Reproductive Strategies and Egg Types of Seven Rio Grande Basin Cyprinids

Reproductive strategy and egg type of Hybognathus amarus, H. placitus, Macrhybopsis aestivalis, Notropis girardi, N. jemezanus, N. simus pecosensis, and N. stramineus were determined from laboratory experiments conducted between 1991 and 1995. The first six taxa were pelagic-broadcast spawners that produced nonadhesive, semibuoyant eggs, whereas N. stramineus was a broadcast spawner that laid demersaladhesive eggs. High-speed cinematography revealed that a spawning event consisted of a single male wrapping around the female's midsection and fertilizing the eggs upon expulsion. The perivitelline space of recently expelled nonadhesive eggs filled rapidly with water, thereby increasing both egg diameter and buoyancy. Semibuoyant eggs remained in suspension as long as water current was maintained. Discovery of the spawning behavior and egg types of these species allowed for the development of hypotheses to explain extirpations and extinctions of several endemic Rio Grande Basin fishes. We believe the synergistic effects of downstream transport of eggs and larval fishes and dam-related modifications of flow and habitat was probably responsible for the decline and demise of these taxa in the Rio Grande Basin.