Efficiency of fish ladders for neotropical ichthyofauna

Abstract

The reproductive strategies of potamodromous fish of the Plata basin involve upstream migration and passive downstream movements of eggs and larvae for dozens of kilometres. At the end of the downstream movements, the fry need marginal lagoons or bays in order for their development to be successful. The construction of reservoirs, often in series, has interrupted the movements, and is perhaps the main factor in the decrease in the abundance of migratory fish in the basin. In an attempt to lessen the impact on the upstream migrations, fish passages (floodgates, elevators and mainly ladders) have been constructed and, despite limitations, they have functioned with relative success. Some studies show the continuity of the migration of individuals captured downstream and released into the reservoir. However, downstream migration of adult migratory species is made difficult by the fact that they are, in this phase, generally reophylic (live in running water) and occupy only the upper stretches of reservoirs (fluvial and transitional zones). It would be feasible only in short reservoirs and with a very short residence time. The fact that reservoirs present calm waters, more transparency and contain a large number of small fish could delay the passive drift of eggs and larvae, exposing them to intense predation and/or promoting their settling toward deeper water, where the conditions of oxygenation are generally critical. In the remote event that the larvae reach the dam, most of them cannot be attracted to ladders (elevators are conceived only for upstream movements) or driven away from the reservoir vicinities, in as much as they have passive movements and would therefore suffer great mortality if they passed through turbines or a spillway. In reservoirs with large free stretches upstream and endowed with ample natural floodplain areas, transposition can be an useful instrument in the maintenance of genetic heterogeneity since the number of transposed specimens does not compromise the stocks of the lower stretch. These limitations, connected to the strategy of neotropical fishes, make fish passages in most situations an inadequate instrument for preservational management.

Preliminary study on the application of radio-telemetry techniques to evaluate movements of fish in the Lateral canal at Itaipu Dam, Brazil

“…Central to these discussions is the role of the passages for the conservation of the fish fauna (Agostinho et al, 2002), and particularly of migratory species which are generally the most affected by dams. In the Paraná River basin there are around 130 dams higher than 10 m, and 26 possess areas greater than 100 km 2 .…”

“…Makrakis et al (2007) describe the ichthyofauna present in the channel and discuss some general aspects that may be limiting the ascent of fish. Other fish passages, particularly ladders, have been evaluated in the Neotropical region over the past two decades (Quirós, 1988;Godinho et al, 1991;Borghetti et al, 1994;Agostinho et al, 2002;Fernandez et al, 2004).…”

Is fish passage technology saving fish resources in the lower La Plata River basin?

“…Moreover, the challenge of preserving migratory species in large rivers of the lower La Plata River basin is made more complex by the life history of South American fishes. For example, downstream passage of post-spawning adults is not considered in the design of dams even though migratory South American fishes are generally iteroparous (Oldani et al, 1998;Agostinho et al, 2002). Passive drift of larvae, active downstream migration of juveniles, and the survival rates of each through dam outlets are also unknown.…”

A barrier to upstream migration in the fish passage of Itaipu Dam (Canal da Piracema), Paraná River basin

“…Despite several attempts to evaluate these fish passages summarized in a special volume of Neotropical Ichthyology (2007, volume 5, number 2), there is still a very limited understanding of how migratory species use these fish passages, and there are many doubts regarding their efficiency, particularly in the Neotropical region (Agostinho et al, 2002;Agostinho et al, 2008).…”

“…Although individuals of some species successfully pass the Canal, available data suggest that passage may be poor (Hahn, 2007;Makrakis et al, 2011), and the system component designated as the Discharge channel in the Bela Vista River (herein named Canal de deságue no rio Bela Vista or CABV), a 200 m long technical section, was the main barrier to the upstream migration. Although detailed data are lacking for other systems, low fish ascension efficiency coupled with species selectivity appears to be a common characteristic of South American fish passages (Oldani & Baigún, 2002;Agostinho et al, 2002Agostinho et al, , 2007Makrakis et al, 2007a;Godinho & Kynard, 2009). However, the fish pass efficiency for South American rivers must consider its capability to maintain viable populations (Pompeu et al, 2012).…”