Image Salmon CSIRO CC BY 2.0
When Fish Go Feral
It might be hard to imagine how a few little escaped captive fish here and there could be a pesty problem. But indeed they are!
Firstly, it's not just a few here and there. It's a systemic problem. The Department of Primary Industries, Parks, Water and Environment (DPIPWE) has estimated that "2%-3% of stocked fish are lost owing to low level leakage in Tasmania, along with occasional large-scale (>1000 individuals) losses" .
In the Table below are some of the more recent escape events which have been reported. Mind you, these are not the only large-scale escapes. For example, following the November 2019 escape event at Macquarie Harbour listed in the Table below, there was another later reported to authorities; these involved young, small fish . However, fishermen were catching many harvest-size salmon and larger, in the range of 4-5 kilos , indicating that there had to have been one or more other sizeable escape events that were never publicly reported.
TABLE. A few of the more notable recent escape events recorded for Tasmanian salmon.
Note that the May 2018 escape of 120,000 was named by Intrafish as one of the world's largest ever farmed salmon escape events . That was before the even larger number of escapes in December 2020 .
Secondly, the salmon industry has asserted that the escaped fish wouldn't be able to find food and survive . However, that's not actually what fishermen and researchers are observing. One study looked at biochemical markers of native fauna in salmon, and found that "15% [of] Atlantic salmon had survived on a diet based on native fauna for a long period of time, as their tissues already reflected the biochemical composition of their new food sources" . This same study concluded that, "a small fraction of escapees conclusively showed changes in biochemical parameters indicative of a shift to feeding on native fauna. Given the numbers and frequency of escapes, this can have an important impact on native species and on the ecology of Macquarie Harbour".
Another study invited fishermen to report their catch and stomach contents of salmon following the abovementioned May 2018 escape event. Of the respondents who checked the stomachs of the fish they caught, 47% noted some food items present, including small baitfish, crabs, shrimp, and soft coral, with a small number of respondents reporting whitebait, juvenile mullet, flounder, toadfish, and anchovy . Also, interestingly, this same study found that fishermen catching salmon up to four months after the escape event reported that the fish were in good condition and most had prey in their stomach.
Although these are only a small number of studies, they make it very clear that it is simply untrue to say that escaped salmon do not eat native prey. Some do. And given the very large number of salmon escaping into Tasmanian coastal waters, this non-native, introduced species can wreak some real havoc.