Invertebrate drift, the downstream transport of aquatic invertebrates, is a fundamental ecological process in streams with important management implications for drift-feeding fishes. Despite long-standing interest, many aspects of drift remain poorly understood mechanistically, thereby limiting broader food web applications (e.g., bioenergetics-based habitat models for fish). Here, we review and synthesize drift-related processes, focusing on their underlying causes, consequences for invertebrate populations and broader trophic dynamics, and recent advances in predictive modelling of drift. Improving predictive models requires further resolving the environmental contexts where drift is driven by hydraulics (passive drift) versus behaviour (active drift). We posit this can be qualitatively inferred by hydraulic conditions, diurnal periodicity, and taxa-specific traits. For invertebrate populations, while the paradox of population persistence in the context of downstream loss has been generally resolved with theory, there are still many unanswered questions surrounding the consequences of drift for population dynamics. In a food web context, there is a need to better understand drift-foraging consumer-resource dynamics and to improve modelling of drift fluxes to more realistically assess habitat capacity for drift-feeding fishes.Résumé : La dérive d'invertébrés, soit le transport vers l'aval d'invertébrés aquatiques, est un processus écologique fondamental dans les cours d'eau qui a d'importantes conséquences pour les poissons qui se nourrissent d'aliments à la dérive. Malgré un intérêt de longue date, de nombreux aspects de la dérive demeurent mal compris d'un point de vue mécaniste, ce qui limite les applications plus larges des réseaux trophiques (p. ex. les modèles d'habitat reposant sur la bioénergétique pour les poissons). Nous passons en revue et résumons les processus associés à la dérive, en mettant l'accent sur leurs causes sous-jacentes, les conséquences pour les populations d'invertébrés et la dynamique trophique plus large, ainsi que les avancées récentes en modélisation prédictive de la dérive. L'amélioration des modèles prédictifs nécessite une meilleure résolution des milieux dans lesquels la dérive est mue, d'une part, par l'hydraulique (dérive passive) ou, d'autre part, par le comportement (dérive active). Nous postulons que cela peut être inféré de manière qualitative à partir des conditions hydrauliques, de la périodicité diurne et de caractères propres aux taxons. Pour les populations d'invertébrés, si le paradoxe de la persistance des populations dans le contexte de perte en aval a généralement été résolu en faisant appel à la théorie, de nombreuses questions demeurent quant aux conséquences de la dérive pour la dynamique des populations. Dans un contexte de réseaux trophiques, il est nécessaire de mieux comprendre la dynamique dérive-consommateur s'alimentant-ressource et d'améliorer la modélisation des flux de dérive pour évaluer de manière plus réaliste la capacité des habitats p...

Most Pacific salmon Oncorhynchus spp. migrate to their natal sites to spawn. Some, however, stray into nonnatal habitats and interact (e.g., reproduce) with individuals from other populations. Pacific salmon straying has been heavily studied for several decades, particularly from the perspective of the populations that donate the stray migrants. Conservation consequences are experienced primarily by the populations that receive strays, though, and there is recent evidence of significant levels of genetic introgression in small recipient populations, which could contribute to the loss of local adaptations. Straying may also provide the benefit of a demographic rescue effect that could save declining recipient populations from extirpation. We highlight the influence of population abundances on the magnitude of straying into recipient populations and demonstrate this using evidence we collected from a small population of Sockeye Salmon O. nerka in British Columbia, Canada. We also review potential factors that might promote higher donor stray rates and therefore recipient straying. Evidence of factors that affect straying is limited and we identify several knowledge gaps, as well as anthropogenic activities that could promote straying. We encourage further discussion and research on the potential effects of recipient straying and the factors that affect straying rates.

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An experiment and a mark‐recapture field study of juvenile coho salmon (Oncorhynchus kisutch) were conducted to identify controls of key energy flow chains in river food webs. In the small‐scale experiment, we investigated the individual and interactive effects of physical habitat structure (PHS) as small wood and resource availability (tissue of adult Chinook salmon, O. tshawytscha) on nutrients, algae, invertebrates, and fish predators including juvenile coho. In the field, we quantified the effects of natural variation in prey availability (invertebrate drift biomass), PHS (wood), and local fish density on summer growth of juvenile coho across multiple stream reaches. Adding salmon tissue to experimental channels resulted in strong bottom‐up effects on select invertebrates including increased population biomass of chironomids and baetids, the numerically dominant invertebrates, and faster growth of juvenile coho. We link the enhanced growth of coho to chironomid productivity: for instance, adult chironomid flux was 4.3× higher and coho consumption of these animals 3× higher in salmon‐subsidized channels. PHS in experimental channels was associated with reduced algal biomass, potentially in response to increased invertebrate consumption, and invertebrate flux or export. The field study revealed coho growth was negatively related to PHS and total fish density and positively related to Diptera drift biomass; however, the effects of fish density and drift biomass on coho growth were relatively weak. The field study also indicated that prey resource availability and coho growth were associated with differences in canopy cover, with prey biomass and coho growth 2–4× higher in reaches receiving more sunlight. As in the experiment, coho in natural stream reaches predominantly fed on adult chironomids and other Diptera, indicating that these taxa and life‐stages are a key link between the benthic food web and mobile vertebrate predators. Our study showed that bottom‐up processes initiated by salmon subsidies and possibly light flux determined key trophic interactions in the Cedar River food web. Moreover, we speculate that PHS may modify some of these interactions indirectly through its effects on the movement of organisms through the environment.

Habitat suitability curves (HSCs) are the biological component of habitat simulation tools used to evaluate instream flow management trade-offs (e.g., the physical habitat simulation model). However, traditional HSCs based on empirical observations of habitat use relative to availability have been criticized for generating biased estimates of flow requirements and for being poorly transferable across locations. For fish like salmonids that feed on drifting invertebrates, bioenergetics-based foraging models that relate habitat conditions to net energy gain offer an alternative approach that addresses some of these shortcomings. To make this technique more accessible for practitioners, we present free and user-friendly software for generating bioenergetics-based HSCs. The software also allows sensitivity analyses of HSCs to factors like fish size or prey abundance as well as direct integration of hydraulic data. While some caveats remain, bioenergetic HSCs should offer a more rigorous and credible means for quantifying habitat suitability for instream flow modeling.