Background
Globally, freshwater crayfish are imperilled, particularly large species harvested for human consumption. Crustacean fisheries typically rely on size- and sex-specific harvest regulations to counterbalance the adverse effects of over-harvesting which manifests within populations as disparate size truncation, and uneven sex ratios.
Aims
Here, we aim to compare the utility of an existing reproductive indicator with a new indicator of reproductive state, to assess the current harvestable slot legal limit (HSLL – 100 to 120 mm, Occipital Carapace Length: OCL), based on the size at onset of maturity (SOM) and not on any relationship between crayfish size and and the likelihood of females being functionally reproductive (SFR - the probability of a female being in berry given its size). Further, harvest regulations include a ban on possessing crayfish with berries, exposing non-berried females within the HSLL to harvest, and if caught, may be kept and unable to contribute to the evolutionary fitness of the species, contravening the principle of of exploited population dynamics.
Methods
A stochastic population model, combined with field data, was used to examine the risk from harvest pressure on a threatened crayfish Euastacus armatus in south-eastern Australia. We compared a new indicator, size at functional reproduction (SFR – females with eggs), with the contemporary index of size at onset of maturity (SOM) when applying the current
regulations of no-take for females with eggs.
Results
SOM under estimates risk compared with SFR. This risk difference was amplified with increasing harvest pressure, proving SOM inadequate for characterizing risk.
Conclusions
As SFR directly represents the biology of a species and will not inappropriately characterize risk compared with SOM, the new indicator SFR must replace SOM for monitoring, conservation
and fisheries management of all decapods. With recent declines in distribution and abundance, we argue that either the harvestable slot limit length be altered to account for our SFR results or that all female E. armatus be protected until the numerous uncertainties outlined herein are resolved.