UPDATE FOR LA SIDRA TROUT

This specimen, about 15 cm in length, was collected in October, 2000.  Unusual for its tall, thin parr marks, as sometimes seen in cutthroats and redband trout.  Specimens are sparsely spotted when young.  Scale rows about 145 - 160.  Stout and stubby body shape, unlike any trout we have seen.    Are these native, or could they have been introduced?  We know of no similar trout, hatchery or otherwise.  Send comments to joe@americanfishes.com

We have received a variety of comments concerning the illustration of the La Sidra trout.  The fin clips from our Sidra trout (we have 40 specimens) have yet to be analyzed for DNA, but we are anxiously awaiting the moment.    Both Ralph Cutter (California School of Flyfishing) and Mike Hatch (New Mexico Game and Fish) suggest that the deep body and stubby shape of the trout could be a result of an abundant food supply in the stream.   Mike also says that he has seen trout in New Mexico that superficially resemble the Sidra trout (called "bastard trout"), populations of Rio Grande Cutthroats hybridized with who-knows-what.   Dr. Robert Behnke, Colorado State University suggests that the scale counts are higher than might be expected from any known hatchery rainbows, but has postulated that the high-degree of phenotypic variability among the population is at least suggestive of hybridization.  

VERTEBRAL COUNTS--Dick Robins at the University of Kansas has counted vertebrae in seven specimens of la Sidra trout.  The counts are: 62, 63, 64, 65, 65, 65, 66.  The native San Lorenzo trout from the Truchas drainage are known to average 61.  McCloud Rainbows, which were introduced into Mexico in l888, (as well as most all hatchery rainbows) average 63 or lower.   Rio del Presidio trout (from the Presidio drainage south of San Lorenzo) average 65.  We no phenotypic similarity between the Presidio trout and the la Sidra trout.

Male Trout from Arroyo la Sidra

If the population is hybridized,  then we are quite curious as to the parental stocks, for several reasons.  A hatchery "grow-out" facility (for rainbow trout--of which we also have specimens) resides on Arroyo la Sidra.  Most of our collections were above a 15m waterfall, which is presumably isolated from hatchery escapes, the proprietor saying that trout have never been stocked in the stream, but rather are only grown to size for food.  It is logical that some trout have escaped the hatchery and made their way into Arroyo la Sidra below our barrier  falls.  The proprietor stated that he and his father used to catch trout with orange bellies in la Sidra prior to the advent of the hatchery in l990, but that the orange-bellied fish had become increasingly rare.  We collected several small specimens above the falls that had orange bellies, and many of the large specimens showed a faint orange blush.   About 5 of our specimens showed a classic cutthroat spotting pattern of spots clustered on the caudal peduncle, with very few spots anterior of the dorsal.  Most of our adult males had coloration similar to rainbow trout, but higher scale counts characteristic of native Mexican trout.  Such a "rainbow appearance" is to be expected from this part of Mexico, as any native trouts are thought to derive from ancient invasions of coastal rainbows.  It is the scale counts and appearance of the juvenile fish that are curious.  The Arroyo la Sidra fish appear to be quite a bit different from known historical populations of San Lorenzo trout in the Truchas watershed.

There are no known historical populations of cutthroat trout in Mexico.  However, several authors (notably Paul Needham) have postulated that some of the Mexican trouts may have arisen from ancient hybrids between Rio Grande cutts and other trouts, suggesting that Rio Grande Cutthroats may have been or may still be found in Mexico.  Edward Drinker Cope reported in the American Naturalist in l886, that his friend Professor Lupton had sent him several specimens of trout from a location south of Guadalupe y Calvo (thought to have been the Sinaloa drainage--native waters of the Mexican Golden Trout).  Cope wrote "The specimens are young, and have teeth on the basihyal bones, as in Salmo purpuratus [Oncorhynchus clarki = cutthroat trout], which they otherwise resemble."  This is unmistakably a reference to cutthroat trout, as Mexican goldens do not otherwise resemble cutthroats and do not have teeth on the back of the tongue.  Was Cope mistaken?  In fact, Cope described a population of Rio Grande cutthroats from southern Colorado in l872 (as Salmo spilurus) so presumably knew what basihyal teeth were and what cutthroat trout looked like.  

Your ideas and comments are welcome.