you're reading...


Analyzing bycatch to better understand natural fish communities

Monk, M.H.; Powers, J.E.; Brooks, E.N. Spatial patterns in species assemblages associated with the northwestern Gulf of Mexico shrimp trawl fishery. Marine Ecology Progress Series 519: 1-12, 2015. doi: 10.3354/meps11150

A picture is worth a thousand words

Schutte 1.26

Why we care

Bycatch is any organism that is unintentionally caught as a result of harvesting a different target organism. Shrimp trawling is notorious for its high amount of bycatch- this type of fishing has the highest discard:catch ratio of any fishery considered in Alverson et al. 1994. Think about how small the typical shrimp you eat is, then think about the size of the net necessary to trap such a small, wily organism. When these fine-mesh nets are dragged through the water, anything bigger than a shrimp is liable to be trapped as well. There have been recent improvements in escape mechanisms built into shrimp nets (like Turtle Exclusion Devices), but these measures are a topic for another post.

Per weight, shrimp trawlers harvest from 3 to 15 times as much bycatch as they do shrimp. The process of hauling in the nets and sorting through the catch takes too long to save non-target species caught with the shrimp- creatures like sharks, rays, and other fish die before they are thrown back into the water. Maximizing the use we get from bycatch, while it doesn’t address the problem of bycatch itself, can at least mitigate the impact of harvesting these non-target species. Only in some places is bycatch used at all (non-target species can be eaten by humans or used in animal feed rather than thrown back in the ocean). This article details another use for non-target species caught as bycatch.

Monk and her colleagues describe the community composition of bycatch from trawls along the Gulf of Mexico’s northwestern coastline. From that information they identified potential “indicator species”- by definition, monitoring just these individual species can indicate the health of the entire ecosystem in which they live. Indicator species are generally characterized by their sensitivity to ecosystem changes. For example, aquatic insect larvae sensitive to oxygen levels will be the first to leave a stretch of a stream if oxygen levels vary from the norm. Keeping track of such sensitive species allows earlier detection of ecosystem change compared to tracking a more robust species that tolerates a wide range of conditions.



The shrimp statistical zones in the northwestern Gulf of Mexico that were analyzed for bycatch community composition.

The authors examined data collected by SEAMAP (the Southeast Area Monitoring and Assessment Program), which conducts surveys along the Texas and Louisiana coastlines in the Gulf of Mexico. Trawls were conducted for scientific purposes only, eliminating any biases inherent in analyzing data collected from fishing vessels. Data was analyzed by season: summer surveys were conducted in June and July from 1982 – 2008 and fall surveys were conducted in October and November from 1986 – 2007. Survey results were further analyzed according into the Gulf’s “shrimp statistical zones” (geographic delineations used by managers to divide the gulf into smaller areas that are more easily overseen).

For those of you interested in the detailed math behind the calculations, please check out the paper for a complete description of the calculations they used when compiling data across years and when analyzing pooled data. The authors do a great job describing their analyses.

If you would rather have a quick summary of their statistical methods, here you go: the authors scaled catches by the effort that resulted in that catch so different trawl times, for example, would not affect their results. They then combined species catches across years. Their analyses not only evaluated whole communities (rather than just one species at a time), they also identified potential indicator species.


Summer and fall fish communities were statistically different and were therefore analyzed independently. There were four distinct community areas indicated by summer trawls, while fall surveys yielded only three distinct communities- one zone’s community that was distinct in the summer was not during the fall.


The four distinct fish communities identified during summer surveys and example indicator species from each community.

There are several fish that could be used as indicator species in this part of the Gulf of Mexico. Species were identified as good indicators of ecosystem condition if they were very abundant but only in one distinct fish community.  Each distinct fish community shared at least one indicator species between summer and fall surveys. Fish that could be used included Atlantic bumper, Atlantic croaker, Atlantic cutlassfish, brown shrimp, dwarf goatfish, longspine porgy, and northern white shrimp.

The authors emphasize that the fish communities they analyzed may differ from the actual fish communities caught as bycatch during shrimp trawls in a few ways. Most notably, data on the potential indicator species identified in this study should be compared with bycatch data from shrimp trawls. Species important in both this study and in bycatch datasets would best serve as ecosystem indicators when analyzing bycatch data.

Environmental conditions, particularly the summer hypoxic zone locations, likely explain the difference in fish communities between summer and fall trawls. Different fish species have different sensitivities to low oxygen areas, and therefore are more or less able to tolerate being in close proximity to hypoxic waters. Life history (specifically, location preferences at different life stages) also influences which species were caught in SEAMAP surveys. Different species exhibit varying affinities for habitats over which trawls were conducted depending on fish breeding status and age.

Knowing where fish communities differ along the Gulf of Mexico coast is the start to effective monitoring and preservation of essential ecosystem services. Not only does this study examine a data set of impressive size conducted over a number of decades, but it also identifies species that could be used by themselves to indicate the health of the entire fish community found in a particular area. I am very excited to see these statistical techniques applied to data collected from actual commercial shrimp trawls. Comparing commercial trawl bycatch data to these SEAMAP results could tangibly advance ecosystem management efforts, which is a great use for thousands of animals that would otherwise be destined for discard.


No comments yet.

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 6 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 9 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