Seasonal variations of abundance and size of recruits of Mugilidae in the coastal lagoon of Merja Zerga (Atlantic coast of Morocco)

International audienceThe seasonal occurrence, relative abundance and recruitment rate of mullet juveniles were studied in the Merja Zerga coastal lagoon (Northern Morocco, Atlantic coast) from January to December 2005. The engine used for sampling was a 10 m length and 6 m wide fixed net, locally named 'zouada'. Used in deep channels with successively 15 mm, 6 mm and 4 turn mesh, it occupies all the water column of the channel. The net is left approximately two hours before being removed. The process is repeated several times per month according to the number of juveniles caught. The period of first appearance of mullets in the lagoon of Merja Zerga was similar with what was observed in the Mediterranean coast (Albertini-Berhaut, 1975; Koutrakis et al., 1994; Rossi, 1986). Liza aurata enters in the lagoon in December; this appearance at the beginning of winter suggests that L. aurata does not require an adaptation to low salinity, in spite of its preference for salted waters (Farrugio, 1975). The date of L. aurata recruitment suggests that the spawning period spends from November to mid-January. In the lagoon of Merja Zerga, the recruitment of L. ramada occurs between August and May, a period characterised by an increase in salinity. The period of reproduction could thus occur between December and February. The first juveniles of Chelon labrosus appeared in February, suggesting a reproduction period between December and February. Juveniles of Mugil cephalus missed in our sampling, confirming the work by Ameur (1994), but adults where present in the lagoon. Similarly, L. saliens is regularly fished as adults, but was absent at the juvenile stage in our sampling, with the exception of only one individual found in February. Our results show that Chelon labrosus, Liza aurata and L. ramada do not confirm their salinity preferences, as described in the literature. Salinity thus cannot explain recruitment chronology. Indeed, Chelon labrosus and L. aurata prefer salted waters, but their recruitment was associated with a decrease in salinity. L. ramada prefers fresh- or desalted waters (De Angelis, 1967; Lasserre and Gallis, 1975; Ben Yami, 198 1; Rossi, 198 1), but its recruitment in the lagoon of Merja Zerga occurred at a period characterised by an increase of salinity. However, this recruitment remains always lower than observed in seawaters. Thus, this species most likely enters into the lagoon before moving quickly towards freshwaters. M. cephalus and L. saliens prefer salted waters in which they found the best living conditions (De Angelis, 1967). The absence of juveniles of these two species in our station can be explained by the low salinity of the lagoonal waters. Various ecological factors can attract juveniles in the lagoons (Blaber, 1987): change of the temperature and salinity, tide and nutrients abundance. In this work, salinity data cannot clearly explain recruitment dates. However, an adaptive strategy limiting interspecific competition can be proposed. The chronology of appearance of juveniles allowed us to deduce the reproduction period of each species. Even if that requires to be validated by otoliths or gonad examination, the period of reproduction of L. aurata could likely be from November to mid-January, and those of C. labrosus and L. ramada between December and February