Fish declines linked to effects of excess nutrients on coastal estuaries

Elkhorn Slough by Brent HughesBy Tim Stephens:

A comprehensive study of a major California estuary has documented the links between nutrient runoff from coastal land use, the health of the estuary as a nursery for young fish, and the abundance of fish in an offshore commercial fishery. The study, published the week of June 8, 2015, in Proceedings of the National Academy of Sciences, focused on Elkhorn Slough and Monterey Bay on California’s central coast.

Lead author Brent Hughes, now a postdoctoral research fellow at the University of California, Santa Cruz, began studying water quality in Elkhorn Slough as a UCSC graduate student. His earlier research showed that virtually every portion of the estuary is adversely affected by high nutrient levels, which stimulate the growth of algae, leading to low oxygen levels when the algae die and decompose. The new study, based on data collected over the past 40 years, shows how low levels of dissolved oxygen (a condition known as “hypoxia”) affects fish populations in the estuary and beyond.

“We found that declines in dissolved oxygen levels were consistently associated with declines in the diversity and abundance of fish in Elkhorn Slough,” Hughes said. “In particular, we saw a drop in certain species of fish that we know use the estuary as a nursery ground for juveniles.”

English sole is one of the fish species that uses Elkhorn Slough as a nursery, and the study found that low oxygen was associated not only with fewer juveniles in the estuary, but also with later declines in the numbers of English sole caught in the commercial fishery and scientific fish surveys in Monterey Bay.

“From a conservation perspective, these findings suggest that improvements in land management and reductions in nutrient runoff could directly benefit estuaries and indirectly benefit offshore fisheries due to the important role of estuaries as nurseries for some species,” said coauthor Mary Gleason, lead marine scientist for The Nature Conservancy in California.

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