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Standardised survey procedures for monitoring handfish populations in the Derwent Estuary
The spotted handfish (Brachionichthys hirsutus) is a small benthic fish endemic to the lower Derwent Estuary, Frederick Henry Bay, northern Storm Bay and the D’Entrecasteaux Channel in south eastern Tasmania (Bruce, Green and Last 1998). It is considered to be vulnerable to extinction due to its highly restricted and patchy distribution, low population density, limited dispersal capabilities and a reproductive strategy of producing low numbers of demersal eggs that are highly susceptible to disturbance.
The recovery program for Spotted Handfish started in 1996 with an initial survey of locations in Frederick Henry Bay, D’Entrecasteaux Channel and the Derwent Estuary (Barrett et al 1996). Only six specimens of handfish were located during this survey, but the locations where they were found were targeted later to try and locate breeding colonies and collect adult fish to develop captive breeding protocols (Bruce et al 1997). The first Recovery Plan was written for the years 1999-2001 (Bruce and Green 1998) and the recovery program has progressed intermittently since then.
Spotted handfish occur primarily on unconsolidated substrate ranging from well sorted coarse sand and shell grit, to areas of fine sand and silt. They have been recorded from depths between 2–30 m but appear to be most common in 5–10 m. Spotted handfish spawn during September and October, females laying eggs on small, vertical, semi-rigid structures. The stalked ascidian, Sycozoa sp., provides the primary spawning substrate within the Derwent Estuary, although spawning around seagrasses, sponges, small seaweeds in the genus Caulerpa and polychaete worm tubes has been recorded in Frederick Henry Bay and other locations. Reduction in available spawning substrate, egg loss and a general decline in habitat quality caused by introduced marine species or urban, rural and industrial development of the Derwent system may be factors which negatively impact on the species. Due to their highly restricted dispersal capabilities, the ability of spotted handfish to recolonise areas from which they have been displaced is considered to be low.
The introduction of artificial spawning substrate has been trialed at two sites in the Derwent estuary after laboratory trials showed that spotted handfish would use plastic rods as spawning substrate (Green and Bruce 2000). These rods were buried in the sand to form vertical structures on the seafloor that spotted handfish may use as alternative spawning substrate to stalked ascidians (Green and Bruce 2001). The success of artificial spawning substrate in captive breeding trials implies that artificial structures may be used to augment depleted or low quality substrate in the wild. Artificial spawning substrate will be less susceptible to destruction and subsequent egg loss.
Ongoing monitoring of handfish populations and installation of artificial spawning substrates to augment available natural spawning substrates are currently believed to be the most appropriate actions. This manual describes the methods recommended for suitably skilled community divers to do these.
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