Overfishing
Overfishing is the removal of a species of fish from a body of water at a rate that the species cannot replenish in time, resulting in those species either becoming depleted or underpopulated in that given area. Overfishing has been present for centuries. Overfishing can occur in water bodies of any sizes, such as ponds, lakes or oceans, can result in resource depletion, reduced biological growth rates and low biomass levels. Sustained overfishing can lead to critical depensation, where the fish population is no longer able to sustain itself; some forms of overfishing, such as the overfishing of sharks, has led to the upset of entire marine ecosystems. The ability of a fishery to recover from overfishing depends on whether the ecosystem's conditions are suitable for the recovery. Dramatic changes in species composition can result in an ecosystem shift, where other equilibrium energy flows involve species compositions different from those, present before the depletion of the original fish stock.
For example, once trout have been overfished, carp might take over in a way that makes it impossible for the trout to re-establish a breeding population. Overfishing has stripped many fisheries around the world of their stocks; the United Nations Food and Agriculture Organization estimated in a 2018 report that 33.1% of world fish stocks are subject to overfishing. Significant overfishing has been observed in pre-industrial times. In particular, the overfishing of the western Atlantic Ocean from the earliest days of European colonisation of the Americas has been well documented. Daniel Pauly, a fisheries scientist known for pioneering work on the human impacts on global fisheries, has commented: It is as though we use our military to fight the animals in the ocean. We are winning this war to exterminate them, and to see this destruction happen, for nothing – for no reason –, a bit frustrating. Strangely enough, these effects are all reversible, all the animals that have disappeared would reappear, all the animals that were small would grow, all the relationships that you can't see any more would re-establish themselves, the system would re-emerge.
Examples of overfishing exist in areas such as the North Sea, the Grand Banks of Newfoundland and the East China Sea. In these locations, overfishing has not only proved disastrous to fish stocks but to the fishing communities relying on the harvest. Like other extractive industries such as forestry and hunting, fisheries are susceptible to economic interaction between ownership or stewardship and sustainability, otherwise known as the tragedy of the commons; the Peruvian coastal anchovy fisheries crashed in the 1970s after overfishing and an El Niño season depleted anchovies from its waters. Anchovies were a major natural resource in Peru. However, the following five years saw the Peruvian fleet's catch amount to only about 4 million tons; this was a major loss to Peru's economy. The collapse of the cod fishery off Newfoundland, the 1992 decision by Canada to impose an indefinite moratorium on the Grand Banks, is a dramatic example of the consequences of overfishing; the sole fisheries in the Irish Sea, the west English Channel, other locations have become overfished to the point of virtual collapse, according to the UK government's official Biodiversity Action Plan.
The United Kingdom has created elements within this plan to attempt to restore this fishery, but the expanding global human population and the expanding demand for fish has reached a point where demand for food threatens the stability of these fisheries, if not the species' survival. Many deep sea fish are at risk, such as orange sablefish; the deep sea is completely dark, near freezing and has little food. Deep sea fish grow because of limited food, have slow metabolisms, low reproductive rates, many don't reach breeding maturity for 30 to 40 years. A fillet of orange roughy at the store is at least 50 years old. Most deep sea fish are in international waters. Most of these fish are caught by deep trawlers near seamounts, where they congregate because of food. Flash freezing allows the trawlers to work for days at a time, modern fishfinders target the fish with ease. Blue walleye became extinct in the Great Lakes in the 1980s; until the middle of the 20th century, it was a commercially valuable fish, with about a half million tonnes being landed during the period from about 1880 to the late 1950s, when the populations collapsed through a combination of overfishing, anthropogenic eutrophication, competition with the introduced rainbow smelt.
The World Wide Fund for Nature and the Zoological Society of London jointly issued their "Living Blue Planet Report" on 16 September 2015 which states that there was a dramatic fall of 74% in worldwide stocks of the important scombridae fish such as mackerel and bonitos between 1970 and 2010, the global overall "population sizes of mammals, reptiles and fish fell by half on average in just 40 years." Several countries are now managing their fisheries. Examples include New Zealand; the United States has turned many of its fisheries around from being in a depleted state. According to a 2008 UN report, the world's fishing fleets are losing US$50 billion each year through depleted stocks and poor fisheries management; the report, produced jointly by the World Bank and the UN Food and Agriculture Organization, asserts that half the world's fishing fleet could be scrapped with no change in catch. In addition, the biomass of global fish stocks have been allowed to run down to the point where it is no longer possible to cat
Slovenia
Slovenia the Republic of Slovenia, is a sovereign state located in southern Central Europe at a crossroads of important European cultural and trade routes. It is bordered by Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the southeast, the Adriatic Sea to the southwest, it has a population of 2.07 million. One of the successor states of the former Yugoslavia, Slovenia is a parliamentary republic and a member of the United Nations, of the European Union, of NATO; the capital and largest city is Ljubljana. Slovenia has a mountainous terrain with a continental climate, with the exception of the Slovene Littoral, which has a sub-Mediterranean climate, of the northwest, which has an Alpine climate. Additionally, the Dinaric Alps and the Pannonian Plain meet on the territory of Slovenia; the country, marked by a significant biological diversity, is one of the most water-rich in Europe, with a dense river network, a rich aquifer system, significant karst underground watercourses.
Over half of the territory is covered by forest. The human settlement of Slovenia is uneven. Slovenia has been the crossroads of Slavic and Romance languages and cultures. Although the population is not homogeneous, Slovenes comprise the majority; the South Slavic language Slovene is the official language throughout the country. Slovenia is a secularized country, but Catholicism and Lutheranism have influenced its culture and identity; the economy of Slovenia is small and export-oriented and has been influenced by international conditions. It has been hurt by the Eurozone crisis which started in 2009; the main economic field is services, followed by construction. The current territory of Slovenia has formed part of many different states, including the Roman Empire, Byzantine Empire, Carolingian Empire and the Holy Roman Empire, the Habsburg Monarchy, the Republic of Venice, the French-administered Illyrian Provinces of Napoleon I, the Austrian Empire and Austria-Hungary. In October 1918 the Slovenes exercised self-determination for the first time by co-founding the State of Slovenes and Serbs.
In December 1918 they merged with the Kingdom of Serbia into the Kingdom of Serbs and Slovenes. During World War II Germany and Hungary occupied and annexed Slovenia, with a tiny area transferred to the Independent State of Croatia, a Nazi puppet state. In 1945 Slovenia became a founding member of the Federal People's Republic of Yugoslavia, renamed in 1963 as the Socialist Federal Republic of Yugoslavia. In the first years after World War II this state was allied with the Eastern Bloc, but it never subscribed to the Warsaw Pact and in 1961 became one of the founders of the Non-Aligned Movement. In June 1991, after the introduction of multi-party representative democracy, Slovenia became the first republic that split from Yugoslavia and became an independent country. In 2004, it entered the European Union. Slovenia's name means the "Land of the Slavs" in Slovene and other South Slavic languages; the etymology of Slav itself remains uncertain. The reconstructed autonym *Slověninъ is derived from the word slovo denoting "people who speak," i. e. people who understand each other.
This is in contrast to the Slavic word denoting German people, namely *němьcь, meaning "silent, mute people". The word slovo and the related slava and slukh originate from the Proto-Indo-European root *ḱlew-, cognate with Ancient Greek κλέος, as in the name Pericles, Latin clueo, English loud; the modern Slovene state originates from the Slovene National Liberation Committee held on 19 February 1944. They named the state as Federal Slovenia, a unit within the Yugoslav federation. On 20 February 1946, Federal Slovenia was renamed the People's Republic of Slovenia, it retained this name until 9 April 1963, when its name was changed again, this time to Socialist Republic of Slovenia. On 8 March 1990, SR Slovenia removed the prefix "Socialist" from its name, becoming the Republic of Slovenia. Present-day Slovenia has been inhabited since prehistoric times. There is evidence of human habitation from around 250,000 years ago. A pierced cave bear bone, dating from 43100 ± 700 BP, found in 1995 in Divje Babe cave near Cerkno, is considered a kind of flute, the oldest musical instrument discovered in the world.
In the 1920s and 1930s, artifacts belonging to the Cro-Magnon, such as pierced bones, bone points, a needle were found by archaeologist Srečko Brodar in Potok Cave. In 2002, remains of pile dwellings over 4,500 years old were discovered in the Ljubljana Marshes, now protected as a UNESCO World Heritage Site, along with the Ljubljana Marshes Wooden Wheel, the oldest wooden wheel in the world, it shows that wooden wheels appeared simultaneously in Mesopotamia and Europe. In the transition period between the Bronze age to the Iron age, the Urnfield culture flourished. Archaeological remains dating from the Hallstatt period have been found in southeastern Slovenia, among them a number of situl
Austria
Austria the Republic of Austria, is a country in Central Europe comprising 9 federated states. Its capital, largest city and one of nine states is Vienna. Austria has an area of 83,879 km2, a population of nearly 9 million people and a nominal GDP of $477 billion, it is bordered by the Czech Republic and Germany to the north and Slovakia to the east and Italy to the south, Switzerland and Liechtenstein to the west. The terrain is mountainous, lying within the Alps; the majority of the population speaks local Bavarian dialects as their native language, German in its standard form is the country's official language. Other regional languages are Hungarian, Burgenland Croatian, Slovene. Austria played a central role in European History from the late 18th to the early 20th century, it emerged as a margraviate around 976 and developed into a duchy and archduchy. In the 16th century, Austria started serving as the heart of the Habsburg Monarchy and the junior branch of the House of Habsburg – one of the most influential royal houses in history.
As archduchy, it was a major component and administrative centre of the Holy Roman Empire. Following the Holy Roman Empire's dissolution, Austria founded its own empire in the 19th century, which became a great power and the leading force of the German Confederation. Subsequent to the Austro-Prussian War and the establishment of a union with Hungary, the Austro-Hungarian Empire was created. Austria was involved in both world wars. Austria is a parliamentary representative democracy with a President as head of state and a Chancellor as head of government. Major urban areas of Austria include Graz, Linz and Innsbruck. Austria is ranked as one of the richest countries in the world by per capita GDP terms; the country has developed a high standard of living and in 2018 was ranked 20th in the world for its Human Development Index. The republic declared its perpetual neutrality in foreign political affairs in 1955. Austria has been a member of the United Nations since 1955 and joined the European Union in 1995.
It is a founding member of the OECD and Interpol. Austria signed the Schengen Agreement in 1995, adopted the euro currency in 1999; the German name for Austria, Österreich, derives from the Old High German Ostarrîchi, which meant "eastern realm" and which first appeared in the "Ostarrîchi document" of 996. This word is a translation of Medieval Latin Marchia orientalis into a local dialect. Another theory says that this name comes from the local name of the mountain whose original Slovenian name is "Ostravica" - because it is steep on both sides. Austria was a prefecture of Bavaria created in 976; the word "Austria" was first recorded in the 12th century. At the time, the Danube basin of Austria was the easternmost extent of Bavaria; the Central European land, now Austria was settled in pre-Roman times by various Celtic tribes. The Celtic kingdom of Noricum was claimed by the Roman Empire and made a province. Present-day Petronell-Carnuntum in eastern Austria was an important army camp turned capital city in what became known as the Upper Pannonia province.
Carnuntum was home for 50,000 people for nearly 400 years. After the fall of the Roman Empire, the area was invaded by Bavarians and Avars. Charlemagne, King of the Franks, conquered the area in AD 788, encouraged colonization, introduced Christianity; as part of Eastern Francia, the core areas that now encompass Austria were bequeathed to the house of Babenberg. The area was known as the marchia Orientalis and was given to Leopold of Babenberg in 976; the first record showing the name Austria is from 996, where it is written as Ostarrîchi, referring to the territory of the Babenberg March. In 1156, the Privilegium Minus elevated Austria to the status of a duchy. In 1192, the Babenbergs acquired the Duchy of Styria. With the death of Frederick II in 1246, the line of the Babenbergs was extinguished; as a result, Ottokar II of Bohemia assumed control of the duchies of Austria and Carinthia. His reign came to an end with his defeat at Dürnkrut at the hands of Rudolph I of Germany in 1278. Thereafter, until World War I, Austria's history was that of its ruling dynasty, the Habsburgs.
In the 14th and 15th centuries, the Habsburgs began to accumulate other provinces in the vicinity of the Duchy of Austria. In 1438, Duke Albert V of Austria was chosen as the successor to his father-in-law, Emperor Sigismund. Although Albert himself only reigned for a year, henceforth every emperor of the Holy Roman Empire was a Habsburg, with only one exception; the Habsburgs began to accumulate territory far from the hereditary lands. In 1477, Archduke Maximilian, only son of Emperor Frederick III, married the heiress Maria of Burgundy, thus acquiring most of the Netherlands for the family. In 1496, his son Philip the Fair married Joanna the Mad, the heiress of Castile and Aragon, thus acquiring Spain and its Italian and New World appendages for the Habsburgs. In 1526, following the Battle of Mohács, Bohemia and the part of Hungary not occupied by the Ottomans came under Austrian rule. Ottoman expansion into Hungary led to frequent conflicts between the two empires evident in the Long War of 1593 to 1606.
The Turks made incursions into Styria nearly 20 times, of which some are c
Sava
The Sava is a river in Central and Southeastern Europe, a right tributary of the Danube. It flows through Slovenia, along the northern border of Bosnia and Herzegovina, through Serbia, discharging into the Danube in Belgrade, its central part is a natural border of Croatia. The Sava forms the northern border of the Balkan Peninsula, the southern edge of the Pannonian Plain; the Sava is 990 kilometres long, including the 45-kilometre Sava Dolinka headwater rising in Zelenci, Slovenia. It is the greatest tributary of the Danube by volume of water, second-largest after Tisza in terms of catchment area and length, it drains a significant portion of the Dinaric Alps region, through the major tributaries of Drina, Kupa, Vrbas, Kolubara and Krka. The Sava is one of the longest rivers in Europe and among a handful of European rivers of that length that do not drain directly into a sea; the population in the Sava River basin is estimated at 8,176,000, it connects three national capitals—Ljubljana and Belgrade.
The Sava is navigable for larger vessels from the confluence of the Kupa River in Sisak, Croatia two-thirds of its length. The name is believed to be derived from the Proto-Indo-European root *sewh1 and the ending *eh2, so that it means'that which waters'; the Sava River is formed from the Sava Dolinka and the Sava Bohinjka headwaters in northwest Slovenia. The river's headwater area encompasses several tributaries, including the 52-kilometre Sora, the 27-kilometre Tržič Bistrica and the 17-kilometre Radovna rivers—flowing into the Sava at confluences located as far east downstream as Medvode; the Sava Dolinka rises at the Zelenci Pools near Kranjska Gora, Slovenia, in a valley separating the Julian Alps from the Karavanke mountain range. The spring is located near the Slovene-Italian border at 833 metres above sea level, in a drainage divide between the Adriatic and Danube basins; the Sava Dolinka spring is fed by groundwater exhibiting bifurcation of source karst aquifer to the Sava and Soča basins.
Nadiža creek, a short losing stream flowing nearby, is the source of Zelenci Pools water. The Sava Dolinka is considered the Sava's 45-kilometre segment; the Sava Bohinjka originates in Ribčev Laz, at the confluence of the Jezernica, a short watercourse flowing out from Lake Bohinj and the Mostnica River. Some sources define the Jezernica as a part of the Sava Bohinjka, specifying the latter as flowing directly out of the lake, while another group of sources include Savica, rising at the southern flank of Triglav as the 78-metre Savica Falls, downstream from Triglav Lakes Valley, flowing into the lake, as a part of the Sava Bohinjka; the watercourse flows 41 kilometres —including the length of the Savica—east to Radovljica, where it discharges into the Sava Dolinka. Downstream from the confluence, the river is referred to as the Sava; the Sava is located in Southeast Europe, flowing through Slovenia, Croatia and along the Bosnia-Herzegovina border. Its total length is 990 kilometres, including the 45-kilometre Sava Dolinka and the 945-kilometre Sava proper.
As a right tributary of the Danube, the river belongs to the Black Sea drainage basin. The Sava River is the third longest tributary of the Danube shorter than the 966-kilometre Tisza and the 950-kilometre Prut—the Danube's two longest tributaries—when the Sava Dolinka headwater is excluded from its course, it is the largest tributary of the Danube by discharge. The river course is sometimes used to describe the northern boundary of the Balkans, the southern border of the Central Europe. Before the breakup of Yugoslavia in 1991, the river was located inside Yugoslav borders and it was the longest river with its entire course within the country; the Sava Dolinka rises in the Zelenci Pools, west of Podkoren in the Upper Carniola region of Slovenia at 833 metres above sea level, flows east, past Kranjska Gora to Jesenice, where it turns southeast. At Žirovnica, the river enters the Ljubljana Basin and encounters the first hydroelectric dam—Moste plant—before proceeding to the east of the glacial Lake Bled towards Radovljica and confluence of the Sava Bohinjka, at 411 metres a.s.l.
Downstream of Radovljica, the Sava proceeds southeast towards Kranj. Between Kranj and Medvode, its course comprises the Lake Trboje and the Lake Zbilje reservoirs, built for the Mavčiče and the Medvode power plants; the Sava flows through the capital of Slovenia, where another reservoir is located on the river, adjacent to the Tacen Whitewater Course. There the river course turns east and leaves the Ljubljana Basin via Dolsko, at 261 metres a.s.l.. The course continues through the Sava Hills, where it passes the Litija Basin with the mining and industrial town of Litija, the Central Sava Valley with the mining towns of Zagorje ob Savi and Hrastnik, turns to the southeast and runs through the Lower Sava Valley with the towns of Radeče, Krško; the course through the Sava Hills forms the boundary of traditional regions of Lower Carniola and Styria, At Radeče, the Vrhovo hydroelectric dam reservoir is located. The latter is site of the Krško Nuclear Power Plant, which uses the Sava River water to dissipate excess heat.
The easternmost stretch of the Sava River course in Slovenia runs to the south of Brežice, where it is joined by the Kr
Lim (river)
The Lim is a river flowing through Montenegro, Albania and Bosnia and Herzegovina. 220 km long, it is the longest tributary of the Drina. The Lim rises below Maglić peak in the Kuči area of eastern Montenegro close to the Albanian border, under the name of Vrmoša, its source is only few kilometers away from the source of the Tara river, but the two rivers go in opposite directions: the Tara to the north-west and the Vrmoša to the east, after only few kilometers it crosses over to Albania. Passing through Prokletije mountains and the village of Vermosh, it re-enters Montenegro under the name of Grnčar. Receiving stream Vruje from the right at Gusinje, it continues as Ljuča for a few more kilometers where it empties into the Lake Plav, creating small delta, it flows out of the lake to the north, next to the high mountain Visitor, under the name Lim for the remaining 193 km. It passes through Murino, continuing to the north through areas of Vasojevići, Gornji Kolašin, Donji Kolašin and Komarani, the Tivran gorge and the cities of Andrijevica, Bijelo Polje and Nedakusi, entering Serbia between villages of Dobrakovo and Gostun.
It receives the right tributaries of Lješnica and Bistrica and left tributary of Ljuboviđa, near Bijelo Polje. In the border area, the Lim carved a long gorge Kumanička klisura, between the mountains of Lisa and Ozren. Lim continues between the mountains of Jadovnik and Zlatar, runs through the northern part of the Sandžak area. Upstream of Priboj, the river is dammed by the hydro power plant "Potpeć", creating Potpeć Reservoir; the villages of Garčanica and Lučice and the towns of Brodarevo, Pribojska Banja and Priboj. After Priboj it turns north-west and enters Bosnia and Herzegovina, but only for a few kilometers when it flows back to Serbia and again to Bosnia at Rudo. In Bosnia and Herzegovina, Lim flows between the mountains of Bić, Javorje and Vučevica from the south, the Varda mountain from the north, before it empties into the Drina near the village of Međeđa; the lowest section of the river is flooded by the Višegrad Reservoir, created by the Višegrad hydroelectrical power plant on the Drina.
Lim belongs to the Black Sea drainage basin through Drina and Danube. Its drainage basin covers the river is not navigable, it receives many smaller streams in Montenegro and Bosnia and Herzegovina, the two most important are its major right tributaries in Serbia, the Uvac and the Mileševa river. Three hypotheses exist about the origin of its name: Albanian origin from the word lumë, which means river Latin origin derived from the word limes, which means border Celtic origin from the root llyn or llym, meaning drink or water The fertile valley of the river is called Polimlje, meaning "Lim valley", it represents area around composite river valley, made of several depressions. The valley is divided in three large parts, Stari Vlah and Bosnian, it is important agricultural region for cultivating fruits and stockbreeding. It is important route for the both road and railways from Serbia to Montenegro and Adriatic coast, most notably, Belgrade-Bar railway. Industry is not much developed, except for industrialized Priboj.
Most use of the river has Serbian electricity production, with power station Potpeć being constructed and several more stations on the Lim's major tributary, the Uvac. Despite the potentials, the entire area the Lim flows through is undeveloped and for decades depopulating. After breakup of Yugoslavia in 1991-1992, on the part of Lim's course entering and re-entering Serbia and Bosnia, several villages of Serbia remained physically cut off from the rest of the republic's territory; the governments in Belgrade and Sarajevo proposed an exchange of territories but no agreement was reached. On 4 April 2004, around 10 PM, a bus carrying Bulgarian tourists back to their native Svishtov from a trip to Dubrovnik in Croatia fell and sank in the river near the Serbian village of Gostun close to the border with Montenegro as the driver lost control over it during a turn on a mountain road. Twelve children died, the other 38 people being saved with the aid of the locals. Mala Prosvetina Enciklopedija, Third edition.
Marković: Enciklopedijski geografski leksikon Jugoslavije.
Endemism
Endemism is the ecological state of a species being unique to a defined geographic location, such as an island, country or other defined zone, or habitat type. The extreme opposite of endemism is cosmopolitan distribution. An alternative term for a species, endemic is precinctive, which applies to species that are restricted to a defined geographical area; the word endemic is from New Latin endēmicus, from Greek ενδήμος, endēmos, "native". Endēmos is formed of en meaning "in", dēmos meaning "the people"; the term "precinctive" has been suggested by some scientists, was first used in botany by MacCaughey in 1917. It is the equivalent of "endemism". Precinction was first used by Frank and McCoy. Precinctive seems to have been coined by David Sharp when describing the Hawaiian fauna in 1900: "I use the word precinctive in the sense of'confined to the area under discussion'...'precinctive forms' means those forms that are confined to the area specified." That definition excludes artificial confinement of examples by humans in far-off botanical gardens or zoological parks.
Physical and biological factors can contribute to endemism. The orange-breasted sunbird is found in the fynbos vegetation zone of southwestern South Africa; the glacier bear is found only in limited places in Southeast Alaska. Political factors can play a part if a species is protected, or hunted, in one jurisdiction but not another. There are two subcategories of endemism: neoendemism. Paleoendemism refers to species that were widespread but are now restricted to a smaller area. Neoendemism refers to species that have arisen, such as through divergence and reproductive isolation or through hybridization and polyploidy in plants. Endemic types or species are likely to develop on geographically and biologically isolated areas such as islands and remote island groups, such as Hawaii, the Galápagos Islands, Socotra. Hydrangea hirta is an example of an endemic species found in Japan. Endemics can become endangered or extinct if their restricted habitat changes, particularly—but not only—due to human actions, including the introduction of new organisms.
There were millions of both Bermuda petrels and "Bermuda cedars" in Bermuda when it was settled at the start of the seventeenth century. By the end of the century, the petrels were thought extinct. Cedars ravaged by centuries of shipbuilding, were driven nearly to extinction in the twentieth century by the introduction of a parasite. Bermuda petrels and cedars are now rare. Principal causes of habitat degradation and loss in endemistic ecosystems include agriculture, urban growth, surface mining, mineral extraction, logging operations and slash-and-burn agriculture
Sana (river)
Sana is a river in the north-western part of Bosnia and Herzegovina. It is a tributary of the Una, it is the longest of the nine rivers that flow through Sanski Most, is 142 km long. It is not a navigable waterway; the name of the river derives from the Latin word sana, which means "healthy". The Sana originates from four springs gushing into a karst plateau of the Bosanska Krajina, near the villages of Pečka and Mrkonjić Grad, not far from the city of Šipovo. After a length of about 1.5 km, these streams meet to form the Sana, which pass near the town of Ključ in a south-north direction that the river maintains for many kilometers. The river combines with the waters of its main tributary, the Sanica River; the river passes through Sanski Most and combines with the river Zdena. The Sana goes through the city of Prijedor; the Sana directs its course to the west until Novi Grad where it flows into the Una river as a tributary. Between Prijedor and Novi Grad, the river serves as a natural border between the two towns of Potkozarje and Podgrmeč.
List of rivers of Bosnia and Herzegovina This article incorporates text translated from the Serbian Wikipedia article Сана_, licensed under cc-by-sa