Papers in the Biological Sciences


Date of this Version



Published in INVESTIGATIONS OF THE ICHTHYOFAUNA OF NICARAGUAN LAKES, ed. Thomas B. Thorson (University of Nebraska-Lincoln, 1976). Copyright © 1976 School of Life Sciences, University of Nebraska-Lincoln.


The shark that occurs in the fresh water of Lake Nicaragua and the Rio San Juan was first described (as Eulamia nicaraguensis) by Gill and Bransford (1877). These authors also first proposed the theory that the sharks, as well as the sawfish and tarpon that occur in the lake, were trapped there by late Pleistocene volcanic activity, which isolated a former bay of the Pacific and resulted in the formation of the present lake. The theory of a landlocked, distinct species, of Pacific origin, has en joyed wide popular acceptance and for many years was also accepted by professional zoologists, although the idea has been questioned frequently in recent years. Carr (1953) pointed out that the closest taxonomic affinities of the marine species in Lake Nicaragua were clearly with their Atlantic, rather than their Pacific relatives. Bigelow and Schroeder (1961) concluded that the lake sharks are morphologically inseparable from the widely distributed marine bull shark, Carcharhinus leucas. Thorson (1964) and Thorson et al. (1966) confirmed the conclusion of Bigelow and Schroeder and presented circumstantial evidence that the lake shark population is not landlocked, but consists of marine bull sharks that enter from the sea. Our evidence was the occurrence, throughout the lake and river, of many sharks of the same euryhaline species that occurs at the river mouth and along the coast; that the same species occurs in similar situations in many rivers and some lakes around the world; and that all the rapids in the Rio San Juan are navigable by barges and other vessels that regularly pass up and down the river.