Catfish

Catfish were positively identified in Lake Rotoiti early in 2016. Catfish are a serious danger to koura and compete with other species for food. Elimination will be very difficult, and innovative measures are being investigated and will be needed. It is an example of the kind of work that is ongoing to protect our waterways from invasive species.

Prior to 2016 there were earlier signs that Catfish might be in the lake. In 1994 a small live catfish was presented to the Department of Conservation. The catfish had exited a boat trailer as it was retrieved from the lake. The previous night the trailer had been left submerged in Lake Taupo where catfish are known to be present. In 2009, a single large catfish was found dead on the shore of the lake.

After each report or finding, netting, electric fishing or dive surveys were undertaken though catfish were never caught.

Systematic surveying through all the Rotorua Lakes had been undertaken over a number of years before the discovery in 2016.

Surveying in 2016 and 2017 showed that the vast majority of catfish were in a small bay, which has now been cordoned off to restrict catfish from getting in or out. Netting inside the cordon in Te Weta Bay has produced catches in the thousands. Netting outside the bay has only caught small numbers of catfish but at numerous sites at the western end of the lake.

Brendan Hicks is professor of Fresh Water Fish Ecology at Waikato University and has been specialising in fish ecology for around 25 years. He says the brown bullhead catfish (Ameiurus nebulosus) is a native of North America. It found its way to New Zealand in the late 1800s. They are nocturnal bottom feeders, eat a diverse range of food and prefer a shallow, weedy habitat in lakes and rivers. They are tolerant of high and low temperatures, low dissolved oxygen, and pollution.

“They are a significant threat to koura and are now found throughout the Waikato River system including Lake Taupo. They also complete for food with other fish species,” Professor Hicks says.

“From previous work at Taupo we understand some of their life cycle. They are sexually mature at 2 years of age and about 220mm long. Their peak gonad development is in September. A female can produce perhaps 20,000 to 50,000 eggs, so we need to avoid those being released into waterways.”

Catfish are very good parents, guarding the nest and keeping away predators. When the young are 50mm or so they disperse and start life on their own. The presence of catfish in Taupo and the Waikato River has been known for some decades but they had not been found in the Rotorua catchment. Possible sightings and evidence of catfish in or around Lake Rotoiti did occur in the 1990s and 2000s. Diving and electric fishing carried out by Waikato University failed to find any. Prof Hicks, who for many years had been monitoring fish around the wall that diverts water from Lake Rotorua down the Ohau channel and away from Lake Rotoiti, says that he had never seen any sign of the species.

“It wasn’t until March 2016 that a contractor harvesting lake weed in Te Weta Bay found two fish amongst the weed being harvested that we were able to positively confirm their presence in the lake,” he says.

“This was a significant find and so incursion response planning began immediately. Two weeks later 21 mesh fyke nets, set and baited with cheese and sardines, caught 52 catfish, most of them juveniles less than 100mm in length.” Fyke nets are fish traps, often cylindrical one-way nets with wings that spread out and guide fish into the middle of the net where they are trapped.

The Bay of Plenty Regional Council’s biosecurity team began systematic netting of the lake starting in April 2016. Team leader Shane Grayling says that after discussing the problem with local agencies they came up with a plan.

“We had nets available and so we engaged a contractor to move methodically around the lake to try to determine how bad the infestation was, how far they were spread and what numbers we were dealing with,” he says.

“We netted up until the end of June and caught 391 catfish, 381 of them in Weta Bay. We found later in the season as the water temperature cooled that we caught more just outside the entrance to the Bay. This didn't surprise us because evidence from Lake Taupo suggests that over winter they have a tendency to move towards a rocky habitat and there was a rocky habitat just outside the entrance to the Bay.”

Over the winter netting stopped because catfish become less active or inactive and catch rates became low but resumed when the water temperature increased in spring. They went systematically around the Lake to try to identify patterns and get a good understanding of the scale of the problem.

“From September 2016 until the end of June 2017 we set over 2300 nets and caught 3272 catfish ranging from 20 mm to 400 mm in length. One night in a single net in Te Weta Bay we caught 1227 juveniles – juvenile catfish tend to shoal together and we must have caught the shoal. The highest catch rates were from February to May when the water temperature was higher,” says Shane.

“At one of the meetings with stakeholders it was suggested that we should block off Te Weta Bay. The Council uses cordons around boat ramps to confine any aquatic weed incursions from boats and trailers, so in April 2017 we modified one to make a physical barrier across the entrance to Te Weta Bay to try and prevent catfish moving in and out and invading other parts of the Lake.”

Only time will tell whether the netting and confinement regime is reducing the population. The local community wants complete eradication but both Shane and Brendan agree that this will be difficult.

“Innovation is going to be very important in this programme. We are currently using what is considered best practice in terms of netting and we have found that fine fyke nets are more effective than coarser ones. However, netting alone will not likely be enough considering the numbers we are dealing with,” says Shane.

“So we are looking at different options to prevent movement and disrupt breeding.   NIWA has developed a pheromone bait for perch which they say has increased their rates of catch and so we will trial that in the near future and also try to find a more attractive bait for catfish. There are other options in terms of acoustic barriers or attractants and electric barriers.”

Shane will soon start an acoustic tag control experiment, which will form part of the Masters thesis he is doing supervised by Brendan.

“We are going to capture 30 large catfish and implant them with acoustic tags. Then we will place acoustic receivers around the Lake, particularly in the Western part and track catfish movement for a year,” says Shane.

“We want to understand their movements at different times of the year, particularly over the spawning period, and hopefully that will allow us to get a better understanding of their behaviour and to target them more effectively at those key periods.”

Netting in likely habitats in other Rotorua lakes has so far found no catfish. This is a good sign. Other positives are that the Rotoiti infestation has been discovered before it has gone out of control, and that the vast majority of catfish are confined to one small bay. Netting in Lake Taupo in 1995 caught up to 92 fish/net/night whereas in Te Weta Bay the catch is 3.5 fish/net/night. So there is hope for a successful outcome.

However, as Shane says, underlying problem is bigger than just catfish – it’s general boat and trailer hygiene.

“Catfish are the latest issue that we have to deal with, but accidental transfer of weeds and pests by boat owners is an ongoing nightmare for us. If boaties don't take really good care to clean and remove any weed, fish and other debris when they exit and before they enter a lake or waterway then we will get new incursions, and the more we have to manage the more it is going to cost.”