Power and precision of warp strike observation rates in the Area 48 krill fishery

During its 2022 meeting, the Working Group on Incidental Mortality Associated with Fishing (WG-IMAF) reviewed results of extrapolations of the number of warp strike incidents, as informed by the protocols for SISO observers in CCAMLR trawl fisheries requiring one 15 min observation period per vessel per day. As a result, WG-IMAF noted that the recommended one 15-minute warp strike observation period per day was a low rate of observation, which may lead to high uncertainty in extrapolated warp strikes, and recommended that a power analysis of required observer sampling for estimating the number of warp strikes across the krill fishery be undertaken. Here we show annual warp strike rates across gear types ranged between 0 and 2.328 birds per hour across Subareas 48.1, 48.2, and 48.3 when considering seasons with more than one hour of observation. Average annual warp strike rates between 2017 and 2022 were 0.042 strikes/hour for traditional trawl vessels and 0.097 strikes/hour for continuous trawling vessels. The power simulation indicates that for the higher warp strike rates (e.g., 0.1 strikes per hour) the current observation rates are adequate to detect changes beyond a presumed strike rate, as well as within the 95% credible limits. However, when the warp strike rates are lower, the current level of observation is unlikely to be sufficient to reliably detect: 1) the true strike rate with any reliability; and 2) a change beyond a critical threshold. The results also indicate that an increase in observation is required to gain better precision when low strike rates are being encountered by the fishery.