Pufferfish species, in Family Tetraodontidae, are commonly found in all estuaries around the Australian coast throughout the full range of estuarine salinities, with some species extending into freshwaters. The two species use in this study are the banded puffer Marilyna pleurosticta (Gunther, 1872) and the common (or Hamilton‟s) puffer Tetractenos hamiltonii (Richardson, 1846). Both species are commonly found in the subtropical estuaries of Queensland and northern New South Wales. This study aims to provide information on aspects of the ecology of these two species that are relevant to their suitability as biomonitors of metals and pesticides pollution, such as longevity and growth, movement patterns, reproductive biology, diet and feeding habits.
The main study location was a tributary of the Richmond River estuary, NSW. Further samples were collected from the Brisbane River and Sandon River estuaries to provide a range of pollution levels from near-pristine to highly pollute.
This study found that banded pufferfish M. pleurosticta and common pufferfish T. hamiltonii were long-lived species that have slow growth rates. Fish of both species can live up to 30 years and reach lengths of with the total length of 13-15 cm. The estimated von Bertalanffy growth parameters for M. pleurosticta were; Lx= 12.70, K = 0.12. The estimated von Bertalanffy growth parameters for T. hamiltonii were; Lx= 14.80, K = 0.08. Attempts to validate the periodicity of growth checks in the otolith were inconclusive although limited edge increment analysis and comparisons with growth checks in toothplates and tag recaptures support the assumption of annual periodicity. Erosion of toothplates during feeding on hard-shelled organisms render them unsuitable for absolute ageing of the fish but they may still contain a microchemical record of environmental conditions for up to 10 years.
This is the first study to provide real-time tracking of pufferfish movements and provides firm evidence for short-term (daily) site fidelity for nearly one month and for medium-term (monthly) site fidelity over six months. The tagging studies with Acoustic transmitter tags and PIT-tags showed that M. pleurosticta and T. hamiltonii probably leave the same series of tidal channels only in spawning seasons and return to the same small area daily for feeding.
Both M. pleurosticta and T. hamiltonii had the same list of prey items. They eat gastropod molluscs such as Tatea sp., Salinator fragilis and S. solida, and crustaceans such as Sesarma erythrodactyla and Metapenaus sp. But they show differences in the habitats they use and their selectivity of prey, which may reduce competition between these two species. There is seasonal variation in feeding rates and species composition, with both species having greatest feeding success in warmer water. Feeding success also showed significant correlations with water salinity, and tide range. However, their reproductive stage does not significantly alter their rates of feeding.
The two species have clearly different spawning seasons. M. pleurosticta spawned in spring while T. hamiltonii spawned in winter and there is circumstantial evidence that M. pleurosticta undergoes migration from the mangrove habitat to spawn. High values for HSI and its strong inverse relationship with GSI demonstrates that energy reserved in liver plays an important role in gonad maturation and spawning process of the fish. Body condition factor varies little throughout the year and seems to play a lesser role in providing energy for reproduction.
This study shows that liver tissue is an unreliable indicator of metal concentrations in the environment. Gonads accumulated higher levels of many metals than other tissues but were variable depending on the reproductive state of the fish. Muscle tissue was less variable and several metals show strong relationships with environmental levels of some metals. Arsenic and mercury in pufferfish muscle show strong relationships with fish size regardless of environmental concentrations. The occurrence of dieldrin in the livers of fish from the Richmond estuary demonstrates the potential of pufferfish as biomonitors of this persistent organic chemical. However, the comparisons between metal and pesticide pollutants in fish tissue and the surrounding environment from different estuaries are confounded by significant differences in fish size and species composition between estuaries.
Finally, this study also showed that both M. pleurosticta and T. hamiltonii met some of the criteria as biomonitors of pollution such as being long lived, having high site fidelity and prey on resident invertebrates that are likely to reflect local pollutant conditions; however care should be taken in choice of sampling location, species, seasonality, and accounting for environmental conditions that may affect feeding success. Monitoring programs should also be mindful of depleting local populations by regular intensive sampling.