Key factors that exacerbate declines in exploited multispecies systems

Abstract

Extinction rates are increasing globally, and direct exploitation is an important driver. Many pathways have been proposed to explain how exploitation can lead to extinction. One of these proposed but understudied multispecies pathways is opportunistic exploitation, which occurs when a highly valuable but rare species is encountered and targeted during exploitation of a less valuable, but more common, target species. Using individual-based simulations of exploiters in a two-species spatial model, we show that opportunistic exploitation increases depletion when compared to single-species exploitation, and is as detrimental to the more valuable, rare species as the anthropogenic Allee effect (where price increases with rarity) and the Allee effect (where population growth declines at low abundance). The most important factors affecting the impact of opportunistic exploitation are gross revenue and abundance of the more common, less valuable species, while ease of capture and growth rate of the more common, less valuable species are less important. Thus, valuable but rare species are most at risk when harvested alongside low-value abundant species; this information is relevant for managers focused on protection of rare species in multispecies systems. Fisheries independent survey data can provide novel insights into global fisheries status and the status of individual taxa but there is no central location where fishery-independent survey-indices of abundance are stored and available for scientific use. Therefore, we compiled a database of estimated abundance or biomass by year for each species from 51 fishery independent surveys. One key feature of this database is a consistent data-sharing agreement allowing for broader use of the database while respecting the rights and wishes of data providers. As an illustration of the usefulness of this database, we examined which factors explain trends in abundance of different fish stocks, finding that declining trends are most closely associated with longer maximum length, highly commercial fished status, and deeper habitats. We anticipate that this database will facilitate new research directions and encourage the contribution of additional surveys.