Evaluation of "pejerrey" (Odontesthes bonariensis) growth and production in relation to zooplankton biomass, using floating cages in shallow lakes (Argentina)

COLAUTTI DARÍO; GARCIA JAVIER; BAIGÚN CLAUDIO; BALBONI LEANDRO

Punta del Este

Congreso; Structure and Function of World Shallow Lakes; 2008

Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay

The “pejerrey” (Odontesthes bonariensis) can be considered as one of the most conspicuous fish species inhabiting the inland waters of Argentina. This is a zooplanktivorous fish that predates mainly on cladocerans, copepods and rotifers during juvenil stages. The pampean shallow lakes are mostly eutrophic, where the zooplankton represents an important community that sustains the “pejerrey” populations. It is currently theorized that in lakes, pelagic trophic level biomass is controlled from below (by resources) and from above (by consumers) named bottom-up and top-down controls respectively. In this context, the main objective of the present study was to evaluate the role of the zooplankton biomass as a bottom-up regulator of “pejerrey” growth and production in floating cages. Four cages (3.5 m length x 3.5 m width x 1.30 m depth and 1.5 mm mesh size) were located in the Lacombe shallow lake where 20 days old juveniles (21.6 mm; 0.019g.) were stocked with 50 fish/m2. Temperature was recorded using a thermo logger during the whole experiment each every hour. Fish and zooplankton samples were taken periodically together with basic limnological data from January to November of 2006. Temperature varied between 10 and 26 °C (average = 19.6 °C) and zooplankton biomass exhibited an increasing trend during the experience (11.4 – 3,066 µg/l mean = 530.9 µg/l). Final fish total lengths and weights (156.1 mm; 23.1g) were above the average size reported for the species. Growth patterns showed a similar behavior in all experimental units. A multiple linear regression analysis was performed in order to relate growth rates with water temperature and zooplankton biomass. These variables presented significant coefficients and explained 89% of the variability in growth rate (r = 0.94 p < 0,01). These results show that it is possible to obtain good individual development in fish reared in floating cages in pampasic shallow lakes. Also, it was possible to identify a significant relationship between two links of the pelagic trophic chain, and the temperature as a modulator of fish production. With this methodology it is possible to perform future tests comparing different stocking densities, and quantify the intensity and the role of resource or consumers controls in this pelagic trophic chain. The relationship between “pejerrey” and the zooplanktonic community has fundamental implications in the management strategies in these environments.