<p>Understanding the determinants of non-compliance is crucial for the effective management and conservation of small-scale fisheries. This study empirically examines the drivers of non-compliance with Territorial Use Rights for Fisheries (TURFs) regulations in Vietnam’s Tam Giang Lagoon. To mitigate response bias associated with sensitive topics, we employed a mixed-methods approach combining the randomized response technique (RRT) and direct questioning (DQ) to estimate the prevalence of illegal fishing activities. Results indicate that the predominant form of non-compliance involves the use of nets with illegal mesh size, whereas violations related to destructive gear and fishing in protected areas are infrequent. Statistical analysis revealed no significant difference between RRT and DQ estimates for illegal mesh-size use, implying minimal social desirability bias in reporting this behavior. Determinants of non-compliance were identified using a binary logistic regression model. Compliance was positively and significantly associated with fishers’ awareness of the negative consequences of illegal fishing, perceived legitimacy of the regulatory framework, and social trust in peers’ adherence to regulations. Conversely, combined instrumental-normative incentives did not significantly influence compliance. The study highlights that the widespread use of illegal nets seems to have established a social norm that replaces traditional incentives, primarily due to insufficient enforcement. These findings contribute to understanding regulatory compliance in multispecies fisheries and offer critical insights for policy design aimed at enhancing adherence to TURF regulations. Policy recommendations include leveraging educational initiatives, promoting participatory governance, and supporting livelihood diversification to reconcile conservation objectives with the socioeconomic needs of fishing communities.</p>