Environmental and biological factors influence the relationship between a predator fish, Gambusia holbrooki, and its main prey in rice fields of the Lower Mondego River Valley (Portugal)

We studied the relationships between a predator fish, Gambusia holbrooki, a nd its main food prey, within the content of a rice field food web. The inf luence of some environmental and biological factors on these trophic intera ctions, in combination with existent quantitative information, allowed us t o evaluate the ecological viability of using a non-ionic surfactant, Genapo l OXD-080, to control a plague caused by crayfish (Procambarus clarkii) pop ulations in the rice fields. In the Lower Mondego River Valley, Portugal, G . holbrooki is abundant in rice fields. It feeds mainly on copepods, cladoc erans and rotifers. Surface insects, such as aphids, collembolans, adult (i mago) chironomids and other dipterans, are additional food. Large G. holbro oki consumed greater amounts of cladocerans and adult chironomids than othe r smaller size groups, while small fish prefered rotifers. Gravid females a te copepods, cladocerans, and adult chironomids and other dipterans in sign ificantly greater amounts than immatures, males, and non-gravid females. No n-gravid females ate collembolans in significantly greater quantities than any other fish group. The population density of copepods, cladocerans, adul t chironomids, and other dipterans, the area covered by aquatic vegetation, and water temperature all had significant effects on the total number of p rey caught by G. holbrooki. In contrast, a negative correlation was found w ith rotifers, collembolans, aphids in higher densities, and of increased wa ter volume, dissolved oxygen and pH. G. holbrooki holds a key intermediate position in the rice field food chain, feeding in large amounts of aquatic invertebrates and being eaten, in rum, by piscivores. With regard to the to xicity of Genapol OXD-080 on non-target organisms, LC50 values for G. holbr ooki and some of its main prey were several times lower than the concentrat ion necessary to decrease the activity of crayfish populations in the rice fields. Thus, Genapol OXD-080 could potentially cause greater damage to the local populations of non-target species and should not be used without tak ing precautions not to contaminate other important biological reservoirs, s uch as the rice field irrigation channels.