Abstract
Interactions between predator and prey contribute towards shaping food webs and can be responsible for the composition of species in an ecosystem. In semi-natural aquaculture ponds, interactions between predators and competitors can be unfavorable for farmed species as they might reduce biomass in ponds and therefore economic growth. Juvenile farmed species are significantly more vulnerable in pond aquaculture as size is usually the factor that dictates trophic levels in aquatic habitats. Odonata are a family of pond invertebrates that are considered opportunistic predators and are known to feed on the prey of many juvenile vertebrate and invertebrate ponds species. Their ability to successfully capture prey makes them top invertebrate predators and how they interact with farmed species, specifically juveniles in aquaculture pond systems has been speculated. In New Zealand (NZ), semi- natural aquaculture ponds are used to cultivate the freshwater water crayfish species Paranephrops zealandicus yet how this species interacts with other pond invertebrates, particularly odonate larvae has not been studied. It was therefore the aim of this thesis to further investigate the predator-prey interactions between NZ odonate larvae and the farmed species P. zealandicus in semi-natural aquaculture ponds. This thesis examines predator-prey interactions based on species spatial patterns and habitat use in ponds, as well as stomach content and stable isotope analysis on study species.
Results indicate odonate larvae have no significant effect on the abundance of P. zealandicus in ponds. Vegetation and the densities of P. zealandicus in ponds are more likely to have a greater impact on odonate larvae and juvenile P. zealandicus abundance. Desirable habitat was occupied by larger P. zealandicus which forced juvenile P. zealandicus to seek refuge in areas of the pond that were not attractive for larger territorial individuals. Based on stomach content analysis, odonate larvae were found to primarily feed on small sedentary pond invertebrates such as Chironomidae P. zealandicus tissue was exceptionally low in odonate larvae stomachs. Stable isotope analysis further showed all odonate larvae had lower trophic levels to juvenile P. zealandicus indicating odonate larvae are unlikely to be major predators of juveniles. P. zealandicus diets reflected their omnivorous nature feeding on both plant detritus and invertebrate tissue, however P. zealandicus were highly cannibalistic with adults found to have high volumes of P. zealandicus tissue in their stomachs. Stable isotope analysis showed between P. zealandicus and juvenile crayfish there was no significant difference in trophic levels. Year 1 crayfish represented much of the P. zealandicus sample used for stable isotope analysis and adults were poorly represented in the sample which could be the reason for this result. Therefore Year 1 P. zealandicus are unlikely to be feeding on juveniles, however more research needs to be carried out looking at the trophic interactions between adult P. zealandicus and year of young.
Ultimately, odonate larvae are not likely to be major predators of juvenile P. zealandicus, and cannibalism is more likely to be the main factor for low juvenile success in ponds. Future research investigating other pond invertebrates and ways to mitigate cannibalism in ponds is speculated