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.

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.

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.