Fish and weed management
Grass carp
On 5 October 2004, Lake Omapere Trust, in a joint venture with New Zealand Waterways Restoration Ltd, gained a special permit from Ministry of Fisheries to remove chinese grass carp from Lake Omapere for the purpose of managing the grass carp population.
The grass carp were originally released into Lake Omapere in 2000 as a method of controlling the aquatic vegetation in the lake, in particular the oxygen weed Egeria densa. Initially 40,643 carp were released, half of which were small fingerlings that were expected to be easy prey to shag colonies. However it is now thought that more of these fingerlings than expected may have survived. As weed biomass had not reduced by 2002, a further 20,000 adult grass carp were released.
It is difficult to assess how many grass carp remain in the lake, because of immeasurable fish losses due to predation, poor water quality and starvation. However it is known that 401 grass carp have been removed from October 2004 to May 2005 and grass carp removal will continue.
January 2007 survey
On 22 and 23 January 2007, a fish survey was carried out in Lake Omapere and contributing streams by the Department of Conservation and Northland Regional Council. The only native fish caught in the lake were short and long fin eels, while the rare northland mudfish was caught in a contributing stream.
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Fish survey in action at Lake Omapere
The introduced fish caught in the lake included grass carp and goldfish. Also the exotic mosquito fish (Gambusia sp.) was sighted in a contributing stream at the southern end of the lake. This is the first confirmed sighting of mosquito fish in the catchment. It is surprising that they are not in large numbers in the lake already, as they are widespread throughout Northland and are prolific breeders. It could be that this is a recent introduction or that the water quality is not suitable for them to survive in the lake. For whatever reason it is good news that they are absent or in very low numbers in the lake, as they are extremely destructive fish.
The results from this survey are similar to the last survey carried out in May 2005, except for the sighting of mosquito fish. It was hoped that one or both of the native fish; common bully and common smelt would be found in this survey by carrying it out in summer when fish are more active with warmer water temperatures. Unfortunately similarly to the 2005 survey, neither of these fish were found.
May 2005 survey
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Short fin eels.
A survey of the fish present in Lake Omapere was carried out on 12 and 13 May 2005 by Lake Omapere Trustees, Department of Conservation and Regional Council staff. The fish survey involved the setting of 34 traps and nets overnight at five different locations around the edge of the lake and in farm drains and the retrieval the next morning.
Only two species of freshwater fish were caught in all of the nets and traps: short fin eels and juvenile goldfish. No mosquito fish or juvenile catfish were caught, which is good news as both are pest fish that can have a detrimental effect on lake systems.
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Juvenile goldfish.
Unfortunately, neither of the native fish; common smelt or common bullies, were caught. Both have been observed in Lake Omapere in the past. This could be due to a loss in emergent vegetation around the margins of the lake, which has caused a decline in numbers, or cooler water temperatures, which means reduced activity by these fish. Another survey will be done in the middle of summer to further assess the status of these two native fish.
It is essential for the weed management of the lake that there is no oxygen weed (Egeria densa) or any other invasive weeds remaining in the catchment that could reinfest the lake, especially now that the grass carp are being removed.
A detailed survey of all significant farm drains and streams was carried out over two days in November 2004 by NRC staff, Lake Omapere Trustees and other volunteers. This involved walking the length of all these waterways and documenting the aquatic plants found.
No plants of the two invasive oxygen weeds, Egeria densa and Lagrosiphon major were found in the catchment streams and drains.
On 24 May 2005, Lake Omapere Trustees and a NRC Biosecurity Officer carried out weed pulling by hand of the potentially invasive swamp lily, (Ottelia ovalifolia) from the Parakataio Stream at the Southern end of the lake to prevent it spreading further. Approximately 20 plants were removed making sure all the root system was taken. However there are still about 50 plants remaining in deeper pools, which will need to be removed by divers.
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Alligator weed
Alligator weed (Alternenthera philoxeriodes), another invasive aquatic plant which is found within the catchment, shows signs of being eaten by the biological control agent, alligator weed beetle (Agasicles hygrophila). The beetle only eats alligator weed and will manage the plants growth; therefore no further action will be required to control alligator weed within the catchment.