Participatory assessment of the mud crab fishery in Southwestern Madagascar

Background
The mud crab fishery makes important economic and social contribution in
Madagascar. However, the state of the resource is not known due to the lack of
reliable fishery monitoring. Indicators are usually used to measure impacts of fishery
management interventions because they can help detect change in the state of the
fishery. The objective of this study was to assess fishery indicators of mud crab
resources such as an abundance index or capture per unit effort (CPUE) and size
structure by measuring cephalothoracic length (CL) between the tips of the crab.
Method
Data acquisition was based on a cost-effective participative approach. A total of 13
voluntary buyers and two community members monitored mud crab fishers’ activities
and catches from April to August 2021 in eight villages in Southwestern Madagascar.
Results
A total of 700 mud crab fishers were recorded in the area, mainly using hooks (68
%), locally-made hoop nets (20%), and lines (12%). Fishing effort averaged 5,550
trips per month. The average size of crabs was 10.0±0.2 cm CL (minimum size : 3.3
cm and maximum size : 15.6 cm) while almost 60% of the catch was composed of
crabs smaller than 11 cm CL (i.e., size at maturity and minimum legal catch size of
that species in Madagascar). Mean CPUE was significantly different across gears
(hook : 5±0.2 kg/trip, hoop net : 0.2±0.01 kg/trip/unit, and line : 0.1±0.01 kg/trip/unit).
Standardized CPUE using a generalized linear mixed model (GLMM) revealed that
crab abundance significantly varied across months and gear types. CPUE followed
the same downward trend over the monitoring period across the three gears
suggesting that the mud crab resource was more abundant from April to June (end
of the high season for mud crabs) that from July to August (winter) in that area.
Conclusion
Our results suggests that the sustainability of the fishery was threatened by the large
number of juvenile crabs in catch. The participative monitoring survey method was
able to detect seasonal change in mud crab abundance in that case study and would
be transferable to other fishery areas so as to capture the trend in resource status at
the national scale.