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Abstract
Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought conditions and under severely restricted river flow resulting from freshwater abstraction and impoundments. Consequently, marine estuarine-dependent fish such as Rhabdosargus holubi may be subjected to extreme conditions, such as hypersalinity prevailing for long periods ( > 2 months). Hypersalinity may impact the energetic demands of fish due to osmoregulation leading to compromised growth. This study assessed the impact of high salinity on the growth and skeletal development of R. holubi juveniles. Skeletons of juveniles grown at different salinities in the wild and in aquaria were analysed for anomalies. The impact of hypersaline conditions on juvenile R. holubi growth was also determined in aquaria. Aquarium experiments indicated that hypersalinity of 50 did not significantly impact growth rates over two months. Overall, anomalies were rare and vertebral-related anomalies specifically did not differ significantly between salinities. However, fin rays were significantly impacted in fish growing at higher salinities in the wild. It was concluded that the strong osmoregulatory ability of R. holubi offers protection against hypersalinity affecting internal structures, but external structures may remain vulnerable. As such, from a locomotory standpoint, R. holubi may be vulnerable to long periods of exposure to hypersaline conditions.
Highlights
Estuaries can be areas of rapid environmental change as physico-chemical variables such as salinity and temperature may vary within tidal scales, localized seasonal weather patterns and inter-annual climatic shifts (Wallace et al 1984, James et al 2013, Potts et al 2015)
Repeated-measures ANOVA indicated that R. holubi length increased weekly (F=223.48, p
Hypersaline conditions may potentially place a significant impact on the physiology of estuarine fish during drought conditions or freshwater impoundments, affecting their ecology (Bœuf and Payan 2001, Harrison and Whitfield 2006, Gonzalez 2012)
Summary
Estuaries can be areas of rapid environmental change as physico-chemical variables such as salinity and temperature may vary within tidal scales, localized seasonal weather patterns and inter-annual climatic shifts (Wallace et al 1984, James et al 2013, Potts et al 2015). The difference in osmotic pressure between the internal structures and the external environment of teleost fish, such as the common bream Rhabdosargus holubi (family Sparidae), results in a loss of water, which must be mitigated by drinking (Brauner et al 2012). This may be especially true for euryhaline species such as R. holubi, which is an estuarine-dependent marine species that is endemic and common in South African estuaries, where salinities may vary considerably (Blaber 1973a, Whitfield 1998, Whitfield et al 2006)
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