Mirta Lidia García

Fish ecology of a seasonal lowland stream in temperate South America

El Pescado (literally ‘the fish’) is a lowland stream in the pampean plain, entering the Rio de La Plata at 35˚55¢S,57˚45¢W, 70 km SW of the city of Buenos Aires. The stream develops as a potamon with variable flood frequency along its 36 km length. The water is slightly soft, with relatively high conductivity and dominance of HCO–3 , Na + and Cl – ions. Three fish families, Characidae (29%), Pimelodidae (14.6%) and Loricariidae (12.8%) are most abundant among 55 species. Differences in species composition among four stations with different environmental traits were observed. Flooding slightly affected the fish fauna. In open-water and rocky-bottom stations, both species number and number of individuals were correlated with conductivity, an indication of the decrease of fish density with floods. Species richness was well over worldwide means. Diversity values, higher than in small tropical streams in Venezuela and southern Brazil, ranged from 0.32 to 2.59, being higher during autumn and spring. The stream has a high persistence, with the composition of the fish community maintained or recovering as soon as flooding ends. One Synbranchus marmoratus specimen was found among rocks, the first record of this behaviour for the species.

From fresh water to the slope: fish community ecology in the Río de la Plata and the sea beyond

Se analizo el patron espacial de las asociaciones de peces y su relacion con los factores ambientales a lo largo del gradiente desde el agua dulce en el Rio de la Plata hasta la parte superior del talud. Los peces fueron muestreados en 22 estaciones con una red de arrastre de fondo tipo Engel (200 mm de abertura de malla en las alas, 120 en el copo, 4 m de abertura vertical y 15 m de abertura horizontal) con una velocidad de arrastre de 4 nudos durante 20 a 30 min. Para definir la distribucion espacial de las asociaciones de peces basadas en la composicion de la ictiofauna (abundancia y biomasa) se aplico el analisis de agrupamiento jerarquico y el analisis de ordenacion MDS y para estimar la relacion entre estas asociaciones con la profundidad, la temperatura y la salinidad del agua en la superficie y el fondo, se utilizo el proceso BIO-ENV. Los resultados obtenidos demuestran que en la comunidad de peces a lo largo de este gradiente riverino-marino se pueden establecer cuatro asociaciones: riverina, estuarial, plataforma y talud. Estas asociaciones fueron significativamente diferentes en la composicion de especies, cada una de ellas fue caracterizada por varias especies comunes y discriminantes y presento diferentes condiciones ambientales. Concluimos que la salinidad y temperatura de fondo fueron las variables ambientales determinantes de la estructura de las asociaciones a traves de las areas. El cambio en la estructura entre las areas fue gradual y con limites flexibles.

Reproductive biology of the lesser guitarfish Zapteryx brevirostris from the south‐western Atlantic Ocean

This study provides the first detailed information on the reproductive biology of the lesser guitarfish Zapteryx brevirostris in the south-western Atlantic Ocean between latitudes 34° and 42° S. A total of 320 males (196-647 mm total length, L(T)) and 336 females (200-640 mm L(T)) were analysed. No significant differences were observed in size at 50% maturity between males (499 mm L(T)) and females (506 mm L(T)). Three seasonal patterns were related to the testes mass, the development of spermatocysts and the presence of spermatozoa in genital ducts. Females showed a 3 year reproductive cycle, with 2 years of ovarian maturation and 1 year of gestation, including a mean annual fecundity of one to two embryos. Synchronicity of both ovulating females and males with sperm in genital ducts indicated that ovulation and hence mating occurs during winter. Birth was assumed to occur during winter, but neither ovulation nor mating is simultaneous with this reproductive event. Despite its low economic value, a decline in abundance in the region, together with a low reproductive efficiency, indicates that the Z. brevirostris population should be monitored closely.

Reproductive biology of the spotback skate Atlantoraja castelnaui in the south-west Atlantic Ocean

This study provides information on the reproduction of spotback skate Atlantoraja castelnaui. A total of 232 individuals (119 females and 113 males) were obtained from surveys carried out between 2003 and 2006, from the south-west Atlantic Ocean, between 34 and 42° S and <50 m deep; another 514 specimens (241 females and 273 males) were obtained between 2005 and 2007 from commercial fishery operations carried out in the same area and landings in the port of Mar del Plata, Argentina. Males ranged from 185 to 1250 mm total length (L(T) ) and females from 243 to 1368 mm L(T) . Length at maturity was estimated to be 980 mm for males and 1089 mm L(T) for females. Lack of variation of testis mass together with the continuous production of mature spermatocyst and spermatozoa in deferent ducts suggested that males can reproduce throughout the year. Females reproduced year-round with peaks of reproductive activity an integral part of a continuous cycle. This conclusion is corroborated by the seasonal variation of ovaries, oviducal gland and the occurrence of females with eggs in the uterus throughout the year. Results from this study indicate that A. castelnaui is very susceptible to fishery pressure.

Is Oreochromis niloticus invading the Samborombón Bay, Río de la Plata, Argentina?

The Nile tilapia (Oreochromis niloticus) is a species widely cultivated worldwide. In recent decades it was an increasing development of fish farming of this species and the red variety in Argentina and Uruguay. From January to March 2010, four specimens of O. niloticus were captured in the south boundary of Samborombon Bay (S 36° 17 W 56° 46 ), which is the external sector of the Rio de la Plata. Probably the collected specimens were released accidentally from hatcheries placed on the banks of aquatic environments in communication with the Samborombon Bay. The patterns and mechanisms of species dispersal are of significant interest, while the interactions among factors determining invasion success often remain poorly understood. Invasion success is influenced by the ability of invading specie to withstand, interactions with native species and oceanographic characteristics of the new habitat. Of four specimens obtained two were females, one in spawning stage. This is the first record of Nile tilapia from a natural environment in Argentina and could indicate the beginning of a new invasion by a non native species.

Fish diversity in the Río de la Plata and adjacent waters: an overview of environmental influences on its spatial and temporal structure.

The fish diversity and the main environmental factors affecting the spatial distribution of species, life history stages and community structure in the Río de la Plata (RdP) and adjacent waters are reviewed and analysed, with emphasis on the functional guild classification. The functional guild classification indicated that most species in the RdP were marine stragglers, zoobenthivores and oviparous species, although the biomass was dominated by estuarine species. Salinity had a stronger influence than temperature on the spatial pattern for all life stages, shallower and fresher waters are the preferred habitats of neonates and juveniles. During the breeding season (spring-summer), adults showed an intrusion into the inner part of RdP or to its adjacent nearshore waters from the offshore waters for spawning or mating, respectively. Variations in river discharge and wind patterns greatly affected the spatial extent of estuarine water, which ultimately influenced the domain of the main life-history stages (juveniles or adults) for both marine and estuarine fishes, as well as species and fish assemblage composition. The strong environmental gradient restricts some species and life-history stages to a particular section and defines three main fish assemblage areas. The composition of the fish assemblage is indicative of the recruitment of freshwater and marine species to the estuary in opposite ways, determined by the vertical stratification. Seasonal changes in the species composition were related to migration as a result of salinity and temperature variations and reproductive migrations to spawning and mating areas. This overview reveals that the RdP is under environmental variations that are likely to produce modifications to fish distribution and abundance that affect its fisheries. This context plus fish stock declines and changes in exploitation patterns could amplify the magnitude of the variations in the fisheries resources availability and affect the sustainability of fishing communities.

Pisces, Atheriniformes, Atherinopsidae, Odontesthes perugiae Evermann and Kendall, 1906: distribution extension, new records and geographic distribution map for the species

In this work, we report the presence of Odontesthes perugiae from the Camba Cue lagoon, in the Apipe Island, upper Parana River. This distribution is the northernmost one for the species.

Sedimentación de partículas con distribución de tamaño fractal

Since several years the fractal fragmentation model has attracted the attention of researchers, as a logic way to describe and interprete observed particle size distributions. Textural analysis has shown to be very important because of its usefulness in the dignosis and inferences about soil functioning and use. Most popular methods of textural analysis employ sedimentation of particles in water using the hydrometer or the pipet. Both have the objective of determining the particle fraction remaining in suspension at predetermined time and to relate them with particle diameters. In the present work a new power law relationship between the mass fraction in suspension and the time was developed. Using this relationship it was possible to determine the fragmentation fractal dimension of a set of particles in sedimentation. The new equation has been checked with data obtained in this research by the Bouyoucoss hydrometer, and others published in the literature, using the Robinsons pipet method. The agreement between the model and the experimental data, using non linear regression, was excellent. Resulting fractal fragmentation dimensions ranged from 2.404 to 2.512, for samples from La Plata, Argentina, and between 2.434 and 2.819 for soils from California, USA. Determination coefficients, R2, were always higher than 0.9.

Length-weight relationships of 29 species of teleost fish from shallow estuarine environment of Samborombon Bay (Río De la Plata, Argentina)

The length–weight relationships (LWR) of 29 teleost fish species from the shallow estuarine environments of the Samborombón Bay wetland (Río de la Plata, Argentina) were estimated. A total of 6,376 individuals belonging to 19 families were measured and weighed. The best represented family was Sciaenidae with 6 species, followed by Clupeidae (n = 3), and Heptapteridae and Characidae with 2 species each. Significant length–weight relationships with high correlation coefficients were found for all species, which mostly presented positive allometric growth. This study reports the first findings regarding the LWR of 29 species in the Samborombón Bay waters. New maximum sizes and geographic distribution records of certain species are also commented.

Relações peso-comprimento de 29 espécies de peixes teleósteos dos ambientes estuarinos pouco profundos da Baía Samborombón (Rio da Prata, Argentina)

The length–weight relationships (LWR) of 29 teleost fish species from the shallow estuarine environments of the Samborombon Bay wetland (Rio de la Plata, Argentina) were estimated. A total of 6,376 individuals belonging to 19 families were measured and weighed. The best represented family was Sciaenidae with 6 species, followed by Clupeidae (n = 3), and Heptapteridae and Characidae with 2 species each. Significant length–weight relationships with high correlation coefficients were found for all species, which mostly presented positive allometric growth. This study reports the first findings regarding the LWR of 29 species in the Samborombon Bay waters. New maximum sizes and geographic distribution records of certain species are also commented.