Variation of stomach content and isotopic niche of puye Galaxias maculatus (Jenyns, 1842) in large river systems of southern Chile

Abstract

Understanding of trophic structure and flow of energy within river systems is essential for informed management of these ecosystems and conservation of native fish fauna and fisheries resources. Food resources used by riverine fish depend on productivity within the main stem and adjacent floodplains, terrestrial inputs, and longitudinal and lateral hydrologic connectivity. Trophic ecology of puye (Galaxias maculatus Jenyns, 1842) in Chilean river systems remains poorly understood and studies that combine short‐term (stomach content) and long‐term (stable isotope analyses) analyses have not been performed to date. In this study, we assessed spatial variability in diet of puye based on both stomach content and analyses of stable isotopes of carbon and nitrogen in 10 river systems of southern Chile (including main stem, headwater lakes, and estuaries). Fish in the main stems of more northern rivers, with predictable flow regimes, were characterised by the most diverse diets, broad niche space, and high proportion of terrestrial prey items and zooplankton in diet. Fish collected in lakes were characterised by narrow isotopic niche space and diet dominated by zooplankton. Diet of G. maculatus in Chilean river systems is associated with characteristics of riverine habitats within each river system (lentic, lotic, and estuarine) and hydrologic connectivity among them. Indeed, connectivity with floodplain habitats and direct supply from headwater lakes appeared to be primary drivers of fish diet in systems where these are present. These supplies maintain fish populations of good condition and allow freshwater recruitment. Therefore, maintaining seasonal flow dynamics and connectivity is essential to preserve the natural function of these river systems and conserve native fish populations.

Unpacking the complexity of longitudinal movement and recruitment patterns of facultative amphidromous fish

“…In the Bueno and Valdivia rivers, a significant proportion of the fish captured reported continuous movement between the upper and middle sections of each river system, with maximum detected displacement of 64 km (Bueno River) and 72 km (Valdivia River). This movement pattern corroborates recent findings of similar resource use and significant overlap of isotopic niche spaces of puye in upper and middle sections of both Valdivia and Bueno rivers 20 . Predictive analyses did not associate any of the fish captured in Valdivia and Bueno river systems with estuarine elemental signatures suggesting restriction of their life-histories to freshwater habitats possibly distinct from estuarine populations.…”

“…To date, however, no estuarine resident populations have been reported. This finding is corroborated by Manosalva et al 20 who reported that isotopic niches of estuarine and freshwater populations of puye in the Imperial and Maullín rivers did not overlap.…”

“…Our results provided evidence of amphidromous and freshwater resident populations, but also estuarine resident populations that were previously considered amphidromous 16,20,27,28 . Amphidromy was described as regular physiologically mediated movements between freshwater and marine habitats, however, we detected that the estuarine populations of the Imperial and Maullín rivers do not move towards freshwater environments and remain in estuaries during their entire lifespan 9 .…”

“…Establishment of resident populations can be driven by several factors such as availability of spawning and nursery habitats and connectivity among essential habitats within the river system that allow recruitment of juveniles 15,19,62 . Juvenile and adult puye inhabit littoral zones of rivers and lakes characterised by low current velocities where they feed on zooplankton and benthic invertebrates 17,20 . As such, successful reproduction and recruitment of puye in freshwater habitats depend on the availability of slow-flowing lotic or lentic habitats that retain larvae and provide small-bodied prey consumed by them 15,19,63,64 .…”

Improving the weaning of larval giant kōkopu, Galaxias argenteus: An emerging aquaculture species

“…Larval fish differ in their diel feeding behaviors by species because of their biological and environmental circumstances (Rønnestad et al, 2013). The diel feeding pattern of wild, marine stage larval giant k okopu is not known; however, studies indicate a high level of feeding flexibility by larval whitebait species, including both opportunistic and selective feeding approaches depending on prey size and density and are to the point of being able to undertake completely freshwater life cycles if required (Catlin et al, 2019;David et al, 2019David et al, , 2022Manosalva et al, 2021). Similarly, studies on adult giant k okopu indicate flexibility between feeding levels between day and night, depedning on season (David & Closs, 2003;Hansen et al, 2004); however, given the relatively poor visual acuity at early stages of development, larvae in this study may in fact benefit from the increased level of natural light intensity at the Midday and Afternoon feeding events, leading to greater capture success and therefore gut fullness (Fiksen & Jørgensen, 2011).…”