Recruitment of migratory fish in free‐flowing rivers with limited floodplain development

Abstract

1. Hydrological connectivity is a basic feature of river ecosystems, responsible for important ecological processes. Human activities, however, have progressively modified natural connectivity, threatening the maintenance of aquatic biodiversity.2. Neotropical potamodromous fish depend heavily on fluvial connectivity to complete their life cycles. However, important ecological aspects remain poorly understood, especially in river systems with limited floodplain development (i.e. channelized rivers and drylands), where the environment poses constraints to fish recruitment. The Uruguay River Basin is such a system, with several migratory fish, small floodplain areas, and limited information about recruitment dynamics and nursery grounds. 3. The lack of information on critical habitats limits conservation and management actions, especially in scenarios of increasing environmental degradation and river regulation. This study investigated recruitment dynamics of migratory fishes in the Middle Uruguay River, a long fluvial segment targeted for hydropower development. The study investigated spatial variations in the abundance, composition and size structure of juvenile fish among different sites along an extensive fluvial gradient (ca. 650 km), comparing the main channel with tributaries.4. Juveniles of migratory fish were widely distributed in the region, totalling nine species, with a predominance of Characiformes. Spatial distribution was variable among species and size classes, with a differential use of tributaries, wetlands and the main channel. Juvenile composition differed between sampling sites and environments, and was correlated with different limnological and spatial variables.Greater abundances occurred in the mouth of a tributary where a small floodplain develops during the wet season.5. The results emphasize the importance of fluvial connectivity for fish migration, dispersal and recruitment, providing valuable information about migratory fishes in river systems with limited wetland development. These findings have significance for the management and conservation of migratory fish under scenarios of hydropower expansion, as comprehensive studies on recruitment are rare in free-flowing rivers.

Spatial and seasonal distribution of fish eggs and larvae in one free‐flowing river in the Neotropical savanna

“…There were no differences in the densities of fish eggs and larvae between the "tributary and main channel"; this result does not corroborate with previous studies that showed the greater importance of tributaries (Pachla et al, 2022;Silva et al, 2015Silva et al, , 2019Vasconcelos et al, 2021). This is likely because the River Araguaia basin still does not experience major human interventions (e.g., dams) in the main watercourses, allowing the entire region to be used as a spawning area, therefore the importance of the whole region for the sustainability of the regional fish stock and species diversity.…”

“…Knowledge of spawning sites provides important information on critical recruitment areas for fish species (Pachla et al ., 2022; Soares et al ., 2022; Zacardi et al ., 2020). Recent surveys on habitat use for fish reproduction found spatial variations in the abundance of fish eggs and larvae in different basins, widely distributed in the main river channel and tributaries (Soares et al ., 2022), as well as preferential use by tributaries (Pachla et al ., 2022; Silva et al ., 2015, 2019; Vasconcelos et al ., 2021). This information is critical for the management of aquatic resources (Zacardi et al ., 2020), mainly in relation to hydroelectric plans (Latrubesse et al ., 2019).…”

Preferential behavior of Tribolodon hakonensis for fishways according to biological characteristics

“…Therefore, not only large fish with nuptial coloration but also small fish that do not have nuptial coloration should be considered. Stream continuity for fish migration and dispersal is also important to maintain diverse fish habitats (Kjaerås et al, 2022;Pachla et al, 2022). Therefore, the fishways of MID have been improved and appropriately maintained to accommodate not only large fish with superior swimming ability but also small fish and bottom-dwelling fish with low swimming ability (Aoki et al, 2021).…”