SEASoNAl AND SPAT�Al VAR�AT�oNS �N THE F�SH ASSEMBlAGE oF SHAlloW HAB�TATS W�TH�N THE B�zERTE

– The diversity and structure of fish populations in southern Mediterranean lagoons are poorly documented. Spatial and temporal variation in uses of shallow habitats for fish fauna in Bizerte lagoon in Tunisia was investigated in relation to environmental variables. Fish sampling was carried out over four seasons at four stations with a 3 mm mesh beach seine. Multivariate statistical analyses were applied to assess environmental influences (temperature, pH, salinity, dissolved oxygen) on fish spatiotemporal assemblages. A total of 3,612 individuals were collected representing 26 species from 14 families. The fish assemblage was quantitatively dominated by resident species, mainly the big-scale sand smelt atherina boyeri accounting for 69.49 % of the total number of fish captured. The fish assemblage was largely dominated in terms of ecological guild by the marine migrant species and fish feeding groups belonging to microbenthivore and detritivore species. The Bizerte lagoon functions as a coastal inlet, with euryhaline conditions throughout the year. Hydrological variables measured in the shallow waters were uniform across the whole lagoon and had low influence on the spatiotemporal fish assemblages. The lagoon could be of great importance in providing nursery habitats for juvenile marine migrant fish species. F�SH ASSEMBlAGE FuNCT�oNAl Gu�lD AB�oT�C FACToRS lAGooN B�zERTE TuN�S�A MED�TERRANEAN 36 R. EMBAREK, M. H. KARA, l. BAHR�-SFAR, R. AMARA Vie milieu, 2019, 69 (1) Directive 2000/60/EC) and are already being considered by many research teams (Pérez-Domínguez et al. 2012, Harrison & Kelly 2013, Alvarez et al. 2013, Smolinski & Calkiewicz 2015). Bizerte lagoon, located on the northwestern Tunisian coast, is an ecologically and economically important water body. �t is connected to �chkeul lake as well as to wetlands, all classified as a Biosphere Reserve and National Park since 1980, and listed as a World Heritage Site. However, several towns in the surrounding area are undergoing rapid population growth and industrial expansion. The lagoon is thus affected by various anthropogenic activities such as domestic sewage and industrial waste inputs, atmospheric pollution, shellfish farming and eutrophication phenomena during the summer period that may affect the quality of the habitats and the living organisms (louiz et al. 2009, Mahmoud et al. 2010, Barhoumi et al. 2014, Ben Ameur et al. 2015, zaaboub et al. 2015). Although Bizerte lagoon supports important fisheries (Béji 2000), its ecological functioning (i.e., nursery, functional ecological groups) and importance for fish has never been examined. The main objective of this year-long study was to describe the fish assemblages of the shallow habitats of Bizerte lagoon, regarding their taxonomic and functional compositions, and to assess their spatial and seasonal changes in relation to environmental factors. MATERIAL AND METHODS Study area: located in the northwest of Tunisia (37°08’37°17’N / 9°46’-9°55’E), Bizerte lagoon is a semi-enclosed ecosystem that is linked to the central Mediterranean Sea (Fig. 1). �t has an average depth of 8 m and a surface area of 121.6 km2 (Quignard & zaouali 1980), making it the third largest lagoon in Tunisia. �t is connected to the Mediterranean Sea through a 7 km-long channel, which is on average 300 m wide and 13 m deep, and to the �chkeul lake (110 km2) through the Tinja Channel (about 5 km long and a few meters deep), which brings fresh water. Since the early 1990s, the channel is equipped with locks to manage trade between the �chkeul lake and Bizerte lagoon. This has resulted in a sharp decrease in freshwater supply to the lagoon (Béjaoui et al. 2008). The lagoon also receives fresh water inputs from non-permanent watercourses such as Rharek, Ben Hassine, and Guenich Streams (Fig. 1). Bizerte lagoon is located in the vicinity of several cities, agglomerations and industrial units (Fig. 1). �t is affected by various anthropogenic activities, such as domestic sewage inputs, atmospheric pollution, industrial waste and aquaculture. The surface sediments of the northern and eastern sectors of the lagoon correspond to silty-sandy-muddy or sandy-silty-muddy bottoms. The western and central areas of the lagoon exhibit muddy-silty to muddy sediments (Ben Garali et al. 2008, Brahim et al. 2013). The macrophytes recorded in the lagoon are most frequently represented by seagrass: Cymodocea nodosa, Zostera noltii, Z. marina, and Caulerpa prolifera (zaouli 1979, Guélorget & Penhuisot 1983). The shallow waters are characterized by Cymodocea nodosa and Zostera noltii in the northeast of the lagoon, Caulerpa prolifera Fig. 1. – Bizerte lagoon and location of the four sampling stations. Fresh water input (wadis), urban zones, industrial zones, agricultural zones and shellfish farming zones are indicated. F�SH ASSEMBlAGE oF B�zERTE lAGooN, TuN�S�A 37 Vie milieu, 2019, 69 (1) in the southwest part and at the outlet of Wadi Tindja with some ruppia spiralis. Zostera marina is encountered mainly in the northwest of the lagoon and in the channel connected to the sea (zaouli 1979). Given the many changes that have occurred in the lagoon since the 1990s (Aleya et al. 2019), monitoring of the macrophytes’ spatial distribution would have certainly exhibited some sharp changes during the last decades, but no data exist. Sampling protocol: Sampling was performed at depths less than 1.5 m at four stations (Fig. 1) in contrasting habitats in terms of distance from the sea, substrates, vegetation and anthropogenic pressures. Station 1 (S1), located in the channel, is influenced by the seawater input and affected by intense fishing, passage of ships, and urban effluents from the city of Bizerte as well as a cement factory; station 2 (S2), located at Menzel Abderrahmen, between the port and the outfall of the National Sanitation utility (oNAS), is characterized by urban effluent (Mahmoud et al. 2010); station 3 (S3), near Menzel Jemil, is located in the main aquaculture part of the lagoon. This station is exposed during summer to the eutrophication process and is characterized by shellfish releases and industrial contamination (louiz et al. 2008); station 4 (S4), located at Menzel Bourguiba, is a heavily industrialized area (e.g., metallurgical industry, boatyard, tyre production factories, etc.). Fish sampling was carried out during daylight hours using a beach seine net (l = 14 m, H = 2 m, mesh size = 3 mm). This gear is appropriate for catching small (< 100 mm Tl) fish, which abound in shallow waters (Pierce et al. 1990, Franco et al. 2012). Hauls were taken parallel to the shoreline at a distance of about 15-20 m. Each seine haul covered an area of about 240 m2. At each station, two replicates were collected. Samples were collected once per season during autumn (2012) and winter, spring and summer (2013). After each sampling, fish were preserved in plastic bags and transported to the laboratory in a cooler with ice. Each individual caught was identified to species level, measured (total length, Tl) and weighed (total weight Wt). For each species, catches were standardised to densities (number of individuals per 100 m2). Environmental variables (temperature, salinity, dissolved oxygen and pH) were measured on each sampling occasion immediately before the fishing pro-