Abstract Few studies have addressed the composition of fish assemblages of the freshwater Río de la Plata (RdlP) and have only been limited to species lists gathered over the last two centuries. As such inventories have never been reviewed or validated by fish sampling, the richness and structure of RdlP fish assemblage are poorly known. Hence, we conducted an exhaustive literature review and a fieldwork in six coastal points of Argentina to update the species composition and determine the hierarchical structure of the fish assemblage. From the 206 species registered in the literature, 48 were not confirmed, 13 were absent, five were taken as synonymized species, 29 were supported by literature and 107 were confirmed; one was an established exotic species, and three were a non-established exotic species. The findings reported here suggest that the fish assemblage currently comprises 141 species, including four new records. Analysis of fieldwork data in number and weight of fish captured resulted in an assemblage hierarchical structure of five dominant, 22 frequent, and 45 rare species; 16 dominant, 11 frequent, and 45 rare taxa, respectively. These results could be used as baseline to monitor, manage, and preserve neotropical fish species in their southern distribution boundary.

Resumen El conocimiento de los ensambles de peces del sector dulceacuícola del Río de la Plata (RdlP) es escaso y limitado a listas de especies de compilaciones realizadas en los últimos dos siglos. Como esos inventarios nunca han sido revisados o validados mediante muestreos de peces, el conocimiento respecto de la riqueza y la estructura de los ensambles del RdlP resulta deficiente. Para ordenar y mejorar la información acerca de la riqueza y estructura de los ensambles de peces del área se realizó una revisión bibliográfica y un muestreo de campo para actualizar la composición de especies y determinar la estructura jerárquica del ensamble de peces. De las 206 especies colectadas de acuerdo a la bibliografía 48 se categorizaron como no reconfirmadas, 13 como ausentes, cinco como especies sinonimizadas, 29 como soportadas por la literatura, 107 como confirmadas, una exótica establecida y tres exóticas no establecidas. Los resultados sugieren que el ensamble de peces actualmente está compuesto por 141 especies, incluyendo cuatro nuevos registros. El análisis de los datos en número y en peso de los peces colectados mostró 5 especies dominantes, 22 frecuentes y 45 raras; y 16 dominantes, 11 frecuentes y 45 raras, respectivamente. Los resultados de este trabajo pueden ser usados como línea de base para el monitoreo, manejo y conservación de las especies de peces neotropicales en su límite de distribución sur.

Abstract Gómez lake (34 ° 39 ‘S 61 ° 01’ W) is a typical shallow lake of Pampas region placed in the upper area of the Salado river. The most abundant fish species in this lake is the pejerrey (Odontesthes bonariensis) valued due to the quality of its flesh and its attractiveness as a game fish. The aim of this study was to describe for the first time in this pejerrey wild population the gonadal stages during three consecutive spawning seasons (August to December) in relation with sexual steroids and temperature in this lake. In general, pejerrey gonadal development, the gonadosomatic index and the plasma levels of estradiol and testosterone varied in relation to air temperature. During the sampling period, pejerrey females started to ovulate in early August (winter), with a peak in October and ending in December with some of them with atretic oocytes. For males, it was possible to find spermiating animals during the whole spawning season and some arrested animals only in December. Our results confirm the relationship with pejerrey maturation and temperature and can be useful for decision making in the management of this natural resource.

Abstract The age and growth of Pseudoplatystoma corruscans, is analyzed on specimens landed in Puerto Antequera, Province of Chaco, Argentina. The study is based on length frequency distribution of 1192 individuals and growth marks of 293 pectoral spines. Previously to age assignation based on spines readings, we performed analyses that ruled out age-associated resorption of rings and corroborated the annual periodicity of mark formation. The average sizes of the radius of each ring were obtained, and the total length of fish were back-calculated to the time of the ring formation, by the regression model fitted between the total radius of the spines to the respective sizes of each fish. Such data showed a good fitting to growth models of von Bertalanffy, Gompertz and logistic for both sexes separately. Results indicate that the study of the species growth must be carried out for each sex separately and that the fishing regulations must consider this characteristic of the species since the current management guidelines could be promoting differential capture by sexes.

Resumen En este trabajo se analiza el crecimiento y la edad del Pseudoplatystoma corruscans, a partir de ejemplares desembarcados en el Puerto Antequera, provincia de Chaco, Argentina. El estudio se basó en la distribución de frecuencias de tallas de 1192 individuos y marcas de crecimiento en 293 espinas pectorales. Para asignar las edades a partir de las lecturas se realizaron análisis que permitieron descartar la reabsorción de anillos asociada a la edad y corroborar la periodicidad anual de la formación de marcas. Se obtuvieron los tamaños medios de los radios de cada anillo y se retrocalcularon las tallas al momento de su formación, mediante la relación del radio total de las espinas con las tallas respectivas de cada pez. Dichos datos mostraron un buen ajuste a las curvas de crecimiento de von Bertalanffy, Gompertz y logística para ambos sexos por separado. Los resultados demostraron que el estudio del crecimiento de la especie debe realizarse para cada sexo por separado y que las normas pesqueras deben contemplar esta característica de la especie ya que las actuales pautas de manejo podrían estar promoviendo la captura diferencial por sexos.

The Martín stream is an urban water course with moderate anthropic impact located in La Plata (Buenos Aires). Gymnogeophagus meridionalis Reis & Malabarba, 1988 is a fish of Cichlids family that is common in natural environments or those modified by human. The paper objective is to characterize the G. merdionalis diet at the mentioned stream. Fish sampling was carried out in autumn, winter and spring 2014 and summer of 2015 in two areas with different land uses M1 (horticultural) and M2 (urban). A total of 173 specimens were collected and composition, volume and number per food item were determinated in the digestive contents. The alimentary importance index indicated that Chironomid larvae and Copepods are the main trophic resources. The Kolmogorov-Smirnov test did not detect qualitative significant differences between the fish diets from M1 and M2. However, the relative repletion index showed very high values in the latter site in spring and summer. The Amundsen method determined two types of feeding strategies one of them as a specialist type in the intake of Chironomid larvae during most of the year and another generalist in the spring. The best relative condition was observed in winter and spring in both sampling sites. The Martín stream provide to G. meridionalis a restrict diet that is reflected in low quantity of item consumed by comparison with item consumed from some fish that inhabit streams with low perturbation in this region. This is likely to be related to a limited supply of food rather than to a selection of prey by the species.

Gymnogeophagus meridionalis Reis & Malabarba, 1988 es un cíclido común tanto en ambientes naturales como modificados. Este trabajo describe la dieta de esta especie en el arroyo Martín, un curso de agua urbano con moderada perturbación antrópica, ubicado en el Partido de La Plata (Buenos Aires). Se recolectaron 173 especímenes en otoño, invierno y primavera de 2014 y verano de 2015, en dos sitios con diferente uso de suelo: M1 (zona hortícola) y M2 (zona urbana). Se determinaron la composición y el volumen del contenido digestivo y el número de individuos por ítem alimentario. El Índice de Importancia Relativa indicó que larvas de Chironomidae y Copepoda son los recursos tróficos más importantes. El test de Kolmogorov-Smirnov no detectó una diferencia significativa cualitativa entre las dietas de M1 y M2. Sin embargo, el índice de repleción relativo mostró valores muy altos en este último sitio en primavera y verano. El método gráfico de Amundsen determinó que la estrategia alimentaria es de tipo especialista en larvas de Chironomidae durante la mayor parte del año, pasando a generalista en primavera. En ambos sitios de muestreo la mejor condición relativa de los peces se observó en invierno y primavera. En el arroyo Martín, G. meridionalis presenta una dieta acotada, reflejada por la baja cantidad de ítems consumidos, en comparación con la de otras especies que habitan arroyos menos impactados de la región. Es probable que esto se deba más a una oferta de alimento limitada que a una selección de presas por parte de la especie.

The pejerrey is the most important recreational species in shallow temperate lakes and reservoirs of Argentina and the attempts to develop its culture have started a century ago. A common constraint of pejerrey aquaculture is its poor growth under traditional intensive rearing techniques. The aim of this study was to evaluate the possibility to achieve and maintain high growth rates of pejerrey throughout the rearing process by semi-intensive culture method . Four floating cages were installed in La Salada de Monasterio Lake and each one was stocked with 300 juveniles (10.22 ±0.38cm; 6.52 ±0.82g). From January through March all fish were exposed to natural zooplankton as food source, whereas from April to September two cages were supplied daily with artificial food. The fish exposed to artificial supplementary diets exhibited significantly higher growth (17.5 ±0.98cm; 41.05 ±8.55g) than those in the control cages (15.02 ±0cm ; 23.5 ±0.84g), and exceeded the known values in pejerrey culture. The results suggest that the species potential growth is not fully achieved by common intensive methods and it can be improved by semi-intensive techniques. Accordingly a better understanding of the species nutritional requirements is needed to improve growth rates and enhance pejerrey culture.

El pejerrey es la especie de mayor importancia deportiva en lagos someros templados y embalses de Argentina y el desarrollo de su acuicultura data de cien años atrás. Un impedimento común para el desarrollo de la acuicultura del pejerrey es el pobre crecimiento que manifiesta bajo cultivo intensivo. El objetivo de este estudio fue evaluar la posibilidad de alcanzar y mantener altas tasas de crecimiento en el pejerrrey mediante cultivo semi-intensivo. Se instalaron cuatro jaulas flotantes en la laguna La Salada de Monasterio, sembrándose cada una con 300 juveniles (10.22 ±0.38cm; 6.52 ±0.82g). Desde enero hasta marzo los peces se alimentaron con el zooplancton natural de la laguna, mientras que desde abril hasta septiembre dos jaulas fueron suplementadas diariamente con alimento balanceado, dejándose las otras dos como controles. Los peces bajo dietas suplementadas mostraron crecimientos significativamente superiores (17.5 ±0.98cm; 41.05 ±8.55g) a los de las jaulas control (15.02 ±0cm ; 23.5 ±0.84g), excediendo incluso los conocidos en el cultivo de pejerrey en períodos similares. Los resultados obtenidos sugieren que el potencial de crecimiento del pejerrey no está siendo explotado en su totalidad bajo las técnicas de cultivo tradicionales y puede mejorarse mediante el cultivo semi-intensivo. En consecuencia es necesaria una mejor comprensión de los requerimientos nutricionales del pejerrey para incrementar sus tasas de crecimiento y así mejorar su cultivo.

Juvenis de pejerrey, Odontesthes bonariensis, foram expostos a Vermelho de Alizarina S (ARS) 0,1% de duas formas, sozinho ou com uma imersão anterior em 2,2% de solução salina (Indução osmótica, IO) para melhorar o método de marcação ARS. Os procedimentos foram realizados no campo e os peixes foram liberados em gaiolas (tanques-rede) de 1 m3 na lagoa "La Salada de Monasterio", Chascomús, Buenos Aires, Argentina. Após 73 dias, marcas claras foram observadas nos otólitos, raios da nadadeira caudal e escamas em ambos os tratamentos, sendo que a intensidade do sinal nas escalas de IO + ARS de peixe tratado foi superior. Por outro lado, não foram observados marcas no grupo controle sobre as mesmas estruturas. Aproximadamente um ano pós-tratamento (385 dias), apenas marcas nos raios da nadadeira caudal foram encontrados claramente nos peixes tratados com IO+ARS. Entre os peixes observados, após este período, não houve diferenças significativas no comprimento total ou peso entre o grupo controle e marcados, ademais, a mortalidade variou entre 30-40% em todas as gaiolas. Estes resultados fornecem fortes evidências e um grande potencial para aplicação desta técnica rentável de marcação que diferencia o pejerrey selvagem e o produzido em cativeiro. O sucesso na marcação dos raios da nadadeira caudal também é de grande importância, pois sua verificação é fácil e não requer o sacrifício de peixes

Juveniles of pejerrey, Odontesthes bonariensis, were exposed to 0.1% Alizarin Red S (ARS) alone or with a previous immersion in 2.2% saline solution (Osmotic Induction, OI) to enhance the ARS marking method. Fish were marked in the field and immediately released in 1 m3 cages in "La Salada de Monasterio" lagoon, Chascomús, Buenos Aires , Argentina. After 73 days, clear marks were observed in the otoliths, caudal fin rays and scales with both treatments, being the intensity of the signal in the scales of OI+ARS treated fish higher. On the other hand, no marks were observed in the control group on the same structures. Approximately one year post-treatment (385 days), only marks in caudal fin rays were found clearly in OI+ARS treated fish. After this period, no significant differences in total length or weight between marked or control fish were observed and the mortality ranged between 30-40 % in all cages. These results provide strong evidence for the potential applicability of this cost-effective marking technique in differentiation of wild and hatchery-produced pejerrey. The success in the caudal fin rays marking is also important because it is easy to do and does not require the sacrifice of fish.

Laguna Chascomús is an eutrophic, turbid, shallow lake typical of the Pampa region of Argentina. This shallow lake is permanently mixed and displays a high degree of spatial homogeneity. The cyclical periods of excess and shortage of rain, characteristic of this area, result in periodic drying and flooding events. According to newspaper articles published during the first half of the XX century, the lake was originally turbid. The first patches of rooted vegetation appeared after the 1913 y 1914 floods. During most part of the XX century the lake remained in a "clear", vegetated state. During this period the dominant fish species was the silverside (pejerrey), Odontesthes bonariensis, which accounted for over 60% of the total fish catches (in biomass). Instead, by the end of the latest century the lake shifted to a turbid state that persisted until present. Currently, the phytoplankton primary production is limited by light and its values are among the highest ones reported for natural aquatic systems. In light limited systems, the transparency may be expected to be controlled by the amount of incident light, through a negative feedback loop with primary production. These predictions have been confirmed, both in mesocosm experiments and "in situ". The phytoplankton biomass, dominated by nanoplanktonic cyanobacteria accounts for 75% of the total amount of carbon in the water column. The equilibrium predicted by the light limitation theory could not hold if the model assumptions were violated. This situation could happen if the phytoplankton biomass were controlled by herbivore zooplankton. However, such a control of primary producer by herbivores seems to be precluded due to the absence of large-sized zooplankton grazers, presumably due to the composition of the fish assemblage. In contrast to earlier reports of the 1960, the proportion of pejerrey biomass is presently quite low (0.04%) and the community is dominated by omnivore microphagous, such as the sabalito (Cyphocharax voga). The dominance of sabalito could not only explain the composition of the zooplankton community, but it also could contribute to prevent or delay the consolidation of sediments.

La laguna Chascomús es un típico lago somero, eutrófico y turbio de la Pampa Deprimida. Se encuentra permanentemente mezclada y presenta un grado elevado de homogeneidad espacial. La alternancia entre períodos de déficit y de exceso de agua, característica de la región, determina que la laguna sufra ciclos periódicos de sequía e inundación. Las primeras crónicas indican que a principios del siglo XX la laguna era turbia y que las primeras matas de vegetación habrían aparecido después de las inundaciones de 1913 y 1914. Hacia mediados del siglo XX, Chascomús se encontraba en un estado de aguas claras y colonizada de manera profusa por macrófitas. En ese momento, aproximadamente 60% de la biomasa de peces correspondía al pejerrey (Odontesthes bonariensis). En las últimas décadas, esta condición se modificó y en la actualidad la laguna está estabilizada en un estado turbio en el que la producción primaria fitoplanctónica es muy alta y se encuentra limitada por luz. La biomasa de fitoplancton representa el 75% del carbono de la columna de agua y predominan las cianobacterias nanoplanctónicas. Estudios de campo y experimentos en mesocosmos indican que la transparencia del agua está controlada por la cantidad de radiación incidente a través de una retroalimentación negativa con la producción primaria. La biomasa fitoplanctónica elevada se mantiene debido a la ausencia de un control efectivo por parte del zooplancton herbívoro. Como resultado de la presión de depredación que ejerce la comunidad actual de peces, la composición del zooplancton no presenta filtradores eficientes (e.g., Daphnia y otros cladóceros de gran tamaño). En la actualidad el pejerrey representa un porcentaje muy bajo de la biomasa total de peces (0.04%), mientras que los micrófagos omnívoros, como el sabalito (Cyphocharax voga), son dominantes. La dominancia del sabalito no sólo explicaría la baja abundancia del zooplancton, sino que al mismo tiempo contribuiría a evitar o retardar la consolidación del material sedimentado.

O peixe-rei Odontesthes bonariensis é a espécie mais importante na pesca de água doce da região temperada da Argentina, e a avaliação de sua condição corpórea tem sido uma prática normal e uma ferramenta de diagnóstico bastante utilizada. Grande parte dos estudos sobre a pesca do peixe-rei utiliza o índice de Fulton (K), sem testar se seus pressupostos são atendidos. Nós analisamos e comparamos a aplicabilidade dos índices K, Kn e Wr para a avaliação do fator de condição em várias populações de peixe-rei em lagos dos Pampas. Nossos resultados mostram que, enquanto o Wr e o Kn dependeram significativamente do comprimento corporal em alguns lagos, o Kn apresentou uma pequena amplitude de variação. Foi também notado que o comprimento máximo e a estrutura de tamanho variaram fortemente entre populações, provavelmente devido às mudanças ontogenéticas no nicho trófico, dependendo das características limnológicas e da disponibilidade de zooplâncton. Concluímos que o índice K deve ser desconsiderado nos casos em que as populações apresentam variações alométricas marcantes no crescimento e na amplitude de tamanhos. O índice Kn parece ser apropriado apenas para avaliações de uma única população, não sendo apropriado para comparações entre populações quando elas exibem diferentes inclinações na relação peso-comprimento. Finalmente, o índice Wr é recomendado se o objetivo for fazer comparações entre populações de peixe-rei, particularmente quando a estrutura populacional não é bem conhecida, a estocagem tenha sido usada para a recuperação populacional, os lagos apresentam grandes variações limnológicas, os dados se limitam a uma ou poucas amostras e suspeita-se da ocorrência de mudanças ontogenéticas na dieta.

The pejerrey Odontesthes bonariensis is the most important target species in temperate freshwater fisheries of Argentina, and assessment of condition has been a regular practice and common diagnostic tool. Most pejerrey fishery studies have used Fulton's (K) index, without testing whether underlying assumptions or requirements were met. We analyzed and contrasted the applicability of K, Kn and Wr indices to assess condition status in several pejerrey populations inhabiting Pampean lakes. Our results showed that whereas Wr and Kn displayed significant condition changes across length at some study lakes, Kn portrayed a small range of variation. We also noted that pejerrey maximum length and size structure strongly varied among populations probably due to the characteristics of trophic niche changes through lifespan, depending on lake limnological characteristics and zooplankton availability. We conclude that the K index should be disregarded in those cases where populations show allometric growth and size ranges strongly vary. In turn, the Kn index appears to be only appropriate for regular within population assessment, being difficult to apply when comparisons between populations are needed and when they exhibit different weight-length relationship slopes. Finally, the Wr index should be strongly preferred if the objective is to perform comparisons between pejerrey populations, particularly when population structure is not well known, stocking has been used for population recovery, lakes are strongly supported by limnological changes, data are limited to only one or few samplings and metaphoetesis is suspected in pejerrey populations.

We developed a standard weight equation Ws to aid in the analysis of pejerrey Odonthestes bonariensis body condition over time and across populations using the relative weight index Wr. Weight - length data were compiled from 73 populations of pejerrey (N=16.022) from the Argentine pampas region. We used the regression-line-percentile technique, which provides a 75th-percentile standard by length intervals of 10 mm, to develop the Ws equation. The proposed equation is log10 Ws=-5,267+3,163 log10 Lst; Ws is weight in grams and Lst is standard length in millimeters. This equation is proposed for use with pejerrey between 120 and 520 mm of Lst. Values for Wr calculated with the Ws equation did not consistently increase or decrease as function of fish length, indicating absence of length bias. We analyze the values and distribution of Wr for pejerrey and suggest how to interpret its results. The equation of Ws that intends to calculate the index of Wr, represents a useful tool of analysis, because not only it allows to statistically compare the physical condition of the pejerrey, independently of its size, capture moment or the individual origin, but also it facilitates to relate it with other variables.

The carp (Cyprinus carpio) is an exotic fish that currently inhabits most of the Pampasic shallow lakes. A better knowledge of its trophic spectrum and of the relative composition of its diet may represent a very useful information to understand its biology, as well as the ecological niche occupied by its populations in these environments. In this work the food habits of the carp were studied in Lobos shallow lake, from the analysis of 75 digestive tracts of fish with a mean standard length of 409.7 mm, captured between March and August of 1994. Food items were counted and classified taxonomically up to the family level. Values for several indexes were calculated with this information, and the seasonal pattern of variation of the digestive contents was analyzed using Correspondence Analysis. The species showed a wide trophic spectrum. The main food items, in order of importance were vegetable fragments, Ciclopoidea, Chidoridae, Chironomidae, Bosminidae, Harpacticoidae and seeds. The second axis of the ordination showed the existence of seasonality in the diet, which could be related to the availability of organisms in the environment. The abundance of zooplanktonic organisms in the diet coincides with that found in fishes from Asian and European shallow lakes, and it established the major difference with several deep lakes of the world, where the carp is exclusively bottom-feeder. This could be due to the opportunistic character and to the trophic plasticity of this species, which would take advantage of the offer of a particularly abundant food resource in the Pampasic shallow lakes.

La carpa (Cyprinus carpio) es un pez exótico que actualmente habita la mayoría de las lagunas pampásicas. El conocimiento del espectro trófico y de la importancia relativa de los componentes de su dieta representa información muy útil para la comprensión de su biología y del nicho ecológico ocupado por sus poblaciones en estos ambientes. En este trabajo se estudió la alimentación de la carpa en la laguna de Lobos, a partir del análisis de 75 tubos digestivos de peces con una longitud estándar promedio de 409.7 mm, capturados entre marzo y agosto de 1994. Los ítems alimentarios fueron contados y clasificados taxonómicamente hasta el nivel de familia. Con la información obtenida se calcularon índices alimentarios y se analizó el patrón de variación estacional de los contenidos digestivos empleando un Análisis de Correspondencia. La especie presentó un espectro trófico amplio. Los principales ítems alimentarios, en orden de importancia, fueron restos vegetales, Ciclopoidea, Chidoridae, Chironomidae, Bosminidae, Harpacticoidae y semillas. El segundo eje del ordenamiento indicó la existencia de estacionalidad en la dieta, que estaría relacionada con la disponibilidad de los organismos en el ambiente. La abundancia de organismos zooplanctónicos en la dieta coincide con lo hallado en ambientes de escasa profundidad de Asia y Europa, y estableció la mayor diferencia con respecto a varios lagos profundos del mundo donde la carpa es netamente bentófaga. Esto se debería al carácter oportunista y a la plasticidad trófica de la especie, la cual aprovecharía la oferta de un recurso alimentario particularmente abundante en las lagunas pampásicas.