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How is the Drought Impacting Fish Habitat ?

from ASMFC Habitat Hotline

(from Jersey Coast Anglers Association November 2002 Newsletter)

No matter where you live in the United States, you are probably familiar with drought. According to the National Climatic Data Center, 36% of the contiguous United States was in severe to extreme drought by the end of June. Drought conditions are impacting the East and West coasts, as well as the South and interior portions of our country (see Figure 1). In cases of severe drought, emergency water use restrictions can be imposed to conserve water such as prohibitions on car washing and watering plants. Drought can result in lower water levels (lakes and rivers) and increased water salinity (bays and estuaries).

During a drought, fresh water flow is reduced to bays and estuaries. This is happening particularly in the mid-Atlantic region.  For example, in the Chesapeake Bay, while there appears to be a natural cycle of high and low water flows, the current severe drought conditions have resulted in the lowest fresh water input since 1941.

A drought can affect a whole ecosystem including the physical, chemical and biological components. In aquatic systems, sea grasses, plankton, invertebrates, fish, and marine mammals are impacted in some way. This article focuses on possible drought impacts to fish and fish habitat in lakes, rivers, estuaries and bays.

 

Drought impacts on water levels, temperature, salinity and dissolved oxygen

Temperature, salinity and dissolved oxygen are important related factors that determine aquatic species abundance and distribution. Aquatic animals and plants have specific ranges or tolerances of these variables and changes can alter abundance and distribution as well as impact reproduction and survival. For example, marine worms need as little as 1 milligram of dissolved oxygen in a liter of water. Blue crabs and bay anchovies require 3 and American shad and striped bass require the most, about 5 milligrams. In addition, young fish are more likely to be sensitive to low-oxygen conditions than adults.

Usually, drought is associated with warm weather conditions. Below average rainfall results in low water levels in lakes, streams, and rivers as well as reduced freshwater flow to bays and estuaries. As a result of lower water levels temperatures in many water bodies can rise. Reduced freshwater flow can result in higher salinity in bays and estuaries and also in adjacent portions of rivers and streams. Temperature and salinity directly affect the amount of dissolved oxygen in water. As water temperature increases, the amount of dissolved oxygen that water can hold decreases. Similarly, the more saline the water, the less oxygen the water can hold.

Higher salinities can impact aquatic ecosystems in other ways as well.  Higher salinity can increase oysters' susceptibility to two diseases, Dermo and MSX, that have been devastating oyster populations. However, this higher salinity can also provide more favorable spawning conditions for oysters. Submerged aquatic vegetation, including sea grasses, can die off because many have adapted to specific salinity levels and they can't tolerate the higher levels found during a drought.

 

Considerations for Diadromouss Fish: Migration, Fish Passage, Habitat Availability and Susceptibility to Disease

Lower water levels in streams can increase susceptibility to disease in fish. Due to the low water levels, fish are stressed from crowding conditions, increased competition for food, and increased predation from birds, making them more susceptible to disease .

During periods of drought, migratory fish can get trapped by low water levels.  Such was the case in May 2002 when thousands of spawning fish died along the Apalachicola River in Florida. Many were concerned about the possible harm to the threatened Gulf sturgeon, whose eggs may have dried up before they could hatch.

During periods of drought, low water flow can reduce available fish habitat in streams. For example, spawning areas can dry out and important cover (vegetation and woody debris) may be inaccessible to juvenile fish increasing their susceptibility to predation. Drought can also affect migratory fish by causing low water flow over or through fish passages making it difficult or impossible for fish to use, especially smaller passages.

In the Pacific Northwest, drought conditions created a fight over water for energy production and migrating salmon, some of which are endangered. With very low water levels in spring 2001, biologists undertook efforts to record salmon strandings and rescue fish. In one count, over 700,000 migrating juvenile fall Chinook were stranded on a beach. Studies have been conducted to try to determine the response of juvenile Atlantic salmon to falling stream water levels. However, the results are contradictory with regard to whether the fish move to deeper pools or remain in shallow water and more research is needed:

Periods of prolonged drought with reduced water flow can affect channel maintenance and fish reproduction in streams. Channel maintenance can become an issue because high water flows are important for scouring and sediment transportation.  Overwintering fish eggs that are laid on stream bottoms and covered with gravel can be harmed by anchor ice produced because of the low water flow conditions. The anchor ice obstructs normal stream water from flowing through the gravel to bring oxygen to and remove wastes from eggs and embryos.  In addition, study results show a correlation between water flow and striped bass recruitment where higher survival related to adequate fresh water flow during the spring spawning period.

 

Drought impacts on Nutrient Levels

Reduced freshwater flow also means reduced nutrients and sediments coming into a bay through runoff. Reduced nutrients could result in fewer harmful algal blooms in the bay. This reduction can help maintain adequate dissolved oxygen levels, especially when the water temperature and salinity are higher. However, while nutrients are reduced into a bay during a drought, this means that nutrients are now trapped in tributaries stimulating algal blooms and affecting dissolved oxygen levels in those areas.

A reduction in nutrients and sediments can also improve water clarity. Increased water clarity allows more light to reach submerged aquatic vegetation (SAV), thus, improving conditions for SAV growth and photosynthesis. SAV provide important habitat for many species, including fish and crabs."

 

Opportunities for Habitat Restoration

Drought conditions and low water levels can provide opportunities for improving fish habitat. A fish and wildlife habitat restoration project was undertaken in Lake Walk In Water (Weohyakapka) a 7,500-acre freshwater lake in Florida renowned as a top bass fishing lake. The project involved removing unwanted decaying plant material around the shoreline that was increasing nutrients to the lake, impeding water flow and choking sport fish spawning areas and waterfowl habitat. In addition, the project sought to check the uncontrolled growth of cattails through use of herbicides and replanting the shoreline areas with beneficial aquatic plants e.g., native grasses and bulrush. Another project underway is Florida's Harris Chain of Lakes that seeks to reestablish beneficial native plants in an effort to improve fish and wildlife habitat in these lakes. In addition, low water and warm weather provide conditions that are conducive for dam removal. Dam removal improves fish habitat by opening passage to migrating fish and improving water flow and water quality.

 

Recovery

How do fish and fish habitat recover from drought impacts? In one study, Paller (1997), examined the recovery of a fresh water reservoir fish community from drawdown impacts. The water level was lowered for 3:5 years. The study results showed that within 9 months of refill, the fish community recovered in terms of number of species and overall fish abundance. However, size structures were different after drawdown. There were fewer large individuals and more smaller individuals.

 

Considerations for Management

Drought conditions impact fish and their habitat and drought severity can vary greatly. Planning for low water flow conditions should be undertaken particularly considering diadromous species spawning and migration. Plans could be developed to respond to severe drought conditions including water release/spillover schedules, monitoring and collection of stranded fish, and alternative fish passage options. In addition, habitat restoration projects could be designed for implementation during drought conditions, especially efforts to restore native vegetation and to improve water quality.

 

Climate and Drought

With a network of data collecting buoys, scientists can now link ocean conditions (sea surface temperature) in the equatorial Pacific to changes in rainfall and temperature in the U.S. El Nino refers to the condition when sea surface temperatures in the tropical Pacific are warmer than normal. El Nino events happen every 3 to 5 years on average. During El Nino events, the southeast US. can receive a lot of rain causing severe flooding and the northeast can experience major storms. La Nina refers to the condition when sea surface temperatures in the tropical Pacific are colder than normal. During La Nina events, rainfall is below normal in the southeastern United States causing drought conditions. La Nina tends to occur after El Nino, but not always. The intensity of these events varies, some are mild and some are strong.

Is there a relationship between global warming and severe drought conditions? Some scientists say that global warming may accentuate El Nino impacts since El Nino events have increased in frequency and have had greater climate impacts over the past century. This change corresponds to a rise in global temperatures. http://www.cpc.ncep.noaa.gov/products/outreach/advance.html under "El Nino and Global Warming: Any Connection?” by NOAA's Climate Prediction Center web team.

Along the East coast of the United States, El Nino and La Nina impacts are beginning to be better understood (e.g., drought in the southeast associated with La Nifia). Further research will hopefully reveal more links between these events and our coastal ecosystems, especially with regard to changes in fish habitat, populations, distribution, abundance, predation, and mortality. The difficult part will be trying to tease out climate impacts to fish and fish habitat from other impacts e.g., human related impacts.

NOAA has just declared (July 11, 2002) that we are officially entering an El Nino phase once again. Hence, scientists have another opportunity for research. This hopefully will signal the end of the severe drought in the U.S.