A Baseline of Fish Species Richness Through <scp>eDNA</scp> Metabarcoding in an Understudied Tropical Mangrove Coast of Java, Indonesia

ABSTRACT Mangrove ecosystems support a diverse array of animal species and also provide pivotal ecosystem services, such as coastal protection, food provisioning, and carbon capture. However, these vital habitats are in decline, leading to coastal degradation in many parts of the globe. To address this, a mangrove restoration project in Demak, Java, Indonesia, introduced the use of semi‐permeable coastal protective barriers made of bamboo pilings to safeguard the shore zone and hinterlands. The introduction of such hard substrate in the marine environment can attract a range of species, and it is important to be able to monitor changes in biodiversity from a restoration point of view. Here, we assessed whether environmental DNA metabarcoding can be applied to monitor fish biodiversity in an understudied area. Our results show slight but significant differences in species richness and fish community composition within a short timeframe of only 4 months, although we cannot disentangle the effects of seasonal variation from those of the introduction of hard substrate. More importantly, this study demonstrates a useful level of temporal resolution of eDNA metabarcoding and establishes a baseline for fish species richness in an understudied mangrove coastal zone in Demak, Java, Indonesia. Our results are of value for informing future restoration efforts and other (metabarcoding) biodiversity studies in the region.