Decline of common smelt (Retropinna retropinna) in turbid, eutrophic lakes in the North Island of New Zealand David K. Rowe* & Aleki Taumoepeau National Institute of Water and Atmospheric Research Ltd., P.O. Box 11-115, Hamilton, New Zealand (*Author for correspondence: Tel.: +64-7-856-7026; Fax: +64-7-856-0151; E-mail: d.rowe@niwa.co.nz) Received 25 July 2002; in revised form 26 February 2004; accepted 26 February 2004 Key words: larval fish, smelt, common bullies, ichthyoplankton, fish density, turbidity, water clarity, lake trophic status, New Zealand Abstract Densities of larval smelt (Retropinna retropinna) and common bullies (Gobiomorphus cotidianus) were estimated by a closing Wisconsin drop-net on two occasions following the spring/summer spawning season in 11 geographically localised lakes varying in size and trophic status. Mean densities of smelt ranged from 0 to 65 fish m )2 and were directly related to water clarity as determined by secchi disc depth. In contrast, densities of larval bullies ranged from 0 to 117 fish m )2 and were inversely related to water clarity. Larval smelt dominated in all the clear water, oligotrophic lakes, but bullies dominated in lakes where water clarity was less than 6 m. The depth distribution and depth range for schools of juvenile smelt was also reduced in turbid lakes. As the ability of juvenile smelt to feed on zooplankton is not reduced by turbidity, and as predation by trout on smelt decreases in turbid lakes, these two factors are not responsible for the decline of smelt in the more turbid, productive lakes. We propose that reduced larval recruitment caused by a higher egg mortality is responsible. A higher egg mortality would be expected in such lakes because of increased siltation of littoral spawning substrates, and increased predation by common bullies. Introduction Anadromous populations of the common smelt (Retropinna retropinna (Richardson)) occur natu- rally in a number of rivers and coastal lakes in New Zealand (Woods, 1968; McDowall, 1970) but, between 1905 and 1949, many landlocked populations were established in inland, mainly North Island lakes by stocking and/or down- stream colonisation (Jolly, 1967; Ward et al., 1989; McDowall, 1990). These fish are small (maximum FL 70 mm), elongate, planktivores (Jolly, 1967; Cryer, 1988) found mainly in the epilimnion. Two size groups of smelt are often present in these in- land lakes; small, mostly juvenile fish ranging in length from 25 to 50 mm that school in the shal- lower waters of the epilimnion (Rowe, 1993, 1994), and larger adults (TL 50–70 mm) that school in deeper waters by day, moving to the lake surface at night (Rowe, 1994). In shallow lakes, some of the larger smelt also occur in the littoral zone where they feed on terrestrial and aquatic inver- tebrates (Stephens, 1984; Boube´e & Ward, 1997). The smaller smelt are an important prey for rainbow trout (Oncorhynchus mykiss (Walbaum)) in many of the central North Island lakes near Rotorua (Fig. 1) and Taupo (Smith, 1959; Rowe, 1984; Stephens, 1984). Here, they are thought to play a key role in maintaining the high growth rates of trout. For example, trout growth rates are fastest (>30 cm year )1 ) in the large, clear, oligo- trophic lakes (Fish, 1968; Kim, 1995) where trout Hydrobiologia 523: 149–158, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 149