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Relationship between inflows, crabs, salinities, and whooping cranes
Document by Tom Stehn, Whooping Crane Coordinator
Aransas National Wildlife Refuge

November 26, 2001


Tom Stehn, Whooping Crane Coordinator
Aransas National Wildlife Refuge
The productivity and quality of winter whooping crane critical habitat is directly dependent on freshwater inflows from the San Antonio / Guadalupe River (TPWD 1998). The survival of one endangered species, the whooping crane, and one candidate species, the Cagle's map turtle, are directly tied to maintenance of inflows (Mendoza 2001b).

The reduction of freshwater inflows is a huge threat to the whooping crane that could lead to extinction. Data shows that the health and survival of the endangered whooping crane flock is directly related with fresh water inflows and blue crab populations. Inflows that carry nutrients and sediments are needed to produce blue crabs that will provide food for whooping cranes.

Data from the last eight winters indicates a direct relationship between freshwater inflows on the Guadalupe River, blue crab populations, and whooping crane survival. Chavez-Ramirez (1996) found that when available, blue crabs can make up 80-90% of the diet of whooping cranes. An individual crane can consume up to 80 crabs per day. Studies by Nelson (1995) of whooping crane food items (crabs, clams, wolfberry, acorns) showed that blue crabs were the highest in protein and nutrition for the whoopers. When crabs are not available, whooping cranes will switch to other foods, but because of the poor nutritive value of these alternate foods, the whoopers may actually burn up fat reserves and have a net loss of energy for periods of the winter (Chavez-Ramirez 1996).

When inflows are high, blue crab populations increase due to enhanced reproduction and survival, and whooping crane mortality is low. With reduced inflows, crabs do poorly and whooping crane mortality rises dramatically. This makes sense since blue crabs make up 80-90% of the whooping crane diet. For an eight-year period starting in 1993, surveys were done to roughly estimate the number of blue crabs available to whooping cranes. The winters of 1993-94 and 2000-01 were poor crab years; the remaining six winters all had adequate numbers of blue crabs present. During the two winters with poor crab numbers, seven and six whooping cranes died respectively. In all six other winters, either zero or one whooper died. Thus, there seems to be a strong inverse correlation between blue crab abundance and whooping crane mortality. Sufficient inflows are required to produce the necessary food for
whooping cranes to survive. Higher winter mortality of whooping cranes occur when blue crab numbers are low, therefore the very survival of the species is dependent on water management strategies that provide inflows for the bays to remain productive (Mendoza 2001a). In addition, following the poor blue crab winter of 1993-94, 37 % of the known adult pairs (17 out of 46) failed to nest following their return to Canada. This was believed to have resulted from their decreased energy reserves that had not built up sufficiently during the previous winter. This was unusual since normally just about all pairs attempt to nest annually.

Inflows which mix with Gulf waters help keep salinity levels moderate. When marsh and bay salinities exceed 23 parts per thousand (ppt), whooping cranes are forced to make daily flights to freshwater to drink. These flights use up energy, reduce time available for foraging or resting, and could make the cranes more vulnerable to predation on the uplands (Stehn pers. comm.). Thus, inflows are crucial in keeping salinity levels below the threshold of 23 ppt in coastal marshes used by whooping cranes .


Status of Guadalupe River flows
Inflows on the Guadalupe River are already insufficient and reduced over historic levels. TWDB data indicate natural droughts already threaten the ecosystem of the Guadalupe Estuary and predict that in less than 50 years withdrawals of surface and ground waters for municipal and industrial growth will leave insufficient inflows to sustain the ecosystem (USFWS 1994).

Kretzschmar (1990) predicted that a net annual loss of gaged inflow amounting to 555,000 acre-feet of water would occur on the Guadalupe by the year 2040. As water development pressures mount, freshwater inflows to the Texas bay systems are being reduced, and blue crab populations are being adversely affected. TPWD projects an 8 % reduction in blue crab populations in 40 years due to reduced inflows. This is going to have an alarming impact on whooping cranes.

The San Antonio and Guadalupe Rivers that empty into whooping crane critical habitat are calculated to need 1.24 million acre-feet per year to maintain ecosystem subsistence (TPWD 1998). Yet between 1941-1976, inflows were less than that amount in 14 out of 36 years, making its status already precarious (TDWR1980). The Service is very concerned about the impacts that planned diversions from the Guadalupe River would have on environmental water needs for adequate inflow to San Antonio Bay. Reduced inflows, including planned diversions from the lower reaches, is a definite threat to the continued survival of the wintering population of the endangered whooping crane and its critical habitat which lies in and around this estuary. With projected diversions, inflows will be insufficient to maintain the ecosystem in an average rainfall year (Stehn pers. comm.). Long before the ecosystem collapsed for lack of inflows, there would be significant adverse impacts to the primary winter food supply of the whooping crane (Kretzschmar 1990). The death of 6 whooping cranes during the 2000-01 winter emphasizes how serious an issue this is. This projected reduction is a major concern in keeping the whooping crane from going extinct (Stehn pers. comm.).

Human consumption of river water in Texas is a growing resource issue as the State's population continues to expand. This is a very worrisome trend since Texas water law reserves water for people but has few provisions for wildlife. Leaving sufficient water in the rivers to provide bay inflows is not explicitly designated as a beneficial use of water in Texas water law. National media attention was received in spring 2001 when the Rio Grande River dried up and flows no longer reached the Gulf. This is not the only river in Texas in trouble. So many people are using water from the aquifer and the rivers in central Texas, that the downstream folks and creatures are already seeing what most Texans do not want to acknowledge; that the rivers are already over-appropriated, and absolutely noone is minding the store, when it comes to making sure any fresh water ever makes it to the bays and estuaries (Wassenich pers. comm.).

With the population of Texas predicted to double in the next 50 years, the Texas legislature has initiated a state-wide water planning effort. As water developers plan to take water from the rivers, there is no direct mechanism in Texas water law to secure freshwater inflows to the bays and estuaries. Basically, the bays get whatever is left over, and portions of Texas rivers, including the Guadalupe, are already over-appropriated.

To prevent impacts and avoid "take" of the whooping crane under the Endangered Species Act, the Service urges planners to quantify the freshwater inflow needs to maintain bay productivity and the critical habitat of the whooping crane and to account for these needs in future versions of the State Water Plan (Mendoza 2001b). Formal consultations on flow reductions must ensure that downstream water needs are met (Stehn 1998).


Management Actions and Needs
The San Marcos River Foundation has applied for a 1.3 million acre-feet water right that would remain in the rivers to provide inflows to the bay. This amount is the minimum flow needed to sustain the bay ecosystem and keep the bays productive (TPWD 1998, Mendoza 2001a). Without sufficient inflows, wildlife resources, including fish, crabs, and shrimp, all decline. The 1.3 million acre-feet figure is often quoted as an amount needed to maximize harvest of 9 marine species of commercial interest in the bays and estuaries. In the case of blue crabs, more than 1.3 million acre-feet is needed to produce high blue crab populations. TPWD data clearly show that increased water inflows result in higher blue crab numbers (Mendoza 2001a). Crab survival of all life stages increases when salinities are generally below 20 ppt, and the very young stages in the blue crab life cycle show much better survival when salinities are moderate. Blue crabs were found to be more abundant in the Guadalupe Estuary in salinities averaging between 10-25 ppt. A simple inverse relationship exists between blue crabs catch rates and mean salinity within an estuary (Longely 1994). Peak crab counts in the bays occur following periods of flooding. In San Antonio bay, the 3 highest blue crab harvest years were all having inflows greater than 3 million acre-feet annually. Thus, to maximize blue crabs for whooping cranes to eat, managers should maximize freshwater inflows on the Guadalupe River. Guaranteed minimum inflows to the bay are essential. Inflow level targets have not been identified to adequately support whooping cranes.

Any withdrawal of water from the San Antonio / Gaudalupe River system is harmful to whooping cranes (Stehn pers. comm.). Inflow modeling is needed specifically for impacts to blue crab populations. Until this is done, water planners cannot make a judgement how much harm river withdrawals will do to blue crabs and thus whooping cranes. The U.S. Fish and Wildlife Service believes that providing a water right to the bays, as proposed by the San Marcos River Foundation, would be a crucial first step in guaranteeing that the bays would continue to function ecologically for all users to enjoy (Mendoza 2001a).

The U.S. Fish and Wildlife Service must take a strong stand on the inflow / whooping crane issue. All conservation groups need to do all in their power to ensure that adequate inflows from the Guadalupe River reach the bays. The Service has written a letter of support for the San Marcos River Foundation's application for 1.3 million acre-feet of water that would remain in the river for wildlife. The San Marcos River Foundation's water rights application is an important step in ensuring inflows on the Guadalupe reach the bays. It would be precedent setting in Texas for a water right to be designated as an inflow. Water developers are contesting the application. They are saying that human needs for water are too great and that there isn't enough water available to provide the water identified by the Texas Parks and Wildlife Department study needed to sustain the bays and estuaries. This issue has everyone's attention, and support is needed if the San Marcos River Foundation's application is to be successful.

Measures to protect instream flow, fish and wildlife habitat, and freshwater inflows to bays and estuaries should be part of each regional water management plan (Sansom 2000). It is essential that the Region L water plan provide a mechanism for providing adequate inflows to San Antonio bay for the health and survival of the whooping crane population.

Any environmental analysis for groundwater use should include a detailed assessment of potential impacts to fish and wildlife found in springs, streams, rivers, and even inflows to bays and estuaries. An example of an area of concern is the Edwards Aquifer and the impacts reduced flows would have on the whooping crane found in the San Antonio and Aransas bays (Mendoza 2001b). Especially in times of drought, groundwater-fed springs can provide 80% of the inflow from the Guadalupe River. Comal and San Marcos Springs combined can make up over 30% of the base flow of the Guadalupe River and nearly 70% during periods of drought (Sansom 2000).

Management actions needed are to

a) model inflows and blue crab populations for the Guadalupe Estuary and relate to ecological needs of whooping cranes.

b) devise strategies to conserve blue crabs populations by maintaining required inflows.

c) maintain inflows to keep marsh salinities below 23 ppt. (Stehn pers.comm.)


Citations
Chavez-Ramirez 1996. Food availability, Foraging Ecology, and Energetics of Whooping Cranes Wintering in Texas. M.S. thesis. TAMU-College Station.

Kretzschmar, G. E. 1990. Exec. Administrator of TWDB, letter to USFWS, Dec. 5, 1990.

Longley et al. 1995. Freshwater Inflows to Texas Bays and Estuaries. TPWD and TWDB, Austin, TX. 386 pp.

Mendoza 2001a. Letter in August, 2001 supporting SMRF.

Mendoza 2001b. Letter commenting on state water plan.

Sansom 2000. Letter to TWDB

Stehn 1998. Memo to Allen White, USFWS-Ecological Services.

Texas Department of Water Resources (1980) Guadalupe Estuary: a Study of the Influence of

Freshwater Inflows. LP-107. Austin, TX. 321 pp.

Texas Parks and Wildlife 1998. Freshwater Inflow Recommendation for the Guadalupe Estuary of Texas. Coastal Studies Technical Report No. 98-1, written by Pulich et. al. 61 pp.

USFWS 1994. Whooping Crane Recovery Plan. Albuquerque, NM.

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