In Chesapeake Bay, water quality dictates where near-bottom fish swim

By on July 25, 2013

Andre Buchheister and Jason Romine prepare to sort the trawl-net catch during a ChesMMAP cruise on Chesapeake Bay. (Credit: ChesMMAP)

In the Chesapeake Bay, one of America’s most productive and stressed fisheries, marine organisms have to be dynamic to keep on swimming.

A study from the Virginia Institute of Marine Science analyzing 10 years of data found many demersal, or near-bottom dwelling, fish are sensitive to water quality and environmental conditions, which seem to dictate where they live and when they leave.

Andre Buchheister, a VIMS graduate student who led the study, wanted to understand how demersal fish distribution over time and space relates to water temperature, dissolved oxygen, salinity and depth.  He and a group of VIMS researchers aggregated and synthesized data to see if they could spot any trends.

They found that water quality in the bay, which is often stricken with low-oxygen dead zones, can greatly impact the distribution of fish in the Chesapeake Bay’s mainstem. “Dissolved oxygen was definitely one of the largest and strongest variables that affect the fish community,” he said.

Once dissolved oxygen in the mainstem dipped below hypoxic levels of 2 milligrams per liter, the variety of species caught quickly declined to zero, according to study data.

Buchheister said many near-bottom fish move north or south away from the mainstem due physiological stress and lack of food when dissolved oxygen become hypoxic.  “There’s evidence that the foraging grounds are compromised due to these hypoxia events,” he said.

Salinity of the bay’s waters also divides near-bottom fish species latitudinally. Fish tend to live in regions where the salinity matches their preferred conditions.  For example, the researchers observed that white perch and striped bass live mainly in the upper bay, which has stronger fresh water influence.  Summer flounder, spot and Atlantic croaker reside more often in high-salinity waters in the lower bay.

An example of the fish caught during a ChesMAPP trawl. Species here include a large sheepshead, striped bass, summer flounder, Atlantic croaker, northern puffer, clearnose skate and a kingfish. (Credit: ChesMAPP)

According to the study, the middle of the bay had a high turnover of fish species moving in and out, as the brackish salinity conditions make it stressful for many fish.

Data for the study came from the ongoing VIMS Chesapeake Bay Multispecies Monitoring and Assessment Program. Researchers analyzed 10 years of data from 3,640 sampling stations during 48 cruises, from 2002 to 2011.  The study was conducted aboard VIMS’ RV Bay Eagle, where 45-foot bottom trawls are dragged for 20 minutes to collect fish.  Researchers use a Hach Hydrolab sonde to collect water quality data.

Researchers sample the bay mainstem by dividing it up and tackling it section by section. At each station the team takes an inventory of the fish they’ve caught. A few are dissected to determine their age and stomach contents.  Environmental data, including water temperature, salinity, dissolved oxygen levels are recorded. The team also notes sea and weather conditions.

ChesMMAP staff sort and measure their catch before returning the fishes to the water. (Credit: ChesMMAP)

Species assessment program researchers sample in March, May, July, September and November. “We sample on these different months to really capture the dynamic nature of the fish in the Chesapeake Bay,” said Buchheister.

Buchheister hopes the study will make it easier for water managers to understand the health of fish populations, which will help them determine fishing regulations and  quotas.

The study was published in the Marine Ecology Progress Series. Buchheister, Chris Bonzek, Jim Gartland, and Rob Latour all contributed to the paper.

Top image: Andre Buchheister and Jason Romine prepare to sort the trawl-net catch during a ChesMMAP cruise on Chesapeake Bay. (Credit: ChesMMAP)

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