Gillnetting is a common fishing method used by commercial and artisanal fishers of all the oceans and in some freshwater and estuary areas. Gill nets are composed of vertical panels of netting that hang from a line with spaced floaters that hold the line on the surface of the water; the floats are sometimes called "corks" and the line with corks is referred to as a "cork line." The line along the bottom of the panels is weighted. Traditionally this line has been weighted with lead and may be referred to as "lead line." A gill net is set in a straight line. Gill nets can be characterized by mesh size, as well as colour and type of filament from which they are made. Fish may be caught by gill nets in three ways: Wedged – held by the mesh around the body. Gilled – held by mesh slipping behind the opercula. Tangled – held by teeth, maxillaries, or other protrusions without the body penetrating the mesh. Most fish are gilled. A fish passes only part way through the mesh; when it struggles to free itself, the twine prevents escape.
Gillnets are so effective that their use is monitored and regulated by fisheries management and enforcement agencies. Mesh size, twine strength, as well as net length and depth are all regulated to reduce bycatch of non-target species. Gillnets have a high degree of size selectivity. Most salmon fisheries in particular have an low incidence of catching non-target species. A fishing vessel rigged to fish by gillnetting is a gillnetter. A gillnetter which deploys its gillnet from the bow is a bowpicker, while one which deploys its gillnet from the stern is a sternpicker. Gillnets existed in ancient times. In North America, Native American fishermen used cedar canoes and natural fibre nets, e.g. made with nettles or the inner bark of cedar. They would attach stones to the bottom of the nets as weights, pieces of wood to the top, to use as floats; this allowed the net to suspend straight down in the water. Each net would be suspended either between two boats. Native fishers in the Pacific Northwest and Alaska still use gillnets in their fisheries for salmon and steelhead.
Both drift gillnets and setnets have long been used by cultures around the world. There is evidence of fisheries exploitation, including gillnetting, going far back in Japanese history, with many specific details available from the Edo period. Fisheries in the Shetland Islands, which were settled by Norsemen during the Viking age, share cultural and technological similarities with Norwegian fisheries, including gillnet fisheries for herring. Many of the Norwegian immigrant fishermen who came to fish in the great Columbia River salmon fishery during the second half of the 19th century did so because they had experience in the gillnet fishery for cod in the waters surrounding the Lofoten Islands of northern Norway. Gillnets were used as part of the seasonal round by Swedish fishermen as well. Welsh and English fishermen gillnetted for Atlantic salmon in the rivers of Wales and England in coracles, using hand-made nets, for at least several centuries; these a few of the examples of historic gillnet fisheries around the world.
Gillnetting was an early fishing technology in colonial America, used for example, in fisheries for Atlantic salmon and shad. Immigrant fishermen from northern Europe and the Mediterranean brought a number of different adaptations of the technology from their respective homelands with them to the expanding salmon fisheries of the Columbia River from the 1860s onward; the boats used by these fisherman were around 25 feet long and powered by oars. Many of these boats had small sails and were called "row-sail" boats. At the beginning of the 1900s, steam powered ships would haul these smaller boats to their fishing grounds and retrieve them at the end of each day. However, at that time gas powered boats were beginning to make their appearance, by the 1930s, the row-sail boat had disappeared, except in Bristol Bay, where motors were prohibited in the gillnet fishery by territorial law until 1951. In 1931, the first powered drum was created by Laurie Jarelainen; the drum is a circular device, set to the side of the boat and draws in the nets.
The powered drum allowed the nets to be drawn in much faster and along with the faster gas powered boats, fisherman were able to fish in areas they had been unable to go into, thereby revolutionizing the fishing industry. During World War II, navigation and communication devices, as well as many other forms of maritime equipment were improved and made more compact; these devices became much more accessible to the average fisherman, thus making their range and mobility larger. It served to make the industry much more competitive, as the fisherman were forced to invest more in boats and equipment to stay current with developing technology; the introduction of fine synthetic fibres such as nylon in the construction of fishing gear during the 1960s marked an expansion in the commercial use of gillnets. The new materials were cheaper and easier to handle, lasted longer and required less maintenance than natural fibres. In addition, multifilament nylon, monofilament or multimonofilament fibres become invisible in water, so nets made with synthetic twines caught greater numbers of fish than natural fibre nets used in comparable situations.
Nylon is resistant to abrasion and degradation, hence the netting has the potential to last for many years if it is not recovered. This ghost fishing is of environmental concern. Attaching the gillnet floats with biodegradable mat
The mallard is a dabbling duck that breeds throughout the temperate and subtropical Americas and North Africa and has been introduced to New Zealand, Peru, Uruguay, Chile, the Falkland Islands, South Africa. This duck belongs to the subfamily Anatinae of the waterfowl family Anatidae; the male birds have a glossy green head and are grey on their wings and belly, while the females have brown-speckled plumage. Both sexes have an area of white-bordered black or iridescent blue feathers called a speculum on their wings; the mallard is 50 -- 65 cm long. The wingspan is 81–98 cm and the bill is 4.4 to 6.1 cm long. It is slightly heavier than most other dabbling ducks, weighing 0.72–1.58 kg. Mallards live in wetlands, eat water plants and small animals, are social animals preferring to congregate in groups or flocks of varying sizes; this species is the main ancestor of most breeds of domesticated ducks. The female lays eight to thirteen creamy white to greenish-buff spotless eggs, on alternate days. Incubation takes 27 to 28 days and fledging takes 50 to 60 days.
The ducklings are precocial and capable of swimming as soon as they hatch. The mallard is considered to be a species of least concern by the International Union for Conservation of Nature. Unlike many waterfowl, mallards are considered an invasive species in some regions, it is a adaptable species, being able to live and thrive in urban areas which may have supported more localised, sensitive species of waterfowl before development. The non-migratory mallard interbreeds with indigenous wild ducks of related species through genetic pollution by producing fertile offspring. Complete hybridisation of various species of wild duck gene pools could result in the extinction of many indigenous waterfowl; the wild mallard is the ancestor of most domestic ducks, its evolved wild gene pool gets genetically polluted by the domesticated and feral mallard populations. The mallard was one of the many bird species described in the 1758 10th edition of Systema Naturae by Carl Linnaeus, he gave it two binomial names: Anas platyrhynchos and Anas boschas.
The latter was preferred until 1906 when Einar Lönnberg established that A. platyrhynchos had priority as it appeared on an earlier page in the text. The scientific name comes from Latin Anas, "duck" and Ancient Greek πλατυρυγχος, platyrhynchus, "broad-billed"; the genome of Anas platyrhynchos was sequenced in 2013. The name Mallard referred to any wild drake, it is sometimes still used this way, it was derived from the Old French malart or mallart for "wild drake" although its true derivation is unclear. It may be related to, or at least influenced by, an Old High German masculine proper name Madelhart, clues lying in the alternate English forms "maudelard" or "mawdelard". Masle has been proposed as an influence. Mallards interbreed with their closest relatives in the genus Anas, such as the American black duck, with species more distantly related, such as the northern pintail, leading to various hybrids that may be fertile; this is quite unusual among such different species, is because the mallard evolved rapidly and during the Late Pleistocene.
The distinct lineages of this radiation are kept separate due to non-overlapping ranges and behavioural cues, but have not yet reached the point where they are genetically incompatible. Mallards and their domesticated conspecifics are fully interfertile. Genetic analysis has shown that certain mallards appear to be closer to their Indo-Pacific relatives while others are related to their American relatives. Mitochondrial DNA data for the D-loop sequence suggests that mallards may have evolved in the general area of Siberia. Mallard bones rather abruptly appear in food remains of ancient humans and other deposits of fossil bones in Europe, without a good candidate for a local predecessor species; the large ice age palaeosubspecies that made up at least the European and west Asian populations during the Pleistocene has been named Anas platyrhynchos palaeoboschas. Mallards are differentiated in their mitochondrial DNA between North American and Eurasian populations, but the nuclear genome displays a notable lack of genetic structure.
Haplotypes typical of American mallard relatives and spotbills can be found in mallards around the Bering Sea. The Aleutian Islands hold a population of mallards that appear to be evolving towards a subspecies, as gene flow with other populations is limited; the paucity of morphological differences between the Old World mallards and the New World mallard demonstrates the extent to which the genome is shared among them such that birds like the Chinese spot-billed duck are similar to the Old World mallard, birds such as the Hawaiian duck are similar to the New World mallard. The size of the mallard varies clinally; the mallard is a medium-sized waterfowl species, slightly heavier than most other dabbling ducks. It is 50–65 cm long – of which the body makes up around two-thirds – has a wingspan of 81–98 cm,:505 and weighs 0.72–1.58 kg. Among standard measurements, the wing chord is 25.7 to 30.6 cm, the bill is 4.4 to 6.1 cm (
The bald eagle is a bird of prey found in North America. A sea eagle, it forms a species pair with the white-tailed eagle, its range includes most of Canada and Alaska, all of the contiguous United States, northern Mexico. It is found near large bodies of open water with an abundant food supply and old-growth trees for nesting; the bald eagle is an opportunistic feeder which subsists on fish, which it swoops down and snatches from the water with its talons. It builds the largest nest of any North American bird and the largest tree nests recorded for any animal species, up to 4 m deep, 2.5 m wide, 1 metric ton in weight. Sexual maturity is attained at the age of four to five years. Bald eagles are not bald; the adult is brown with a white head and tail. The sexes are identical in plumage; the beak is hooked. The plumage of the immature is brown; the bald eagle is both the national bird and national animal of the United States of America. The bald eagle appears on its seal. In the late 20th century it was on the brink of extirpation in the contiguous United States.
Populations have since recovered and the species was removed from the U. S. government's list of endangered species on July 12, 1995 and transferred to the list of threatened species. It was removed from the List of Endangered and Threatened Wildlife in the Lower 48 States on June 28, 2007; the plumage of an adult bald eagle is evenly dark brown with a white tail. The tail is moderately long and wedge-shaped. Males and females are identical in plumage coloration, but sexual dimorphism is evident in the species, in that females are 25% larger than males; the beak and irises are bright yellow. The legs are feather-free, the toes are short and powerful with large talons; the developed talon of the hind toe is used to pierce the vital areas of prey while it is held immobile by the front toes. The beak is hooked, with a yellow cere; the adult bald eagle is unmistakable in its native range. The related African fish eagle has a brown body, white head and tail, but differs from the bald in having a white chest and black tip to the bill.
The plumage of the immature is a dark brown overlaid with messy white streaking until the fifth year, when it reaches sexual maturity. Immature bald eagles are distinguishable from the golden eagle, the only other large, non-vulturine raptorial bird in North America, in that the former has a larger, more protruding head with a larger beak, straighter edged wings which are held flat and with a stiffer wing beat and feathers which do not cover the legs; when seen well, the golden eagle is distinctive in plumage with a more solid warm brown color than an immature bald eagle, with a reddish-golden patch to its nape and a contrasting set of white squares on the wing. Another distinguishing feature of the immature bald eagle over the mature bird is its black, yellow-tipped beak; the bald eagle has sometimes been considered the largest true raptor in North America. The only larger species of raptor-like bird is the California condor, a New World vulture which today is not considered a taxonomic ally of true accipitrids.
However, the golden eagle, averaging 4.18 kg and 63 cm in wing chord length in its American race, is 455 g lighter in mean body mass and exceeds the bald eagle in mean wing chord length by around 3 cm. Additionally, the bald eagle's close cousins, the longer-winged but shorter-tailed white-tailed eagle and the overall larger Steller's sea eagle, may wander to coastal Alaska from Asia; the bald eagle has a body length of 70–102 cm. Typical wingspan is between 1.8 and 2.3 m and mass is between 3 and 6.3 kg. Females are about 25% larger than males, averaging as much as 5.6 kg, against the males' average weight of 4.1 kg. The size of the bird varies by location and corresponds with Bergmann's rule, since the species increases in size further away from the Equator and the tropics. For example, eagles from South Carolina average 3.27 kg in mass and 1.88 m in wingspan, smaller than their northern counterparts. One field guide in Florida listed small sizes for bald eagles there, at about 4.13 kg. Of intermediate size, 117 migrant bald eagles in Glacier National Park were found to average 4.22 kg but this was juvenile eagles, with 6 adults here averaging 4.3 kg.
Wintering eagles in Arizona were found to average 4.74 kg. The largest eagles are from Alaska, where large females may weigh more than 7 kg and span 2.44 m across the wings. A survey of adult weights in Alaska showed that females there weighed on average 5.35 kg and males weighed 4.23 kg against immatures which averaged 5.09 kg and 4.05 kg in the two sexes. An Alaskan adult female eagle, considered outsized we
Willows called sallows and osiers, form the genus Salix, around 400 species of deciduous trees and shrubs, found on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier, some broader-leaved species are referred to as sallow; some willows are creeping shrubs. Willows all have abundant watery bark sap, charged with salicylic acid, soft pliant, tough wood, slender branches, large, fibrous stoloniferous roots; the roots are remarkable for their toughness and tenacity to live, roots sprout from aerial parts of the plant. The leaves are elongated, but may be round to oval with serrated edges. Most species are deciduous. All the buds are lateral; the buds are covered by a single scale. The bud scale is fused into a cap-like shape, but in some species it wraps around and the edges overlap; the leaves are simple, feather-veined, linear-lanceolate. They are serrate, rounded at base, acute or acuminate; the leaf petioles are short, the stipules very conspicuous, resembling tiny, round leaves, sometimes remaining for half the summer.
On some species, they are small and caducous. In color, the leaves show a great variety of greens. Willows are dioecious, with male and female flowers appearing as catkins on separate plants; the staminate flowers are without either calyx with corolla. This scale is square and hairy; the anthers are orange or purple after the flower opens. The filaments are threadlike pale brown, bald; the pistillate flowers are without calyx or corolla, consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale, borne on the rachis of a catkin. The ovary is one-celled, the style two-lobed, the ovules numerous. All willows take root readily from cuttings or where broken branches lie on the ground; the few exceptions include the goat peachleaf willow. One famous example of such growth from cuttings involves the poet Alexander Pope, who begged a twig from a parcel tied with twigs sent from Spain to Lady Suffolk; this twig was planted and thrived, legend has it that all of England's weeping willows are descended from this first one.
Willows are planted on the borders of streams so their interlacing roots may protect the bank against the action of the water. The roots are much larger than the stem which grows from them. Willows have a wide natural distribution from the tropics to the arctic zones and are extensively cultivated around the world. Willows are cross-compatible, numerous hybrids occur, both and in cultivation. A well-known ornamental example is the weeping willow, a hybrid of Peking willow from China and white willow from Europe; the hybrid cultivar'Boydii' has gained the Royal Horticultural Society's Award of Garden Merit. Numerous cultivars of Salix L. have been named over the centuries. New selections of cultivars with superior technical and ornamental characteristics have been chosen deliberately and applied to various purposes. Most Salix has become an important source for bioenergy production and for various ecosystem services; the first edition of the Checklist for Cultivars of Salix L. was compiled in 2015, which includes 854 cultivar epithets with accompanying information.
The International Poplar Commission of the FAO UN holds the International Cultivar Registration Authority for the genus Salix. The ICRA for Salix produces and maintains The International Register of Cultivars of Salix L.. Willows are used as food plants by the larvae of some Lepidoptera species, such as the mourning cloak butterfly. Ants, such as wood ants, are common on willows inhabited by aphids, coming to collect aphid honeydew, as sometimes do wasps. A small number of willow species were planted in Australia, notably as erosion-control measures along watercourses, they are now regarded as invasive weeds which occupy extensive areas across southern Australia and are considered'Weeds of National Significance'. Many catchment management authorities are replacing them with native trees. Substantial research undertaken from 2006 has identified that willows inhabit an unoccupied niche when they spread across the bed of shallow creeks and streams and if removed, there is a potential water saving of up to 500 ML/per year per hectare of willow canopy area, depending on willow species and climate zone.
This water could benefit the environment or provision of local water resources during dry periods. To aid management of willows, a remote sensing method has been developed to map willow area along and in streams a
The Canada goose is a large wild goose species with a black head and neck, white cheeks, white under its chin, a brown body. Native to arctic and temperate regions of North America, its migration reaches northern Europe, it has been introduced to the United Kingdom, New Zealand, Argentina and the Falkland Islands. Like most geese, the Canada goose is herbivorous and migratory. Successful at living in human-altered areas, Canada geese have proven able to establish breeding colonies in urban and cultivated areas, which provide food and few natural predators; the success of this common park species has led to its being considered a pest species because of their depredation of crops and their noise, aggressive territorial behavior, habit of begging for food. The Canada goose was one of the many species described by Carl Linnaeus in his 18th-century work Systema Naturae, it belongs to the Branta genus of geese, which contains species with black plumage, distinguishing them from the grey species of the genus Anser.
Branta is a Latinised form of Old Norse Brandgás, "burnt goose" and the specific epithet canadensis is a New Latin word meaning "from Canada". According to the Oxford English Dictionary, the first citation for the'Canada goose' dates back to 1772; the Canada goose is colloquially referred to as the "Canadian goose". The cackling goose was considered to be the same species or a subspecies of the Canada goose, but in July 2004, the American Ornithologists' Union's Committee on Classification and Nomenclature split them into two species, making the cackling goose into a full species with the scientific name Branta hutchinsii; the British Ornithologists' Union followed suit in June 2005. The AOU has divided the many subspecies between the two species; the subspecies of the Canada goose were listed as: Atlantic Canada goose, B. c. canadensis Interior Canada goose, B. c. interior Giant Canada goose, B. c. maxima Delacour, 1951 Moffitt's Canada goose, B. c. moffitti Aldrich, 1946 Vancouver Canada goose, B. c. fulva Dusky Canada goose, B. c. occidentalis Lesser Canada goose, B. c. parvipes The distinctions between the two geese have led to confusion and debate among ornithologists.
This has been aggravated by the overlap between the small types of Canada goose and larger types of cackling goose. The old "lesser Canada geese" were believed to be a hybrid population, with the birds named B. c. taverneri considered a mixture of B. c. minima, B. c. occidentalis, B. c. parvipes. The holotype specimen of taverneri is a straightforward large pale cackling goose however, hence the taxon is still valid today and was renamed "Taverner's cackling goose". In addition, the barnacle goose was determined to be a derivative of the cackling goose lineage, whereas the Hawaiian goose originated from ancestral Canada geese. Thus, the species' distinctness is well evidenced, A recent proposed revision by Harold C. Hanson suggests splitting Canada and cackling goose into six species and 200 subspecies; the radical nature of this proposal has provoked surprise in some quarters. The black head and neck with a white "chinstrap" distinguish the Canada goose from all other goose species, with the exception of the cackling goose and barnacle goose.
The seven subspecies of this bird vary in size and plumage details, but all are recognizable as Canada geese. Some of the smaller races can be hard to distinguish from the cackling goose, which overlap in mass. However, most subspecies of the cackling goose are smaller; the smallest cackling goose, B. h. minima, is scarcely larger than a mallard. In addition to the size difference, cackling geese have a shorter neck and smaller bill, which can be useful when small Canada geese comingle with large cackling geese. Of the "true geese", the Canada goose is on average the largest living species, although some other species that are geese in name, if not of close relation to these genera, are on average heavier such as the spur-winged goose and Cape Barren goose. Canada geese have a 127 -- 185 cm wingspan. Among standard measurements, the wing chord can range from 39 to 55 cm, the tarsus can range from 6.9 to 10.6 cm and the bill can range from 4.1 to 6.8 cm. The largest subspecies is the B. c. maxima, or the giant Canada goose, the smallest is B. c. parvipes, or the lesser Canada goose.
An exceptionally large male of race B. c. maxima, which exceed 8 kg, weighed 10.9 kg and had a wingspan of 2.24 m. This specimen is the largest wild goose recorded of any species; the male Canada goose weighs 2.6–6.5 kg, averaging amongst all subspecies 3.9 kg. The female looks identical, but is lighter at 2.4–5.5 kg, averaging amongst all subspecies 3.6 kg, 10% smaller in linear dimensions than the male counterparts. The female possesses a different, less sonorous, honk than the male; this species is native to North America. It breeds in the northern United States in a wide range of habitats; the Great Lakes region maintains a large population of Canada geese
A shipwreck is the remains of a ship that has wrecked, which are found either beached on land or sunken to the bottom of a body of water. Shipwrecking may be accidental. In January 1999, Angela Croome estimated that there have been about three million shipwrecks worldwide. Historic wrecks are attractive to maritime archaeologists because they preserve historical information: for example, studying the wreck of Mary Rose revealed information about seafaring and life in the 16th century. Military wrecks, caused by a skirmish at sea, are studied to find details about the historic event. Discoveries of treasure ships from the period of European colonisation, which sank in remote locations leaving few living witnesses, such as Batavia, do occur as well; some contemporary wrecks, such as the oil tankers Prestige or Erika, are of interest because of their potential harm to the environment. Other contemporary wrecks are scuttled in order to spur reef growth, such as Adolphus Busch and Ocean Freeze. Wrecks like Adolphus Busch and historic wrecks such as Thistlegorm are of interest to recreational divers that dive to shipwrecks because they are interesting to explore, provide large habitats for many types of marine life, have an interesting history.
Well known shipwrecks include the catastrophic Titanic, Lusitania, Empress of Ireland, Andrea Doria, or Costa Concordia. There are thousands of wrecks that were not lost at sea but have been abandoned or sunk; these abandoned, or derelict ships are smaller craft, such as fishing vessels. They may be removed by port authorities. Poor design, improperly stowed cargo and other human errors leading to collisions, bad weather and other causes can lead to accidental sinkings. Intentional reasons for sinking a ship include forming an artificial reef. A ship can be used as breakwater structure. Many factors determine the state of preservation of a wreck: the ship's construction materials the wreck becoming covered in sand or silt the salinity of the water the wreck is in the level of destruction involved in the ship's loss whether the components or cargo of the wreck were salvaged whether the wreck was demolished to clear a navigable channel the depth of water at the wreck site the strength of tidal currents or wave action at the wreck site the exposure to surface weather conditions at the wreck site the presence of marine animals that consume the ship's fabric temperature the acidity, other chemical characteristics of the water at the siteThe above - the stratification and the damages caused by marine creatures - is better described as "stratification and contamination" of shipwrecks.
The stratification not only creates another challenge for marine archaeology, but a challenge to determine its primary state, i.e. the state that it was in when it sank. Stratification includes several different types of sand and silt, as well as tumulus and encrustations; these "sediments" are linked to the type of currents and the type of water, which implies any chemical reactions that would affect potential cargo. Besides this geological phenomenon, wrecks face the damage of marine creatures that create a home out of them octopuses and crustaceans; these creatures affect the primary state because they move, or break, any parts of the shipwreck that are in their way, thereby affecting the original condition of amphorae, for example, or any other hollow places. In addition to the slight or severe destruction marine animals can create, there are "external" contaminants, such as the artifacts on and around the wreck at Pickles Reef and the over-lapping wrecks at the Molasses Reef Wreck, or contemporary pollution in bodies of water, that affect shipwrecks by changing the chemical structures, or further damaging what is left of a specific ship.
Despite these challenges, if the information retrieved does not appear to be sufficient, or a poor preservation is achieved, authors like J. A. Parker claim that it is the historical value of the shipwreck that counts as well as any slight piece of information or evidence, acquired. Exposed wooden components decay quickly; the only wooden parts of ships that remain after a century are those that were buried in silt or sand soon after the sinking. An example of this is Mary Rose. Steel and iron, depending on their thickness, may retain the ship's structure for decades; as corrosion takes place, sometimes helped by tides and weather, the structure collapses. Thick ferrous objects such as cannons, steam boilers or the pressure vessel of a submarine survive well underwater in spite of corrosion. Propellers, condensers and port holes were made from non-ferrous metals such as brass and phosphor bronze, which do not corrode easily. Shipwrecks in some freshwater lakes, such as the Great Lakes of North America, have remained intact with little degradation.
In some sea areas, most notably in Gulf of Bothnia and Gulf of Finland, salinity is low, centuries-old wrecks have been preserved in reasonable condition. However, bacteria found in fresh water cause the wood on ships to rot more than in seawater unless it is deprived of oxygen. Two shipwrecks, USS Hamilton and USS Scourge, have been at the bottom of Lake Ontario since they sunk during a violent storm on August 8, 1813, during the War of 1812, they are