Adriatic Fish Biodiversity and Review of Bibliography Related to Croatian Small-Scale Coastal Fisheries

Adriatic fish biodiversity and review of bibliography related to Croatian small-scale coastal fisheries Dulčić J.,* Soldo A.* and Jardas I.* Abstract The paper provides some basic bio-geographic features of the Adriatic ichthyofauna with attention to the new species occurring in the Adriatic region. A bibliographic outline of the scientific research carried out on coastal fish of Croatia is also given together with the historical background of the coastal fishery research. Investigations on small-scale coastal fisheries are described with emphasis to the effects of fishing gear on the fish assemblage in the Croatian coastal areas. Keywords: artisanal fishing; marine fish; coastal fisheries; fishing gears; zoogeography; MED, Adriatic Sea; MED; Croatia 1. General features of the Adriatic Sea The Adriatic Sea is a long depression or a synclinal. Its present shape and form originated at the beginning of the Quaternary period, when it was formed by a transgression. In the late Tertiary period in this area, the Earth’s crust sagged forming a depression. Later, this was filled with seawater creating the northern Adriatic. During the Pleistocene period, a part of the sag, the north Italian plain, became dry again. The deeper southern Adriatic was formed when the Earth’s crust collapsed in the late Tertiary period. The Strait of Otranto that connects the Adriatic to the Ionian Sea was formed the same way. The coastline of the Adriatic was unstable during the Tertiary period. The eastern, Dinaric part was flooded in the beginning. Later the water withdrew to the west flooding almost the entire Apennine coast and leaving the eastern coast dry. After that, the western part raised and the eastern part subsided, so water entered among the islands all the way to the present coastal line. The raising of the western and the lowering of the eastern coast probably still goes on. Excluding the Black Sea, the Adriatic Sea is the northernmost part of the Mediterranean. This fact influences some important physical properties of even its southernmost areas. The Adriatic is a shallow sea. Most of the bottom, about 102 415 km2 or 73 %, is less than 200 m deep. The depth gradually decreases from south to north. The Jabuka/Pomo Pit (273 m) and the south Adriatic Pit (1 330 m) are the only areas where the water is over 200 m deep, averaging 231 m therefore, most of the bottoms are on the continental shelf and a significantly smaller part on the continental slope (sea bottom under 200 m deep). The Adriatic Sea covers about 800 km by latitude and from 100 to 200 km by longitude. In the * Institute of Oceanography and Fisheries – Split. S. Ivana Meštrovica, 63 - 21000 Split, Croatia. Email: [email protected] 103 south, the Adriatic Sea is separated from the Ionian Sea, by the 72 km-wide Strait of Otranto, where a submarine sill of 800 m exists. A series of thousands of large and small islands lies along the eastern coast, while there are almost no islands along the western shore. The eastern and western coasts have different morphological and topographic properties: the eastern coast is composed of limestone, its steep and narrow shelf deepens fast while the western coast has a wider shelf because of sediments brought by the river. The largest river input into the Adriatic comes from the Po River, with annual mean inflow of 1700 m s-1. High variability of this inflow is the major factor that determines a number of parameters like temperature, salinity and transparency in the North Adriatic. The sea bottom of the Adriatic shelf is covered with recent sediments of various structural and mineral-petrographic composition. The muddy and sandy sediments cover the largest part of the shelf. Indeed this type of sediment covers almost the entire the south and central Adriatic region, together with the channel area of the north-eastern Adriatic, Gulf of Trieste and a narrow belt along the north-eastern side of the Italian coast. The muddy sediments also cover the most of the northern, a smaller part of the central and only some limited areas of the southern Adriatic. The thermohaline properties of the Adriatic Sea are determined mainly by the air-sea interaction, water exchange through the Otranto Strait, river discharge, mixing, currents, and topography of the basin. The annual temperature range at the surface is 18 ºC in the South and 25 ºC in the North Adriatic. As a whole, the Adriatic is a temperate warm sea. The extremes of surface temperature have a large range, from 6 ºC to 29 ºC. Temperatures of even the deepest layers are for the most part above 10 ºC. The south Adriatic is 8-10 ºC warmer than its central and northern parts during winter. In other seasons the horizontal temperature distribution is more uniform. Generally, the open sea is warmer than the coastal waters. At the Split-Gargano transect the highest temperature occurs in the central part, the lowest surface temperatures occurs are near the eastern coast, and the lowest bottom temperatures are found towards the western coast. Salinity of the Adriatic is relatively high and its ranges are significant. The southern part has salinity between 38.4 to 38.9 psu, and is especially high in the intermediate layer. In the northern part and in the coastal zones salinity is lower, and also more variable. The lowest salinity is found close to the Po River mouth. It could be generally said that the Adriatic Sea water salinity decreases from south to north and from the open sea to the coast. Long-term measurements in the coastal and open waters of the middle and southern Adriatic have shown a salinity-increased trend up to 1993/94 (Zore-Armanda et al., 1999). This phenomenon probably, together with considerable year-to-year variability in three time series (Dubrovnik, Split, Trieste) of sea surface temperature (SST), plays an important role in the apparition of some rare fish in the Adriatic (Dulčić et al., 1999a). Currents are generally low speed and variable in direction. The current field shows a simple cyclonic circulation regime. Generally, streamlines follow the isobath lines along both costs, but wavelike patterns or meanders appear, influenced by the topography of the middle Adriatic (Palagruža sill). Meteorological conditions favourable for the formation of water masses are outbreaks of cold and dry winter weather, when the bora wind (cold, north to northeast wind) is blowing. Three water types of Adriatic origin have been identified (Zore-Armanda, 1963; Artegiani et al., 1997) and characterized with respect to the temperature and salinity (NadDW: North Adriatic Deep Water; MadDW: Middle Adriatic Deep Water; SadDW: South Adriatic Deep Water). 104 Due to its high density, the NAdDW fills up the Jabuka/Pomo Pit and only occasionally spreads to the South Adriatic. The MAdDW is formed in the Jabuka/Pomo Pit area, when there is no intensive northwestward flow, i.e. during the period of a low Mediterranean water inflow. The SAdDW originates in the South Adriatic Pit. Due to its high density, this water spreads into the bottom layer of the eastern Mediterranean. The fourth water type is not of Adriatic origin. The Levantine Intermediate Water (LIW) is formed in the Levantine Basin. This water experiences a salinity decrease on its way to the Adriatic, and eventually enters the Adriatic through the Strait of Otranto. This water type can be recognized in the intermediate layer of the South and Middle Adriatic as mLIW (modified LIW) water type (Orlić et al., 1992). Seasonal distributions of water masses are closely related to the current fields. The Adriatic shows considerable year-to-year variations in the oceanographic parameters, as highlighted by the amount of LIW present. The horizontal pressure difference varies between the northern and the southern Adriatic, which influence the intensity of water exchange between the Adriatic and the eastern Mediterranean as a consequence of the presence of a large pressure centre over the wider Mediterranean region. Year-to-year fluctuations of water exchange between these basins influence long-term fluctuations of a variety of parameters like salinity, temperature, transparency and nutrient salts density, the latest having a consequence on chlorophyll concentration fluctuations. In addition, the man-made impact on at least some of the oceanographic properties has become clear over the past three decades. Nowadays, Adriatic oceanography, especially in the North Adriatic area, faces the necessity of dealing with increasingly urgent problems of the sea pollution, which might influence toxic phytoplankton blooms as well as biodegradation of the environment thus having consequences on biodiversity too. The Adriatic Sea, especially its northern part, is one of the richest fishing grounds in the Mediterranean. The rich river inflows over the shallow shelf of the north Adriatic and in addition the mixing of bottom sediments make this area highly productive. The central and southern Adriatic are less productive, but their exposure to the influence from the north Adriatic and to the periodically stronger influence of the Mediterranean waters, enable high productivity in these areas as well. Open Adriatic waters are, therefore, distinguished as areas for pelagic fisheries. The areas off Ancona and around Palagruža Sill both are known as rich fishing grounds, especially for pelagic fisheries. The coastal area of the eastern Adriatic is traditionally the most important fishing area for the category of professional, sport and other fishermen who use small-scale fishing gears in their fishing activities. The coastal area is characterised by greater productivity rate compared to the open sea thanks to relatively lower depth, vicinity of the land and fresh-water inflow. In the eastern Adriatic channel areas and in the areas under the influence of fresh-water inflow, coves, bays and mouth of rivers, where the coastal fishing takes place, mean gross primary production ranges between 60 and 150 gC m-2 year-1, while in the open sea it is 55 gC m-2 year-1.

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