Lo Stretto di Messina: via di migrazione per pesci e mammiferi marini

Being situated in one of the main migration routes of the Mediterranean Sea, the Strait of Messina is undoubtedly a key point of transit for the migration of many species. Certainly, the most famous and important ones, from the economic and environmental point of view, are the large pelagic species, including swordfish (Xiphias gladius), the bluefin tuna (Thunnus thynnus), the albacore (Thunnus albacore), the Garfish Imperial (Tetrapturus belone) and bonito (Sarda sardinian). All these species are considered migratory, although only the bluefin tuna fully meets the requirements of migratory movement, while the other species, which are certainly characterized by a wide range of motion related to their condition of great swimmers, still show cognitive emptiness on their life cycle. The term «migrate» comes from Latin and it means «move», moving from one place to another and it is just what many fish populations are doing, not hesitating in travelling thousands of miles driven by the need to find the best conditions to carry out their life functions, in search of environments, rich in food resources, of appropriate sites for their reproduction, survival and optimal increasing. Normally, these movements occur in well-defined seasonal periods for the various species although, sometimes, they may be unusual because of the occurrence of particular adverse conditions. One of the behavioral aspects that characterizes many species of pelagic animals – which, in addition to many species of fish, cetaceans and some cephalopods belong to – is their constant movement in the open sea, environment in which they lead mostly all their lives and to which they have adapted by developing specific morphological and functional characteristics developed in relation to the need to overcome the resistance of the water, tirelessly bringing to face marine currents and wave motion. As regards the studies about the presence of swordfish in the Strait of Messina, it is known that this species appears in April in an irregular and sporadic manner. Its presence at the end of April and early May is becoming more frequent in the waters of the Tyrrhenian Sea overlooking Sicily and even more in the Calabrian waters between Cannitello, Scilla , Bagnara and Palmi. Thus the phenomenon goes rarefying towards the end of June to be extinguished in the early days of July, together with the appearance of the swordfish along the Sicilian coast of the Strait, where it sometimes stops until September. By the third decade of June it reaches full sexual maturity, which continues throughout the first decade of August. Coinciding with this biological stage, from the first days of June it is often sighted together with a female. Important studies on the nature, characteristics and causes of the traditional and very regular gamic migration of the swordfish, mainly based on valid practical observations, clarified manifold phenomena and behavioral manifestations once shrouded in legend and mystery. It is certain that in the intergenetic period (autumn-winter) this fish stays in fresh and deep waters of the southern Tyrrhenian Sea, as evidenced by the frequent catches off the coast of Sicily and Calabria through longline fishing (between 100 and 300 meters ). The fact that no winter catches has ever occurred with the same tool in the Strait of Sicily and in the Ionian waters of Sicily, constitutes valuable feedback to the above statements. In spring it appears on the Tyrrhenian coast of Sicily, coinciding with the gradual increase in the temperature of surface water (which remains between 18° and 21°) and with the approach of their sexual maturity. Therefore, in this period, which lasts until the middle of June, the swordfish is in the sea area between Cape Milazzo-Aeolian Islands- Palmi and the entrance of the Strait of Messina (specifically in the Calabrian stretch between Punta Pezzo and Palmi, where for thousands of years fishing has been practiced with the harpoon). Thereafter, with the advancing of the heating of Tyrrhenian surface waters, it disappears from the Tyrrhenian and the upper Calabrian coasts, to appear along the coast of Sicily and the Ionian Sea, where, throughout the month of July and August, the temperature of ‘water best suits their physiological needs. For these reasons, a very substantial group of specimens stops in the Ionic waters between Scaletta and Giardini, where the reproductive activity is carried out. Nearly in July the Ionic surface waters face with an increase in temperature and therefore the swordfish, reaching its limit of tolerance, are induced to abandon those areas and gradually move along the Sicilian coast of the Strait , where they certainly find a still suitable temperature and food. In August the last stage of the migration cycle slowly starts: the fish gradually abandon the Strait to reach the deep waters of the Tyrrhenian Sea, where they spend their intergenetic time until the following spring. As regards the presence of tuna in the Strait of Messina, it was noticed for the first time in autumn and winter by E. Giglioli in 1881 and 1882. Later, other authors have found this characteristic presence of tuna throughout the year. With the establishment in 1928 of the Observatory of sea fishing in Ganzirri, more detailed studies were done by Prof. Scordia, from 1925 to 1937, on the transition and on the movements of tunas, on their biology, on the physico-chemical conditions of the waters of the Strait, and on the collection of statistical data about the catches of the fishermen from the coast of Faro. There is an important publication of 1938 «The gamic migration of tuna in the Mediterranean» by the same author who, trying to give a correct interpretation to all observations collected during the long years of study, came to the deduction that tuna in the Mediterranean are different from the Atlantic ones, representing a «distinct race». The Mediterranean population, as reported by Scordia, is divided into different groups, and each group performs separate migration at the same time and in the same direction. The group of the Tyrrhenian- Ionian tuna dwells in autumn and winter in the deep waters of the lower Tyrrhenian Sea: when these waters begin to warm up with a consequent decrease in density, they abandon this site, moving in the Ionian Sea through the Strait of Messina. The transition is usually from the end of May and in the first half of June. When even the Ionian waters start to become warmer and therefore less dense, firstly the larger and more sensitive tuna come back into the Tyrrhenian Sea, gradually followed by smaller specimens. Numerous scientific results obtained in about 20 years of research have led to confirm, that the movements of tuna and their breeding areas depend in large part by physical and chemical parameters of water and by the food chain. It is also not to be forgotten that the Strait of Messina is an obligatory point of passage for the migration of whales, probably the most important one in the Mediterranean Sea in terms of species diversity. Worthy of note, as well as all the species of dolphins found in the Mediterranean, are the whales (Balaenoptera physalus) and particularly sperm whales (Physeter macrocephalus) that cross the Strait to migrate in the Aeolian Islands, probably for reproductive purposes. The biogeographical characteristics of the Strait make it a very peculiar environment from an ecological point of view. Cetaceans are key elements of marine biodiversity and important indicators of the environmental quality status in which they live. The complex interaction levels of these species with their environment make these animals sensitive and are therefore good indicators of different aspects related to the overall quality of the marine environment. The availability and distribution of food affect those species of cetaceans at different levels of the food chain, from the abundance and distribution of planktonic blooms for the whale, to the highest level for toothed predators, which can be affected by the bioaccumulation of pollutants and the inclination of attack by viruses and bacteria. Other forms of interaction and disorder concern noise, fishing (for which cetaceans are involved both by catch and by competition and direct interaction), the effects of mariculture, marine traffic etc.. The monitoring of cetaceans in the Strait of Messina allows to measure, through their presence and their migratory rhythms, any signs of stress and critical environmental impacts in the two seas: the Southern Tyrrhenian and in the Western Ionian.