Variation of the Fish Community Associated with Soft Bottoms in a Coastal Lagoon on the Pacific Side of B.C.S, México

The San Ignacio lagoon is located inside the "Biosphere reserve El Vizcaíno". The study of fish within coastal lagoon systems allows a better understanding of how these organisms are responsible for performing various activities that are necessary for the stability of the ecosystem. There is very general and outdated research of the ichthyofauna in the San Ignacio lagoon. In the present study, the structure of the fish community associated with soft bottoms and the ecological role of the most dominant species was analyzed. Six bimonthly samplings were carried out in 11 localities from April 2013 to April 2014 resulting in 66 replicates, each time an experimental trawl net was used to capture the organisms and physicochemical parameters were recorded. 2,887 organisms belonging to 26 families, 38 genera and 46 species were captured. There were significant differences in seasonal richness but not spatial richness, and there were no significant differences in diversity or evenness. According to the BVI, 12 species were the most biologically important.


Materials and Methods
The San Ignacio lagoon is located on the west coast of the Baja California peninsula, Mé xico, in the city of Mulegé , in the state of Baja California Sur. Geographically is located between parallels 26°43' and 26°58' N and between meridians 113°08' and 113°16' W. Sampling localities were specifically chosen to cover most of the lagoon, from entry to end ( Figure 1; Table 1).  The San Ignacio lagoon has a coastline with sandy beaches, muddy lowlands, mangroves, marshes and a few rocky areas. It is a shallow water body with average depths from 2 to 4 meters and a depth of 20 meters in the channel zone that communicates with the ocean (Swartz and Cummings, 1978). The substrate of this lagoon is variable (Barjau-Gonzá lez, 2003) (Table 1). Geographically it is in the Californian province (Robertson and Cramer, 2009).
Six bi-monthly samplings were carried out in 11 localities from April 2013 to April 2014. An experimental trawl net with a length of 9 m, a vertical opening of 4.5 m, a mesh size of 1.5 inches, and metal doors of 95x50 cm, was used to catch the fish. The trawl speed was 3.5 km/h, sweeps were carried out with a duration of 20 minutes at an average depth of 5 m at each locality. Sweeps in which no capture was recorded were considered as water sweeps. A 22 feet boat with a 90 HP four stroke outboard motor was used as a trawler. Ecological indexes used in this study were relative abundance, Margalef's species richness, Shannon-Weiner diversity (log 2), Fisher's alpha diversity, Pielou's evenness and Biological Value Index (BVI). Fish species were classified in four groups based on abundance and catch frequency: abundant, frequent, common and rare. At each locality, the physicochemical variables were recorded using YSI 2030 Pro multiparameter instrument to determine the salinity and bottom temperature. Species identifications were carried out using specialized literature (Miller and Lea, 1972;Thomson et al., 2000). Ecological indexes were analyzed using the software Primer-E & Permanova 6 and statistical analysis were performed using Statistica v.7.
Frequent species: Nine species were classified as frequent because their relative abundance ranged from 0.10% to 0.96%. The species with greater abundance and frequency within this group were E. dowii, E. asper and B. polylepis.
Common species: Eight species were included in this group since their relative abundance ranged from 0.09% to 0.02%. Within this group the species with greater abundance and frequency were Haemulon flaviguttatum, Halichoeres semicinctus and Cynoscion xanthulus.
Rare species: This group comprised 15 species with a relative abundance value less than 0.01%. Within this group the species with greater abundance and frequency were Ancylopsetta dendritica, Atherinops affinis and Cynoscion phoxocephalus.

Physicochemical variables
According to physicochemical variables, studies in nearby lagoons such as Bahia Magdalena (south of San Ignacio) (Gutié rrez-Sá nchez, 1997), Laguna Ojo de Liebre (north of San Ignacio) (Acevedo-Cervantes, 1997) and Barjau et al. (2015), found similar results to this study. Based on the temperatures recorded in our study, there are two climatic seasons, warm (June to October) and cold (December to April). Temperature records in San Ignacio show that minimums are in the mouth of the lagoon while maximums are in the end, and are similar to previous studies from this lagoon and adjacent lagoons. We infer a similar pattern because the deepest area of the lagoon is in the entrance while the shallow areas are in the end, suggesting warm-up of the water, coupled with a replacement of the water that takes around three to five months (Ocean. Leonardo Álvarez Santamarí a, Personal Conv. UABCS) ( Table 2; Table 3).
High values of salinity recorded in this study with a maximum of 38‰ in June 2013 and a minimum of 31.1‰ in April 2014, are similar to values recorded by Núñez-López (1996), with a minimum salinity of 32‰ in winter and a maximum of 37‰ in autumn during 1992-1993. On the other hand, Barjau-Gonzá lez (2003) recorded higher values, a maximum of 42‰ in summer of 1998, and a minimum of 36‰ in winter of 1999, with oceanographic events such as El Niño in 1998 and La Niña in 1999, which may have caused these differences.

Ecological indexes
The species registered in this study have been previously recorded in this lagoon (Barjau-Gonzá lez, 2003) and in lagoons near San Ignacio (Acevedo-Cervantes, 1997; Gutié rrez-Sá nchez, 1997). From the 44-species recorded by Barjau-Gonzá lez (2003), 18 were also recorded in this study. Gutié rrez-Sá nchez (1997) recorded 75 species of which 19 species were also recorded in this study. Acevedo-Cervantes (1997) recorded a total of 59 species, 15 species of these were also captured during our samplings. It is important to mention that these studies do not show the same list of species nor the same methodology, which highlights the selectivity of the fishing gear used, sampling effort, characteristics of the area, mobility of the species and habitat diversity (Gutié rrez-Sá nchez, 1997; Barjau-Gonzá lez, 2003).
Main factors that affect abundance and distribution of the species are the physical, chemical and biological properties of their habitat, as well as the capacity of each species to tolerate these conditions (Lagler et al., 1984;Torres-Orozco and Castro-Aguirre, 1992;Barjau-Gonzá lez, 2003;Barjau et al., 2015).
In the present study we recorded a higher abundance than the one recorded by Barjau-Gonzá lez (2003) with an abundance of 1,361 organisms. A difference between this studies was the number of samplings, in this study six samplings were carried out, while in 2003 four were carried out.
Regarding seasonal abundance, the greater abundances were recorded in April, August and December, and the lower abundances were recorded in June and October. These results differ from those found by Barjau-Gonzá lez (2003), with higher abundances recorded during the fall season, and lower values recorded during spring. This is probably due to the events El Niño in 1998 and La Niña in 1999.
Regarding spatial abundance, all the localities presented a similar spatial variation, except for locality 6, in which only 2,294 organisms were captured, of which 1,964 were captured in a single sweep. One factor that could have caused this was that locality of El Remate is the furthest from the entrance of the lagoon, with muddy sediment because is a discharge area during rainy seasons, also is one of the shallowest localities and is influenced by low tides.
In contrast to this study, Barjau-Gonzá lez (2003)  icistia. When comparing these studies, there are similar species that exist within these lagoons, and some of these species were found in the lagoon throughout the whole year, which suggests they are adapted to this type of environment, therefore they are considered resident species with euryhaline and eurythermal adaptations. Physiological and biological processes such as reproduction, breeding, feeding and shelter are carried out in these lagoons by these species (Barjau-Gonzá lez, 2003;Barjau-Gonzá lez et al., 2015).
In this study, significant differences in the monthly values of specific richness were found, higher values were recorded in the warmer months (June-August-October) and lower values were recorded in the temperate months (December-April). This may be because in the warmer months there are more species with tropical affinity (Barjau-Gonzá lez, 2003;Kosegarten-Villarreal et al., 2016). These results were partially similar to what Gutié rrez-Sá nchez (1997) recorded in Magdalena-Almejas Bay, higher values in the colder months and lower values in the warmer months. This may be due to ecosystem differences and that this area is in a subtropical zone, considered an ecotone between California (cold) and sub-equatorial (warm) currents.
The α-Fisher diversity analysis of this study revealed no seasonal or spatial differences. These results are similar to what was found by Barjau-Gonzá lez et al. (2014), who carried out the same analysis in this lagoon. In both studies, it was expected that diversity would be different, because variations of temperature and salinity throughout the year and difference of substrates and depths along the lagoon have been described as factors of great relevance in the role of the diversity of ecosystems (Allen and Horn, 1975;Amezcua-Linares, 1977;Quinn, 1980;Amezcua-Linares, 1996;Manjarrez-Acosta, 2001). The Shannon-Wiener diversity analysis, as with the α-Fisher diversity, revealed no significant differences between seasons and localities. Diversity values recorded in the present study had a temporal variation of 0.3 to 1.9 bits/ind. and a spatial variation of 0.1 to 2.3 bits/ind.
Based on results of the evenness analysis, there was not a seasonal and spatial distribution of evenness, probably because of the dominance of some species. Barjau-Gonzá lez (2003)  In these studies, the species that was always present is P. maculatofasciatus. The species previously mentioned present a spatio-temporal constancy within the lagoon systems, which directly influences their dominance (Loya-Salinas and Escofet, 1990). These dominant species present certain biological strategies that allow them to dominate over others, such as their reproductive capacity, eating habits (opportunistic, generalists), tolerance to changes in environmental factors, etc., which allows them to carry out processes such as feeding, breeding, and the use of the area as protection (Acevedo-Cervantes, 1997). Most dominant species show similar feeding habits, suggesting a proximity to the benthos and that they belong to the first levels of the food web. These feeding habits have been noted as of high importance because fish have been recognized by difference authors as important regulators of the community structure of the benthos, which can be of economic importance for the area, like some of the crustaceans that inhabit the lagoon.
Classification of species by their relative abundance and their frequency of occurrence in the samplings, is useful to identify which species are present throughout the year. This analysis contrasted from previous studies such as those by Aburto-Oropeza (2000) and Barjau-Gonzá lez (2012), who mentioned that according to this classification, depending on time of sampling, a species can be abundant or may be rare.
Based on the previous, we conclude that pattern of physicochemical parameters showed a defined seasonal variation, similar to what was recorded by other authors. The lagoon presents a low to medium diversity (0.3 to 2.3 bits/ind). According to the BVI analysis, most dominant species were, C. zonatus, P. maculatofasciatus, S. lobatus, P. nebulifer, S. annulatus, Pleuronichthys guttulatus, C. brachysomus and A. ischana.