Invasive species are known to have negative impact on native species and the environment. New Zealand has numerous introduced fish species, with the brown trout being the most widespread. Trout have caused massive declines in native fish species and the most devastating effect has been on non-migratory Galaxias species.

Native fish tend to disappear when trout occupy the steam. They coexist only when trout are less than 150 mm long. 

Trout Facts

• Brown trout – from 1867 in NZ
• Fast growing active predator and competitor (30-70 cm)
• Eat native fish and invertebrates
• Compete with native fish for food
• New Zealand is well known for its trout fishery and trout are highly valued

How to Eradicate Trout

• In small streams and sanctuaries
• Use rotenone (poison toxic to fish)
• Return native fish  to the streams once rotenone levels dissipate.
• Rotenone breaks down rapidly in sunlight and air
• Does not accumulate in the sediments of water bodies
• At 15-22°C it is undetectable in three days.
• Alternative techniques: netting, electric fishing and low pH

To Eradicate or Not to Eradicate (Trout)

Pros
  • Restore habitat for native fish species
  • Control introduced species (trout)
  • Imposible in large systems
  • In small headwaters, where eradication is possible, trout is not longer then 20cm
  • Dispersal of galaxiid' juveniles downstream – food for trout
  • Higher whitebait runs
  • Currently, trout removals are done with electrofishing; trout are caught and translocated to other trout fisheries
Cons
  • Bad image: people wanting to kill trout can be perceived as “Environmental vandals” 
  • Eradication can be done not only by electrofishing, but also by poison small streams, which might be an emotional topic and would kill some trout
  • Trout is better protected than most species of native fish  
(Pham, Closs “Silently spreading death”)

Dual Conservation

The conservation of galaxiids in New Zealand emphasizes the preservation of trout fisheries. This is due to the high cultural, recreational and economic value that the introduced trout has gained over time in New Zealand. Hence the challenge is to maintain trout fisheries whilst conserving galaxiids.

The angling community has contributed much to the protection of aquatic habitats, which have had a positive impact on native fishes. However, it is insufficient for the mitigation of negative impacts of trout on galaxiids and in areas where the trout fisheries are not as significant, conservation priority should be given to the indigenous galaxiids. 

Zealandia initiatives

Picture
• ZEALANDIA Karori Sanctuary

• 2011 – first use of rotenone in flowing water in NZ

• Brown trout was eradicated from the upper lake and streams

• One year later, there was complete eradication of trout and an increase in the population of banded kōkopu (Galaxiid)

Trout Removal Experiments

  • 2008, Twizel (South Island) – 300m stream à electric fishing & barrier 
  • Above Fraser Dam – Fraser Stream spring trout removal operation à trout removal &insulating fish passage barrier
  • 2001, Motueka (South Island) – small irrigation ponds à eradication of Koi carp and Gambusia
  •  Auckland – pond – European gudgeon
  • *ZEALANDIA – small streams of Broun trout removal. First use of rotenone for trout eradication in a stream system

Research

McDowall (2006) reviewed the current state of knowledge on all Galaxioid species in his paper: 

"Crying wolf, crying foul, or crying shame: alien salmonids and a biodiversity crisis in the southern cool-temperate galaxioid fishes?"
Credits: scicomm