Assessment of Invertebrate diversity and fishing by mosquito net trawl in seagrass beds, Southwestern Madagascar

Background
In the Indian Ocean, the use of mosquito trawl nets has much developed on
seagrass beds that provide appropriate, easily accessible areas for that gear type. In
the bay of Toliara (Southwestern Madagascar), harboring one of the major Malagasy
coral reef fisheries, the number of mosquito trawl nets has burst over the past
decade, which has led to intense exploitation of seagrass fish and invertebrate
communities.

Method
This study aimed to characterize the abundance and diversity of macroinvertebrates
in seagrass beds by analyzing the spatial and temporal patterns of mosquito trawl
net catches in the bay of Toliara. Monthly catches of 12 fishers were monitored for 7
months in 2018-2019. The taxonomic richness, diversity, biomass and size structure
of invertebrate community were assessed in relation to the nature and composition
of the seagrass habitats.

Results
Overall, 40 taxa of macroinvertebrates were observed in catches, mainly crabs
(mainly Portunidae) associated with shrimps (mainly Processidae, Hippolytidae, and
Palaemonidae) and Cephalopods (Octopodidae and Loliginidae). The crab
community was dominated by the genus Thalamita (two species) which represented
the large part of catches both in total abundance (67%) and biomass (70%).
Macroinvertebrates constituted the by-catch of mosquito trawl nets, which mainly
targeted juvenile reef fishes. Invertebrate by-catch yielded from 1 to 72 % of the
fishers’ income and was used for animal feed (93,9 %) and human consumption
(6,1%).

Conclusion
Our results suggest that mosquito trawl nets have strong ecosystem effects on
seagrass-associated macroinvertebrate community while sustaining the livelihood of
> 100 households, which makes management of the fishery difficult. Implementing
gear restrictions would involve building social capacity within the coastal
communities while improving knowledge on the negative effects of the use of
mosquito trawl nets and other gears (i.e., gillnet, beach seine) in the fishery.