A Mixed-Method Approach for Quantifying Illegal Fishing and Its Impact on an Endangered Endemic Grayling in Lake Hovsgol National Park, Mongolia

Managers must know as much as possible about illegal fishing to effectively develop regulations but illegal activity is difficult to quantify. We used a mixed-method approach to evaluate the extent, character, and motivations of illegal fishing in Lake Hovsgol National Park, Mongolia and its impact on fish populations, especially that of the endangered endemic Hovsgol grayling (Thymallus nigrescens). We used: (1) surveys for derelict fishing gear to indirectly describe the extent, distribution, and character of illegal fishing; (2) interviews with herders and park rangers to contextualize these surveys; (3) long-term biological monitoring to identify species vulnerable to gillnet fishing and evaluate population-level impacts potentially caused by fishing; and (4) meta-analysis of data-poor stock assessment methods to estimate the effort required to overexploit the grayling population. Surveys for derelict fishing gear indicate that gillnet fishing is widespread and increasing. Interviews suggest that locals, non-locals, and foreigners participate in subsistence, commercial, and recreational fishing, fishing primarily targets grayling during the spring spawning migration, and fish population sizes are decreasing. Biological monitoring indicates that the gillnets efficiently target grayling and grayling, burbot, and roach populations are declining. Data-poor stock assessment methods demonstrate the plausibility of current or future fishing pressure to overexploit grayling.