M. P. Olivar á A. SabateÂs Vertical distribution of ®sh larvae in the north-west Mediterranean Sea in spring Received: 10 March 1997 / Accepted: 4 April 1997 Abstract The vertical distribution of the larvae of shelf- dwelling ®sh species that spawn in the NW Mediterra- nean Sea in spring was studied in relation to environ- mental data. Two sampling cycles were carried out at ®xed stations on the continental shelf in May and June 1992. Three patterns of larval vertical distribution for the various taxa represented in the samples were ob- served. The larvae of most species (e.g. Boops boops, Diplodus sargus) were mainly located in the surface layer (10 m), others (e.g. Arnoglossus sp.) had broader distri- butions in the upper 40 m of the water column, and but a few (e.g. Gobiidae) were present in large concentra- tions at greater depths. The vertical distribution patterns of the various species showed no variations, despite high hydrographic variability during the study. The vertical distribution of only a few species (e.g. Arnoglossus sp., Crystallogobius linearis and Engraulis encrasicolus) var- ied over the diel cycle. The possible in¯uence of the vertical distribution of ®sh larvae on their horizontal distribution patterns is discussed. Introduction Many factors are involved in the inshore±oshore dis- tribution of ®sh larvae, chief amongst which is the spawning strategy employed by the parent stock (Sher- man et al. 1984). Other factors include the length of time that eggs and larvae remain in the plankton, egg and larval vertical distributions, and advective processes ± all of which combine to determine the distribution pattern for each species (Cushing 1972; Sherman et al. 1984; Heath 1989; Govoni and Pietrafesa 1994). It follows that the location of eggs and larvae in the water column is a key factor aecting dispersal. There have been many reports of the role of local environmental conditions in determining the vertical and horizontal distributions of ®sh larvae in many areas around the world (e.g. Loeb 1980; Hamann et al. 1981; Fortier and Leggett 1983; Sameoto 1984; Leis 1986, 1991; Heath et al. 1988; Olivar 1990; Palomera 1991). Although most ®sh larvae are located above the thermocline, vertical distribution range and migration patterns are characteristic for each species (e.g. Ahlstrom 1959) and for each ontogenetic stage (Brewer and Kleppel 1986; Fortier and Harris 1989; Heath et al. 1991). The ability of larvae to remain at a given depth depends not only on their swimming ability, but also on their motivation to stay at that depth. In their review of vertical migration, Neilson and Perry (1990) concluded that changes in the depth at which ®sh larvae are located are probably regulated by endogenous mechanisms, but appear to be triggered by environmental factors such as light levels, prey and predator density, hydrographic conditions, and turbu- lence. The most common distribution pattern is for ®sh larvae to remain close to the surface at night (Kendall and Naplin 1981; RoÈpke 1989). Some species do not present diel variations or else have deeper distributions at night than in the daytime (Boehlert et al. 1985; RoÈ pke 1989; Lycskowski-Schultz and Steen 1991). Other spe- cies have been reported to form aggregations during the daytime and then to disperse at night (Munk et al. 1989; Heath et al. 1991; Leis 1991). The apparent discrepan- cies in the day±night patterns are probably due to the fact that light levels themselves are not the single de- termining factor for vertical migrations, but rather act as a synchronizing factor (Heath 1992). Research intended to establish the spawning areas and periods of many ®sh species in the NW Mediterra- nean Sea and the eect of mesoscale physical structures on patterns of horizontal larval distribution has multi- plied during the past ten years (e.g. Sabate s 1990; Marine Biology (1997) 129: 289±300 Ó Springer-Verlag 1997 Communicated by A. RodrõÂguez, Puerto Real M.P. Olivar (&) á A. SabateÂs Instituto de Ciencias del Mar (CSIC), Paseo Joan de Borbo s/n, E-08039 Barcelona, Spain