The extent of die-back of the kelp Ecklonia radiata in the Cape Rodney to Okakari Point Marine Reserve

The extent of die-back of the kelp Ecklonia radiata in the Cape Rodney to Okakari Point Marine Reserve. Summary The kelp Ecklonia radiata has undergone high rates of mortality in a region of northeastern New Zealand stretching from Whangarei Heads south to Cape Rodney and east to Great Barrier Island. Die-back of this kelp began in late 1991 and continued up to May 1993, with mortathy of over 90% in many areas. The area most heavily affected by die-back appeared to be centered on the Cape Rodney to Okakari Pt. Marine Reserve where mature kelp plants are virtually absent below 15m, and densities overall are an order of magnitude lower than in 1991. Die-back was patchier in other areas such as Great Barrier Island where affected and unaffected areas may be separated by distances of less than 100m. High densities of recruits became established at the Marine Reserve between January and May 1993 and full re-establishment of kelp populations should proceed over the next year, unless recruiting kelp becomes more susceptible to die-back in the post-recruitment phase. The causes of mortality are not yet known but so far human influences are not implicated. Hypotheses put forward to date include the action of pathogens such as bacteria, fungi or viruses, environmental effects related to temperature and phytoplankton blooms, and episodic demographic processes associated with the longevity of Ecklonia.