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　主题：everal methods for stopping the spread of snakeheads

　　First, adopting certain laws may prevent snakeheads from spreading from one body of water to another. People at fish markets almost certainly created the snakehead problem by releasing unsold snakeheads into North American lakes and rivers. Making it illegal to transport and release snakeheads will prevent them from spreading to new waterways and thus will limit the harm this invasive species is doing to North American ecosystems.

　　Making it illegal to transport and release live steakheads may help, but the problem is that steakheads, already in north america, can spread from one body of water to another, all by themselves. You see, steakheads differ from most other fish by being able to live and move on land. Stickers can get from one lake or pond to another by crossing stretches of land as long as they remain moist. They do this by wriggling across mud or from puddle to puddle, managing to stay wet for days before reaching a new lake or river. So steakheads will spread to new waterways even without humans transporting them.

　　Another method of control involves educating North American fishers about the fact that snakeheads might be a worthwhile species to catch. Because the snakehead is rarely part of the North American diet, many American fishers do not know that the fish can be a good food source, so they do not catch them, either for themselves or to make money by selling them. But in Asia, the snakehead is widely sold and consumed. Educating North American fishers about the virtues of snakeheads will encourage the fishers to pay increased attention to these fish, removing them from lakes and rivers in the process.

　　Second, educating north american fishers about snakeheads in order to get them to catch the species could lead to another problem. You see, when fishers see a benefit to catching certain fish, they often intentionally introduce those fish to other lakes and rivers so that they can catch them there. Fishers who find they enjoy eating snakeheads, or who make money by selling snakeheads, well, those fishers will want to catch more snakeheads. And fishers know they can make that happen by introducing snakeheads to other fishing spots even illegally. So fishers who get educated about snakeheads could end up spreading snakeheads, not reducing their numbers.

　　Finally, scientists are experimenting with different fish poisons to eliminate snakeheads in waters that have large populations of the species. By treating a body of water with toxic chemicals that target fish, scientists should be able to wipe out all of the snakeheads in the water. While this treatment might wipe out many of the native fish as well, the waters can be restocked with new populations of native fish.

　　Lastly, poisoning and then restocking lakes or ponds is not a very good idea. That's because poisoning kills more than just snakeheads and other fish, it also kills many smaller organisms, such as insects and amphibians. These animal populations will not be restocked and often cannot fully recover after poisoning. Also, these smaller animals are actually the food source that many of the native fish need, and without this food source, the fish populations won't do well. So once we use poison. It's not as easy to restore the ecosystem as the reading suggests

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　主题：several hypotheses to explain the decline of these endangered whales

　　One hypothesis is that industrial chemicals are inhibiting the whales' ability to reproduce. The area of ocean in which the whales live is contaminated by industrial waste, such as dangerous chemicals called PCBs. PCBs become concentrated in the bodies of marine animals. When Southern Resident orcas feed on fish such as salmon, they consume the PCBs accumulated in the bodies of the salmon. PCBs can affect certain marine animals' ability to reproduce, and this could also be the case for the Southern Resident orcas.

　　First, these whales do ingest the chemicals known as PCB. But it's not clear that this is affecting their reproduction. You see, the southern resident orpas are not the only orpha community living near the pacific coast. Other communities of orcas also live in the area. However, the southern resident orcas are unique in that they feed mainly on fish such as salmon. Orcas in other communities feed mostly on larger prey, such as seals. And those larger prey that the other whales eat actually contain higher levels of pcbs than fish. So the whales that eat seals consume more pcbs than the fish eating southern residents. But the other whales are not having problems reproducing. So PCB S probably aren't the main problem.

　　Another possibility is that too much noise in the whales' environment has affected their ability to find food. Orcas rely on echolocation, which allows them to use sound waves to locate objects, including food. This ability could be affected by the presence of noisy whale-watching boats. Because of increased public interest in seeing whales, these boats became more numerous in the Pacific Northwest at about the same time that the whale population started declining. The loud sounds of the boats' engines could be interfering with the whales' ability to use echolocation and thus with their ability to find food.

　　Second, about noise. Well, researchers recently studied the noise from whale watching boats. They put sensors on the whales, and measured the noise, the whales hear. And it turns out that the noise of the boats is not very loud to the whales. In fact, the noise made by the boat's engines is about as loud as a rainstorm. Rainstorms are clearly something that whales experience regularly and storms do not affect whales ability to use echolocation. Since the boats are not louder than rainstorms, the noise is not a problem.

　　Finally, bacterial infections could be causing the orcas' population decline. Researchers have been able to identify bacteria that live in the whales' lungs by collecting the mist (water droplets) that the whales exhale. The bacteria they have found in these samples include staphylococcus bacteria, which can make whales and other marine animals sick with dangerous diseases such as pneumonia.

　　Third, bacteria. Yes, small numbers of staphylococcus bacteria were detected in the water droplets breathed out by the whales. But animals can tolerate small amounts of bacteria without being sick. To be sure that the bacteria are really making the whales sick. We need to see some actual signs of illness among the whales. We need to see signs like trouble breathing or damage to their lungs. But these whales are difficult to examine. They're swimming freely in the ocean. Without confirming signs of actual illness, finding bacteria doesn't mean much.

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　主题：Several hypotheses to explain why the Vikings left

　　First, climate change might have made farming in the colony impossible, exposing the Vikings to starvation. In the tenth century, the North Atlantic was somewhat warmer than it is today; this, historians believe, is what allowed the initial success of the Greenland colony. When the coast of Greenland was warmer, the Vikings were able to grow crops and raise animals there. However, the regional climate gradually cooled, making farming more difficult. Without a reliable source of food, the Vikings were eventually forced to abandon their colony.

　　First, the starvation theory. Even if agriculture became difficult, the vikings still had an excellent source of food, the ocean. Historians used to think that the vikings were unable or unwilling to fish, since they preferred to farm. But new evidence contradicts that idea. Researchers chemically analyzed the skeletons of Vikings in Greeland and learned the Vikings diet was mainly based on fish. Since the supply of fish was not affected by the cooling climate, it seems unlikely that the Greenland Vikings would have starved or abandoned the colony for lack of food.

　　Second, the Vikings could have been driven out by other groups of people who also inhabited Greenland. Some of these groups came to Greenland before the Vikings did. The Vikings had to compete with them for the land and resources available in the region, and conflicts may have resulted. The Vikings likely would have been at a disadvantage when defending against populations that had been in Greenland longer and were better established there.

　　Second, it's unlikely that a conflict with another group forced the vikings to leave.

　　Archaeologists examining the remains of the colony found that the vikings took all of the valuable objects away with them when they left the colony. This shows that the vikings left the colony in an orderly way and had plenty of time to carefully pack their valuable belongings. If the colony had been attacked, the vikings would have been forced to leave very quickly and would not have had time to pack up their valuables.

　　Third, a decline in trade with Europe could have caused the collapse of the Greenland colony. The colony initially traded directly and extensively with Europe, exchanging valuable animal products, such as walrus tusks, for European goods, such as metal tools. Later, however, the colony came under the control of Norway. The government of Norway forbade the colonists to trade with European traders and demanded that all Greenland products be transported to Norway first. The loss of direct trade with Europe would have made it much more difficult for the Greenland colonists to obtain the supplies they needed.

　　Third. A decline in trade with Europe. Well, even though official trade had to be conducted through Norway, there was still plenty of unofficial illegal trade. Many European traders ignored Norway's rules and continued to sail directly to Greenland to trade there. In fact, this illegal trade happened so frequently that the king of Norway himself complained about it. You see, we've discovered an actual document in which the king of Norway complained that the Greenland vikings were trading so extensively with other countries that they had no good products left over to transport to Norway. All of this illegal trade probably provided enough supplies to the Greenland colony.