(DRAFT) - Taxonomy
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
TAXONOMY
NAME - GAMBUSIA, SAN MARCOS
OTHER COMMON NAMES - GAMBUSIA and SAN MARCOS
ELEMENT CODE -
CATEGORY - Fish
PHYLUM AND SUBPHYLUM - CHORDATA,
CLASS AND SUBCLASS - OSTEICHTHYES,
ORDER AND SUBORDER - ATHERINIFORMES,
FAMILY AND SUBFAMILY - POECILIIDAE,
GENUS AND SUBGENUS - GAMBUSIA,
SPECIES AND SSP - GEORGEI,
SCIENTIFIC NAME - GAMBUSIA GEORGEI
AUTHORITY -
TAXONOMY REFERENCES -
COMMENTS ON TAXONOMY -
San Marcos Gambusia
Gambusia georgei Hubbs and Peden, 1969
KINGDOM: Animal GROUP: Fish
PHYLUM: Chordata CLASS: Osteichthyes
ORDER: Atheriniformes FAMILY: Poeciliidae
The San Marcos gambusia (Gambusia georgei Hubbs and Peden, 1969)
is a member of the Poeciliidae and belongs to a genus having more than
30 species of live bearing freshwater fishes of Central American
origin. The genus Gambusia is well defined and mature males may be
distingushed from related genera of this family by their thickened
upper pectoral fin rays (01). Only a limited number of Gambusia are
native to the U.S. and of this subset, the San Marcos gambusia has the
most restricted range (02,03). The San Marcos gambusia is plainly
marked and is subtly different from the mosquitofish (G. affinis) with
which it may be confused. Scales on the San Marcos gambusia tend to
be strongly crosshatched in contrast to the less distinct markings on
the scales of G. affinis. In addition, G. georgei tend to have a
prominent dark pigment stripe across the distal edges of the dorsal
fins. A diffuse mid-lateral stripe from the base of the pectoral fin
extending posteriorly to the caudal peduncle is also often present,
Taxonomy - 1 (DRAFT) - Taxonomy
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
especially in dominant individuals. As in G. affinis, a dark
subocular bar is visible and is elicited easily from frightened fish
(02,03).
Compared to G. affinis, G. georgei has fewer spots and dusky
pigmented regions on the caudal fin. The median fins of wild caught
specimens of the San Marcos gambusia tend to be lemon yellow (02). In
a dominant or "high" male, this color can approach a bright yellowish-
orange, especially around the gonopodium. A bluish sheen is evident
in more darkly pigmented individuals, especially near the anterior
dorsolateral surfaces of adult females (03).
Gonopodial structures of males classically have been employed in
dealing with Gambusia systematics and G. georgei is unique
morphologically from other species in several characters, including
the presence of more than five segements in ray 4a which are
incorporated into the elbow and also by the presence of a compound
claw on the end of ray 4p (02).
The holotype is stored at the University of Michigan Museum of
Zoology (UMMZ 187447) and other specimens are housed at the Texas
Natural History Collections at the University of Texas, Austin (02,
03).
Taxonomy - 2 (DRAFT) - Status
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
STATUS
Coded Status
E: Federal Endangered
COMMENTS ON STATUS -
U.S. STATUSES AND LAWS:
The San Marcos gambusia (Gambusia georgei) has been designated an
Endangered species pursuant to the Endangered Species Act of 1973
(50 CFR 17.11; P.L. 93-205, 87 Stat. 884; 16 U.S.C. 1531-1540),
as amended. The species has this status wherever found including
the State of Texas. Critical Habitat has been designated in Spring
Lake and its outflow, the San Marcos River system downstream
approximately 0.5 miles below the Interstate Highway 35 bridge,
Hays County, TX (50 CFR, Sec. 17.95(e)).
This species is protected by the Lacey Act (P.L. 97-79, as
amended; 16 U.S.C. 3371 et seq.) which makes it unlawful to import,
export, transport, sell, receive, acquire, or purchase any wild
animal (alive or dead including parts, products, eggs, or offspring):
(1) in interstate or foreign commerce if taken, possessed,
transported or sold in violation of any State law or
regulation; or
(2) if taken or possessed in violation of any U.S. law,
treaty, or regulation or in violation of Indian tribal law.
It is also unlawful to possess any wild animal (alive or dead
including parts, products, eggs, and offspring) within the U.S.
territorial or special maritime jurisdiction (as defined in
18 U.S.C. 7) that is taken, possessed, transported, or sold in
violation of any State law or regulation, foreign law, or Indian
tribal law.
RESPONSIBLE FEDERAL AGENCIES:
USFWS -Responsible for the management/recovery, listing, and
law enforcement/protection of this species.
All Federal agencies have responsibility to ensure that any
action authorized, funded, or carried out by that agency is not likely
to jeopardize the continued existence of the species or result in the
destruction or adverse modification of Critical Habitat (50 CFR 402),
and to utilize their authorities to carry out programs for the
conservation of the species.
STATE STATUSES AND LAWS:
STATE: Texas
DESIGNATED STATUS: Endangered
ADMINISTRATIVE AGENCY: Texas Parks and Wildlife Department
Status - 1 (DRAFT) - Status
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
STATE STATUTE: 31 Texas Administrative Code Sec. 57.133
(127.30.003) July 1977
INTERNATIONAL STATUSES, TREATIES, AND AGREEMENTS:
The San Marcos gambusia is listed as Endangered in the IUCN Red
Data Book, Vol. 4, 1977.
ECONOMIC STATUSES:
This species has value to naturalists and as a component of
Texas' natural heritage of biotic diversity.
78/07/14:43 FR 30316/30319 - Proposed Endangered
80/07/14:45 FR 47355/47364 - Listing as Endangered w/ Critical Habitat
85/07/22:50 FR 29901/29909 - Five year review
Status - 2 HABITAT ASSOCIATIONS
HABITAT - AQUATIC
INLAND AQUATIC
LAND USE -
Transportation, communications, and Util
Mixed Urban or Built-up Land
Streams and Canals
Reservoirs
NATIONAL WETLAND INVENTORY CODES
NWI NWICLS NWIMOD NWISPEC
Riverine, upper perennial UB3
COMMENTS ON HABITAT ASSOCIATIONS -
The San Marcos River arises in a series of spring openings along
the Balcones Fault Zone in the City of San Marcos, Hays County, TX.
The second largest spring system in Texas, the springs at San Marcos
historically have exhibited the greatest flow dependability and
environmental stability of any spring system in the southwestern U.S.
Records indicate that the San Marcos Springs have never ceased
flowing, although the flow has varied and is tied to fluctuations in
their source, the Edwards Aquifer underlying the Balcones Fault Zone.
Partly because of the consistancy of its waters, the San Marcos Spring
ecosystem, including its springrun, the San Marcos River, has a
greater known diversity of aquatic organisms than any other ecosystem
in the southwestern United States (07).
The biological uniqueness of this system has been known for many
years. Many of the species found in the San Marcos River ecosystem
are found nowhere else and are restricted to the first few kilometers
or less of the San Marcos springrun. Other forms are nearly as range
restricted and are limited to the largest spring system in Texas---the
springs issuing into the Comal River in nearby New Braunfels, Comal
County, TX---in addition to the San Marcos River ecosystem (07).
Due to a variety of factors, including increased use of the
aquifer waters for human activities, increased urbanization in the
San Marcos region resulting in increases in flood intensity,
pollution, recreational use and alterations of the river, the San
Marcos ecosystem is in a precarious situation.
The Balcones Fault Zone is the principal geological feature
characterizing the San Marcos area. This fault zone separates the
Edwards Plateau vegetation region from the Blackland Prairies and
South Texas Plains regions. These regions correspond to the
Balconian, Texan and Tamaulipan Biotic Provinces respectively, of
Blair (1950). The headwaters of the San Marcos River issue from
several large fissures and numerous smaller solution openings along
the San Marcos Springs fault. Early Spanish explorers estimated that
a series of 200 springs made up the main spring area. The springfed
San Marcos River flows primarily southeastward for approximately
110 km before joining the Guadalupe River in the vicinity of Gonzales,
Gonzales County, TX. The upper San Marcos River is a rapidly flowing,
unusually clear springrun some 5-15 m wide and up to approximately
4 m deep. For the first few kilometers, to near the Blanco River
confluence, the river flows mostly over a firm gravel bottom with many
shallow riffles alternating with deep pools. The section between the
Blanco River confluence and the Guadalupe River has fewer attributes
of a springrun. Upstream from the junction of the Blanco River with
Habitat Associations - 1 the San Marcos, three creeks, various storm sewers, and one wastewater
treatment plant discharge into the river. Sink Creek, largest of the
three creeks, discharges large quantities of storm runoff from the
north into Spring Lake. Spring Lake Dam backs water approximately
1.6 km up Sink Creek. The other two creeks, Willow Springs and
Purgatory Creeks, are normally dry except during periods of high
rainfall.
The San Antonio Region of the Balcones Fault Zone extends as a
series of faults and fracture lines from the vicinity of Brackettville
(Kinney County) east to San Antonio (Bexar County) and then northeast
to near Kyle (Hays County). A major aquifer (Edwards Balcones Fault
Zone Aquifer) underlies this fault zone and is the source of water for
San Marcos Springs. Runoff from the southern and eastern portions of
the Edwards Plateau recharge this aquifer through the porous
Cretaceous-aged limestones found in this region. Water from this
recharge flows along the fault zone from west to east and then
northeast. Major springs located along this fault system include the
two largest springs in Texas, Comal Springs in New Braunfels (Comal
County) and San Marcos Springs in San Marcos (Hays County).
The flow of San Marcos Springs has been monitored intermittently
since 1894. Average annual spring flow is 4.4 cubic meters per second
(161.0 cf/s). During drought years much lower flows occurred,
especially during the mid-1950's when Comal Springs did not flow for
part of one year. As of 1979 the lowest recorded monthly flow from
San Marcos Springs was 1.53 cubic meters per second (54 cf/s) during
1956. The lowest measured daily flow rate occurred on 15 and 16
August 1956 when the San Marcos River flowed at only 1.29 cubic meters
per second (45.55 cf/s). Maximum daily spring flows can be greater
than 8.33 cubic meters per second (294.13 cf/s), especially following
periods of high local rainfall and runoff.
The thermally constant water from the San Marcos Springs has long
been noted and generally varies annually by less than 1-2 degrees C
in the headwaters. At the lower end of the springrun habitat only a
slightly greater range of variation in temperature (from 25.5 degrees
C in August to 20.4 degrees C in February) has been recorded between
1967-1971. Waters tend to be alkaline or neutral due to the limestone
aquifer. The pH range of the San Marcos Springs is 6.9-7.8. The
stability of this stream, both in terms of flow dependability and
thermal characteristics, probably provided the appropriate ecological
conditions necessary to allow the unusually high degree of endemism
of the San Marcos biota.
Spring Lake, created on the San Marcos River not far from the
headsprings is the site of a major tourist attraction, Aquarena
Springs, Inc., a private amusement park.
Within its restricted range of part of the upper San Marcos
River, the San Marcos gambusia (Gambusia georgei) prefers quiet waters
adjacent to sections of moving, but seemingly of greatest importance,
thermally consistent waters (approximately 21 degrees C) (02,03).
Substrates on which the San Marcos gambusia (Gambusia georgei) have
been found are mostly muddy but generally not silted habitats, and
shade from overhanging vegetation or bridge structures is one factor
common to all localities where San Marcos gambusia (Gambusia georgei)
have been found (02,03). Few localities along the upper San Marcos
River have this combination of thermal consistency, shallow muddy
habitats and partial shading (02,03,07).
Habitat Associations - 2 (DRAFT) - Food Habits
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
FOOD HABITS
TROPHIC LEVEL -
CARNIVORE
LIFESTAGE FOOD FOOD PART
General Crustaceans
General Zooplankton
General Arthropods
Food Habits - 1 (DRAFT) - Environment Associations
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
ENVIRONMENTAL ASSOCIATIONS
G = General A = Adult
LIM = Limiting RA = Resting Adult
J = Juvenile FA = Feeding Adult
RJ = Resting Juvenile BA = Breeding Adult
FJ = Feeding Juvenile P = Pupae
L = Larvae E = Egg
RL = Resting Larvae
FL = Feeding Larvae
LIFESTAGE ENVIRONMENTAL ASSOCIATIONS
G Aquatic Features: Springs [flowing]
G Coastal Features: Vegetated offshore islands
G Human Association: Public residential parks
G
Environment Associations - 1 (DRAFT) - Life History
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
LIFE HISTORY
FOOD HABITS:
Nothing is known of the food habits of the San Marcos gambusia
(Gambusia georgei) (02,03,07). Presumably, as in other Poeciliids,
insect larvae and other invertebrates account for most of the dietary
intake of this spcies (07).
HOME RANGE/TERRITORY:
No information is known concerning territoriality of the San
Marcos gambusia (Gambusia georgei). Presumbly, as in other spring run
adapted Poeciliids, males will maintain territories of approximately
1 square meter as breeding territories (08).
PERIODICITY:
The periodicity of the San Marcos gambusia (Gambusia georgei) is
not known. However it is presumed similar to other Gambusia in which
individuals are most active diurnally with several activity peaks
during the day (08,07).
MIGRATION PATTERNS:
No migrations are known for the San Marcos gambusia (Gambusia
georgei).
COVER/SHELTER REQUIREMENTS:
The San Marcos gambusia (Gambusia georgei) requires shallow,
muddy substrates in thermally consistent (21 degrees C) habitats with
partial shading from trees overhanging the San Marcos River or from
bridge structures crossing the river (02,03,07,09,10).
REPRODUCTIVE SITE REQUIREMENTS:
The San Marcos gambusia (Gambusia georgei) requires shallow,
muddy, but not silted, substrates in thermally consistent (21 degrees
C) habitats with partial shading from trees overhanging the San Marcos
River or from bridge structures crossing this river (02,03,07). These
breeding sites are not separate from non-breeding habitats of this
species (02,03).
REPRODUCTIVE CHARACTERISTICS:
While all members of the family Poeciliidae are livebearers with
internal fertilization, there is little information on the
reproductive capabilities of the San Marcos gambusia (Gambusia
georgei). Two individuals kept in laboratory aquaria produced 12
young, 30 young, and 60 young, although the largest clutch appeared to
have been aborted and did not survive (03). Presumably, as in other
spring inhabiting Poeciliids, individuals mature in approximately
three to six months of age and females can produce broods at
approximately bimonthly intervals for about 6 months to a year (08).
PARENTAL CARE:
Female Poeciliids provide care in utero during gestation;
however, no further parental care is known in Gambusia after birth
(08).
Life History - 1 (DRAFT) - Life History
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
POPULATION BIOLOGY:
Little is known concerning the population biology and dynamics of
the San Marcos gambusia (Gambusia georgei). Estimates of population
size of the species range from 1000 individuals in 1969 (02), to
approximately 100 individuals in 1980 (03), to only a few dozen in
1982 (09). Attempts to secure individuals of G. georgei in 1984 were
unsuccessful, suggesting that the species, if not extinct, is
extremely rare (10).
SPECIES INTERRELATIONSHIPS:
No direct causal relationships are known for the San Marcos
gambusia (Gambusia georgei) (02,03,07).
OTHER LIFE HISTORY DESCRIPTORS:
Hybridization between the San Marcos gambusia (Gambusia georgei)
and the mosquitofish (G. affinis) was noted in the original
description of G. georgei (02) and the production of hybrid
individuals between these species has continued for many years without
obvious introgression of genetic material into either of the parental
species (03). Since 1980, samples of Gambusia from the San Marcos
River contain a relatively greater number of hybrid individuals than
in previous decades suggesting that the hybrid individuals may be
placing an additional stress through competitive interference with the
small native population of San Marcos gambusia (03,07,09,10).
Life History - 2 (DRAFT) - Management Practices
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
MANAGEMENT PRACTICES
RESULT MANAGEMENT PRACTICE
Beneficial Controlling/Restricting Boating Activities
Beneficial Restricting/regulating human disturbance of populations
Beneficial Maintaining/Controlling Water Flow
Beneficial Developing/maintaining/protecting wetlands
Beneficial Controlling water levels
Beneficial Restricting/regulating human use of habitats
Beneficial Land Acquisition
Beneficial Water Right Acquisition
Beneficial Controlling pollution [thermal, chemical, physical]
Beneficial Controlling/Restricting Pesticide Use
Beneficial Controlling/Restricting Herbicide Use
Beneficial Reforestation
Beneficial Stocking captive-reared wild-strain animals
Beneficial Controlling/Removing Exotic Vertebrates
Adverse Low Gene Pool
Existing Low Gene Pool
Adverse Hybridization
Existing Hybridization
Adverse Predation
Existing Predation
Adverse Rural Residential/Industrial Areas
Existing Rural Residential/Industrial Areas
Adverse Siltation
Existing Siltation
Adverse Strip mining
Existing Strip mining
Adverse Flooding
Existing Flooding
Adverse Groundwater drawdown
Existing Groundwater drawdown
Adverse Dredging
Existing Dredging
Adverse Developing/maintaining stream bank vegetation
Existing Developing/maintaining stream bank vegetation
Adverse Applying fertilizers
Existing Applying fertilizers
Adverse Applying herbicides
Existing Applying herbicides
Adverse Applying pesticides
Existing Applying pesticides
Adverse Environmental Contamination/Pollution
Existing Environmental Contamination/Pollution
Adverse Competition
Existing Competition
Adverse Exotic/Feral/Introducted Species
Existing Exotic/Feral/Introducted Species
Adverse Erosion
Existing Erosion
Adverse
Management Practices - 1 (DRAFT) - Management Practices
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
RESULT MANAGEMENT PRACTICE
Existing
COMMENTS ON MANAGEMENT PRACTICES -
A number of factors are currently threatening the San Marcos
gambusia (Gambusia georgei) with extinction. These include the
potential elimination of the spring flow from San Marcos Springs due
to water withdrawals from the Edwards Aquifer, especially from urban
and agricultural users of this water (04,05). On a local scale, the
City of San Marcos is growing rapidly (06) and increased urbanization
has caused increased flooding; erosion, pollution, pesticides and
herbicides which runoff into the San Marcos River; exotic species
which compete for resources and space and may prey on the San Marcos
gambusia; and effluent discharges from the Secondary Sewage Treatment
Plant of the City of San Marcos. Potential genetic contamination
from hybridization with the mosquitofish (G. affinis) and a very
limited gene pool in its extremely restricted range (02,03,07).
In addition to spring flow, the San Marcos gambusia also requires
relatively constant temperature regimes and shading in its habitats.
Modifications arising from increased urbanization must take these
factors into account. A secondary portion of the Soil Conservation
Service Plan is to upgrade recreational facilities in the Rio Vista
Park area. Since the entire range of G. georgei is immediately
downstream from these facilities, extreme care must be taken during
the construction phases to insure the protection of this species and
its extremely limited habitat.
Exotic species pose a significant threat to G. georgei; this is
especially true with Poecilia, which share many similarities in
habitat use with G. georgei. Although Poecilia sp. in the San Marcos
River exhibit broad thermal tolerances (especially to high
temperatures), overlap in habitat with G. georgei appears especially
great during winter and spring when thermally moderated, quiet,
shallow habitats are chosen by all of these poeciliids. Juvenile
centrarchids and cichlids in the San Marcos River also appear to share
habitat similarities with G. georgei populations. In addition, the
abundance of the predaceous characin (Astyanax mexicanus) may have an
additional adverse impact on the abundance of San Marcos gambusia.
Competition for resources may be one factor which imposes extreme
limits on the abundance of G. georgei. In addition to expected high
levels of interspecific competition from other Gambusia, especially
G. affinis, other less closely related species also have been found
associated with G. georgei.
The following species are exotics in the San Marcos River but
have been taken in G. georgei habitats: Astyanax mexicanus, Poecilia
latipinna, P. formosa, Micropterus dolomieui, Lepomis microlophus, L.
auritus, Ambloplites rupestris, Cichlasoma cyanoguttatum, Sarotherodon
mossambicus and S. aureus. Abundance of Poecilia (both species),
Lepomis (especially L. auritus) and the cichlids (all species) are
high in the habitats where G. georgei are found. Interference from
these species may inhibit the ability of G. georgei to recolonize the
San Marcos River following perturbations such as flooding.
Management Practices - 2 (DRAFT) - Management Practices
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
Studies have shown that many fishes (especially when small) have
very similar food habits (09). If exotic, or nonnative, species are
added to aquatic systems, greater competition or overlap among species
is possible. These exotic species may be able to acquire resources
with greater efficiency than native species. Also, during the
exponential population growth phases of recently introduced exotics,
even short term extensive niche overlap with G. georgei is likely to
impact this species negatively.
Stability within the San Marcos River system apparently is the
key to survival of that ecosystem. This stability will have the
added benefit of not only insuring the protection of G. georgei, but
conserving the other unique elements of the San Marcos aquatic
environment as well.
APPROVED PLAN:
U.S. Fish and Wildlife Service. 1985. San Marcos Recovery Plan.
U.S. Fish and Wildlife Service, Albuquerque, New Mexico. 109 pp.
The San Marcos gambusia may be extinct. Efforts should be made
to recover any individuals alive in the wild and culture them in a
hatchery. The highest priority is to locate and capture any
surviving individuals.
If this species is relocated and its numbers increase in
captivity, its survival and that of three other Threatened or
Endangered San Marcos River species (Texas wild rice, fountain
darter and San Marcos salamander) is tied to the survival of the San
Marcos River ecosystem. If the flow of the San Marcos River, with its
cycle of variation, is assured, the chances of long-term survival of
these species increases. Short term threats also must be addressed,
but the two types of concerns are totally and inextricably
intertwined. The following items need to be determined and addressed
for the San Marcos River system:
1) Aquifer characteristics and recharge patterns and zones that
influence flow at San Marcos Springs.
2) Impacts of recreational (i.e., boating, swimming, "tubing",
etc.) use of the river on listed species.
3) Characteristics of the river's watershed.
4) Pesticide and herbicide use in the watershed.
Management guidelines for the San Marcos River ecosystem
need to be established, including:
1) Reduction of pollution load (i.e., herbicides, pesticides,
fertilizers, waste water and other pollutants).
2) Possible augmentation of aquifer recharge.
3) Controls on groundwater pumping of the aquifer.
4) Preparation of water wells to ensure continued flow of the
river.
5) Restoration of damaged and enhancement of marginal habitats.
6) Removal of nonnative organisms (i.e., Gambusia affinis and
others).
7) Establishment of river recreational use guidelines.
8) Encouragement of proper management by private and public
Management Practices - 3 (DRAFT) - Management Practices
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
landowners. Reduction of impacts on the ecosystem can be
enhanced by obtaining management authority along the San
Marcos River (i.e., agreements, acquisition, zoning, water
rights, etc.).
Finally, public awareness of the uniqueness of the San Marcos
River ecosystem should be increased through information pamphlets,
news releases and public involvement.
Management Practices - 4 (DRAFT) - References
Species GAMBUSIA, SAN MARCOS
Species Id ESIS252003
Date 14 MAR 96
References
***** REFERENCES FOR ALL NARRATIVES EXCEPT N-OCCURRENCE *****
01 Rosen, D.E. and R.M. Bailey. 1963. The Poeciliid fishes
(Cyprinodontiformes), their structure, zoogeography, and
systematics. Bull. Amer. Mus. Nat. Hist. 126:1-176.
02 Hubbs, C. and A.E. Peden. 1969. Gambusia georgei sp. nov. from
the San Marcos River. Copeia 1969:357-364.
03 Edwards, R.J., E. Marsh and C. Hubbs. 1980. The status of the
San Marcos gambusia, Gambusia georgei. Endangered Species Report
9, U.S. Fish and Wildlife Service, Region 2, Albuquerque, NM.
34 pp.
04 Texas Department of Water Resources. 1977. Continuing water
resources planning and development for Texas. Phase I. Draft.
05 Klemt, W.B., T.R. Knowles, G.R. Elder, and T.W. Sieh. 1979.
Groundwater resources and model applications for the Edwards
(Balcones Fault Zone) Aquifer in the San Antonio Region, Texas.
TX Dept. Water Res. Rept. 239. 88 pp.
06 U.S. Bureau of the Census. 1982. 1980 census of the population.
Vol. 1. Characteristics of the population. Chap. B. General
population characteristics. Pt. 45. Texas. 791 pp.
07 U.S. Fish and Wildlife Service. 1985. San Marcos Recovery Plan.
U.S. Fish and Wildlife Service, Albuquerque, NM. 109 pp.
08 Hubbs, C. 1971. Competition and isolation mechanisms in the
Gambusia affinis x G. heterochir hybrid swarm. Bull. No. 19, TX
Mem. Mus., University of TX. Austin, TX. 47 pp.
09 Hubbs, C., T. lucier, E. Marsh, G.P. Garrett, R.S. Edwards and E.
Milstead. 1978. Results of an eradication program on the
ecological relationships of fishes in Leon Creek, Texas.
Southwest. Nat. 23(3):487-496.
***** REFERENCES FOR N-OCCURRENCE NARRATIVE ONLY *****
01 Edwards, R.J., E. Marsh and C. Hubbs. 1980. The status of the
San Marcos gambusia, Gambusia georgei. Endangered Species Report
9, U.S. Fish and Wildlife Service, Region 2, Albuquerque, NM.
34 pp.
02 Hubbs, C. and A.E. Peden. 1969. Gambusia georgei sp. nov. from
the San Marcos River. Copeia 1969:357-364.
03 Longley, G. 1975. Environmental assessment, upper San Marcos
River watershed. Cont. No. AG-48-SCS-02156 for the SCS.
Environmental Sci. of San Marcos, TX. 367 pp.
04 U.S. Fish and Wildlife Service. 1985. San Marcos Recovery Plan.
U.S. Fish and Wildlife Service, Albuquerque, NM. 109 pp.
References - 1