Modoc sucker - U.S. Fish and Wildlife Service

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transport and release. The first is not subject to regulatory control. The second is already prohibited by state laws (CDFG 2007; ODFW 2008b). Although it is possible that someone would illegally introduce additional non-native fishes into a Modoc sucker stream, it is unlikely to occur (see Factor C) and, if done, would probably represent a relatively small number of individuals when compared to natural dispersal events (e.g., dispersal by reservoir overflows). Although natural drought can adversely affect the Modoc sucker it is not subject to regulatory control. There are no substantial diversions affecting water quantity and flow in Modoc sucker streams, with exception of an existing diversion in Rush Creek (located at the lower extent of designated Critical Habitat), which maintains in-stream base flows. Reservoirs in the upper Turner Creek watershed influence winter and early spring flow regimes but are no longer connected by late spring and have little, if any, impact on flow during the dry season (July- October). Therefore, flow quantity and duration during the crucial dry season are dependent primarily on natural flows, which are subject to inter-annual variability in precipitation. Factor D Conclusion No threats have been identified as a result of inadequate of regulatory mechanisms. Throughout its range, the Modoc sucker is protected by the Act and a variety of state laws. FACTOR E: Other Natural or Manmade Factors Affecting Its Continued Existence Hybridization The 1985 listing identified hybridization with the Sacramento sucker, also native to the Pit River drainage, as a principal threat to the Modoc sucker. Hybridization can be cause for concern in a species with restricted distribution, particularly when a closely related non-native species is introduced into its range, and can lead to loss of genetic integrity or even extinction (Rhymer and Simberloff 1996). In 1985, it was assumed that hybridization between Modoc and Sacramento suckers had been prevented in the past by natural physical barriers, which had been recently eliminated by human activities, allowing contact between the two species. Modoc sucker populations from streams in which both species were present were considered hybrid populations and were excluded when evaluating the Modoc sucker’s distribution in 1985. The assumption that extensive hybridization was occurring was based solely on the opportunity presented by cooccurrence and the identification of a few specimens exhibiting what were thought to be intermediate morphological characters. At that time, genetic information to assess this assumption was not available. Modoc and Sacramento suckers are naturally sympatric (occurring in the same streams) in the Pit drainage. There is no indication that Sacramento suckers are recent invaders to the Pit River or its tributaries. Both morphological and preliminary genetic data suggests that the upper Pit River population of Sacramento suckers is distinct from other Sacramento River drainage populations (Ward and Fritsche 1987; Dowling, unpub. data. 2005). There is also no available information suggesting Modoc and Sacramento suckers were geographically isolated from each other in the recent past by barriers within the Pit Drainage (see Factor A – Barriers). Separation of the two species appears to be primarily ecological, with Modoc suckers occupying smaller, headwater 22
streams typically associated with trout and speckled dace, while Sacramento suckers primarily occupy the larger, warmer downstream reaches of tributaries and main-stem rivers with continuous flow (Moyle and Marciochi 1975, Moyle and Daniels 1982, Reid 2008c). Further reproductive isolation is probably reinforced by different spawning times in the two species and their size differences at maturity (Reid 2008c). The morphological evidence for hybridization in 1985 listing was based on a limited understanding of morphological variation in the Modoc and Sacramento suckers, derived from the small number of specimens available at that time. Subsequent evaluation of variability in the two species, based on a larger number of specimens, shows that the overlapping character states (primarily lateral line and dorsal ray counts), interpreted by earlier authors as evidence of hybridization, are actually part of the natural meristic (involving counts of body parts such as fins and scales) range for the two species and are not associated with genetic evidence of introgression (Kettratad 2001, Reid 2008c). Furthermore, the actual number of specimens identified as apparent hybrids by earlier authors was very small and in great part came from streams without established Modoc sucker populations. In 1999, the Service initiated a program to examine the genetics of suckers in the Pit River drainage and determine the extent and role of hybridization between the Modoc and Sacramento suckers using both nuclear and mitochondrial genes (Palmerston et al. 2001; Wagman and Markle 2000; Dowling 2005a; Topinka 2006). The two species are genetically similar, suggesting that they are relatively recently differentiated and/or have a history of introgression throughout their range that has obscured their differences (Wagman and Markle 2000; Dowling 2005a; Topinka 2006). Although the available evidence cannot differentiate between the two hypotheses, the genetic similarity in all three sub-drainages, including those populations shown to be free of introgression based on species-specific genetic markers (Topinka 2006), suggests that introgression has occurred on a broad temporal and geographic scale and is not a localized or recent phenomenon. Consequently the evidence indicates that introgression is natural and is not caused or measurably affected by human activities. In a study of nuclear anonymous frequency loci polymorphisms in the two species, Topinka (2006) did identify species-specific markers indicating low levels of introgression by Sacramento sucker alleles into most Modoc sucker populations. However, there was no evidence of first generation hybrids, and it is not clear whether introgression occurred due to local hybridization or through immigration by individual Modoc suckers carrying Sacramento alleles from other areas where hybridization had occurred (see below). Topinka (2006) did find extensive bi-directional introgression in upper Ash Creek, where Modoc and Sacramento suckers are sympatric. Phenotypic Modoc suckers in this population reach 15 inches standard length, the largest size encountered in the species, but are dominated by Sacramento markers and show extensive variability in characteristic Modoc sucker markers. Similarly, Sacramento suckers in the stream have a relatively high frequency of Modoc sucker markers, as do populations in neighboring Willow Creek, which is not known to contain Modoc suckers. Ash Creek is a large, warm-water stream having habitat more characteristic of Sacramento suckers and includes other native species, such as Sacramento pikeminnow and Pit roach, which are not typically found with Modoc suckers. Willow Creek is ecologically similar 23
Page 1 and 2: Modoc Sucker (Catostomus microps) 5
Page 3 and 4: the time of listing. In this review
Page 5 and 6: Information on the Species and its
Page 7 and 8: Figure 2. Map showing detailed dist
Page 9 and 10: introgressed character and full sym
Page 11 and 12: Willow (15), Ash (200), Dutch Flat
Page 13 and 14: Table 2. Comparison of available an
Page 15 and 16: Five-Factor Analysis The following
Page 17 and 18: suckers and were, therefore, exclud
Page 19 and 20: needed. However, current informatio
Page 21: with Modoc suckers contain no non-n
Page 25 and 26: There is no record of how frequentl
Page 27 and 28: local extinction in at least one of
Page 29 and 30: VII. ACKNOWLEDGEMENTS The Service a
Page 31 and 32: ODFW [Oregon Department of Fish and
Page 33 and 34: USFWS [U.S. Fish and Wildlife Servi
Page 35 and 36: 1. Install and maintain fish barrie
Page 37: 4. Cooperate with private land owne

Modoc sucker - U.S. Fish and Wildlife Service

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