Intensive netting suppresses invasive fish, improves numbers of NZ species
Friday, 7 June 2019
Relatively little is known about how to successfully control or eradicate introduced fishes in New Zealand fresh waters.
Invasive fish don't get anything like the attention, money or scientific scrutiny as this country's introduced land pests.
But 'as the extent and severity of introduced fish impacts becomes increasingly apparent, so too has the interest of waterway managers and stakeholders in controlling or eradicating introduced species,' wrote Amber McEwan and Philippa Crisp recently in the journal Ecological Restoration.
They were describing a trial that tested whether repeated netting could suppress invasive fish numbers and improve native New Zealand fish abundance.
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After two years of work, they found that 'repeated netting was an effective means of suppressing introduced fish biomass and could potentially also result in eradication if the lagoon was closed off to immigration from the adjacent lake'.
They also found that repeated netting 'increase[d] numbers of small indigenous fishes.'
Repeated netting works, at least in some circumstances, McEwan said in an interview.
She acknowledged that many recreational fishermen don't want to suppress, let alone eradicate, introduced species such as brown trout.
McEwan – principal scientist at consulting firm Riverscapes Freshwater Ecology and a PhD student at Victoria University – said there were places where native fish should be preferred over introduced ones.
'In many locations, such as those where angler interest is low and the potential for ecological restoration is high, mutually agreeable resolutions between freshwater management agencies may be possible,' she wrote with Crisp, who works at the Greater Wellington Regional Council.
The trial occurred at Barton's Lagoon, a shallow 7.5 hectare lake connected to the north end of Lake Wairarapa and the wider Wairarapa Moana wetland complex.
The lagoon hosted invasive species such as brown trout, perch and rudd, as well as native species such as longfin and shortfin eels, black flounder, common bullies, īnanga, common smelt, brown mudfish, and freshwater crayfish or kōura.
McEwan and colleagues intensively caught exotics using nets designed to capture adult fish. Nets were set in the evening and retrieved at first light.
In 2014, 498 fish (77 trout, 186 perch, 235 rudd) were removed in six netting nights. In 2015, 196 fish (15 trout, 87 perch, 94 rudd) were removed in 10 netting nights.
'Overall, the control trials showed that repeated netting was successful at suppressing introduced fish biomass in Barton's Lagoon,' they reported.
Meanwhile they also captured native species, using nets and traps.
'In total, 4508 native fish and 177 decapod crustaceans were captured over the course of the trials.'
Īnanga, common bullies and decapod shrimp catch abundance increased by 845 per cent, 534 per cent, and 144 per cent respectively over the trial period. No obvious difference in catch rate was recorded for common smelt or kōura, and shortfin and longfin eel catch rates decreased by 93 per cent and 88 per cent respectively,' they found.
McEwan said eel numbers may have been skewed by a flood just before the first native survey. Eels use floods to find new food resources or begin spawning migrations.
Repeated netting could be seen as relatively expensive, but this study suggested perch migrated back into the lagoon in spring to spawn. If so, then netting could efficiently target that migration and take fewer resources.
A barrier could also be built to prevent migration, McEwan said.
An analysis of the stomach contents of trout and perch showed they were feeding on native species.