Connectivity of Sacramento splittail populations

For more information contact Melinda Baerwald:

Background and Significance of Study

Splittail FieldOur research study is comparing key characteristics and potential long-term viability of the two splittail populations by integrating genetic, physiological, and modeling approaches. The genetic component will investigate the potential overlap in range distribution of the two populations during the non-spawning season. This information will enable more focused monitoring and/or habitat conservation if either population is considered threatened or endangered in the future. Genetics will also enable us to estimate effective population size, which is extremely useful for predicting each population’s current and future viability and can be considerably smaller than census population size. The physiological component will examine responses of each population to different environmental factors that vary between the spawning grounds, such as temperature and salinity, and investigate the potential for these populations to adapt to  local conditions. Both juvenile and adult life stages will be used to determine conditions that can be  tolerated and conditions that are preferred in order to gain insight into each population’s environmental preferences and potential selective factors that may influence splittail population structure and long-term viability.


SplittailResultsThe genetic component was used to investigate: 1) the effective population size of each population, and 2) the potential overlap in range distribution of the two populations at two different life stages (age-0 and adult). We found that the Central Valley population has an effective population size that is considerably (~3x) larger than the San Pablo (Petaluma/Napa) population. This finding suggests that more focused monitoring and/or habitat conservation should occur for the San Pablo Bay population since its population size is considerably reduced in comparison to the Central Valley population. When examining age-0 splittail distribution patterns, we found that the two populations are predominantly spatially segregated when water flow rates are average or below average, but substantial geographic overlap may occur during years of high flow rates. Similarly, adults exhibit increased spatial overlap between the populations during spawning seasons with high flow rates. Intermittent spatial intermixing, however, has not weakened the observed genetic distinctions between the two populations during the past decade. The mechanisms enabling continued population differentiation despite flow-dependent spatial overlap are currently unknown.

The physiological component examined responses of each population to different environmental factors (i.e. temperature and salinity) to investigate the potential for locally adapted differences between these populations. Wild-caught splittail of both juvenile and adult life stages were used for preference and tolerance challenges in order to gain insight into each population’s environmental preferences and potential selective factors that may influence splittail population structure and long-term viability. We showed that splittail populations varied little in upper thermal tolerance limits (critical thermal maxima ranging from 33.7-34.6°C), but did vary in preferred temperatures. In general, juvenile San Pablo and Central Valley fish preferred 19-21°C, as did San Pablo adults. Central Valley adults selected cooler temperatures of 14-19°C. Salinity tolerance and associated osmoregulatory capacities also differed among splittail populations and lifestages. In hatchery-born and wild caught juvenile San Pablo splittail, we found upper salinity tolerances to be 16 ‰, which was higher than the upper salinity tolerance of 14 ‰ for wild caught juvenile Central Valley splittail. This, in conjunction with differential magnitudes of osmoregulatory disturbances between Central Valley and San Pablo splittail in response to salinity, supported our hypothesis of inter-population variation. In wild-caught adults, we found both populations to tolerate salinities of up to 11 ‰, with 100% survival for 336 hr. Cellular and tissue osmoregulatory disturbances, assessed by measuring plasma osmolality and ions, skeletal and ventricular muscle moisture and Na+-K+-ATPase during a 24 to 336 hr 11 ‰ salinity treatment, showed evidence for impaired osmoregulatory capabilities in adult Central Valley relative to San Pablo splittail. Osmoregulatory disturbances under this salinity treatment corroborated findings for juvenile splittail and further supported our hypothesis of variation in salinity tolerance between Central Valley and San Pablo splittail. Overall, the improved salinity tolerance of San Pablo juvenile and adult splittail is consistent with its higher salinity habitat. Population differences in salinity tolerance and temperature preference support the recommendation of population specific management that acknowleges differential habitat preferences and preserves resiliency to salinity demonstrated in San Pablo fish.

Results from the genetic and physiological studies were used to inform a more comprehensive splittail model, which saught to determine the degree of connectivity between the two splittail populations and assess extinction risk for each population. The modeling effort has resulted in two models, which demonstrate that the two populations are capable of surviving independently. The Central Valley population is essentially invulnerable to extinction. While the San Pablo population is stable, its extinction risk is higher because it is considerably smaller and exists in a very limited environment. However, considerable flow of Central Valley individuals into the San Pablo population results in the distinctive San Pablo genetic composition being swamped by the Central Valley. Since it is in conflict with existing data, considerable reproductive linkage between the two populations is not a realistic alternative.


Nann Fangue, UC Davis. Role: lead PI of physiology studies

Ted Foin, UC Davis. Role: lead PI of modeling studies

Fred Feyrer, USGS. Role: project initiation, grant writing, sampling, and advising



Feyrer F, Hobbs J, Acuna S, Mahardja B*, Grimaldo L, Baerwald M, Johnson RC, Teh S. In press. Metapopulation structure of a semi-anadromous fish in a dynamic environment. Canadian Journal of Fisheries and Aquatic Sciences, DOI: 10.1139/cjfas-2014-0433.

Mahardja B, May B, Feyrer F, Coalter R, Fangue N, Foin T, Baerwald MR. 2015. Interannual variation in connectivity and comparison of contemporary effective population size between two splittail (Pogonichthys macrolepidotus) populations in the San Francisco Estuary. Conservation Genetics 16(2): 385-398.

Mahardja B, May B, Baerwald MR. 2012. Characterization of 36 additional microsatellite loci in splittail (Pogonichthys macrolepidotus) and cross-amplification in five other native Californian cyprinid species. Conservation Genetics Resources 4: 917-921.

Sommer T, Reece K, Feyrer F, Brown R, Baerwald M. 2010. Splittail persistence in the Petaluma River. IEP Newsletter 23: 7-8.

Baerwald MR, Feyrer FV, May B. 2008. Distribution of genetically differentiated splittail populations during the non-spawning season. Transactions of the American Fisheries Society 137: 1335-1345.

Feyrer F, Hobbs J, Baerwald M, Sommer T, Yin QZ, Clark K, May B, Bennett W. 2007. Otolith microchemistry provides information complimentary to microsatellite DNA for a migratory fish. Transactions of the American Fisheries Society 136: 469-476.

Baerwald M, Bien V, Feyrer F, May B. 2007. Genetic analysis reveals two distinct splittail (Pogonichthys macrolepidotus) populations. Conservation Genetics 8: 159-167.

Baerwald MR, May B. 2004. Characterization of microsatellite loci for five members of the minnow family Cyprinidae found in the Sacramento – San Joaquin Delta and its tributaries. Molecular Ecology Notes 4: 385-390.