FAUNA SURVEY OF THE DECAPOD CRUSTACEANS, REPTILES AND COASTAL BIRDS OF THE ESTERO EL VERDE, SINALOA, MEXICO, WITH SOME NOTES ON THEIR BIOLOGY

Trabajo recibido el 25 de agosto de 1982 y aceptado para su publicación el 20 de enero de 1983.

RESUMEN

INTRODUCCIÓN

For their commercial importance, the coastal lagoons of the Pacific coast of Mexico have been the subject of intensive studies which were all basically orientated towards the fishery and fisheries biology of penaeid shrimp (Edwards, 1978; Blake et al., 1981). Indeed, together with the shrimp catch on the continental platform of western Mexico, the lagoon shrimp fishery actually represents a major source of revenues for Mexico.

In many cases though, and as a direct consequence of this prime necessity for shrimp study, the rest of the fauna has been neglected and global faunistic studies of western Mexico lagoons are almost inexistent if one excepts the intensive studies realized on the fish fauna (Warburton, 1978; Yáñez, 1978; Carranza, 1970; Amezcua, 1972; Chan-González, 1980).

For their reduced size, some of the coastal lagoons of Sinaloa evidently have a very limited economical importance. Nevertheless, because of the variety of ecological niches they offer, some of them play an interesting role as habitat for a wide variety of species.

This is the case of the Estero EI Verde, a small coastal lagoon located approximately 30 Km North of Mazatlan, Sinaloa, Mexico, where a survey of the fauna and flora was initiated in 1978.

The present paper gives the results originated from an intensive study of the fauna of decapod crustaceans and marine birds inhabiting the estero as well as basic information dealing with the presence of one species of crocodile and one species of sea turtle that uses the beach of the system as nesting area.

The Estero EI Verde is located in the state of Sinaloa, on the Pacific coast of Mexico (23°25'30"N and 106°33'30"W) and is therefore included in the Gulf of California geographic area (the Cortés zoogeographic province of Briggs, 1974). It has the typical morphology of a coastal lagoon as described by Phleger (1969). It is composed of one small inlet and two main channels. running in a parallel direction with the coast line (see figure) and is almost entirely bordered by mangroves (Laguncularia racemosa, Rhizophora mangle). In its middle part, the Rio Quelite constitutes the river channel of the system.

The system is connected to the sea by means of a single inlet which is closed by a sand bar for most of the year. During the rainy season (june to October) the combined effects of the river flow and of the tides destroy the sand bar and keep the inlet open for as long as up to four months.

Although it is approximately 7 Km long, the total area is only of about 2 Km² which makes of the Estero EI Verde one of the smallest coastal lagoons of Sinaloa.

The system includes a wide variety of habitats including: (1) sandy beach, unprotected, extending seawards all along the estero; (2) dunes; (3) mud banks and flats, sometimes partly shaded by mangrove; (4) muddy sand banks; (5) channel and inlet delta waters, shallow to 4 meters depth; (6) mangrove trees; (7) mangrove aerial roots.

Fig. 1. Map of the Estero El Verde, Sinaloa, México.

Since 1978, numerous field trips were made by the authors to the study area with a view of collecting, observing and registering the local fauna. All field trips were made by road to the estero and small motorboats were used to go over the channels of the system. Invertebrates were usually collected by hand but a small bottom dredge was occasionally used in the channels.

Identifications of invertebrates were realized in the laboratory using fixed material while identifications of vertebrates were performed in the field or were based on color prints and slides. Crabs and shrimps were identified using the monographs of Rathbun (1930), Holthuis (1952) and Crane (1975); additional literature included more recent works by Brusca (1980) and Abele (1981). The identification of birds was done using the field guide to western birds of Peterson (1961) and reptiles were recognized with the help of the monograph of Hardy and Mc Diardim (1969).

This is the only group of invertebrates of the area that has been subject to a complete investigation. A total of 30 species have been found to occur in the Estero EI Verde (Table 1) including 3 species of Caridea, 3 species of Penaeidea, 2 species of Macrura, 2 species of Anomura and 19 species of Brachyura; 4 have recently been found for the first time in the Gulf of California area: Sesarma rhizophorae, S. magdalenense, Uca zacae (Hendrickx, 1979; Hendrickx and van der Heiden, in press; Paul et al., 1981) and Pinnixa valerii (unpublished data).

TABLE 1 SPECIES OF DECAPOD CRUSTACEANS INHABITING THE ESTERO EL VERDE

Although most of the other species of decapod crustaceans are commonly found in other coastal lagoons or estuaries of Sonora and Sinaloa, a faunistic assemblage such as the one of Estero EI Verde has so far never been reported.

The great majority of the species of Brachyura (13 out of 19) occurring in the Estero EI Verde are either terrestrial or semi terrestrial. Most of them live among mangrove roots (Grapsidae), on mud bank and flats (genus Uca) or on sandy beach (Ocypode occidentalis) where they make, their burrows. The land crabs of the family Gecarcinidae are the better adapted to the terrestrial environment and are found among bushes in the dunes and among mangrove trees. All these habitats are similar to those previously reported for these species (Crane, 1947; Bright and Hogue, 1972; Brusca, 1980).

The Pacific coast of western tropical Mexico is an important region for the winter visiting birds proceeding from United States and Canada. The Estero EI Verde serves as an important shelter for these birds which are undoubtedly an important component of the ecosystem.

A total of 38 species of marine and shore birds have been found to inhabit the lagoon (see Table 2) of which as many as 20 species are migrating birds observed only during winter time in Mexico (Peterson, 1961). The remaining 18 species are present all year round.

Among the most spectacular resident species are the roseate spoonbil (Ajaia ajaja) the white ibis (Eudocimus albus) and the 6 species of Ardeidae which can be observed on the edge of the channels and above mangrove trees where they build their nests.

The migrating species (marked M in Table 2) comprise of five species of ducks of the genus Anas, the white pelican (Pelecanus eryth rorhynchus), three species of plover, two species of gull, two species of tern, the American avocet (Recurvirostra americana), the cared grebe (Podiceps nigricollis) and four species of Scolopacidae.

All these species have been included in Table 2) in taxonomic order. In addition to the American common name in use, the Mexican most popular names have been indicated when available as well as some collective names used in Mexico for groups of species (e.g. "garzas" for Ardeidae).

As in the case of the decapod crustaceans, the major feature of the marine bird fauna consists in the high species diversity that can be observed in such a reduced area; diversity which provides unique condition for the study of each individual species and of its relationships with the surrounding system.

If we consider the littoral fringe of the Estero EI Verde, i.e. the sandy beach, we will come to the conclusion that one of the most interesting aspects of the system consists in the fact that it represents an important nesting area for the ridley turtle Lepidochelys olivacea, an endangered species of marine turtle which was once plentiful along the coast of Sinaloa (Briseño-Dueñas, 1980).

Due to the uncontrolled slaughter that occurred in the sixties, the population of sea turtle has declined dramatically along the Pacific coast of Mexico. In Sinaloa only, the most recent data (1979-80) indicate a total yearly capture of less than 2,500 animals while it used to be of about 300,000 units in 1966 (BriseñoDueñas, op. cit.).

The nesting area of the Estero EI Verde actually represents the most important one for the entire state of Sinaloa, reason for which, since 1975, a protection program has been established by the Fishery Department of Mexico in Mazatlan with the collaboration of the Instituto de Ciencias del Mar y Limnología, Estación Mazatlan, UNAM.

The main target of this program is to protect turtles coming to lay eggs and these eggs from predation. Major predators are (in order of importance) man, dogs and the ghost crab (Ocypode occidentalis).

TABLE 2 LIST OF MARINE AND SHORE BIRDS (INCLUDING MIGRATING SPECIES MARKED M) INHABITING THE STERO EL VERDE

At the end of the 1979 nesting season (July to December), a total of 136,000 young turtles had been safely relased into the sea and estimation showed the reproductive female population to account for between 600 and 800 individuals of wich 459 had been marked.

The precense of crocodile in the state of Sinaloa has been reported on several ocasions in the past (Boulenger, 1889; Cope, 1900; Zweifel, 1959; Scott, 1962). It used to be quite numerous in several coastal lagoons (Caimanero lagoon), esteros (Estero de Teacapán; Estero den Urías) and estuaries (Rio Presidio) of the area. Today, however, most of these populations have definitely disappeared, victims of the fear of local fishermen and of uncontrolled hunting.

Although the especies (Crocodylus acutus) is actually protected by law, there is virtually no way for the population to recover and the newly born specimens are usually killed when they reach over 60 cm long.

A reliable estimation of the crocodile population of the Estero El Verde does not exist; two young specimens of 37 and 62 cm long were captured once a released after identification. Some larger specimens have been observed occasionally, including one of over 2 meters long and another of about 3 meters long. There is, so far, no reliable report of aggression to man in Sinaloa and crocodiles feed basically on fishes and crabs and occasionally on small mammals and birds.

Because they represent such an important part of the coastal ecosystem, the lagoons of Sinaloa deserve a much more careful attention than the one they are presently receiving and comprehensive studies should be encouraged.

The lagoons fisheries on the Pacific coast of Mexico are presently controlled by the Fishery Department through a complex system of cooperatives. The annual catch of shrimp, in the Estero EI Verde has so long been insignificant if compared to the harvest in other coastal lagoons of southern Sinaloa (i.e. less than 500 Kg/year vs. a peak value of more than 5,000 Kg/day in Huizache-Caimanero; see Edwards, 1978).

One can not dispel the feeling that the Estero EI Verde could perhaps find a better accomplishment through a protected-area statute that would leave it accesible to non-destructive research. Indeed, there might be there a unique opportunity to study simultaneously and in a relatively small area, the complex interactions that occur among a series of interconnecting habitats and a highly diversified fauna.

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