Do juveniles of the estuarine-dependent dusky kob, Argyrosomus japonicus, exhibit optimum growth indices at reduced salinities? Andrea K. Bernatzeder a,1 , Paul D. Cowley b, * , Thomas Hecht a,1 a Department of Ichthyology and Fisheries Science, Rhodes University, Grahamstown 6140, South Africa b South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa article info Article history: Received 22 October 2009 Accepted 25 May 2010 Available online 2 June 2010 Keywords: Argyrosomus japonicus estuarine fish growth salinity South Africa abstract The natural distribution of early juvenile (<150 mm TL) dusky kob Argyrosomus japonicus (Sciaenidae) along the east coast of South Africa appears to be restricted to the reduced salinity upper reaches of estuaries. This study experimentally tested the effect of salinity on growth indices in an attempt to explain the estuarine distribution of early juvenile A. japonicus. Growth under laboratory conditions was used as an indicator of relative energy expenditure for osmoregulation at salinities 5 (hypoosmotic), 12 (isosmotic) and 35 (hyperosmotic), as energy used for osmoregulation becomes unavailable for growth. Juveniles (81  1.5 mm SL and 9.6  0.5 g) survived and grew at all three salinities over a nine-week period. Contrary to what was hypothesised, growth of juveniles increased with increasing salinity (average weight 35.57  1.50 g, 42.31  2.70 g and 48.86  1.30 g for the 5, 12 and 35 treatments respectively), with specific growth rate of juveniles at the hypoosmotic salinity (2.05  0.15) significantly lower than at isosmotic (2.52  0.09) and hyperosmotic salinities (2.59  0.09). Food conversion ratio was significantly higher at the hypoosmotic (1.30  0.28) than the hyperosmotic salinity (0.94  0.05), but there was no difference between the isosmotic (1.01  0.08) and hyperosmotic salinity. Salinity did not affect condition factor (range: 2.67  0.03e2.75  0.10) or feed intake (range: 2.43  0.17e2.75  0.53). The results, in part, indicated that early juvenile A. japonicus expend more energy at salinities below isosmotic levels and that salinity alone does not explain their natural distri- bution within estuaries. It is possible that high conductivity in the upper reaches of certain South African estuaries may mitigate the negative effect of reduced salinity on growth and hence the estuarine distribution of early juveniles. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Estuarine associated fishes are exposed to a wide range of salinities; however, many species exhibit specific habitat affinities that are primarily associated with salinity (Monaco et al., 1998). The ability to tolerate specific salinity regimes is controlled physiolog- ically through osmoregulation, which requires energy. The proportion of metabolic energy used for osmoregulation may range from as low as 1e2% (Jobling, 1994) to 10e50% (Boeuf and Payan, 2001). Irrespective of the amount of energy used, the physiolog- ical consequence is that energy used for osmoregulation is no longer available for growth. Therefore, fish should grow best within a salinity range at which energy required for osmoregulation is reduced (Brett, 1979). Jobling (1994) suggested that juvenile fish, in particular, grow best at isosmotic salinities at which the energy expenditure for osmoregulation is minimised. However, the “isos- motic hypothesis” does not apply to all species (Denson et al., 2003; Partridge and Jenkins, 2002). The optimum salinity for growth is generally below isosmotic levels for freshwater and eurohaline (anadromous) species and above isosmotic levels for true marine species and euryhaline species (Deacon and Hecht, 1999). Furthermore, salinity responses are also dependent on the size/age of fish and life history stages (Allen and Cech, 2007; Cardona, 2000). The dusky kob Argyrosomus japonicus (Temminck and Schlegel, 1843), is a widely distributed sciaenid fish found in the southern and northern hemispheres, along the entire south and east coasts of Australia, where it is known as mulloway, and from Hong Kong northwards along the Chinese coast to southern Korea and Japan (Griffiths and Heemstra, 1995). In southern Africa, it occurs on the east coast from Cape Point to Mozambique, but is especially abundant between Cape Agulhas and KwaZulueNatal (Griffiths, 1996). Adults spawn in the near-shore marine environment (beyond the surf to depths of approximately 100 m) and early * Corresponding author. E-mail address: p.cowley@saiab.ac.za (P.D. Cowley). 1 Tel.: þ27 (0) 46 6035800; fax: þ27 (0) 46 6222403. Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2010.05.016 Estuarine, Coastal and Shelf Science 90 (2010) 111e115