• elastomer;
  • growth;
  • life span;
  • monogamy;
  • programme MARK;
  • Syngnathidae

The aim of this study was to collect data on population dynamics and life history for White's seahorse Hippocampus whitei, a geographically restricted species that is listed as data deficient under the IUCN Red List. Data from H. whitei populations were collected from two regions, Port Stephens (north) and Sydney Harbour (south) in New South Wales, Australia, covering most of the known range of H. whitei, from 2005 to 2010. Over 1000 individuals were tagged using fluorescent elastomer and on subsequent recaptures were re-measured for growth data that were used in a forced Gulland–Holt plot to develop growth parameters for use in a specialized von Bertalanffy growth-function model. Growth parameters for Port Stephens were: females L = 149·2 mm and K = 2·034 per year and males L = 147·9 mm and K = 2·520 per year compared with estimates from Sydney Harbour: females L = 139·8 mm and K = 1·285 per year and males L = 141·6 mm and K = 1·223 per year. Whilst there was no significant difference in growth between sexes for each region, H. whitei in Port Stephens grew significantly quicker and larger and matured and reproduced at a younger age than those from Sydney Harbour. The life span of H. whitei is at least 5 years in the wild with six individuals recorded reaching this age. Data collected on breeding pairs found that H. whitei displays life-long monogamy with three pairs observed remaining pair bonded over three consecutive breeding years. Baseline population densities were derived for two Port Stephens' sites (0·035 and 0·110 m−2) and for Manly in Sydney Harbour (1·050 m−2). Even though the life-history parameters of H. whitei suggest it may be reasonably resilient, precaution should be taken in its future management as a result of its limited geographical distribution and increasing pressures from anthropogenic sources on its habitats.