Introduction

Mollusks are one of the most important food resources in Mexico, representing approximately 11.13 % of fisheries and 23.24 % of aquaculture production. The Mexican Pacific contributes 8.35 % of the catch and 2.42 % of the culture of these organisms (^{Ríos-Jara et al., 2008}). On Guerrero state offshore, mollusks are an essential resource in coastal fisheries and are highly valued, generate important income and contribute to the food security of hundreds of people (^{Castro-Mondragon et al., 2016}). In Guerrero there are 97 coastal fishing Cooperative Fisheries Production Societies (CFPS) registered. In the Costa Grande 47 Cooperative Societies land their products in 19 arrival sites, where there are species with high commercial value such as red snapper, snapper, shark, lobster, octopus, oyster, snails and oysters among others. In the region, the municipality of “La Unión” has a greater number of sites to land their fishery products, the area has a narrow continental shelf that widens due to the transport of sediments in the “Balsas” River delta, in this area most of the cooperatives are dedicated mainly to the capture of scale fish, and mollusks in a smaller percentage (^{Gutiérrez & Cabrera, 2012}).

The capture of marine species in the state is regulated through the registration of fishing permits with a catch regime for scale fish, rock oyster, snail, sea roach, lobster or shark; the largest number of fishing permits is concentrated in the cooperatives located in the municipalities of “La Unión” and “Acapulco” with 64 % of the CFPS (^{Morales-Pacheco et al., 2010}). From the total number of cooperatives registered for the state (97), approximately 30 of them have a valid permit for catching oysters, conch or sea roach. In the “Costa Grande” 10 CFPS have permits for catching some type of mollusk (^{CONAPESCA, 2016}).

In the state of Guerrero, coastal fishing lacks special infrastructure; panga-type boats are used, which can beach and/or land their products on a wide variety of beaches. The main methods used in coastal mollusk fishing are free diving and, much less frequently, semi-autonomous diving with the use of a compressor. Few places along the coast have collection centers for fish products that facilitate both the activity and commercialization. (^{Castro-Mondragón et al., 2015}; ^{Galeana-Rebolledo et al., 2018}).

In the southern Mexican Pacific, there is little information on the species of mollusks that are subject to coastal fishing and the current status of their populations is unknown, some of the existing studies are those carried out for the coasts of Chiapas and Oaxaca (^{Ríos-Jara et al., 2008}), and Michoacán offshore (^{Gorrostieta-Hurtado y Trujillo-Toledo, 2012}). Particularly in these areas, mollusks are caught in an artisanal manner for self-consumption, and are only marketed locally (^{Ríos-Jara et al., 2008}).

In Guerrero state, although there are reports on exploited fishery resources, the data and information provided on commercially fished mollusk species is limited and most of the studies that address this issue have been carried out in the Acapulco and Costa Chica regions (^{Galeana-Rebolledo et al., 2007}; ^{Villegas-Maldonado et al., 2007}; ^{Flores-Garza et al., 2012}; ^{García-Ibáñez et al., 2013}; ^{Olea-de la Cruz et al., 2013}; ^{Torreblanca-Ramírez et al., 2014}; ^{Castro-Mondragon et al., 2015}; ^{Castro-Mondragon et al., 2016}; ^{Galeana-Rebolledo et al., 2018}).

In the Costa Grande region, information on mollusks caught by coastal fishing is scarce, including the work done by ^{Baqueiro & Stuardo (1976)}, who made observations on the biology, ecology and exploitation of three species of clams, Megapitaria aurantiaca (Sowerby, 1831), M. squalida (Sowerby, 1835) and Dosinia ponderosa (Gray, 1838), thus recognizing the high commercial value of these three species for the Zihuatanejo Bay and Ixtapa Guerrero Island fisheries. Moreover, based on an analysis of the gonad cycle and minimum catch sizes, they propose recommendations for a controlled exploitation, with alternating closures within the exploitation banks, in addition to the publication of ^{Gutiérrez & Cabrera (2012)}, who described some of the species of mollusks that are captured for commercialization and some additional information reported by fishermen in the region.

Due to the lack of information regarding the current state of the mollusk fishery in the Costa Grande, this research aimed to determine the richness of mollusk species caught by coastal fishing, the most important species caught, and which species generate the greatest economic income in the region.

Material and Methods

Study Area

The Costa Grande region is located in the southwestern part of the Mexican state of Guerrero, between (16°53’56” to 17°54’00” N and 99°58’26” to 102°09’50” W). It is composed of seven municipalities of which only six (Coyuca de Benítez, Benito Juárez, Tecpan de Galeana, Petatlán, Teniente José Azueta and La Unión de Isidoro Montes de Oca) border the Pacific Ocean (Figure 1). The Costa Grande has approximately 260 kilometers of coastline, where fishing activity takes place.

The coastal zone of the state of Guerrero has a tropical sub-humid Aw climate. The annual temperature variation is less than or equal to 5 °C, with an annual average of 27.5 °C. In the coastal zone, tidal conditions show a mixed type pattern, with two high and two low tides in a 24-hour period; these characteristics allow fishing activity to be carried out during most of the year (^{García, 1973}).

This research was carried out in four of the coastal municipalities of the Costa Grande region; Tecpan de Galeana, Petatlán, José Azueta and La Unión de Isidoro Montes de Oca, where nine cooperatives dedicated to mollusk fishing are located (Figure 1, Table 1).

Figure 1 Geographical location of the Costa Grande de Guerrero region and the Cooperative Fisheries Production Societies (CFPS). 

Table 1 General data of the Fisheries Production Cooperative Societies analyzed in the Costa Grande region of Guerrero state. 

Cooperative	Location	Municipality	N.S.	Permission record	Fishing regime
Costa Grande de Guerrero	Puerto Vicente Guerrero	Tecpan de Galeana	10	11201356001	Ostión de roca
Buzos y Pescadores La Barrita	Playa La Barrita	Tecpan de Galeana	12	-------	--------
Cerro de Guamilulle	Barra de Potosí	Petatlán	15	11202156004	Ostión de roca
Vicente Guerrero	Zihuatanejo	José Azueta	27	11202156014	Ostión de roca
Buen Rumbo	Zihuatanejo	José Azueta	36	11202156027	Ostión de roca
Isla de Ixtapa	Zihuatanejo	José Azueta	20	11202156020	Ostión de roca
Los Pulpos	Zihuatanejo	José Azueta	20	11202156017	Ostión de roca
Morros de Ixtapa	Las Salinas	José Azueta	19	11202156019	Ostión de roca
Rivereña Lagunillas	La Majahua	La Unión	24	11202156009	Ostión de roca

N.S. = number of members that belong to each fishing cooperative.

Results and Discussion

The present analysis included 1,184 specimens of mollusks caught by artisanal fisheries and identified 17 families (10 bivalves, 6 gastropods and one polyplacophore) and 24 species (13 bivalves, 10 gastropods and one polyplacophore) (Table 2). For the states of Chiapas and Oaxaca, ^{Ríos-Jara et al. (2008)} have reported 47 species considered to be of commercial interest or with potential use; among these species, 40 are reported for human consumption (31 bivalves and 9 gastropods), and the remaining species are for artisanal use. The difference in the number of species determined by this work with respect to that reported by ^{Ríos-Jara et al. (2008)} is related to the size of the area studied, given that Oaxaca and Chiapas together have a coastline of approximately 788 km and their sampling was carried out in 9 localities located within the littoral zone and 55 stations on the continental shelf of both states. In contrast, the coastal zone of the Costa Grande in Guerrero, which was the area studied, has an approximate extension of 260 km (^{Gutiérrez & Cabrera, 2012}), and only those species that were caught by the fishing production cooperatives dedicated to mollusk fishing were inventoried. Of the species that were inventoried, we coincided in five with those reported by the aforementioned author, which are: S. prismatica, P. multicostata, Spondylus limbatus, M. ringens and H. radix.

For the Costa Chica region of the state of Guerrero, ^{Galeana-Rebolledo et al. (2018)}, reported 27 species of mollusks that are caught by riverine fishing, notwithstanding that the number of species reported for the Costa Chica and Costa Grande regions is similar, only 18 species that are caught are similar between both regions. The species that are caught in Costa Chica and not recorded in Costa Grande are: Lobatus peruvianus, Triplofusus princeps, Chama equinata, Chama mexicana, Megapitaria aurantica, Donax puntactostratus, D.caelatus, D. kindermani and Chaetopeura lurida (Table 3).

In the region of Acapulco, Guerrero, ^{Castro-Mondragon et al. (2016)} reported 48 species that are caught by coastal fishing. Twice as many species are caught in this region as in Costa Grande. The capture of a greater number of species, is directly related to the demand for mollusks that comes from the City and Port of Acapulco de Juarez, given that it is a tourist and trade center of global importance and experiences a high demand for seafood for human consumption, which has led to an increase in fishing effort to meet market demand. The problem is that the mechanisms for regulating the fishing effort are scarce, which causes a depletion of commercially important populations, to the extent that some have ceased to be profitable and use has been made of other species that were not originally part of the coastal fishery to try to satisfy market demand and the personal income of the fishermen (^{Castro-Mondragon et al., 2015}). This situation is not noticed in the Costa Grande region.

In the state of Guerrero, 66 species of marine mollusks have been reported, caught by coastal fishing (^{Flores-Garza et al., 2012}; ^{Gutiérrez & Cabrera et al., 2012}; ^{Torreblanca-Ramírez et al., 2014}; ^{Castro-Mondragon et al., 2016}; ^{Galeana-Rebolledo et al., 2018}), of the species reported by this research, only T. trochiformis was recognized as a new record in the state of Guerrero, thus complementing the inventory of marine mollusk species caught by coastal fisheries to 67 species (Table 3).

The most important genus in the Costa Grande is Hexaplex, since three species of this genus were reported. For the states of Chiapas and Oaxaca, authors reported that the genus Anadara is the most important in the marine mollusk fishery (^{Ríos-Jara et al., 2008}), since eight species of this genus are caught. For the Costa Chica and Acapulco, the genera Hexaplex and Chama have been reported with the highest number of species within commercial catches in the state (^{Flores-Garza et al., 2012}; ^{Gutiérrez & Cabrera et al., 2012}; ^{Castro-Mondragon et al., 2016}; ^{Galeana-Rebolledo et al., 2018}). This research also recorded the genera Hexaplex (three species) and Chama (two species) as the most representative.

The species Striostrea prismatica and Hexaplex princeps were recorded as the main target of fishing by CFPS in the Costa Grande region. This trend was also reported for the Acapulco and Costa Chica regions (^{Castro-Mondragon et al., 2016}; ^{Galeana-Rebolledo et al., 2018}), these species are distributed throughout the coastal zone of the state of Guerrero in depths ranging from 3 to 15 m, due to the fact that the main methods used in the coastal fishing of mollusks in the state are apnea diving and much less frequently semi-autonomous diving with the use of a compressor facilitates fishermen access to these species, in addition, the populations are relatively abundant and in the local market the demand is high and its commercial value is profitable, this represents good economic income for the fisherman and their families.

The species Chiton articulatus and Plicopurpura columellaris in the Costa Grande, were recorded as occasional fishing species and are mainly used for self-consumption by the fisherman and his family, however, it has been reported that C. articulatus in the other coastal regions of the state, is collected in large volumes and generates significant economic income (^{Galeana-Rebolledo et al., 2007}; ^{Olea- De la Cruz et al., 2013}). During fieldwork, evidence was found of fishermen from the Acapulco region catching C. articulatus in Costa Grande, for transport and sale in Acapulco.

Regarding the species P. columellaris, it is protected by the Official Mexican Standard ^{NOM-059-SEMARNAT-2010}, in Acapulco this species is reported in commercial catches and its meat is used for human consumption (^{Castro-Mondragon, et al., 2016}). Table 2 shows the species of mollusks that are commercialized in different fishing cooperatives in Costa Grande.

The average number of days per week worked by each fisher is 5.4 days, which represents 259.2 days worked per year, this result is similar to what has been reported for the Acapulco and Costa Chica regions (^{Castro-Mondragon et al., 2016}; ^{Galeana-Rebolledo et al., 2018}) and for the state of Guerrero by ^{Gutiérrez & Cabrera (2012)}, who describe that fishing activity is carried out during most of the year, except for the months of greatest tropical disturbance, which are the months of rain and hurricanes, where fishing is practically suspended for safety reasons. According to the authors, in the first five months of the year the best catches can be obtained and in the following months these gradually decrease because of “bad weather”, but recover in the months of November and December.

Ten species were found to have the highest volumes of catch and these generate the highest economic income for fishermen (Table 4, Figures 2 and 3). This result is lower than that reported in Acapulco, (^{Castro-Mondragon et al., 2016}), who recorded 16 species of marine mollusks, with the highest catch volumes by artisanal fisheries, four of these 16 species, were also reported in this research with the highest catch volumes: S. prismatica, H. princeps, N. muricata and H. hyotis. Eight species have a higher catch volume in Acapulco waters: Opeatostoma pseudodon (Burrow, 1815), Leucozonia cerata (Wood, 1828), Crucibullum umbrella (Deshayes, 1830) Fissurella nigrocincta (Carpenter, 1856), Polygona tumens (Carpenter, 1856), Vasula speciosa (Valenciennes, 1832), Fissurella gemmata (Menke, 1847) and Chiton articulatus (Sowerby in Broderip & Sowerby, 1832). In the case of the Costa Grande, fishermen do not focus their fishing effort on these species and some do not even catch them, because these species are not attractive for commercialization and do not generate economic income for the fishermen of the region.

a-a1) Chama buddiana; b-b1) Hyotissa hyotis; c-c1) Spondylus limbatus; d-d1) Pinna rugosa; e-e1) Striostrea prismatica.

Figure 2 Species of the Bivalvia Class that obtained the highest capture volumes and generated the highest economic income in Costa Grande, Guerrero. 

a-a1) Hexaplex princeps; b-b1) Hexaplex radix; c-c1) Hexaplex regius; d-d1) Neorapana muricata; e-e1) Trochita trochiformis.

Figure 3 Species of the Gastropoda Class that obtained the highest capture volumes and generate the highest economic income in Costa Grande, Guerrero. 

The observed difference in the type and number of commercially exploited species, between the two nearby regions, can be explained, as reported by ^{Castro-Mondragon et al. (2016)}, by the fact that in Acapulco, there is a high demand for mollusks, mainly due to tourism and local consumption, to meet the demand, fishing effort is increased and species are diversified, in addition, other factors also influence, such as the lack of regulation and monitoring of fishing activity.

On the other hand, for the Costa Chica region (^{Galeana-Rebolledo et al., 2018}), reported that five species are the most important in terms of catch volume: S. prismatica, H. princeps, H. radix, H. regius and L. galeatus. In the present research it was found that the first four species also contribute the highest catch volumes in Costa Grande. In the case of L. galeatus, Costa Grande fishermen, specifically those belonging to the municipality of La Unión reported that this species is bycatch, it is caught by scale fishing nets and its sale is through intermediaries that transport them to the municipality of Zihuatanejo. According to reports from interviewed CFPS members in Zihuatanejo, abundances of the species are low, because they were previously caught in large volumes to be transported to Acapulco for sale.

The survey with CFPS members showed evidence that at least five species have drastically reduced their populations: L. galeatus, M. ringens, P. multicostata, M. squalida and P. praestantior. The fishermen report that, due to the high price that these species command in the market, the fishing effort on these species has increased, which, added to the poor supervision by the authorities, led to a reduction in their populations and it is increasingly difficult to catch organisms of these species. The fishermen interviewed also reported that the clam populations of A. formosa and G. panamensis have decreased to such an extent that it is difficult to find them, so they have stopped directing their fishing effort towards these species.

Regarding the overexploitation and the changes in abundance that the populations of some mollusks have suffered, in the Costa Grande ^{Baqueiro & Stuardo (1976)}, reported three species of clams D. ponderosa, M. aurantiaca and M. squalida as target species for fisheries in Zihuatanejo and Isla Ixtapa regions, which generated great economic income for fishermen almost 40 years ago. However, according to the results of the surveys applied to the fishermen, they mentioned that the first two species have completely disappeared from the catches, and in the case of the last species, M. squalida, the results showed a decrease in its populations, to such a degree that fishermen no longer focus their efforts on catching it.

From the total number of target species (10) in the Costa Grande, only two have a fishery management plan. S. prismatica has a period in which its capture is illegal from June 1 to August 31, established by Mexican law (^{CONAPESCA, 2015}) and T. trochiformis has a period of fishing prohibition from June 1 to September 30, this prohibition was not enacted by the Mexican government, but was voluntarily adopted by fishermen with the purpose of protecting the species and making a rational use of this resource, since it occupies the second place in commercial importance in the region, this shows the importance of the CFPS in the process of catching and commercializing the mollusk resource and the fundamental role that these cooperatives could play in order to carry out fishing activities in a more sustainable manner in the Costa Grande region and in other areas such as Acapulco and Costa Chica where there is overexploitation.

Conclusion

This research concludes the first inventory of commercially important mollusk species that are caught by coastal fishing in the coasts of the state of Guerrero.

It was found that two genera of mollusks, Chama and Hexaplex, are the ones that contribute the most species to commercial catches in the state with five and three species, respectively.

In the Costa Grande, T. trochiformis represents the second largest catch and is only fished in this region, displacing Hexaplex to third place.

The species C. articulatus and P. columellaris, were recorded within the commercial catch in the Costa Grande region, these species do not generate income for the cooperatives, however, they are an essential part of the food for fishermen and their families.

This research found evidence of overexploitation of some species of mollusks such as Scutellastra mexicana (Broderip & GB Sowerby I, 1829), Megapitaria aurantiaca and Dosinia ponderosa, the fishing pressure suffered by the populations of these species was so high that they disappeared from the commercial catches in the region, of these species only S. mexicana has some kind of regulation, within the Official Mexican Standard (^{NOM-059-SEMARNAT-2010}), it is registered as an endangered species.

With respect to the above mentioned and according to the results of this research, the lack of regulation of the mollusk fishing activity and the disappearance of some species within the catches in the Costa Grande region of the state of Guerrero, for this reason it is proposed to carry out ecological-reproductive studies mainly of the species of commercial importance that are being subjected to greater fishing pressure, with the purpose of being able to determine aspects such as; reproductive cycles of the species, sizes of reproduction initiation and reproduction size, minimum catch sizes and fishing quotas, in order to regulate the fishing activity and allow local fishermen to also take actions with good practices oriented towards a responsible use of the resource.