Benthic dynamics within a land-based semi-intensive aquaculture fish farm: The importance of settlement ponds

The present work aims to assess the importance of settlement ponds (SP) in semi-intensive fish farms by studying benthic dynamics in an aquaculture fish farm, more specifically in the water reservoir (WR) and SP and also in production (P) and nonproduction (C) ponds during a 16-month period. In Portugal, a SP is only mandatory for intensive fish farms, and another objective of the present study is to assess the importance of these areas in semi-intensive fish farms. The WR was the area with highest diversity and evenness, as well as the higher number of exclusive taxa and taxa sensitive to organic enrichment. P and SP samples showed signs of higher disturbance levels, emphasized namely by the association of the opportunistic annelids Capitella spp. and Tubificidae. However, the benthic data from SP points to lower disturbance levels than P both due to an increase in the percentage of sensitive taxa observed in June and October 2004 and by the association of this latter sample with water reservoir samples as evidenced by canonical correspondence analysis. Moreover, a higher and increasing number of taxa when compared with the P area were also observed. Therefore, in semi-intensive fish farms, where effluents from P ponds are directly discharged to the lagoon, the potential environmental impacts would be more severe. In conclusion, the imposition of SP in semi-intensive fish farms should be considered, especially because most fish farms are located within relevant wetland areas.