First Results Of Reef Fish Spatial And Temporal Dynamics In The Bay Of Ranobe Fishery, Southwestern Madagascar

Background
Small-scale multi-gear and multi-species fisheries in the tropics are an important
source of food and income for coastal populations. However, fishing is the main
anthropogenic threat to marine ecosystems such as coral reefs that coral reef
fisheries depend. Indeed, worldwide coral reefs face an increasing degradation due
to pollution, sedimentation and climate change. The reef fishery of the Bay of
Ranobe is the main exploitation area in southwestern Madagascar (143 km²), where
these problems related to reef degradation have been reported for many years.
Besides, this reef fishery suffers from poor management due to the lack of reliable
data and/or resources to allow an accurate diagnosis of the risk of their
overexploitation and to ensure their sustainability.These situations highlight the great
need to rebuild this ecosystem and the resources it provides. Artificial reefs are
among large spread solutions proposed to rebuild fisheries production. Known as an
artificial reef, standard ARMS (Autonomous Reef Monitoring Structures) can collect
representative local communities and up to 85% of all known reef biodiversity.
Through the ARMS Restore project, implemented in the Bay of Ranobe (2021-2024),
new tool ARMS will be used to sample the diversity of coral around and seed
artificial reefs. The project’s primary goals are to increase the number of fish and
other marine species that can be harvested for food and income and better continue
to protect shorelines.This study aims to provide complete and fine scale baseline
data to assess the effectiveness of artificial reefs seeded with ARMS in
enhancement of fisheries production, in the one hand, and to evaluate exploitation
level of this fishery based on fisheries indicators, in the other hand.

Method
To achieve these goals, a fishery survey and a fish biodiversity monitoring of 14
months have started in 12 fishing villages along Bay of Ranobe (October 2021 -
December 2022). To evaluate fishing effort and fishing power, a record of reef
fishers and gear measurement (gillnet, beach seine and mosquito trawl net) was
done before the start of the survey. Each 30 days, we monitor the activity of 103
fishers targeting reef fish using gillnet, handline, fishing speargun, trawl net, beach
seine and mosquito trawl net. Fisheries survey includes three components: boat
movement monitoring using GPS trackers, participatory survey of fishers catches,
the study of fish biodiversity via catches landing survey (fish images collection) and
DNA barcoding approach for a precise species identification. Also, indicators based
on size is recorded to determine length at first maturity (Lmat) of each species
caught.

Results
A total of 1,460 reef fishers were recorded in the 12 villages of the bay, the majority
of fishers are using gillnet (40%) and localized in Andrevo village (16%). For the
fishing gears, 248 gillnets, 83 mosquito trawl nets and 13 beach seines have been
measured within the bay. To date our results, concern the warm season (November
– April). With 279 fishing landings monitored, we collected 850 fish images. It has
been found that 60 families of reef fish are exploited in Bay of Ranobe with the
dominance of Labridae in terms of abundance. These fish will be identified at lowest
taxonomical rank after DNA barcoding analyses of the 1823 tissues collected during
the survey. Analysis related to indicators based on size are yet in progress.

Conclusion
This exhaustive study is the first within the bay that characterize the fishery with
complete and accurate data thanks long period survey and with innovative methods.
It actually involves: catch diversity at the species level and their spatial repartition in
habitats, effective fishing effort (in hour) and production distribution via GPS survey,
seasonal variation and size structure of catches within the bay of Ranobe. Our
results demonstrate the necessity of multidisciplinary approach to better manage the
multi-gear fishing effort within a community and regulatory approach.