Fishes in the lower San Pedro Mezquital River, Nayarit, Mexico

%e San Pedro Mezquital River is the seventh largest river in Mexico, and &ows through the Sierra Madre Occidental into the Marismas Nacionales Biosphere Reserve, on the coast of the state of Nayarit. %e present study is to conform a systematic checklist of 'shes in the lower basin of the San Pedro Mezquital River. In total, 52 species were collected from 24 families. Four native species were collected (Atherinella crystallina, Poecilia butleri, Poeciliopsis latidens and Poeciliopsis proli"ca) that are federally protected. Five of the collected species were new records for the state of Nayarit. %is checklist constitutes a 'rst approximation of the 'sh fauna present in the San Pedro Mezquital River. However, the construction of the Las Cruces dam upstream, will modify the basin hydrology, worsen the introduction of exotic species and create habitat loss, which can have immediate negative impacts on the 'sh communities in this region.


INTRODUCTION
Biological inventories have contributed to the knowledge of Mexican sh fauna, and have allowed scientists and natural resource managers to evaluate the ecological and biological attributes of aquatic communities in marine, estuarine and freshwater ecosystems. However, in many regions of Mexico, the ichthyofauna is not well documented. erefore, additional evaluations of the biodiversity are needed to develop e ective conservation strategies for freshwater biodiversity.
e San Pedro Mezquital basin is one of least studied freshwater ecosystem in the state of Nayarit, Mexico (González-Díaz and Soria-Barreto ). With a length of km and a surface area of , , ha, the river ows through the states of Durango, Zacatecas and Nayarit.
e San Pedro Mezquital is the seventh largest river in Mexico. It ows through the Sierra Madre Occidental and links the desert of Chihuahua to the Gulf of California, e ectively linking Nearctic and Neotropical regions. e watershed begins north of Durango City and includes the Tunal, Santiago Bayacora and Súchil and the Mezquital rivers. After the river crosses the Sierra Madre Occidental it ows into the Laguna Grande de Mexcaltitán, part of the Marismas Nacionales Biosphere Reserve in Nayarit. Near to the ocean, the morphology of the river is formed by lacustrine deltas in lakes and estuarine marshes (Tamayo ; INEGI ; WWF ; Blanco y Correa ). Historically, human populations in the region have depended on the San Pedro Mezquital River for water and food ( shes) (WWF ). However, inadequate management of the basin has led to a decline in the quality and quantity of the ecosystem services provided by the river. roughout the basin, freshwater sheries are declining due to over-exploitation and pollution from sewage (INEGI ). Other land-use activities, such as livestock grazing, agricultural development and deforestation have also negatively a ected the river. Moreover, the imminent construction of the hydroelectric dam Las Cruces upstream of San Pedro Ixcatán, planned for (SENER ), threatens freshwater communities of the river.
Although the basin has enormous cultural, economic and ecological value in western Mexico, research on the sh fauna is limited. Fish community structure

MATERIALS AND METHODS
e study was conducted in the watershed of the San Pedro Mezquital River in the state of Nayarit, Mexico.
ere were only six historical records in this basin. We established sites including these records to collect shes (Figure ; Table ). All of the samples were collected between May to May . Sampling gear was selected to re ect habitat characteristics and environmental conditions. Sampling equipment employed included cast nets ( m diameter in the upper basin has been documented in the state of Durango (Huidobro-Campos et al.
; Charre-Medellín et al. ; López-González ). ere are limited and spatially scattered records of shes in the middle and lower parts of the basin that only can be found in national sh collections (Miller ; González-Díaz and Soria-Barreto ); therefore the aim of this investigation was to describe the composition and distribution of sh species present in the lower part of the San Pedro Mezquital River.  with mm mesh), gill nets ( m long, m high and mm mesh), scoop nets and baited hooks in an attempt to document all of the species at each of the sites. Each site was georeferenced with a GPS receiver (Magellan explorist ). e shes collected were preserved in a formalin solution and were transported to the laboratory, where they were washed with running water and nally preserved in ethanol. Collections were carried out with the permits of Fishing of Development SGPA/DGVS/ / and DGOPA . . and were stored at the ichthyological collection at El Colegio de la Frontera Sur, San Cristóbal de Las Casas, Mexico (ECOSC).
All of the shes collected were identi ed to species level. is identi cation was based in the published keys and species descriptions of Hubbs ( ), Hubbs  ) were used. A systematic checklist was made with taxonomic categories above the genus level following the classi cation of Nelson ( ). Genera and species within families were arranged in alphabetical order. Nomenclature, authorities and years of description of each species were obtained from the on-line work of Eschmeyer and Fricke ( ). e ecotonic classi cation was listed according to classi cation of Castro-Aguirre et al. ( ). e protection categories were obtained from American Fisheries Society list (Jelks et al. ), the Mexican O cial Norm NOM-( ) and the Red List (IUCN ). e frequencies of occurrence were calculated, using the percentage of occurrence of each species for all sampling sites.

RESULTS
e sh fauna in the San Pedro Mezquital River, Nayarit, consisted of families, genera, and species (Table ). Centropomidae (six species), Poeciliidae ( ve species) and Gerreidae ( ve species) were the most diverse sh families in the study region (Table ). According to species descriptions (Castro-Aguirre et al. ), species were classi ed as freshwater species, while were marine species with some tolerance to freshwater. Five exotic species were collected: Grass Carp, Ctenopharyngodon idella; Common Carp, Cyprinus carpio; Yucatan Gambusia, Gambusia yucatana; and two Tilapia, Oreochromis mossambicus and O. niloticus (Table ).
Moreover, four species were collected with some level of international and national protection:  Table ).

DISCUSSION
e sh fauna present in the lower San Pedro Mezquital River in the state of Nayarit re ects the hydrological and geological history of the region and the strong links between freshwater and marine-estuarine ecosystems. For example, freshwater species such as A. crystallina, C. beani, P. butleri and P. latidens are also found in nearby watersheds of the Santiago, Ameca and Baluarte rivers, among others (Miller  ,  ). ese fauna are similar as a result of connections and isolations events between the upper part of San Pedro Mezquital River and its basins, during the Pleistocene (Domínguez-Domínguez et al.
). e sh fauna is dominated by marine-estuarine species, suggesting there is a strong in uence marine from tidal, waves, saline intrusion and storm surges in the Paci c Coastal Plain (Blanco y Correa ). Fish diversity in the lower San Pedro Mezquital River is high, and species were documented that correspond to of the entire basin. ese species live in a relatively small area compared with other nearby aquatic systems. For instance, the Agua Brava-Teacapán lagoon, a much larger and more environmentally complex system located near the river basin, has species of shes (Álvarez-Rubio et al.
). e recorded data suggest that the San Pedro Mezquital River also has greater species richness than both the Ameca River ( species; Guzmán and Lyons ) and the Santiago River (up to species; Gómez-Balandra et al. ) . Notably, the present study reported four species that had not been previously documented in the state of Nayarit (González-Díaz and Soria-Barreto ): Ctenopharyngodon idella, Gambusia yucatana, Oreochromis niloticus and Aboma etheostoma.
Additionally, four of the native species we collected are classi ed as protected. It is necessary to do more research to provide biological and ecological information of these species, and to determine their real conservation status and the possible threats they face in the basin.
e San Pedro Mezquital River is under anthropogenic activities such as waste water discharges (INEGI ), presence of exotic species and over shing. In addition the construction of the hydroelectric Las Cruces dam, will strongly a ect the aquatic habitats and sh communities. Dams alters the ow regime, temperature and nutrients in rivers, and also act as a barriers for species dispersal, resulting in fragmentation of habitat, migration interruption and changes of habitat and structure of aquatic communities (Marmulla , Guzmán et al.
). Damming limits and reduces the distribution of shes and diminishes population of native and endemic species (Terra et al.

ACKNOWLEDGEMENTS
is work was nanced by Gonzalo Río Arronte Foundation, IAP and WWF. We thank Abraham Aragón, Antonio Guerra, Pedro Rivas, Sarahy Reyes, Erika Hernández, Jorge García and Yesicca Barreto for their assistance in the eld. anks to Dario Alejandro Navarrete Gutiérrez for the preparation of the map. Also thanks to community shermen of Campo Los Limones, Mexcaltitán, Tuxpan and Ruíz. Unidad Académica de Agricultura of Universidad Autónoma de Nayarit (UAN) o ered us the infrastructure to develop the work. anks to Allison Pease, Krista Capps, Rebecca Quinones and anonymous reviewers for their valuable comments that improved the manuscript.