Field Evaluation of a Non-Conventional Mobile Square Cylinder Fish Enclosure with Variable Aft-End Constriction

This study presents field measurements for a non-conventional mobile square cylinder fish enclosure, with permeable ends, evaluated under both towed and moored conditions at a semi-open ocean test site. Novelty lies within the enclosure design that enables both mobility and control of internal flow through the incorporation of an adjustable aft-end perimeter constriction to regulate internal flow and support fish welfare by enabling control of swimming conditions. Enclosure motion, internal flow speeds and hydrodynamic loads were measured for three constrictions (0%, 40% and 60%). The primary objective was to assess the effectiveness of aft-end constriction in regulating internal flow to levels compatible with sustainable swimming speeds for finfish culture. The enclosure remained stable at approximately 9 m depth across all vessel speeds and constriction settings in both towed and moored scenarios. During towing, increasing constriction to 40% reduced time-averaged internal flow by up to 24% without a significant increase in hydrodynamic load. A 60% constriction achieved a larger reduction (~51%) but resulted in a substantial load increase (~90% relative to 0% constriction), indicating a trade-off between flow control and towing resistance. Under moored conditions, aft-end constriction had minimal influence on both internal flow and hydrodynamic load. Mooring loads showed no clear relationship with wave height and only a weak correlation with ambient current speed. Overall, the results demonstrate that aft-end constriction is an effective mechanism for controlling internal flow during towing, but has limited impact when moored. The enclosure’s stability and controllability highlight its potential advantages over conventional gravity cages for mobile open-ocean finfish aquaculture applications.