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Keywords:

  • biodiversity;
  • habitat;
  • harvesting;
  • introduced;
  • Madagascar

Summary

1. Freshwater crayfish of the genus Astacoides are endemic to the highlands of eastern Madagascar. Very little is known about their ecology and how this affects their vulnerability to threats. Working in the Fianarantsoa forest corridor, we used a combination of ecological research (>29 000 crayfish caught and released) and interviews (>130 interviews in 38 villages) to investigate the ecology and status of four of the seven described species.

2. Astacoides species studied showed very slow growth, with growth rates of Astacoides granulimanus and Astacoides crosnieri among the slowest known in any species of crayfish. We found individuals of all three species for which we had growth data which we estimate at more than 20-years old. The size at which females became ovigerous varied among species. Astacoides betsileoensis withheld reproduction until a large size (only 30% of females were gravid at 60-mm carapace length, compared with 90% for A. granulimanus). This is likely to make A. betsileoensis particularly vulnerable to overexploitation and we found that only 10% of individuals measured in a market (n = 909) would have reproduced before being caught compared with 35% of the more common A. granulimanus (n = 30 561).

3. Habitat loss is a serious threat to the genus; even A. granulimanus, the most widespread species, was only found in rivers or streams flanked by natural vegetation. Astacoides caldwelli, the rarest species in this study, was found only at low altitudes (<800 m) in rivers draining forested catchments. Habitat loss is particularly rapid in low elevation forest. A. crosnieri was restricted to swampy land that is rapidly being converted to rice fields. Introduced Asian snakehead fish (Channa maculata) may pose a hitherto unrecognised threat to some species, particularly A. betsileoensis.

4. Madagascar's freshwater habitats have great significance for global biodiversity yet conservation effort, as in much of the world, has focused on terrestrial ecosystems. Until recently almost nothing was known about the ecology of Astacoides crayfish, a diverse and economically important genus. Here, we show that members of the genus vary markedly in their reproductive biology, growth rates, habitat requirements and the threats they face. We suggest that habitat loss is an urgent threat, especially to A. caldwelli and A. crosnieri, while overharvesting is probably the most immediate threat to the larger A. betsileoensis. We call for more attention to be paid to Madagascar's exceptional, yet understudied, freshwater biodiversity.