Freshwater mussel research
2007 Survey results
The freshwater mussel population in Lake Ōmāpere was resurveyed by divers in October 2007, following the same methods as used in November 2004.
Results show that there is still a high mussel density at the northern end of the lake. The overall mussel density in the lake was 22 mussel/m², which is slightly lower than the last survey in December 2005 (25 mussels/m²).
The mussel density in the northern end of Lake Ōmāpere is lower than in previous years, with an average of 28 mussels/m² compared to an average of 31.7 and 40.5 mussels/m² in 2004 and 2005, respectively. The mussel density was also lower in the centre and southern end of the lake with an average of 3.6 mussels/m² (compared to 7.6 in 2004 and 4.4 in 2005).
The number of mussels in Lake Ōmāpere appears to be reasonably stable and it is likely the slight fluctuation over the last two years could be a reflection of their patchy distribution, rather than a decrease/increase in numbers (Figure 1).
Figure 1: Total average mussel densities in Lake Ōmāpere from November 2000 to December 2007.
The graph below shows the frequency of mussels in each size class and compares the results from the November 2004, December 2005 and October 2007 surveys (Figure 2).
Figure 2: The size of the freshwater mussels surveyed in Lake Ōmāpere in November 2004, December 2005 and October 2007, showing best-fit lines for each year.
The best-fit line shows that there was a slight shift in the population towards larger mussels, between 2004 and 2005, but no change between the 2005 and 2007. No small live mussels (<59mm) were found, which indicates that there may be a lack of recruitment and/or a high mortality rate in juvenile mussels. It is therefore recommended that this survey be repeated in November or December 2008.
2004 Survey results
Improving water clarity in Lake Ōmāpere is a key factor in allowing the re-establishment of native plant biota, an important component for restoring the lake. Being filter feeders, freshwater mussels have the ability to improve water clarity.
On 1 November 2004, a survey was undertaken to map the extent of freshwater mussels in the lake. Mussel densities were calculated at 17 sites in total.
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Figure 1: Diagram showing the layout of the November 2004 mussel survey in Lake Ōmāpere.
Scuba divers collected the mussels from five 0.1m2 quadrats at each site. The mussels from each quadrat were put into a separate catch bag and brought to the surface for counting and measuring. Only live mussels were counted and the longest axis of each live mussel was measured. All mussels were returned to the lake after measuring.
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Hyridella menziesi being measured on the “v board”.
Sediment samples were collected at some sites in order to establish any relationship between sediment nutrients and numbers of mussels. Spot water quality samples were collected at site P, where the highest mussel density was found.
Survey results
A strong population of freshwater mussels was found in the northern end of Lake Ōmāpere in November 2004 with an average of 31.7 mussels/m2, while there was an average of 7.6 mussels/m2 at the five sites in the centre and south end of the lake. The diagram below shows the mussel densities recorded at each site (Figure 2). The mussel distribution in Lake Ōmāpere was patchy, even in the areas that had higher mussel numbers.
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Figure 2: Diagram showing mussel densities (no/m2) at the 17 sites sampled in Lake Ōmāpere in November 2004.
The results from sediment sampling showed that the amount of nutrients in the sediment could be an influencing factor on the number of mussels. The highest mussel density was found where nutrients were the lowest and the lowest mussel numbers were found where nutrients were the highest. On the other hand, it could be that there are less nutrients at the northern end of the lake because the mussel population is stronger there and the mussels are reducing the amount of nutrients in the sediment. This is inconclusive at this stage.
Water clarity experiment
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Trustees built an enclosure in Lake Ōmāpere where freshwater mussel numbers are high.
Freshwater mussels are filter feeders, that can filter algae and sediment out of the water. Therefore they can play an important role in improving water clarity in a lake. Freshwater mussels are in high numbers in several locations in Lake Ōmāpere.
On 9 June 2005, Regional Council staff and Lake Ōmāpere Trustees built an enclosure in the lake where freshwater mussel numbers are high. The theory is that the enclosure will restrict water movement, reduce the amount of sediment being stirred up and prevent the water that the mussels have filtered from being moved away. This means that the mussels in the enclosure will have a greater chance of filtering the algae and sediment out of the entire water column, with the overall aim of improving water clarity.
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Freshwater mussels are in high numbers in several locations in Lake Ōmāpere.
Once water clarity improves, native aquatic plants can be introduced to the enclosure and the grass carp will not be able to access the plants. The enclosure will then need to be left there for up to 14 months to allow an extensive bed of plants to grow.