# Sediment and Salinity Thresholds Govern Natural Recruitment of Manila Clam in the Xiaoqing River Estuary: Toward a Predictive Management Framework

## TL;DR

Manila clams in the Xiaoqing River estuary require specific sediment and salinity conditions to survive and reproduce, which can help guide habitat restoration and management.

## Contribution

The study identifies precise sediment and salinity thresholds that govern natural recruitment of Manila clams in a degraded estuary.

## Key findings

- Manila clams almost disappear when sediment particles exceed 95 μm or salinity drops below 17.50‰.
- Dissolved inorganic nitrogen has a minor positive effect but cannot compensate for poor sediment or low salinity.
- The identified thresholds can guide habitat mapping, early-warning systems, and restoration prioritization.

## Abstract

In some degraded coastal areas, Manila clams continue to survive and reproduce naturally, but it was unclear what specific environmental conditions allow this. Our study aimed to find these key conditions. We surveyed the Xiaoqing estuary and analyzed how clam presence relates to their habitat. We found two strict limits: clams almost disappear where the sediment particles become coarser than 95 μm or where the water salinity drops below 17.50‰. While nutrients in the water had a minor positive effect, they could not overcome poor sediment or low salinity. These clear thresholds provide science-based tools for managers: to map and prioritize areas for clam habitat restoration, and to create an early-warning system that alerts to low-salinity events which risk mass clam mortality. This work helps protect this ecologically and economically important shellfish in our changing coasts.

Natural recruitment of Manila clam (Ruditapes philippinarum) often persists in degraded estuaries, yet the environmental thresholds enabling this resilience remain quantitatively undefined. We employed binomial generalized additive model (GAM) coupled with field surveys (n = 168) in the Xiaoqing River estuary (Laizhou Bay, China) to identify critical limits for adult occurrence, which served as a field-based proxy for recruitment potential. Sediment median grain size (D50), salinity (Sal) and dissolved inorganic nitrogen (DIN) were identified as the key factors, collectively explaining 79.30% of the deviance (AUC = 0.98). The probability of occurrence decreased sharply beyond two distinct thresholds: D50 > 95 μm and salinity < 17.50‰. While DIN had a positive effect, it did not offset the strong negative associations with coarse sediment or low salinity. These field-validated thresholds provide quantifiable criteria to guide habitat suitability mapping, activation of early-warning systems against salinity-driven mortality, and site prioritization for ecological restoration in the Xiaoqing River estuary. Our findings offer a framework for developing management strategies to support clam resilience under environmental stress.