Collaboration among countries in marine conservation can achieve substantial efficiencies.

Aim Multinational collaboration is important for successfully protecting marine environments. However, few studies have assessed the costs and benefits incurred by taking collaborative action. One of the most complex marine regions in the world is the Mediterranean Sea biodiversity hotspot. The sea is shared by over 20 countries across three continents with a vast array of socio-economic and political backgrounds. We aimed to examine how collaboration between countries of the Mediterranean Sea affects conservation plans when costs and threats are considered. Location The Mediterranean Sea. Methods We compared three collaboration scenarios to test the efficiencies of coordinated marine conservation efforts: full coordination between Mediterranean countries, partial coordination within continents and no coordination where countries act in isolation. To do so, we developed four basin-wide surrogates for commercial and recreational fishing effort in the Mediterranean Sea. Using a systematic decision support tool ( Marxan), we minimized the opportunity costs while meeting a suite of biodiversity targets. Results We discovered that to reach the same conservation targets, a plan where all the countries of the Mediterranean Sea collaborate can save over two-thirds of the cost of a plan where each country acts independently. The benefits of multinational collaboration are surprisingly unequal between countries. Main conclusions This approach, which incorporates biodiversity, costs and collaboration into a systematic conservation plan, can help deliver efficient conservation outcomes when planning spatially explicit actions within marine environments shared by many countries. [ABSTRACT FROM AUTHOR]