A multidecadal Bayesian trend analysis of harbor porpoise (Phocoena phocoena) populations off California relative to past fishery bycatch

Publication Date

4-1-2021

Document Type

Article

Publication Title

Marine Mammal Science

Volume

37

Issue

2

DOI

10.1111/mms.12764

First Page

546

Last Page

560

Abstract

Harbor porpoises, Phocoena phocoena, off California, comprise four recognized population stocks: Morro Bay (MOR), Monterey Bay (MRY), San Francisco-Russian River (SFRR), and Northern California-Southern Oregon (NCSO). The three southernmost stocks experienced substantial bycatch in gill net fisheries during the 1970s and 1980s. While the SFRR stock received full protection from gill nets in 1989, the MOR and MRY stocks continued to experience at least some bycatch through 2001–2002. We examined long-term population trends for these four harbor porpoise stocks, based on two sets of systematic, aerial line-transect surveys conducted off California during summer/fall of 1986–2017. We applied a Bayesian hierarchical framework to specify a process model of population density and an observation model of porpoise counts during line-transect surveys. Growth rates were estimated for periods with and without bycatch. Posterior distributions indicate the MOR, MRY, and SFRR stocks, respectively, grew at 9.6%, 5.8%, and 6.1% per year after gill nets were largely or fully eliminated for each stock. Abundance off northern California appears stable or slightly increasing. This study provides a first empirical estimate of maximum net reproductive rate for harbor porpoise (at least 9.6%), and demonstrates that porpoise populations can recover from substantial gill net impacts if bycatch is eliminated.

Funding Number

14097

Keywords

abundance, Bayesian hierarchical model, bycatch, California, harbor porpoise, maximum net productivity rate, Pacific, Phocoena phocoena, population recovery, population trends

Department

Moss Landing Marine Laboratories

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