The ichthyofauna of the Chacahua Lagoon in the western Oaxaca State of Mexico was sampled every 2 months, using a trawl net over seven sampling cycles. The estuary mouth closed in late January, generating hypersaline conditions in the system. A total of 33 species belonging to 20 families were recorded; most of them marine species, showing tropical and subtropical affinities. The most abundant species were Diapterus peruvianus, Centropomus robalito, Anchovia macrolepidota, Lile stolifera, and Lutjanus novemfasciatus. Total fish abundance and species richness were not significantly affected by the mouth closing, and this was related to the presence of a permanently open entrance channel with an adjacent lagoon, which allowed easy exchange of fish between these two systems. Canonical correspondence analysis (CCA) revealed that salinity and dissolved oxygen were the most important environmental variables in determining the observed variability in fish community composition. Two fish groups were evident: the fish assemblage of the lowsalinity period (open-mouth phase), in which Eucinostomus currani, Sciades guatemalensis, Centropomus armatus, Citharichthys stigmaeus, and Caranx caninus constituted the majority of the catch; and the fish assemblage of the high-salinity period (closed-mouth phase), with A. macrolepidota, L. stolifera, and Harengula thrissina as the most abundant species. Multivariate analyses showed differences in the composition of the fish community between both periods. Other species, such as the permanent residents D. peruvianus and C. robalito, which showed the widest range in tolerance of ambient salinity, were caught during both periods. Seasonal succession of fish populations may be related to differences in life cycle and tolerance of the environment among those species adapting to ecological conditions.

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We hypothesized that temporal variation in fish species composition and community structure in a low complexity habitat in the Pueblo Viejo Lagoon, Mexico, is influenced by diel light/dark cycles and tidal stage, and by seasonal changes in salinity and temperature. We collected a total of 17,661 individuals during 2‐h interval sampling over six bi‐monthly 24‐h sampling cycles representing 53 species, of which 11 (∼20%) were previously unknown in the system. Diel variation indicated that significantly higher numbers of individuals and species were caught at night, whereas diversity and evenness were higher during the day. Species richness was significantly higher in July and January, whereas diversity and evenness peaked around May; both were correlated with temperature. Diel variation in species composition was influenced primarily by the light/dark cycle. Cluster analyses of each diel cycle separated fish assemblages from midday samples from those of nocturnal samples, separated by an extended wide transition period as fish moved at dawn and during the late afternoon/dusk. Significant shifts (as determined by MANOVA) in assemblage structure occurred between months. Canonical correspondence analysis showed that temperature and day/night effects were the most important environmental variables structuring the fish community. This constrained ordination also defined species with specific habitat preferences as follows: (i) diurnal, warm temperature species (mainly planktivores) (Brevoortia gunteri, Cetengraulis edentulus, Diapterus auratus, and Membras martinica); (ii) nocturnal, warm temperature species (mainly predators) (Citharichthys spilopterus, Cathorops melanopus, and Bairdiella spp.); and (iii) low temperature, diurnal species (Brevoortia patronus and Mugil curema) or those with twilight and nocturnal distributions (Anchoa mitchilli, the most numerically abundant species). Our results indicate that diel and seasonal changes in fish community structure were mainly related to day/night cycles and temperature regimes.

ABSTRACT. To extend our knowledge about spatial and temporal variations in the structure of estuarine fish communities, diel samplings (collections every 2 h) were carried out every two months over a one-year period in two different habitats (vegetated and nonvegetated). We collected a total of 34,766 individuals, corresponding to 72 species, most of them with marine and tropical affinities. Species richness was significantly higher in the vegetated habitat, whereas abundance (number of fish) was higher in the nonvegetated habitat. The diel pattern of abundance showed peaks at dusk and dawn, whereas richness was higher at night. Seasonally, fish abundance and species richness showed peaks during January, March, and July-September, and these peaks were related to the entry of temperate species, production processes in the system, and rainfall pattern. Fish assemblages showed significant differences among habitats, day/night, and months. In the vegetated habitat, Lagodon rhomboides, Menidia beryllina, and Orthopristis chrysoptera were the predominant species. Microphagus species such as Brevoortia gunteri, Brevoortia patronus, and Mugil curema were predominant during the warm, dry season (March-May) and they were more abundant during the day. In contrast, predatory species such as Citharichthys spilopterus and Opsanus beta were more abundant during the rainy season and their capture was higher at night. The two most numerically abundant species, Anchoa mitchilli and Membras martinica, showed a twilight pattern, with abundance peaks at dusk and dawn, respectively. Hence, in Pueblo Viejo Lagoon (Mexico), the spatiotemporal changes in species composition are in part related to predator avoidance and feeding strategies, through a coupling of life histories with the diel and seasonal variation in the availability of resources, the presence of submerged vegetation playing an important role in this dynamic.Key words: Ruppia maritima, mean trophic level, predator avoidance, diversity, fish assemblages. RESUMEN. Con el objetivo de contribuir al conocimiento de las variaciones espaciotemporales en la estructura de las comunidades de peces estuarinos, cada dos meses se realizaron muestreos nictímeros (colectas cada dos horas) en dos ambientes diferentes (con vegetación y sin vegetación) durante un año. En total se capturaron 34,766 individuos, correspondientes a 72 especies, de origen principalmente marino y tropical. La riqueza de especies fue significativamente mayor en el hábitat con vegetación sumergida, mientras que la abundancia lo fue en el hábitat sin vegetación. El patrón nictímero de la abundancia mostró pulsos al anochecer y al amanecer, mientras que la riqueza fue mayor durante la noche. Estacionalmente, la abundancia y la riqueza presentaron pulsos en enero, marzo y julio-septiembre, los cuales se relacionan con el ingreso de especies templadas, procesos de producción dentro del sistema y el régimen de lluvias. El conjunto de especies mostró diferencias significativas entre hábitat, día/noche y meses. En el hábit...

Spatial, seasonal and diel distribution patterns were analysed over a 2-year period for Diapterus auratus and Eucinostomus melanopterus in Pueblo Viejo lagoon, a tropical coastal lagoon in Veracruz, Mexico. A total of 216 samples were collected with 329 individuals of D. auratus and 170 of E. melanopterus. Diapterus auratus individuals were captured throughout a wider range of environmental conditions, and consequently its spatial niche breadth was significantly greater (P < 0.001) than that of E. melanopterus. In general, more individuals were captured at localities with submerged vegetation (Ruppia maritima), with 70% for D. auratus and 87.2% for E. melanopterus, although only the latter species showed significant spatial differences. A spatial segregation influenced by distance from the lagoon inlet and salinity was observed between the species. It is considered that this segregation may allow resource partitioning and minimize interspecific competition. A relatively low niche overlap (4.9%) was observed for the two species. Seasonal capture peaks of D. auratus were related to rainfall and the productivity pattern in the lagoon system. Seasonal fluctuations in monthly mean number of D. auratus were correlated with preceding variations in local rainfall (r ¼ 0.86, P < 0.02). Moreover, seasonal capture peaks of D. auratus coincided with ecosystem primary production peaks. Although E. melanopterus exhibited no significant seasonal differences, its maximum numbers also coincided with an ecosystem primary production peak and recruitment patterns. In 24-h cycle analysis, both species were captured mainly around mid-day, with minor peaks at dawn and/or at dusk. Generally, few individuals were collected during hours of darkness. It was proposed that diel abundance variation of gerreids might be related to both light penetration in the water column and feeding activity. Food availability within Pueblo Viejo lagoon appears to be an important factor influencing fish abundance.