First evidence of avian predation of threatened freshwater mussels (

Abstract

Abstract Context Conservation efforts for New Zealand freshwater mussels (kākahi) are challenging because of their longevity and their complex life history, but also by the scarcity of research that has investigated causes of decline. Reproductive constraints may impair recruitment, but other key aspects that influence population structure, such as predation pressure, remain unknown. Predation is an emerging risk for kākahi because, although bivalve predation has been observed in New Zealand waterways, its prevalence and impacts remain unquantified. Aims The present study resolves trophic interactions with predator populations by identifying kākahi predators and examining characteristics of freshwater mussels (e.g. species or size) that may indicate vulnerability to predation. Methods Intensive monitoring was conducted over 6 weeks (January to March 2020) by using trail cameras to observe kākahi predators at the Orongo Stream, a western Waikato (New Zealand) site with high densities of Echyridella aucklandica (threat status: Vulnerable) and E. menziesii (Declining). Predated kākahi shell remains were analysed to (i) identify typical patterns of shell damage, and (ii) determine prey selectivity via size distribution. Key results Eight observations of an avian predator (pūkeko, Porphyrio porphyrio melanotus) interacting with both E. aucklandica and E. menziesii were recorded; however, E. menziesii comprised a significantly greater proportion of the shell remains that had direct evidence of predation. Both species of mussel displayed significantly more damage to the posterior end of the valves but size selectivity was detected only for E. menziesii. Conclusions Pūkeko actively search for mussels and frequently consume them as part of a wider diet when they are accessible during summer. Whereas healthy populations of freshwater mussels may withstand predation by native species, populations that face substantial pressure from other natural stressors and anthropogenic change may suffer. Predation impacts on kākahi populations should be a factor considered for the future management of New Zealand mussel species. Implications Identifying pūkeko as kākahi predators clarifies food webs for conservation managers and highlights the need to quantify their impact on mussel populations; particularly for E. menziesii, which may be more vulnerable to predation.