The occurrence and paleolimnologic significance of cutthroat trout (Oncorhynchus clarki) in pluvial lakes of the Estancia Valley, central New Mexico: Discussion and reply Discussion ROBERT J BEHNKE 1 T TIT A TTO f Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado 80523 WARKEN J. rLAI IS ) F. W. Bachhuber's (1989) review of the pluvial lakes of the Estancia Therefore, whenever the cutthroat trout is the only fish species com- Valley in central New Mexico will surely provoke renewed interest in the mon to two neighboring basins, the most parsimonious interpretation is Quaternary history of the eastern extensional basins and in the use of that a high-elevation headwater stream capture involving only cutthroat fresh-water fish faunas for the interpretation of paleohydrology. With trout caused the observed pattern. A lake overflow generally implies a respect to Bachhuber's overflow model of pluvial Early Lake Estancia, lower-elevation event which should allow more fish species to disperse. however, we believe Bachhuber has overestimated the significance of ap- In conclusion, Bachhuber's overflow model of Estancia Valley his- parent Rio Grande cutthroat trout (Oncorhynchus clarki virginalis) fossils tory must await other kinds of confirmation besides the presence of cut- found in the sediments of Late Lake Estancia as evidence in support of an throat trout as the only fish fossil to be found in the Lake Estancia earlier overflow into the Pecos River watershed. Indeed, the apparent sediments. One of Bachhuber's explanations for the absence of other fish absence of any other fish species should be taken as evidence against a late species from Late Lake Estancia—that other fish species could not survive Pleistocene overflow. interpluvial conditions between Early Lake Estancia and Late Lake The cutthroat trout is known for its tendency to colonize high- Estancia—is unlikely because most other fish species native to the Pecos elevation headwater streams. This trout is often the only fish species to be River watershed are more extinction resistant than the cutthroat trout found in the uppermost reaches of a watershed (personal observation). In when faced with a hot, dry climate. Bachhuber's suggestion that no other addition, the cutthroat trout had a wider, historic, native distribution than fish species reached Lake Estancia because non-salmonid species were not any other obligate cold-water fish species in the western United States vagile enough to negotiate the proposed waterway leading from Lake (Smith, 1978; Behnke, 1988). Smith noted that, at least within the Great Estancia to the Pecos River, however, is somewhat plausible, which per- Basin, the cutthroat trout has been more susceptible to transbasin dispersal haps offers an opportunity to test the overflow hypothesis. via the phenomenon of headwater stream capture than any other fish Modern playas with outcrops similar to those found in the Estancia species. This statement seems to hold true for other areas of the cutthroat Basin also exist within the Pinos Wells and Encino Basins. If Bachhuber's trout's range as well. For example, during the latter part of the Pleistocene, overflow hypothesis is correct, one would predict finding a more diverse cutthroat trout crossed the Continental Divide eastward via independent fossil fish assemblage within the Lake Pinos Wells and Lake Encino sedi- headwater stream captures into the South Saskatchewan, upper Missouri, ments. Alternatively, if the cutthroat trout fossils within Lake Estancia Yellowstone, South Platte/Arkansas, and Rio Grande drainages (Behnke, sediments are the result of a high-elevation headwater stream capture, then 1988); this feat was matched by no other fish species. one would not necessarily expect to find any fish fossils within either the Lake Pinos Wells or Lake Encino sediments. Reply FREDERICK W. BACHHUBER Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, Nevada 89154-4010 Behnke and Platts bring up two separate issues regarding my paper tive interpretation, put forth by Behnke and Platts, of fish introduction by (Bachhuber, 1989). How did cutthroat trout (Oncorhynchus clarki) get headward-stream capture, is a plausible mechanism for the transfer offish into the Estancia Valley, and, what I believe to be the larger question, did a into adjacent drainage basins, and I recognized it as such in my article pluvial system overflow, ultimately into the Pecos River drainage basin? (Bachhuber, 1989, p. 1548). If trout were introduced by stream capture, Whereas these issues may or may not be related, I used the occurrence of the implications are that fish could have arrived at any time or multiple fish as one line of evidence supporting a basin-overflow event. The alterna- times during late Pleistocene time, such as during the various fresh-water Geological Society of America Bulletin, v. 102, p. 1731-1732, December 1990. 1731 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/12/1731/3380924/i0016-7606-102-12-1731.pdf by guest on 27 September 2021 1732 DISCUSSION AND REPLY phases of Late Lake Estancia and Lake Willard (both late Wisconsin in overflow problem. The only exposed lacustrine sediment in the Encino age). In contrast, I proposed that fish arrived in the valley via overflow of Basin contains Ruppia (ditchgrass) and the ostracode Megalocypris ingens, Early Lake Estancia during the Illinoian(?), remaining in the watershed for both indicators of saline conditions (Bachhuber, in press). Although time some 130,000 yr in high-elevation refugia, returning to the lacustrine equivalency has not been established, the Encino sediment may be correla- environment only when limnologic conditions permitted. Since the timing tive with the fresh-water phase of Lake Willard, suggesting that, although of arrival of fish is used as the basis for paleoclimatic speculations as well late Wisconsin fresh-water systems existed in the Estancia Valley, only as supporting evidence for basin overflow, Behnke and Platts' arguments saline lakes evolved in the Encino Basin. From a geomorphic standpoint, are more than academic. They suggest that the occurrence of cutthroat this seems reasonable because the Encino watershed is small in area, trout and the "absence of any other fish species should be taken as evi- elevations are restricted to less than 2,134 m (7,000 ft), and the basin is dence against a Late Pleistocene overflow." I recognized the paradox of leeward of the Manzano/Sandia Mountains. As a result, the Encino Basin monospecific occurrence of trout and offered two alternatives: multiple may have contained a fresh-water lake only during an Illinoian(?) over- fish species entered the valley during overflow, but only cutthroat trout flow episode. Fish migrating into the basin during overflow would have survived in high-elevation, cold-water refugia during the Sangamon(?), little chance of long-term survival after the lake was isolated from Early eventually migrating into the late Wisconsin lakes; or non-salmonid spe- Lake Estancia. Even if a fresh-water phase existed during the late Wiscon- cies were unable to negotiate a physical barrier, such as a waterfall, located sin, fish would not be expected because of an apparent lack of high- somewhere along the overflow route and thus never entered the valley elevation refugia within the Encino watershed. (Bachhuber, 1989). All explanations for the monospecific occurrence of Behnke and Platts' alternative interpretation is well taken, and I agree fish are plausible, but the problem cannot be resolved with existing paleon- that the occurrence of fish, by itself, is not prima fades evidence that tologic data. In the realm of speculation, however, if fossils from multiple overflow occurred. The monospecific occurrence of cutthroat trout, how- species of fish are eventually found in Early Lake Estancia sediment, an ever, does not disprove an Early Lake Estancia overflow, an event sup- overflow event probably occurred. If it can be demonstrated that fish never ported by a high-elevation, geomorphically old (albeit, poorly docu- existed in Early Lake Estancia, the late Wisconsin occurrences would then mented) strandline sequence that stands above the present topographic sill. be attributed to stream capture because neither Late Lake Estancia or Lake I appreciate Behnke and Platts' comments and recognize that there are Willard overflowed. Conversely, if only cutthroat trout are found in Early various mechanisms for the introduction offish into topographically closed Lake Estancia, a case could be made for either (or both) overflow or basins. We agree upon one major point, however; fish got into the Estancia stream capture. Clearly, resolution rests in Early Lake Estancia sediment. Valley by swimming. Unfortunately, it is known only in subcrop. As an alternative to studying Early Lake Estancia sediment, Behnke COMBINED REFERENCES CITED and Platts propose a different test of my overflow model. They predict Bachhuber, F. W., 1989, The occurrence and paleolimnologic significance of cutthroat trout (Oncorhynchus clarki) in that, if overflow occurred, the downdrainage, satellite Pinos Wells and pluvial lakes of the Estancia Valley, central New Mexico: Geological Society of America Bulletin, v. 101, p. 1543-1551. Encino basins (see Fig. 4, Bachhuber, 1989) would contain a more in press, A pre-late Wisconsin paleolimnologic record from the Estancia Valley, central New Mexico, in Clark, P. U., and Lea, P. D., eds., The last interglaciation/glaciation transition in North America: Geological Society of diverse fossil fish assemblage than that of the Estancia Valley, whereas the America Special Paper. absence of fish in the satellite basins could indicate stream capture for the Behnke, R. J., 1988, Phylogeny and classification of cutthroat trout, in Gresswell, R. E., ed., Status and management of Interior stocks of cutthroat trout: American Fisheries Society Symposium, Volume 4, p. 1-7. Estancia Valley occurrences. This test is applicable only if there was no Smith, G. R., 1978, Biogeography of intermountain fishes, in Harper, K. T., and Reveal, J.