Use ‘mammal-safe’ nets to capture and release mammals trapped in fishing structures

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • One study evaluated the effects on marine mammals of using ‘mammal-safe’ nets to capture and release mammals trapped in fishing structures. The study was in the Bay of Fundy (Canada).



  • Survival (1 study): One controlled study in the Bay of Fundy found that using ‘marine mammal nets’ with a larger mesh size to release harbour porpoises from herring weirs resulted in lower porpoise mortality compared to using conventional herring nets.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A controlled study in 1992–2001 at coastal fishing sites in the Bay of Fundy, Canada (Neimanis et al. 2004) found that using specialised ‘marine mammal nets’ with a large mesh size to release harbour porpoises Phocoena phocoena from herring weirs resulted in lower porpoise mortality compared to when conventional nets were used. Overall porpoise mortality rates were lower when released from weirs with ‘marine mammal nets’ (6 of 240 porpoises, 3%) than with conventional herring nets (44 of 239 porpoises, 18%). Porpoises trapped in herring weirs were captured and released using two types of purse-seine net: ‘marine mammal nets’ (buoyant polypropylene with mesh size 7.5 cm; 240 porpoises) and conventional herring nets (mesh size 0.75–1.25 cm; 239 porpoises). Herring weirs (comprising 1-cm nylon mesh strung between wooden stakes in a kidney-shape, 3–20 m deep) were built near the shore to catch Atlantic herring Clupea harengus. Trapped porpoises were enclosed within the purse-seine nets, transferred to boats and released outside of the weirs. Researchers recorded porpoise deaths during each of the 479 release attempts in 1992–2001.

    Study and other actions tested
Please cite as:

Berthinussen, A., Smith, R.K. and Sutherland, W.J. (2021) Marine and Freshwater Mammal Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marine and Freshwater Mammal Conservation

This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation

Marine and Freshwater Mammal Conservation - Published 2021

Marine and Freshwater Mammal Synopsis

What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 21

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.

Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust