Quantifying Causal Mechanisms in Shipping EIA Under the BBNJ Agreement with an Anchor Scour Case Study

The BBNJ Agreement requires science-based, replicable environmental impact assessments (EIAs) for shipping, yet current practices remain largely descriptive and correlational, lacking a formal causal framework for quantitative impact assessment. Anchoring and mooring, as routine shipping operations, physically disturb seabed sediments and benthic communities, yet their ecological effects remain poorly quantified and rarely assessed within a causal framework. This study integrates Pathways of Effects (PoE) with DoWhy’s workflow and mediation analysis to translate qualitative PoE into Directed Acyclic Graphs and estimate causal effects. A proof-of-concept case study on anchor scour demonstrates the framework’s feasibility. Results show that point estimates for total abundance were negative across the total, direct, and indirect effects; for taxonomic richness, the indirect effect was weakly positive while the total and direct effects were negative. All confidence intervals crossed zero, and those for the proportion mediated were wide, reflecting limited statistical power and substantial uncertainty. Nevertheless, all refutation tests passed, confirming the internal robustness of the estimation. This demonstration shows how qualitative PoE models can be operationalized into quantitative frameworks for hypothesis generation and mechanism exploration, making causal assumptions explicit and uncertainty quantifiable. The framework is extensible to other shipping activities and adaptable to various EIA scenarios, given a well-specified PoE model and sufficient data, offering a methodological foundation for BBNJ-aligned EIAs that support scientifically credible assessments, the precautionary principle, and cumulative impact evaluation.