Best Practices for Assessing and Managing Bycatch of Marine Mammals

Abstract

Bycatch in marine fisheries is the leading source of human-caused mortality for marine mammals, has contributed to substantial declines of many marine mammal populations and species, and the extinction of at least one. Schemes for evaluating marine mammal bycatch largely rely on estimates of abundance and bycatch, which are needed for calculating biological reference points and for determining conservation status. However, obtaining these estimates is resource intensive and takes careful long-term planning. The need for assessments of marine mammal bycatch in fisheries is expected to increase worldwide due to the recently implemented Import Provisions of the United States Marine Mammal Protection Act. Managers and other stakeholders need reliable, standardized methods for collecting data to estimate abundance and bycatch rates. In some cases, managers will be starting with little or no data and no system in place to collect data. We outline a comprehensive framework for managing bycatch of marine mammals. We describe and provide guidance on (1) planning for an assessment of bycatch, (2) collecting appropriate data (e.g., abundance and bycatch estimates), (3) assessing bycatch and calculating reference points, and (4) using the results of the assessment to guide marine mammal bycatch reduction. We also provide a brief overview of available mitigation techniques to reduce marine mammal bycatch in various fisheries. This paper provides information for scientists and resource managers in the hope that it will lead to new or improved programs for assessing marine mammal bycatch, establishing best practices, and enhancing marine mammal conservation globally.

Estimating Bycatch Mortality for Marine Mammals: Concepts and Best Practices

“…Bycatch estimates need to be compared to conservation reference points, which are derived for marine mammal populations mainly from estimates of population size. Population size and reference point estimation are not covered here, but we have provided references on these topics, and a more complete treatment of estimating abundance and reference points can be found in Hammond et al (2021) and Wade et al (2021), respectively, in this issue. Scientific observer programs are the only known way to obtain the data needed to estimate bycatch accurately.…”

“…It is widely understood that certain gear types represent a predictable threat to particular groups of marine mammals (Wade et al, 2021). For example, vertical buoy lines used to mark and retrieve fixed gear such as crab, lobster or fish traps have the potential to entangle large whales, and to result in their serious injury and death, but may not be a threat to smaller species.…”

“…Reference point estimation is tied to estimating population size, a topic thoroughly reviewed by Hammond et al (2021), this issue. An in-depth description of the broader assessment framework is found in Wade et al (2021), this issue. Readers should examine these papers to understand the broader management context within which bycatch estimation takes place under the MMPA Import Provisions and how estimating marine mammal bycatch and population size relate to each other.…”

“…There are important precursors to designing a program to estimate bycatch in a fishery (discussed more thoroughly by Wade et al, 2021). The first is making use of exploratory data (which may need to be collected anew) to characterize the fishery (number of vessels, vessel types, gears used, when and where fished, target species, etc.)…”

Estimating Bycatch From Non-representative Samples (II): A Case Study of Pair Trawlers and Common Dolphins in the Bay of Biscay

“…Using a Potential Biological Removal (PBR) approach (Wade, 1998), ICES (2020d) estimated a removal limit of common dolphin for the whole North-East Atlantic of 4, 926 individuals. An annual bycatch no greater than PBR would allow the population of common dolphins to recover to or be maintained at or above 50% of carrying capacity with a probability of 0.95 (Wade et al, 2021). This conservation objective is, however, different from the ASCOBANS interim objective "to restore and/or maintain stocks/populations to 80% or more of the carrying capacity.…”