Fish Sci (2013) 79:569–577 DOI 10.1007/s12562-013-0615-8
ORIGINAL ARTICLE
Biology
The relationship between migration speed and release date for chum salmon Oncorhynchus keta fry exiting a 110-km northern Japanese river Kiyoshi Kasugai • Mitsuru Torao • Mitsuhiro Nagata James R. Irvine
•
Received: 7 November 2012 / Accepted: 6 March 2013 / Published online: 24 May 2013 Ó The Japanese Society of Fisheries Science 2013
Abstract The relationship between release date and migration speed was examined for hatchery chum salmon Oncorhynchus keta fry exiting the Nishibetsu River in eastern Hokkaido, northern Japan so that future releases might be scheduled so that fry arrive at the ocean during periods favoring high survival. Separate marked groups of chum salmon released in early April, mid-April, and early May in 2008, late March and mid-April in 2009, and midApril in 2010 were recaptured with a rotary screw trap 12 km above the river mouth. Chum salmon in later release groups tended to migrate downstream faster than fish in earlier release groups. Those released after mid-April arrived in the lower river on average 9 days after release, while those released before mid-April arrived on average 26–28 days after release. Most marked fish arrived in the lower river during late April to mid-May. These results suggest that chum salmon are adapted to adjust their migratory speed so as to arrive at the ocean during a relatively discrete period, presumably during a time of high productivity favoring good survival.
K. Kasugai (&) M. Torao Doto Research Branch, Salmon and Freshwater Fisheries Research Institute, Hokkaido Research Organization, 3-1-10 Maruyama, Nakashibetsu, Hokkaido 086-1164, Japan e-mail:
[email protected] M. Nagata Salmon and Freshwater Fisheries Research Institute, Hokkaido Research Organization, 3-373 Kitakashiwagi-cho, Eniwa, Hokkaido 061-1433, Japan J. R. Irvine Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC V9T 6N7, Canada
Keywords Chum salmon Downstream migration Release date Hatchery Nishibetsu River
Introduction Japanese chum salmon Oncorhynchus keta are largely sustained by hatcheries. Surveys in coastal regions of Japan have been conducted to better understand the distribution, growth, and feeding of young chum salmon to reduce the possibility of mismatches between the timing of their ocean entry and suitable conditions in the coastal environment [1–3]. These surveys suggested that, when chum salmon arrive at coastal nearshore areas too early, survival can be poor due to low seawater temperature, while fish that arrive too late can be disadvantaged as a result of smaller body sizes, due to shorter stays in the nearshore area [2]. However, understanding of optimal ocean entry timing is limited. In the 1960s, pink salmon O. gorbuscha fry were stocked into the Nishibetsu River shortly after emergence during January and February. Wild chum and pink salmon fry arrived in the lower Nishibetsu River between late April and mid-May [4, 5]. It was concluded that hatchery releases of pink salmon fry early in the year may have negatively affected the pink salmon resource [6]. Most Japanese hatcheries are near the coast, but in some large rivers, hatcheries are well upstream, close to water suitable for rearing salmon. In smaller rivers, chum salmon fry migrate seaward soon after release [7, 8]. However, with some exceptions, migration periods and speeds are generally unknown in large rivers [9, 10]. The 110-km Nishibetsu River has several hatcheries in its upper reaches (Fig. 1). Hatchery staff wished to select release dates so that young salmon arrived at nearshore marine areas when the environment was well suited for
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growth and survival, i.e., when sea surface temperatures (SSTs) were between 8 °C and 13 °C. A preliminary study in 2006 suggested that, when chum salmon fry were released later in the year, they migrated faster than those released earlier in the Nishibetsu River (Takeuchi K, unpubl. data, 2006). It was important to investigate the relationship between release date and migration speed, because release date could be adjusted if a relationship was discovered, in order to improve the likelihood of young salmon arriving at the ocean at an optimal time, and increase their probability of survival. It was therefore hypothesized that migration speed was correlated with release date, which is closely connected with photoperiod, the environmental cue that triggers a complex series of physiological events for downstream migration [11, 12]. In addition to photoperiod, proximate factors initiating downstream migration are water temperature, velocity, and discharge [11]. Downstream migration speed in rivers can be influenced by water speed, date, location where fish commence migration, fish size, and extent of parr–smolt transformation [13, 14]. Large fish can swim faster than small fish, so that larger fish may arrive at the ocean earlier than smaller fish [13, 15, 16]. Change in discharge may promote downstream migration, and greater discharge may move fish faster [17–20], therefore greater discharge may cause earlier arrival of fish at the ocean. Since higher water temperature can induce downstream migration [21, 22], fish may arrive earlier when temperatures are higher. Therefore, fish size, discharge, and water temperature were monitored during the study, in addition to release date, and the influence of these factors on migration was investigated.
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46˚
Hokkaido Island 44˚
Ishikari River
Tohoro River
Nishibetsu River Chitose River Utabetsu River 42˚
40˚
Otsuchi River Kitakami River 38˚ 140˚
142˚
144˚
146˚
43˚36' Okunishibetsu Hatchery Nemuro Bay Sampling site 43˚24' Betsukai
Betsukai ST
Nishibetsu River 10 km
43˚12'
144˚36'
144˚48'
145˚00'
145˚12'
145˚24'
Fig. 1 Location of release site and sampling site (solid circles), gauging station (solid triangle) on the Nishibetsu River, and data logger for sea surface temperature [Betsukai Station (ST), solid square]
Materials and methods Release of marked chum salmon fry The study was carried out on the Nishibetsu River in eastern Hokkaido, northern Japan (Fig. 1). The otoliths of chum salmon were marked with alizarin-complexone (ALC), a fluorescent material [2, 23]. Eyed eggs were immersed in 200 ppm solution of ALC for 24 h. Groups were differentiated with three discrete marking patterns by exposing developing eggs to different cumulative daily temperatures after fertilization: at about 340 °C (small ring), at about 450 °C (large ring), and both (double rings). The precise timing of smoltification, with its accompanying physiological changes, is not clear for chum salmon [24]. Seawater tolerance is part of smoltification [25], so that delayed smoltification (i.e., development of seawater tolerance) may postpone entry to the ocean. Seawater tolerance, a surrogate for smoltification, was assessed, and the ability to osmoregulate was examined by transferring
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groups of 100 otolith-marked fish into seawater (30 %) and recording mortality during the next 48 h. Otolith-marked chum salmon were released at the Okunishibetsu Hatchery, 110 km from the river mouth, in 2008–2010 (Fig. 1; Table 1). Approximately 40 million fry were released into the Nishibetsu River annually in 2008 (18 March to 20 May), 2009 (26 March to 27 May), and 2010 (27 March to 31 May) (Fig. 2), of which about 1 to 3 million were marked (Table 1). Recapture of chum salmon fry A 1.5-m-diameter rotary screw trap (E.G. Solutions, Inc., OR, USA) was operated 12 km upstream from the river mouth (Fig. 1) from 7 April to 17 June in 2008, 1 April to 19 June in 2009, and 2 April to 17 June in 2010. Captured chum salmon fry were counted daily, and fish were sampled on Mondays, Wednesdays, and Fridays. Sampled fish were anesthetized with eugenol (FA100; Dainippon
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Table 1 Numbers, sizes, and seawater survival of marked chum salmon fry released from 2008 to 2010 Year
2008
2009 2010
Date of release
Number of marked fish released
Number of fish measured
Fork length (mm)
Body weight (g)
Seawater survival (%)
Mean
SD
Multiple comparisons
Mean
SD
Multiple comparisons
3 April
1,057,000
100
52.26
3.32
c
1.279
0.282
bc
17 April
1,018,000
101
50.58
3.90
ab
1.172
0.354
ac
100
3 May 26 March
1,017,000 862,000
99 90
50.76 51.78
3.41 3.83
abc c
1.197 1.329
0.270 0.343
abc c
100 98
17 April
1,037,000
102
50.88
4.22
abc
1.179
0.324
abc
100
16 April
1,085,000
101
49.98
3.17
a
1.137
0.236
a
100
100
Number of chum released
Multiple comparisons followed by the same letter(s) did not differ significantly from each other. Letters indicate size, with the letter ‘‘a’’ indicating the smallest fish and ‘‘c’’ indicating the largest fish
2008 Released
Number of juvenile chum salmon
1200 1000 800 600 400 200 0
2008 Recaptured 7 April trap installed
17 June trap removed
rotary screw site and the Okunishibetsu Hatchery and in the nearshore area 1.5 km from the mouth of the Nishibetsu River [Betsukai Station (ST)] [26]. River discharge was recorded hourly at the gauging station at Betsukai (Fig. 1). Daily precipitation at Betsukai (Fig. 1) was obtained from databases of the Japan Meteorological Agency (http:// www.data.jma.go.jp/obd/stats/etrn/index.php).
2009 Released 1200 1000 800 600 400 200 0
Statistical methods 2009 Recaptured 1 April trap installed
19 June trap removed
2010 Released 1200 1000 800 600 400
2010 Recaptured 2 April
trap installed 200 0 25-Mar 4-Apr 14-Apr 24-Apr
17 June trap removed
4-May 14-May 24-May
3-Jun
13-Jun
Fig. 2 Number of released (upper panel) and recaptured (lower panel) chum salmon fry in the Nishibetsu River in 2008, 2009, and 2010. Open bars in the upper panel indicate ALC-marked chum salmon. Figures in the bar of the upper panel indicate numbers of released chum salmon fry exceeded 4 million fish. The symbol ‘‘x’’ in the lower panel for 2009 and 2010 indicates no catch because the cone was temporarily removed due to a flood
Sumitomo Pharma Co., Ltd., Osaka, Japan), fixed for 4 h with neutral 5 % formaldehyde, and preserved until measurement in 70 % ethanol. Each fish was measured for fork length and body weight, and the otoliths were extracted and stored. Otoliths were examined with a fluorescence microscope to verify the presence of ALC marking and to determine the mark patterns. Water temperature and river discharge Hourly water temperatures were recorded using data loggers (TidBid; Onset Computer Corporation, MA, USA and ACLW-CMP; ALEC Electronics, Kobe, Japan) at the
Migration speeds (km/day) of each marked group were compared with Kruskal–Wallis and Scheffe´ tests. To evaluate the river temperature and discharge that each marked group underwent, these tests were also used to compare: (1) daily mean river temperatures at the upper and lower reaches during the study period (early April to mid-June) among years, (2) daily mean discharges between 20 March and 20 June among years, and (3) means of daily discharge during 60 days after release, which covered the migration period, for each group each year. The potential importance of release date, fish size, water temperature, and river discharge on migration speed was examined by determining correlation coefficients. Mean fork length at release (FL) and number of days from 26 March to the release date (RELEASE) represented body size and release date. The Okunishibetsu Hatchery, being spring-fed, had relatively constant water temperatures, while temperatures in the lower river varied seasonally. It was not possible to install water temperature data loggers before early April because the lower Nishibetsu River was frozen. Therefore, water temperatures were estimated using relationships between air temperature at Betsukai (data obtained from the Japan Meteorological Agency: http://www.data.jma.go.jp/obd/stats/etrn/index.php) and water temperature at the lower reaches before early April in 2008 and 2009. Relationships between air temperature (AT) and water temperature (WT) were: 2008: WT = 5.741 ? 0.618AT (n = 72, P \ 0.01); 2009: WT = 5.798 ? 0.564AT (n = 79, P \ 0.01).
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P \ 0.05; Table 2). Variability in upper river temperatures was less than for lower river temperatures. Lower river temperatures exceeded those at the Okunishibetsu Hatchery commencing on 20 April in 2008, 13 April in 2009, and 1 May in 2010. SSTs in the nearshore area 1.5 km off the mouth of the Nishibetsu River exceeded 8 °C after early May (Fig. 3) [26]. Daily discharges were generally constant except for sharp increases due to snowmelt and rainfall (Fig. 4). Mean daily discharge during 20 March–20 June was the lowest in 2008 (Kruskal–Wallis test, v2 = 155.5, df = 2, P \ 0.01; Scheffe´, P \ 0.05; Table 2); marked fish experienced different discharge regimes each year. Differences in mean discharge during the 60 days following release were not significant in any year (v2 = 208.3, df = 5, P \ 0.01; Scheffe´, P [ 0.05; Table 3), therefore mean discharge, which marked fish were assumed to have experienced, were not different in any year.
Recorded and estimated water temperatures at the sampling site at the release dates, and mean daily discharge during the 60 days following release, were used to indicate temperature (TEMP) and discharge (DISCHARGE), respectively (recapture of marked fish was observed during 60 days following release). Comparisons of variables and correlation analyses were conducted using R 2.10.1 [27].
Results Changes in water temperature and river discharge Water temperatures at the Okunishibetsu Hatchery ranged between 7.7 and 9.4 °C during the study, while those at the lower river site ranged between 3.3 and 16.1 °C (Fig. 3; Table 2). Upper and lower river temperatures were lower in 2010 than other years (upper: Kruskal–Wallis test, v2 = 13.5, df = 2, P \ 0.01; Scheffe´, P \ 0.01; lower: Kruskal–Wallis test, v2 = 8.2, df = 2, P \ 0.05; Scheffe´,
Comparisons of size and seawater tolerance of marked chum salmon fry Salmon fry released earlier in the year tended to be larger than fish released later, although these differences were not always significant (Table 1). Seawater tolerance of each marked group was more than 98 % (Table 1).
Water temperature ( C)
2008
Migration of chum salmon fry
2009
Chum salmon fry were captured most days during the study (Fig. 2). The number of fish recaptured for each marked group ranged from 24 to 137 (Table 3). Recapture rates ranged from 0.0023 to 0.0132 %, and groups released later had higher recapture rates than those released earlier in each year. Most marked fish were recaptured from late April to mid-May each year (Fig. 5). Migration periods for marked fish released before mid-April exceeded those for fish that were released on or after mid-April in each year (Table 3). Mean migration speeds differed significantly
2010
Fig. 3 Changes in water temperature at the upper (open triangles) and lower (solid circles) Nishibetsu River, and at the Betsukai ST (gray rhombuses) in 2008, 2009, and 2010
Table 2 Water temperature and discharge during study period in the Nishibetsu River from 2008 to 2010 Year
2008 2009 2010
Study period
7 April–17 June 1 April–19 June 2 April–17 June
Water temperature at release site (°C)
Water temperature at sampling site (°C)
Mean discharge from 20 March to 20 June (m3/s)
Mean
SD
Range
Multiple comparisons
Mean
SD
Range
Multiple comparisons
Mean
SD
Range
Multiple comparisons
8.64
0.27
7.90–9.16
bc
10.48
2.99
4.91–16.13
b
8.33
1.45
6.04–13.00
a
8.75
0.32
7.83–9.38
c
10.05
2.56
3.27–14.40
ab
13.63
7.09
8.73–57.44
b
8.54
0.39
7.68–9.27
ab
9.01
3.01
3.90–14.91
a
14.65
5.28
8.77–35.72
c
Multiple comparisons followed by the same letter(s) did not differ significantly from each other. Letters indicate size, with the letter ‘‘a’’ indicating the smallest and ‘‘c’’ indicating the largest
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Percentages of number of marked fish recaptured with the trap against number of marked fish released
Multiple comparisons followed by the same letter(s) did not differ significantly from each other. Letters indicate size, with the letter ‘‘a’’ indicating the smallest and ‘‘c’’ indicating the largest
a
c
b 4.64
2.68 13.85
12.56 b
b 6.51
3.15 7.68
8.99 b
b 4.8
4.8 13.9
14.1 38
22 3–24
3–40 0.0132
0.0075 81
137 17 April
16 April 2010
a
b 3.66
1.43 8.65
11.89 a
c 3.05
2.67 4.54
11.75 a
c 12.4
2.9 8.9
27.7 48
15 6–20
8–55 0.0050
0.0060 61
43
3 May
26 March 2009
a
a 1.55
1.44 8.63
8.64 a
b 1.13
0.95 3.99
8.22 b
c 5.7
2.3 12.1
25.5 24
12 11–22
16–39 0.0023
0.0060
24
61
3 April
Multiple comparisons SD
2008
17 April
SD Mean Multiple comparisons Mean Mean
SD
Migration speed (km/day) Migration period (days) Recapture period (days) Range of migration period (days)
Our study showed that chum salmon released earlier in the year took longer to travel the approximately 100 km to the lower Nishibetsu River than fish that were released later. Even though chum salmon fry were released during quite different time periods (i.e., from late March to mid-April), most of them arrived at the lower reaches during a limited time frame (from late April to early May). Migration speeds of fish released at similar times (i.e., before midApril and mid-April) did not differ, although discharge and river temperatures varied among years. Therefore, our results suggest that chum salmon fry in the Nishibetsu River adjust their migration speed, presumably to arrive at the ocean during a time period that would, on average, favor good survival. Migration speed was primarily affected by release date, and not by water temperature or discharge. Fish size and seawater tolerance also had little effect on the migration behavior and migration speed of chum salmon in the present study. Release date is likely a proxy for photoperiod, which has been shown to affect smoltification and downstream migration [12, 28]. Chum salmon fry released in the upper reaches of the Nishibetsu River, where water
Recapture rate (%)a
Discussion
Number of marked fish recaptured
among groups (Kruskal–Wallis, v2 = 172.5, df = 5, P \ 0.01; Table 3). Fish released later generally arrived in the lower river more quickly than fish released earlier. Correlations between migration speed and FL and DISCHARGE were negative, while the correlations with RELEASE and TEMP were positive (Fig. 6). However, the only significant correlation was with RELEASE. Migration speed appeared to increase when the release date was delayed.
Table 3 Migration periods, speeds, and discharge of recaptured marked chum salmon fry during 2008 to 2010
Fig. 4 Changes in the Nishibetsu River discharge (line) and precipitation (bar) at Betsukai in 2008, 2009, and 2010
Date of release
2010
Year
2009
Precipitation (mm)
Water discharge (m3/s)
2008
Multiple comparisons
573 Discharge during 60 days after release of marked fish (m3/s)
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Fig. 5 Number of ALC-marked chum salmon fry recaptured at the sampling site in the Nishibetsu River in 2008, 2009, and 2010. Open arrows indicate release time of those fish
2008
2009
2010
Migration speed (km/d)
25-Mar
r2 = 0.633 P = 0.058
4-Apr
14-Apr
r2 = 0.656 P = 0.051
FL
TEMP
r2 = 0.909 P = 0.003 RELEASE
r2 = 0.002 P = 0.935 DISCHARGE
Fig. 6 Relationships between migration speed and independent variables in 2008, 2009, and 2010 for chum salmon in the Nishibetsu River. Dashed lines indicate 95 % confidence interval. FL, mean fork length (mm); RELEASE, days from 26 March to release date (days); TEMP, water temperatures recorded and estimated at the sampling site at the release dates (°C); DISCHARGE, means of discharges during 60 days after releases (m3/s)
temperature and discharge were virtually constant due to the spring water, did not experience the water temperature and discharge fluctuations that occurred in the lower reaches; therefore, the fish were primarily affected by photoperiod as the season progressed. In another study, Atlantic salmon Salmo salar smolts started their downstream migration
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24-Apr
4-May
14-May
24-May
3-Jun
13-Jun
mainly after 10 May in 2 years, although water temperature increased earlier in 1 year than in the other year [29]. This finding is consistent with the idea that the onset of migration is strongly influenced by photoperiod. Many Japanese hatcheries use spring well water that is relatively constant in temperature. Therefore, earlier hatched chum salmon fry may grow larger than those hatched later. Hatchery-origin chum salmon fry tended to emerge earlier than wild fry because of higher winter water temperature in hatcheries than in nearby streams [30]. Chum salmon fry released in February were recaptured in the lower Nishibetsu River in April [9], indicating that chum salmon fry can significantly extend their migration period. Information on the downstream migration of wild chum salmon is not well understood in Japan because few chum salmon spawn naturally and their study has been neglected [1]. Information on optimal marine arrival periods has been obtained primarily from the study of hatchery fish. Wild pink salmon fry arrive in the lower Nishibetsu River from late April to early May [4, 9] and coexist with juvenile chum salmon in the nearshore area (M. Torao, unpubl. data, 2010). Ocean arrival may be timed to occur when salmon mortalities are lowest in the nearshore area [31], or when nearshore marine production is at a spring maximum [32]. Therefore, young pink and chum salmon that arrive at the lower reaches of the Nishibetsu River during late April to early May may have a survival advantage over fish arriving earlier or later. Travel times in the Chitose River varied with release date similarly as in the Nishibetsu River [10] (Table 4). In the Chitose River, chum salmon fry
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Table 4 Previously reported migration periods for Japanese chum salmon released into rivers at different dates River
Distance (km)
Fork length at release (mm)
Weight at release (g)
Date of release
Arrivals at the lower reaches
Estimated migration period
Nishibetsu
97
34
0.3
Mid-February to late February
15 April–30 April
About 2 months \2 km/day
34
0.3
Late March to early April
15 April–30 April
20–30 days
3.2–4.9 km/ day
48–53a
0.90–1.27a
Mid-April to late April
Early May
7–10 days
7.2–10.3 km/ day
48–52a
0.93–1.28a
Mid-May
Late May
4–5 days
14.4–18.0 km/ day
45.5
0.80
Early March
1–2 monthsb
42.1
0.71
Mid-April
10 daysb
51
Early March
About 1 monthb
53
Mid-April
10 daysb
Late May
1 dayc
Tohoro
Chitose Kitakami Utabetsu
72
ca. 80 ca. 60 3.7
53.9 46.2
a
1.29 0.92
Migration speeds
Sources
[9]
[33]
[10] [34] [8]
c
Early June
1 day
Data provided by Nemuro Salmon Enhancement Programs Association
b
Duration from release to date when fish were not observed around the release point
c
About half of fish released migrated seaward within one day
adjusted their migratory period to arrive at the nearshore area after mid-April [10]. In the Nishibetsu, Tohoro, and Ishikari Rivers, all with distances from release points to river mouths exceeding 50 km, chum salmon released earlier had longer freshwater residence periods than those released later (Table 4). In contrast, chum salmon fry that were released into small coastal streams less than 50 km in length, such as the Utabetsu and Otsuchi Rivers, migrated seaward rapidly [7, 8]. The timing and size at release of coho salmon O. kisutch smolts can affect returns at maturity, indicating that their survival is influenced by ocean entry time [35, 36]. However, optimum release timing and size for coho salmon differed among nearby hatchery streams [36]. Masu salmon O. masou smolt emigration timing varied by region [37, 38], and was thought to be genetically controlled [38–40]. It is possible that the outmigration timing for chum, coho, and masu salmon smolts has adapted to variations in river and nearshore marine temperatures. If this is so, then the enhanced chum salmon fry that are released early in the spring may delay their movement downstream to control the timing of the seaward migration in a long river, such as the Nishibetsu River. Chum salmon that originated from the Nishibetsu River (1.35 g, 1 million released on 13 April 1987) remained near the release point for more than 1 month, although chum salmon originating from the Chitose River (0.81 g, 1.5 million released on 17 April 1986) migrated seaward quickly (Kobayashi M, unpubl. data, 1987). It is not known whether differences in
migration between Chitose River and Nishibetsu River fish were the result of genetic or environmental differences. Photoperiod acts on circadian rhythm, which is based on a molecular mechanism, to represent seasonal responses in the biochemistry, physiology, and behavior of animals [41]. Genetic correlations between the circadian rhythm gene and behavioral characters were demonstrated for some animal taxa including Pacific salmon [42–45]. One circadian rhythm gene might be related to the timing of migration and spawning, corresponding to a latitudinal cline in photoperiod for chum salmon in North America [24, 46]. The results of this study suggest that juvenile chum salmon enter the ocean during a relatively discrete period regardless of release date; therefore, fish released earlier stay in the river longer than fish released later. In the Nishibetsu River, chum salmon fry fed little during March and April [9], and nutritional conditions in the lower river declined during the year (Mizuno S, unpubl. data, 2009); so a longer migration time might result in a higher mortality rate. Lower recapture rates of earlier released fish in the lower reaches (Table 3) and the nearshore areas off the Nishibetsu River mouth [26] than for later released fish in the same year would be expected if these fish delayed their movement downstream. SSTs exceeded 6 °C in the nearshore area about 1.5 km off the Nishibetsu River mouth (Betsukai ST, Fig. 1) generally in early May, although times when SSTs exceeded 8 °C varied among years [26]. SSTs at the littoral zone were 1–2 °C higher than those at
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the Betsukai ST (Kasugai K, unpubl. data, 2010), therefore SSTs at the littoral zone may be approximately 8 °C, which was preferable to juvenile chum salmon [2, 26] in the time when SSTs at the Betsukai ST exceeded 6 °C. Stability in the time when SSTs exceeded 6 °C in the nearshore area in early May may result in adaptation of arrival of chum salmon at the ocean in that time, so that survival might be higher. We conclude that, to reduce mortality, chum salmon fry released into the Nishibetsu River after late April minimized their stay in fresh water and entered the ocean in what appeared to be an optimal period. This conclusion is supported by the results of other surveys in the nearshore area [26]. In order to delay hatchery release dates, it may be appropriate to delay the development of early fertilized eggs by rearing them in cool waters from nearby rivers or artificially cooled water rather than relatively warm spring water or well water [47]. There may be some variability in the optimal ocean entry timing for chum salmon, as has been found for coho salmon [36], so additional research is encouraged on the downstream migration of wild and hatchery chum salmon in different areas of Japan. Acknowledgments We express our sincere thanks to Yoshimi Toda, Yuichi Akiyama, Kotaro Ono, Jun-ichi Nakano, Tetsuo Nakazawa, Shigeo Iguchi, Yoshifumi Shimo, Hitoshi Kawakami, Masayuki Kimura, Masaki Ohno, Takeo Sasaki, and Hiroshi Kakizaki of Nemuro Salmon Enhancement Programs Association, who helped to install the screw trap, and reared chum salmon fry. Yutaka Ogasawara, Toshinori Ohashi, Shinji Kawahara, Katsuhiko Ohashi, Yoshitaka Sasaki, Hiroyuki Sakamoto, and Mahito Miyamoto also assisted with field sampling. Chisato Suzuki, Seiko Ito, Mikiko Minami, and Chikako Shimokawara measured and extracted otoliths. The Nakashibetsu District Public Works Management Office, Kushiro Department of Public Works Management provided discharge data for the Nishibetsu River. We thank Dr. Yasuyuki Miyakoshi for his constructive comments on drafts of this paper, Lana Fitzpatrick for editorial input, and the editor and two anonymous referees for their useful comments.
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