The ecological effects of collecting Callianassa kraussi Stebbing and Upogebia africana (Ortmann) for bait : impacts on the biota of an intertidal sandflat
The ecological effects of collecting the sandprawn Callianassa kraussi and the mudprawn Upogebia africana for bait were assessed at Langebaan Lagoon, South Africa, through surveys, observations and long-term experimental analyses. While only a small proportion of the prawns is removed from the lagoon annually, amounting to no more than 0.01 for the total lagoon, the physical disturbance inflicted by collecting is likely to be more detrimental and longer-lived than the actual removal of the prawns. Bait-collecting activities are concentrated on the centre sandbanks where some 400 000 prawns are removed annually - less than 5 of the total prawn population on these sandbanks. In terms of numbers removed, bait-collecting is thus no threat to the prawn populations at Langebaan Lagoon although there are signs that sandprawn densities and modal body sizes have been reduced in areas of intense bait-collecting activities. Massive quantities of sediment are disturbed through bait-collecting activities and this, inadvertently, results in the disturbance of other components of the biota. It is estimated that approximately 543 g of macrofauna are disturbed with each bag limit of prawns (50) removed, amounting to approximately 10 000 kg of macrofauna being disturbed annually. While the fate of this macrofauna is not completely determined, approximately 80 is subsequently preyed upon by scavenging gulls. The long-term effects of high-intensity disturbance on the biota of a sandflat were assessed through simulated bait-collecting activities, viz "digging" and "sucking" for C. kraussi and U. africana. The recovery of both C. kraussi and U. africana was far more protracted than predicted - probably being completed only 18 months after the initial disturbance. Meiofaunal and macrofaunal numbers decreased immediately following the initial disturbance.
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