One Ocean, One Index – a 'Composite Essay' on Opportunities and Limits.
The ocean-health index is a challenging attempt, namely to describe a set of assets through a single index. Setting up the index and reviewing it teaches lessons on the human-ecological intersections of the human-ocean system, including the issue of appropriate mathematical methods "how to calculate its scores".
My first insight:
Setting up an ocean-health index  was an a lasting contribution to the management of the human-ocean system. An ocean-health index can be a tool for comparison of national and regional policies, benchmarking, and qualification of development options. That is much needed to manage global commons like the ocean.
Implications of the (simple) mathematics to calculate the score of ocean-health index have been analysed  and suggests that the mathematical method chosen for calculating the score is causing bias of the index. The method, “weighted arithmetic average”, makes the score insensitive to less appropriate balances between low-performance assets and better-performing assets. The feature “unconstrained mutual substitution between assets” that is implicit to the averaging method to obtain the score of the index limits its usefulness : "policy assessment and advice based on an index with unconstrained substitution possibilities could result in (a) certifying a healthy human-ocean system for countries that in reality neglect important aspects of ocean health and (b) identifying development trajectories as sustainable although this is not the case."
My second insight:
Constrained mutual substitution of assets should improve the assessment of the various oceanic features that are relevant for societal wealth and human development. Evidently, the substitution of different assets is a societal endeavour. It requires knowledge, social choices and norms and particular the latter may evolve and vary among societies.
Substitution possibilities should be constrained by the boundaries to the elasticity of the ocean system. If we do not know this ‘elasticity’ then “strong sustainability concept” or even the “precautionary principle” should be applied. Substitution possibilities should provide for a margin for management decisions - not everything goes, not all is forbidden – to render the ocean-health index a practical tool with operational value.
My third insight:
For better or for worse, a common and robust ocean-health index is a welcomed management tool, and should be part of any mature ‘blue economy strategy’. Thus, it is important to strengthen the index in a manner that enables its sound use in practice. Thus, furthering the analysis is needed, be it of suitable asset substitution or how to describe the substitution process in mathematical terms, to properly evaluate benefits, risks and development options of the ocean-human system.
In the absence of such an index, the alternative would be to manage all assets one-by-one using the "strong sustainability concept" or even the "precautionary principle". Such a choice has the intrinsic risk of a political process to retain only those assets that the strongest lobby considers to be most relevant. Such a situation certainly will be detrimental for the overall balance among assets, to the comparison of national and regional policies, benchmarking, and qualification of development options.
Thus, one composite index has a strong appeal. However, attention should be given to the averaging procedure, which, if too complex or perceived as too complex, would hamper application. To recall, the attractiveness of estimating the ocean-health index by a weighted arithmetic average is the simplicity of the mathematical procedure.
An afterthought:Possibly a two tiers approach may provide a useful compromise for now. Tentatively, such a compromise could be: (i) apply the "strong sustainability concept" to divide the entire set of assets in two sub-sets; one sub-set for the assets that match the respective threshold and the other sub-set for the assets that fail the respective threshold. (ii) calculate the score of the ocean-health index for both sub-sets and the entire set, and (iii) present the score for the full set with the scores for the sub-indexes as lower and upper bounds.
,  for references see "One Ocean, One Index – a 'Composite Essay' on Opportunities and Limits"