Survival of Chinook Salmon, Oncorhynchus Tshawytscha, from A

812 Abstract.-To characterize the im Survival of chinook salmon, pact ofspring floods on the survival of juvenile chinook salmon in the un Oncorhynchus tshawytscha, from a stable, braided rivers on the east coast ofNew Zealand's South Island, I exam spawning tributary of the Rakaia Rivet. ined correlations between spring and summer flows in the mainstem of the New Zealand, in relation to spring and Rakaia River and fry-to-adult survival for chinook salmon spawningin a head summer mainstem flows water tributary. Flow parameters that were investigated included mean flow, maximum flow, and the ratio of mean Martin J. Unwin to median flow (an index of flow vari National Institute of Water and Atmospheric Research (NIWA) ability), calculated during peak down PO Box 8602, Christchurch, New Zealand river migration ofocean-type juveniles (August to January). Survival was E-mail address:[email protected] uncorrelated with mean or maximum flow but was positively correlated with the ratio of mean to median flow dur ing spring (October and November). The correlation suggests that pulses of suits can be derived (Kope and Bots freshwater entering the ocean during To understand the population dy floods may butTer the transition offin namics of anadromous Pacific ford, 1990). gerlings from fresh to saline waters and salmonids <Oncorhynchus spp.), it Despite the importance of in thus partly compensate for the lack of is important to isolate and charac stream habitats for rearingjuvenile an estuary on the Rakaia River. A posi terize the influence of varying en chinook salmon <0. tshawytscha), tive correlation between spring flow variability and the proportion ofocean vironmental factors on annual pro the relation betweenflow and brood type chinook in relation to stream-type duction. In the course of their life year survival has received compara chinook is also consistent with this hy cycle salmon inhabit a succession of tively little attention. Interannual pothesis. All correlations were rela freshwater and marine environ trends in the abundance ofchinook tively weak. reinforcing earlier results ments, where prospects for survival salmon in the Fraser River, British that production is primarily controlled Columbia, have been linked to flow by marine influences. These findings depend on prevailing conditions. further demonstrate the considerable Spawning and incubation success variations inthemainstem(Beamish ability of chinook salmon to adapt to may be adversely affected by sub etaI., 1994) andinthe Nechako River new habitats. strate disturbance during floods; tributary (Bradford, 1994). In the the suitability ofriverine waters as former study, annual production habitat for rearing juveniles is de was inversely related to mean an pendent on both flow and tempera nual discharge, whereas in the lat ture and may be reduced by flows ter study, juvenile survival in the that are too low or too high; and upper Nechako appeared to decline adult survival within the marine as a result offlow diversion for hy environment is at least partly de droelectric generation, and the pro termined by environmentally con portion of spawning fish using the trolled factors such as oceanic wa upper river appeared to be nega ter masses and the availability of tively correlated withAugust flows. suitable prey. Numerous studies However, in the Nechako River have demonstrated significant cor study, as in some other studies link relations between environmental ing downriver migration to river variables and indices of survival flows (e.g. Kjelson et aI., 1982), low and growth, at scales ranging from flows were often associated with local to global. Although correlation increased water temperatures, analysis in fisheries studies has making it d~fficult to differentiate been criticized for its potential for between flow-related and tempera misuse and for a propensity to pro ture-related effects. Williams and duce weak results oflittle practical Matthews (1995) found that sur value (Walters and Collie, 1988), vival of Snake River spring and other authors have noted that pro summer chinook salmon juveniles Manuscript accepted 7 May 1997. vided the method is used with dis was reduced during low flow condi Fishery Bulletin 95:812-825 (1997). cretion, biologically meaningful re- tions but concluded that these Unwin: Survival of chinook salmon in relation to spring and summer mainstem flows of the Rakaia River. New Zealand 8 J3 losses were primarily due to problems with passage able flows (Jowett and Duncan, 1990>, flooding quickly through hydroelectric dams rather than to low dis whenever snow and ice melt in the headwaters is charge per se. The effects of flow variability on sur augmented by heavy orographic rainfall. These floods vival have also received little attention. Increased occur at any time ofyear but are particularly common downstream movement of newly emerged salmonid in spring. Inrivers such as the Rakaia theycause mas fry following sudden increases in discharge has been sive bedmovement <Ibbitt, 1979) and reduce the abun well documented (e.g. Irvine, 1986; Saltveit et aI., dance anddiversity ofinvertebratefauna (Sagar, 1986) 1995I, but only rarely has flow variability been used for up to one month afterwards. The impact of these as a predictor variable in population studies events on seaward-migratingjuvenilechinookhas gen (Berggren and Filardo, 1993). erawd some debate. Several authors have remarked Whenever brood year survival is estimated from that survival ofNew Zealand chinook fry may be ad stock-recruitment or similar data, a search for cor versely affected duringfloods (McDowall, 1990; Flain1). relations between river flow and survival will usu Other studies suggest that, although some fry may be ally involve deriving a single flow index, such as the lost during extreme floods, flow fluctuations in a more annual mean, for each cohort. Most such studies con typical season do not have a serious negative impact ducted to date have used flow averaged over periods on migration (Hopkins and Unwin, 1987). from three months (Kope and Botsford, 1990) to one Chinook salmon spawning populations in Glen year (Beamish et aI., 1994), but it is by no means ariffe Stream, a headwater spawningtributary ofthe obvious that these are the most informative or bio RakaiaRiver(Fig. 1), have been monitored since 1965 logically meaningful parameters to use. A single cata by means ofan upstream counting fence (Quinn and strophic flood duringthe incubation period may cause Unwin, 1993). In this study I analyzed brood year large-scale destruction of redds and loss of alevins survival, for chinook spawning in Glenariffe Stream, through bed scour (Montgomery et aI., 1996) with in relation to Rakaia mainstem discharge during out having much effect on the mean annual flow. springand summer (August to January). My primary Prior to smolting, fry may be susceptible to short objectives were to examine various flow statistics as term floods that carry them prematurely into sea possible correlates with survival and to determine the water, whenthe same floods a few months laterwould sensitivity of any resulting correlations to changes in have little impact. In addition, mean flow is not nec the interval used to calculate each statistic. A second essarily the most relevant statistic for characteriz aryobjective was to examineevidence thatspringfloods ing flow regimes; it is possible that in the two ex were detrimental to brood year survival. amples given above, some other parameter (such as maximum flow or the coefficient ofvariation) might be more informative (e.g. Hvidsten and Hansen, Chinook salmon in New Zealand 19881. For example, in New Zealand, where high flow variability is a defining characteristic ofriverine eco New Zealand chinook salmon are broadly similar to systems (Biggs, 1995), statistics such as the propor their Sacramento ancestors in terms of both their tion of the time the flow exceeds three times the general life history (Unwin, 1986) and genetic make median (Clausen and Biggs, in press) and the ratio up (Quinn et aI., 1996), Present day stocks comprise a ofmean flow to median flow (Jowett, 1990), havebeen mixture ofocean- and stream-type fish (Gilbert, 1913; successful in elucidating relations between flow re Healey, 1983), corresponding to juveniles that spend gime and biological parameters. To explore fully the 3--6 mo or 12-15 mo in fresh waterbefore enteringthe relation between flow and survival, therefore, it is ocean (Unwin and Lucas, 1993). In the Rakaia River, necessary to consider not only the type of flow sta the most thoroughly studied ofthe major salmon pro tistic that is likely to be ofinterestbut also the dura ducingrivers, ocean-typefish makeup about two-thirds tion and seasonal timing of the period over which ofthe returning adults (Quinn and Unwin, 1993). the statistic is to be calculated. The migration patterns ofage-O+ juvenile chinook Since the introduction offall-run Sacramento River salmoninthe Rakaia River and a key spawningtribu stock to New Zealand in the early 1900's (McDowall, tary, Glenariffe Stream (Fig. 1), have been studied 1994a; Quinn et aI., 1996), chinook salmon have in some detail, and are relatively well understood maintained self-sustaining populations in all major (Unwin, 1986; Hopkins and Unwin, 1987). From rivers onthe eastcoastofthe South Island(McDowall, 1990; Quinn andUnwin, 1993). Like mostNew Zealand 1 Flain. M. 1982. Quinnat salmon runs, 1965-1978, in the rivers,

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