Deep-dwelling fish higher in mercury


SeafoodSource staff

Published on
August 31, 2009

Researchers at the University of Hawaii at Manoa’s School of Ocean and Earth Science and Technology say fish that hunt closer to the ocean floor have higher mercury concentrations than those that feed near the ocean surface.

The study, published in the August edition of the journal Proceedings of the National Academy of Sciences, reveals that large pelagic fish like bigeye tuna and swordfish that feed deeper in the ocean have elevated mercury levels compared to their shallower-dwelling counterparts like yellowfin tuna and mahi mahi. This is due to the fact that deeper-living micronekton prey, such as small fish, squids and crustaceans, have higher mercury levels.

The study may help provide information to the public on mercury levels in popular commercial fish species, though that wasn’t the study’s focus.

To conduct the study, researchers collected nine predatory pelagic fish species with different diets in waters surrounding Hawaii, along with a representation of the types of prey these fish eat. The predatory fish collected represented a wide variety of depths at which they search for food, from shallow (0 to 300 meters) to deep (up to 1,000 meters).

The mercury levels of these fish were measured, along with an analysis of animals in their stomachs. The authors found that while the sex of a fish and its location did not affect mercury concentrations, the size, age and species of the fish did.

“After looking more closely at these different mid-water prey organisms, a number of interesting questions have opened up,” said Anela Choy, the study’s lead researcher. “As these organisms are the primary food items for large pelagic fishes that humans like to eat, we need to understand more about how they fit into the open-ocean ecosystem in order to sustainably manage our fish populations. We hope this will provide crucial information for ecosystem-based fishery managers and ecosystem modelers.”

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