Tropical rivers and their associated floodplain habitats are dynamic habitat mosaics to which fishes are challenged to respond in an adaptive manner. Migratory fishes create linkages among food webs that are partitioned along a nested hierarchy of spatial scales. Such linkages are examined across a hierarchy of spatio‐temporal scales, ranging from small streams to entire drainage basins, for rivers in South America and Africa. Migratory herbivorous fishes originating from eutrophic, productive ecosystems may subsidize resident predators of oligotrophic river ecosystems, which may result in cascading direct and indirect Effects on other species in local food webs. Successful management of many of the most important stocks of tropical river fishes requires conceptual models of how fish movement influences food web structure and dynamics.

We use stomach contents and stable isotope ratios of predatory fishes, collected over a 10‐year time span from a species‐rich river in Venezuela, to examine potential body‐size–trophic‐position relationships. Mean body size of predator taxa and their prey (determined by stomach content analyses) were significantly correlated, but trophic position of predators (estimated by stable isotope ratios) was not correlated with body size. This reflects no apparent relationship between body size and trophic position among prey taxa. Primary consumer taxa (algivores and detritivores) in this system are characterized by diverse size and morphology, and thus predatory fish of all body sizes and feeding strategies are able to exploit taxa feeding low in the food web. Regardless of relative body size, predators exploit short, productive food chains. For any given food chain within a complex web where predators are larger than their prey, trophic position and body size are necessarily correlated. But in diverse food webs characterized by a broad range of primary consumer body size, apparently there is no relationship between trophic position and body size across all taxa in the web.

In channel and floodplain habitats of the Cinaruco River, Venezuela, Cichla temensis was more abundant and larger than C. intermedia and C. orinocensis. Seasonal variation in hydrology influenced habitat use, spawning, and predator-prey interactions. The three piscivores partitioned habitat, with C. intermedia showing a strong affinity for structured habitats in the main channel during all water level fluctuations. C. orinocensis was most abundant in shallow areas with submerged structure in lagoons and, to a lesser extent, in low velocity regions of the channel, and C. temensis occupied a wide range of lotic and lentic habitats. During the low-water period, the feeding frequency and body condition of all three species declined, and this was related, in part, to preparation for spawning near the end of the low-water season. The diet of C. intermedia was least similar to its two congeners during falling and rising water. C. orinocensis and C. temensis had lowest diet overlap during the low-water conditions, the period when many individuals of these two species move into lagoons for nesting. Prey in stomachs were significantly larger during the falling-water than the rising-water period, and predation by Cichla and other large piscivores during the falling-water period may have reduced the abundance of large prey, particularly Semaprochilodus kneri. These migratory detritivorous fish were important prey for C. temensis during the falling-water period and probably contributed a substantial fraction of the annual energy intake for this species. Together, the three Cichla species consume a wide spectrum of prey from a diverse fish assemblage, but prey are subdivided based on habitat, prey type, and season.

2002. Structure of tropical river food webs revealed by stable isotope ratios. -Oikos 96: 46 -55.Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre-identified trophic category. Nutrient-poor, black-water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish d 15 N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient-poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species-level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.

The development of miniature acoustic transmitters and economical, robust automated receivers has enabled researchers to study the movement patterns and survival of teleosts in estuarine and ocean environments, including many species and age‐classes that were previously considered too small for implantation. During 2001–2003, we optimized a receiver mooring system to minimize gear and data loss in areas where current action or wave action and acoustic noise are high. In addition, we conducted extensive tests to determine (1) the performance of a transmitter and receiver (Vemco, Ltd.) that are widely used, particularly in North America and Europe and (2) the optimal placement of receivers for recording the passage of fish past a point in a linear‐flow environment. Our results suggest that in most locations the mooring system performs well with little loss of data; however, boat traffic remains a concern due to entanglement with the mooring system. We also found that the reception efficiency of the receivers depends largely on the method and location of deployment. In many cases, we observed a range of 0–100% reception efficiency (the percentage of known transmissions that are detected while the receiver is within range of the transmitter) when using a conventional method of mooring. The efficiency was improved by removal of the mounting bar and obstructions from the mooring line.

Hoeinghaus DJ, Winemiller KO, Layman CA, Arrington DA, Jepsen DB. Effects of seasonality and migratory prey on body condition of Cichla species in a tropical floodplain river. Abstract -In this study we examine trends in body condition of three Cichla species native to the Cinaruco River, Venezuela. Specifically, we characterise the potential subsidy provided by migratory Semaprochilodus kneri to Cichla by comparing condition among species, hydrologic seasons and different size-classes (based on the ability to consume S. kneri assuming gape-limitation). All three species had lowest body condition during the rising-water season (following the reproduction period). Only Cichla temensis had greater body condition during the falling-water season than low-water season. During the falling-water season, C. temensis >300 mm SL had significantly greater condition than smaller conspecifics. Annual migration of S. kneri into the river during the falling-water season provides a resource subsidy to large C. temensis. Transfer of whitewater production into nutrient-poor rivers by migratory fishes appears to be a general phenomenon in the major Neotropical river basins and may partially account for high standing stocks of large piscivores in these unproductive ecosystems.

We analysed stable carbon and nitrogen isotopic ratios of dissolved inorganic carbon (DIC), plants, detritus and fishes to estimate the relative importance of dominant production sources supporting food webs of four Venezuelan rivers with divergent geochemical and watershed characteristics. Based on samples taken during the dry season at each site, fishes from two nutrient-poor, blackwater rivers had significantly lower d 13 C values (mean ¼ )31.4& and )32.9&) than fishes from more productive clearwater and whitewater rivers (mean ¼ )25.2& and )25.6& respectively). Low carbon isotopic ratios of fishes from blackwaters were likely influenced by low d 13 C of DIC assimilated by aquatic primary producers. Although floodplains of three savanna rivers supported high biomass of C 4 grasses, relatively little carbon from this source appeared to be assimilated by fishes. Most fishes in each system assimilated carbon derived mostly from a combination of microalgae and C 3 macrophytes, two sources with broadly overlapping carbon isotopic signatures. Even with this broad overlap, several benthivorous grazers from blackwater and whitewater rivers had isotopic values that aligned more closely with algae. We conclude that comparative stable isotopic studies of river biota need to account for watershed geochemistry that influences the isotopic composition of basal production sources. Moreover, isotopic differences between river basins can provide a basis for discriminating spatial and temporal variation in the trophic ecology of fishes that migrate between watersheds having distinct geochemical characteristics.

We estimated the survival of juvenile salmonids out-migrating through the lower Columbia River to the Pacific Ocean. We tested the null hypotheses that no association exists between survival and transportation type (including barge transportation and in-river migration with no transportation), release date, river flow, mean body weight, and tag type. During 2002-2004 and fall Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss were implanted with uniquely coded radio or acoustic transmitter tags and released during the early, middle, and late out-migration periods. A series of receiver lines were used to detect these fish and estimate survival. Estimated survival varied considerably with transportation type and release date for fall Chinook salmon and steelhead, but the results were dependent upon year. Estimated survival between the lowermost dam (Bonneville Dam) and the upper estuary (river kilometer 46) was relatively high for SS Chinook salmon and steelhead, whereas estimates for fall Chinook salmon were comparatively low. This order of high to low survival for SS Chinook salmon, steelhead, and fall Chinook salmon corresponds with the chronology of out-migration timing and waning river flows. After accounting for a number of factors, estimated survival between Bonneville Dam and the upper-estuary site did not differ significantly between steelhead that were barged and those that migrated in-river during 2002 and 2003. Conversely, estimated survival to the upper-estuary site was quite low for barged fall Chinook salmon in comparison with fish migrating in-river during the middle and late release dates in 2003. Hypotheses for the low survival of barged fall Chinook salmon are presented and discussed.