Abstract/Summary

A host of regulations should protect fish, a common-pool resource, from overexploitation, but detecting violations of these regulations is challenging, both at sea and in port. We present a novel approach to uncover a supposedly widespread and particularly harmful illegal fishing practice, the use of nets with illegally small mesh size. Our approach relies on readily available data on reported fish landings. We focus on bottom trawling, the world’s most widely used fishing method. We exploit the fact that using illegally small mesh size increases the share of small fish in the catch. Using quasi-random variation in nautical patrol as a source of variation in the incentive to comply, we show that in weeks without patrol the share of small fish in the landed catch is systematically larger than in adjacent weeks with patrol. Our results are in line with widespread use of illegally small mesh. The resulting catch and discard of juvenile fish is many times larger than the gain in the catch of marketable fish. This harm has thus far been largely ignored in estimates of the externalities from fishing.