Ecology and life history characteristics of black bream, Acanthopagrus butcheri, in Tasmanian estuarine ecosystems

Estuaries are characterised by high biological productivity associated with relatively variable environmental conditions. Estuarine dependent fish fauna must cope with such variability and typically exhibit a range of life history adaptations in conjunction with migratory capabilities, to make use of a range of habitats throughout their life time. Their utilisation of these habitats is known to vary at a number of temporal scales, including tide, diurnal, seasonal and life history stage. Studies of the interrelationships between estuarine dependent fauna and environmental conditions have attracted much attention because of the physiological and/or behavioral adaptations to the unstable environment. The black bream, Acanthopagrus butcheri, is an estuarine resident species that completes its whole life cycle within an estuary. It occurs extensively in temperate estuarine waters in southern Australia. This species is the subject of major recreational and commercial fisheries throughout its distributional range and has been the subject of several studies on mainland Australia, though very little work has been conducted in Tasmania. Black bream is known to exhibit significant variability between populations in some key biological and ecological characteristics (i.e. growth, size at maturity and spawning period), though relationships between life history stages, habitat utilisation and environmental conditions remain poorly understood. This information is required to guide conservation and management strategies. Using black bream as an example of estuarine resident species, this study describes key biological and ecological characteristics possessed by the species to successfully inhabit the unstable estuarine environment. Biological and ecological characteristic of black bream, including distribution and movement patterns within an estuary, age and growth, aspects of reproductive biology such as age and length at maturity, spawning period, spawning locations, and feeding habits were studied in the Little Swanport Estuary and Swan River, east coast of Tasmania. The study involved extensive field sampling including collection of fish specimens and measurements of physical environmental parameters between April 2004 and April 2006, and fish tracking study using acoustic telemetry in the Little Swanport Estuary from August 2005 and January 2006. The distribution patterns of adult black bream varied between the Little Swanport Estuary and Swan River. Adults occurred mainly within the middle estuary, moving into the upper estuary of the Little Swanport Estuary during the spawning season. By contrast, fish were largely distributed within the upper estuary of the Swan River throughout the year. This difference was probably due to more suitable habitats (i.e. submerged trees) and higher food availability in the upper estuary of the Swan River than in that part of the Little Swanport Estuary. Juveniles appeared largely restricted to the upper estuary, but as they grew they became widely distributed within the estuary. Black bream in these Tasmanian estuaries had a long life span of up to 30 years, with slow growth rate. Females grew larger than males. Based on gonadosomatic index and back-calculated birth dates, spawning occurred from early October to early January with a peak in November-December. This study indicated that spawning was strongly influenced by the environmental conditions, especially salinity on the spawning ground. Successful spawning probably required salinities above approximately 10%0 and flood events during the spawning season negatively influenced spawning success. The ability to tolerate a wide range of salinities, a prolonged spawning season and long life span are the key strategies that have enabled the species to adapt successfully to this highly variable environment. Black bream fed on a variety of prey including molluscs, crustaceans, polychaetes, teleosts and insects; they also consumed considerable volumes of plant materials such as seagrass and algae. The diel feeding activity pattern of this species reached a peak prior to sunset with constant low values of gut contents from early morning to late afternoon. No substantial seasonal variation in the composition of the diets was observed, but plant material was consumed more during winter when food availability was lower. The diet of this species showed clear ontogenetic changes with increasing size, which would help to reduce the potential for intraspecific competition. A study of the movements of individual fish using acoustic telemetry conducted in the Little Swanport Estuary demonstrated that adult black bream mainly utilised the upper and middle estuary regions, and showed that an upstream migration occurred from early August to middle January with a peak in November-December. There was no firm evidence that tagged fish moved out of the estuary, even during the periods of heavy freshwater discharge. However, during excessive freshwater inflows (flood events), fish moved or were washed away from the upper estuary region, and they remained the middle estuary region until water conditions in the upper estuary became favorable. Clearly, freshwater inflow was one of most important physical factors influencing movement and distribution of this species within the estuary. This study has improved understanding of the key biological and ecological characteristics of black bream that enable them to successfully live in estuarine environments and knowledge of the interrelationship between the species and its environment by adding data on its performance at the southern-most extent of its range. This study has also provided scientific information to support the recreational fishery management of black bream in Tasmania