There is long-running debate concerning the conservation benefits of marine protected areas (MPAs) in seascapes that are already ‘well-regulated’ by other forms of fisheries management. Resolving this issue is particularly important given recent calls for expanding MPA coverage. Here, we adapt an influential model from agriculture – the land sparing/sharing framework – to an archetypal managed trawl fishery to determine how catches can be obtained at least cost to biodiversity in such seascapes. This new sea sparing/sharing framework fixes catch across scenarios so that fishing effort displacement is not overlooked. We find that a sparing strategy combining MPAs with fishing effort regulation is best for biodiversity when avoiding the local extinction of sensitive species is a priority. However, if there are no sensitive species, or if biodiversity is instead measured in a way that emphasises species abundances, then a sharing strategy relying on fishing effort regulation alone instead prevails. Extending these findings globally to ‘well-regulated’ crustacean trawl fisheries, we find that ≈ 72% may benefit from no-trawl MPAs (≈ 57–100% depending on the methodology chosen). However, these MPAs could also necessitate increased fishing effort if catches are to be maintained. Our framework thus suggests that whether MPAs increase biodiversity in a well-regulated seascape depends heavily on the presence of highly sensitive species there, as well as conservation and management priorities.