Instream structures like culverts, dams, and other anthropogenic barriers fragment river networks worldwide, posing a significant threat to the connectivity and migration of freshwater fish communities. Designing effective fish passage solutions requires a comprehensive understanding of the swimming abilities, behaviours, and environmental tolerances of diverse fish species. This thesis aims to address this need by investigating the factors influencing the swimming performance and passage success of multiple migratory fish species in New Zealand, with implications for improving fish passage design and connectivity restoration efforts. Two methods for assessing swimming performance, critical swimming speed (Ucrit) and sprint swimming speed (Usprint), were compared across two fish species. Results revealed no significant statistical difference between swimming speeds estimated using Ucrit versus Usprint protocols for the pelagic Galaxias maculatus and the benthic-associated Galaxias fasciatus. This suggests that shorter time-stepped swimming speeds tests can be used to measure swimming abilities of benthic-associated species, allowing comparisons across a broader range of fish for passage design. Inter- and intraspecies variation in swimming speeds across nine migratory New Zealand fish species was quantified. Galaxias brevipinnis, Galaxias argenteus, and Galaxias postvectis exhibited the strongest swimming abilities. Galaxias maculatus was among the weakest swimmers. Body length was positively correlated with maximum speed, indicating that barriers select against weaker swimming species and smaller individuals within species. Maximum allowable culvert velocities should be significantly lower than previous standards to accommodate most individuals across species. The impact of varying acute water temperatures on critical swimming speeds of four migratory species was investigated. At higher temperatures (26°C), three species (Galaxias maculatus, Galaxias brevipinnis, Gobiomorphus cotidianus) exhibited significant reductions in swimming performance compared to lower temperatures (8°C, 15°C). In contrast, Galaxias fasciatus showed no water temperature-related changes. These findings underscore the importance of designing fish passages to accommodate acute temperature fluctuations, to ensure successful migration under changing environmental conditions. Potential benefits of collective navigation were explored using the small-bodied Galaxias maculatus. Experiments with an artificial velocity barrier revealed that fish swimming in groups had faster entry and passage rates, as well as lower metabolic rates indicating reduced energy expenditure, compared to solitary individuals. These findings highlight the importance of designing fish passes to facilitate movement of gregarious species by accommodating group dynamics. The effect of repeated exposure on passage performance through an experimental raceway with high water velocities was examined. Over five consecutive days, passage success increased significantly, suggesting a role for cognition and spatial memory in improving passage performance. However, approach and entry rates did not improve, indicating other factors like attraction flows or fish physiology may be important for locating and entering structures. This thesis provides insights into the factors influencing swimming performance and passage success for migratory New Zealand species. By studying variation across species in water temperature effects, group behaviour, and cognitive abilities, this research provides a comprehensive understanding to guide the development of more inclusive and effective fish passage solutions. The findings highlight the importance of accounting for many sources of variation when designing instream structures to facilitate unimpeded fish migration.