Catfish are significant invaders that are widespread in the North Island of New Zealand (Invasive fish management handbook). The North American brown bullhead catfish (Ameiurus nebulosus) was introduced to NZ in 1877 and is recognised internationally as an invasive species. This catfish species is commonly 200–300 mm in length in New Zealand but can grow to 480 mm and more than 2 kg in weight (McDowall 1990). Catfish in NZ present potential adverse ecological impacts on lakes due to their benthic feeding, which can add to nutrients and sediment in the water column. They also prey on and compete with native freshwater crayfish (kōura, Paranephrops planifrons) and other fish (Francis 2019).
Catfish have most likely been in Lake Rotoiti for more than 20 years because in 1995 a juvenile catfish was observed to fall out of a hollow-framed boat trailer after a boat launching. This boat had been parked on its trailer overnight in Lake Taupo at Motuapa, where catfish are abundant, some hours before the boat was driven to and launched in Lake Rotoiti. There were no further confirmed sightings of catfish in Lake Rotoiti until January 2009 when a dead adult catfish 450-500 mm long was found on the shore (Blair and Hicks 2009). In March 2016, catfish were caught first by a weed harvester and then by fyke netting in Te Weta Bay, Lake Rotoiti (Hicks and Allan 2019).
Warm water temperatures in early summer 2017 probably allowed a rapid expansion of catfish abundance through a very successful breeding season (Below); Hicks and Allan 2019). One reason that catfish have high juvenile survival in suitable environmental conditions is that unusually in fish they exhibit parental care, in which parents guard small juveniles.The recruitment to the Lake Rotoiti catfish population from this breeding season was seen in the vast increase of young of the year (juveniles < 100 mm fork length) seen in late summer 2018, when 13,935 catfish were caught, compared with the same season in 2017, when similar netting effort caught only 2,889 catfish.
Temperature anomaly in Lake Rotoiti surface waters from July 2017 to January 2018 compared to the mean of data from 2005-2017. Source: unpublished lake buoy data, Chris McBride, The University of Waikato
Potential catfish habitat (red shading) in Lake Rotoiti. Source: Hamilton et al. (2005).
Research Lead: Brendan Hicks
Brown bullhead catfish, Ameiurus nebulosus. Fork length 415 mm, 1035 g. Photo: Brendan Hicks.
Adult catfish guarding a school of juvenile catfish. Photo H J Bannon.
Length frequencies of catfish (n catfish = 32,738) caught in Te Weta Bay between March 2016 and September 2018 by fyke netting (n net nights = 5,812). Relative frequency scales are normalised the modal length to 1. (Source: Francis 2019).