Understanding the movement patterns of breamRyuji Sakabe and Jeremy Lyle
Over the past decade the sporting qualities of black bream have been recognised increasingly amongst Tasmanian anglers, as has the overall quality of the Tasmanian fishery. Unlike mainland states, the commercial sale of bream is prohibited in Tasmania, effectively achieving recreational-only status for the species and avoiding many of the problems of overfishing experienced elsewhere.
In this article PhD student Ryuji Sakabe and Dr Jeremy Lyle of the Tasmanian Aquaculture and Fisheries Institute (TAFI) report initial findings of a research project tracking movements of black bream in the Little Swanport Estuary, a popular bream fishing area. This project is part of a wider study of bream ecology and was supported through funding provided by the Fishwise Community Grants scheme.
The Little Swanport Estuary occupies an area of approximately 6.5 km2 and is located on the western side of Great Oyster Bay. The estuary is relatively shallow ranging from approximately 1 to 9 m deep at high tide, has a narrow entrance of approximately 30 m wide and 5 m deep, and a long channel, with numerous shoals and irregular shape (refer map next page).
During the winter of 2005 thirty-five bream, ranging from 272 to 430 mm fork length, were tagged and released in the Little Swanport Estuary. An acoustic pinger tag, about the size of a cigarette butt, was surgically implanted into the gut cavity of the fish, which were then tracked over a period of up to seven months using moored hydrophone receivers. Ten of these acoustic receivers were deployed throughout the estuary, from the Little Swanport Bridge to the estuary entrance (see map), continuously recording each time an individual tagged fish passed within approximately 500 m of a receiver. In addition, environmental data loggers and regular sampling provided information about water quality, in particular salinity and temperature.
Unfortunately, the acoustic tag inserted into the largest bream (430 mm) malfunctioned and the two upstream hydrophone receivers (B01 and B02) flooded, resulting no movement information from the upper reaches of the estuary. The remaining 34 acoustic tags and 8 hydrophone receivers worked well and have provided an extensive dataset of bream movement data that can now be correlated with environmental and biological (spawning) parameters.
According to the results, few tagged fish were detected at the estuary entrance and when they were they were typically redetected within a short period further up into the estuary, providing no evidence to indicate that bream left the estuary within the study period. Most of the tagged fish showed localised movements within the middle reaches of the estuary, the most conspicuous pattern being influenced by the tidal cycle. As indicated in the graph that relates tidal cycle (tide height) and location within the estuary (receiver number) (see graph), bream regularly moved upstream during the flood tide (to the vicinity of receiver B03) and then returned downstream with the ebb tide (typically to either receivers B08 or B09). During the experimental period several flood events were experienced which resulted in a dramatic lowering of the salinity throughout the estuary. Under such circumstances bream were observed to move downstream to the lower reaches of the estuary, remaining there until salinities started to increase to more normal levels.
Spawning in bream occurs from late August to mid-January and in Little Swanport spawning fish are known to aggregate in the upper estuary, in water that is brackish. As receiver B03 was positioned where the Little Swanport River entered the estuary, any fish moving further upstream would have been detected at this point. Assuming that movements upstream of B03 during the spawning season were primarily related to spawning, it was evident that individual fish probably spent up to two weeks on the spawning grounds before moving back into the estuary proper. It was also apparent that individual fish moved on and off the spawning beds several times within the spawning season. However, during the study period, there was constant rainfall around the Little Swanport Estuary and the level of freshwater input was much higher than usual. The combination of upstream and downstream movement for spawning and in response to freshwater discharge may have influenced the observed behaviour such that in more typical spawning season bream may tend to remain on the spawning grounds for longer periods.
There is a considerable amount of work still to be done in order to fully analyse our dataset but preliminary results suggest little or no dispersal of adult fish out of the estuary; a strong correlation between tidal cycle and movement patterns, the relationships between salinity and bream distribution; and existence of small-scale spawning migrations. In many respects this study will have confirmed what experienced bream fishers understand about bream behaviour but it also serves to highlight some vulnerabilities about bream stocks. If, as our findings suggest, there is little or no mixing of bream populations between estuaries, localised stock depletions are possible, especially in some of the smaller systems. Furthermore, aggregating behaviour during the spawning season has the potential to attract increased fishing activity, further pressuring the stocks. In order to maintain the quality of the bream fishery we need to better understand the species ecology and nature of the fishery. Current research at TAFI should go someway to achieving this outcome.