Growth and population dynamics of crayfish Paranephrops planifrons in streams within native forest and pastoral land uses

Population dynamics of crayfish (Paranephrops planifrons White) in streams draining native forest and pastoral catchments, Waikato, New Zealand, were investigated from September 1996 to July 1998. Crayfish densities were generally greater in native forest streams because of high recruitment over summer, but varied greatly between streams in both land uses. Peak densities in summer were 9 crayfish m-2 in native forest and 6 crayfish m-2 in pasture streams, but peak biomass in summer was much greater in pasture streams. Mark-recapture data showed that crayfish, particularly juveniles, in pasture streams grew faster than in native forest streams, through both greater moult frequency and larger moult increments. Females reached reproductive size at c. 20 mm orbit-carapace length (OCL) after their first year in pasture streams, but after 2 years in native forest streams. Annual degree days >10°C appeared to explain the differences in the timing of life cycles. Estimates of annual crayfish production (range = 0.8-3.4 g dry weight m-2 year-1) were similar in both land uses, and P/B ratios were between 0.95 and 1.2. Despite deforestation and conversion to pasture, crayfish in these Waikato hill-country streams have maintained similar levels of annual production to those in native forest streams, although juvenile growth rates have increased and longevity has decreased

Evaluating techniques for sampling stream crayfish (paranephrops planifrons)

We evaluated several capture and analysis techniques for estimating abundance and size structure of freshwater crayfish (Paranephrops planifrons) (koura) from a forested North Island, New Zealand stream to provide a methodological basis for future population studies. Direct observation at night and collecting with baited traps were not considered useful. A quadrat sampler was highly biased toward collecting small individuals. Handnetting at night and estimating abundances using the depletion method were not as efficient as handnetting on different dates and analysing by a mark-recapture technique. Electrofishing was effective in collecting koura from different habitats and resulted in the highest abundance estimates, and mark-recapture estimates appeared to be more precise than depletion estimates, especially if multiple recaptures were made. Handnetting captured more large crayfish relative to electrofishing or the quadrat sampler

New Zealand stream crayfish: Functional omnivores but trophic predators?

1. The feeding ecology of the crayfish Paranephrops planifrons in streams draining catchments in native forest and pastoral land was investigated using analyses of both stomach contents and stable isotopes of carbon and nitrogen. We aimed to (1) identify the energy sources of crayfish, (2) determine whether these were affected by ontogeny or land use change, and (3) assess the functional and trophic roles of crayfish in New Zealand hill-country streams. 2. In native forest streams, crayfish stomach contents were dominated volumetrically by leaf detritus (>60%), but in pasture streams aquatic invertebrates constituted more than 40% of stomach volumes and leaf detritus <30%. Stable isotope analyses revealed that crayfish from both native forest and pasture streams incorporated energy from aquatic invertebrates into their body tissue but did not appear to utilize detritus for growth. Therefore, deforestation had little impact on crayfish energy sources. 3. In native forest streams, adult crayfish (≥20 mm orbit-carapace length (OCL)) consumed greater amounts of leaf detritus than juvenile crayfish, but a corresponding change in stable isotope signatures was not detected. Ontogenetic shifts in diet were not consistent between land use suggesting that change in local habitat and food resources, as a result of land use change, affect crayfish food choice more than factors related solely to age or size. 4. Crayfish appear to occupy the trophic position of a predator, but by functioning as omnivores, they have dual roles as both predators and processors of organic matter. The use of gut content analysis in conjunction with stable isotope analyses revealed that the functional and trophic roles of P. planifrons differed, with implications for the interpretation of diet studies and understanding of the role of omnivores in food webs