Estimates of marine mammal, sea turtle, and seabird bycatch in the California large-mesh drift gillnet fishery: 1990-2021

Bycatch of marine mammals, sea turtles, and seabirds in the California thresher shark / swordfish drift gillnet fishery from 1990 to 2021 is estimated with random forest classification and regression trees. Tree estimates are compared with annual ratio estimates using the same observer data. Biases associated with ratio estimators (systematic under- and overestimation of bycatch) are notable when observed bycatch is rare, when each annual bycatch estimate is informed only by data from the same year, and observer coverage is low. Tree methods provide more stable annual bycatch estimates with better precision, because estimates are informed by all available data. Even in years without observed bycatch, expected values from regression trees may be positive and include estimates of error, whereas corresponding ratio estimates are zero and lack error estimates. Regression tree bycatch models include oceanographic, location, and gear variables used as predictors to estimate bycatch at the fishing set level. Model variables were identified using ‘balanced random forest’ classification trees that deliberately oversample sets with observed bycatch to address zero-inflated data signal-to-noise challenges. This method was previously validated with a simulated rare bycatch dataset where significant predictor variables were correctly identified in most cases, even when simulated bycatch events represented <1% of all data (Carretta et al. 2017).