Abstract
I investigated the relationship between landuse and the diet and growth of the fresh-water crayfish Paranephrops zealandicus in the Taieri River catchment, South Island, New Zealand. The thesis presents a detailed dietary and growth analysis of P. zealandicus from native bush and pastoral streams. The use of two different methods to assess the crayfish diet gives greater insight into the relative importance of and utilisation of various food items. The marking method presented in this thesis allows for identification of individual crayfish for their lifetime overcoming the problem associated with growth studies in species that retain no annuli.
Stomach content analysis was used to determine food items important to P. zealandicus and assess the influence of land-use, season and size class on the diet.
1. Allochthonous detritus was the main food item and proportionatey more was ingested in pastoral streams and less in native bush streams in every season except autumn.
2. An ontogenetic shift in diet was observed with larger crayfish consuming greater amounts of allochthonous detritus than medium or smaller size classed individuals.
3. Aquatic invertebrates were the second most commonly ingested food item but on average made up less than 4% of the total stomach volume.
4. P. zealandicus ingested a wide range of invertebrates with Deleatidium (Ephemeroptera: Leptophlebiidae), Potamopyrgus (Prosobranchia: Hydrobiidae) and non-biting midges (Diptera: Chironomidae) the most commonly ingested invertebrate species.
5. P. zealandicus from native bush streams contained more aquatic invertebrates in their stomachs than those from pastoral streams (average 1.4 and 0.9 respectively).
6. Moss, diatoms and terrestrial invertebrates made up little of the volume of P. zealandicus stomach contents; only one diatom and no algae was found in P. zealandicus stomachs.
Stable isotopes of carbon (ratio of 12C to 13C) and nitrogen (ratio of 14N to 15N) were used to investigate the energy utilisation of P. zealandicus in streams with native bush or pastoral catchments.
1. Epilithon and aquatic invertebrates were identified as important to the diet of P. zealandicus from pastoral streams, whilst aquatic invertebrates were more important to native bush stream crayfish.
2. Crayfish from all streams had isotopic values consistent with an omnivorous nature.
3 Little evidence for an ontogenetic shift in diet was found.
4. Allochthonous detritus and moss were identified as of little importance as an energy source for P. zealandicus.
5. The results suggest that there was an important 13C depleted food source in both stream types that was not identified by this study.
Annual growth rates were measured and compared for P. zealandicus occupying streams in native bush or pastoral catchments.
1. Large P. zealandicus (>30mm orbital carapace length) grew faster in native bush and slower in the pastoral streams.
2. In general crayfish were larger in native bush streams, and there were greater numbers of small crayfish in pastoral streams.
3. No difference was found in the number of annual degree-days, however, the number of degree days >l0°C was significantly higher in pastoral streams.
4. Native bush streams had higher pH, alkalinity and calcium levels.
5. The different combination of abiotic factors present in the two
categories of streams appear to produce similar rates of growth for
smaller P. zealandicus.
6. The combination of alkalinity and calcium concentrations appear to
be more important than temperature in dictating growth in larger
crayfish.