Bottom Trawling Makes Fish Work Harder for Their Next Meal

Written by on December 10, 2014 in Fish, Marine Life
How bottom trawling works. Image credit: NOAA.

How bottom trawling works. Image credit: NOAA.

Bottom trawling, the process of dragging an enormous weighted net across the seafloor for miles at a time, is an unselective and incredibly destructive method of fishing. The net collects anything and everything in its path, and it can permanently damage or destroy seafloor habitats. A new study reveals that bottom trawling has another interesting impact on marine life: it makes fish work harder for their next meal.

Previously, scientists assumed fish that weren’t swept up in the trawler went hungry, but this study suggests something different. The fish left behind are skinnier than normal fish, but they’re still full. This is most likely because trawling alters their food sources so they have to forage in a different way and expend more energy in order to obtain the same amount of food.

Researchers studied the effect of bottom trawling on commercially two important flatfish species, plaice (Pleuronectes platessa) and dab (Limanda limanda), in a heavily trawled area of the Irish Sea. They compared the preferred prey, range of prey, and overall stomach energy contents and fullness of plaice and dab, and found that the overall body condition of plaice decreased, while no change was observed in dab.

Photo taken from the back of the research vessel, where seagulls are awaiting the catch. Photo courtesy of Andrew Johnson.

Photo taken from the back of the research vessel, where seagulls are awaiting the catch. Photo courtesy of Andrew Johnson.

Unlike previously thought, they found that plaice left behind were not going hungry due to a reduction in local prey availability. Surprisingly, the plaice fish had full stomachs no matter where they were found. This is because as trawling frequency increased, both fish and prey biomass decreased, leaving the ratio of fish to prey unchanged. After further investigation, the researchers found that decreased body condition was due to plaice shifting to “energy-poor prey items,” and having to search longer (and use more energy) to find the same amount of food. They observed no real changes in dab because they are more generalist feeders and are capable of switching prey more easily than plaice.

This study shows for the first time that “fish living in highly trawled areas are still able to maintain food intake when the consumption and quantity of food is changed as a result of chronic bottom trawling,” the authors write. However, because the overall prey density decreases, the fish left behind have to spend more time and energy searching for food.

The results, which challenge a commonly held belief in fisheries science, may have implications for fishery management. Knowing which species are the most impacted by bottom trawling, and understanding the relationship between trawling, seafloor habitats, and body conditions, are important factors for determining effective management policies.

Photo of plaice taken with an underwater camera sled. Photo courtesy of Andrew Johnson.

Photo of plaice taken with an underwater camera sled. Photo courtesy of Andrew Johnson.

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Copyright © 2014 by Marine Science Today, a publication of Marine Science Today LLC.

About the Author

About the Author: Emily Tripp is the Publisher and Editor of MarineScienceToday.com. She holds marine science and biology degrees from the University of Miami's Rosenstiel School of Marine and Atmospheric Science and a Master of Advanced Studies degree in Marine Biodiversity and Conservation from Scripps Institution of Oceanography. When she's not writing about marine science, she's probably running around outside or playing with her dog. .

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