Abstract
In open ocean systems, trawling with nets remains one of the most common methods for measuring animal biomass in a volume of seawater, calculated as distance traveled multiplied by net mouth area. The effective distance traveled by the net can be measured using a flowmeter, but in the absence of a flowmeter is often estimated as the distance traveled by the trawling vessel. Studies that do not use a flowmeter to quantify trawling effort assume that the rate at which water flows into the net (flow rate) is constant within and among trawls. Similarly, studies that cannot directly measure net mouth area assume its variability is negligible. However, assumptions of constant flow rate and net mouth area are rarely scrutinized and their impacts on volume estimates are poorly understood, especially across sampling depths. We tested these assumptions using a 10 m 2 Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and observed ~ 10% variability in both flow rate and net mouth area when trawling between the surface and 2500 m. We then compared measurements of net volume and animal biomass from 160 MOCNESS nets to estimates based on six combinations of common net distance and mouth area metrics. Volume estimation methods using vessel-based estimates of distance traveled or the nominal net mouth area overestimated measured volumes by 15–42% on average, resulting in an average 12–28% underestimation of animal biomass across methods. Estimation errors increased with sampling depth and were largest at depths greater than 1000 m. Using a novel method to predict MOCNESS flow rates based on ship velocity, net depth, and other variables describing trawling conditions, estimation errors relative to measured volumes were minimized and became consistent across depths. The findings from our case study suggest trawl studies that do not directly measure flow rates likely have depth-dependent bias in volume estimates, which has broad implications for our understanding of pelagic ecology and ecosystem services.