In 1978 much of the sea floor between 5 and 9 m depths was rock flat barrens dominated and maintained by sea urchins.
Between 1978 and 2000 these barrens all changed to habitats dominated by large brown seaweed –kelp forest or shallow mixed seaweeds. It is considered that these habitat changes were probably due to lobster and snapper preying on urchins, causing a decline in sea urchin densities, and thereby allowing seaweed regeneration (this is called a trophic cascade).
The densities of a limpet and a gastropod species in the reserve have also changed, probably a response to the habitat changes, and therefore another indirect effect of the above trophic cascade. The change from barrens to seaweed habitat has been slow, as one urchin per square metre is sufficient to maintain a barren. The productivity of seaweed habitats in the reserve increased by 58% between 1978 and 1996 due to these habitat changes.
Regular surveys of both fish and lobsters are undertaken in the Cape Rodney-Okakari Point Marine Reserve. A summary of the findings of these surveys can be found below.
In autumn 2003, estimates made using Baited Underwater Video indicated that legal-sized snapper were 27.7 times more abundant inside the reserve than outside, an even greater difference than detected in previous autumn surveys.
The increase was due to an approximate doubling in numbers of legal-sized fish within the reserve since autumn 2002. This was probably due to an exceptionally large influx of individuals from offshore waters. The increase in numbers, however, may be short-lived, as many of the additional fish can be expected to migrate out of the reserve again during the winter of 2003, and further surveys are required to determine the long-term effects on the numbers of “resident” snapper within the reserve.
A BUV survey undertaken in the autumn of 2005 indicated that legal-sized snapper were 12.8 times more abundant inside the marine reserve, less than half the number estimated in the 2003 survey. The general snapper density was also estimated to be lower than 2003 levels.
The drop in density is most likely due to seasonal and inter-annual variability rates of immigration and emigration, superimposed on a reasonably stable resident snapper population.
A further BUV survey undertaken in autumn 2007 estimated that legal-sized snapper were 30.2 times more abundant than in unprotected areas outside the marine reserve, higher than the levels recorded in 2003. Again, the general snapper density was estimated to be lower than the levels estimated in 2003.
The average fork length of snapper inside the marine reserve was over 100 mm greater than those measured in unprotected areas outside the marine reserve, which was consistent with pervious surveys.
The spatial distribution of snapper was also found to be consistent with previous surveys, with highest densities of snapper recorded in the centre of the marine reserve, and lowest densities on the western boundary.
The autumn 2008 survey found that snapper densities had again declined, with snapper abundance being four times higher within the marine reserve than in adjacent unprotected areas.
Consistent with previous surveys, the autumn 2011 survey indicated species diversity is higher within the marine reserve than in non-reserve areas. Snapper, butterfish, john dory, banded wrasse, parore, blue cod and silver drummer are all more abundant inside the reserve than outside.
Snapper abundance levels within the marine reserve have increased since surveys undertaken between 2002 and 2008. The 2011 survey found a higher abundance of 0+ and 1+ age snapper individuals both within and outside the marine reserve than in 2008, indicative of strong recruitment over the last few years. A higher frequency of legal sized snapper was also recorded within the marine reserve in 2011 compared to 2008 levels.
The average number of lobsters has almost doubled inside the marine reserve between 2002 and 2004.
The most recent lobster survey was undertaken in May 2009. This was the seventh survey of lobster within the marine reserve. Lobster abundance was found to be 13.4 lobsters per 500m2, more than five times higher than the level found in unprotected sites surveyed.
The mean lobster size, based on carapace length was 105 mm, compared to a lobster length of 85 mm in unprotected sites.
Trends over time for the Cape Rodney-Okakari Point Marine Reserve indicate that the lobster population remained static between 2006 and 2009. However, the population is about 2.5 times lower than the population measured in 1995.
The number of legal sized lobster found inside the marine reserve was also lower than the number recorded in the last survey in 2006. This decline indicates that lobster abundance levels are maintained largely through recruitment and high numbers of less than legal sized lobsters. Patterns of this nature suggest the recovery of the marine reserve is most probably affected by fishing that routinely occurs along its boundary.
Fish monitoring reports
- 2011 fish monitoring report (PDF, 362K)
- 2008 fish monitoring report (PDF, 494K)
- 2007 fish monitoring report (PDF, 114K)
- 2005 fish monitoring report (PDF, 206K)
- 2003 fish monitoring report (PDF, 423K)
Lobster monitoring reports
Cape Rodney–Okakari Point and Tāwharanui Marine Reserve rock lobster monitoring - 2014, 2009 and 2004 reports