Monitoring fish populations

Sea-Scope has been involved with devising and executing an annual census of reef fishes on a discrete shallow water (8-12 m deep) reef in Cardigan Bay, Wales. This monitoring study has been running since 2004 on behalf of Natural Resources Wales (formerly the Countryside Council for Wales).

The seaweed-covered bedrock outcrop, known as Holden’s Reef, is barely 50 m in diameter and is surrounded by an extensive area of flat sand. The diversity and abundance of reef fishes on the reef have been obtained by getting pairs of divers to sit back-to-back and to note down all of the fishes which swam within a 3 m arc and within their 180° fields of vision within given time periods.

The most frequently recorded resident species of fish on the reef (by an order of magnitude) was the goldsinny wrasse Ctenolabrus rupestris. Numbers of this species remained consistent throughout the monitoring period (2004-2011). Resident species which have undergone a noticeable decline include the tompot blenny Parablennius gattorugine; whilst amongst the migratory species, numbers of poor cod Trisopterus minutus have declined markedly and lesser-spotted dogfish Scyliorhinus canicula were only recorded in 2004 but not since.