To assess the trophic impact of the planktonic larvae of the invasive Asian bivalve Limnoperna fortunei, introduced in South America around 1990, we investigated the gut contents of fish larvae collected at monthly intervals between October 2000 and March 2001 at three locations along a 600 km stretch of the Paraná River, and during November 2004 in two areas of marginal lagoons connected to the river near the city of San Nicolá s, Argentina. Zooplankton was also collected in the lagoons in 2004. In total, 11,956 fish larvae were retrieved, of which 1,511 were used for detailed analyses. Of the 15 fish taxa collected, 11 had veligers of L. fortunei in their gut. Fish larvae with empty guts represented 60% (San Nicolá s) to 72% (Paraná River) of the total number of fish. Proportions of feeding fish larvae with L. fortunei veligers in their guts varied between 20% (San Nicolá s) and 56% (Paraná River); in 15% of the guts analyzed, L. fortunei was the only food item recorded. For those specimens that had consumed L. fortunei larvae and any other food item, L. fortunei was the most important item in 55% (Paraná River) to 71% (San Nicolá s) of the animals in terms of biomass. No major temporal or spatial changes in the diet were observed along the Paraná River, but the relative biomass contribution of L. fortunei larvae differed strongly in fishes of different developmental stage. In protolarvae and mesolarvae, veligers accounted for 30-35% of the gut contents. In metalarvae, veligers accounted for only 3%, indicating enhanced food supply for the earliest fish life stages. Comparison of the relative proportions of the three main zooplankton types (L. fortunei veligers, cladocerans, and copepods) in the water and in larval fish guts indicates that L. fortunei is always selected positively over the other two prey types. While our results strongly suggest that the expansion of L. fortunei results in an enhanced food supply for local fish populations, they do not necessarily imply that Handling editor: S. M. Thomaz the overall effect on the ecosystem in general, and on the fish fauna in particular is beneficial.

We evaluated the passage of early‐stage fishes through the Salto Grande Dam using high‐frequency downstream ichthyoplankton monitoring and five surveys involving samples taken upstream and downstream of the dam. Eggs and larvae of migratory fishes were captured downstream of the dam, usually during high discharges. Upstream and downstream larvae were frequently unyolked, which corresponds to individuals aged 4+ days, and represents a time significantly longer than that required for the displacement of the water mass from the dam to the sampling location. In low flow rate surveys, fish larvae of the same species and degree of development were captured immediately upstream and at 1, 10, 24 and 40 km downstream of the dam. The densities and percentage of Pimelodinae larvae captured alive by short time and low speed tows were similar upstream and downstream of the dam, indicating that larval mortality was a result of sampling and not to the passage through the turbines. The results show that the larvae of fish that spawn in the middle section are partly transported to the lower section, and suggest that both spillway and turbine discharge should be considered part of the passage. We also found evidence that the passage of small and fragile Pimelodinae larvae through the Salto Grande Kaplan turbines does not significantly affect survival rates. Copyright © 2016 John Wiley & Sons, Ltd.

Prochilodus lineatus (Val.) is one of the main target species of South American freshwater fisheries. The following null hypotheses regarding juvenile recruitment of P. lineatus were tested: (a) recruitment is not determined by variations in larval abundance and (b) recruitment is not determined by variations in discharge. For this purpose, variations in abundance of larvae in main channel drift and monthly captures of juveniles in a floodplain lake of the River Paraná were examined weekly between 2009 and 2016. Mean annual abundances of larval densities and CPUE of juveniles were correlated with a set of hydrometric variables. A positive correlation was found between the abundance of juveniles with high and persistent flood pulses. By contrast, larval abundance was not correlated with juvenile abundance. Pronounced contrast was found in the recruitment of P. lineatus between years of high and low discharge, which supports the hypothesis that floods are the main determining factor for recruitment of this species.

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