Illicit trading of seafood with hotels, restaurants, and caterers (the HORECA sector) by small-scale fisheries is suspected to be widespread, but estimates rely on heuristics and limited evidence. We hypothesized that when the HORECA selling channel became unavailable, due to the COVID-19 lockdown, an increase in the reported catch in official data indicates the amount of unreported fishing and illicit seafood trade. We analysed a comprehensive dataset of 5 million first sale of seafood transactions by 3,715 small-scale fishers catching 351 species in Portugal from 2017 to 2020. The analysis shows an overall decline in reported catch and number of fishers during the lockdown. Albeit there were significant increases in reported catch volume in regions where historically high tourism activity declined, often for high-value species. This study offers an alternative way to estimate potential illicit seafood trade among small-scale fisheries and practical examples of proactively addressing the illicit trade issue.