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To advance current environmental flow standards in Texas, we developed field, laboratory, and analytical methods to model recruitment dynamics of indicator species selected to cover a range of population responses to flow variation. Preliminary investigations were conducted in three central Texas river basins to evaluate the feasibility of these methods for formulating and evaluating the state's environmental flow standards. Age analysis allowed back‐calculation of dates of fish hatching, mussel recruitment, and tree germination for estimation of temporal variation in recruitment. Recruitment data were then analyzed in relation to flow regime components. Statistical models were derived for flow–recruitment relationships of Shoal Chub Macrhybopsis hyostoma, Smallmouth Buffalo Ictiobus bubalus, threeridge mussel Amblema plicata, and boxelder Acer negundo. The Shoal Chub model was used to hindcast recruitment dynamics in the Brazos River, revealing a decline in recruitment after dam construction. The Smallmouth Buffalo model revealed that strong year‐classes were positively associated with springtime pulses of long duration but negatively associated with frequent short‐duration pulses. Results for the threeridge mussel were not as easily interpreted, but for all three river basins generally showed weaker recruitment during periods with extreme low and high flows. The boxelder model revealed positive associations between recruitment and high flow components and was used to forecast the probability of germination under five simulated flow regime scenarios, thereby providing an additional means for evaluating the suitability of environmental flow standards. Assessment of how hydrology influences recruitment provides an evidence‐based method to hindcast responses to past flow regimes and forecast responses to future flow scenarios, thereby providing a basis for evaluating environmental flow standards and water management options.

Regulation of river flow regimes by dams and diversions impacts aquatic biota and ecosystems globally. However, our understanding of the ecological consequences of flow alteration and ecological benefits of flow restoration lags behind our ability to manipulate flows, and there is a need for broader development of flow–ecology relationships. Approaches for establishing flow–ecology relationships have recently shifted away from state‐based methods that analyze snapshots of ecological conditions and towards rate‐based methods focused on mechanisms that link hydrology with dynamics of important ecological components and processes. We used a rate‐based approach to validate environmental flow standards developed for the lower Brazos River, Texas, by analyzing the relationship between flow regime components and recruitment strength of imperiled Shoal Chub Macrhybopsis hyostoma, a fluvial specialist and pelagic‐broadcast‐spawning fish. We collected 254 age‐0 Shoal Chub (9–40 mm total length), extracted their otoliths to estimate age in days, and used a generalized additive model to regress the number of captured recruits that hatched on a calendar date against flow regime metrics, such as pulse magnitude, flow rate of change, and pulse timing in relation to environmental flow standards proposed by a science advisory committee (Brazos Basin and Bay Area Expert Science Team). The model revealed that flow magnitude, rate of change, and timing were all significant predictors that collectively explained 60% of variation in the recruitment strength index. Hindcasting for 1919–2020 showed a general reduction in recruitment strength following commencement of flow regulation in the lower Brazos River and revealed that high recruitment correlated with years in which most or all proposed flow tiers were attained, whereas low recruitment correlated with years when less than half of the targeted tiers were attained. Our work represents an effective validation method for environmental flow recommendations and reveals specific flow regimes that benefit an imperiled fish species. Impact statementEnvironmental flows science guides reservoir releases to promote river water security for humans and nature, but natural water needs are challenging to estimate. We derive flow–recruitment relationships for Shoal Chub and validate environmental flow standards in a regulated river. Environmental flows science guides reservoir releases to promote river water security for humans and nature, but natural water needs are challenging to estimate. We derive flow–recruitment relationships for Shoal Chub and validate environmental flow standards in a regulated river.

Headwater streams and wetlands are integral components of watersheds that are critical for biodiversity, fisheries, ecosystem functions, natural resource‐based economies, and human society and culture. These and other ecosystem services provided by intact and clean headwater streams and wetlands are critical for a sustainable future. Loss of legal protections for these vulnerable ecosystems would create a cascade of consequences, including reduced water quality, impaired ecosystem functioning, and loss of fish habitat for commercial and recreational fish species. Many fish species currently listed as threatened or endangered would face increased risks, and other taxa would become more vulnerable. In most regions of the USA, increased pollution and other impacts to headwaters would have negative economic consequences. Headwaters and the fishes they sustain have major cultural importance for many segments of U.S. society. Native peoples, in particular, have intimate relationships with fish and the streams that support them. Headwaters ecosystems and the natural, socio‐cultural, and economic services they provide are already severely threatened, and would face even more loss under the Waters of the United States (WOTUS) rule recently proposed by the Trump administration.

High‐flow pulses affect river ecosystem dynamics in many important ways including by forming connections between the channel and oxbow lakes. This study assessed the influence of discharge on fish assemblage structure and diversity in the channel and oxbows of the Guadalupe River, Texas. Local assemblages of two oxbows and two channel sites were surveyed using standardized methods to test four hypotheses: (1) assemblage structure in oxbow lakes differs from those in the river channel, (2) α diversity decreases during extended periods of low discharge, (3) β diversity decreases during high discharge and increases during extended periods of low discharge, and (4) species turnover and assemblage nestedness decline during periods of high discharge. We found evidence to support the first three hypotheses. Unsurprisingly, lotic‐adapted fishes were observed more frequently in the river channel, whereas lentic‐adapted species generally were more common in the oxbow lakes. Species richness declined during periods of low discharge possibly due to harsher environmental conditions or stronger species interactions (e.g., predation). Discharge was inversely associated with both β diversity and species turnover, suggesting a stronger mass effect during high‐flow pulses, and stronger species sorting during low‐flow conditions. Contrary to our fourth hypothesis, assemblage nestedness increased during periods of high discharge. Finally, we also found evidence to support the hypothesis that species turnover decreased as discharge declined. The results of this study demonstrate the importance of flow pulses for maintaining fish diversity and assemblage structure in floodplain river systems. With additional research involving more sites and longer time series, it should be feasible to define and identify thresholds for flow regime changes that alter assemblage structure and species diversity.

AbstractFrog diets are influenced by multiple factors, including morphological constraints, habitat use, and seasonal variation in environmental conditions and food availability. This study combined stomach content analysis (SCA), stable isotope analysis (SIA), and estimates of prey availability to investigate the influence of body size and microhabitat use on seasonal variation of the trophic ecology of two sympatric hylids (Pseudis minutaand Scinax squalirostris). We evaluated two hypotheses: (1) the species with larger body and mouth sizes or broader use of microhabitats will have greater diet breadth, and (2) regardless of differences in morphological traits and microhabitat use, diet breadth of both species will be greater during the warmer of two periods. Pseudis minutaexhibited larger body size and mouth width and revealed broader use of microhabitats (mostly within and near major water bodies), whereas S. squalirostrishad smaller body size and mouth gape and was found exclusively within or near phytotelmata (plant-held water bodies). SCA revealed that P. minutahad a more diverse diet than S. squalirostris. Only P. minutashowed temporal dietary differences, but these findings did not corroborate our prediction of greater diet diversity during the warmer and drier period when prey densities were higher. The two species had distinct carbon and nitrogen stable isotope ratios, indicating assimilation of different resources, except during the colder wetter season when their isotope spaces overlapped partially. We concluded that the two hylids did not use the same food resources on account of their differences in morphology and microhabitat use, and environmental seasonality did not influence their feeding strategies.

AbstractSeasonal flow pulses in rivers facilitate spawning, dispersal, and early life stage survival of many fish species. To evaluate the effectiveness of current flow standards to sustain threatened fish populations, we investigated the relationship between hydrology and recruitment of the Shoal Chub Macrhybopsis hyostoma, a broadcast-spawning minnow in the Brazos River, Texas. From March 2013 to March 2014, we collected metalarval and juvenile Shoal Chub bimonthly at night using arrays of stationary drift nets. Otoliths were examined to estimate age, and the relationship between hatch date and discharge was analyzed. Shoal Chub recruited under both base-flow and pulse-flow conditions, including intervals of increasing, decreasing, and stable discharge. However, hatch dates of surviving fish indicated greater levels of recruitment during flow pulses, particularly on the rising limb. Greatest recruitment occurred during flow pulses of a magnitude defined as two per season according to the method of hydrological analysis adopted by the state’s environmental flow program. Our findings imply that the state’s current environmental flow standards for the lower Brazos River may be insufficient to sustain Shoal Chub populations and additional research on this issue is warranted.Received October 11, 2015; accepted March 30, 2016 Published online July 28, 2016

Seasonal flow pulses in rivers facilitate spawning, dispersal, and early life stage survival of many fish species. To evaluate the effectiveness of current flow standards to sustain threatened fish populations, we investigated the relationship between hydrology and recruitment of the Shoal Chub Macrhybopsis hyostoma, a broadcast-spawning minnow in the Brazos River, Texas. From March 2013 to March 2014, we collected metalarval and juvenile Shoal Chub bimonthly at night using arrays of stationary drift nets. Otoliths were examined to estimate age, and the relationship between hatch date and discharge was analyzed. Shoal Chub recruited under both base-flow and pulse-flow conditions, including intervals of increasing, decreasing, and stable discharge. However, hatch dates of surviving fish indicated greater levels of recruitment during flow pulses, particularly on the rising limb. Greatest recruitment occurred during flow pulses of a magnitude defined as two per season according to the method of hydrological analysis adopted by the state's environmental flow program. Our findings imply that the state's current environmental flow standards for the lower Brazos River may be insufficient to sustain Shoal Chub populations and additional research on this issue is warranted. Received October 11, 2015; accepted March 30, 2016 Published online July 28, 2016

During and following lateral connections, aquatic organisms residing in the river channel may assimilate material from sources imported from oxbows, and oxbow residents may consume and assimilate material imported from the channel. Hydrology, lateral connectivity, and stable isotope ratios of fishes and mussels were analyzed for evidence of spatial food web subsidies between the active channel and oxbow lakes in the floodplain of the Guadalupe River, Texas. During surveys conducted between March 2016 and April 2017, fish, mussel, periphyton, seston, and riparian plant samples were collected in and around two oxbows and adjacent channel sites for analysis of stable isotope ratios. Biplots of δ13C and δ15N were graphed for basal sources and specimens of six common fish species, four sunfish species (Lepomis spp.combined), and two mussel species (Unionidae combined) captured from oxbows and the channel. Within each graph, polygons were drawn to indicate the space occupied by animals that could have assimilated feasible combinations of source materials originating from either oxbows or the river channel. Based on positions of animals within source polygons, riparian C4 grasses were not an important source of organic matter supporting biomass of fishes and mussels within the channel or oxbows. Overall, 84% of organisms had isotopic signatures consistent with assimilation of in situsources, but also 76% of all organisms were inconclusive with regards to cross-habitat exchanges. Outliers that may have assimilated ex situsource material were observed for only 4% of 313 organisms from oxbows and 9% of 232 organisms from the channel, and some but not all of these cases followed high flow pulses that connected oxbows for extended periods. Several issues that compromise inferences from stable isotope analysis were identified, and estimation of spatial food web subsidies in fluvial systems could be enhanced by analyzing additional biomarkers, such as isotopic ratios of other elements and compound-specific stable isotopes, as well as additional sources, time-specific biotracers, and experimental approaches that directly track movement of sources and organisms in spatially structured food webs.

We investigated the influences of local and landscape-scale environmental variables on fish assemblage structure among 64 stream reaches in two large river basins in central Texas. The broad spatial extent of this study region provided an opportunity to examine fish assemblage–environment relationships at multiple scales across a range of stream types in landscapes exposed to varying degrees of anthropogenic alteration. We used nonmetric multidimensional scaling (NMS) combined with permutational analysis of variance, k-means cluster analysis, and indicator species analysis to evaluate broad-scale influences of ecoregional and large river basin boundaries on fish assemblage structure. We also estimated relationships between fish assemblage structure and environmental factors with NMS and rotational vector fitting across all ecoregions and within ecoregions. Ordinations of sites based on species composition grouped stream reaches together according to ecoregion, and k-means clustering identified three groups that corresponded with ecoregional membership. Several species had high affinities with specific ecoregions, a pattern that tracked broad-scale physiographic differences in climate, topography, terrestrial vegetation, and instream habitat. Within ecoregions, we observed that local-scale stream habitat variables as well as larger-scale landscape features were significantly related to fish assemblage composition. Substrate composition was a key local-scale habitat factor, and a gradient of rocky substrate to predominance of mud and silt correlated strongly with assemblage structure within all three ecoregions. The abundance of instream woody debris was also an important local-scale correlate for fish assemblage structure. At the landscape scale, patterns of agricultural and urban land development in the surrounding watersheds were consistently associated with fish assemblage structure in each ecoregion. This study adds important information toward a better understanding of how environmental factors structure fish assemblages across scales, which should facilitate refinement of existing habitat and biological indices for conservation of stream habitats and their biota.

Ecosystem processes and biological community structure are expected to change in a relatively predictable manner along fluvial gradients within river basins. Such predictions are heavily based on temperate rivers, and food web variation along fluvial gradients in Mesoamerican rivers has received limited attention. In this study, we analyzed carbon and nitrogen stable isotope ratios of basal carbon sources and dominant consumer species to examine aquatic food web structure along the fluvial gradient of the Monkey River Basin, Belize. Similar to previous studies in other regions, consumer species richness and functional diversity increased along the downstream fluvial gradient, due in part to the addition of estuarine species in lower reaches and increasing diversity of piscivorous species along the gradient. Aquatic food webs in upstream reaches were primarily supported by allochthonous production sources, and in‐stream sources increased in importance along the downstream gradient. Our study system traversing the Maya Mountain Marine Area Transect also provided a unique opportunity to test the utility of primary consumer δ15N as an indicator of watershed impacts within a tropical basin with a diverse biota and a different type of agricultural impact than typically studied (i.e. banana plantations vs. tilled row cropping). As expected, primary consumer δ15N at sites draining impacted watersheds was enriched compared to values from forested reference sites. Assessment of primary consumer δ15N may be a feasible option for monitoring watershed impacts on aquatic food webs in service of the ridge‐to‐reef conservation strategy adopted for this watershed as well as in other tropical river basins. Copyright © 2010 John Wiley & Sons, Ltd.

Seven sandbanks in the main channel of the Brazos River, Texas, were sampled a total of 35 times during both day (1800–1900 hours) and night (2200–2300 hours) in June and July 2008 to examine the diel turnover of fish assemblages. Day samples had an average of 10.1 species, whereas night samples had an average of 12.9 species. Average abundance (catch per unit effort, here defined as the number of individuals per 10-m seine haul) for day samples was 41.5, compared with 80.5 for night samples. Species richness and abundance were significantly greater on sandbanks at night. Additionally, nonmetric multidimensional scaling analysis and cluster analysis indicated consistent differences in assemblage structure between day and night samples. Most of the diel change in assemblage structure was due to ictalurids and palaemonids (freshwater decapod crustaceans) that were only common on sandbanks nocturnally. Catastomids, clupeids, cyprinids, and poeciliids were present on sandbanks both diurnally and nocturnally. Species that exclusively use sandbank habitats may be foraging during the day and seeking refuge from predators at night. Nocturnal species apparently move onto sandbank habitats at night for foraging and then migrate to deeper, more structurally complex habitats during the day to escape predation. Diel turnover of sandbank fish assemblages may be a general phenomenon in lotic ecosystems, but it is apparently not as pronounced in temperate rivers as in tropical rivers. Diel changes in habitat use deserve greater study because species assemblage structure and habitat use are the basis for ecological niche models used to predict species distributions based on climate change scenarios as well as physical habitat simulation models designed to determine instream flows.

Three oxbow lakes with different connection frequencies and an adjacent reach of the middle Brazos River, Texas, were surveyed quarterly from summer 1993 to summer 1996 to examine the effects of hydrology and physicochemical attributes on fish assemblage structure. During flood events, oxbows usually were colonized by about 20 fish species from the river channel, and several fluvial specialists were rarely or never sampled from oxbows. Multivariate analyses of seine samples revealed divergent patterns of assemblage structure during periods of isolation that also were associated with a gradient of maximum water depth, temperature, and conductivity. In contrast, analysis of gill-net samples that targeted large fishes revealed low between-site and temporal variation in assemblage structure among oxbow lakes but significant differences in the structure of oxbow versus channel assemblages. The shallowest oxbow (which dried out with greatest frequency) had the most variable fish assemblage across seasons and was dominated by small colonizing species, whereas deeper oxbows and the river channel tended to have higher species richness and more stable assemblages. Despite large differences in connection frequency and distance from the river channel, the deepest oxbows had the most similar fish assemblage structure across all seasons. Multiple regression analysis suggested that the timing and frequency of flood events (yielding colonization) in the most frequently connected oxbow interact with predator abundance (yielding mortality) to influence the abundance of common lotic-adapted minnows (red shiner Cyprinella lutrensisand bullhead minnow Pimephales vigilax) that periodically dominated the species assemblage of that oxbow after floods. Our results suggest that current hydrologic and geomorphologic dynamics in the middle Brazos River produce oxbow lakes with a range of physical characteristics yielding different disturbance and colonization regimes that strongly influence fish species assemblages.

We evaluated the effect of salt and formalin-ethanol sample preservation on carbon and nitrogen isotopic signatures of fish muscle tissue. We found statistically significant effects of the tissue preservation technique on both d13C and d15N; however, the magnitude of change was small and directionally uniform. Isotopic shifts were similar to those observed in previous studies in which formalin was used to preserve samples of quail blood and muscle and sheep blood. Because salt preservation caused minimal isotopic shifts (+0.13‰ d13C, +0.72‰ d15N), we propose salt as an easy, inexpensive preservation technique for biological samples collected in remote field settings. Specimens preserved with formalin and ethanol were minimally affected by preservation (-1.12‰ d13C, +0.62‰ d15N) and therefore may be suitable for ecological applications of stable isotope analysis when carbon and nitrogen sources are differentiated by more than 2‰. Further research is required to evaluate potential long-term storage effects of formalin fixation and alcohol preservation on isotopic signatures of fish tissues.

Fish assemblages and habitat conditions of oxbow lakes and the main channel of the middle-lower Brazos River, a meandering lowland river in east central Texas, were investigated during summer 1994. All oxbows were eutrophic, with chlorophyll-alevels of up to 640 µg/L. Assemblage structure showed large between-lake variation that was explained by both physical and biotic variables, with combinations of water depth, dissolved oxygen, dissolved nutrients, turbidity, and plankton densities accounting for 45–59% of the variation in abundance of the dominant species. Water depth and dissolved nutrient concentrations were the best predictors of species diversity and fish abundance. Periodic desiccation of shallow, vegetated oxbows created harsh conditions that favored small fishes that are efficient colonizers. The two youngest oxbows were relatively deep and contained a high diversity and biomass of fishes. Of the 42 fish species collected, several were largely restricted to oxbow lakes, and others were either entirely restricted to or common only in the river channel. The flood dynamics of Brazos River floodplain habitats are less predictable (both intra- and interannually) than are those of large temperate rivers that receive runoff from snowmelt or predictable spring rainfall. As a result, Brazos River oxbow lakes remain separated from the river channel for many months or years, such that faunal exchange between oxbows to the channel should be pulselike and irregular.

Ten traits related to life history theory were measured or estimated for 71 freshwater fish species from two locations in the Venezuelan llanos. Multivariate statistics and cluster analysis revealed three basic endpoint patterns bounding a two-dimensional continuum. A suite of attributes associated with parental care and aseasonal reproduction appeared to correspond to an equilibrium strategy. A second group of small fishes was distinguished by traits associated with rapid colonizing ability: early maturation, continuous reproduction, and small clutches. The third basic pattern was associated with synchronized reproduction during the early wet season, high fecundity, absence of parental care, and breeding migrations. A subset of mostly small fishes exhibiting little or no parental care, small clutches, and two to four month reproductive seasons was intermediate between the opportunistic (rapidly colonizing) and seasonal strategies. All ten life history variables showed significant effects of phylogeny. The cluster of species corresponding to the equilibrium group was dominated by siluriform fishes and perciforms of the Cichlidae. The opportunistic cluster was dominated by cyprinodontiform and characiform fishes, whereas the seasonal cluster contained primarily characiform and siluriform fishes. Seven of nine traits were significantly correlated with body length. The three reproductive patterns are interpreted as being adaptative with respect to relative intensity and predictability of temporal and spatial variation in abiotic environmental parameters, food availability, and predation pressure.

Abstract:The feeding ecology and morphological diversification of fish assemblages of two coastal lakes in southern Benin (West Africa) were examined to compare patterns of community organization. Though located only 18 km apart, the two fish assemblages had dissimilar species composition, and this is largely derived from differences in connectivity to the Gulf of Guinea, salinity and pH. Among the 35 and 20 species sampled from Lake Nokoue (connected with a sea) and Lagoon Toho-Todougba (no sea connection), respectively, only a few species dominated each system. Lake Nokoue was dominated by the perciform Gerres melanopterusand the clupeiform Ethmalosa fimbriata, and Lagoon Toho-Todougba was dominated by the characiform Brycinus longipinnis.In Lake Nokoue, the akadja habitat (dense stands of woody debris installed by humans to attract fishes) had high species richness compared to the other lake habitats. Multivariate procedures were used to examine trophic guilds and the relationship between morphology and diet. Overall, thirteen trophic guilds were identified, with detritivore and piscivore guilds having the most species. Lagoon Toho-Todougba was dominated by detritivorous and insectivorous species and Lake Nokoue, a larger lake with a connection to the sea, was dominated by piscivores. In Lagoon Toho-Todougba, consumption of vegetation and insects was common, but these items were very rare in Lake Nokoue fish diets, possibly because the destruction of its fringing mangroves has reduced their availability for fishes. Despite major differences in taxonomic composition, the two fish assemblages had similar mean values for most morphological traits and morphological-based centroids from multivariate analyses were remarkably similar. Because members of the same feeding guild tended to have similar morphologies, some degree of diet specialization has accompanied morphological diversification in the taxa comprising these communities.

Guri Reservoir in Venezuela supports sport fisheries for two cichlid species (pavón venado or speckled pavon Cichla temensisand pavón mariposa C. orinocensis) and payara Hydrolycus scomberoides.The lake receives acidic, hyperoligotrophic “black waters” from the Caroní River and experiences water level fluctuations associated with seasonal precipitation and hydroelectric operations. During the early 1980s, piscivore populations expanded in the new reservoir, but the quality of sportfishing declined during the 1990s. During 1993–94, we studied the ecology of the two Cichlaspecies, H. scomberoides, and a fourth piscivore, Plagioscion squamosissimus(curvinata or silver croaker) and compared our data with those from similar surveys performed 7–9 years earlier (0–2 years after attainment of the reservoir's current crest height). Seine and gill-net samples from the lake's northern region produced 50 fish species from 18 families. Seine samples were dominated by a small characid, Hemigrammus micropterus, whereas gill-net samples were dominated by a large detritivorous characiform, Prochilodus rubrotaeniatus.Gill-net catch rates were low in all three surveys. All four piscivores from our recent survey had better body condition than in a 1985–1986 study, an indication of possible growth compensation. Diet breadths of all piscivores were low, and diet overlap was low for H. scomberoideswith the two Cichlaspecies and for P. squamosissimuswith C. orinocensis. Hydrolycus scomberoidesand C. temensisconsumed significantly larger prey than P. squamosissimus, and differences in the relative proportions of specific characid and cichlid fishes consumed by the two Cichlaspecies indicate that they forage at different depths within the littoral zone. The combined effects of hyperoligotrophic waters entering the reservoir, reduced inputs of dissolved nutrients from submerged terrestrial vegetation, low retention time of the reservoir, and harvest probably influence the low abundance of the two Cichlaspecies in Guri Reservoir.

The distribution, habitat association, group size, population structure, and prey availability of river dolphins (Inia geoffrensis) were studied from November 1993‐June 1994 in the Cinaruco River, a tributary of the Orinoco River that forms the southern boundary of Venezuela's Santos Luzardo National Park. Dolphins were sampled from a boat using modified strip‐width transects, for a total of 418 h. The study area was 1.67 km2, and contained 20 km of water courses. Like other rivers of this region, the Cinaruco River undergoes a seasonal flood cycle. Dolphins were seen most often during the period of falling water (41% of total sightings) and least often during the rising water period (24% of total sightings). Dolphins were seen most often in confluence areas (35% of total sightings) and were seldom seen in side channels (13% of total sightings). The presence of rocks or sandbanks was associated with a greater frequency of dolphin sightings, and sightings increased with habitat heterogeneity. Average group size for the 8‐mo study was 2.0 (±1.0) and was largest during the rising water period. Calves were first sighted during the end of the dry season and became more common during the early flood season. Six individuals were photo‐identified and resighted with one sighted eight times over 186 d. The fish diversity of the study area was high, with 161 species documented in our samples. The stomach of one Iniacontained 15 fishes representing at least 4 species.

Reproductive ecology, population structure, and diets of three common livebearing poeciliid fishes (Alfaro cultratus, Phallichthys amates, Poecilia gilli) from rainforest streams in Costa Rica were investigated over ten continuous months. The region experiences little annual temperature variation, and although monthly rainfall is continuous each year, two brief dry seasons typically occur. Monthly changes in indices of ovarian condition, percentages of females with developing embryos, and population size structure revealed that reproductive output by females of all three species varied seasonally. Based on testicular condition, males were reproductively active year-round, however the mean gonadal index for males of two algivorous species showed low levels of seasonal cycling that largely coincided with female variation in reproductive effort. All three species had seasonal differences in the female size-brood size relationship, whereby larger females tended to carry more embryos during the wet season. Several important adult and neonate food resources are more available in the flooded forest during the wet season, which is also the period when conspecifics and predators are at their lowest per-area densities. Three hypotheses are discussed: (1) brood size in relation to conspecific density-mating frequency, (2) reproductive allocation in response to variation in adult food resources, and (3) selection for greater reproductive effort during conditions optimal for juvenile growth and survival. Data for Alfaro were consistent with the latter two hypotheses. In Phallichthys and Poecilia, diets were poorer during wet seasons, indicating that reproductive effort does not coincide with availability of adult food resources, and that selection probably favors greater reproductive effort during periods optimal for juvenile growth and survival.

We link life history theory, environmental setting, and individual-based modeling to compare the responses of two fish species to environmental change, Life history theory provides the framework for selecting representative species, and in combination with information on important environmental characteristics, it provides the framework for predicting the results of model simulations. Individual-based modeling offers a promising tool for integrating and extrapolating our mechanistic understanding of reproduction, growth, and mortality at the individual level to population-level responses such as size-frequency distributions and indices of year-class strength. Based on the trade-offs between life history characteristics of striped bass Morone saxatilisand smallmouth bass Micropterus dolomieuand differences in their respective environments, we predicted that young-of-year smallmouth bass are likely to demonstrate a greater compensatory change in growth and mortality than young-of-year striped bass in response to changes in density of early life stages and turnover rates of zooplankton prey. We tested this prediction with a simulation experiment. The pattern of model results was consistent with our expectations: By the end of the first growing season, compensatory changes in length and abundance of juveniles were more pronounced for smallmouth bass than for striped bass. The results also highlighted the dependence of model predictions on the interplay between density of larvae and juveniles and characteristics of their zooplankton prey.