How Fish Fare
by Zoë Hoyle
Saltwater surges from Hurricane Katrina pushed up into sections of the large and medium-size rivers closest to the Gulf Coast; meanwhile the flooding of wetlands may have flushed water low in oxygen into the rivers. Saltwater surges, water depleted of oxygen, and large inputs of small organic material can have immediate adverse effects on fishes and other aquatic organisms in lowland, coastal areas. Over a longer timeframe, dramatic increases in large and small downed wood indirectly affect fishes in small streams by changing the habitats and functions of streams and rivers.
When Hurricane Katrina came through, Susan Adams, fisheries research scientist with the SRS Center for Bottomland Hardwoods Research, was working on an ongoing research project on Alabama shad funded by the FishAmerica Foundation, NOAA Fisheries, the Mississippi Museum of Natural Science, the University of Southern Mississippi (USM), and the Forest Service.
Adams, with USM cooperators Jake Schaefer, Paul Mickle, and Brian Kreiser, had been collecting data on fish communities, habitat, and migration in the Pascagoula River drainage in southeast Mississippi. The researchers had been surveying at 10 large river sites: 4 sites each in the Leaf and Chickasawhay Rivers that merge to form the Pascagoula, and 2 sites in the Pascagoula River itself. Schaefer and graduate students had additional historic and recent data from small and medium streams in the area.
On August 29, 2005, Hurricane Katrina made landfall about 70 miles west of the mouth of the Pascagoula River. Adams and her colleagues had to delay the sampling trip they had planned for September 2005.
“When we finally returned to the Leaf and Chickasawhay in October, I was surprised to find the rivers looking essentially the same as they did in August, before the hurricane,” says Adams. “The sand bars had rearranged a bit, but the channels weren’t choked with wood, nor the trees toppled. More importantly, the fish community was essentially unchanged.”
But when the researchers got to the downstream site on the Pascagoula River, they caught only 10 fish where they had normally caught 140, and local fishermen and residents reported fish kills all the way up the mainstem of the river. Similar affects were seen in the Pearl River as well. “The presumed cause of fish death on the Pascagoula River was a combination of salt water and low oxygen,” says Adams. “The huge tidal surge forced salt water much further up the river than normal. Salt water overlaid with anoxic (low in oxygen) freshwater flushed from wetlands could have killed most of the fish in the river.”
The good news is that most of the fish deaths were the result of a shortduration “pulse disturbance.” Habitat and water-quality conditions quickly returned to within a normal range, and fish populations began to recover immediately.
When they compared poststorm and prestorm data, the researchers found that short-term effects on the fish community were greater in areas closest to the Gulf of Mexico. Adverse effects were short-lived for most fish species, with species composition recovering within 1 to 2 years, and fish size structure taking longer to recover. Long-term benefits from the addition of downed wood were expected in smaller streams and rivers.
“The long-term impact of the storm in smaller streams was from the addition of rootwads and fallen trees—an integral component of stream and river ecosystems—to streams and smaller rivers,” says Adams. “Large dead wood helps retain sediment and organic matter, helps stabilize some stream channels, and creates complex habitats such as pools and side channels.” Fish community data collection is ongoing, and the researchers expect to assess some longer term results of the storm within the next year. One important question that remains unanswered is how such a storm affects slowgrowing, long-lived species such as Gulf sturgeon.
As a natural disturbance, Katrina is expected to have positive long-term impacts on river and stream habitats and their aquatic inhabitants. For Adams, the more interesting question is how the long-term impacts and recovery processes have played out. “The answer to that question depends not so much on the degree of impact which the hurricanes had on ecosystems as on how humans altered ecosystems prior to the storm and how humans respond to posthurricane conditions.” —ZH
For more information:
Susan Adams at 662–234–2744, x267 or