英语翻译Comparatively few fresh-water species of fishes are limi

英语翻译
Comparatively few fresh-water species of fishes are limited in their distribution to a single river system,yet not many are found on both sides of a high mountain ridge,such as the Rocky Mountains in North America.That is to say,the fishes of the Mississippi Valley are generally different from those of the Pacific slope.While it is a well-known fact that the fish life in no two river systems,even though they empty into the sea on the same side of a divide（分水岭）,is exactly identical,such streams do have many species in common.The principal rivers of the Atlantic slope of the United States,for example,contain several species common to all of them,including the bullhead catfish,the bluegill sunfish,and the largemouth bass.None of these species can endure salt water,so they cannot move from one river system to another.On the other hand,the more northern streams contain species not found in the southern ones,and vice versa.The common pike,for example,is found in the Atlantic streams from Maryland northward,and the brook trout and yellow perch occur only in the streams from North Carolina southward.How the present distribution came about must remain a matter of guess.It is quite probable that some of the streams,including those on opposite sides of a divide,may have been connected at one time.) Again,streams may be entirely separate during normal weather,but an exceptionally heavy rainfall or the sudden melting of snow in the uplands sometimes causes floods which may form a temporary connection between them,providing a passageway for fishes.It is also possible that water birds may accidentally carry fish or spawn （卵） from one stream to another,or that man may be instrumental in such a transfer.

相对较少的淡水鱼种受限于其生活的独立的淡水（河流）生态系统,然而,并不是大部分（鱼）都在高海拔山脊（例如：北美的Rocky山脉）的两侧被发现.换言之,Mississippi Valley的鱼类会有异于the Pacific slope的鱼类.然而,“鱼类不会生活在两个淡水生态系统”是妇孺皆知的事实,纵使（两个淡水/河流生态系统）皆于同一分水岭进入大海,纵使（两个系统）有如出一辙的物种多样性,此事实也亦是如此.例如,大美利坚的流入Atlantic slope的几个主要河流,有着几个共有的鱼类,包括：the bullhead catfish（鲶鱼）,the bluegill sunfish（蓝鳃太阳鱼）,and the largemouth bass（大嘴鲈鱼）.以上枚举的几种鱼类均为淡水鱼（在咸水环境中难以生存）,因此,这些鱼类几乎不能从一个系统移至另一个系统.另一方面,南北鱼类（PS：直接将species意译为鱼类）种类差异大.又比如,常见的pike（梭子鱼/秋刀鱼）在Atlantic streams（自Maryland北流入）中被发现;the brook trout（鳟鱼）和yellow perch（金鲈）仅在自North Carolina南流的河流中出现.当下,上述的这种生态分布特征尚有一定程度上的猜测.也很有可能,一些河流（包括那些在分水岭的相反方向的河流）,在某一特定时期下,可以相互沟通连接.另外呢,一些河流在平时正常的天气条件下,是完全被分开的,然而,如遇大雨倾盆抑或雪水消融这类特殊天气条件,河流也会出现短暂的互通,这就会为鱼类转移（离开原有系统,到一个新的系统）提供条件.另外,同时水鸟也可能会偶然地带着鱼类的卵进行地点上的转移（人类也有可能成为鱼卵的携带者）,这也为鱼类转移系统开启了便宜之门.