Desert Fishes Council, Death Valley, CA, November 2000.

Winter reproduction of introduced poeciliids in warm springs.

J. Phelps, M. Wald & P.J. Unmack

Numerous poeciliid fishes have been introduced into warm springs in the western United States, a novel environment due to their constant warm temperatures. This provides an ideal situation for testing of hypotheses relating to factors influencing reproduction, e.g., changes in photoperiod versus seasonal temperature variation. Unfortunately, these introductions have also resulted in declines of native species, and the only real option for controlling exotics is removal, usually achieved via ichthyocide application. Removal is more likely to succeed when populations are at their lowest level and/or non-reproductive, since young can inhabit the extreme shallow margins and avoid poisoning. We sampled multiple populations of exotic poeciliids: Gambusia affinis, Poecilia mexicana, P. latipinnis, and P. reticulata, autumn through spring, to test if reproduction was occurring based on the presence of developing embryos. Principal sites included springs in Ash Meadows, and Rogers Spring (Lake Mead), both in Nevada, and Watson Wash, Arizona. Single samples were obtained from an additional seven springs. In all species, winter reproductive output was lower than in spring and varied among localities at the same time of year. All guppy populations were reproductive through winter, while the remaining species had some populations which were non-reproductive, and others reproducing only at relatively low levels. These results demonstrate considerable variation, even between geographically proximate springs with similar habitat parameters. If eradication efforts were to be attempted and one were concerned about juvenile survival, January would be the most appropriate time to treat a spring. Reproductive potential would be lowest and marginal habitats coldest (at least at night), hence more likely avoided by juvenile poeciliids.