Translocation Ecology of New Zealand Freshwater Mussels

Freshwater mussels are a diverse and important group of animals that provide multiple ecosystem services as well as direct services to humans. They are also one of the most imperiled taxa in the world, due to a suite of anthropogenic stressors that are destroying what little suitable habitat remains and making scarce the symbiont fishes that mussels require in order to complete their life cycle. Threats to mussels such as sedimentation and eutrophication are typically neither quick nor simple to fix, and translocation is increasingly being used to rescue populations, restore defaunated habitats, and increase species’ distributions to reduce stochastic risk. The strong filtering ability of these animals has also aroused interest in their potential use as bioremediation organisms in degraded waterways. Freshwater mussel translocation ecology has been a growing field around the world for decades, but remains in its infancy in New Zealand, which has three extant species in the genus Echyridella, with all classified as Threatened, Declining, or Data Deficient. With this thesis, I aim to provide a robust starting point for future mussel translocation work in New Zealand. In Chapter 1, I expand on the aforementioned themes and explain how I designed and implemented New Zealand’s first comprehensively monitored mussel translocation project in order to achieve this. I also outline several laboratory-based experiments that I conducted to investigate additional aspects of mussel behaviour in relation to translocation. Overall, I covered:• Mussel transport methods informed by Mātauranga Māori (Chapter 2)• Mussel personalities and how they might influence translocation planning (Chapter 3)• The responses of mussels to disturbance simulations (Chapter 4)• The movements made by mussels following translocation (Chapter 5)• The growth rates and trophic niche of translocated freshwater mussels (Chapter 6)Handling methods are an important determinant of release site behaviour. In New Zealand, tangata whenua (indigenous people) traditionally used insulated kete to translocate aquatic animals to new environments as part of ahumoana tawhito (ancient aquaculture). In Chapter 2 I investigated the influence of three transport methods (traditional [flax kete], modern [bucket], and a hybrid of the two [bucket with flax support structures]) on the short-term performance (burrowing speed) of Echyridella menziesii. I also tested whether assisted release (planting mussels in the substrate) resulted in enhanced burrowing speeds. Mussels that were transported using the traditional method were slower to begin probing the substrate, but there was no difference between methods in overall burrowing speed. I also found that assisted release resulted in faster burrowing speeds.

I concluded that handling and release procedures can influence the short-term performance of translocated mussels, and recommend procedures for future translocation projects, including transporting animals in immersion vessels where practical, and planting them at the release site. Release site behaviour is an important determinant of translocation success. Knowledge of behaviour patterns, and the consistency of those patterns can help predict how animals will respond to translocation. The use of ‘behavioural syndrome’, or more simply ‘animal personality’ theory can provide insight into species’ behaviour patterns as well as potentially help to endear them to the public. The latter can be of particular use when raising funds for expensive translocation projects involving animals that many view as uncharismatic. In Chapter 3, I examined three behavioural variables (probing time, movement distance, and burrow depth) during a series of lab and field experiments conducted on Echyridella aucklandica and E. menziesii. Overall, I observed individual behavioural consistency that was indicative of animal personalities, as well as changes in behaviour, and in behavioural consistency, in relation to species, time, and space. I concluded that E. aucklandica behaviour may be more flexible than that of E. menziesii, which may allow more informed planning regarding focus species in future translocation projects. I also suggested that the use of longer test sequences in animal personality research may capture greater behavioural variability.

Knowing how animals may behave in response to environmental disturbance is of value to translocation ecology given the disturbance that is inherent in this practice, and to better incorporate the disturbance regime of release sites in translocation planning. In addition, novel disturbance such as human mediated translocation may elicit different behaviours compared to more familiar disturbance such as failed predation attempts or floods. Also, behaviour may be influenced by release site characteristics such as the presence or absence of predators and conspecifics. In Chapter 4 I observed the laboratory-based reburial behaviour of E. aucklandica and E. menziesii that I exposed to two types of disturbance: novel (involving transport in either immersion or emersion vessels), and familiar (involving simulated disturbance in the laboratory—either flood or predation attempt). During the novel disturbance experiment, I also tested the influence of conspecific odour on reburial behaviour. I found that mussels that were transported in immersion vessels and released in the presence of conspecific odour were quicker to initiate reburial. I also found that mussels that were exposed to the predation simulation were quicker to initiate reburial than those exposed to the flood simulation, and that overall, E. aucklandica initiated reburial earlier than E. menziesii. I suggested that factors such as transport and release methods, as well as the disturbance regime and presence of predators and conspecifics at the release site be considered when planning future translocation projects.

In addition to careful planning, the implementation of follow up monitoring is essential to determining project outcomes as well as collecting robust knowledge for future work. Adequate monitoring has been scarce in mussel translocations and has focussed mostly on biological measurements such as survival rather than behavioural measurements such as movement. The movements that translocated animals make once released into new habitats can influence their performance as well as the effectiveness of follow up monitoring. In Chapter 5 I measured the distances moved by E. aucklandica and E. menziesii following translocation from two lakes in the lower North Island to the Zealandia wildlife sanctuary. I included multiple factors in the design relating to habitat type, release density, and release group species composition. I found no effect of release group composition on post-translocation movement, and no strong evidence for differences between species. I found that mussels that were released into stream habitat moved further than those in other habitats, and that stream mussels that were released in higher densities moved further than those released in lower densities. I concluded that E. aucklandica and E. menziesii make relatively small movements directly following translocation, and that reasonable recapture rates can be expected during short term monitoring. I also made suggestions regarding how the different movement patterns I observed may relate to release density and habitat characteristics and concluded that optimal habitat selection by managers should allow translocated mussels to spend their energy on burrowing and feeding rather than searching for better habitat. Optimal mussel release habitats will obviously include adequate food resources, however there is little knowledge presently available to robustly assess this parameter. Most translocations projects rely on the assumption that mussels are generally flexible feeders and will adapt to new food resources. In Chapter 6, I used stable isotope technology to examine the trophic niche of E. aucklandica and E. menziesii in two lakes. I also used 2 years of mussel growth data in the same lakes to assess how trophic niche may influence performance. I found that mussels grew more in Zealandia compared to Lake Wairarapa, that E. menziesii grew more in Zealandia than E. aucklandica, and that E. aucklandica grew more in Lake Wairarapa than E. menziesii. Isotope signatures indicated that mussels in Lake Wairarapa may be more reliant on benthic food sources and that mussels in Zealandia may be more reliant on pelagic food sources. The isotope signatures also indicated that the species were partitioning resources in both lakes and that E. aucklandica may have a more flexible trophic niche than E. menziesii. I concluded that the interspecific performance differences I observed are at least in part influenced by differences in trophic niche size and fidelity and suggested that environmental conditions in Lake Wairarapa may be contributing to poor growth of mussels in this lake.

In Chapter 7 I summarised my main findings in the context of mussel translocation literature. I then integrated and discussed several recurring themes I noticed that are related to mussel ecology and translocation practices. Finally, I discussed the strengths and limitations of my work, and made recommendations for future research in the mussel translocation ecology field. In conclusion I ventured that the findings contained in this thesis 1) provide a significant contribution to literature regarding the ecology of E. aucklandica and E. menziesii, 2) contribute to the global mussel translocation literature, and 3) provide a robust starting point for future mussel translocation work in New Zealand.