Evaluation participative d’indicateurs d’état des ressources en crabes de mangrove dans le sud-ouest de Madagascar (zone de Morombe)

Abstract

The mud crab fishery is important in the world and in Madagascar because of its
contribution to the economic and social plan. Due to the lack of reliable fisheries monitoring,
the state of the resource is not known. So, a participatory monitoring protocol was implemented
in the mud crab fishery in the Morombe area, southwest of Madagascar during april until august
2021. The objective was to assess fishery indicators of the state of crab resources: abundance
index (CPUE) and size structure by measuring cephalothoracic length (CL). Data acquisition
was based on a participatory and collaborative approach by collecting information on
fishermen's activities and their catches. The main gears used in the area were: hook, scale and
line.
A total of 699 crab fishermen were recorded, 68% of whom fished with hooks, 20%
with scales and 12% with lines. The fishing effort recorded during the period was 27 733 trips
(an average of 5 550 trips per month). The minimum and maximum sizes of crabs caught in the
villages were 3.3 cm and 15.6 cm respectively. The average was 10.0±0.2 cm, and almost 60%
of the catches were crabs smaller than 11 cm (minimum legal catch size). The catch per unit
effort (CPUE) was significantly different for the three gears. It was on average 5±0.2 kg/trip
for the hook, 0.2±0.01 kg/trip/hoop net and 0.1±0.01 kg/trip/line. Standardized CPUE using a
generalized linear mixed model (GLMM) revealed that the variables months, gear and their
interaction had a significant effect on crab abundance. It followed the same regressive
downward trend over the monitoring period as well for the different gears. The study of these
indicators shows that crab resources were abundant during different periods in the Morombe
area and that the sustainability of the fishery was threatened with catch of juveniles.