Genetic investigation of the large-scale escape of a tropical fish, barramundi Lates calcarifer, from a sea-cage facility in northern Australia

Expansion of sea-cage aquaculture has increased incidences of large-scale escapes of farmed fish. Whilst escape events are relatively well-characterised for temperate fish species, such as salmonids, there are no studies that report the occurrence and persistence of farmed tropical fish escapees. Given the rapid rise in tropical finfish aquaculture, there is a need to understand the persistence of escaped farmed tropical species so that informed risk assessments in the planning of future sea-cage aquaculture developments can be made. In 2011, a barramundi Lates calcarifer sea-cage facility in Hinchinbrook Channel, Queensland, was destroyed by a tropical cyclone, resulting in the escape of similar to 280 t of >= 500 mm fish into the surrounding wild population. Between 12 and 18 mo after the escape event, 403 genetic samples were collected from barramundi caught within Hinchinbrook Channel. DNA parentage analyses were used to assign sampled fish as either 'farm escapees' or 'wild' by matching microsatellite genotypes to the hatchery broodstock used to produce seedstock for the farm. Following parentage analysis, fish identified as escapees comprised 31% of the sampled barramundi population. A single male-female broodstock pairing accounted for 32% of the escaped fish identified. The allelic richness of escapees was 56% of that observed in wild barramundi, while the average pairwise relatedness of escapees was 0.164 (+/- 0.003) versus 0.002 (+/- 0.001) within wild fish. This study is the first to show that farm barramundi can survive and integrate into natural populations in large numbers up to at least 18 mo after they escape. Due to the large proportion of escapee fish currently in the Hinchinbrook Channel population and their high levels of relatedness, longer-term concerns are raised over potential accumulation of inbreeding effects. This makes ongoing monitoring and documentation of impacts critical to quantify the true effects these large escape events in this tropical fish have on wild populations