The Ocean Health Index measures how well the ocean sustainably delivers a range of benefits to people now and in the future. It does this by assessing the cumulative stressors on ecosystems and tracking the capacity for sustainable delivery of services to people. It also incorporates measures of governance to understand the potential resilience of the system.

Ocean Health Index scores for the 15 high seas regions currently range from 53 to 79 out of 100, with all high seas regions together scoring 67. The lowest scoring high seas regions were in each of the main ocean basins (Arctic, Pacific, Indian and Atlantic), as were the highest scoring regions (excluding the Arctic).

One of the greatest challenges for resource management is to fully understand the condition of coupled human-natural systems, and make informed decisions about how best to improve them. Too often monitoring, assessments, indicators, and decisions are made within a single sector or with a single objective in mind, without full consideration of the broader implications of those actions.

Both ecosystem-based management and marine spatial planning aim to overcome these management barriers, but relatively few tools exist to directly inform and support such approaches. Without a tool to measure overall ecosystem health – and track improvement – multiple integrated objectives cannot be effectively managed. In the marine environment, this challenge particularly persists in areas beyond national jurisdictions, also called the high seas. The Ocean Health Index helps address this need.

The Ocean Health Index measures how well the ocean sustainably delivers a range of benefits to people now and in the future. It does this by providing an assessment of the status and likely future state of ten goals for healthy oceans, ranging from food provision to coastal protection to biodiversity (see table for the goals and their definitions). Each goal is measured against a reference point of the best possible status for the goal, and scored 0-100 in relation to this reference point. The likely future state is predicted based on the recent trends, negative human pressures exerted on the goal, and governance, social and ecological ecosystem traits in place that help provide resilience to pressures.

In the high seas, only 3 goals are relevant and have available data: food provision (wild caught fisheries), sense of place (iconic species), and biodiversity (species). Results for each of these goals were assessed and summarized for each of 15 high seas regions.

Fisheries catch data were provided by the Sea Around Us project for each stock in each high seas region. Time series of these catch data were then used to estimate the biomass of each stock in the ocean relative to its biomass at maximum sustainable yield (B/Bmsy), which can be used to estimate sustainability of harvest. There are many known problems with estimating stock status for data-poor fisheries; methods used here are considered the best available science.

Species data (used for both iconic species and biodiversity sub-goals) come from Aquamaps and IUCN red list. Range maps are known to be approximations, and extinction status is rarely updated for a species, making it difficult to track trends in risk.

For several goals relevant to the high seas we currently lack sufficient data to conduct assessments. These include natural products (such as fish oil), clean waters, special places that define culture value, and habitat biodiversity. For example, large ‘garbage patches’ are known to exist in most oceans, but the location and extent of them remains largely unknown. These data would allow assessment of the clean waters goal. Similarly, the condition of most deep sea habitats is poorly known, such that the habitats sub-goal could not be assessed.

On average, the high seas scored lower according to the Ocean Health Index than coastal regions, confirming the need for coordinated international action to better manage these areas. For all regions, there remains substantial opportunity to improve the sustainability of wild caught fisheries. Achieving this outcome could benefit global food security. Yet many aspects of ocean health in the high seas remain poorly monitored, hindering our ability to comprehensively assess ocean health. Improving data reporting standards from all UN member states, especially for fisheries catch and monitoring of deep-sea habitats, would significantly aid assessments of ocean health and decision making based on those assessments.

The management of high seas regions will benefit greatly from coordination with surrounding countries. Because relatively few goals are relevant to the high seas, the status of fisheries and biodiversity drive assessments in these regions. As such, meaningful management aimed to improve overall ocean health needs to focus on improving fisheries management and mitigating threats to biodiversity.