Striking a balance between retaining populations of protected seahorses and maintaining swimming nets

1. The fish family Syngnathidae (seahorses, pipe. sh, pipehorses and seadragons) is fully protected in New South Wales, Australia, but in some countries certain species are threatened by unsustainable collecting, capture as incidental bycatch, and habitat degradation. 2. Within Sydney Harbour, two species of seahorses (Hippocampus abdominalis and Hippocampus whitei) have been found to colonize artificial structures such as jetty pylons and protective netted swimming enclosures. These protective nets are subject to fouling from epibiotic growth (algae, ascidians, bryozoans, etc.) and rubbish, which causes the nets to collapse from the additional weight. Local authorities employ diving contractors on an ad hoc basis to remove the epibiota from nets. 3. Surveys showed a significant decline in the numbers of both seahorse species at one site following the replacement of a net, and recovery of the H. whitei population took more than 15 months. 4. A manipulative experiment tested the importance of epibiotic growth for seahorses. H. whitei, tagged with individual marks, were allocated to sections of a net that had undergone different cleaning procedures. Seahorse size, position on the net and total population abundance were recorded every 2 weeks over a 3 month period. It was demonstrated that seahorses have a signi. cant positive association with epibiotic growth and proximity to the sea floor. Seahorse populations also showed seasonal variation in abundance with increased numbers on the net during the breeding season (spring-summer). 5. This project has led to the development of best practice net cleaning procedures for local authorities in Sydney Harbour to manage growth on the nets while minimizing impacts on seahorse populations. Copyright (C) 2009 John Wiley & Sons, Ltd.