Effects of demersal trawling along the west coast of southern Africa: Multivariate analysis of benthic assemblages

  • Atkinson L
  • Field J
  • Hutchings L
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Abstract

This study is the first to examine the benthic impacts of the otter-trawl fishery on hake in the southern Benguela upwelling region. Infauna were sampled at 4 sites, from southern Namibia to Cape Town by means of 5 replicate grab samples at each of 2 trawling treatments (heavily and lightly trawled areas), paired at each site. The large invertebrate epifauna was also sampled at 2 of these sites using a fine-meshed otter trawl. Sites ranged in depth from 350 to 450 m. Environmental attributes (sediment particle size, total organic carbon, depth, salinity, temperature and dissolved O2 concentra- tion) were examined along with faunal assemblage composition. Vertical profiles of water mass char- acteristics showed little long-shore variation, apart from slightly lower O2 concentrations in the north. Difficulties of pseudo-replication in benthic impact studies are discussed, and methods for circumvent- ing these suggested. There were significant differences in sediment characteristics among the 4 sites, but only 2 sites showed different sediment characteristics between trawling treatments. Studies of spe- cies richness, evenness and numbers of infaunal individuals showed little difference between trawling treatments at 3 sites and species diversity was similar between treatments at all 4 sites. Multivariate analyses show marked differences in both infaunal and epifaunal assemblages among the sites and between trawling treatments at all sites. The analyses suggest that differences in trawling intensity are at least partially responsible for significant variation in benthic assemblage composition between heavily and lightly trawled areas. These findings contrast to those in shallower waters in the northern hemisphere, where infauna are more sensitive to trawling than epifauna. This study shows that epifau- nal abundances, number of species and species diversity decrease with increasing trawling intensity, and that there are also considerable changes in epifaunal assemblages in more heavily trawled sites.

Author-supplied keywords

  • Benguela
  • Benthic assemblages
  • Benthic biota
  • Demersal trawl fishing
  • Epifauna
  • Fishing impacts
  • Infauna
  • Pseudo-replication

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Authors

  • L. J. Atkinson

  • J. G. Field

  • L. Hutchings

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