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Copepoda is an essential marine organism in marine ecosystems. Its support fish production. The Plankton Laboratory at the Faculty of Fisheries Science, Hokkaido University, has been a core laboratory for global research on plankton ecology for 70 years, and continues to report new research results.
This time, Associate Professor YAMAGUCHI Atsushi of the Plankton Lab will lead us through an overview of his newly launched project, KAIASHI (Copepod) Fishery.
Catching Copepoda!? We hope that we will persuade you of the significance and importance of this project, help understand its methodology including acoustic detection and net gear development, as well as its nutritional compositions, and that you will share our fascination with copepods in marine ecosystems.
Plankton will be collected on board the T/S Oshoro-Maru and other vessels using NORPAC nets. We will undertake the net collection on a working deck stage called the Otachi-dai. Soon the results of years of plankton observation will reach its pinnacle as the KAIASHI fishery.
FoM Editorial
18 March 2022 posted
Toward The KAIASHI (Copepod) Fishery: the Importance of Copepoda in the Marine Ecosystem and the Project Outline
Are you familiar with the term Copepoda? Probably not many people have ever heard of it. Copepoda, however, is one of the important organisms constructing lower trophic levels of marine ecosystems. Here we’ll introduce you to the Biology of Copepoda, and the importance of the KAIASHI (Copepod) fishery.
Copepoda is zooplankton belonging to the crustaceans and is1 to 10 mm in length, depending on the species. The waters around Japan are dominated by the warm Kuroshio Current and the cold Oyashio Current. Around Hokkaido Island, cold Oyashio is dominated. In the Oyashio region, Copepoda composes 33% biomass of the whole biota (from bacteria to seabird)) (Figure 1, Yamaguchi 2011). The large Copepoda (5-10 mm in length) that dominates in the Oyashio region has a one-year generation length and has a role to transfer the energy of the spring phytoplankton bloom to the higher trophic organisms, making them a renewable resource with high production potential.
During the phytoplankton bloom in spring, Copepoda feeds on phytoplankton and grows at the sea surface layer, depositing oil (lipids) in their bodies. These lipids are utilized as the energy source for the rest of the year staying in the deep layer, and their body fat percentage in mass reaches as high as 80% of dry weight. During summer to winter, when phytoplankton is scarce at the surface layer, Copepoda sinks to 200-2000 m of the deep sea and spends the resting phase termed dormancy (Figure 2). Their reproduction occurs at that depth, they are hatched from eggs the morphologically different from adults termed nauplius. This is an example of metamorphosis, and nauplius is suitable for growth, feeding, and survival at that size. Nauplius has six stages and molts six times. The sixth stage of the nauplius metamorphoses into the first stage of the copepodite, which has the same shape as the adults. There are also six copepodite stages, and the sixth stage is the adults (male and female). Dormancy takes place during copepodite stage five and other stages.
Copepodite stage five of the genus Neocalanus, the dominant deep-sea dormant Copepoda, molt into adults at depth, mate, and lay eggs in the deep sea in winter. N. cristatus, the largest of the Neocalanus species, their eggs laid in the deep sea have a light specific gravity than seawater due to the high lipid content, so they hatch in the water and ascend up to the surface through a series of molting in the no-priest stage. The young reach the surface as copepodite stage one when it is the first feeding stage and grows quickly to copepodite stage five at the surface layer, where they encounter the spring phytoplankton bloom.
The zooplankton community includes both herbivorous species such as Copepoda and carnivorous species such as Chaetognatha. When the production of herbivores compared with the predation by carnivores, both were almost balanced in the subtropical zone, while in the subarctic zone, the production of herbivores greatly exceeded the predation of carnivores. In contrast, in the subarctic, herbivorous production greatly exceeds predation of carnivores, and this surplus production provides food for the growth of epipelagic fishes such as Pacific Saury and Japanese Anchovy, which reproduce in the subtropical zone in winter and migrate northward to the subarctic zone where zooplankton is abundant in summer (Figure 3, Yamaguchi et al. 2017). Thus, large Copepoda in the subarctic region is a renewable resource with high production potential and enormous resource volume. The KAIASHI fishery is an attempt to collect these underutilized lower trophic level organisms and utilize them as food resources for human beings.
It is known that the amount of energy and resources in marine ecosystems decreases by a factor of 10 as the one trophic level increases. This is the concept of ecological efficiency. In other words, fishing for higher trophic organisms is a very fragile and vulnerable way of obtaining food resources. On the other hand, it is easily understood that the amount of energy and resources in marine ecosystems is greater for the lower trophic levels, which are closer to primary production. Therefore, our goal is to establish the KAIASHI fishery that catches Copepoda, which are primary consumers and can be caught with small mesh nets, and to explore the possibility of using them as food resources.
YAMAGUCHI Atsushi・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor／Hokkaido University Arctic Research Center・Associate Professor
Acknowledgements: Exploring the Possibility of a "Copepod Fishery" targeting on the Marine Lower Trophic Organisms supported by KAKEN.
References
18 March 2022 posted
Acoustic Detection of Copepoda
Ultrasonic sound waves are used to estimate the distribution and quantity of organisms in the water. The principle is the echo. Sound is transmitted toward the seafloor. The sound propagates through the sea and reflects the seafloor. By measuring the time between the transmit and receive, we can determine the distance to the seafloor, i.e., the depth of the water. Also, the signature of the echo can be used to determine the target. This is the principle of the echo sounder. The device that transmits and receives ultrasonic waves is called a transducer, and it is generally attached to the bottom of a ship. The echo sounder can detect not only fish but also zooplankton. Even Copepoda, which are only a few millimeters in size, reflect ultrasonic waves. Of course, the size of the reflection is much smaller, but this allows us to determine where the copepods are. Ultrasonic sound waves do not reach everywhere but attenuate with distance as they propagate. Therefore, if the same fish is located at 10 m from the transducer and 30 m from the transducer, the magnitude of the returned ultrasound will be different. A quantitative echo sounder compensates for these characteristics so that the ultrasonic signal can be quantified and the size of the fish or the number of fish in the school can be measured. How and to what extent fish and plankton reflect ultrasonic waves have been studied in various ways, and the general rule is that the higher the sound (higher frequency), the smaller the organism that can be detected. The figure for acoustic detection of copepods shows how different frequencies look at the sea simultaneously. This figure also shows the echograms of an echo sounder looking at the sea at three different frequencies: 38 kHz, 120 kHz, and 200 kHz, from top to bottom. The horizontal direction represents time, and the vertical direction represents the distance from the bottom of the ship or depth. In this figure, the echogram is displayed from the bottom of the ship to a depth of 100 m over about 7 minutes. The grains in the echogram are the response to something being hit, and the warmer the color, the stronger the return sound, and the colder the color, the weaker the return sound. We can see that the echograms are different for the same area of the ocean but at different frequencies. The echograms differ in the range of 10 to 40 meters. The higher the frequency, the echo is strong, so we can predict that the echo shows the plankton. We found out that these are copepods using the plankton net. As you can see, even organisms as small as a few millimeters can exist in dense concentrations, and by using high-frequency ultrasound, we can detect Copepoda. By quantifying these signals and surveying them over a wide area, it is possible to determine the distribution and quantity of Copepoda. Related basic information is also available below. In addition, an example of investigating the distribution of Copepoda in an actual sea area can be found in Kim et al. (2016).
MUKAI Toru・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
LASBOS: 音波を用いた水中観測 ～音で海中をのぞく.
LASBOS: 魚群探知機のいろいろ.
18 March 2022 posted
Acoustic Scattering Characteristics of Copepoda
Surveys of the distribution and abundance of marine organisms using acoustic instruments such as quantitative echo sounder shave been conducted not only for fish but also for zooplankton. In an acoustic survey using a quantitative echo sounder, the amount of living organisms is calculated by dividing the sum of the energy bounced back from ultrasonic waves emitted into the sea by the amount of sound reflected per individual organism in the sea. This index of the amount of sound reflected per individual organism is called the target strength (TS), and it is a very important value in acoustic surveys. This TS varies depending on factors such as the type of organism, its size, the frequency of the ultrasonic wave used, and the direction of sound incidence to the organism. As a result, the TS of various organisms has been investigated so far. There are two main ways to determine the TS of an organism: one is to fix the organism in the water using tegus, etc., and actually apply ultrasonic waves to it and determine the TS from the echoes bounced back; the other is to calculate the TS using a computer with an acoustic scattering model that takes into account the shape of the organism and the reflection coefficient on its body surface. To measure the TS, we have to know the true TS of the organism, but it is time-consuming to prepare organisms of various sizes, use several frequencies, and change the angle of incidence of sound waves to conduct experiments. Therefore, we use the acoustic scattering model. If we can confirm that the acoustic scattering model is correct to some extent, we can use the TS calculated by the acoustic scattering model for actual biomass estimation. In the case of fish, actual measurements of TS have been conducted for a long time because of the large size of the organisms, and acoustic scattering models suitable for each fish species have been investigated. On the other hand, in the case of zooplankton, there are some examples of actual measurements for large species (up to 60 mm in length) such as Antarctic krill, but actual measurements of TS have been impossible for species the size of Copepoda (less than 10 mm in length).
Recently, however, the development of measurement technology has made it possible to measure TS in Copepoda. Sawada et al. (2006) developed an experimental system that can measure TS down to -100 dB while minimizing the effects of noise and Sawada et al. (2011) and Fukuda et al. (2012) used the system to measure TS in Diaphus theta (fork length: 65 mm) and in Euphausia pacifica (body length: 10-20 mm), respectively, off the coast of Hokkaido and Tohoku. We also measured the TS of Neocalanus cristatus (about 8 mm long), a species of Copepoda that inhabits the northern coast of Japan, using this TS measurement system. The figure for the acoustic scattering of Copepoda shows the comparison between the measured TS of N. cristatus and the value calculated by the acoustic scattering model. N. cristatus feeds on phytoplankton, which is produced in large quantities in spring, and stores oil in its body. The upper part of the figure shows a specimen with almost no oil sac, and the lower part shows a specimen with a large oil sac. In the case of a specimen with no oil sac, the main lobe TS and the estimated model values are in agreement with each other, forming a profile near the middle. The lack of agreement at both ends is due to measurement limitations. On the other hand, for specimens with large oil sac, the measured value is convex downward, and the model estimate is convex upward, indicating that the acoustic scattering model cannot be used. In the future, it is necessary to develop an acoustic scattering model that can calculate the TS of Copepoda that have large oil sac.
FUKUDA Yoshiaki・Faculty of Fisheries Sciences, Hokkaido University・Specially appointed assistant professor
References
澤田浩一・石井 憲・安部幸樹・甘糟和男、2006. 小型水槽でのターゲットストレングスの測定限界 ―ターゲットストレングス測定−100 dBへの挑戦―. 海洋音響学会講演論文集69-72.
Sawada K. et al. (2011) In situ and ex situ target strength measurement of mesopelagic lanternfish, Diaphus theta (family Myctophidae). J. Mar. Sci. Tech. 19, 302-311.
福田美亮・向井 徹・澤田浩一・飯田浩二 (2012). 懸垂法を用いたツノナシオキアミEuphausia pacificaの側面方向ターゲットストレングス測定. 日本水産学会誌 78, 388-398.
18 March 2022 posted
Commercial Fisheries Targeting Copepoda
As for zooplankton fisheries, the Antarctic krill fishery targeting Euphausia superba is well known, and the Japanese fishery targeting Euphausia pacifica has been conducted off the Pacific coast of Tohoku. However, fisheries targeting Copepoda are not well known. In Norway (Nordic Sea), however, the fishery for Copepoda is not new and has been conducted in the fjord area since the late 1950s. At that time, small beam trawls and anchored net-like Japanese Komase-Ami were used on an experimental basis, and the scale of fishery was small with an annual catch of 20-50 tons (Figure 1: Wiborg, 1976). The Calanus finmarchicus caught in this fishery were used as food for salmon farming and aquarium fish. Later, in the 1990s, interest in the use of zooplankton such as Euphasiacea and Copepoda increased again, and since the 2010s, the fishing operations for C. finmarchicus have been experimented by a company called Calanus AS. In recent years, Norway has designated C. finmarchicus as a target species for fishing and has set an annual quota of 254,000 tons for 2019 (Nutraceutical, 2019).
In the project of Calanus AS, the fishing operations have been conducted by large vessel using newly developed trawl nets for Copepoda, targeting the depth zone up to 50 m where the C. finmarchicus distribute. This is a large-scale fishery in which Copepoda are caught in the net, that are pumped on board in real time by submersible pump without hauling the net, and quickly processed into frozen blocks by equipment in vessel. Prior to 2018, the fishery was operated as usual, catching and hauling fish without the use of a pump. The size of the nets used at that time was up to 12 m wide and 8 m high near the mouth of the net, and the maximum catch was reported as about 2 tons per hour (Grimaldo and Gjøsund, 2012). In this operation, the nets are towed at a very low speed of 0.5 to 1.5 kt, which is much slower than the typical fin-fish trawl net, which are towed at about 3 to 4 kt. This is due to two main reasons. The first reason is that this trawl net is made of very fine mesh nets to catch very small creatures such as Copepoda. This means that when the nets are towed fast, the resistance of the nets becomes very high, which not only puts a strain on the vessel, but also increases fuel consumption. The situation was further aggravated by the clogging of mesh caused by the Copepoda caught when towing for a long time, but this problem has now been largely eliminated by the use of a pump. The second reason is to protect the ecosystem, that is to avoid bycatch of organisms other than Copepoda as much as possible. Organisms with high swimming ability, such as fish, can easily avoid trawl nets being towed at a slow speed. In Norway, fishing gear and methods for the shallow-water copepod C. finmarchicus have been established through the experimental operations described above. However, there have been no studies and experiments on deep-sea Copepoda.
FUJIMORI Yasuzumi・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
Wiborg, K. F. (1976). Fishery and commercial exploitation of Calanus finmarchicus in Norway. J. Cons. int. Explor. Mer. 36, 251-258.
Grimaldo, E. and Gjøsund, S. (2012). Commercial exploitation of zooplankton in the Norwegian Sea. The Functioning of Ecosystems (Ed. M. Ali), InTech. DOI: 10.5772/36099.
Nutraceutical, business review (2019 Dec 9).
18 March 2022 posted
Copepoda as A New Source of Marine Lipids
Copepoda is mainly herbivorous zooplankton, plays an important role in the marine food web, transporting the energy produced by phytoplankton to the higher trophic organisms. It is well known that fish oil is rich in n-3 polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are good for health, but animals generally cannot synthesize these n-3 PUFAs on their own. Fish accumulate n-3 PUFAs in their bodies by feeding on Copepoda, which feed on phytoplankton that can biosynthesize EPA and DHA. Since Copepoda account for about 80% of the zooplankton biomass in the ocean, it can be said that they are an important part of the food web in terms of n-3PUFA supply.
In recent years, stable catches of fish have become difficult not only in Japan but also worldwide, and securing fish oil resources for use in aquaculture fish feed and food supplements has become a major issue. Zooplankton and small crustaceans are attracting attention as a potential solution to this problem. During the fifth stage of copepodite growth, copepodites have dormant in the deeper layers through large-scale vertical downward migration and accumulate large amounts of lipids in their bodies accumulating the nutritions from phytoplankton for energy to produce the eggs. At that time, n-3 PUFAs are also stored abundant, so they can be expected to become a very valuable feed ingredient in the cultivation of fish, which are higher-trophic predators.
On the other hand, it has been reported that the lipids accumulated in the body of various Copepoda are mainly wax esters (WE) (Richard et al., 2006). In particular, the WEs of Neocalanus and Calanus spp. have been found to be highly composed of n-3 PUFAs. WEs have a variety of structures depending on the type of fatty acids and aliphatic alcohols and are known to cause diarrhea when eaten because WEs from deep-sea fish such as Abrascomorpha are not digestible. However, the copepod lipids, which are rich in n-3PUFA-bounded WEs, do not cause diarrhea even when consumed at 2 g or 4 g per day, and the results of human intervention studies have shown that they are useful as an important source of EPA and DHA (Cook et al., 2016; Tande et al., 2016).
Table 1. n-3 PUFA ratio in TG and WE.
Recently, insect diets have attracted attention as a potential food source for human beings. Protein- and lipid-rich insects are considered to have very high nutritional value. On the other hand, EPA and DHA are hardly contained in the fatty acids that make up the lipids. Although alpha-linolenic acid derived from food plants may be present, the percentage of alpha-linolenic acid that is converted to DHA in the human body is very small, less than 10% at most (Burdge et al., 2002; Burdge and Wootton, 2002; Plourde and Cunnane, 2007), so Copepoda may be a better direct source of EPA and DHA.
The lipids that caecilians accumulate are bright red in color and contain astaxanthin, a marine carotenoid. Astaxanthin shows excellent antioxidant properties and has recently been commercialized and widely recognized as a cosmetic and functional food. However, while organically synthesized astaxanthin has been used to enhance the color of cultured fish, the sources of naturally occurring astaxanthin available for human consumption are limited to krill and Hematococcus algae. In such cases, the copepod lipids may be a useful resource as a natural source of astaxanthin, but the extent of astaxanthin absorption and functional effects when consumed as food needs further study.
A Norwegian company has already targeted the copepod Calanus finmarchicus for fisheries and has begun selling its caecilian oil as a health supplement. On the other hand, since the lipid composition of caecilians varies depending on the species, developmental stage, sea area, etc., the identification of species with more functional lipids and the discovery of new unknown functional components are also expected in the future. Although there are still many issues to be resolved, such as quality and safety assurance, and cost, it is quite possible that Copepoda collected in the neighboring waters of Japan will be used as a source of highly functional marine lipids in the future.
BEPPU Fumiaki・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
References
Burdge, G.C., Jones, A.E., and Wootton, S.A. (2002). Eicosapentaenoic and docosapentaenoic acids are the principal products of alpha-linolenic acid metabolism in young men. Br. J. Nutr. 88, 355-363.
Burdge, G.C., and Wootton, S. A. (2002). Conversion of alpha-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women. Br. J. Nutr. 88, 411-420.
Cook, C.M., Larsen, T.S., Derrig, L.D., Kelly, K.M., and Tande, K.S. (2016). Wax ester rich oil from the marine crustacean, Calanus finmarchicus, is a bioavailable source of EPA and DHA for human consumption. Lipids 51, 1137-1144.
Plourde, M., and Cunnane, S.C. (2007). Extremely limited synthesis of long chain polyunsaturates in adults: implications for their dietary essentiality and use as supplements. Appl. Physiol. Nutr. Metab. 32, 619-634.
Richard, F.L., Wilhelm, H., and Gerhard, K. (2006). Lipid storage in marine zooplankton. Mar. Ecol. Prog. Ser. 307, 273-306.
Tande, K.S., Vo, T.D., and Lynch, B.S. (2016). Clinical safety evaluation of marine oil derived from Calanus finmarchicus. Regul. Toxicol. Pharmacol. 80, 25-31.
18 March 2022 posted
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