Assessment for seawall modifications to reduce land crabs roadkills

Abstract

Vertical seawalls increase the difficulty and roadkilled risk for land crabs to cross in Gaomei Wetland. The Third River Management Branch of Water Resources Agency, the competent authority in charge, in Taiwan conducted seawall modification project in 2022, which modified vertical seawalls to sloped seawalls for mitigating roadkill mortality of land crabs. In the present study, the effectiveness of seawall modifications was assessed, climbing behavior experiments, and simulations using agent-based modeling (ABM). Results suggested that Chiromantes haematocheir exhibited a higher climbing preference and climbing success rate on sloped seawalls compared to vertical ones. Climbing duration was also shorter on sloped seawalls. Following seawall modification and traffic control, the roadkill rate of land crabs decreased. Simulation result indicated that the climbing success rate will increase, whereas duration spent on the ground will decrease, provided that sloped seawalls are constructed near the roadkill hotspot. Based on the results, seawall modification can increase the climbing willingness and mitigate roadkill risk of land crabs. Agent-based modeling is suitable for estimating the climbing preference and roadkill risk. Based on our finding, the Third River Management Branch of Water Resources Agency, the competent authority in charge, continue to implement seawall modifications in Gaomei Wetland to mitigate the roadkill rate of land crabs in 2026.

關鍵字：陸蟹、路殺、海堤改善、代理人模型

Methodology and Implementation Steps

There were three groups in climbing willingness and climbing duration experiments, including vertical seawalls, sloped seawalls, and free-to-choose group that crabs could choose either vertical and sloped seawall for climbing. Climbing results were recorded, including climbing success, fail to reach checkpoint, and no climbing. Detail behavioral patterns were recorded based on the climbing sequential experiments. Ovigerous females were collected on the road. Each individual was randomly and alternately assigned to one experimental group and tested only once. After experiment, all individuals were released to the seaside of seawalls. Roadkilled surveys were conducted July and September in 2017, 2018 and 2023. In 2017 and 2023, surveys were started from spring tide and five days after spring tide. In 2018, surveys were conducted from spring tide and three days after spring tide. Individual state, including alive, roadkilled and non-readkilled death (Fig. 1), species, gender, whether it carry egg or not and location were recorded. Photos of dead individuals were taken. Climbing willingness were analyzed using Test of Goodness-of-Fit and Fisher’s Exact Test. Climbing duration were analyzed using Kruskal-Wallis rank sum test and Dunn’s test. Climbing pattern were analyzed using sequential analysis. The effects of seawall modification and traffic control on numbers of roadkilled and non-roadkilled individual were analyzed using GLMM. The parameters of agent-based modeling (ABM) were based on the results from climbing behavioral experiments. ABM was used to simulate the climbing success rate, the total time spent on the ground and the proportion of individual on the ground when simulation was completed. The simulation results can be used to estimate the roadkilled risk of land crabs.

Innovation and Cross-Disciplinary Collaboration

Different climbing behavioral experiments were conducted to understand the behavioral patterns of land crabs under various conditions. A three-year roadkill survey provides a scientific evidence on effectiveness of seawall modification on roadkill mitigation. Our results also suggest that agent-based modeling can be applied on roadkill mitigation in small invertebrate.

Expected Results and Contributions

Our results support the effectiveness of seawall modification on roadkill mitigation, which could increase climbing willingness and reduce 26% of roadkilled and non-roadkilled death for land crabs (Fig. 2). In April, 2026, Third River Management Branch of Water Resources Agency, the competent authority in charge, expanded the seawall modification in Gaomei Wetland (Fig. 3). The roadkill risk of land crabs in Gaomei Wetland is expected to be further mitigated.