Pesticide bioavailability, bioaccumulation, and bioconcentration in zebrafish: a critical review with emphasis on methodological inadequacies.

Zebrafish (Danio rerio) have become the pre-eminent vertebrate model in aquatic ecotoxicology, yet the evidence base on pesticide bioavailability, bioaccumulation, and bioconcentration in this species is characterised by profound methodological fragmentation that limits the derivation of reliable risk estimates. This review critically appraises the primary literature across five major pesticide classes, namely organophosphates, pyrethroids, organochlorines, triazole and succinate dehydrogenase inhibitor (SDHI) fungicides, and herbicides, and argues that the field's central limitation is not a deficit of primary studies but rather a systematic failure to implement methodological standards capable of generating internally consistent, cross-comparable toxicokinetic data. Bioconcentration factors (BCFs) in zebrafish span four orders of magnitude, from less than 2 for glyphosate to over 1300 for chlorfenapyr, with log Kow as the primary predictor; yet BCF values derived from embryo stages cannot be extrapolated to adults given the profound life-stage dependency of CYP450 metabolic capacity. A recurring pattern of nominal-concentration reporting, pure active-ingredient testing, single life-stage design, and single-compound exposure has generated a body of evidence that diverges systematically from real-world pesticide exposure conditions. Quantitative analysis of methodological bias reveals that biotransformation alone can reduce internal concentrations by 5- to 90-fold; sediment partitioning can confine pyrethroid bioavailability in the water column to as little as 0.07% of total added mass; and formulation adjuvants can redirect pesticide distribution entirely between water and sediment compartments. A cross-study quality assessment of the ten primary studies forming the evidence core of this review shows that the majority score 7 or below on a 10-point methodological adequacy scale, with no study simultaneously verifying concentrations, capturing tissue-specific BCFs, tracking metabolites, and incorporating realistic mixture or formulation conditions. This review provides the first integrated, methodologically-focused, quantitative synthesis of these issues in zebrafish, identifies five priority knowledge gaps, and proposes a minimum standards framework for future zebrafish pesticide bioaccumulation studies.