Abstract
Protected species bycatch can be rare, making it difficult for fishery managers to develop unbiased estimates of fishing-induced mortality. To address this problem, we use Bayesian time-series models to estimate the bycatch of humpback whales (Megaptera novaeangliae), which have been documented only twice since 2002 by fishery observers in the United States West Coast sablefish pot fishery, once in 2014 and once in 2016. This model-based approach minimizes under- and over-estimation associated with using ratio estimators based only on intra-annual data. Other opportunistic observations of humpback whale entanglements have been reported in United States waters, but, because of spatio-temporal biases in these observations, they cannot be directly incorporated into the models. Notably, the Bayesian framework generates posterior predictive distributions for unobserved entanglements in addition to estimates and associated uncertainty for observed entanglements. The United States National Marine Fisheries Service began using Bayesian time-series to estimate humpback whale bycatch in the United States West Coast sablefish pot fishery in 2019. That analysis resulted in estimates of humpback whale bycatch in the fishery that exceeded the previously anticipated bycatch limits. Those results, in part, contributed to a review of humpback whale entanglements in this fishery under the United States Endangered Species Act. Building on the humpback whale example, we illustrate how the Bayesian framework allows for a wide range of commonly used distributions for generalized linear models, making it applicable to a variety of data and problems. We present sensitivity analyses to test model assumptions, and we report on covariate approaches that could be used when sample sizes are larger. Fishery managers anywhere can use these models to analyze potential outcomes for management actions, develop bycatch estimates in data-limited contexts, and guide mitigation strategies.
Highlights
Estimating the mortality of marine mammals and other protected species incidentally caught during commercial fishing operations is an important, but often, challenging task
We focus on two sectors in the sablefish pot fishery: the limited entry (LE) sector (∼ 90 vessels/year), where fishers have individual quota to catch sablefish during the seven month season (April–October), and the open access (OA) sector (∼ 472 vessels/year), which is managed by pertrip landing limits on sablefish and is open year-round
We did not estimate a single model for both sectors combined because the fishing areas, targets, and tactics are sufficiently different between the two sectors to warrant separate models for each sector
Summary
Estimating the mortality of marine mammals and other protected species incidentally caught during commercial fishing operations (bycatch) is an important, but often, challenging task. Managers must rely on estimates of bycatch to accurately assess marine mammal stocks (Wade, 1998), and to set fishing impact reference points (Moore et al, 2013), population recovery goals, and species-specific protective status. The rarity of bycatch events, which vary by fishery and marine mammal species involved, makes it challenging to develop robust bycatch estimates that are critical for setting recovery goals and conservation priorities. Bycatch can be rare for a number of reasons, including: fishing vessels and animals only occasionally overlap in time or space; vessel-mammal interactions are unobserved (i.e., cryptic; Gilman et al, 2013) or observation rates are low (Moore et al, 2011; Wakefield et al, 2018; Curtis and Carretta, 2020); fishers deliberately avoid marine mammals; or because the species itself is rare, sometimes as a consequence of fishery or other humaninduced mortality
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