Hydrodynamic exposure metrics for mariculture sites (Lojek et al., 2024)

Kelson Marine is an active member of the ICES Working Group for Open Ocean Aquaculture (WGOOA). As a Working Group, we have convened farmers, engineers, regulators, and scientists to collaboratively advance the open-ocean aquaculture industry. 

One challenge that arose quickly was the need to resolve the present ambiguity in the term "Offshore Aquaculture". The Working Group now recommends two distinct metrics to quantitatively describe aquaculture sites:

"Distance from shore": Sites are described by their distance from the coast.

"Exposure": Sites are further described with an exposure index that incorporates the design wave, current velocity, and depth of the site.

This distinction addresses a need to define the terminology to reduce ambiguity for characterizing aquaculture sites to:

1. Promote a common understanding and avoid misuse of different classifications;
2. Enable regulators to identify the characteristics of a marine site;
3. Allow farmers to assess or quantitively compare sites for development;
4. Equip developers and producers to identify operational parameters for the equipment and vessels; and
5. Provide insurers and investors with better means to assess risk and premiums.

To help farmers, developers, regulators, investors, and members of the public apply these definitions, Kelson Marine worked with ICES to turn the vague idea of a site’s “exposure” into a set of six Exposure Indices that combine waves, currents, depth and simple structural properties into single, physically meaningful numbers (Lojek et al. 2024). This allows engineers, biologists, economists and regulators could compare sites and design consistently rather than rely on imperfect proxies; Kelson’s motivation was to make exposure a practical input to engineering and economic decisions.

In a new study, Kelson Marine and collaborators around the world propose six hydrodynamic exposure indices to quantify the energy environment around mariculture structures by combining wave height, wave period, current velocity, and water depth. Exposure Velocity (EV) is the characteristic horizontal velocity at the surface, while Exposure Velocity at Reference Depth (EVRD) represents the mean velocity at a specified depth. Specific Exposure Energy (SEE) calculates the wave and current energy per unit mass. Depth-integrated Energy Flux (DEF) integrates energy flux through the water column. Structure-centered Depth-integrated Energy (SDE) and Structure-centered Drag-to-Buoyancy Ratio (SDBR) incorporate gear dimensions, evaluating the energy acting on the structure and the ratio of hydrodynamic drag to the buoyancy capacity. EV, EVRD, SEE and DEF depend only on environmental conditions, whereas SDE and SDBR account for the size and drag area of the gear.

Applying the six exposure indices to 29 aquaculture sites around the world revealed a wide range of exposure categories. Some open‑ocean sites exhibited moderate exposure while certain nearshore sites experienced high energy conditions. The study’s sensitivity analysis linked the indices to storm return periods, showing that extreme events strongly influence exposure and that hindcast data for the German Bight can be used to estimate regional extremes. Exposure maps demonstrated that hydrodynamic exposure is not simply a function of distance from shore; local bathymetry and current patterns govern energy levels. The indices performed plausibly in classifying sites and highlight the importance of site-specific assessments.

Quantitative exposure metrics provide practical tools for mariculture developers, regulators and researchers. By linking exposure categories to structural design and operational demands, regulators can set objective criteria for site permitting, while farmers can choose sites and gear that balance productivity with environmental stress. These metrics support risk assessments and help unify terminology across regions, facilitating international best practices for offshore aquaculture. As offshore farms expand to deeper waters, applying hydrodynamic exposure indices will help ensure designs remain safe and economically viable.