Patterns of dispersal in riverine populations of Australian smelt ( Retropinna semoni ) were examined using otolith chemistry (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) and genetic markers (allozymes, mtDNA, microsatellite DNA). During a period of prolonged low flows, young-of-year smelt were collected from 13 streams within three catchments of the southern Murray–Darling Basin, Australia. Spatial differences in otolith core and edge chemical signatures and high levels of genetic assignment to sampling locations were observed, suggesting that most recruits were retained in natal areas after spawning. Following a subsequent period of hydrological connection, the same cohort was sampled as 1-year-olds. Maximum likelihood estimation using otolith core chemistry data from these fish suggested that retention in natal areas was highly variable between years and a similar, though less pronounced, pattern was evident in genetic assignments. Partitioning of genetic variation among catchments was not significant (FCT < 0.004) and probably reflects disequilibrium between migration and genetic drift due to an historical population expansion (~270 000 years ago). Taken together, otolith chemistry and genetic analyses suggest that contemporary dispersal of smelt within these catchments is relatively restricted and may be mediated by changes in hydrological connectivity.

Few studies measure multiple ecological tracers in individual organisms, thus limiting our ability to differentiate among organic matter source pathways and understand consequences of dietary variation and the use of external subsidies in complex food webs. We combined two tracers, stable isotope (SI) ratios and fatty acids (FA), to investigate linkages among ecological compartments (water column, benthos, riparian zone) in food webs in waterholes of a dryland river network, the Border Rivers in southwestern Queensland, Australia. Comprehensive analyses of sources (plankton, periphyton, leaf litter, riparian grasses) and animals (benthic insects, mollusks, large crustaceans, fishes) for SI and FA showed that all three zones contribute to animal biomass, depending on species and life stage. Large fishes derived a subsidy from the riparian/floodplain zone, likely through the consumption of terrestrial and semi-aquatic insects and prawns that fed on detritivorous insects. Importantly, post-larval bony bream (Nematalosa erebi) and golden perch (Macquaria ambigua) were tightly connected to the water column, as evidenced by 13C-depleted, 15N-enriched isotope ratios and a high content of plankton-derived polyunsaturated fatty acids (eicosapentaenoic acid [EPA; 20:53] and docosahexaenoic acid [DHA; 22:6003]). These observations were consistent with expectations from nutritional requirements of fish early life stages and habitat changes associated with maturity. These results highlight the importance of high-quality foods during early development of fishes, and show that attempting to attribute food-web production to a single source pathway overlooks important but often subtle subsidies that maintain viable populations. A complete understanding of food-web dynamics must consider both quantity and quality of different available organic matter sources. This understanding can be achieved with a combined SI and FA approach, but more controlled dietary studies are needed to estimate how FA profiles are modified by animals when consuming a diverse range of diets of variable quality.

Summary Amphidromy, the dispersal of larvae to the estuarine or marine environment with juveniles recolonising fresh waters to complete development, is common on tropical islands. This has led to the suggestion that amphidromy is an adaptation to hydrologically unpredictable environments, allowing recolonisation after local extinction (dispersal‐limitation hypothesis). Alternatively, amphidromy may be more common in streams with stronger flow, as high tractive forces flush larvae into the estuarine/marine environment, forcing them to recolonise (passive dispersal hypothesis). We tested these hypotheses in a continental gobiid fish, the Roman Nose Goby (Awaous acritosus) that inhabits hydrologically and physically diverse catchments on the east coast of Australia. We measured 87Sr/86Sr along transects of the otoliths of 92 adult fish from 12 catchments, identified migratory life histories from these data and correlated the proportion of migratory fish with key environmental variables. This information was augmented with data from mtDNA (n = 276) and six microsatellite loci (n = 429) from 19 catchments, to explore gene flow among catchments and to assess whether local adaptation was likely in the face of high or restricted gene flow. Estimates of 87Sr/86Sr along transects revealed high variation in life history, with 63% of individuals showing no evidence of marine/estuarine residence. The slope of catchments was the only significant variable associated with the proportion of larvae retained in catchments, with steep catchments having a greater proportion of adults exhibiting a marine/estuarine phase during early life. Total panmixia was detected with mtDNA and six microsatellite loci, suggesting that gene flow is high for this species. These results support the passive dispersal hypothesis. Steep slopes are likely to have higher tractive forces in their flow, leading to a greater proportion of larvae being flushed into the estuary or sea. In addition, high gene flow would inhibit local adaptation, making the dispersal‐limitation hypothesis unlikely. This study provides a framework for understanding how local adaptation may be constrained by gene flow across the landscape so that the adaptive or passive mechanism underlying facultative amphidromy can be explored.

Although considerable knowledge has been gathered regarding the role of fish in cycling and translocation of nutrients across ecosystem boundaries, little information is available on how the energy obtained from different ecosystems is temporally allocated in fish bodies. Although in theory, limitations on energy budgets promote the existence of a trade-off between energy allocated to reproduction and somatic growth, this trade-off has rarely been found under natural conditions. Combining information on RNA:DNA ratios and carbon and nitrogen stable-isotope analyses we were able to achieve novel insights into the reproductive allocation of diamond mullet (Liza alata), a catadromous, widely distributed herbivorous-detritivorous fish. Although diamond mullet were in better condition during the wet season, most reproductive allocation occurred during the dry season when resources are limited and fish have poorer body condition. We found a strong trade-off between reproductive and somatic investment. Values of δ13C from reproductive and somatic tissues were correlated, probably because δ13C in food resources between dry and wet seasons do not differ markedly. On the other hand, data for δ15N showed that gonads are more correlated to muscle, a slow turnover tissue, suggesting long term synthesis of reproductive tissues. In combination, these lines of evidence suggest that L. alata is a capital breeder which shows temporal uncoupling of resource ingestion, energy storage and later allocation to reproduction.