Downstream and Early Marine Migratory Behaviour of

Downstream and Early Marine Migratory Behaviour of Sockeye Oncorhynchus nerka Smolts Entering Bark ey Sound, Vancouver Chris C. Wood, N. Brent Hargreaves, Dennis T. Rutherford, and Brian T. Emmett' Department of Fisheries and Oceans, Biological Sciences Branch, Pacific Bioisgical Station, Manairno, BC V9R 5K6, Canada

Wood, C.C., N.B. Hargreaves, D.T, Rutherford, and B.T. Emmett. 1993. Downstream and early marine migratory behaviour of sockeye salmon (Owcorhynchus nerka) smolts entering Barkley Sound, Vancouver Island. Can. j. Fish. Aquat. Sci. 50: 1329-1 337. Behaviour of sockeye salmon (Oncsrhynchus nerka) srnolts was studied during their migration down the Stamp and Somass rivers and through Alberni lnlet and Barkley Sound, Vancouver Island. Emigration from Great Central Lake was typical of other sockeye lakes: smolt abundance peaked in early May and most srnolts left the lake at dusk. Migration down the rivers occurred only during twilight or darkness and most smolts made the 25-km journey to tidal waters within a single night. Those that did not held position in tight schools in the surface waters of deep pools during daylight. During downstream migration, ground speed averaged about 2.5 kmlh, and srnolts did not appear to be especially vulnerable to predation owing to their short residence time, with few smosts remaining in the river during daylight. Catch rates at purse-seine sampling sites throughout Alberni lnlet and Barkley Sound indicated that sockeye srnolts migrated more slowly in tidal waters and were sometimes highly vulnerable to predaceous fish (especially Pacific hake, Mer8ucciusproductus). Estimated migration speeds through Alberni lnlet were 1.9 kmld for smolts entering tidal waters during April and May, and 4 - 3 kmld for those during lune and July; speed through Barkley Sound was 1.6 kmld throughout May-July. Nous avons ktudi6 le comportement des srnolts de saumon rouge (Oncorhynchus nerka) pendant leur avalaison dans les rivieres Stamp et Somass, puis dans B'inlet ABberni et la baie Barkley, prGs de t'ile de Vancouver. L16migration du Grand lac Central 6tait typique d'autres lacs saumon rouge : I'abondance des srnslts atteignait un pic au d&but de mai, et la plupart quittaient le lac au crkpuscule. L'avalaison se produisait seulement dans la p6nombre esu l'obscuritk, et la plupart des smolts parcouraient les 25 km de leur voyage jusqu'aux eaux 3 rnarke en une seule nuit. Ceux qui ne migraient pas formaient des bancs serres dans Des eaux superficielles des bassins profonds pendant le jour. Pendant I'avalaison la vitesse sol moyenne ktait de 2'5 krnlh environ, et les smolts ne semblaient pas particuli&rement vuln6rables 3 la prkdation du fait de leur faible temps de r6sidence dans les rivieres; ils ktaient peu nornbreux y rester pendant le jour. Les taux de capture aux points d16chantillsnnagei la senne i poche, sur I'ensernble de I'inlet Alberni et de la baie Barkley, indiquaient que les srnolts de saumon rouge rnigraient plus lenternent dans les eaux 5 mar6e et 6taient parfois trGs vulw6rabies aux psissons pr6dateurs (notamment le merlu du Pacifique, Merluccius productus). O n estime les vitesses de migration dans l'idet Alberni 2 1,9 kmld pour les smolts qui pkn6traient dans les eaux 3 mar6e en avril et en mai, et 1 , 3 kmld en juin et juillet; dans la baie Barkley, la vitesse etait de f,6 kmld de mai A juillet. Received june 10, 1992 Accepted December 9, 11 992

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onsiderable effort has been directed towards understanding the seaward migration of anadromous salmonids (see reviews by Thorpe et al. 1985; Grcsst and Margolis 11991). In particular, much is howam about the behaviour and physiological characteristics of sockeye salmon (Opecsrhyreehus peer&) srnolts just prior to emigration, and during emigration from "nursery" lakes in which sockeye fry typically spend their first year of life (Burgner 1962;Johnson and Groot 1963; Groot 1965; Hartman et al. 196'7; Fwrster 1968; Clarke 1989). However, few studies have documented the behaviorar of sockeye smolts during both the downstream migration and subsequent period of early sea life, no doubt swing to logistic difficulties. Early investigators (Foerster 1929; Bmaby 1944) reported that sockeye srnolts migrated downstream passively, often fac'Resent address: Archipelago Maine Resemh, La., 525 Head St., Victoria, BC V9A 5S I, Canada. Can. $. Fish. Aquat. Sci., VoL 50,1993

ing into the current of the outlet stream. More recently, other investigators (Hartman et A. 1967; Burgner 1991) concluded that the migration downstrem is active where water flow is uniform and quiet, but passive where water flow is turbulent. In contrast, sonic tracking studies indicate that Atlantic sdmsn (Sa&mosalar) srnolts migrate downstream passively, and are continually displaced from feeding territories through behavioural interactions (Fried et al- 1978; LaBar et al. 1978; Tytler et al. 1978; Thorpe et al. 1981). Similar studies show that coho salmon (8.kisutch) smolts migrate downstream actively in sections of high water velocity but hold position for extended periods in sections of Iow water velocity (Moser et al. 1991). Burgner (199 1) also pointed out in his recent review that it is not known whether sockeye smolts migrate downstream continuously, day and night. If srnolts are especially vulnerable to predation during the downstream migration, as often supposed (Hartman et a%.196'7; Tytler et aH. 19'78), they 1329

Sampling sites

IZ,

Great

j ~ e n w o i Lake

migration in freshwater with that in tidal waters and comment on the apparent vulnerability of sockeye smolts to predators in these two environments.

Study Area Sockeye smolts examined in this study emigrated from two Vancouver Island lakes: Great Central Lake and Sproat Lake. Great Central M e (surface area = 5085 ha) is situated at m elevation of 82 m and is drained principally by the S t m p River (21 km long) but also by Robertson Creek. Robertson Greek flows through the Robertson Greek Hatchery and joins the Stamp River at Stamp Lagoon, a shallow I&e about 1 km Hong (Fig. 1). Sproat Laate (3775 ha) is situated at an elevation of 28 m and is drained by the Sproat River (2.8 km). The Sproat and S t m p rivers converge into the Somass River which flows for another 5 -0km to the sea (Albemi Inlet and Barkley Sound). The zone of tidal influence extends upstream as far as site L in Figure 1 . Thus, sockeye smolts from Great Central Lake must migrate a total of 24.9 km at an average gradient of 0*33% before reaching tidal waters. Those from Sproat Lake must migrate 7.8 km at an average gradient s f 0.36%. Once in tidal waters, smolts must swim 37 km though Albemi Inlet to reach Barkley Sound and the Pacific Ocean. Since 1987, annual smolt production from Great Central and Sproat lakes has averaged 7 and 9 million smolts, respectively. Most smolts emigrate as yearlings, with annual mean sizes ranging from 3.74 to 6.80 g for Great Centrd Lake smolts and from 3.50 to 4.74 g for Sproat Lake smolts over the s m e period (Hyatt et aH. 1990).

Methods Downstream Migration FIG. 1. Map showing locations of sampling sites.

might be expcted to seek refugia and hold position in the stream during daylight. The migration of sockeye during the period of early sea life varies considerably among populations and also depends on latitude (Burgner 1991). Sockeye from southern populations be. g ., the Fraser River) typically enter the ocean in April and May. Groot and Cooke (1987) found that Fraser River sockeye concentrated near the river mouth for a short time, but by the end of May, most had dispersed away from the estuary and were broadly distributed in the Strait of Georgia; the rate of migration during this early period was estimated to be 6-7 k d d . Marine survival of sockeye appears to be controlled by conditions encountered during early sea life (Brett 1986). Predation is widely assumed to be the major source of mortality for all species of salmon during this period (e.g ., Parker 1971 ; Mathews amd BuckHey 1976; Brett 1986). To elucidate mechanisms that determine variations in recruitment of Pacific salmon, the Canadian Department of Fisheries and Oceans initiated the Marine Survival of Salmon (MASS) Program. This p r o p m has supported a variety of research activities, most focussing on factors relating to the survival of juvenile salmon in Albemi Inlet, Bmkley Sound, and along the west coast of Vancouver Island. In this paper, we consolidate new infomation from these studies concerning the behaviour sf sockeye smolts during their seaward migration in a coastal river system and estuary on Vancouver Island. We examine the speed s f downstream migration and compare migratory activity during daylight and darkness. We d s o compare the speed of

Fyke nets Fyke nets ("traps") were deployed facing upstream in the main current of the Stamp River at three sites to detemine daily and seasonal trends in abundance of migrating smolts. The nets measured 2 x 2 m at the upstream opening, were 7.5 m long, and consisted of a graded series of mesh sizes from 5.0 cm at the opening to 1.3 cm at the downstream end. These led via 15-cm-diameter PVC tubing to 1 x 0.75 x 2.2 rn collection boxes with baffles to minimize current in the holding chamber. Trap B was deployed at site C from 10 t s 27 April, trap 2 at site E from B 1 April to 9 August, and trap 3 at site I fmm 13 April to 16 June 1989. Trap 1 was used mainly to estimate the s p e d of smolt migration across the still waters sf Stamp Lagoon by comparing the relative timing of peak catches during the wight at the inlet (trap I) and outlet (trap 2) ends of the Stamp Lagoon. Trap 2 was considered to provide the best index of relative abundance over the season because fishing efficiency at this site appeared to be least influenced by fluctuations in river level. During the main period of smolt migration (26 April - 12 May), catches were usually recorded (and smolts released) at 20:00, W:W, W:80, and 08:W PDT. At other times, catches were usually recorded between 07:W and 09:W PDT. Smples of approximately 50 smslts were collected for size and age composition data from trap 3 each week (except the week s f 13-19 May). Catches were d s o recorded and sampled for size composition (n = 50) at approximately 2-h intervals at all traps on 26-27 April and at traps 2 and 3 on 1-2 and 9-10 May. To investigate correlations in daily catches at the upstream and downsteam traps, we divided the period of migration into Can. J. Fish. Aqucmt. Sci., Voi. 50, 6993

the "increasing phase" prior to 2 May and a "decreasing phase'' after 2 May. We fitted linear trends to the daily catches for each trap in each phase after log transformation to stabilize variances (a = 0.85 and 0.95 for traps 2 and 3, respectively, in the increasing phase; r - 0.9 1 and - 0.59, respectively, in the decreasing phase). We then removed the Linear trends and calculated cross-correlation functions for the residuals at lags of 0-5 d.

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Umderwater sbsensrtisns Underwater surveys by divers using SCUBA or snorkels were conducted to observe the die1 patterns in migratory behaviour of sockeye smolts. Underwater lights were used during darkness. Divers with SCUBA swam fixed-lengthtransects in Stamp Lagmn (site D, 650 m) and in the Stamp River just above its confluence with the Sproat River (sites I and J, 580 m) at various times of day, 25-27 April, and recorded the number, swimming behaviour, depth, and schooling habits of sockeye smolts. Divers tethered in position and using SCUBA recorded similar observations during 20-min intervals at 20:0Q, 22:W, 01:30. 03:30, and 05~30PDT, 9-10 May, both in the section of maximum current and at midchannel in the lower Somass River (site L). Divers using either SCUBA or snorkels also estimated the abundance and behaviour of sockeye smolts during daylight hours at several other sites (G, H, K. and M) where smolts could hold position in the river (i.e., slackwater sections, or deep pools). Emigration porn Great Central Lake The daily timing of smolt emigration from Great Central Lake was estimated by enumerating smolts against a submerged, white, 1.2 x 2 rn flashbard using photographic techniques (e.g., Goodlad et al. 1974), both at the outlet of Boot Lagoon (site A) and in a fast-flowing section of Robertson Creek (site B). The flashboards were photographed (using a flash but no other external light) every minute during 10-min intervals at various times of the day and night from 4 to 18 May. Later, silhouettes of smolts were counted from the photographs. Physical measurements Water temperature and level were measured at site E each morning that trap 2 was inspected (usually daily) from 11 April to 20 July. Percent cloud cover and air temperature were also recorded at the same times. On 16 June, we measured maximum river velocity at 0.5-km intervals between sites E and L (40 stations in all), except for a 1-km reach at the Stamp Falls and another 1-km reach of turbulent water on the Somass River. Measurements were taken 20 cm below the surface in the main current of the river using two General Oceanics model 2030 flow meters; at each site, the higher of the two readings was taken as the correct value. River velocity was not measured downstream of site & because there it was obviously influenced by tidal action. The minimum time required for water to flow between sites E and L was calculated by plotting the inverse of the maximum velocity measurement at each station against distance downstream and integrating under the curve.

Marine Migration The relative abundance and distribution of juvenile sockeye in Alberni Inlet and Barkley Sound were determined by purse seining from the 12.5-m-long vessel RV KETA . The purse seine was 275 m long, with a nominal depth of 26 m tapering to 18 rn in the bunt. The bunt of the net was constructed of five panels with stretched mesh sizes of 50,25, 19, 13, and 10 mm. PurseCart. .J. Fish. Aqucmt. Sci., Vo'sl. SO, 1993

seine sampling in Alberni Inlet and Barkley Sound began on 13 April and ended on 14 July. From April until mid-May ,one purse- seine sample was collected at each of 24 fixed sites every third week. Thereafter, one sample was collected weekly at each site except for a 1-wk gap after the second sample. A total of 192 purse-seine sets were completed in 1989, all during daylight between 06:00 and 21:00 PDT. All juvenile sockeye caught with the purse seine were immediately identified, counted, and most were released alive. A random subsample of 30- 100 fish was retained from each catch; these fish were anesthetized with 2-phenoxyethanol, measured (fork length) to the nearest millimetre, and allowed to recover from the anesthetic before being released alive. The abundance of juvenile sockeye in Alberni Inlet and Barkley Sound is presented as catch-per-unit-effort (CPUE), defined as the total catch in one set of the net at each site. We estimated the speed of migration of sockeye through Alberni Inlet and Barkley Sound by plotting the dates of peak abundance of juvenile sockeye at 24 marine sampling locations against their respective distances from the first marine sampling location (site N). Dates of peak abundance were determined by inspection of graphical plots and then regressed on distance travelled using several least-squares models (linear, exponential, and polynomial).

Results Downstream Migration The downstream migration of sockeye smolts began in midApril 1989, reached a peak at the beginning of May, and then declined gradually throughout May and June (Fig. 2). Catches in traps 2 and 3 showed similar trends, with peak catches of 1488 and 2887ld occurring on 344 April and 2 May, respectively. Sockeye smolts continued to migrate downstrem in small numbers as late as July; the last trap (trap 2) was removed on 20 July when only one smolt was caught. In d l , 27 540 sockeye smolts were counted during the sampling period. Daily catches of sockeye smolts were not correlated with river level, water temperature, cloud cover, or lunar phase in any obvious way (Fig. 2). However, after 10 May, the capture efficiency of trap 3 appeared to increase slightly relative to that of trap 2 because at lower river levels, a greater proportion of the main channel flow was sampled by trap 3 than by trap 2. For this reason, catch data from trap 2 probably gave the most consistent index of relative abundance over the season. These data indicate that $596 of the downstrem migration occurred between 23 April and 25 May, during which time daily catches at trap 2 exceeded 200 sockeye smolts. The average size of smolts declined from 80.5 mm (SD = 7.0) during the first 2 wk of enumeration (period 1, 11-24 April) to 63.7 mm (SD = 5.1) in period 4 (23 May 5 June) (Fig. 3). Age 1 smolts accounted for 99.9% of the smolt migration, averaging 2.9 g and 71 mm fork length (range 54-95 mm). Only 6 of 928 specimens collected were age 2 , and these were captured in periods 1 and 2 and ranged in fork length from 85 to 96 mm.

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Diel Migratory Behaviour Cohesive schools of 50-10Q smolts were observed at the outlet of Boot Lagoon (site A) in late afternoon between 15:W and 20:W PDT (6-10 May), but these resisted the current at the

Period

SAMPLING PERlODS

lunar phase

a0.0

a0.

50

be8

0

0 APRIL

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FIG. 2. Seasonal trends in daily fyke-wet ("trap") catches of sockeye smolts and water depth and temperature in the Stamp River.

outlet and probably remained in the lake until the outlet was in shadow (around 19:W PDT). Emigration from Boot Lagoon (determined by flashboard counts) reached a peak between 19:00 and 21:OO PDT and had virtually finished for the night by 23:00 PDT. The same pattern was observed 0.5 km downstream in Robertson Creek (site B) where small schools (