Habitat suitability maps for juvenile tri-spine horseshoe crabs in Japanese intertidal zones: A model approach using unmanned aerial vehicles and the Structure from Motion technique

Abstract

The tri-spine horseshoe crab, Tachypleus tridentatus, is a threatened species that inhabits coastal areas from South to East Asia. A Conservation management system is urgently required for managing its nursery habitats, i.e., intertidal flats, especially in Japan. Habitat suitability maps are useful in drafting conservation plans; however, they have rarely been prepared for juvenile T. tridentatus. In this study, we examined the possibility of constructing robust habitat suitability models (HSMs) for juveniles based on topographical data acquired using unmanned aerial vehicles and the Structure from Motion (UAV-SfM) technique. The distribution data of the juveniles in the Tsuyazaki and Imazu intertidal flats from 2017 to 2019 were determined. The data were divided into a training dataset for HSM construction and three test datasets for model evaluation. High accuracy digital surface models were built for each region using the UAV-SfM technique. Normalized elevation was assessed by converting the topographical models that consider the tidal range in each region, and the slope was calculated based on these models. Using the training data, HSMs of the juveniles were constructed with normalized elevation and slope as the predictor variables. The HSMs were evaluated using the test data. The results showed that HSMs exhibited acceptable discrimination performance for each region. Habitat suitability maps were built for the juveniles in each region, and the suitable areas were estimated to be approximately 6.1 ha of the total 19.5 ha in Tuyazaki, and 3.7 ha of the total 7.9 ha area in Imazu. In conclusion, our findings support the usefulness of the UAV-SfM technique in constructing HSMs for juvenile T. tridentatus. The monitoring of suitable habitat areas for the juveniles using the UAV-SfM technique is expected to reduce survey costs, as it can be conducted with fewer investigators over vast intertidal zones within a short period of time.

Spawning habitat suitability maps for the conservation of the tri‐spine horseshoe crab Tachypleus tridentatus in Tsuyazaki Cove, Fukuoka, Japan

“…However, this was impractical for the sedimentary parameters. Koyama et al (2020) applied a habitat suitability model for T. tridentatus juveniles using Maxent, and sedimentary parameters, one of the factors influencing the distribution of juveniles, was not included for the same reason. Furthermore, Iyooka et al (2015) pointed out that since remote sensing is still a developing technology, it is expected that further developments will improve the ability to estimate habitat factors, such as sediment that influence the target species' distribution.…”

“…T. tridentatus is not commercially harvested in Japan (Sekiguchi, 1989), thus overfishing is not considered an issue. Annual juvenile records from 2003 to 2019 using a standard method show a steady decline in juvenile numbers in the area; on average, 97 juveniles were recorded per survey between 2003 and 2006 (Wada et al, 2008), whereas the 2007–2016 surveys averaged 24 individuals per survey (Itaya & Shuuno, unpublished data) and an average of only seven juveniles per survey from 2018 to 2019 (Koyama et al, 2020). The suitable nursery habitat in Tsuyazaki was estimated to be 61,000 m 2 (out of the investigated area of 195,000 m 2 ) (Koyama et al, 2020).…”

“…Annual juvenile records from 2003 to 2019 using a standard method show a steady decline in juvenile numbers in the area; on average, 97 juveniles were recorded per survey between 2003 and 2006 (Wada et al, 2008), whereas the 2007–2016 surveys averaged 24 individuals per survey (Itaya & Shuuno, unpublished data) and an average of only seven juveniles per survey from 2018 to 2019 (Koyama et al, 2020). The suitable nursery habitat in Tsuyazaki was estimated to be 61,000 m 2 (out of the investigated area of 195,000 m 2 ) (Koyama et al, 2020). There have been no landfills or development works within the juvenile habitat between 2003 and 2019, hence no further habitat destruction in the area.…”

“…There is a tidal mud flat on the north side of the cove, which is the habitat for T. tridentatus juveniles. Numerous studies on T. tridentatus have been conducted in the cove, including on the juvenile population (Wada et al, 2008), the suitable juvenile habitat (Koyama et al, 2020), the adult habitat and movement patterns (Wada et al, 2016), the spawning population (Wada, Itaya & Shuuno, 2010; Itaya et al, 2019; Itaya, Wada & Shuuno, 2022) and the spawning environment (Itaya et al, 2019; Itaya et al, 2022). According to Wada, Itaya & Shuuno (2010), Sites 1–4 are essential for the local population as about 70% of T. tridentatus spawning is concentrated on these small beaches (Figure 2).…”

“…There is a tidal mud flat on the north side of the cove, which is the habitat for T. tridentatus juveniles. Numerous studies on T. tridentatus have been conducted in the cove, including on the juvenile population (Wada et al, 2008), the suitable juvenile habitat (Koyama et al, 2020), the adult habitat and movement patterns (Wada et al, 2016), the spawning population (Wada, Itaya & Shuuno, 2010;Itaya, Wada & Shuuno, 2022) and the spawning environment .…”