Variabilityof MarineSurvival of Pacific Salmonids: a Review S. B. Mathews School of Fisheries Universityof Washington Abstract Time series of total marine survival rate estimates for a number of North Americansalmonfd stocks were reviewed. Only species with extendedfresh-water rear- ing phaseswere considered. Variability fn marinesurvfval tends to be quite variable, and such variability is largely unexplaineble fram smolt data such as mean smolt size, agecomposition, or abundance.The causes of such variability in the roarine envi ronment are poorly understood. The best correlation so far found between marinesurvival anda marineenvironmental factor strengthof ocean-upwelling! i s for cohosalmon originating from the Oregon coast andColumbia River. However,the biological mechanismeven for this case is not known. Introduction This paper revi ews case hi stories of salmonruns for which the marine survfva1 rate fromsmolt outmigratfon to adult return wasie.11 estimated or indexed!for a series of consecutiveyears. Thepurposes were I! to clarify the extent of vari- ability i n marine survival; ! to reviewthe state of know1edge about mechanisms for observedvariability; ! to discuss the extent to which such mechanisms are knownto be relatedto broadchanges in nearshoremarine or oceanographic conditiois; and! to point out anycormoonalfties in marinesurvival trendsamong regians wit i- in a species or across specfes. Mychoices of casehistories for inclusionfn this reviewmay be arbitrary. I includedNorth American stocks of thosespecies with extendedfreshwater rearing phases:coho salmon Oncorhynchus kisutch!, sockeyesalmon . nerka!, chinooksai- for wRic smot out-migrationhad been e~s imate wit relativelyhigh precision, suchas fromdirect countthrough a wier, knownhatchery release numbers, marking, or acoustical means;and ! the numberof adults that resulted from knownsmolt num- bers had been closely measured or indexed. This reviewwas contended to broad1yelucidate whethermarine survival of salmon- ids with extendedfreshwater rearing phases tends to be relatively constantor rela- tively variable. If constant,then freshwaterfactors maybe moreimportant than marinefactors for explainingrun-strength variability; ff highly variable, this wouldamplify the needfor further understandingor study of salmonidsin thei r marine environment. Theterm marine survival in the contextof this reviewmay .be somewhat inappro- priate, since for all case histories presented there fs a freshwater transitional phasebetween times of smoltenumeration and entrance to the marine environment per se. Perhapsa better term is post-smolt survival rate, which is the fraction or 162 percentageof smoltsthat surviveto adulthood.I usethe terms"marine surv1val rate" and"past-smolt survival rate" synonymously,1mplying that mostof the mortal- ity occurringbetween smolt enumeration and adult enumeration probably takes place in the marine environment, including both estuary and open sea. Data from the following stocks are considered in this review: Years of smolt Stock 1960-1981 Coho 1. OregonProduction Index OR, WA!, hatchery 2. Big gualicum River BC!, hatchery 1973-1980 3. Puget Sound WA!, hatchery 1959-1973 Sackeye 1. Lake Washington WA!, nanhatchery 1969-1981 2. Chi lko Lake BC!, nonhatchery 1951-1974 3. Babine River BC!, nonhatchery 1963-1979 4. Karluk Lake BC!, nonhatchery 1926, 1929-1933 5. Bristol Bay AK!, nonhatchery 1971-1981 1962-196 5 Chinook 1. Columb~aRiver fall chinook WA, OR!, hatchery 1978-1980 Stee 1 head 1. Snow Creek WA!, nonhatchery 2. Keogh River BC!, nonhatchery 1977-1980 Coho Ore on Production Index OPI The term OPI refers to hatchery and nanhatcheryproduction from the Columb1a River andOregon coast. Overthe past 25 years mostof this productionhas been fr ompublic hatcherysmolt releases Mathews1980!. Theocean distribution of the catch is relatively well knownfrom markingexperiments. Consequently,estimates a ' post-smoltsurvival rates,developed by the OregonDepartment of Fish andWildlife 982! from catch andescapement data, public hatcherysmolt release data, and judicious apportionmentof total catch into public hatchery, private hatchery, and nonhatcherycompanents T. Nickelson,ODFhW, Corvallis, Oregon,personal communica- tion! provide a good,long-tenn series for considerationof variab1lity in marine surv1valof hatcherysmolts releasedat meansizes rangingfrom 15 to 30 g per i ndi- vidual release group Washington,1982!. Accordingto Table I, post-smolt survival rate over a 22-year period has rangedfram 2.4$ to 10.7$, or about fivefold between low and high. The averagehas been5. 3'4. In all of the most recent six years, sur- vival has been less than average, which has promptedconsiderable effort to explain variability in surv1val. Of manyfactors cans1deredby ODFSW 1982!, 1ncluding Oregoncoastal oceanupwe111ng during spring andsunmer of smolt outmigration, over- planting the ocean density dependentmortal 1ty!, anddeclining trends in hatchery smolt quality, upwellingoffers a relatively high degreeof explanat1on Gunsolus 1978;ODF8W 1982!. Thereis a 1~nearcorrelation r! of .71 betweenpost-smolt survival rate and the upwell1ng i ndexdeveloped by Gunsolus F1g. I!. According :o T. Nickelson personalcommunication! there is an evenh1gher correlation with an upwelling index that includes a broaderrange of spr1ngand summer months than used by Gunsolus. Theability to explain 51$ r2 x 100! of the variability in survival over such a long time per1odwith a single environmentalmeasurement, is uniquein the study of populationfluctuat1ons of salmonids. But the biological mechanismlink1ng acean upwelling ta survival is not known. This linkage could be related to production food for the salmon. If so, a negative correlation betweensurvival and numbersof smolts released at a g1ven upwell1ng 1ntensity should be expected. The data of Table 1 were subjected to a multiple regression analysis, with upwelling and smolt 10 oW I 0 100 200 300 400 500 Upwelllng Fi g. 1. Relationship between OPI upwelling index and post-smolt survival rate of public hatchery coho salmon. Brood years are indicated by the numbers. numbers the independent variab1es and post-smolt survival rate the dependent vari- able. The partial correlation coefficient between survival and smolt numbers, given upwelling, was -.27, which only marginally implicates smolt numbers as a factor negatively affecting survival t -1.18 with 18 d.f.; .10<P<.15!. Another mechanism suggested to explain the upwelling correlation involves predationby seabirds, specifica11ythe commonmurre, Uria ~alae D. Yaroujean, University of Oregon, Charleston, Oregon, personal communscatson;and Yaroujean anc Matthews 1983!. According to this hypothesis, out-migrating coho salmon smolts would becomemore widely dispersed as they enter the sea in cold water high upwel1- ing! years than warm water low upwelling years!. If the coho are more widely 164 Table 1. Post-smolt survival rate estimates af hatchery coho salmon in the Oregon Production Index region, 1958-79 brood years. Year Public of smolt hatchery re- Brood outmigra- leased smolts Post-smolt Upwelli ng year tion yri yeari x 106 survival 5 indexyear>> 1958 1960 7.5 4.5 127. 0 1959 1961 16. 6 2.4 103. 0 1960 1962 16. 1 5.1 178. 3 1961 1963 23. 3 5.0 159 ~ 3 1962 1964 19. 0 8.7 353. 5 1963 1965 24. 0 8.0 351. 0 1964 1966 27. 4 9.5 403. 8 1965 1967 27. 7 6.9 396. 5 1966 1968 24. 3 5.8 405. 8 1967 1969 32.5 7.8 182. 8 1968 1970 29.5 10. 7 305. 0 1969 1971 35.7 5.1 205. 3 1970 1972 36.3 4,8 231. 3 1971 1973 34. 5 8.2 356. 0 1972 1974 33. 7 4.5 272. 3 1973 1975 36.1 10. 6 379. 8 1974 1976 35.1 2.4 225. 8 1975 1977 37.5 3.5 234. 5 1976 1978 39. 4 3.2 187. 0 1977 1979 34.7 3.0 221. 0 1978 1980 34.1 2.9 181. 5 1979 1981 38.0 2.8 176. 3 X=5.3 1 Index developed by Gunsolus 978!: July Cascade Head I April + Ma +June !] + Brookings [April + Source: Oregon Department of Fi sh. Wi1dli fe 982!, Appendix Table 3F-1, and Appendix Fig. 3G-6. dispersed they maybe relatively less attractive to murre predation than alternatf~e schooling prey species such as anchovies. 2. This hatchery has released groups of wire-tagged coho smolts into Georgia Strait for a series of years beginning in 1973. Nark sampling in the catch and escapementhas provided estimates of post-smolt survival for sevenconsecutive years T. Perry, Canadian Department of Fisheries and Oceans, Vancouver, B.C., personal communication!. Table 2 indicates that post-smolt survival has ranged from I 5. 5%to 41.6%, averaging 25.64. Muchof the variability in post-smolt survival may be d~e to variation in date of release. It is well known that date of release is one of the most important freshwater variables affecti ng survival Mathews and Buckley, 1976; Bi lton, 1980! . For the Big Oualium data series there is a strong correlation between release date and survival rate r = .76, Fig. 2!. I statistically removed the variability due to this linear relationship by addi ng the deviation in each survival value from its predicted value on the regression line to the mean surviva rate for all data points, The resulting adjusted post-smo1t survival values ranged 165 Table 2. Post-smolt survival rate estimates of Big gualicum British Columbia! hate he ry-reared coho s alman, 1971-1978 broods. Year o S ze at 0 ~ Brood smolt outmi - Dates af release ma rks Post-smolt surviva'l rate % ear ration release fish/lb! released atc sca ement ata 1971a 1973a 5/11/73 16. 96. 9! 32, 156 17. 5 8.4 25. 9 1971b 1973b 5/25/73 18. 4 4. 7 ! 35,553 15. 3 6.6 21. 9 1971c 1973c 5/25/73 19. 23.6! 13,652 15. 4 9.0 24. 4 1971d 1973d 6/01/73 17.16.6! 33,187 20. 1 9.0 29. 1 1972a 1974a 5/11/74 16. 77. 2! 30,076 10. 9 9.1 20. 0 1972b 1974b 5/23/74 21.21.4! 35,474 19. 9 15. I 35.