Abstract
Globally wild fish stocks are declining due to increased demand and overharvest, causing problems across the marine food webs and ecosystems. Aquaculture has risen to meet this increase in demand and will soon overtake wild fisheries in global protein production. To continue to feed the world’s increasing need for protein as the global population rises, there needs to be continuing innovation and research into all forms of aquaculture to become more efficient, use less area and resources as well as being environmentally friendly.
Crayfish aquaculture (or astaciculture) occurs worldwide with at least 10 freshwater crayfish species used in large scale production. These species are used due to high growth rates and fecundity, low energy inputs and scope to intensify. Currently 150,000 tonnes of crayfish are produced in inland astaciculture systems annually, and this production expected to increase as systems become more efficient and widespread.
In New Zealand, Paranephrops zealandicus is a relatively new species to astaciculture. Grown in natural earthen ponds it requires no minimal inputs and low maintenance resulting in a high profit system. These ponds have been identified as an opportunity for landowners to establish ponds on land not suitable for agriculture or forestry as a means of diversification. P.zealandicus is a sought after food source with high cultural significance and is sought after for high end dining, resulting in high demand indicating a bright future. Currently P.zealandicus production is poorly understood compared to other astaciculture operations and the information on best practice is not as researched as international examples. P.zealandicus production systems are often restricted by variation in success of ponds as well as restrictions to production due to limiting nutrients within these systems. Research is needed to increase the production of P.zealandicus to improve the viability of these operations.
The first aim of this study was to determine which variables from pre-established ponds influence the production of P.zealandicus biomass. Twelve ponds within Ernslaw One Ltds ‘Keewai’ P.zealandicus aquaculture operation were compared across several key variables likely to influence P.zealandicus production. Of the variables assessed, there were no significant relationships with P.zealandicus biomass production. However, the results did indicate that nutrient limitation was likely occurring across all the study ponds.
The second aim of this study was to determine whether P.zealandicus ponds would benefit from fertilisation with key nutrients. Many of New Zealand freshwater systems are nutrient limited. Elsewhere, fertilisation of ponds to boost production is common in astaciculture systems. Other studies on P.zealandicus have shown that calcium addition increases P.zealandicus growth. Three treatment levels of superphosphate were added to a series of ponds in Ernslaw One Ltds ‘Keewai’ P.zealandicus aquaculture operation. Responses of chlorophyll-a, water nitrogen and phosphorus and invertebrate abundance, richness and diversity were assessed prior to, and for the following three months post- fertilisation. There were no statistically significant relationships between phosphate addition and water chemistry or invertebrate abundance or diversity. However superphosphate did slightly increase invertebrate taxa richness. This study did confirm that these ponds are nutrient limited and that added nutrients did not remain in the water column long enough to have a strong impact on pond productivity. Future studies should investigate how these nutrients are removed from the water column and whether it is being assimilated by ponds food webs and ultimately P.zealandicus, or is it being sequestered in pond sediments and therefore removed from pond food webs.
This study outlines the importance of further study into the relatively new field of P.zealandicus aquaculture in New Zealand to ensure that success of ponds is increased and more are able to join this exciting venture.