Monitoring

Crayfish are noticebly larger and more
abundant within the marine reserve
than outside the reserve

How has marine reserve status helped the sea life in the area? DOC is monitoring the changes in the Tonga Island Marine Reserve to measure the project's results.

Biological baseline data were collected within and outside the marine reserve over the year following the establishment of the reserve in November 1993. Sub-tidal data collected included:

• Quantitative data on community structure for rocky and sediment habitats.
• Density and size frequency data for key invertebrate grazers, scallops, horse mussels and rock lobsters.
• Finfish size and density data from underwater fish transects.

Follow-up sampling was undertaken in 1999 for:

• Reef fish size and density using underwater fish transects.
• Scallop size/density and horse mussel density.

From 2001-2007, a regular monitoring programme has been undertaken at Tonga Island Marine Reserve assessing the following variables within the reserve and at control sites elsewhere along the Abel Tasman National Park coastline:

• Fish abundance and size using underwater dive transects (every second year).
• Rocklobster density, size and sex from underwater transects (every second year).
• Benthic species presence/absence and % cover (once every 5 years).
• Kina and gastropods: densities and kina size (once every 5 years).
• Scallop and horse mussel density and horse mussel size frequency (once every 3 years).
• Subtidal profile transects (once every 5 years).

Monitoring at Tonga Island Marine Reserve up until and including 2007 is described in Davidson et al (2007). Download a copy of the report (PDF, 842K)

Key results from the 15 year monitoring programme are summarised below.

• Monitoring has found many more larger-sized rock lobster and blue cod inside Tonga Island Marine Reserve than outside it.  They are now also far more plentiful inside the reserve than when it was created in November 1993.
• Most changes in the rock lobster population did not occur until about eight years after the reserve’s formation.
• In 2007, large reproductive male lobsters were 12 times more abundant inside the reserve than outside the reserve’s boundaries and reproductive-sized females were almost 30 times more abundant in the reserve than outside it.
• In the same year, the mean size of lobsters in the reserve (measured by body carapace length) was 18.4 mm larger than the mean size of lobsters outside the reserve. No lobsters longer than 160 mm were found outside the reserve but lobsters up to 200 mm long were regularly recorded inside the reserve.
• The increased abundance in large reproductive lobsters will result in dramatically greater production of lobster larvae from the reserve.
• Blue cod numbers within the reserve have steadily increased since the reserve was established in 1993.
• In 2007, blue cod over 330 mm in length were far more numerous in the reserve than outside it and represented close to half of the cod population inside the marine reserve. No large blue cod over 330 mm in length were recorded outside the reserve in this year.
• The mean size of blue cod within the reserve has gradually climbed from an average of 271mm in 2000 to 311 mm in 2007. In comparison, the mean size of blue cod from sites outside the reserve was 227 mm in 2000 and was much the same in 2007 at 228 mm.
• Blue moki also tended to be larger and more abundant in the marine reserve than outside it.
• Scallop numbers have increased in the reserve while remaining relatively low outside, but there is no difference in the average size of scallops.
• The monitoring has found that although some species are doing better inside Tonga Island Marine Reserve than outside its boundaries, there were few differences with some other species, including spotty, red moki and tarakihi.

The monitoring results published in 2007 show the marine reserve ecosystem is continuing to recover since the marine reserve was established in 1993 and fishing and taking of marine life was prohibited there.  However, while monitoring is showing some very positive changes, the overall process is a slow one.  Effects on other species in the reserve, especially from changes in the food chain, are expected to take many decades.

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