To date, p rotected marine vertebrates have rarely been injured or killed during physical interactions with aquaculture gear. Although the evidence for this observation is compelling when compared with observed effects of fisheries, concerns persist because it is not backed by a substantial body of scientific research or modeling. In 2019, under funding from the NOAA Saltonstall -Kennedy grant program, we conducted a workshop on the science and engineering of interactions between marine vertebrates and open-ocean aquaculture facilities. The workshop focused on evidence of injuries and mortalities; practices adopted by farms to collect observations and minimize interactio ns; design and engineering of aquaculture gear ; and physics and physical models of the behavior of lines and cages .
E vidence presented during the workshop and since supports the rarity of entanglement and injury , amounting to fewer than 25 incidents involving protected species over the last four decades . Biologist p articipants noted that this count does not include the effects of permitted lethal takes on seals and sea lions after depredation attempts. They also described gaps in effort to collect data. D etailed observations of actual interactions are still very limited and forensic approaches are rarely applied to incidents . There is also substantial granularity in the information available on abundance and movements of species of concern . Farmers, including two successful West Coast finfish operations, described best practices for eliminating gaps, which included ongoing documentation and maintenance procedures. Both farmers and engineers pointed to significant differences between aquaculture and fishing gear, foremost in the tension applied to all types of lines. Engineers described ongoing efforts to model the behavior of aquaculture gear under the influence of waves and currents, and the dynamics of entanglement . To date models of entanglement have focused only on fishing gear.
Line tension is hypothesized as an important factor in takes. It is simplest to study in line-based operations (e.g., bivalves) . W e made initial physical measurements of a sample of line types used in both aquaculture and fisheries. We included samples of line (10 – 13 mm [3/8” to &fraq12;”] nominal diameter ) weathered by a macroalgae grower for up to 5 years. Based on measurements of elasticity as tension was applied, weathering made the lines stiffer but less resistant to strain . A preliminary finite element model describing deflection as transverse force was applied ( e.g., when an animal contacts a line) showed that even modest tension had a more important effect on line displacement than elasticity. In summary, both the workshop and feasibility measurements emphasized the value of obtaining detailed, direct measurements of marine life interactions with aquaculture gear.