Disturbance associated with bait-collection for sandprawns (<I>Callianassa kraussi</I>) and mudprawns (<I>Upogebia africana</I>): Long-term effects on the biota of intertidal sandflats

Abstract

The sandprawn Callianassa kraussi and the mudprawn Upogebia africana are used extensively as fish bait in southern Africa. A holistic analysis of disturbance associated with experimental prawn-collecting was undertaken to determine its repercussions upon the sediment and associated macrofaunal, meiofaunal, microbial and microalgal communities. Patterns of recovery were examined for 18 months following the disturbance. The recovery of both C. kraussi and U. africana was far more protracted than predicted, taking 18 months for completion. Sedimentary compaction, associated with the removal of prawns, could account for these prolonged recoveries. Both C. kraussi and U. africana suffered greater depressions of population densities (ca. 70%) than would have been expected from the proportions removed (ca. 10% and 46% respectively). This suggests that disturbance and sedimentary compaction have greater effects than the removal of sand- and mudprawnsper se. One month after the disturbance of C. kraussi, chlorophyll levels increased above control levels and remained elevated for a further 2-3 months. In contrast, the removal of U. africana resulted in nett decreases in chlorophyll levels for approximately one month following the disturbance. A short-lived decline in bacterial numbers was apparent following the removal of C. kraussi but not U. afiicana. Meiofaunal numbers declined immediately after disturbance of both C. kraussi and U. aficana, but this depression was followed by explosive increases and then a return to control levels. The macrofauna was slower to recover and, after initial reductions of numbers, biomass and species richness, still showed signs of depression 18 months after the disturbance. Three response patterns were apparent: species which were immediately reduced by the treatments and were also slow to recover; species which appeared to have their recruitment suppressed relative to the control; and species which were unaffected by the treatment. Only a single macrofaunal species, the hermit crab Diogenes brevirostris, benefited from the disturbance. Similar trends were observed following the harvesting of both C. kraussi and U. africana.