fish-mangrove interactions, the spatial aspects of small-scale fisheries locations, and the productivity of these fisheries within the Derawan Marine Conservation Area) will be integrated, and new directions for research will be presented.
6.2.1 The fish-mangrove interaction, the structural complexity, and the
nursery function
Mangrove forests are home to many wildlife species. The increasing human population in the delta where the mangrove forests occur has led to an increase in disturbance to this ecosystem. Human disturbances in mangrove forests might be dominant factors driving a transition in land use types, such as coastal developments for settlements, aquaculture ponds, roads, and industrial areas, triggering coastal erosion and mangrove conversion (Bengen 2003, Sukardjo et al. 2014). In chapter 2, I focused on the anthropogenic distur- bances in the mangrove ecosystem. The decrease in the extent and quality of mangrove forests in the world has been well documented (FAO 2004, Spalding et al. 2010, Giri et al. 2011, Sukardjo et al. 2014). However, as discussed in the general introduction, the function of mangroves as a nursery area for juvenile marine species is not entirely un- derstood, and we do not know how human disturbance affects the relationships between nursery grounds and juvenile fish.
The nursery function of structurally complex coastal habitats is useful for growing marine organisms (Edgar and Shaw 1995, Able 1999, Edgar et al. 2000, Nagelkerken et al. 2001, Nagelkerken et al. 2002, Manson et al. 2005b, Dahlgren et al. 2006, Nagelkerken et al. 2012, Lee et al. 2014, Nagelkerken et al. 2015). These services depend on the habitat size, root structural complexity as one of mangrove attributes, and standardized coverage per unit area. If the mangrove habitats are disturbed then the extent, root structural com- plexity, mangrove cover and the number of species will decrease. These changes have also been documented in previous studies (Blasco et al. 1996, Valiela et al. 2001, FAO 2004, Blaber et al. 2010, Spalding et al. 2010, Sukardjo et al. 2013b). To obtain a clearer picture of the effects of disturbance on the mangrove attributes, I studied mangrove roots, seedlings, saplings, and size of mangrove trees. I measured tree densities and root struc- tural complexity concerning human disturbance and natural sources of disturbance such as coastal erosion through wave action (Chapter 2). Mangrove forests in the Berau Delta were dominated by Rhizophora sp. and Nypa fruticans. The estimated area of mangrove forests that was lost reached 30%, from 78,000 ha originally (1991) to 56,000 ha (2009) mainly due to infrastructure and aquaculture ponds from Nypa palm areas (Chapter 2; Ta- ble 2.3b). Three reasons have been given for the selection of mangrove palm conversion: i) farmers do not need permits for the conversion, ii) the soil type of mangrove palms is more suitable for pond construction than that of other mangrove trees, iii) mangrove trees protect the aquaculture ponds against wind and wave erosion. This disturbance affects the spatial configuration of the mangrove habitat and led to fragmentation of the nursery habitats. Moreover, in chapter 2, I also showed that mangrove root length and sapling density increased closer to villages in the Delta, while the trees diameter decreased. The trees diameter decline could have been caused by the removal of large trees by local
communities, e.g., for construction material, leaving smaller trees closer to settlements, and opening the forest. The remaining gaps near villages’ increases the availability of sunlight in the area, facilitating germination and seedling growth, partly explaining the high abundance of sapling density and larger root lengths there.
To quantify the changes in mangrove cover and its extent, I used remote sensing tech- niques. There are various techniques to quantify spatial changes at the scale of an entire delta. The question remains whether commonly applied methods, such as remote sensing (Field et al. 1998, Valiela et al. 2001, Lucas et al. 2007, Vaiphasa et al. 2007b, Mcleod et al. 2011, Fauzi et al. 2014), are suitable for quantifying anthropogenic disturbances in mangroves forests, especially when root structural complexity is essential. Moreover, mangroves extent and tree structure might not only influence the productivity of fisheries but also be related to species diversity in the area. Currently, the field of LiDAR offers news prospects to quantify tree structure (Naidoo et al. 2012, Wulder et al. 2012) in inac- cessible places such as mangrove forests, although its application to mangrove structural complexity is still not fully exploited (Kamal et al. 2015).
Interestingly, changes in mangrove habitats might not only influence the marine commu- nity but also in the long term, the livelihood of local communities who depend indirectly on the marine resources (Chong 2007, Lee et al. 2014). The disappearance of a single mangrove species, such as the pioneer species Rhizophora sp. in and alongside cannals due to aquaculture pond construction, might negatively affect the quality of the habitat, because this species has a unique and extensive root structure. The complex root system attracts a wide variety and vast abundance of juvenile fish and shrimp during high tides, offering food and shelter (Primavera 1995, Laegdsgaard and Johnson 2001, Cocheret de la Moriniere et al. 2004, Tong et al. 2004). A lower mortality during the juvenile stages is therefore expected, due to a lower natural predation pressure (because of the higher shelter possibility and larger food availability in the nursery area). The nursery habitat is therefore expected to increase the abundance of young sub-adult fish and shrimp, and hence sustain the fishing, activities, supporting the future livelihood of local communities. Comparing the four deltas in East Kalimantan Province, the degree of anthropogenic disturbance on the mangroves in the Berau Delta is probably smaller than in three other Deltas. Regarding extent habitats and species distributions as documented in various pub- lications for the Mahakam Delta (Zuhair 1998, Dutrieux 2001, Suaib 2004, Sidik 2008, Nursigit et al. 2013, Fauzi et al. 2014), Bulungan-Nunukan Delta (Prasetyo 2010, Ilman et al. 2011) and Pasir Delta (MacKinnon et al. 1997, Sukardjo 2009).
The underlying mechanisms of anthropogenic disturbance on mangroves habitat for ma- rine juveniles
Mangrove in the delta and estuaries hold different levels of structural complexity (Rön- nbäck et al. 1999, Rönnbäck et al. 2002). Different zones within a mangrove ecosys- tem have different mangrove species that influence the complex nature of the root struc- ture (Robertson and Duke 1987, Robertson and Alongi 1992, Laegdsgaard and Johnson 2001). So, human disturbance in mangroves can affect not only the species diversity, the