How to manage assets that jointedly provide a common resource? How to measure health of the commons? How to assess a composite of assets? How to assess changes of a composite? To consider these questions one may turn to indexes as descriptors, or even to a single index that is designed to provide a middling representation. This kind of descriptors, such as the "Gross Domestic Product per Capita" [#] to name one of the most widely used indexes, appeal to provide first-hand guidance for management options and policy choices. That is their charm and their risk.
[#] averaging economic contributions weighted by their monetary value over a group of people
Since the last 3.5 Billion years the ocean, possibly is the essential ecosystem-service-provider of Earth. The ocean produces half of the free oxygen in the atmosphere, stabilises our climate and facilitates plate tectonics. The ocean is a global resource exploited by humankind. The interplay between humankind's economy and marine ecosystems can be described as "the human-ocean system". The ocean is beneficial for societal wealth and human development. It offers access to food, materials, energy, and recreational opportunities. Therefore managing the human-ocean system in a sustainable manner, is paramount to maintain the "health of the ocean".
Regional seas are theatres of competing economic interests that significantly influence the stage of the global ocean. Already today, many seas are in a poor state. Overfishing shifts balances of ecosystems. Pollution caused by extractive industries threatens living resources. Marine litter spoils recreation. Plastic disrupts marine life along the entire food web with eventual health effects on humans. Alterations of coastal zones destroy unique habitats and resilience of living spaces, etc. Rapid deployment of advanced technologies enables penetration and extraction of resources in areas previously too harsh or difficult to get to. And thus, are eliminating the last natural refuges of marine life and this without adequate "compensation" by marine sanctuaries.
On one side, the risk is high that widespread implementation of “blue economy strategies” will increase the general pressure on marine environments. On the other side, treating the marine environment as a jointly managed common resource increases the possibility to foster its sustainable use. Management however needs standardized tools; e.g. to compare options. In that context, a means to assess the ‘health of the ocean’ in a standardized manner would be much appreciated.
How to generalize this experience? What has been discussed above for the ocean health index applies mutatis mutandis to other indices that are calculated as a single score for a composite of assets that can substitute each other.
Reference for all sections:
[c] The single assets of the ocean-health index are: (1) Artisanal Fishing Opportunities, (2) Biodiversity i.e. species and habitats, (3) Coastal Protection, (4) Carbon Storage, (5) Clean Waters, (6) Food Provision i.e. fisheries and aquaculture, (7) Coastal Livelihoods & Coastal Economics, (8) Natural Products, (9) Sense of Place i.e. iconic species’ and special places, and (10) Tourism & Recreation [x].
[e] http: //en.wikipedia.org/wiki/Generalized_mean
 An index to assess the health and benefits of the global ocean (2012). Benjamin S. Halpern, Catherine Longo, Darren Hardy, Karen L. McLeod, Jameal F. Samhouri, Steven K. Katona, Kristin Kleisner, Sarah E. Lester, Jennifer O’Leary, Marla Ranelletti, Andrew A. Rosenberg, Courtney Scarborough, Elizabeth R. Selig, Benjamin D. Best, Daniel R. Brumbaugh, F. Stuart Chapin, Larry B. Crowder, Kendra L. Daly, Scott C. Doney, Cristiane Elfes, Michael J. Fogarty, Steven D. Gaines, Kelsey I. Jacobsen, Leah Bunce Karrer, Heather M. Leslie et al., Nature 488.
 How healthy is the human-ocean system? (2014). Wilfried Rickels, Martin F. Quaas and Martin Visbeck. Environmental Research Letters Vol. 9(4). doi:10.1088/1748-9326/9/4/044013