Abstract:
Freshwater fish were collected mainly by electrofishing in 27 localities in rivers and streams
within the Area de Conservacion Guanacaste in northwestern Costa Rica, in June 1998 (rainy
season), February 1999 and March 2001 (dry season). The fish were identified and analyzed
for macroparasites. Fourteen localities were in rivers draining to the Atlantic, and 13 draining to
the Pacific. On the Pacific slope, three localities were seasonal streams with small catchment
areas. Five localities lacked fish, probably due to effluents from volcanic areas. A total of 36
fish species were recorded, including one new to science; Poecilopsis santaelenae. The most
commonly occurring species were the catfish Rhamdia rogersi (11 sites), the cichlid Astatheros
alfari (10 sites), and the guppy Poecilia gillii (9 sites). A total of 25 fish species were recorded
on the Pacific slope, and 26 species on the Atlantic slope. On the Pacific slope, sampling sites
were at altitudes from 8 to 880 m a.s.l.; on the Atlantic slope from 205 to 675 m a.s.l. On both
slopes the number of species per site decreased with increasing altitude. Fish communities at
lower altitudes included species feeding on detritus and plant material as well as species feeding
on invertebrates and fish. At higher altitudes, species feeding on invertebrates became increasingly
dominant. At the uppermost sites, the only recorded fish species were invertebrate
feeders.
