Brown Shrimp life history
- Louisiana’s shrimp industry is based around the brown shrimp and white shrimp.
- Post-larvae the shrimp move to inshore estuaries (May-November). Juveniles move to nursery areas in estuaries and migrate out as they grow larger (August – September). Adults inhabit saltier offshore waters. Juvenile shrimp prefer marsh edge habitats, flooded grasses, & aquatic vegetation in upper estuaries.
- The life expectancy of brown shrimp is about 18 months. They reach harvestable sizes in 2–3 months under favorable environmental conditions.
- Louisiana is number one in commercial shrimp landings in the United States. Shrimp have been harvested commercially since the 1800s and are the most valuable and second-largest commercial fishery in Louisiana.
Factors impacting shrimp growth
- Salinity, water temperature, and dissolved oxygen can influence function, distribution, growth, survival, and movement of shrimp.
- Hydrological conditions within nursery (particularly in spring) play a large role in dictating next season’s potential harvest.
- They commonly tolerate lower salinity levels and exhibit a slower growth rate at higher salinities.
- Shrimp population size relates to river discharge and its effect on estuarine salinities.
- Extremes in salinity have been shown to reduce the growth rates of juvenile shrimp.
Factors affecting shrimp abundance
- Marsh habitat loss may affect shrimp yield because the yield of an estuary is directly related to marsh acreage.
- Low dissolved oxygen (hypoxia) areas have less shrimp. The appearance of Dead Zone corresponds with peak shrimp fishing and may impact shrimp harvests (shrimp avoid low oxygen areas).
The CRMS design includes a suite of sites encompassing a range of ecological conditions of swamp habitats and fresh, intermediate, brackish, and salt marshes. Approximately 390 sites are monitored using standardized data collection techniques and fixed sampling schedules. The CRMS sites are located within nine coastal basins and four CWPPRA regions, covering the entire Louisiana coast. Comparisons of changing conditions are not limited to project influences but are possible throughout the coastal zone because CRMS was designed as a reference network.
The reference network approach enables assessment of ecological conditions at multiple scales. Within a CRMS site, there are many CRMS stations or plots. At each site, data are collected at a broader 1 km2 and a finer 200 m2 scale (Figure 3). At the 1 km2 scale, high-resolution aerial photography is used to calculate the ratio of land to water to investigate land change trends through time. Within the 200 m2 area, data are collected in the field using standardized protocols and consistent sampling intervals. CRMS data include water level, salinity, sediment accretion, surface elevation change, composition and abundance of vegetation, the ratio of land to water, and soil characteristics. The ten vegetation stations are in a diagonal transect across the 200 m2 area. The rod surface and accretion stations are nested around a boardwalk. The hydrologic station is generally in a bayou or water body near the boardwalk.
Excerpt below from the National Ocean Service Monitering Estuaries Tutorial
"Under laboratory conditions, pure water contains only oxygen and hydrogen atoms, but in the real world, many substances are often dissolved in water, like salt. Salinity is the concentration of salt in water, usually measured in parts per thousand (ppt). The salinity of seawater in the open ocean is remarkably constant at about 35 ppt. Salinity in an estuary varies according to one's location in the estuary, the daily tides, and the volume of fresh water flowing into the estuary.
"In estuaries, salinity levels are generally highest near the mouth of a river where the ocean water enters, and lowest upstream where fresh water flows in. Actual salinities vary throughout the tidal cycle, however. Salinity levels in estuaries typically decline in the spring when snowmelt and rain increase the freshwater flow from streams and groundwater. Salinity levels usually rise during the summer when higher temperatures increase levels of evaporation in the estuary.
"Estuarine organisms have different tolerances and responses to salinity changes. Many bottom-dwelling animals, like oysters and crabs, can tolerate some change in salinity, but salinities outside an acceptable range will negatively affect their growth and reproduction, and ultimately, their survival.
"Salinity also affects chemical conditions within the estuary, particularly levels of dissolved oxygen in the water. The amount of oxygen that can dissolve in water, or solubility, decreases as salinity increases. The solubility of oxygen in seawater is about 20 percent less than it is in fresh water at the same temperature."
The Gulf of Mexico Dead Zone
Excerpt below from an article by Elizabeth Carlisle in The Louisiana Environment
"The Gulf of Mexico hypoxic zone is a seasonal phenomenon occurring in the northern Gulf of Mexico, from the mouth of the Mississippi River to beyond the Texas border. It is more commonly referred to as the Gulf of Mexico Dead Zone because oxygen levels within the zone are too low to support marine life. The Dead Zone was first recorded in the early 1970s. It originally occurred every two to three years, but now occurs annually. In the summer of 1999, the Dead Zone reached its peak, encompassing 7,728 square miles.
"Hypoxic conditions arise when dissolved oxygen levels in the water fall below two milligrams per liter of water, too low to sustain animal life in the bottom strata of the ocean. The Dead Zone forms each spring as the Mississippi and Atchafalaya Rivers empty into the Gulf, bringing nutrient-rich waters that form a layer of fresh water above the existing salt water. It lasts until late August or September when it is broken up by hurricanes or tropical storms. The nutrients provide favorable conditions for the excessive growth of algae that utilize the water’s oxygen supply for respiration and when decomposing.
"The Mississippi River Basin covers forty-one percent of the continental United States, contains forty-seven percent of the nation’s rural population, and fifty-two percent of U.S. farms. The waste from this entire area drains into the Gulf of Mexico through the Mississippi River. Included in this agricultural waste are phosphorus and nitrogen, the primary nutrient responsible for algal blooms in the Dead Zone. Nitrogen and phosphorus were first used in fertilizers in the United States in the 1930s. Concentrations of nitrate and phosphate in the lower Mississippi have increased proportionately to levels of fertilizer use by agriculture since the 1960s when fertilizer use increased by over two million metric tons per year. Overall, nitrogen input to the Gulf from the Mississippi River Basin has increased between two and seven times over the past century. In addition to agricultural waste, inadequately treated or untreated sewage and other urban pollution are also dumped into these waters. Nitrogen is normally a limiting factor, meaning its restricted quantities limit plant growth and reproduction. However, excessive amounts of nitrogen can lead to eutrophication, the takeover of nutrient-rich surface water by phytoplankton or other plants. If nutrient pollution is not greatly reduced, fish and shellfish may someday be permanently replaced by anaerobic bacteria."
This activity was developed for Project Resilience, funded by the Gulf Research Program of the National Academies of Sciences, Engineering, and Medicine.