Abstract
The movement of postflexion larvae of marine estuarine-dependent species into estuaries is critical for the survival of fishes reliant on estuaries as nurseries. However, detailed studies focused on environmental variability experienced by postflexion larvae entering a range of estuary types under varying conditions are rare. This study assessed the in situ conditions (temperature, salinity and water clarity) under which the southern African endemic fish Rhabdosargus holubi (Sparidae) recruits into estuaries. Postflexion larvae were sampled in three biogeographic regions (cool temperate, warm temperate and subtropical boundary), which included three estuary types (permanently open estuaries (POEs), temporarily open/closed estuaries and estuarine lake systems) on a seasonal basis, independent of each other. Rhabdosargus holubi larvae were more abundant in spring and summer, in POEs in the warm temperate region. Models predicted that higher larval occurrence in estuaries is a function of lower salinity (e.g. mesohaline zones of 5-17.9 salinity) and lower water clarity (e.g. 0-0.2 Kd, light extinction coefficient), particularly for warm, temperate POEs. This re-emphasizes the importance of freshwater for optimal nursery functioning, which may be compromised by impoundments, abstraction and climate change in water-short countries like South Africa.
Highlights
Many marine estuarine-opportunist and -dependent fish species in the Southern Hemisphere enter estuaries at the postflexion larval stage for protection, shelter, food, optimal growth and development (Neira et al 1992, Whitfield 1994, Strydom 2015)
Detailed studies focused on environmental variability experienced by postflexion larvae entering a range of estuary types under varying conditions are rare
Postflexion larvae were sampled in three biogeographic regions, which included three estuary types (permanently open estuaries (POEs), temporarily open/closed estuaries and estuarine lake systems) on a seasonal basis, independent of each other
Summary
Many marine estuarine-opportunist and -dependent fish species in the Southern Hemisphere enter estuaries at the postflexion larval stage for protection, shelter, food, optimal growth and development (Neira et al 1992, Whitfield 1994, Strydom 2015). Compared with their counterparts in the marine environment, the larvae experience dynamically changing salinity, water temperature and turbidity, which influence their recruitment, survival, growth and distribution (Boehlert and Mundy 1988, Whitfield 1994). Environmental variables affect the distribution of fish species among estuaries, as driving factors such as seasonal spawning and recruitment times, regional climate (e.g. temperate vs. tropical), inter-annual climate variability such as El Niño–Southern Oscillation (ENSO) and estuarine typology (e.g. estuaries that are permanently open, temporarily open or exhibiting lake basins, i.e. permanently open estuaries (POEs) vs. temporarily open/closed estuaries (TOCs) vs. estuarine lake systems (ELS) determine the structure of fish populations and communities (Harrison and Whitfield 2006, Koehn et al 2011, Strydom 2015)
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