Regulated tidal exchange
Guadalquivir Estuary (Isla Mayor near Sevilla)
This is a 3,200ha fish farm and a fundamental part of the much larger Andalusian National Park. It stocks sea bass, bream, mullet etc. and is a waterbird refuge (60-70% of birds in the wider Doñana area use the farm). The functional heart of the scheme is a pumping station located at its centre. Tidal water reaches it via a straight 8.5km (5 mile) channel from the Guadalquivir and the pumps transfer around 1 million m3 into the site while raising its elevation by 2m. The water then disperses gravitationally throughout the site and back out into the estuary (with an improved quality). In transit through 300km of canal it enters 130 small semi-extensive pools and then 45 main large (70ha) extensive pools. As the site is within the National Park, the owners (Pesquerías Isla Mayor, S.A.) must carefully manage it (e.g. fish stocked at low densities, limits on catch sizes, water maintained at good quality). This inherently limits the profits that can be made. Water quality monitoring is important and there is good understanding about the nutrient cycling processes taking place. Artificial food is provided to fish in the semi-extensive pools only. The system has ecosystem/management resilience and is tolerant of rains, droughts, pollution events and economic crises. Ecological processes such as flamingo feeding act to control eutrophication and the site can close down and operate on recycled water only if required. To cope with siltation, every 5-6 years the extensive ponds are dredged (on a rotational basis) with the sediment being used to create topographically heterogeneous islands.