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Mazama Newt Conservation

Welcome to Project Mazama Newt, where we discuss conservation of the Mazama newt.

Introduction

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Anatomical variation between the rough-skinned newt and its subspecies the Mazama newt

The Mazama newt (Taricha granulosa mazamae) is a subspecies of the rough-skinned newt and is endemic to Crater Lake. While the life history of the Mazama newt is incomplete, it is known that the newt colonized Crater Lake around 6,000 years ago.[1] The Mazama newt survives on a diet of snails, aquatic insect larva, and terrestrial insects. [2] The Mazama newt can produce a low toxicity level of tetrodotoxin, a potentially lethal neurotoxin. [3]

Biology

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Life history

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While not thoroughly studied, the life history of the Mazama-newt is presumably similar to the main population of rough-skinned newts; the latter having been extensively studied. Rough-skinned newts begin their life growing in their eggs the mother newt produces, one at a time, underwater and latches onto vegetation.[4][5] When the newts hatch after three to four weeks, they live as aquatic larvae for another four to five months before becoming juveniles and leaving the water.[5] These juveniles will stay near the water location, and will return to the water as adults at the age of four to five years to reproduce.[5] The times when these newts reproduce vary depending on where the elevation of their habitats are located; if in higher elevations, the newts reproduce during the late summer. In lower elevations, they reproduce during the spring months.[5] Examples of different habitat are mountain lakes, flood plains, streams, marshes, and other similar locations.[6]

Newts, in general, can be found in many different locations from North America to Europe to all through Asia. In these locations, there is a large diversity in newt populations, and the populations have their own unique range for distribution.[6] Newts are considered to be carnivores, and the different populations have similar diets. Their diets include worms, flies, insect larvae, beetles, snails, tadpoles, spiders, slugs, and other similar prey.[6] When dying from natural causes, most newts can live up to 15 years and can reach up to 20 centimeters long and 50 grams in weight.[4] As amphibians and a part of the salamander family, they have a salamander-type body, long torso and long tail, with colors varying from red, black, yellow, white, orange, grey, and brown.[6] Even though Mazama newts have been found to be similar to the rough-skinned newts, they are still a mystery to scientists and others.

Physiological ecology

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Rough-skinned newts are unique semi-aquatic animals due to their production of a deadly and powerful paralyzing  neurotoxin called the tetrodotoxin (TTX).[7] According to scientific studies, a group of bacteria called Pseudomonas lives on the newts’ skin and produce TTX.[8]  The newts utilize their production of TTX as a chemical defense against their predators such as snakes, herons, or foxes.[7][4] When predators swallow the TTX on the newts, the toxin will discontinue the signals from the nerve cells telling muscles to move. Therefore, predators who swallow the toxin in low doses may experience numbness and those who swallow the toxin in high doses may experience paralysis or even death.[8] However, newts are resistant to TTX and not immune to it. Therefore, the production of TTX on the newts is energy extensive and causes newts to be slow and mellow.[9] Additionally, newts keep TTX on its skin because even a few milligrams of it in their guts could be lethal; most toxic newts will only have 14 mg to 15 mg of TTX at a time.[9] Ultimately, TTX is a deadly toxin for all living organisms including newts, however, newts have evolved to develop resistance against a certain amount in order to defend themselves against their predators.

Since the production of TTX does have its trade-offs, rough-skinned newts tend to produce less TTX if they do not need it to fight against predators. Mazama newts specifically have grown accustomed to the predator-free environment of Crater Lake which has resulted in their minimal production of TTX. In fact, regular rough-skinned newts in nearby populations can produce 4,000 times more TTX than Mazama newts. Unfortunately, when humans introduced crayfish into Crater Lake, the Mazama newts have been struggling to defend themselves due to their low dosage of TTX.[7] Due to their adaptation to living predator-free, Mazama newts are near extinction because they are the crayfish’s primary food source. Fundamentally, unlike regular newts, the Mazama newts are not able to utilize the production of TTX and can only attempt to hide from the crayfish.[7]

Habitat

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Crater Lake

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Crater Lake is located in Klamath County in the southern part of Oregon, approximately 100 miles east of the Pacific Ocean and 80 miles northeast of Medford, the closest well-known town. The caldera, in which the lake occupies, is the result of the violent eruption of Mt. Mazama that took place 7,700 years ago [10]. The Crater Lake caldera is considered to be a dormant volcanic site today, with the most recent eruption occurring between 6,600 and 4,800 years ago [11] [12], which resulted in the creation of Wizard Island in the lake. Mt. Mazama, now the foundation of Crater Lake, was a composite volcano comprised of mostly andesite and decite [13], that started forming about 400,000 years ago and at its peak used to stand approximately 12,000 feet tall [14]. Mt. Mazama itself is part of the High Cascades volcanic chain, which is made up of volcanoes that range from Washington to California.

 
Aerial photo Crater Lake in Southern Oregon

The Cascadian Mountain chain, and the vulcanism that comes with it, is a result of the subduction of the Juan de Fuca plate underneath the North American plate. This subduction generated the magma supply for each of this volcanoes from Mt. Lassen in California to Mt. Baker in Washington, including Mt. Mazama. Vulcanism and mountain chain creation is a by-product of subduction is because once the subducting plate reaches 100 km the subducting plate starts to melt into magma, and the magma then rises to the surface of the other plate and creates volcanoes. Because it is a former volcanic giant, the lakes altitude is 8,159 feet [15].

The weather at Crater Lake is typical of a high altitude. During the winter months this area experiences significant snowfall with average temperatures ranging from 40*F to 18*F. During the summer months snowfall still occurs but is scarce and temperatures range from 70*F to 35*F depending on the time of day. The average annual lake temperature of Crater Lake is between 59*F and 38*F[16].

Mazama newt habitat

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Mazama newts colonized the shorelines of Crater Lake around 6,000 years ago when the lake was still relatively young [17] and active volcanically. The shoreline of the lake, the primary habitat of the Mazama newt, primarily consists of cobble and boulder-sized rocks weathered and deposited from the cliffs above the shore with alternating bands of bedrock [18]. As adults these newts live mostly on the shoreline, returning to the water to lay their eggs. Larvae and juveniles newts live solely in the waters of Crater Lake. Mazama newts have a longer relationship with water throughout their lives as compared to other species of newts, which makes them more dependent on the conditions and ecology of the water where they spend so much time. When they are in the water, Mazama newts are benthic zone dwellers, meaning they live in the zone of water closest to the floor of the lake [19]. The benthic zone of Crater Lake is typical of that of a caldera; it has a minimal shoreline, steep walls, a fairly flat bottom with the exception of the two newer volcanic features (e.g., Wizard Island), and some volcanic debris [20].

Biological history of Crater Lake

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Crater Lake has an intricate, if small, food web. There are four species of blue-green algae, four species of green algae, and over 112 species of diatom algae [21]. There’s also the Drepanocladus moss, which forms massive colonies and can grow in waters up to 460 ft. deep, and the water buttercup, a flowering plant that grows on the surface of the water. The primary consumers are the water flea, the freshwater shrimp, snails, copepods, and bloodworms/midge larvae. The secondary consumers are dragonfly nymphs, tardigrades, and whirligig beetles. The tertiary consumers are the Mazama newts and signal crayfish, the latter being. At the top of the food-web are kokanee salmon and the rainbow trout. In 1888, officials began stocking the lake with seven different species of fish, only two of which survived to today[22].

Conservation status

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Currently the endemic population of Mazama newts are facing the threat of a species commonly known as the signal crayfish, which was introduced to crater lake in 1915. Due to the fact that this species has dominated nearly 80% of the Crater Lake shoreline there has been massive reduction of Mazama newt abundance, leaving them to inhabit the few locations along the shoreline that the signal crayfish have not overrun [23]. Tests have been conducted indicating that the invasion of the crayfish not only reduced the abundance of the newt, but displace the newts from cover which render the newts even more vulnerable to other prey and alter their behavior [24]. Further expansion of the signal crayfish could lead to the eventual extinction of the unique Mazama species [25].

Signal crayfish

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The signal crayfish was introduced to the Crater Lake ecosystem

The species of signal crayfish (Pacifastacus leniusculus ), was introduced to Crater lake in 1915 in an effort by state park officials to create a working food system for the introduced species of fish, including trout and salmon [26]. This system would attract visitors and generate revenue for the park, but had unintentional impacts on the endogenous population of Mazama newts, (Taricha granulosa mazamae). After its introduction in 1915, the signal crayfish has taken over the littoral zone of the lake (approximately 80% of the shoreline), which is also the habitat of Mazama newts, and they share many aspects of the same food web niche [27]. The signal crayfish is omnivorous, having a diet that consists of aquatic insects, detritus, and plant matter; this wide food range allows for the out-competition of endogenous species that have a narrower range of food sources [28]. The signal crayfish expansion and domination of the littoral zone of Crater Lake has caused the severe population decline of the Mazama newt; this is due to competition for food, energy expenditure to avoid aggression, and predation of the crayfish on newts [29].

Conservation strategies

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Here, we provide various suggestions for conserving the endangered population of Mazama newts.

Proposal 1: Captive breeding and release

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Captive breeding and release programs are done by first capturing sexually mature male and female newts for the purpose of breeding. Once the newts are breed and lay their eggs the adult newts are released back into their natural habitat. However, the eggs are kept and cared for in captivity until they reach their juvenile stage of their life; and then they are reintroduced into their natural habitat.

Captive breeding and release programs have been known to be successful in the past. For example, researchers were able to help raise the population size of the yellow-spotted mountain newts in Western Iran by using this conservation method. [30] Amphibians such as the yellow-spotted mountain newts are ideal organisms for the captive breeding method because of their small body size, high fecundity, rapid growth, and low maintenance requirements.[30] However, this method has continued to be controversial because of issues such as interbreeding depression or adaptation to captive life. [30]

Despite these risks, researchers in Iran continued with this strategy because it was the newts’ best chance of survival.[30] Researchers began by beginning and implementing a conservation management plant with the help of the Mohamed bin Zayed Species Conservation Fund including the establishment of a captive breeding facility in Razi University at Iran.[30] Their facility was ventilated by an air conditioner and had ten aquariums were designed to be similar to the newts’ natural habitat with specific temperatures and small pebbles from the wild as the terrestrial habitat.[30] Researchers kept 9 to 12 mature newts together with a sex ratio of 2 males and 1 female in one aquarium to maximize breeding.[30] Once the newts reached 5 to 7 months old, they were considered able enough to be reintroduced into the reintroduction sites because they have formed resistance to environmental factors and combat predation.[30] Reintroduction sites are carefully picked by the researchers depending on factors including water temperature and distance from human settlements. Fortunately, researchers in Iran have been successful as their captive populations increase every year with an average survival rate of 20.5%.[30]

This method should be used for the Mazama newts in Crater Lake as the last possible solution because it may not be as successful. Crater Lake researchers would be able to increase population size by mating the few Mazama newts left, however, they would have a difficult time ensuring their survival after reintroducing them to Crater Lake. Researchers in Iran had five carefully chosen sites to ensure maximal survival while Crater Lake researchers would only have one reintroduction site dominated by the crayfish population. In order for this method to have a possibility of success on the Mazama Newts, Crater Lake researchers need to find a way to limit crayfish population or attempt to find a different site where Mazama newts could begin a new population.

Proposal 2: Genomic modification

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A potential option for eradicating the negative impacts of invasive species in ecosystems is a gene drive. A gene drive is when organisms reproduce and pass down a preferred gene to their offspring in order to create a population with only that specific gene. Scientists can create a gene drive using the CRISPR/Cas9 system, which is a way to alter genes. A gene drive could be used in invasive species to eradicate them from ecosystems if, for example, the altered gene caused the offspring to all become males.

A gene drive is accomplished by altering an organism's genes and then breeding these genetically modified individuals with wild individuals that lack the altered genes. When these organisms breed and reproduce, their offspring will gain one chromosome from each parent: one altered chromosome and one 'wild type' chromosome.[31] The altered chromosome will hold three different parts: the altered gene, a gene for the Cas9 enzyme (an enzyme that cuts DNA at a specific location[32]), and DNA corresponding to multiple guide RNAs.

Following reproduction, the offspring's altered chromosome will use the guide RNA as a message to tell the Cas9 enzyme to cut and destroy the unaltered, wild type gene. Then, the altered gene can be copied and added onto the wild type chromosome, becoming a new copy of the altered gene. Each generation this will occur and each time the frequency of the altered gene increases in the population while the frequency of the wild type gene decreases. Eventually, the wild type gene will be dominated by the genetically altered one.[31] A gene drive could potentially eradicate the invasive crayfish in Crater Lake by introducing an altered gene that negatively affected their reproduction.

Gene drives have been studied in mosquitoes, for example. In one approach, the mosquitoes were genetically altered so that they were immune to diseases such as malaria and the Zika virus. Additionally, another approach was to create daughterless offspring, thus eradicating mosquitoes[33]. In the spring of 2021, a biotechnology company by the name of Oxitex released genetically modified male mosquitoes in the Florida Keys. When these males mate with wild, female mosquitoes, a gene will be passed down to the next generation which causes the offspring to not survive into adulthood.[34] This same concept could be applied to the signal crayfish in Crater Lake. By making daughter-less offspring eventually the crayfish will be removed from the ecosystem and the newts will be able to return to normal abundance. This certainly leads to questions such as, do we want to completely remove crayfish from the ecosystem? What other impacts would the removal of the crayfish have on the ecosystem? Gene drive and CRISPR/Cas9 technology is a not yet perfected but many studies and tests are being conducted.

Proposal 3: Biological control

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To control the expansion and aquatic dominance of the signal crayfish over the Mazama newt a course of action could be to introduce a freshwater parasite called Psorospermium haekeli. This species was discovered in 1883 and affects species of crayfish. It resides in the thoracic cavity of the crayfish where it will parasitize the crayfish from its connective tissues and thoracic endothelial tissues that lead to the brain. By the introduction of this freshwater parasite that is specific to only the crayfish population of Crater Lake, we can hinder the relative fitness of the while not disrupting other endogenous aquatic species[35]. This method would be applicable to a cooperative approach with other methods due to it's very specific ecological effect and less than devastating effects. Caution would be advised with the consumption of crayfish with this parasite as studies have not concluded if it is safe for human consumption. Psorospermium haekeli is an amoeba that is speculated to posses many different morphologies, and if cyst morphology exists it could allow for the amoeba to survive in boiling or freezing temperatures, potentially risking consumer health if found to be pathological.

Proposal 4: Physical barriers

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Currently, placing fencing to keep the crayfish from advancing further along the shoreline is being considered for containing the signal crayfish population in Crater Lake. Because signal crayfish don't swim high or burrow deep, all that would be needed is a foot and a half tall metal fence funning in a straight line from the shore into the water. There are several concerns about this. The main concern is that the fences would ruin the lake's aesthetic. Another concern is how far down the fences have to stretch because crayfish have been found at depths of up to 800 feet. Studies are currently being done on whether or not these deepwater residing crayfish return to the shallows[36]. In the Investigation of Crayfish Control Technology Report by the Arizona Fish and Game Department [student 2: insert proper reference], physical barriers such as fencing were found to be at best temporary solutions, although they can make trapping easier.

Proposal 5: Trapping, predation, and augmentation

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In order to preserve the Mazama newt and protect it from extinction, something has to be done about the increasing signal crayfish population, even if it’s only a temporary solution. Unfortunately once these crayfish are introduced to an ecosystem and allowed to take hold, they are almost impossible to remove or control. In fact, almost all conventional methods, trapping, manual catching, habitat barriers, controlled diseases, increased predation, etc. by themselves have been widely unsuccessful in removing or even managing invasive crayfish populations [37]. North American crayfish are resilient, rapid reproducers, and eat almost anything, from insects, snails, and small fish, to others of their species if necessary [38]. They can even remain fully functioning, albeit sterile, after being exposed to 40 Gy of radiation (four times the lethal dose to a human) [39].

In other invasive populations, such as the European green crab (Carcinus maenas) in British Columbia, the method of annual trapping has been shown to be an effective temporary solution to the population size. However due to other factors, immigration of more crabs, a surviving larval population, and a lack of trapping frequency, trapping alone hasn’t been enough to suppress the population to the desired levels for a large amount of time (i.e. longer than a year) [40]. Luckily, Crater Lake is an enclosed ecosystem, so there are no crayfish immigrating to the area; which only leaves the challenge of how to remove or lessen the already existing signal crayfish population from the lake. There is evidence that intensive trapping along with increased fish predation of crayfish populations can drastically reduce crayfish population size [41] and at this time, these measures seem to be the best option for decreasing the crayfish population of Crater Lake. However, though they seem to solve the immediate threat that the crayfish pose to the Mazama newt, these measures also take time and effort, and do not completely remove the crayfish from the area, meaning they would continue to be a threat in the future if not carefully controlled.

Right now the Mazama newt population is very low, with fewer than 12 newts found at any single testing site in the lake (compared to the crayfish found in the hundreds per site) [42]. In other populations, such as North American freshwater mussels (Mollusca: Bivalvia: Unionoida), controlled propagation, augmentation, and reintroduction of the species has worked to boost population size in dire circumstances [43]. In the case of the Mazama newt, controlled propagation, augmentation and reintroduction would be to their benefit in conjunction with heavy trapping and fish predation of the signal crayfish. This step would help return the Mazama newt population to reasonable numbers along with keeping a balance with the available food supply of the lake as to not exceed its capabilities to support the population.

In conclusion, unless we are able to completely remove the signal crayfish population from the lake or create a balance to keep their numbers in check, we will constantly be involved in manually managing the newt and crayfish population numbers. However, if nothing is done, if no steps are taken to ensure the survival of the Mazama newt, it will become extinct while we are all watching.

References

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  1. ↑ National Park Service, "Mazama Newt Toxicity", 2018
  2. ↑ Farner, Donald S., Kezer J., "Notes on the Amphibians and Reptiles of Crater Lack National Park." The American Midland Naturalist, Vol.50, no.2, 1953
  3. ↑ National Park Service, "Mazama Newt Toxicity", 2018
  4. ↑ 4.0 4.1 4.2 "Amazing Facts about Newts | OneKindPlanet Animal Education & Facts". OneKindPlanet. Retrieved 2021-06-08.
  5. ↑ 5.0 5.1 5.2 5.3 Amanda Lorenz. "Taricha granulosa (Rough-skinned Newt)". Animal Diversity Web. Retrieved 2021-06-08.
  6. ↑ 6.0 6.1 6.2 6.3 Oldham, Cydni (2019-10-24). "Newt - Description, Habitat, Image, Diet, and Interesting Facts". Animals Network. Retrieved 2021-06-08.
  7. ↑ 7.0 7.1 7.2 7.3 "Meet the Mazama Newt". Oregon Wild. Retrieved 2021-06-08.
  8. ↑ 8.0 8.1 "Toxic germs on its skin make this newt deadly". Science News for Students. 2020-06-23. Retrieved 2021-06-08.
  9. ↑ 9.0 9.1 Martin, Shelby (2008-03-12). "Snakes slither past toxic newts in evolving race". Stanford University. Retrieved 2021-06-08.
  10. ↑ Orr, Elizabeth and William Orr. Oregon Geology. 6th ed., Oregon State University Press, 2014, p. 164.
  11. ↑ “Crater Lake - Home.” U.S.G.S., www.usgs.gov/volcanoes/crater-lake/. Accessed 11 June 2020.
  12. ↑ Orr, Elizabeth and William Orr. Oregon Geology. 6th ed., Oregon State University Press, 2014, p. 160.
  13. ↑ “Crater Lake - Home.” U.S.G.S. www.usgs.gov/volcanoes/crater-lake/. Accessed 11 June 2020.
  14. ↑ Orr, William and Elizabeth Orr. Geology of the Pacific Northwest. 2nd ed., Waveland Press, Inc., 2002, p.94.
  15. ↑ “Crater Lake - Home.” U.S.G.S., www.usgs.gov/volcanoes/crater-lake/. Accessed 11 June 2020.
  16. ↑ “Weather.” N.P.S., 29 Mar. 2020, www.nps.gov/crla/planyourvisit/weather.htm. Accessed 11 June 2020.
  17. ↑ “Mazama Newt Distributions.” N.P.S., 28 Feb. 2015, www.nps.gov/crla/learn/nature/newtdistribution.htm. Accessed 12 June 2020.
  18. ↑ Girdner, S.F., Ray, A.M., Buktenica, M.W. et al. “Replacement of a unique population of newts (Taricha granulosa mazamae) by introduced signal crayfish (Pacifastacus leniusculus) in Crater Lake, Oregon.” Biol Invasions 20, 721–740 (2018). doi.10.1007/s10530-017-1570-6. Accessed 12 June 2020.
  19. ↑ Girdner, S.F., Ray, A.M., Buktenica, M.W. et al. “Replacement of a unique population of newts (Taricha granulosa mazamae) by introduced signal crayfish (Pacifastacus leniusculus) in Crater Lake, Oregon.” Biol Invasions 20, 721–740 (2018). doi.10.1007/s10530-017-1570-6. Accessed 12 June 2020.
  20. ↑ Marshak, Steve. Earth Portrait of a Planet. 5th ed., W.W. Norton & Company, Inc., 2015, p.283.
  21. ↑ Bacastow, Wesley
  22. ↑ “Fishing” Crater Lake National Park Service
  23. ↑ Buktenica, M. W., et al. “The Impact of Introduced Crayfish on a Unique Population of Salamander in Crater Lake, Oregon (U.S. National Park Service).” National Parks Service, U.S. Department of the Interior, 4 Sept. 2015, www.nps.gov/articles/parkscience32_1_5-12_buktenica_et_al_3815.htm
  24. ↑ Buktenica, M. W., et al.(2015).The impact of introduced crayfish on a unique population of salamander in Crater Lake, Oregon. Park Science, 32 (1), 5-12. https://www.nps.gov/articles/parkscience32_1_5-12_buktenica_et_al_3815.htm
  25. ↑ Buktenica, M. W., et al. “The Impact of Introduced Crayfish on a Unique Population of Salamander in Crater Lake, Oregon (U.S. National Park Service).” National Parks Service, U.S. Department of the Interior, 4 Sept. 2015, www.nps.gov/articles/parkscience32_1_5-12_buktenica_et_al_3815.htm
  26. ↑ Umek, John., Dr. Sundeep, Chandra. The ecology of signal crayfish in two large ultra-oligotrophic ecosystems: Crater Lake and Lake Tahoe. Dissertation. University of Nevada, Reno. 2016.
  27. ↑ Buktenica, Mark & Girdner, Scott & Ray, Andrew & Hering, David & Umek, John. The impact of introduced crayfish on a unique population of salamander in Crater Lake, Oregon. Park Science. 32. 5-12. (2015).
  28. ↑ Hogger JB. Ecology, population biology, and behaviour. In: Holdich DM, Lowery RS, eds. Freshwater Crayfish. Biology, Management and Exploitation. London, Portland: Croom Helm Ltd., 114-144, 424-479 (1988).
  29. ↑ Buktenica, Mark & Girdner, Scott & Ray, Andrew & Hering, David & Umek, John. The impact of introduced crayfish on a unique population of salamander in Crater Lake, Oregon. Park Science. 32. 5-12. (2015)
  30. ↑ 30.0 30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 Mozafar S. Somaya V., Endangered Species Research, "Captive breeding and trial reintroduction of the endangered yellow-spotted mountain newt neurergus microspilotus in western Iran", Vol. 23, 2014
  31. ↑ 31.0 31.1 "Gene Drives". Wyss Institute. 2017-08-24. Retrieved 2021-06-12.
  32. ↑ "What is CRISPR-Cas9?". yourgenome. Retrieved 2021-06-12.
  33. ↑ Keele, Jacque. "Using Genetic Manipulation to Control invasive Species. Sept. 2017. https://www.usbr.gov/research/projects/download_product.cfm?id=2670
  34. ↑ "First GMO Mosquitoes to Be Released In the Florida Keys". Undark Magazine. 2021-04-12. Retrieved 2021-06-12.
  35. ↑ Vogt, G, and M Rug. “Microscopic Anatomy and Histochemistry of the Crayfish Parasite Psorospermium Haeckeli.” Diseases of Aquatic Organisms, vol. 21, 1995, pp. 79–90., doi:10.3354/dao021079.
  36. ↑ Burns, Jess "Perfect Invaders: How Crayfish Are Threatening Crater Lake's Smallest Creatures"
  37. ↑ Hyatt, Matthew W., “Investigation of Crayfish Control Technology.” Arizona Fish and Wildlife, www.usbr.gov/lc/phoenix/biology/azfish/pdf/CrayfishFinal.pdf. Accessed 12 June 2020.
  38. ↑ Kelly, Daniel. “Struggling Crater Lake Mazama Newts.” Lake Scientist, 8 Dec. 2015, www.lakescientist.com/struggling-crater-lake-mazama-newts/. Accessed 12 June 2020.
  39. ↑ Manfrin, Chiara, et al. “Detection and Control of Invasive Freshwater Crayfish: From Traditional to Innovative Methods.” Diversity, 4 January 2019, www.mdpi.com/1424-2818/11/1/5/pdf Accessed 12 June 2020.
  40. ↑ Duncombe, Lynda G. and Thomas W. Therriault. “Evaluating trapping as a method to control the European green crab, Carcinus maenas, population at Pipestem Inlet, British Columbia.” REABIC, 14 Feb. 2017, https://www.reabic.net/journals/mbi/2017/2/MBI_2017_Duncombe_Therriault.pdf). Accessed 12 June 2020.
  41. ↑ Hein, Catherine, et al. “Intensive trapping and increased fish predation cause massive population decline of an invasive crayfish.” Research Gate, May 2007, www.researchgate.net/publication/262945675_Intensive_trapping_and_increased_fish_predation_cause_massive_population_decline_of_an_invasive_crayfish. Accessed 12 June 2020.
  42. ↑ “Mazama Newt Distributions.” N.P.S., 28 Feb. 2015, www.nps.gov/crla/learn/nature/newtdistribution.htm. Accessed 12 June 2020.
  43. ↑ Stephen E. McMurray and Kevin J. Roe. "Perspectives on the Controlled Propagation, Augmentation, and Reintroduction of Freshwater Mussels (Mollusca: Bivalvia: Unionoida)," Freshwater Mollusk Biology and Conservation, 20(1), 1-12, 1 Mar. 2017, bioone.org/journals/freshwater-mollusk-biology-and-conservation/volume-20/issue-1/fmbc.v20i1.2017.1-12/Perspectives-on-the-Controlled-Propagation-Augmentation-and-Reintroduction-of-Freshwater/10.31931/fmbc.v20i1.2017.1-12.full Accessed 12 June 2020.
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