The ichthyocidal activity of Pfiesteria piscicida dinospores was examined in an aquarium bioassay format by exposing fish to either Pfiesteria-containing environmental sediments or clonal P. piscicida. The presence of Pfiesteria spp. and the complexity of the microbial assemblage in the bioassay were assessed by molecular approaches. Cell-free water from bioassays that yielded significant fish mortality failed to show ichthyocidal activity. Histopathological examination of moribund and dead fish failed to reveal the skin lesions reported elsewhere. Fish larvae within "cages" of variable mesh sizes were killed in those where the pore size exceeded that of Pfiesteria dinospores. In vitro exposure of fish larvae to clonal P. piscicida indicated that fish mortality was directly proportional to the dinospore cell density. Dinospores clustered around the mouth, eyes, and operculi, suggesting that fish health may be affected by their direct interaction with skin, gill epithelia, or mucous surfaces. Molecular fingerprinting revealed the presence of a very diverse microbial community of bacteria, protists, and fungi within bioassay aquaria containing environmental sediments. Some components of the microbial community were identified as potential fish pathogens, preventing the rigorous identification of Pfiesteria spp. as the only cause of fish death. In summary, our results strongly suggest (i) that this aquarium bioassay format, which has been extensively reported in the literature, is unsuitable to accurately assess the ichthyocidal activity of Pfiesteria spp. and (ii) that the ichthyocidal activity of Pfiesteria spp. is mostly due to direct interactions of the zoospores with fish skin and gill epithelia rather than to soluble factors.
Odontesthes bonariensis is an inland water fish from the Pampas region, valued due to the quality of its flesh and its attractiveness as a game fish. Chasico ´Lake located in the south of the pampean region is appreciated by fishermen due to O. bonariensis abundance. However, in the last year, a reduction in their catches was recorded. In this work, it was analyzed the changes of O. bonariensis biomass in relation to climate change and salinity in Chasico ´Lake from 1997 to 2013. From 2004 to 2013, when a dry period began, a reduction in lake area (68.1-47.4 km 2 ) and an increase in salinity (18.9-41.5 g l -1 ) were observed. O. bonariensis catches showed a significant decrease in the same period as a result of salinity increments. Possibly, at high salinity, O. bonariensis may still develop but with an impaired reproduction and limited embryo and juvenile survival. In conclusion, after 2004, the fishery quality in Chasico ´Lake showed a decreasing trend from the maximum registered in the Pampas lakes. This was in agreement with a marked decrease in the lake area which in turn increased water salinity up to levels close to the O. bonariensis tolerance limit.
The habitat use, seasonality and demography of the broadnose sevengill shark Notorynchus cepedianus were investigated in central Patagonia, where a data gap exists for the species. Catch and effort and video‐derived indices indicated high relative abundance of sharks during warm months. Video stations revealed differences in the spatial use by sharks, being more frequently observed in the inner section of the bay. Complementary tagging efforts evidenced both a seasonal residence pattern and site fidelity between consecutive warm seasons. Juveniles outnumbered adults, which suggests that they may use the study area as a secondary nursing ground. Evidence from spontaneous regurgitation further suggests that prey abundance may be driving the seasonal occurrence of sharks in the region. This study allows for a more comprehensive understanding of the population structure and dynamics of sevengill sharks in the Southwest Atlantic.Abstract in Spanish is available with online material.
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