decreased with an increase in cut-off value for horse mackerel, whereas variograms of sardine and round herring showed no structure at larger cut-off values. There were large variances in estimated biomass for sardine (77%) and round herring (90%) between surveys, whereas biomass estimates of horse mackerel were relatively similar between surveys.

Keywords: abundance estimates, acoustics, geostatistics, spatial distribution

African Journal of Marine Science 2001, 23: 99–109

Journal Identifiers

eISSN: 1814-2338
print ISSN: 1814-232X