Demographics and vulnerability of a unique Australian fish, the weedy seadragon Phyllopteryx taeniolatus

The weedy seadragon Phyllopteryx taeniolatus is a vulnerable and endemic Australian fish and also an icon and flagship species for marine conservation. However, little is known about its population dynamics, which hinders the establishment of conservation policies. We have previously demonstrated seadragons to be highly site-attached, so we estimated population densities, growth and survival of weedy seadragons using mark-recapture techniques at 5 sites in New South Wales (NSW, 34 degrees S) and Tasmania (TAS, 43 degrees S), near the northern and southeastern limit of distribution for the species, over a 7 yr period. Population densities varied from ca. 10 to 70 seadragons ha(-1) depending on site and year. There was a significant decline in the number of weedy seadragon sightings per unit area searched in 2 out of 3 study sites near Sydney, NSW, from 2001 to 2007. There was also a decline at one of the 2 sites surveyed in the lower Derwent Estuary, TAS, in 2009 compared to 2003 and 2004. Survival rates at NSW sites ranged from 0.62 to 0.65 yr(-1) and were higher at TAS sites where they ranged from 0.71 to 0.77 yr(-1). Birth occurred approximately 3 mo later and seadragons exhibited significant slower growth in TAS (maximum adult size x growth rate parameter, L-infinity x k = 31.02) compared to NSW (L-infinity x k = 55.15). This study is the first population assessment of seadragons over ecologically relevant spatial and temporal scales, and shows differences in the dynamics of populations at different latitudes. It also shows declines in some populations at widely separated sites. Determining whether these declines are natural interannual fluctuations or whether they are caused by environmental or habitat changes must be a priority for conservation.