Assessing a mud crab fishery and integrating suitable management in Borneo, Indonesia
In association with Planet Indonesia
Third world and poverty-stricken areas like southeast Asia have the most under-reported and over-exploited fisheries in the world. As demand increases, so does effort. In a fishery that is not managed, this leads to over-exploitation and unsustainable catch rates. Technological advancements in fishing equipment further increase catch rates and without management or monitoring, mass reductions in population number (and likely extinction) will occur.
Oceanwise Australia and Planet Indonesia, are working to provide environmental management and socio-economic benefits to remote villages in West Kalimantan, Indonesia. This project aims to promote conservation, manage natural resources, improve living standards and provide community-based services to create long-term, sustainable solutions to these remote fishing villages. One of Oceanwise Australia’s roles is to monitor the significant mud crab fishery in the village of Sungai Nibung. We assess the population status with the aim to design a management plan ensuring the longevity of the mud crab populations. Once the framework has been tried and tested in one village, the management framework will be applied to surrounding villages to. This strategy is largely monitored and enforced by local fishermen and community members, making it unique to many other fisheries management projects around the world.
Mud crab species (Scylla sp.) are a commercially important species worldwide, including Indonesia. Four species occur globally, of which at least three are found in the West Kalimantan area (Figure 1). They are nocturnal and forage on incoming tides within and around mangrove and estuarine habitats. It is critical to understand the behaviour and ecology of the species to create sustainable practices and ensure the livelihoods of the local fishermen. Firstly, a Temporary Mangrove Reserve (TMR) system was setup with three-month fishing closures, which are enforced biannually. Specific rivers were selected by community members and protected mud crabs, as well as fish and prawns in the area. To assess the population of the mud crabs, a survey of year-round seasonal data was collected to understand the migratory patterns of the species. At this point in the project, data has been collected in November 2018 and February 2019, along with data from the fishermen. This included abundance, weight, carapace size and sex of the crabs. This will yield information on the catch rates of the fishery as well as an indication on the effects of having managed closures in the village.
Figure 1. Variations in mud crab morphology, Scylla sp., in Sungai Nibung and surrounding rivers [Photos: Caroline Hart & Ben Fitzpatrick].
The data we have collected so far indicates that, overall, the fishery has historically been overfished, and that catch rates per unit effort (CPUE) is not being maximised. Just prior to the implementation of the closures, male crabs accounted for 69.8% of the recorded catch and were generally larger within the TMR rivers. This suggests that the closure does benefit the overall population of crabs but there are seasonal migrations of the females crabs that affect the results. This was supported by in February, which yielded extremely low numbers of females. The sex ratio for the November sampling period was 1.3:1 compared to 2.3:1 at the end of the closure in February. Results for both sampling periods are summarised below (Table 1).
Table 1. Summary of crab population data within and outside of Temporary Mangrove Reserve rivers obtained by Oceanwise, across two sampling periods.
Across the two sampling periods, a notable difference was also the average size of the crabs in the TMRs. Although there was a high capture rate of male crabs outside of the TMR, these individuals were smaller in size, suggesting the larger mud crabs have been fished out in the open rivers. It also suggests the larger, more dominant crabs inside the reserve drive away smaller crabs into areas where the large males have been fished out, explaining the smaller crab capture rate outside of the TMR.The average carapace length within the TMRs increased significantly from 8.1cm to 14.61cm. In the openly fished areas, however, this change was relatively insignificant (10.48cm to 11.9cm) (Figure 2).
Figure 2. The mean size of the crabs captured in the two sampling periods was noticeably different in the TMR, with an increase from 8.1cm to 14.61cm average carapace length. The 3-month closure appears to have allowed larger crabs to establish territories and not be caught in these rivers. There was a much smaller change observed outside of the TMR, which could be due to the level of fishing effort remaining the same in these areas.
Conclusions and future directions
Based on the data collected from two sampling periods, the dynamic of the mud crab population in Sungai Nibung is highly complex. Overall, the implementation of the TMR system does lead to a number of benefits and should be continued. However, the nature of these benefits are not simple and further monitoring and data collection is required to get an idea on the long-term effects. At the beginning of the year, during the female migratory season, the TMR has the potential to protect the territorial male crabs that remain in the rivers all year round. When the fisheries are opened, these mature males will be caught and the benefits realised will be quickly diminished. Ideally this reserve would be closed year-round in order to ensure that females migrate back to an abundant and reliable male population. Without the closures for protection male crabs, in particular, will be over-fished throughout the year and the breeding cycle will be disrupted when females migrate back to the rivers. This will cause a decline in average size, shift in sex ratio and reduction in crab populations overall. It must also be stressed that numbers of crabs found in these rivers is dependent upon the supply of new recruit settling into the area.
This is a summary of some preliminary findings of two field trips. Future trips will yield more results and expand on the long-term effects of the TMR system. Local fishermen will continue to collect data from their catches before and after closures and aid in the development of sustainable fishing practices for Sungai Nibung and surrounding villages.
Note on fishery management and monitoring
The aim of the TMR and other management initiatives is to create a sustainable fishery while maintaining income for the local villagers. CPUE is one way of measuring the efficiency of this fishery and will help us to understand how much effort per fisherman is required to maximise overall income from the fishery while maintaining a sustainable long-term mud crab population. In fisheries, the highest amount of effort does not lead to the highest income or best result. Initially catch number will increase with increased fishing effort, but after a while more traps will be required to catch fewer crabs, until the population crashes (Figure 3). Optimising CPUE will help to identify the ideal number of traps per fishermen that will yield the most efficient catch. Along with the TMR system, this will maximise income and sustain the fishery for the future.
Figure 4. Maximum sustainable yield (MSY) curve, showing practice of exploiting a fishery is not to maximise effort. The curve represents the population numbers of a fishery, where too much effort leads to voerfishing. An ideal level of effort (unique to every fishery) is highlighted to show the point at which yield (income) is maximised.