The ECHA Workshop ‘OECD TG 236 & 249: Alternative Methods for Acute Fish Toxicity’ was a 2-day hybrid event and gathered over 250 participants (50 in person, 200 online) from industry, regulatory authorities, academia, and NGOs to advance the development and regulatory acceptance of alternative methods to replace traditional in vivo fish acute toxicity testing (OECD TG 203).
The meeting focused on regulatory needs, scientific reliability, and the applicability of new approach methodologies (NAMs) such as OECD TG 236 and 249 within different regulatory frameworks. A key message throughout was the need for caution: removing a central test from regulatory assessments requires robust scientific confidence to avoid destabilising chemical safety evaluations.
New ECHA analysis demonstrated a roughly equal distribution of most‑sensitive taxa between fish, algae, and daphnia, challenging claims that fish rarely represent the most sensitive species. Participants examined variability, reproducibility, and protective capacity across test methods (OECD TG 203, 236, 249), emphasising the importance of clearly defining applicability domains—chemical classes, physicochemical properties, and modes of action.
Triggered by presented case studies a substantial portion of the workshop addressed difficult‑to‑test substances, including adsorptive, volatile, ionisable, or highly hydrophobic chemicals. These challenges affect both traditional tests and NAMs, with issues such as adsorption to plasticware, unstable exposure concentrations, and analytical limitations. Participants discussed to use more innovative and less established exposure preparations such as passive dosing, pre‑conditioning test plates, frequent media renewal but particularly stressed the need for reliable exposure analytics and clearer guidance such as given in OECD GD 23.
Predictive modelling tools (e.g., VEGA, iSafeRat) were benchmarked, revealing the importance of high‑quality training data. Historical OECD TG 203 data often lack GLP or modern guideline robustness, limiting their value for validation. The successful regulatory integration of QSARs into regulatory frameworks supporting chemical hazard assessment was referenced repeatedly as a potential model on how to approach this —highlighting the need for explicit criteria, well‑defined applicability domains, and transparent validation processes for NAMs. Establishing checklists or structured acceptance frameworks to guide both regulators and method developers were recommended.
Regulatory acceptance emerged as a central barrier. While regulators state that OECD TG 236 and 249 may be used in weight‑of‑evidence approaches, clear acceptance thresholds do not yet exist. The workshop stressed the need to identify chemical groups where alternatives are already (most) suitable, improve access to high‑quality data, and focus testing where it most advances regulatory confidence. Data sharing remains a major obstacle despite broad agreement on its importance.
Breakout groups agreed that progress requires distinguishing between issues of test methodology and issues of regulatory process. Participants emphasised defining boundaries and practical domains of applicability, identifying priority data gaps, and developing processes that give regulators sufficient confidence to replace OECD TG 203 test requirements.
Overall, the workshop reflected significant momentum toward reducing animal testing within EU chemicals legislation – supported by the upcoming roadmap on phasing out animal testing (publication to be expected late April 2026) – but also highlighted the substantial scientific, technical, and policy challenges that must be addressed to achieve reliable non‑animal approaches for acute fish toxicity.
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