Length-based population dynamics of the horseshoe crab Tachypleus gigas and management implications for shrimp fisheries in Kotabaru Waters, South Kalimantan, Indonesia

Abstract

Shrimp fisheries in Kotabaru, South Kalimantan, represent an integral part of Indonesia’s small-scale coastal fishing sector, primarily operating with trammel nets. While targeting penaeid shrimp, these fisheries frequently capture non-target species such as the horseshoe crab (Tachypleus gigas), resulting in incidental bycatch. Despite their ecological significance, localized management frameworks and monitoring efforts for T. gigas populations remain insufficient, particularly in data-limited fisheries. This study assessed the stock status of T. gigas in Kotabaru waters using a length-based approach. A total of 234 individuals, representing the size fraction vulnerable to trammel nets, were sampled between December 2024 and March 2025, of which 105 females were used for gonad maturity analysis. Key biological parameters were estimated, including the length–weight relationship, length at first maturity ( cm), and length at first capture ( cm, to support a length-based spawning potential ratio (LB-SPR) analysis. Results showed that T. gigas exhibits negative allometric growth ( ), indicating faster increase in length relative to weight. The estimated L_c was below the L₅₀, indicating that the catch is dominated by immature individuals. The LB-SPR model estimated a spawning potential ratio (SPR) of 14%, falling below the 20% limit reference point, which indicates recruitment overfishing. These findings highlight the vulnerability of horseshoe crabs to bycatch pressure and underline the need for improved fisheries management. Recommended measures include enhancing gear selectivity, implementing seasonal closures during spawning periods, and strengthening bycatch monitoring to support sustainable shrimp fisheries in Kotabaru waters. This study demonstrates the utility of the LB-SPR framework for assessing non-target bycatch species in data-limited conditions, providing a baseline for localized conservation strategies.