Despite the perception that it is rare, by-catch of marine mammals can lead to population decline

Abstract

By-catch, the unintentional capture of non-target species in fishing gear, is often described as a rare event when it pertains to marine mammals. Yet, it is the greatest threat to many marine megafauna species, especially small cetaceans. How can both of these statements be true simultaneously? The adjective “rare” is itself rarely defined in a precise and quantitative way. Data collection features are crucial to understand the processes leading to a perception of rarity in samples on by-catch of marine mammals and other protected, endangered and threatened species (PETS). Statistical principles in the design of monitoring schemes must be upheld to ensure sample representativeness when scaling up impact estimates to the level of whole fisheries. Random allocation of observers or monitoring technologies (e.g. Remote Electronic Monitoring) can be strengthened to test the hypothesis that by-catch is a rare event and not only a rarely registered event, especially in monitoring schemes that are not dedicated to PETS by-catch. Even if by-catch is a rare event at the level of a single fishing operation, given the large total number of the latter, and their expansive spatial and temporal extent, by-catch may still impact significantly marine mammal populations.