This study determined how Black Bream used the fish gate on the Vasse Surge Barrier by tagging them state of the art internal electronic tags. The tags, known as PIT tags, detected fish that passaged upstream and downstream through the fish gate over an 18 month period in 2017 and 2018. The local community helped us tag and release 322 Black Bream.
The study revealed that movements through the fish gate were unrelated to spawning activity of this species and supported early work that the Vasse Estuary is not a key breeding site; instead they use the Deadwater to reproduce. Up until May 2018, Bream passaged through the fish gate 440 times (265 downstream and 175 upstream). Black Bream preferred to passage when the water velocity in the fish gate chute were lowest, which occurred when the water levels upstream and downstream of the surge barrier were relatively similar. When the dissolved oxygen upstream of the surge barrier was good, fewer fish passaged downstream to the Wonnerup Inlet although this was a relatively weak effect. However, the dissolved oxygen around the Vasse Suge Barrier during the current analysis period was relatively high compared to other years and therefore we anticipate that the effect of low dissolved oxygen on fish passage would be even stronger in those years. They also preferred to pass downstream through the fish gate during the evening, whereas upstream passages mostly occurred during the dawn and dusk periods.
The findings greatly increase our understanding of the conditions that Black Bream require or prefer to use the fish gate on the Vasse Surge Barrier. However, as the PIT tags last for 20 years, additional data analysis is recommended to compare the factors influencing the passage of the species over multiple years of fish gate operation; including those years that experience poor oxygen levels. It is also recommended that additional fish PIT tagging occur, including other species, so that long-term fish passage through the structure may be further quantified
