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Several hypotheses and conceptual models propose to explain mechanisms mediating riverine fish abundance, but few empirical studies to date have explored their utility in tropical systems. This study assesses key components of previous fish recruitment models by exploring spatiotemporal variation in larval fish assemblages in response to predicted key drivers in a tropical Australian river catchment. Data on larval fish composition and abundance, alongside hydrological, hydraulic, habitat and food variables, were collected monthly to bimonthly over one year at eight sites. Variables which best predicted larval fish abundance and diversity were determined with Boosted Regression Trees. The most commonly important predictors were microfauna abundance, structural habitat complexity and temperature, with high values of each predicting high larval fish abundance and diversity. Maximum larval diversity occurred when discharge was highest because several wet-season spawning taxa occurred alongside aseasonally spawning taxa. These findings support previous generic fish recruitment models, demonstrating the utility of their inclusion in the recent Riverine Recruitment Synthesis Model and the applicability of this model for describing processes important for tropical riverine fish recruitment. Key words: habitat complexity, zooplankton, hydrology, Daly River, northern Australia Plusieurs hypotheses et modeles conceptuels proposent d'expliquer les mecanismes qui modulent l'abondance des poissons de riviere, mais peu d'etudes empiriques a ce jour ont explore leur utilite pour les systemes tropicaux. La presente etude evalue des composants cles de modeles anterieurs de recrutement de poissons en explorant les variations spatiotemporelles d'assemblages de larves de poisson en reponse a des facteurs d'influence cles predits dans un bassin versant australien tropical. Des donnees sur la composition et l'abondance de larves de poisson, ainsi que sur des variables hydrologiques, hydrauliques et reliees a l'habitat et a la nourriture ont ete recueillies sur une base mensuelle et bimensuelle pendant un an dans huit sites. Les variables qui predisent le mieux l'abondance et la diversite des larves de poisson sont determinees a l'aide d'arbres de regression stimules. Les predicteurs les plus souvent importants sont l'abondance de la microfaune, la complexite structurale des habitats et la temperature, des valeurs elevees de ces variables predisant une grande abondance et une grande diversite de larves de poisson. La diversite de larves maximum se produit quand le debit est le plus important, puisque plusieurs taxons frayant durant la saison humide sont cooccurrents avec des taxons au frai non saisonnier. Ces constatations appuient des modeles de recrutement generiques anterieurs, demontrant l'utilite de les inclure dans le recent modele Riverine Recruitment Synthesis Model et l'applicabilite de ce modele pour decrire des processus importants pour le recrutement des poissons de riviere tropicaux. [Traduit par la Redaction] Mots-cles : complexite des habitats, zooplancton, hydrologie, riviere Daly, Australie septentrionale, Introduction The early life stages of fishes are well established as being the most perilous, with development and survival of eggs and larvae largely controlling the strength of subsequent year [...]

Riverine fish assemblages are strongly influenced by attributes of the flow regime. Tropical savannah river systems have distinct and predictable hydrologic seasonality, reflecting the wet‐dry climate, but can vary substantially in terms of dry season flow permanency and wet season flow‐pulse characteristics. Understanding how flow permanence and variability influence fish assemblages, and whether these factors can be used to predict responses to future hydrological change, are key knowledge gaps that impede effective management. We examined the influence of hydrological variability on the structure and diversity of freshwater fish assemblages across rivers of the wet‐dry tropics of northern Australia. We found distinct fish assemblages that varied predictably across three hydrological river types: Intermittent, Perennial Stable and Perennial Flashy flow regimes. This distinction emerged despite a common species pool across the region. Species richness was greatest in rivers with Perennial Stable flow regimes, whereas beta‐diversity was greatest in Intermittent rivers. However, life history strategies of constituent species were generally poor predictors of species abundances within each hydrological river type. The distinct fish assemblages evident among hydrological classes may provide some cautious ability to both predict potential fish assemblage changes with future hydrological changes (e.g. if perennial streams became more flashy or intermittent), and to predict fish assemblages expected in unsampled rivers with particular hydrological characteristics. Our findings provide further support for the importance of maintaining regional flow‐habitat heterogeneity and the connectivity between hydrological river types, and their essential role for conserving tropical fish species diversity into an uncertain hydrological future. [ABSTRACT FROM AUTHOR]

Parental care purportedly promotes individual reproductive success and improves offspring survival, but is thought to come at a cost to the adult carer. Reproductive costs can be physiological such as the resources parents spend on caring but also the morphological changes that are necessary to accomplish parental care activities. This study considers morphological and body condition changes in brooders of two paternal mouthbrooder fishes – which incubate eggs and guard young inside the buccal cavity – mouth almighty (Glossamia aprion) and blue catfish (Neoarius graeffei), from rivers of northern Australia. Potential costs associated with mouthbrooding were examined by quantifying body condition, feeding deterrence and changes in gut length and gill form across individuals from varying sex‐breeding state category (brooder males, nonbrooder males and females). Brooders of both species exhibited significantly lower values in some condition indices, and did not feed, suggesting that body reserves were being used and not renewed during mouthbrooding. Brooders of both species had shorter gut lengths than nonbrooders, potentially to reduce tissue maintenance costs during a time of restricted food intake and requirements. Both species also exhibited changes in gill form and structure compared to nonbrooders; indicating structural adaptive plasticity to mouthbrooding by allowing more space for egg brooding in the buccal cavity. For G. aprion and N. graeffei, caring for progeny seemingly required similar trade‐offs including nonphysiological activities (e.g. feeding) and responses, such as changes in condition, respiratory and digestive morphology, that potentially allowed brooders to compensate for parental care energy expenditure. [ABSTRACT FROM AUTHOR]

The Nurseryfish Kurtus gulliveri is found in estuarine and freshwater reaches of tropical rivers in northern Australia and southern New Guinea. Nurseryfish have been recorded in salinities ranging from fresh-water to seawater (salinity <0.5-36‰), but almost nothing is known of their movements. We conducted core-to-edge transect analyses of 87Sr/86Sr in the otoliths of 16 male and 14 female Nurseryfish collected from a tropical river in northern Australia to examine their movements across a freshwater--estuarine salinity gradient. Comparisons between water and otolith 87Sr/86Sr showed that most fish had 87Sr/86Sr values indicative of periods of saline (salinity > 4‰) residence during their early life history, usually followed by a distinct transition into brackish (0.5-4‰) or fresh water (<0.5‰) later in life. No differences were discernible between the salinity histories of male and female Nurseryfish. Analysis of the relationship between the otolith core-to-edge distance and fish mass indicated that most growth occurred in brackish water. We conclude that Nurseryfish are euryhaline rather than diadromous and that analysis of water and otolith 87Sr/86Sr is an effective method for assessing the movements of tropical riverine fishes and the potential importance of fish migration in driving foodweb subsidies. [ABSTRACT FROM AUTHOR]