Bibliography Background About KRIS

Hypothesis #1: In the Ten Mile River basin, coho salmon have decreased in distribution and abundance to where the long term viability of a self-sustaining population is at risk.

Support for the Hypothesis from the Literature

The minimum viable population size for adult anadromous salmon to maintain genetic diversity for long term survival is estimated as 200 by Bjornn and Horner (1980), and as 500 by Gilpin and Soule (1990). Groot and Margolis (1991) provide projected survival rates that allow estimation of recruitment of juvenile coho salmon to adult spawners. Based on typical survival rates of 1.27%-1.71% for the fry-to-smolt stage (Godfrey 1965) and 1%-2% (Neave and Wickett 1953) to 5% (Shapavolov and Taft 1954) for the ocean stage, a viable coho population could be associated with a minimum 233,000 to 585,000 coho fry.  

Ten Mile River Data and Reports

In the early 1960's, the Ten Mile River was estimated to have a coho run of 6,000 fish (California Wildlife Plan 1965; Taylor, 1978). This estimate reflects the Ten Mile River basin's abundance of accessible, low-gradient, stream habitat in which coho salmon are known to thrive. Adult spawner and redd counts of coho salmon in the Ten Mile River come from Maahs (1996a, 1996b, 1997a, 1997b). Hawthorne Timber Company (HTC) electrofishing surveys provide juvenile distribution and abundance indices from 1993-2000.

Spawning surveys in the Ten Mile River Basin estimated a minimum population of 32-52 adult coho for the 1989/1990 season, and 14-42 adult coho salmon for the 1991/1992 season (Maahs and Gilleard,1994).  Estimates from the 1989/90 and 1991/92 surveys may be low because only two to three surveys were conducted per year.  A more intensive survey effort, utilizing carcass tag and recapture methods, estimated 190-250 adult coho for the 1995/1996 season (Maahs 1996a).  Although the survey was not exhaustive, it was thought to cover most major coho producing reaches. Repeat spawning surveys in 1996/97 were conducted in only Smith and Campbell Creeks, two of the most productive Ten Mile River tributaries. These surveys found a total of one coho carcass and were used to estimate 60-80% fewer adult coho across the basin compared the previous year (Maahs, 1997b). 

Electrofishing data from 24 sampling stations throughout the Ten Mile River Basin from 1993-2000 provide an index for juvenile coho numbers, density and biomass in summer (see chart below).  These data reveal that coho numbers and densities were low and most often zero for most years at most sites, and that the highest juvenile coho densities were found in the South Fork Ten Mile River and its tributaries until recent years. The highest juvenile numbers were found in 1996 following the largest return of adult coho (190-250; Maahs 1996a) in all recent surveys. 

The eight-year set of electrofishing data provides valuable trend information and shows a pattern of declining coho population abundance and distribution, particularly in South Fork tributaries (see chart below). Densities of juvenile coho were less than one fifth of those found in 1996 in all other years surveyed, which likely indicates adult populations of fewer than 100 individuals in the preceding year. Alternatively, adult populations may have been higher but survival very low due to adverse spawning and winter rearing conditions. The three-year life cycle of coho salmon is reflected in the data with 1994-95 and 1997-98 returns representing a very weak year class that seems to be recurring and would predict a low abundance in 2001. Diminished returns from the 1995-96 and 1997-98 cohort in 1998-99 and 1999-2000, respectively, suggests a down trend and give cause for concern for the viability of the coho population in the Ten Mile River basin overall. The decline of coho salmon in the Ten Mile River in the 1990's proceeded despite artificial supplementation efforts which attempted to reverse declines (Maahs, 1999).

A comprehensive survey by divers in 2000 counted 1731 juvenile coho throughout the entire basin. The 2000 count was indirectly corroborated by very low electrofishing densities in that year.  The count showed a dramatic decrease in the South Fork Ten Mile where only 11 coho juveniles were seen in 2000.  This number of smolts is two orders of magnitude lower than needed to maintain a viable adult spawning population. If fry to smolt survival were conservatively estimated at 2% and smolt to adult survival at 5%, then 1731 fry in the Ten Mile River in 2000 would result in roughly one pair of adult coho salmon (1.7 adults) from this year class returning to spawn in 2002. Although this is only one year class of Ten Mile River coho salmon, juvenile surveys in other years show very low densities which indicates that other cohorts are also weak.

Supporting Data Topics in KRIS Ten Mile: 

The charts below are derived from the Topics listed here. Check these Topics in the KRIS database to see the information in context and to have access to Info Links and Metadata. All data shown was provided by Hawthorne Timber Company.

(LT) Fish: Average Coho and Steelhead Density, 1993-2000

(LT) Fish: Basin wide Coho Dive Survey 2000

(SF) Fish: SF Ten Mile at Camp 28 (SFT6) Catch, 1993-2000

The chart at left shows the mean density for juvenile coho salmon and steelhead trout  from 24 electrofishing stations throughout the Ten Mile River Watershed. Note that there are two axes and that steelhead density was at least 10 times greater than coho density in every year. The highest coho juvenile densities by far were found in 1996 following an adult return estimated at 190-250 fish. The other years coho densities were roughly one fifth or less of 1996 levels indicating significantly lower adult returns in previous spawning years. 

This chart shows results from a snorkeling survey of all potential coho salmon rearing streams in the Ten Mile River Basin in the summer of 2000.  A total of 1731 coho juveniles were counted in more than 30 miles of stream.  Coho were found in 6 of 28 streams and results agree with the distribution and abundance of coho indicated by electrofishing index sites in 2000. Only 11 coho juveniles were found in the South Fork Ten Mile and its tributaries, whereas Maahs (1997a) found this area to formerly be the most productive for coho. 

The chart at left shows the number of fish captured by the Hawthorne Timber Company (formerly Georgia Pacific) on Lower Campbell Creek (SFT2), in the South Fork Ten Mile River drainage, from 1993 to 2000. The total number of fish captured in three passes is displayed for steelhead trout, coho salmon, stickleback, and lamprey. These data show a decline in coho juveniles from the early (1993-1996) period to the latter (1997-2000) with no coho captured in 1998 and 2000. This is indicative of patterns in South Fork Ten Mile tributaries and loss of their productivity has contributed to the decline of the species in the basin.

Alternate Hypothesis

The observed change in the distribution and abundance of coho juveniles is within the natural range of variability and populations may rebound.

Valentine and Jameson (1994) advance this hypothesis in the conclusion of their report on the Little North Fork Noyo River. This alternative could be further explored in the same way as Hypothesis 1 (see below). 

Monitoring Trends to Test the Hypotheses 

Annual monitoring of coho juvenile population size in summer and adult population size by carcass or redd counts would reveal whether coho populations rebound, remain at low levels, or continue to decline.  Adult coho carcass and redd surveys should cover a large and representative sample of basin tributaries and need to be conducted with equal effort annually.  Snorkel and electrofishing surveys (e.g. Hankin and Reeves, 1984) may provide more accuracy for less cost but these alone will not provide information on spawning population size or the number of smolts produced.  Genetic tests of the existing coho population for family size should be conducted to detect whether inbreeding depression is occurring and to help determine the long-term viability of the coho population (Simon et al., 1986). 


Bjornn, T.C. and N. Homer. 1980. Biological criteria for classification of Pacific salmon and steelhead as threatened or endangered under the Endangered Species Act. Report to National Marine Fisheries Service, Seattle, WA.

Gilpin, M.E. and M.E. Soule. 1990. Minimum Viable Populations: Processes of Species Extinction. In: M. Soule (ed) Conservation Biology: The Science of Scarcity and Diversity University of Michigan Press. pp 19-36.

Godfrey, H. 1965. Coho salmon in offshore waters, p 1-39. In: Salmon of the North Pacific Ocean. International No. Pacific Fish Commission, Bulletin 16.

Groot, C. and L. Margolis (Eds). 1991. Pacific Salmon Life Histories. University of British Columbia Press, Vancouver, Canada. 561 p.

Maahs, M. 1996 (a). A salmon spawning survey for portions of Ten Mile, Caspar Creek and Garcia River. 1995-96. Prepared for Humboldt County Resource Conservation District. Salmon Trollers Marketing Association, Inc. Fort Bragg, CA. 31 pp.

Maahs, M. 1996 (b). 1996 South Fork Ten Mile River and Little North Fork Noyo Outmigrant Trapping. Final Report for Humboldt County Resources Conservation District. 43 pp.

Maahs, M. 1997 (a).  1997 Outmigrant trapping, coho relocation and sculpin predation survey of the South Fork Ten Mile River. Prepared for Humboldt County Resource Conservation District. Salmon Trollers Marketing Association, Inc. Fort Bragg, CA. 37 pp. without appendices.

Maahs, M. 1997 (b). The 1996-97 salmonid spawning survey for portions of the Ten Mile River, Garcia River and Caspar Creek. Prepared for Humboldt County Resource Conservation District. Salmon Trollers Marketing Association, Inc. Fort Bragg, CA. 31 pp.

Maahs, M. 1999. Fort Bragg's World's Biggest Salmon BBQ: a brief history of the Salmon Restoration Association and their work. Salmon Restoration Association. Fort Bragg, CA.

Maahs, M. and J. Gilleard.1994.  Anadromous salmonid resources of Mendocino county coastal and inland rivers, 1990-91 through 1991-92: An evaluation of rehabilitation efforts based on carcass recovery and spawning activity. Final report. Salmon Trollers Marketing Association. Fort Bragg, CA. 60 pp.

Neave, F. and W.P. Wickett. 1953. Factors effecting the freshwater development of Pacific salmon in British Columbia. Proc. of Seventh Pacific Sci. Congress. 1949 (4):548-556.

Shapavalov, L. and A.C. Taft. 1954. The history of steelhead trout and silver salmon with special reference to Waddell Creek, California, with special recommendations regarding their management. CA Dept. of Fish and Game Bull. 98:375 p.

Simon, R.C., J.D. McIntyre, and H. Hemminigson. 1986.  Family size and effective population size in a hatchery coho salmon population. Canadian Journal of Fisheries and Aquatic Sciences 43: 24-34.

Taylor, S.N. 1978. The status of salmon populations in California coastal rivers. California Department of Fish and Game. Salmon/Steelhead Program, Anadromous Fisheries Branch. 14 pp.

Valentine, B.E. and M. Jameson. 1994.  Little North Fork Noyo Fishery Study, 1992. CA Department of Forestry and Fire Protection, Coast Cascade Region. Santa Rosa, CA. 48 pp.