An investigation into the impact of the Geoffrey Blundell Barrage Gates on tuna (eel) migration in and out of Lake Wairarapa
Part 1: Introduction
Kia Ora. My name is David Famularo and these are the Geoffrey Blundell Barrage Gates.
The gates are located in what used to be a stretch of the Ruamahanga River, where it used to leave Lake Wairarapa and flow into Lake Onoke further downstream.
Map: Greater Wellington Regional Council
So what is this investigation all about? For some years I have been concerned about the survival of the Long Fin Eel - tuna kuwharuwharu, which is classified as “declining/at risk” by the Department of Conservation and which I will refer to as tuna, although there is a more common short fin variety as well.
Photo: Jason Gunn Burton
Tuna can live up to 80 or more years, and only breed once, at the end of their life, migrating down the streams, rivers and waterways they had originally migrated up when elvers, and out to sea where they meet en masse and spawn at an unknown location thought to be near Tonga.
But for tuna and other migratory native fish species, things changed drastically when the barrage gates and diversion were constructed in the early 1970s.
Out of personal curiousity, I wanted to find out what impact these had had on native fish migration so did some online research. I was surprised at how much material on this subject is publicly available online.
It seems this question has been asked quite a few times over the past 50 years in scientific papers and reports.
In this Part 1: Introduction I am going to provide a the background to this question, and in Part 2 answer the question.
First we must imagine what Lake Wairarapa, and the other parts of Wairarapa Moana which includes a number of lakes, rivers, streams wetlands and lagoons, were like before Europeans arrived in the Wairarapa in the mid-1900s.
Wairarapa plains at Featherston from the foot of the Remutaka Hill. Mid 1870s James Bragge, Te Papa (0.032446)
Think of Wairarapa Moana as being like a bowl of water and wetlands, surrounded by forests, small mountains, plains, and ocean.
Lake Wairarapa used to breath, so to speak. The lake level’s repetitive rise and fall, saw the lake dramatically bexpand and contract through the seasons, a process that encouraged life forms that made it rich in plant, fish and bird life, and an important food source for the local Maori.
During spring, southerly storms would close off the gap in the Lake Onoke spit which was the passage for water from Wairarapa Moana to the sea, backing water up the Ruamahanga River and causing Lake Wairarapa to double in size.
Lake Onoke Spit Photo: David Famularo
Rainfall in the Wairarapa valley in late summer and autumn would see Lake Onoke rise, spill over the spit, and reopen the gap, providing a route for thousands of tuna out to sea, but not before many had been harvested by Maori.
Things started to change when European pastoralists bought land around the lake to graze their animals.
The annual floods restricted the available land for grazing and first steps were taken to reduce the flooding, most notably by keeping permanently open the gap in the sand spit between Lake Onoke and the sea, despite protests from local Maori.
The signing over of ownership of Lake Wairarapa from Wairarapa Maori to the Crown, at Papawai Pa. National Library
After years of pressure and non-violent conflict, in 1896 Ngati Kahungunu gifted Lake Wairarapa to the Crown with the understanding that tangata whenua could retain their existing use of the lake. This enabled the crown to start taking steps to control the level of the lake.
However, despite ongoing flood control measures, severe flooding occurred in 1947, covering 200 square kilometres. This lead to the establishment of the Wairarapa Catchment Board which subsequently planned the Lower Wairarapa Development Scheme.
Works undertaken by the Board between 1964 and 1984 included the installation of the barrage gates in 1974, and the diversion of the lower Ruamahanga River to bypass Lake Wairarapa.
The scheme planned to reclaim another 2600 hectares of lake bed along the eastern shore but this never went ahead after widespread concern about the impact it would have on the wildlife-rich eastern shore.
While the scheme has impacted fish migration as we shall see in Part 2, it has been very successful in its goal of maintaining a relatively stable lake level, and reducing the duration of the floods to usually less than ten days each year.
Part 2:The impact of the Barrage Gates on fish migration
The Geoffrey Blundell Barrage Gates are located half way along the stretch of water that connects Lake Wairarapa to the current course of the Ruamahanga River.
The Ruamahanga River used to enter Lake Wairarapa and then exit again a few kilometres further south. A diversion was dug from just before where the river enters the lake to just after leaving the lake so that the Ruamahanga River now completely bypasses the lake.
Barrage Gates looking south Photo: David Famularo
The Barrage Gates are a set of six radial arms which are raised and lowered at certain times of the day to maintain a fixed level in the lake. There are also a number of floodways to the east of the lake where in times of heavy rain floodwater can spill into Lake Wairarapa where it is held temporarily until the threat of flooding of farmland has passed.
In the introduction I gave background context to my question of how much the barrage gates are affecting fish migration.
Here I will supply an answer to that, as much as one is possible.
Most native New Zealand freshwater fish are diadromous. That is, they must migrate to and from the sea to complete their life-cycle with the journey varying from species to species.
Not surprisingly, these migrations were affected when the Barrage Gates and Diversion were constructed in 1974.
Just how much seems uncertain as all knowledge of the state of the species prior to 1974 is based on anecdotal evidence and scientific surveys that didn’t have this question as their main focus. But from the material I have read, it seems there was a noticeable decline in species numbers after 1974.
As an example, commercial flounder fishing collapsed within a year, and commercial eel fishers and local Maori both noticed a significant drop in tuna catches.
While the construction of the Barrage Gates and Diversion had an immediate effect, there has been a steady decline in species populations in the 100 plus years that flood management has been going on.
As we shall see below, migrating through the barrage gates, has contributed to species decline. But far more significant has been the fact that the seasonal expansion and contraction of Lake Wairarapa has all but permanently ceased, with the level of the lake permanently maintained at around ten metres above sea level (the lake itself never being more than around 2.5 metres at its deepest).
This has severely reduced the size of the riparian margins which were such important part of the food chain.
Lake Wairarapa looking east Photo: David Famularo
In the 2023 Cawthron report Wairarapa Moana Fish Futures: Current State and Initial Priorities For Fish Restoration, commissioned for Greater Wellington Regional Council, its author Simon Stewart said that floodplain inundation is a primary driver of fish production in shallow coastal lakes, facilitating invertebrate emergence from flooded soil, fuelling the growth of smaller fish such as inanga and subsequently larger piscivorous fish.
In his conclusion, Stewart argued that an immediate focus should be placed on improving growth rate of tuna through enhancing their diet, and that this best achieved through enhancing native fish population productivity through such means as protecting and increasing macrophyte beds. Stewart also said the barrage gates potentially remain a major obstacle to fish movement.
Wetlands on the north east shore of Lake Wairarapa Photo: David Famularo
When I started doing my research for this investigation, I was surprised to find that numerous scientific research projects and reports had been carried out as to the impact of the barrage gates on fish migration.
In 1995 the Department of Conservation even commissioed a design for a better fish passage through the gates than the small fish passage that was part of the original barrage gates. The original pass had proved to be ineffectual, not least because the flow of water is too strong for fish to swim against.
The proposed fish passage would have allowed fish to navigate through the two gates closest to the left and right river banks through slower moving currents. I don’t know why this fish passage was never constructed.
One of the barrage gates up Photo: David Famularo
Probably the most informative survey of fish migration through the barrage gates was carried out by NIWA and Greater Wellington Regional Council, with assistance from the Department of Conservation, in 2002.
In the introduction to Monitoring Of Fish Movements At The Lake Wairarapa Barrage Gates, G J Glova and D J Jellyman noted that 20 species of fish had been recorded in the lake and its surrounding wetlands, with many of them migrating between fresh water and the sea at certain times of the year to complete their life cycles.
They noted that it was likely that fish recruitment into Lake Wairarapa had declined once the barrage gates and diversion were constructed.
The 2002 survey found that whitebait, smelt and bullies were the main recruits of the ten species found, with eels, mullet and others comprising a minor proportion of fish moving into the lake. Fish movement occurs year-round, with a peak in spring and low during winter.
The authors thought the bulk of fish movement is now up the Ruamahanga River via the diversion.
A much larger volume of water used to flow out of Lake Wairarapa before the barrage gates were constructed. This would have been attractive to fish migrating upstream.
They use the physical flow of water to orientate themselves, as well as smell to follow the scent of their species to suitable habitat.
Over the years a number of recommendations have been made to aid fish migration, most of which have been adopted.
These include opening of the gates nearest the two banks at certain hours of the day, at certain times of the year when fish are most likely to be migrating upstream or downstream.
The only recommendation which has never been acted upon is allowing the level of Lake Wairarapa to rise and fall more than it currently does.
This would help species for whom the riparian edges of the lake are an essential part of their life cycle. But it would increase the flooding of farm land.
One thing I noticed in much of the material I read was how often the importance of ongoing monitoring of fish migration and numbers was stressed. But as far as I can tell, little actual monitoring has been going on so far, perhaps due to lack of funds.
View looking north to Lake Wairarapa Photo: David Famularo
Part 3: Where do things stand as of November 2026?
In 2018 Beca Ltd prepared an Operations and Environmental Management Plan for Greater Wellington Regional Council, outlining the protocols and procedures for the operation of the Barrage Gates and the requirements for ongoing environmental monitoring.
According to the plan, the primary purpose of the Barrage Gates is firstly to manage the risk of flooding to land owners within the Lower Wairarapa Valley Development Scheme area.
Secondly, to maintain specified target water levels during different periods of the year.
A third objective is to allow for fish passage by operating the gates in the following way:
Opening the two lateral gates for one hour before low tide from December to March when fish are migrating out of the lake, if there is no inflow into the lake.
And one hour before high tide from September to November when fish are migrating into the lake, if there is no flow out of the lake.
There is also a fourth objective of research and monitoring to enable alternative gate operation modes to be studied, and assessing the effect of the barrage gates’ operation effects on water quality, flood risk, fish passage, and mana whenua, terrestrial and aquatic ecological values.
Every year, since the plan was put in place, Greater Wellington Regional Council Flood Protection has had to supply an annual monitoring report to the Manager, Environmental Regulation, Wellington Regional Council.
By 30 September, 2026, it also had submit a comprehensive catchment wide review to understand the effects the Lower Valley Development Scheme is having on the environment.
In summary, it would be fair to say there seems to be a high level of awareness that the barrage gates may be a barrier to fish migration, and steps have been taken to alleviate this as much as possible, as long as this doesn’t have a negative impact on the primary purpose of the gates which is to maintain a static level of water in the lake.
