you're reading...


Untangling the issues with longline fishing

The Paper:

Werner, T. B., Northridge, S., Press, K. M., & Young, N. (2015). Mitigating bycatch and depredation of marine mammals in longline fisheries. ICES Journal of Marine Science: Journal du Conseil72(5), 1576-1586 DOI:10.1093/icesjms/fsv092


Longline fishing is a prevalent form of commercial fishing and allows for massive fish yields. Longlines can be used near the surface (pelagic longlines) to catch open-water fish such as tuna and swordfish, or near the seafloor (demersal longlines) to catch bottom-dwelling fish such as cod or halibut. Longlines consist of a very long mainline (up to 60 miles long) that is set and dragged behind a boat. The mainline has thousands of attached branchlines, each containing baited hooks used to lure and capture target fish. An unfortunate consequence of the longline design is that it attracts and easily snags non-target marine life (known as bycatch). A wide range of animals such as sea turtles, sharks, seals, seabirds, and marine mammals can get caught on hooks or entangled in fishing line (Fig. 1). Interactions with longlines can cause non-target animals, many of which are endangered, to suffer injury and even mortality.

Although bycatch is seen as a moral dilemma, fishers have been more concerned with the loss of the target catch due to marine mammal depredation or theft. Large marine mammals (i.e. killer whales, dolphins, and sperm whales) are drawn to the baited longlines, find easy access to prey, and steal the bait or target fish from the hooks. Nearby marine mammals may also spook target fish, causing them to avoid the baited hooks altogether. Furthermore, marine mammals may damage fishing gear, specifically hooks or other components of longlines as they try and break free from capture or entanglement.


Figure 1: Diagram of longline fishing with mainline and attached branchlines. Baited hooks attract a variety of marine life.

As is, longline fishing appears to be both detrimental to marine life and inefficient in maximizing target fish catch. In an attempt to help reduce future bycatch and increase the efficiency of longline operations, experts gathered for an intensive workshop. Participants refined a list of possible solutions and ranked them (low, medium or high) based on potential mitigation promise and research priority. Many solutions were considered, but only a handful were ranked as “medium to high”. A few of the most promising solutions are discussed below, they include: terminal gear modifications, fishers accountability, and real-time monitoring.

Terminal Gear Modifications:

According to participants, the best way to reduce unwanted catch and mortality from longlines is to modify the fishing gear itself. Using hooks that are slightly weaker and more flexible may allow caught animals to better release themselves. Circle hooks with an attached appendage also seem to reduce catch rates of multiple species (especially sea turtles) when compared to normal circle hooks (Fig. 2 A, B). The appendage is thought to increase the surface area of the hook, form a physical barrier to ingestion and make it harder for the hook to attach in the gut. Improved hooks would need to remain strong enough to retain the target catch.


Figure 2: A) Circle hook with attached appendage. B) Catch per unit effort produced by standard circle hooks (black columns) vs. appendage hooks (blue columns) for a variety of marine animals.

Fishery Closures and Real-time Monitoring:

Another way to lessen unwanted bycatch would be to implement triggered closures of the fisheries. This strategy would hold fishers accountable for reporting bycatch and keeping it below a certain threshold number. If bycatch were to increase above this threshold, the entire fishery would be closed for the remainder of the fishing season. In fear of losing profit, fisheries would comply with regulations and in the process lower harmful bycatch. This method would only work if vessels strictly adhere to protocols and correctly report bycatch.

It is challenging to monitor marine mammals near longlines and decrease theft of the target fish. The best suggestion from the workshop was to further explore near real-time monitoring. Satellite tags enable fishers to track the location of marine mammals and avoid setting longlines in the vicinity of tagged animals. This method would be most effective in confined areas with small marine mammal populations (such as killer whales in the Crozet Islands). It would be physically and financially improbable to track the activity of hundreds of marine mammals!

Conclusion and Significance:

Over the last few decades, commercial fishing has been made easier with the use of longlines. Though good for catching target fish, longline fisheries involve frequent interactions with other marine animals. Non-target species become hooked on baited lines and suffer injury or even mortality. Large marine mammals tend to steal target fish off the baited hooks and damage fishing gear in the process. Numerous suggestions have been made to improve longline operations and lower deadly bycatch, but most are hard to put into action.

Hook modifications, triggered closures, and monitoring devices may be the most practical solutions, but further research into their success in the field is needed. The pursuit of a solution to longline bycatch is complicated and will likely involve a combination of different gear modifications and fishing regulations. A quick solution would be ideal, especially for fisheries that impact threatened species and endure economic losses through marine mammal theft. A more realistic approach is to continue engaging in collaborative research between science and industry and to further evaluate the most promising solutions.


What are your thoughts on ways to improve longline fishing? How can we reduce bycatch?



  1. […] fishing methods, including longlines, gillnets, trawling, and scallop dredging (check out this oceanbites article about longline fishing impacts on animals like sea […]

Post a Comment


  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com