Marine protected areas can benefit biodiversity even when bycatch species only partially overlap fisheries

1. Marine protected areas (MPAs) are increasingly relied upon to preserve biodiversity and ensure fisheries sustainability. Although recent analyses have shown that MPAs can provide overall biodiversity benefits even when they only redistribute fishing effort, these did not account for refuges outside of fishing grounds for non-target species. It has been argued that such refuges could render MPAs obsolete where their chief benefit is to ensure species persistence. 2. Here, we modelled the outcomes associated with placing MPAs within a seascape where non-target species' ranges can extend beyond fishing grounds. 3. In scenarios with no explicit fishing-induced habitat damage, we found that MPAs provided a net biodiversity benefit so long as there was at least an approximately 60% overlap between fishing grounds and non-target species' ranges. In scenarios with explicit fishing-induced habitat damage for non-target species, we found that MPAs instead benefitted biodiversity at all overlaps, but that this was reduced if target species' abundances also declined with damage. 4. Additionally, we identified counterintuitive mechanisms by which MPAs could either help or harm the same species depending on their location by changing fishing effort distribution patterns. However, MPAs did always increase the total fishing effort required to reach a catch target. 5. Policy implications: Our results showed that MPAs could still deliver biodiversity benefits in otherwise well-regulated fisheries when species' ranges and fishing grounds did not fully overlap, and particularly when sensitive species and habitats were present. Well-placed MPAs should thus be considered in such settings. However, MPAs were also less likely to be useful when range overlaps were low and always resulted in higher fishing effort to maintain the same catches.