Population ecology of the freshwater crayfish Paranephrops zealandicus and its effect on the community structure of a lowland bush stream

Abstract

My research describes aspects of the population ecology of the freshwater crayfish Paranephrops zealandicus, its influence on the community of a lowland bush stream in Otago, New Zealand, and the ecosystem consequences of their removal.

Production and longevity were assessed for a population of the crayfish in Powder Creek, a headwater stream draining a catchment of regenerating coniferous-broadleaf forest in eastern Otago. 1. Crayfish abundance was 3-4 individuals m-2 in riffles and 4-12 individuals m-2 in pools. 2. Crayfish biomass (4-33 g AFDW m-2) and production (2-11 g AFDW m-2y-1) were high in comparison to equivalent overseas studies. 3. Production was dependent primarily upon the magnitude of biomass rather than growth rate. 4. Individual crayfish were found to be slow growing and long lived in comparison to other crayfish species. Crayfish with ages estimated to be 16+ years were not uncommon. Female crayfish were not reproductively active until 6+ years. 5. Fewer than 4% of all females carried eggs, and the young remained attached to the female for a minimum period of 15 months.

Enclosure experiments in Powder Creek were used to examine the influence of freshwater crayfish on the invertebrate community.

1. Changes in the structure of the stream community occurred according to whether crayfish were present or absent. 2. Enclosures without crayfish accumulated silt. 3. Crayfish of different size did not influence the stream community differently. 4. Orthocladiinae and Chironominae (Diptera: Chironomidae) larvae were negatively affected by crayfish presence, while the dominant mayfly, Deleatidium (Ephemeroptera: Leptophlebiidae) was more abundant in the presence of crayfish. 5. Bioturbation, caused by the foraging and movement of crayfish, appears to be responsible for these patterns. 6. Like other freshwater decapods, the impact of P. zealandicus on the headwater community appears to be as an indirect result of their foraging behaviour and high biomass.

The stomach contents of crayfish captured from six headwater streams of Otago were analysed to determine the predatory capabilities of P. zealandicus. 1. Leaf litter fragments dominated the stomach contents of all crayfish sizes. 2. Crayfish preyed on a wide range of aquatic invertebrates. 3. Whole invertebrates were frequently found in the gut, while plant material was well masticated. 4. Deleatidium, Aoteapsyche and chironomids were the most common prey. The high incidence of these taxa appears to be attributable to their availability. 5. P. zealandicus shows an ontogenetic shift in diet, characteristic of many crayfish species, with small crayfish consuming a lower proportion of plant tissues than large crayfish.

The work contained in my thesis represents the first ecological study on New Zealand crayfish in streams which combines both population and community ecology. The methods used in my study are limited to identifying a number of the possible mechanisms by which crayfish may affect their community. As such, the research contained within this thesis is insufficient to predict generalised effects of their local extinction.