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Assessment of impacts of mechanical spat harvesting on the surf clams of Te Oneroa a Tōhē

Much of the mussel spat used to seed mussel aquaculture farms throughout New Zealand is sourced from Te Oneroa a Tōhē (90 Mile Beach) in Te Hiku (far north of New Zealand). The mussel spat washes ashore attached to seaweed and hydroids where it is collected by spat harvesters and transported to mussel farms around the country to be on-grown. The collection of Te Hiku spat and seaweed was originally done by hand. However, the methods used have evolved over time and mussel spat is now primarily collected using mechanical loaders operating in the surf zone. Local iwi, hapu and the wider community have recently voiced their concerns about the impacts of this harvesting method. Specifically, there are concerns about the impact of loaders operating on top of toheroa (Paphies ventricosa) and tuatua (Paphies subtriangulata) beds or other shellfish inhabiting in the intertidal and subtidal areas of Te Oneroa a Tōhē. Consequently, research into the ecological impacts of mechanical spat harvesting was conducted to address the concerns of the community and inform the management of the Te Hiku spat fishery. This research aimed to examine the ecological impacts of a ‘worst case scenario’ by (1) simulating an intensity of spat harvesting activity much greater than would be anticipated in the ‘real world’ and (2) conducting the research at high density tuatua beds where impacts would potentially be greatest. Adult toheroa were not targeted in the experiment to avoid unnecessary impacts on already depleted toheroa populations. At each of three sites on Te Oneroa a Tōhē, nine spat harvesting loaders were repeatedly driven in and out of the surf, over shellfish beds, for 75 minutes leading up to the low tide. Once loader activity ceased, a sampling team made up of mana whenua, local primary school children, spat harvesters and staff from MPI, Te Ohu Kaimoana and the University of Waikato collected clams from the intertidal area. Collections were made from both impact and control (non-impact) sites, where (a) the number of crushed shells was quantified and (b) the self-righting and burying abilities of tuatua from impact and control areas were compared to provide an estimate of sub-lethal effects. There was no detectable impact of mechanical spat harvesting loaders on tuatua (or the juvenile toheroa that were present in the sampled tuatua beds) when these surf clams were either fully or partially buried in sands of Te Oneroa a Tōhē. Rates of shell damage were less 2 than 6.1% and were similar in impact and control areas indicating much of the observed shell damage was caused by the sediment corers that were used to extract samples from the beach. Similarly, being driven over by loaders did not affect the ability or speed at which tuatua were able to right and bury themselves in the sediment. When standing on end these clams appear robust to the forces exerted by loaders upon the sediment surface. However, beach cast clams (both tuatua and toheroa), lying on top of the sediment on their sides are vulnerable to crushing by vehicles. This includes juvenile clams that ‘float’ to the beach surface where multiple loader passes liquefy the surface sediment. Mortality in these beach cast clams was not well captured by the sampling methodology used in this study, but was observed at two of the three sites sampled. It was not possible to quantify this type of mortality during this study. However, the number of clams crushed on the beach surface was small relative to the high abundances of clams within the sediment and the impact is not considered ecologically significant at the scale the individual shellfish beds or the entire beach. Overall, there was nothing observed during the course of this experiment to suggest that mechanical harvesting is having a significant ecological effect on the toheroa or tuatua of Te Oneroa a Tōhē. Mechanical spat harvesting did result in some clam mortality during the course of this experiment, and it is likely that some mortality will occur in the future when clams are present on the beach surface. However, the level of mortality observed, and anticipated, is unlikely to compromise the viability of either clam species. To minimise clam mortality caused by spat harvesting, loader operators should endeavour to avoid areas where large numbers of surf clams are either beach cast or are observed washing around in the shallow subtidal. It would also be prudent, because of the depressed state of toheroa populations on Te Oneroa a Tōhē, for spat harvesters to have up-to-date information about the location of toheroa beds and avoid them where possible. It should be noted that concerns have been raised as to whether the driving of loaders during this study was equivalent to real world operations. However, the collection of operational data during this experiment, the proposed installation of GPS trackers on all loaders and the use of observers will allow for monitoring and enforcement of operating procedures established in the industry code of practise.
Type of thesis
Ross PM. 2020. Assessment of impacts of mechanical spat harvesting on the surf clams of Te Oneroa a Tōhē. Environmental Research Institute Report No. 138. Client report prepared for the Te Oneroa a Tōhē Spat Working Group. Environmental Research Institute, School of Science, The University of Waikato, Hamilton. 32pp.