Illegal harvest is recognized as a widespread problem in natural resource management.
The use of multiple methods for quantifying illegal harvest has been widely recommended
yet infrequently applied.We used a mixed-method approach to evaluate the extent, character,
and motivations of illegal gillnet fishing in Lake Hovsgol National Park, Mongolia and its
impact on the lake’s fish populations, especially that of the endangered endemic Hovsgol
grayling (Thymallus nigrescens). Surveys for derelict fishing gear indicate that gillnet fishing
is widespread and increasing and that fishers generally use 3–4 cm mesh gillnet. Interviews
with resident herders and park rangers suggest that many residents fish for subsistence
during the spring grayling spawning migration and that some residents fish commercially
year-round. Interviewed herders and rangers generally agree that fish population sizes are
decreasing but are divided on the causes and solutions. Biological monitoring indicates that
the gillnet mesh sizes used by fishers efficiently target Hovsgol grayling. Of the five species
sampled in the monitoring program, only burbot (Lota lota) showed a significant decrease in
population abundance from 2009–2013. However, grayling, burbot, and roach (Rutilus rutilus)
all showed significant declines in average body size, suggesting a negative fishing
impact. Data-poor stock assessment methods suggest that the fishing effort equivalent to
each resident family fishing 50-m of gillnet 11–15 nights per year would be sufficient to overexploit
the grayling population. Results from the derelict fishing gear survey and interviews
suggest that this level of effort is not implausible. Overall, we demonstrate the ability for a
mixed-method approach to effectively describe an illegal fishery and suggest that these
methods be used to assess illegal fishing and its impacts in other protected areas.
