Lake Rototoa
This lake is an ecologically and culturally significant dune lake on the north west coast of Auckland. The lake is home to one of the last remaining populations of threatened freshwater mussels (Kakahi) in the region. These mussel population have been declining over the past two decades and the reasoning is not well understood.
The aim of this project is to collaboratively develop a standardised survey method that will assess various aspect of mussel population dynamics and habitat condition. These surveys will be used to benchmark and track mussel populations overtime in response to anthropogenic stressors and restoration efforts. This project is the first of its kind in New Zealand and the data forms a key part of a local government funded research programme focused on enhancing the biodiversity values of this lake.
Full Project Report coming soon
How it started
We began diving this lake in 2014 and became accustom to seeing extensive mussel beds. Over time we noticed a general decline in lake condition and wanted to know how the mussel populations are being effected.
The Aotearoa Lakes Dive Team mapped almost 1 km of mussel bed during our initial scoping dives and quickly realized that the situation was far worse than we imagined. Majority of the mussels were dead and there were clear signs of a mass die off.
Working together
We approached the Auckland Council to see what they had planed for the lake and to notify them of our rather scary observations.
The Council had just started a long term project aimed at managing and restoring native biodiversity in Lake Rototoa. They were already investigating plant and fish populations as well as invasive species management but, did not consider freshwater mussels.
We began developing a collaborative monitoring programme that aims to assess the current population dynamics, potential limiting factors and habitat preferences of these mussels.
The project consists of four phases: method development, survey testing, lake wide surveys and repeat surveys.
Through these four phases we will be able to monitor mussel population responses to pest management and restoration techniques as well as building the most comprehensive freshwater mussel data set in the region.
Phase 1
Phase 1 involved developing a standardised survey methodology that would allow us to capture the data we need to achieve the project objectives.
There was no existing survey methods that would satisfy all of our needs so we designed one from scratch.
The survey method involved teams of 3 - 4 divers that would run a 50 m transect parallel to shore along the upper extent of a mussel bed. They then run perpendicular transects every 10 m along the parallel transect, these transects extend to the maximum depth extent for that specific mussel bed. Quadrats are placed every 2 m along the perpendicular transects and all the mussels in each quadrat are counted and measured. The divers also collect a variety of metadata across the mussel bed as well as photos and videos.
Phase 2
Once the survey methods and data collection protocols were developed we tested them on a variety of mussel bed types and locations to ensure that they equally applicable in all situations.
We tested the methods on three different mussel beds that represent the varying bed formations seen in Lake Rototoa as well as other lakes.
Bed 1 was deep and had a relatively steep gradient, this bed was also representative of open water beds.
Bed 2 had a narrow band of mussels between the reed edge and the macrophyte beds and was very shallow with a gentle slope.
Bed 3 had characteristics that were between beds 1 and 2 with a moderate depth and slope.
The method performed well across all three beds and with some fine tuning and minor tweaking we began Phase 3.
Phase 3
Our initial survey were done on the three test beds and the results thus far are frightening.
These initial surveys covered 2120 m2 of lake bed. A total of 1604 Kākahi (Echyridella menziesii) were counted with an average count of 10.35 mussels per quadrat. The combined mussel density across all three sites was 41.39 mussels per m2. Out of the 1604 mussels found, 1320 (82.29%) were dead and only 284 (17.71%) were still alive.
No juveniles were seen during any of these surveys and all the mussels were larger than 51 mm. The surveyed population is composed entirely of mature adults (approximately 20 - 30 years old), 64.08% of live mussels were larger than 70 mm in length, 30.63% were between 61 to 70 mm and the remaining 5.28% was in the 51 – 60 mm size class.
This ageing population and lack of younger individuals is a sign that limited to no viable recruitment had occurred in the surveyed area for more than a decade. Considering that the majority of the live mussels were at the upper end of their life expectancy and no recent recruitment was evident, the long-term viability of the surveyed population is low.
It is clear that a mass extinction event is occurring in Lake Rototoa and if nothing is done we risk losing these key stone species for good.
Whats next
We will continue with Phase 3 and survey new mussel beds across the lake. We also intend to install a variety of water quality sensors to get a better understanding of the environmental conditions in the lake as well as across the mussel beds. This information is critical to our understanding of this lake and its inhabiting biodiversity.
