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Fly High
The year 2021 was a year of flight for FoM. In this memorial year, we will release Seabirds as the last chapter.
Seabirds, as top predators, are indispensable organisms in marine ecosystems. The Faculty of Fisheries Sciences, Hokkaido University has first class laboratories and scientists in the field of seabird ecology. In particular, Professor WATANUKI Yutaka, who is a current leader in the seabird research, and the researchers and graduate students in his research group have compiled comprehensive and advanced seabird research findings. I hope you will enjoy reading about it.
The importance of seabird research was first brought to my attention about 20 years ago, when I was on an educational cruise to the Ogasawara Islands on the Oshoro Maru, a training ship used by Hokkaido University. The ship traveled through the waters near Torishima Island, I was able to see with my own eyes albatross breeding grounds. I feel that many of the students in the marine training program have still not forgotten the actual experience of that time. For Japanese students, it was a fascinating field education experience that only Hokkaido University, with its own training ship, can provide in this country.
I wish you all a wonderful 2022, and we will keep flying to increase your interest in the environment and food through FoM, including the ocean, marine life, food, and biotechnology.
FoM Editorial
25 December 2021 posted
Origin and Evolution of Seabirds
Seabird are members of the huge group of of marine organisms, highly adapted to marine life and spending 90% of their time at sea and collecting all their food from the sea (Figure: オオミズナギドリ Calonectris leucomelas, photo by B. Nishizawa). They, however, live under the life-history constraints of birds. Birds, of the class aves, are in the clade Theropoda, which also includes dinosaurs such as the well-known Velociraptor, specialist predators hunting in groups (綿貫 2008). Birds share many derived characters with Theropoda, such as having three fingers on their forelimbs: thumb, index finger and middle finger. These three digits can be seen when we eat chicken wings. Fossils of the earliest bird, the Archaeopteryx, have been found in Late Jurassic strata dated 157 to 146 million years ago. Fossils of other primitive birds, such as Enantheornis have been found from later periods.
Hesperornithids were seabirds living between the Early and Late Cretaceous (~100 million years ago to 65.5 million years ago). Hesperornithids were flightless, but they had developed foot webs between the toes of their hind limbs (綿貫 2013; 綿貫 2019). Hesperornithids used these webs to swim and collect shellfish and fish on the sea bed. After the extinction of the dinosaurs 65.5 million years ago, Plotopterum lived between 35 and 18 million years ago and were distributed in the northern part of the Pacific Rim. Plotopterum propelled themselves through the water with small penguin-like wings and were not able to fly. Another seabird group, Pelagornithidae, specialized in flight. Like the extant albatrosses, they had long and narrow wings and were able to glide. Pelagornithidae lived from 55 million to 3 million years ago and were distributed across the oceans of the world. Whilst sharing many similar traits, these extinct seabirds are not the ancestors of any current living seabird species (綿貫 2013; 綿貫 2019).
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
綿貫豊 (2008) ジュラ紀＝白亜紀温室世界とその終焉—恐竜から鳥類へ. 地球と生命の進化学：新・自然史科学I, 北海道大学出版会. In Japanese.
綿貫豊(2013) ペンギンはなぜ飛ばないのか？海を選んだ鳥たちの姿. 恒星社厚生閣. In Japanese.
綿貫豊 (2019) 空中と水中でのストローク. 鳥の不思議な世界. 一色出版. In Japanese.
25 December 2021 posted
Taxonomy and Current Status of Seabirds
There are 354 species of extant seabirds. In addition, four species have become extinctin recent times. The definition of seabirds is ambiguous. Some bird species of various families occur in both fresh and saltwater habitats depending on season. Some members of the same families are exclusively marine whereas others occur in exclusively freshwater habitats. With this is mind there are six main groups considered to be seabirds: penguins, tubenoses, pelicans, tropicbirds, gannets/cormorants, and a diverse group, charadriiform, consisting of auks, gulls/terns and skuas.
Of the 16 penguin species of the order Sphenisciformes, nine breed in Antarctica and sub-Antarctica (Figure アデリーペンギン Pygoscelis adeliae, photo by A. Takahashi). They are relatively heavy among seabirds, ranging from 1.2 kg for the Little penguin to 30 kg for the Emperor penguin. The order Procellariiformes (‘tubenoses’) is comprised of over 90 species including 22 species of albatross weighing more than 2 kg, over 70 species of shearwaters (Figure: シロハラミズナギドリ Pterodroma hypoleuca, photo by Y. Watanuki) weighing 0.4-4 kg (two of which are extinct), four species of diving petrels weighing 0.1-0.2 kg, and 24 species of storm petrels weighing less than 0.1 kg, most of which breed in the Southern Hemisphere. There are only three species of tropicbird in the family Phaethontidae. The order Suliformes is comprised of 56 species and includes cormorants, gannets、boobies and frigatebirds (Titile figure オオグンカンドリ Fregata minor, photo by Y. Watanuki). The cormorants are 1 to 3 kg in weight and are well adapted to swimming and diving. Gannets and boobies are aerial divers, frigatebirds engage in piracy by stealing other birds’ food. Cormorants are widely distributed across all oceans whereas frigatebirds and boobies are mainly tropical. The order Pelicaniformes consists of eight species of pelican.
The last order, Charadriiformes, consists of skuas, gulls/terns and alcids, although other groups of Charadriiformes such as plovers and sandpipers are not considered seabirds. Skuas often practice piracy other seabird species. Five species of skuas breed in the Arctic and two species in the Antarctic. The gull/tern family includes 101 species: the subfamily Gullinae (‘gulls’), weighing 0.2 to 1.5 kg, and the smaller subfamily Terninae (’terns’), weighing about 0.1 to 0.2 kg. Terns are distributed in both the northern and southern temperate zones, and tropical zones, while most gulls are distributed in the northern hemisphere. Gulls/terns feed mainly on the water surface. Alcids are comprised of 25 species (one of which is extinct), weighing between 0.2 and 1.3 kg, and dive by propelling their short wings. Unlike penguins, alcids are restricted to the northern hemisphere and are able to fly.
The number of seabirds is declining worldwide. Global seabird population is estimated using annual breeding population data from many seabird colonies around the world. Global seabird population has declined by about one-third in the 60 years between 1950 and 2010 (Paleczny et al. 2015). Humans continues to have a variety of impacts on the oceans and islands of the world. Since the Industrial Revolution, the development of larger and more powerful ships enabled us to accelerate the expansion of settlement and fishing over all areas of the ocean. These human activities may be partly or fully responsible for the decline in seabird populations.
According to the IUCN Red List, four species of seabirds are listed as globally extinct, which means that these species have been permanently removed from the world during the period of record: the Great Auk, the Spectacled Cormorant, the Saint Helens Petrel and the Olson’s Petrel. Even before the age of these historical records, there were many seabird species that became extinct due to human settlement. Since then, the number of seabirds has been declining rapidly, and about one-third of the existing species are listed as threatened. This is especially true for tubenoses, where nearly 70% (15 out of 22 species) of albatrosses are listed as threatened.
A study on the status of seabirds in Japan prior to the 1980s noted a decline in the numbers of several seabird species (Hasegawa 1984). A study on the status of seabirds in Hokkaido up to 2000 showed that the numbers of Common Murres, Spectacled Guillemots, and Tufted Puffins declined sharply from the 1950s to the 1960s (Osa and Watanuki 2002). A recent study indicates that Short-tailed albatrosses, Pelagic Cormorants and Rhinoceros Auklet have increased, but Common Murres and Tufted Puffins had declined before the 1980s and have not recovered, and Japanese Cormorants, Black-tailed Gulls and Slaty-backed Gulls had increased until 2000 but have been declining since then (Senzaki et al. 2019) . According to the 2018 Red List of the Ministry of the Environment, 18 species are listed as threatened, including almost half of Japan's breeding seabird species including albatrosses, petrels, cormorants, crested murrelets, murrelets, and petrels.
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
Hasegawa (1984) Status and conservation of seabirds in Japan, with special attention to the short-tailed albatross (Status and Conservation of the World’s Seabirds, International Council for Bird Preservation Technical Publication).
Physical Ability of Seabirds
Based on the characteristics of birds in general, such as feathers, a strong thorax skeleton, large pectoral muscles, and air sacs, seabirds have evolved the abilities to search for and catch prey in the ocean. These include the ability to dive as deep as a nuclear-powered submarine (over 300 meters) in the case of the Emperor Penguin, the swimming speed of an olympian (2 meters per second) in the case of the Japanese Cormorants, the flight speed of a light aircraft (nearly 150 km/h) in the case of the Wandering Albatross, and the range of a jet airliner (over 8,000 km) in the case of Short-tailed Shearwaters.
Because seabirds need to fly over the ocean to search for and catch prey on the open sea, they live in two media with very different physical properties: water and air. The major difference between the two media is density. The density of water is about 800 times that of air, so the major forces acting on objects in motion are different. When moving in water, the main forces are resistance and buoyancy, and when moving in air, the main force is gravity. Because of the different forces at work, the size and stroke speed of the wings or fins suitable for airborne and underwater motion are also different (Taylor et al. 2003). The wings suitable for airborne motion need to be large to generate a large lift force against gravity (Figure ワタリアホウドリ Diomedea exulans, photo by M. Itoh), and the wings or fins suitable for underwater motion need to be small to stroke faster and generate a propulsive force against greater resistance. The wings or flippers suitable for underwater movement are small (Figure オウサマペンギン Aptenodytes patagonicus, photo by K. Sato), and the stroke speed is slow.
Seabirds use their forelimbs (hands) or hindlimbs (feet) or both for propulsion. For this reason, seabirds have evolved five types of morphological groups to move in the air or/and water from the basic form of birds such as gulls (綿貫 2013) (Figure: the evolution of sea birds). The three types that propel wings are: (1) the gliding type, such as the tubenoses, which glide on long and narrow wings; (2) the flying/diving type, such as the auks, which propel their small wings to fly in the air and swim in the water; and (3) flightless-diving type, the penguins, which use their wings as small "fins" for underwater use and not for flying.
On the other hand, there are two types that use their feet for propulsion in the water: (4) the flying/diving type, like the cormorant family, which flies with its large wings in the air but folds its wings in the water and dives with its webbed feet; and (5) the Galapagos flightless Cormorant type, which has larger feet webs and dives with its feet strokes but can’t fly due to its smaller wings. The foot-propelling flying/diving types are equipped with two propellers of different sizes suitable for both underwater and airborne use, but they may have difficulties in maintaining large leg and breast muscles to drive each propeller.
In order to enhance locomotion, seabirds have evolved not only propellers but also other characteristics. One example is the regulation of buoyancy. Seabirds have a lot of air in their feathers, which gives them a certain buoyancy, and they also dive with air in their lungs. When diving, buoyancy, which is an upward force, must be reduced (綿貫 2010). A typical example is cormorants. When floating on the surface of the water, the body of non-diving albatross floats high in the water, while that of diving cormorant sinks significantly (Figure チシマウガラス Phalacrocorax urile, photo by M. Senzaki; Figure アホウドリ Phoebastria albatrus, photo by B. Nishizawa), because each feather of the cormorant has a structure that allows water to penetrate easily.
Such morphological diversification is independent from phylogeny. According to molecular phylogeny, four clades of seabirds (penguins, procellariiformes, pelicans, tropicbirds) were separated more than 60 million years ago (Prum et al. 2015). Among Charadriiformes, skuas and gull/terns were separated between 36 and 30 million years ago (Paton et al. 2003). It is likely that evolutionary expansion into the oceans occurred independently at two different times: over 60 million years ago and between 36 and 30 million years ago in the case of seabirds.
Among seabirds, some species have also lost the ability to fly. Among extinct species three clades were flightless. The evolution of the loss of flight ability occurred independently in six different lineages at different times. Currently penguins are flightless and there have been several species of flightless auk and cormorant which have become extinct in recent times (綿貫 2019).
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
綿貫豊 (2013) ペンギンはなぜ飛ばないのか？海を選んだ鳥たちの姿. 恒星社厚生閣. In Japanese.
綿貫豊 (2019) 空中と水中でのストローク. 鳥の不思議な世界. 一色出版. In Japanese.
綿貫豊 (2010) 海鳥の行動と生態: その海洋生活への適応. 生物研究社. In Japanese.
Taylor et al. (2003) Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency. Nature 425:707-711.
Prum et al. (2015) A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing. Nature 526:569–573. doi:10.1038/nature15697.
Paton et al. (2003) RAG-1 sequences resolve phylogenetic relationships within Charadriiform birds. Molecular Phylogenetics and Evolution 29:268-278
Foraging Behavior and Food of Seabirds
Morphological diversity is related to foraging behavior (綿貫 2010) . Many species of seabird including gulls/terns, albatrosses, some procellariiformes and pelicans are surface feeders. They feed on fish and squid close to the surface of the sea (surface feeding) as well as fish discarded from fishing boats (scavenging). They can also plunge into the surface layer of the water while floating (surface plunge), or plunge from a height in the air into 1 m underwater by inertia (aerial plunge) to catch krill and other large floating fish on the surface.
Gannet/boobies also plunge from 20 m height into the water. Because of the heavy weight, their inertia is high, and their plunge depths can reach several meters. Many of them plunge one after another into schools of fish at considerable depths. They plunge to the water's surface with their wings folded to minimize drag during the plunge. Pelicans and prions feed on planktons in their mouths with the water and use their beaks to scrape up these. Skuas and frigatebirds often steel fish that have been caught by smaller seabirds to feed their young. There are two types of diving: foot propelling, as in the cormorants, and wing propelling as in the penguins and auks. Wing propellers chase and catch fish and krill swarming in the surface water or shoaling in the water column while foot propellers find bottom dwelling fish in rocky crevices on the sea floor.
Seabirds eat a wide variety of foods. Auks and terns bring fish between the bills to their young and the food can easily be analyzed when specimens are collected (Figure イカナゴを嘴にくわえて来たニシツノメドリ Fratercula arctica, photo by A. Takahashi). Other species such as penguins carry their food in their stomachs. The food in the stomach can be collected by stomach flushing techniques. Information about prey can also be obtained using organic matter that has been digested, absorbed, and synthesized into body tissues. Chemical markers such as the stable isotope ratios of nitrogen and carbon and the fatty acid composition of the blood cell and subcutaneous fat can tell us information on seabird diet (綿貫・高橋 2016) .
The prey of seabirds can be categorized as millimeter-sized zooplankton (mainly copepods) and fish eggs with low swimming power, centimeter-sized micronekton with swimming power (krill, Japanese anchovy, young squid, etc.), and 10-centimeter-sized nekton that live in the surface and middle layers (綿貫 2010). The marine invertebrates in intertidal zone can also be important food.
The most important food for seabirds is fish. Seabirds in the northern part of Northern hemisphere feed on Arctic cods Boreogadus saida, capellin Mallotus villosus, sand lance Ammodytes, the herring Clupea, pollack Theragra chalcogramma, krill and squid. Seabirds breeding in the southern hemisphere feeds mainly on Antarctic krill, Myctophids and squid. In the mid-latitude upwelling areas, sardines Sardinops and Anchovy Engrauris are important prey and these fish species show large decadal fluctuations.
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
綿貫豊 (2010) 海鳥の行動と生態: その海洋生活への適応. 生物研究社. In Japanese.
綿貫豊・高橋晃周 (2016) 海鳥のモニタリング法. 生態学フィールド調査. 共立出版. In Japanese.
Biologging and Seabirds
Recent development of biologging technique including GPS, acceleration, water pressure, and temperature records, seabird-mounted cameras and video loggers has made it possible to conduct physiological, behavioral and ecological research of seabirds at sea. There are two reasons why we use such information collected by biologging tools, especially image data.
Research in the high seas require a lot of money and effort, but it has become clear that the stress associated with human activities are extremely high in some places. In the high seas, seabirds find hotspots or important areas passively, where there is a stable supply of food and they use these areas repeatedly. Therefore, they are the candidate as devices for monitoring environmental change in these important areas.
Camera/video loggers attached to seabirds can record images from the seabird's perspective, allowing us to collect a variety of information. Since albatrosses and petrels approach operating fishing boats from a distance, they may be useful for monitoring stress in fishing activities. Video images give us information on the interaction between seabirds and fishing activities. By attaching GPS and video loggers on the backs of Laysan Albatrosses breeding on Oahu Island, Hawaii and recording images over their foraging tracks, we were able to collect images clear enough to determine the type of fishing vessel, its mode of operation, and in some cases even the name of the vessel (Figure GPS on the back of Phoebastria immutabilis; Figure the bird view). This information may be useful for clarifying the risk of bycatch risk and the impact of discarded fish. In addition, in areas where surveys by ships are not feasible, data loggers that record radar radiation waves can be used to find the location of IUU vessels (poaching vessels) that can’t be detected with the Vessel Monitoring System (Weimerskirch et al. 2020).
Camera/video loggers can record marine debris in the open ocean. Albatrosses, which feed on a wide range of food floating on the surface of the sea, frequently ingest marine debris such as plastic. They may also be killed by entanglement in discarded fishing gears. In collaboration with the Yamashina Institute for Ornithology, we simultaneously attached GPS recorders (recording positions at 2-minute intervals) and video recorders (recording 3-second videos at 2-minute intervals during daylight hours) on Black-footed Albatrosses raising their young on Torishima in the Izu Islands, about 580 km south of Tokyo. We collected and analyzed data from 13 birds and found that a total of 16 pieces of relatively large marine debris, such as Styrofoam and fishing gear floating on the sea surface, were captured in the videos of 9 birds (Nishizawa et al. 2021) (Figure debris in ocean). Analysis of the locations of these debris showed that Black-footed Albatrosses forage in the waters around the Izu Islands, and that the most frequent encounters with debris were in the waters with no strong currents south of the Kuroshio Current. This area may have a high potential risk of entanglement in discarded fishing gear which is sadly common in the area.
The other reason is to study the foraging behavior and ecology of seabirds in more detail. In the past, we collected food from seabird’s parents when they returned to their breeding grounds and regurgitated the contents of their stomachs or their mouths. Their foraging behavior was only observed from ships. In recent years, the records of the opening and closing of the beak and the movement of the neck via biologging are used to detect feeding events. However, it was not possible to determine what they ate. We, in collaboration with a Scottish research team, have obtained the image and dive data from European Shags breeding on the Isle of May in Scotland. Using video and acceleration data loggers on the back feather of birds, we were able to obtain images of the prey and microhabitat in which the individual was foraging. The results showed that in some years, the shags repeatedly dove straight down to the seafloor, nearly 40 meters depth, mainly in habitats where rocks and sponges prevailed, and ate butterfish Pholis hiding in the rocks (Watanuki et al. 2008) (Figure the capture photo by Y. Watanuki). In other years, they often used areas where the seafloor was sandy and ate sand eel Ammodytes by sticking their beaks into the sand on the seafloor and driving the fish out of sand.
Beaks of squid had been often found in the stomach contents of many species of albatrosses in previous studies. The size (mantle length) of the squid estimated from the size of these beaks could be nearly 1 meter. It has long been the mystery how they ate such large squid. Our study of Laysan Albatrosses breeding in Hawaii equipped with GPS and video loggers showed that they pecked giant squid Taningia that had died after spawning and were floating on the surface of the subtropical waters (Nishizawa et al. 2018).
NISHIZAWA Bungo・NIPR・JSPS Research Fellow
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
Consumption by Seabirds
Compared to cetaceans, fish such as tuna and sharks, seabirds seemed to be less important as predators in the ocean. However, seabirds feed entirely on marine organisms (zooplankton, fish, squids, etc.), making them true top predators in the marine ecosystem. The annual prey consumption by seabirds worldwide (70 million tons) (Bax 1991) is almost equal to the annual catch by fisheries (80 million tons) (Brooke 2004). Local prey consumption by seabirds can sometimes be greater that of other top predators in some areas of the world and should not be ignored.
Seabirds breed in large numbers in colonies of tens to millions of individuals on remote islands. Parents must return to the colonies repeatedly to swap incubation duties and feed chicks, and so their foraging area is limited to the area around the colonies. Therefore, in the waters around large colonies, huge amounts of food are temporarily consumed by seabirds breeding there. A recent study reported annual fluctuations in seabird prey consumption in the five areas of the world where seabird colonies are concentrated (Saraux et al. 2020). The average prey consumption by seabirds in each area is about 1% of the prey fish abundance. In years when fish stocks were reduced to less than 18% of the maximum level, it was estimated that seabirds consumed up to 20% of that amount. This indicates that seabird predation can sometimes have a significant top-down effect.
Seabird prey consumption is usually estimated by model calculations from the population size of seabirds, energy requirement per individual per day (kJ/day), prey composition, and prey energy density (kJ/g). Multiplying the energy requirement by the number of individuals gives the daily energy requirement of the entire population (kJ/day/population). On the other hand, from the diet composition and the energy density of each prey species, the proportion of the bird's energy requirements by each prey species (energy ratio) can be calculated. Multiplying the energy requirement of the population by this energy ratio, we get the amount of energy consumed by each prey species per day by the population (kJ/day/population). By dividing this amount by the energy density of each prey species again, we can estimate the amount of each prey species consumed by the population (g/day/population), and by multiplying this amount by the period of time (days, for example, the length of the breeding season), we can estimate the amount of each prey species consumed by the population (g/population per season) (綿貫・高橋 2016).
Seabirds nest on land in colonies, it is easy to count (or estimate) the number of individuals in a population. The energy requirements of free-ranging parent birds in the field have also been estimated by examining their oxygen consumption rate using methods such as the double-labeled water method. Furthermore, there is information available on the food composition of the chicks brought home by the parent birds in the breeding grounds, as well as on the stomach contents of the parent birds themselves. Because these lines of information is readily available, the prey consumption by seabirds (especially during the breeding season) can be estimated accurately.
Here is our study on seabird prey consumption. The salmon Oncorhynchus keta is one of the most important fishery resources in Japan, and it is known that there are large annual fluctuations in its abundance. One of the reasons for this fluctuation is thought to be changes in survival due to predation in the early stages of marine life, immediately after the juvenile salmon descend from rivers to the sea. In Japan, where many hatcheries are operated to increase salmon stocks, many studies indicate that juvenile salmon are prone to predation, but little is known about the types of predators at sea that eat them and to what extent.
During the study of the foraging ecology of the Rhinoceros Auklets breeding around Hokkaido, we found that those breeding on Daikoku Island, eastern Hokkaido, frequently ate juvenile salmon. Because they bring whole fish for their chicks in their beaks (Figure Cerorhinca monocerata eating fish photo by M. Itoh), it is possible to measure the length and weight of the fish. Intact otoliths can be sampled. The otoliths of juvenile salmon from hatcheries sometimes have temperature markings, which are unique to the hatcheries in which they originate. Examination of these markings revealed that the Rhinoceros Auklets on Daikoku Island fed on juvenile salmon mainly came from the hatchery at Yurakubu River in the southern Hokkaido. They presumably caught juvenile salmon while these were on migrating to the Sea of Okhotsk where they spend first winter (Figure The map). Then, we estimated the number of juvenile salmon eaten by Rhinoceros Auklets during the early half of the chick rearing period (about 26 days) using the bioenergetics model mentioned above. We estimated that Rhinoceros Auklets on Daikoku Island consumed 0.3-9.2% of the salmon fry released from the hatcheries along the Pacific side of Hokkaido. This study is the first to suggest that predation by seabirds can be an important cause of the mortality of juvenile salmon in the latter half of their early life at sea.
Okado Junpei・School of Fisheries Sciences, Hokkaido University
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
Referenes
綿貫豊・高橋晃周 (2016) 海鳥のモニタリング法. 生態学フィールド調査. 共立出版. In Japanese.
Bax N.J. (1991) A comparison of the fish biomass flow to fish, fisheries, and mammals in six marine ecosystems. ICES Mar Sci Symp 193:217-224.
Brooke M. de L. (2004) The food consumption of the world's sea birds. Proc R Soc Lond B Suppl Biol Let:246-248.
Saraux et al. (2020) Seabird-induced natural mortality of forage fish varies with fish abundance: Evidence from five ecosystems. Fish Fisheries 22:262-279.
Plastics and Seabirds
Plastics are essential to our lives today. However, only about 10% of the plastic discarded in the past 65 years has been recycled, and about 60% has ended up in landfills. In addition, some of the discarded plastic has been released into the ocean, and the cumulative amount of plastic in the ocean continues to grow because it is not degraded. This oceanic plastic is shredded smaller than 5 mm by waves and ultraviolet light and ingested by a wide variety of organisms, from zooplankton, shells, fish, seabirds, and whales (山下ら 2016) . In particular, albatrosses, which are large seabirds that forage on the ocean surface, have been found to ingest plastics at a high frequency and in high quantities (Wilcox et al. 2015). Various sizes of plastic waste have been found in their stomachs, ranging from debris to disposable lighters, sometimes with adverse effects such as stomach ulcers and intestinal blockage. In addition, it is known that seabirds that have plastics in the stomach take in toxic chemicals (POPs etc.) added to the plastic or absorbed from seawater while drifting on the surface of the sea as microplastics during the digestion process (Tanaka et al. 2020).
During the project supported by the National Research Institute of Fisheries Science, the stomach contents of Laysan and Black-footed Albatrosses caught as bycatch in the Pacific around Japan were analyzed in order to investigate their feeding habits. In these analyses, we found plastics in about 90% of the stomachs of Laysan Albatrosses and 50% of those of Black-footed Albatrosses. It may be that Laysan Albatrosses foraging more in offshore waters were more likely to encounter plastics in garbage patches where marine debris are concentrated by winds and water current. Laysan Albatrosses might be attracted to debris and ingested various types of plastics (e.g. pieces of plastic products of various colors; Figure: Plastic debris in the stomach of sea birds). Black-footed Albatrosses often took up plastic in the form of strings (e.g. fishing line). These results supported the results of previous studies (Gray et al. 2012). However, we found that the occurrence of Styrofoam and sponges was higher in our samples. Our study indicates the acceleration of marine plastic pollution in the seas around Japan as in other oceans.
Many questions still remain; the reasons why seabirds take marine plastics and how they are affected. These studies on the change of marine environment using seabirds as indicator help in making our ecosystem sustainable.
SAKAI Lisa・School of Fisheries Sciences, Hokkaido University
WATANUKI Yutaka・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
山下ら (2016) 海洋プラスチック汚染：海洋生態系におけるプラスチックの動態と生物への影響. 日本生態学会誌 66:51-68. In Japanese.
Wilcox et al. (2015) Threat of plastic pollution to seabirds is global, pervasive, and increasing. Proc Natl Acad Sci 112: 11899-11904.
Gray et al. (2012) Incidence, mass and variety of plastics ingested by Laysan (Phoebastria immutabilis) and Black-footed Albatrosses (P. nigripes) recovered as by-catch in the North Pacific Ocean. Mar Pollut Bull 64: 2190-2192.
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