Brief floodplain inundation provides growth and survival benefits to a young-of-year fish in an intermittent river threatened by water development

Abstract

Riverine floodplains are highly productive habitats that often act as nurseries for fish but are threatened by flow regulation. The Fitzroy River in northern Australia is facing development, but uncertainty exists regarding the extent to which floodplain habitats deliver benefits to fish, particularly given the brevity of seasonal floodplain inundation. We investigated the growth rate of young-of-year bony bream (Nematalosa erebi) in main channel and ephemeral floodplain habitats using age derived from otolith daily increments. We also investigated potential mechanisms influencing growth and modelled the consequences of differential growth rate on survival. Our results revealed higher growth occurred exclusively on the floodplain and that zooplankton biomass was the best predictor of growth rate. Modelling indicated that elevated growth rate in high-growth floodplain pools (top 25th percentile) could translate into substantial increases in survivorship. The positive effect of zooplankton biomass on growth was moderated under highly turbid conditions. Temperature had a minor influence on growth, and only in floodplain habitats. Our results indicate ephemeral floodplain habitats can deliver substantial growth and survival benefits to young-of-year fish even when floodplain inundation is brief. This study highlights the need to ensure that water policy safeguards floodplain habitats due to their important ecological role.

The utility of otolith weight in growth studies of young-of-year bony bream (Nematalosa erebi), Australia’s most widespread freshwater fish

“…In the Fitzroy River, north-western Australia, the primary mechanism influencing the growth of juvenile bony bream was zooplankton biomass, which varied 250-fold between floodplain pools sampled within the same week (Pratt et al 2023). Given the observed variability in the increment count-otolith weight relationship and potential differences in site-scale growth rate, we caution against the use of otolith weight to assign individual ages to young-of-year bony bream.…”

“…Our results demonstrated that when paired with bodylength data, otolith weight is a reliable metric for use in relative growthrate analysis of young-of-year bony bream. Recent modelling of the species suggests that faster growth boosts survival, increasing the number of individuals that recruit into the adult spawning stock (Pratt et al 2023). A greater understanding of the factors that influence growth can help shape management strategies to ensure that bony bream thrive in environments that are increasingly threatened by anthropogenic disturbance (Arthington and Pusey 2003;Brodie and Mitchell 2005;King et al 2015) and climate change (Finlayson et al 2013).…”

“…Two studies from opposite ends of the species range have directly measured growth using otolith daily increment counts from young-of-year individuals. Stocks et al (2019) constructed size-at-age growth models in the Macquarie River (NSW) and Pratt et al (2023) linked growth rate to environmental factors (food availability, temperature, habitat) in the Fitzroy River (WA). Southwell et al (2015) used otolith increments as a proxy for age to back-calculate hatch dates when evaluating the influence of environmental water release on spawning and recruitment, but used bodylength frequency analysis to assess growth.…”