i37of considerable size, such as Lago de Guzman at the mouth of the Rio Casas Grandes, Lago de Patos at the mouth of the Rio Carmen, etc., etc.From the upper part of these river courses the lakes into which they flow become stocked with fishes. There is considerable alkali in the beds of these lakes which the water takes up in solution. As the dry season comes on the lakes gradually become smaller and the water in them more alkaline. When each lake is reduced to about three-fourths of its original size the water becomes so charged with alkali that the fishes die in great numbers. During the latter part of the dry season there is very little water in the Rio Casas Grandes below Terrasas, more than half of its bed being dry. What is true Introduction.xxvii of this river is also true of many other rivers in the dryer portions of Mexico, and especially so of those streams whose waters never reach the sea. RECENT GEOLOGICAL CHANGES IN MEXICO AND THEIRGENERAL EFFECT ON THE FISH FAUNA.Geologically speaking, within more recent times the climatic and hydrographic conditions of Mexico have been less stable than in theMississippi Valley.The central portion of Mexico has been subject to considerable volcanic disturbances which have continued to within recent times.The northern portion has evidently at some former time been much better watered than it is now. Such streams as the
Atlantic coast, the canal is provided with locks, consisting of three flights of 15 feet each. Beyond the locks lies Lake Gatun, a large, artificial body of fresh water, made by the construction of a very large dam across the Rio Chagres and part of its valley. This lake is approximately 45 feet above sea level and it extends partly as a broadly expanded body of water and partly confined to the channel of the canal from Gatun to Pedro Miguel, a distance of about 30 miles. Locks with a single flight of 15 feet occur at Pedro Miguel, and below these locks lies another fresh water lake which contains about 3 or 4 miles of the channel of the canal. Then at Miraflores are locks containing two flights of 15 feet each, which bring a ship passing through the canal from the Atlantic to the Pacific back to sea level.It is, therefore, evident that the Panama Canal provides no passageway from ocean to ocean for strictly marine fishes. However, it does not appear impossible for some of the marine forms which frequent brackish, or even fresh water, such as some of the Centropomida, Gerridce and Gobiida, to endure the fresh water and to find their way from ocean to ocean. THE COMMERCIAL FISHERIES OF PANAMA.Nearly all fishing on the coasts of Panama is done at night for two reasons; first, because it is quite generally believed that fish can see the nets during the day and that, therefore, larger catches can be made at night; and second, because the fish must be placed on the markets, which usually are open only in the forenoon, early and disposed of the same day in order to prevent decomposition, as recourse to icing is rarely taken.Both drag nets and set nets are used, and it is feared that altogether too frequently for the welfare of the fishery dynamite is employed; at least it was during the periods of our visits to Panama. Bamboo traps, such as are also used in the West Indies, are in general use, particularly on the Atlantic coast. The cast net is very common and at times quite successful. Hook and line fishing also is engaged in to a limited extent.Very few power boats are used, the sail or oar and paddle being depended upon. Most of the boats are of the "dugout" type, which are rather heavy and which capsize easily and must be quite carefully manipulated.Class II. Elasmobranchii.Order II. Asterospondyli.Family II. OrectolobidaB. THE NURSE SHARKS.Body short and subcylindrical to moderate and depressed ; nostrils with a nasoral groove and with a cirrus or barbel; mouth transverse, with labial folds around angles; teeth compressed, with or without lateral cusps on each side of the median one; eyes very small, without nictitating membrane; spiracle minute to large, and behind the eye or more or less below it; gill-slits small to medium, the posterior 2 or 3 above base of pectoral; caudal fin narrow, usually without exerted lower lobe ; other fins short and broad ; no fin spines ; no caudal pits.
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