Abstract:
In the 1960s, overfishing of the green-lipped mussel, Perna canaliculus, by benthic dredging in the Hauraki Gulf extirpated extensive natural mussel beds and the associated benthos. Despite 50 years since the cessation of dredging, the mussel beds have shown no sign of recovery. Recent research indicates that the lack of recovery might due to the scarcity of suitable settlement surfaces for the mussel larvae, which have a strong preference for settling onto finely-branched red macroalgae, such as Vidalia colensoi, Melanthalia abscissa, Pterocladia lucida, Pterocladiella capillacea and Laurencia thyrsifera. All of these red macroalgal species are thought to have become less abundant in the Hauraki Gulf over the last century, possibly as a result of increased water turbidity. The aim of the research presented in this thesis was to determine the light requirements of these species with laboratory experiments, to help identify those species which may be more tolerant of reduced light conditions, as might be expected in turbid coastal waters in the Hauraki Gulf. In addition, novel methods were developed to translocate red macroalgal species and apply these techniques to determine if translocated red macroalgae could re-established in turbid coastal waters. Historic records of the presence of red macroalgal species in the Hauraki Gulf were collated and used to identify five species of red macroalgae that are known to be associated with promoting the settlement behaviour of green-lipped mussel larvae. From which, V. colensoi, M. abscissa, P. lucida, P. capillacea and L. thyrsifera were selected as the primary focus for the remainder of this research. Elevated turbidity of seawater is known to increase light attenuation in the water column. Consequently, the light requirements of V. colensoi, M. abscissa, P. lucida, and P. capillacea were determined by experimentally measuring the respective irradiance-growth compensation points. M. abscissa and P. capillacea have emerged as the two species with the lowest irradiance-growth compensation points and therefore were more likely to sustain biomass in turbid environments with reduced light availability. Three different mooring blocks and three different attachment techniques for translocating various red macroalgal species were devised and experimentally evaluated in the absence of any published methodology. Red macroalgal thalli attached to polypropylene rope onto galvanised steel racks was determined as the best technique for the translocating various red macroalgae, due to the high thalli retention and consistent positive growth rate for all experimentally translocated macroalgae. Using the previously developed novel translocation technique, two species of red macroalgae (M. abscissa and P. lucida) were experimentally deployed into six coastal sites with varying turbidity and light availability for a period of 28 days. Changes in thalli colouration and biomass indicated that environmental conditions at all six sites were unsuitable for both macroalgal species. For all sites, M. abscissa appeared to perform better than P. lucida, possibility related to the comparatively low irradiance-growth compensation point of M. abscissa. Overall, the results of this study showed that red macroalgae can be translocated into new locations, however, a better understanding of their habitat requirements are needed to ensure their subsequent growth and survival.