AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. (2009) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.1066 Striking a balance between retaining populations of protected seahorses and maintaining swimming nets DAVID HARASTI a,Ã , TIM M. GLASBY a and KEITH M. MARTIN-SMITH b a NSW Department of Primary Industries, Locked Bag 1, Nelson Bay, NSW 2315, Australia b Australian Antarctic Division, 203 Channel Highway, Kingston, TAS 7050, Australia ABSTRACT 1. The fish family Syngnathidae (seahorses, pipefish, 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 significant 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 r 2009 John Wiley & Sons, Ltd. Received 22 January 2009; Revised 11 June 2009; Accepted 8 July 2009 KEY WORDS: artificial structures; habitat; seahorses; epibiota; Hippocampus abdominalis; Hippocampus whitei; Sydney INTRODUCTION Estuarine ecosystems around the world are threatened by increasing coastal development and declining water quality (Weinstein, 2008). Within many Australian estuaries and harbours there are various types of artificial structures (such as wharfs, pylons, sea walls, pontoons) that provide important habitat for various marine species. These structures provide unique marine habitats (Glasby and Connell, 1999; Connell, 2001; Holloway and Connell, 2002) and are known to be utilized by a variety of fish (Coleman and Connell, 2001; Brickhill et al., 2005; Clynick, 2008) and molluscs (Cole et al., 2005; Blockley, 2007). Seahorses have been found to utilize artificial structures as habitat, with species such as Hippocampus abdominalis, H. reidi and H. subelongatus recorded on pilings and piers (Kvarnemo et al., 2000; Pogonoski et al., 2002; Martin-Smith and Vincent, 2005; Rosa et al., 2007), H. abdominalis and H. kuda being recorded on fish cages (Marshall, 2004; Choo and Liew, 2006), and H. whitei on protective swimming nets (Pogonoski et al., 2002; Clynick, 2008). All syngnathids are legislated as protected species in New South Wales (NSW) under the Fisheries Management Act (1994). The White’s seahorse, H. whitei, is one of the most common seahorse species in shallow estuarine waters of NSW (Kuiter, 2000; Pogonoski et al., 2002). The pot belly seahorse, *Correspondence to: D. Harasti, NSW Marine Parks Authority, Locked Bag 800, Nelson Bay, NSW 2315, Australia. E-mail: david.harasti@ environment.nsw.gov.au Copyright r 2009 John Wiley & Sons, Ltd.