Lake opening critical for fish recruitment

 

Dr. Phil Jellyman (left) and Dr. Shannan Crow (right) collecting longfin and shortfin eels in Te Waihora (Lake Ellesmere). Photograph by Dave Allen.

Dr. Phil Jellyman (left) and Dr. Shannan Crow (right) collecting longfin and shortfin eels in Te Waihora (Lake Ellesmere). Photograph by Dave Allen.

Recent research has reinforced that the timing of lake openings is critical to fish recruitment in Te Waihora/Lake Ellesmere.

NIWA freshwater fish ecologist Shannan Crow says that if the lake is closed at the wrong time it can restrict fish species from coming into the lake.

“The lake opening regime is critical to fish numbers within the lake,” says Crow.

“In the last third of 2013, the lake was open for a longer period than normal, which enabled us to get a lot of extra data on the recruitment timing of species like pātiki (flounder).”

“We had good pātiki recruitment in 2013 with the lake being open for this long period.”

The data showed researchers the peak recruitment periods for pātiki, which in turn enables the team to better advise Ngāi Tāhu and Environment Canterbury on the best times to consider for future lake openings.

Now in its second year, the review of fisheries’ management, led by Crow and including freshwater ecologist Dr Phillip Jellyman and a team of NIWA scientists, is a multi-faceted research project aimed at understanding factors affecting mahinga kai species within Te Waihora. The project is funded by Whakaora Te Waihora and it sits within the Science Investigations Workstream.

The research, undertaken in collaboration with Ngāi Tahu, covers seven components: an overview of existing information, recruitment of juvenile fishes, factors limiting productivity, managing prey species, the influence of macrophytes, population estimates and customary monitoring.

To date, existing information from the 1970s to the present has been collated. This includes published and unpublished data and represents over 60,000 records of fish length and weights. The study of this resource has highlighted knowledge gaps, particularly around pātiki recruitment says Crow.

Dr. Shannan Crow holding a by-catch of bullies and smelt at Te Waihora (Lake Ellesmere).  Photograph by Dave Allen.

Dr. Shannan Crow holding a by-catch of bullies and smelt at Te Waihora (Lake Ellesmere). Photograph by Dave Allen.

 

Recruitment studies were aimed at an improved understanding of the timing of different species entering the lake. Crow says it also threw up some surprises – like the high numbers of juvenile torrentfish found recruiting into the lake. “They’re generally a faster-current species so we weren’t expecting to find them in Te Waihora.”

“We knew torrentfish were in the lake tributaries, but they are considered to be a species at risk. Our recent data shows that they seem to come back into the lake at the beginning and end of summer, with some even crossing the bar when the lake is closed.”

“That’s a positive thing, and it gives us another piece of information that Ngāi Tahu and Environment Canterbury can use when they’re making decisions to manually open the lake.

 

There are only a limited number of manual lake openings allowed per year so if fish are recruiting over the bar, lake openings could be saved for a different part of the season.”

Studies are also being carried out on mahinga kai prey species – on bullies in particular. NIWA staff are now using MBIE research funding to conduct a survey of bully populations around the lake, comparing results with previous data gathered in 2007-2008, to assess any change in numbers.

“Bullies are key species in the lake as they support tuna (eel) and pātiki (flounder) abundance, so it’s important for us to get a handle of them.”

“We’ve also been looking into spawning preferences to see if they are limited by spawning substrate availability in the lake. If possible, we want to enhance prey species spawning areas in the lake.”

To that end, the team is working with Mary de Winton from NIWA in Hamilton to determine the influence macrophytes have on fish. Crow says that most of projects are necessarily long term.

“That can be frustrating at times waiting to gather enough data, but we’re enthusiastic about the information we’ve accumulated so far.”