Effects of juvenile size at release and early marine growth on adult return rates for Hokkaido chum salmon (Oncorhynchus keta) in relation to sea surface temperature

Using path analyses, we investigated relationships between size at release from hatcheries, the early marine growth of juveniles, and adult return rates for chum salmon from five river stocks of Hokkaido, Japan, in relation to sea surface temperature during ocean residence. Marine growth was estimated using scales collected from 11 760 adults of age 0.3 (1980–2004). The growth and survival of each stock appeared to have a different suite of regulatory processes. Interannual variability in return rates was mainly regulated by size at release in two stocks from the Sea of Okhotsk. A similar relationship was found in one stock from the Sea of Japan, but growth during coastal residency also affected their return rates. In two stocks from the Pacific coast of Hokkaido, variability in return rates was not related to size at release or to the coastal growth of juveniles, but with offshore growth in the Sea of Okhotsk, the nursery area for juveniles after leaving Japanese coastal waters. Whereas coastal growth tended to be negatively correlated with size at release in some stocks, offshore growth was positively associated with the August–November sea surface temperature in all stocks. This study confirmed that mortality of juvenile salmon occurred in two phases, during the coastal residency and the late period of the growing season, but the relative importance of both phases varied by stock and region, which probably regulated year‐class strength of Hokkaido chum salmon.     

Relationship between zooplankton abundance and the early marine life history of juvenile chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> in eastern Hokkaido,Japan

Interannual variations in abundance, timing of outmigration from rivers, growth rate and condition of juvenile chum salmon (Oncorhynchus keta) were studied in the Nemuro Strait (eastern Hokkaido, Japan) during 1999–2002 to establish a possible relationship to zooplankton abundance. The otolith microstructure of juveniles was examined each year in late June to determine their time and size at sea entry (i.e., outmigration), and to estimate the early marine growth rates. Salmon outmigration peaked in mid- or late May, which coincided, in three of the four study years, with the peak release of juveniles into rivers within the study area. Abundance, growth rate and condition of fish were higher in 2001, when—compared to other years—smaller fish experienced higher growth rates, coinciding with greater zooplankton abundance for that year. Our results suggest that high zooplankton abundance positively influenced juvenile chum salmon growth and the condition of the fish during their early marine life despite their small size at sea entry.     

Management actions in relation to the controversy about salmon lice infections in fish farms as a hazard to wild salmonid populations

Risk analysis provides a logical strategy for the identification of hazards in an area of concern leading to their management. This approach has rarely been used for research studies on the possible links between salmon lice in fish farms and variations in wild salmonid populations. Although correlation has been found between some data sets, a series of international meetings of experts has not been able to establish cause–effect relationships between lice on farms and the current low level of salmonid stocks in Europe and eastern Canada. Research in this area is currently in progress in western Canada. Views on the meaning of the correlations found have become highly polarized in both the scientific and non‐scientific communities, with the same data often being interpreted in different ways. Farm lice levels are being managed by industry codes and by official strategies in Europe, but these programmes have not so far resulted in associated benefit to the size of wild salmonid stocks. The large array of factors that cause variations in both the size of fish stocks and the level of fish diseases is frequently overlooked. Using a risk analysis approach, the key contributory factors may be identified and appropriate remedial management actions designed to meet achievable objectives. Regular assessment of the effectiveness of the measures introduced would then be possible.     

Feed intake, growth rate and body composition of juvenile Baltic salmon exposed to different constant temperatures

Feed intake, specific growth rate and changes in body composition of age 1+ and 2+ Baltic salmon, Salmo salar L, were studied for fish held under constant temperature conditions. The 1+ fish (60 g) were reared for 6 weeks at 11, 15, 17, 19 or 23 °C and 2+ fish (250 g) were held at 15 °C. Feed intake of 1+ salmon increased from 176 kJ kg-1 day-1 at 11 °C to 275 kJ kg-1 day-1 at 17 °C and decreased to 229 kJ kg-1 day-1 at 23 °C. Specific growth rate increased from 1.18% day-1 at 11 °C to 1.59% day-1 at 15 °C and decreased to 0.56% day-1 at 23 °C. Optimum temperatures for feed intake and growth were estimated at 17.8 °C and 15.6 °C, respectively, and estimated upper thermal limits for feeding and growth were 29.0 °C and 24.1 °C, respectively. Models for feed intake and growth rate in relation to temperature and fish size are presented. The utilization efficiency of ingested energy decreased from 57% at 11 °C to 22% at 23 °C. For all groups of 1+ fish, most (approximately 86%) of the weight gain consisted of water. Lipid deposition accounted for about 52% of the body energy gain irrespective of rearing temperature.     

Dietary protein/energy ratios for Atlantic salmon in relation to fish size: growth, feed utilization and slaughter quality

Four extruded diets differing in protein/fat concentrations, 378/389 g kg^?1, 425/346 g kg^?1, 480/308 g kg^?1 and 524/256 g kg^?1 were tested in a digestibility trial and a growth study. Apparent digestibility of protein and fat were not significantly different among the diets when tested in 1-kg Atlantic salmon, Salmo salar L., in sea water. The diets represented a range of digestible protein to digestible energy ratios (DP/DE ratios) of 14.1, 16.4, 18.8 and 21.9 g MJ^?1. The 138-day growth study was performed with triplicate groups of Atlantic salmon of 1.0 and 2.5 kg initial weight. Irrespective of size; growth, feed conversion ratio (FCR), nitrogen and energy retention were poorer in fish fed the diet with DP/DE ratio of 14.1 g MJ^?1 compared with the fish fed the other diets. A DP/DE ratio of 16.4 g MJ^?1 was sufficient to produce maximum growth for the large fish, while the DP/DE ratio of 18.8 g MJ^?1 produced the highest growth in the small fish. In the large fish, the lowest FCR was obtained on a DP/DE ratio of 16.4 g MJ^?1, while there was no clear difference in FCR within the small fish when diets of DP/DE ratios of 16.4–21.9 g MJ^?1 were fed. The carcass-to-body ratio in the small fish decreased with decreasing DP/DE ratios. The fish fed the diet of 21.9 g MJ^?1 had significantly lower fat and dry matter and higher protein content than fish of similar size fed the other diets. Increased dietary lipid content seemed to improve astaxanthin deposition in the small fish, while the large fish showed no significant differences in astaxanthin deposition due to dietary treatment. This study indicates that a DP/DE ratio of 14.1 g MJ^?1 in high-energy diets for Atlantic salmon in sea water is below the optimal DP/DE ratio for growth and feed utilization, and that the optimal DP/DE ratio decreases with increasing fish weight. DP/DE ratios around 19 g MJ^?1 for fish weighing 1 to 2.5 kg, and 16–17 g MJ^?1 for fish weighing 2.5 to 5 kg, are suggested to be optimal.     

Feeding and growth of exercised and unexercised juvenile Atlantic salmon in freshwater,and performance after transfer to seawater

Juvenile Atlantic salmon (Salmo salar), selected from the upper modal group, were first held for 122 days (from December to May) in freshwater, and then for 49 days in seawater (34% salinity). In freshwater, the fish were exposed to either standing water or water currents corresponding to a swimming speed of 1 Bl s^–1. Natural light conditions prevailed throughout the whole experiment.In the freshwater phase, food intake, growth rate and condition factor of the fish increased with increasing daylength. Condition factors, however, decreased slightly during the last month of the freshwater period, suggesting that the fish underwent parr-smolt transformation. The growth rate and food intake of the fish subjected to standing water were approximately 15% lower than those of fish exposed to water currents.Transient suppression of appetite and growth, and an increase in plasma concentration of Cl^–, were seen after exposure of the fish to seawater. Within 28 days, all these parameters had been restored to pre-transfer levels. Previous exercise did not appear to have influenced the hypoosmoregulatory capacity of the fish. Growth rate and food intake observed during the seawater phase were similar, irrespective of the treatment experienced by the fish during freshwater rearing.     

Sublethal effects and safe levels of carbon dioxide in seawater for Atlantic salmon postsmolts (Salmo salar L.): ion regulation and growth

Atlantic salmon (Salmo salar L.) postsmolts (0.17–0.26 kg) were exposed to four different levels of carbon dioxide partial pressure for 43 days in an open flow system: 0.6 mm Hg (control), 4.9 mm Hg (low), 12 mm Hg (medium), and 20 mm Hg (high). The water temperature was 15–16°C and the salinity 34‰. In the low carbon dioxide group (P_CO2=4.9 mm Hg; 10.6 mg/l), no significant differences were found in blood parameters (haematocrit, plasma chloride and plasma sodium) or in growth parameters (weight, length and condition factor) when compared to the control group. After 43 days, the mean plasma chloride concentration for the medium group (P_CO2=12 mm Hg; 26 mg/l) was significantly reduced, while weight and condition factor were slightly, although not significantly, lowered. For the high carbon dioxide group (P_CO2=20 mm Hg; 44 mg/l) plasma sodium and plasma pH were significantly increased and plasma chloride, oxygen consumption, weight, length and condition factor were significantly reduced at the end of exposure. There was no mortality in the control group or in the low carbon dioxide group. The mortalities in the medium and high carbon dioxide groups were 1.1 and 4.3%, respectively. Nephrocalsinosis was not observed in any of the groups. The results of the present investigation indicate that the CO₂ concentration of the low group may represent a safe level for Atlantic salmon postsmolts when the temperature is 15–16°C and the oxygen level is 6–7 mg/l. Further studies are required.     

Changes in plasma prolactin and growth hormone concentrations during freshwater adaptation of juvenile chum salmon (Oncorhynchus keta) reared in seawater for a prolonged period

Freshwater adaptability of chum salmon was examined in juvenile fish reared in seawater for 4 months. The fish, weighing about 40g, were transferred directly to fresh water in October, when their cohorts are migrating in the North Pacific Ocean. Plasma sodium concentration decreased from 167 mM in seawater to about 130 mM during the first 24h, and increased gradually during 2–7 days after the transfer. No immunoreactive prolactin (PRL) was detected in the plasma of the seawater-adapted fish nor during the first 24h in fresh water. Significant levels of PRL were detected after 2–3 days. The maximal level (2.6 ng/ml) was observed after 5 days and became undetectable again after 7 days; no significant correlation was seen between the changes in plasma sodium and PRL levels during the transfer. Plasma growth hormone levels were relatively constant, except for a significant decrease 12h after the transfer. Although plasma thyroxine levels were highly variable during the experiment, a significant decrease and an increase were observed 12h and 5 days after the transfer, respectively. The present study indicates that juvenile chum salmon retain hyperosmoregulatory ability even after prolonged rearing in seawater. Examination of turnover rates, rather than changes in plasma levels, seems to be essential to clarify the osmoregulatory roles of the hormones.     

Declines in chinook salmon catches in the Strait of Georgia in relation to shifts in the marine environment

Chinook, Oncurhynchus tshawytscha, catches in the Strait of Georgia increased in the 1970s and reached maximum levels from 1976 to 1978. Catches then declined until they stabilized through regulation at levels approximately one-quarter of the 1976 to 1978 levels. The timing of the decline in catch was synchronous with an increase in the mean temperature of the Strait of Georgia, a decline in annual Fraser River flows, and an abrupt decrease in the marine survival of hatchery-reared chinook released into the Strait of Georgia. Surprisingly, the number of young chinook salmon (smolts) more than doubled over the period of declining catches compared with the number produced during the period of high catches. The increase in smolt abundance was a consequence of the production from hatcheries that was approximately equal to wild production. We conclude that there was a change in the carrying capacity for chinook salmon in the Strait of Georgia in the late 1970s that contributed to the declines in abundance and that rebuilding stocks to the high abundance of the late 1970s is unlikely until the carrying capacity for chinook salmon changes either naturally or through manipulation. Although we did not separate density-dependent and density-independent effects on marine survival, the current total number of chinook smolts produced appears larger than required for the current marine carrying capacity.     

Influence of feeding regime on intraspecific competition, fin damage and growth in 1+ Atlantic salmon parr (Salmo salar L.) held in freshwater production cages

The feeding behaviour, growth and feed conversion ratio (FCR) of cage‐held Atlantic salmon parr (Salmo salar L.) were studied when in 576 m³ (12 m × 12 m × 4 m) commercial freshwater cages under ambient water temperature (8.84±3.53°C) and photoperiod (11.02±2.05 h) for 205 days. The effect of feeding regime on fin damage was also investigated. Six groups (n=31 234±2051 fish group⁻¹, initial stocking density 1.25±0.14 kg m⁻³) were fed to satiation using either (a) an imposed regime involving scheduled, fixed ration feeding every 10 min from dawn till dusk or (b) on demand from dawn till dusk using commercial interactive feedback systems. During feeding, there were no significant differences in aggression although swimming speeds and turning angles were significantly higher in fish under the imposed regime. On‐demand feeding significantly reduced the incidence of dorsal fin damage. There was no clear relationship between fish size, feed regime and the incidence of fin damage until 1 week before the fish were transferred to marine cages, when the smallest fish under each feeding regime had the highest incidence of fin damage. Interestingly, growth did not differ between regimes, but fish under the imposed regime were significantly overfed and achieved higher FCRs.     

Feed intake of Atlantic salmon parr Salmo salar L. in relation to temperature and feed composition

The design and testing of a feed intake monitoring system based on feed waste collection is described. The system was used in a study of feed intake and growth of Atlantic salmon Salmo salar L. parr in relation to temperature (2 °C or 8 °C) and feed composition (21% fat, 50% protein and 22.5 MJ kg^?1 or 34% fat, 40% protein and 24.8 MJ kg^?1). Fish had lower feed intake and slower growth at the lower temperature, but temperature‐corrected growth (TGC) was better in fish held at the lower temperature. There was an increase in TGC over time at low temperature, probably as a result of long‐term thermal acclimation. Feed conversion for an increase in body size from 19 to 38 g was better for the fish held at low temperature. Feed consumption of low‐fat feed was higher than that of high‐fat feed, and growth of the fish fed the low‐fat feed was better at the higher temperature. These data are consistent with the ideas that the fish compensated for differences in feed energy densities to maintain energy and nutrient intakes, and that lipostatic factors may be operating to regulate feed intake and growth.     

Interannual variability in the feeding and condition of subyearling Chinook salmon off Oregon and Washington in relation to fluctuating ocean conditions

Chinook salmon (Oncorhynchus tshawytscha) is one of several economically‐important species of salmon found in the Northeast Pacific Ocean. The first months at sea are believed to be the most critical for salmon survival, with the highest rate of mortality occurring during this period. In the present study, we examined interannual diet composition and body condition trends for late‐summer subyearling Chinook salmon caught off Oregon and Washington from 1998 to 2012. Interannual variability was observed in juvenile salmon diet composition by weight of prey consumed. Juvenile subyearling Chinook salmon were mainly piscivorous, with northern anchovy (Engraulis mordax) being especially important, making up half the diet by weight in some years. Annual diets clustered into two groups, primarily defined by their proportion of invertebrate prey (14% versus 39% on average). Diet composition was found to influence adult returns, with salmon from high‐invertebrate years returning in significantly larger numbers 2–3 yrs later. However, years that had high adult returns had overall lower stomach fullness and poorer body condition as juveniles, a counterintuitive result potentially driven by the enhanced survival of less fit individuals in better ocean conditions (top‐down effect). Ocean conditions in years with a higher percentage of invertebrates in salmon diets were significantly cooler from May to August, and bottom‐up processes may have led to a fall plankton community with a larger proportion of invertebrates. Our results suggest that the plankton community assemblage during this first fall may be critical in predicting adult returns of Chinook salmon in the Pacific Northwest.     

Survival of reared Atlantic salmon and sea trout in relation to marine conditions of smolt year in the Baltic Sea

The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970–2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.     

Introduction and comparison of two methods of assessment of coronary lesions in Atlantic salmon, Salmo salar L

This study investigated and compared quantitative and semi-quantitative coronary lesion evaluation in Atlantic salmon, Salmo salar. A total of 121 immature farmed and 47 sexually mature wild Atlantic salmon were included. Coronary arteries from all fish were morphometrically evaluated using a semi-quantitative method. A subsample of 76 salmon was additionally evaluated using a quantitative method. Another subsample of 71 salmon was used for reliability testing of the semi-quantitative method. Ten cross-sections of coronary arteries located on the ventral surface of the bulbus arteriosus were semi-quantitatively scored with regard to myointimal coronary lesions. Quantitative measurements comprised maximal intimal thickness and area lesion (%) while semi-quantitative measurements comprised maximal intimal thickness and lesion degree (%). Scores for individual fish were expressed as the average of lesion score, denoted as mean range lesion. One person performed all evaluations blindly. Two pathologists tested repeatability of the semi-quantitative lesion score measurements blindly. Both semi-quantitative and quantitative variables increased significantly with increasing lesion score. Semi-quantitative estimation gave significantly higher maximal intimal thickness values than quantitative measurement. Semi-quantitative measurements were found to explain 63.2% of variation in quantitatively measured maximal intimal thickness. Due to lack of agreement, the two methods are not directly comparable and one method cannot replace the other. Repeatability within and between observers with regard to semi-quantitative classification was very good with Kappa values larger than 61.5%. The semi-quantitative method was a valid and reliable method for coronary lesion evaluation in Atlantic salmon.     

Return migration of adult Atlantic salmon, Salmo salar, in relation to water diverted through a power station

Abstract  The migration of Atlantic salmon, Salmo salar L., returning to the River Suldalslågen, Norway, was studied in relation to redirection of freshwater flow through a power station. The outlet of the power station is situated in the Hylsfjord, a fjord adjacent to the river mouth. Seventy-two salmon were tagged with acoustic transmitters, released in the outer part of the fjord system and automatically recorded when entering the Hylsfjord or the river. Data were collected during one period when the power station was running and two periods when the power station was closed. The release of water from the power station did not greatly attract the salmon during their return migration. Proportions of salmon entering the river or time from release to entering the river did not differ among salmon tagged in the different periods. The salmon were recorded in the Hylsfjord both when the power station was running and closed and there were no differences in number of times, number of days or hours recorded in the Hylsfjord among salmon tagged in the three periods. The only significant difference found among periods was duration of continuous stays in the inner part of the Hylsfjord. This may indicate a slight attraction to the freshwater release, but the difference seems small (1.8 vs. 0.7 h) compared with the time the fish stayed in the fjord system before entering the river (16–85 days).     

Use of juvenile salmon growth and temperature change indices to predict groundfish post age‐0 yr class strengths in the Gulf of Alaska and eastern Bering Sea

Juvenile marine growth (SW1) of salmon and a new temperature change (TC) index were evaluated as ecosystem indicators and predictors for the post age‐0 year class strength (YCS) of groundfish in the Gulf of Alaska (GOA) and eastern Bering Sea (EBS). Our hypothesis was that SW1, as measured on the scales of adult Pacific salmon (Oncorhynchus spp.), is a proxy for ocean productivity on the continental shelf, a rearing area for young salmon and groundfish. Less negative TC index values are the result of a cool late summer followed by a warm spring, conditions favorable for groundfish YCS. In the GOA, SW1 was a positive predictor of age‐1 pollock (Theragra chalcogramma), but not age‐2 sablefish (Anoplopoma fimbria) YCS, indicating that the growth of the Karluk River sockeye salmon that enter Shelikof Strait is a proxy for ocean conditions experienced by age‐0 pollock. Contrary to our hypotheses, the TC index was a negative predictor of GOA pollock YCS; and the SW1 a negative predictor of EBS pollock and cod YCS since the 1980s. Recent fisheries oceanography survey results provide insight into possible mechanisms to support the inverse SW1 and YCS relationship. For the EBS, the TC index was a significant positive predictor for pollock and cod YCS, supporting the hypothesis that a cool late summer followed by a warm spring maximizes the over‐wintering survival of pollock and cod (Gadus macrocephalus), especially since the 1980s. The TC and SW1 index showed value for the assessment of pollock and cod, but not sablefish.     

Effects of dietary protein level on muscle cellularity and flesh quality in Atlantic salmon with particular reference to gaping

Growth performance, muscle cellularity and flesh quality were investigated in Atlantic salmon (Salmo salar L.) fed either of two diet ranges [high protein (HP), or low protein (LP)], which differed in digestible protein/digestible energy ratios but were of equivalent digestible energy content (21.4 MJ kg⁻¹ wet weight). Smolts from an early maturing (Lochy) and a late maturing (Mowi) strain were PIT-tagged and reared together in duplicate 5×5×5-m sea cages for each diet. The Lochy and Mowi fish were harvested in May and August, respectively, after 417 and 515 days in seawater. The average body weight of fish in each cage at harvest was in the range 3.8–5.4 kg, with no significant difference between diets. The total cross-sectional area of white muscle and the number and diameter of muscle fibres was determined at the level of the first dorsal fin ray. The distribution of muscle fibre diameters was investigated using nonparametric smoothing and bootstrapping techniques. Diet had no effect on fibre size distribution or fibre number in the Mowi strain, and small but significant effects for the Lochy strain. At harvest, in Lochy salmon of average fork length 69 cm there were around 15% more fibres in fish fed the HP than LP ratio diets. However, the 50th percentile of fibre diameter was 20% greater in fish fed the LP than HP diets, such that the total muscle cross-sectional area was similar. The lipid content (14.1–15.3% wet mass), astaxanthin pigment concentration (7.0–8.5 mg kg⁻¹ wet mass) and colour (RocheSalmoFan™ and Minolta Chromatometer readings) of the flesh were similar for both strains and diets. There was no significant difference in the average muscle fibre density between strains and diet, which varied between 60 and 140 fibres mm⁻² muscle cross-sectional area. Gaping during processing of the fillet was in part related to muscle cellularity. Little or no gaping was observed in any fish with a fibre density in excess of 95 fibres mm⁻² muscle. It was concluded that individual variation in fibre density is important in the development of gaping, but that muscle cellularity and flesh quality are relatively insensitive to the protein to energy ratio in the diet over the range studied.     

An assessment of the potential for the application of two simple models to Atlantic salmon, Salmo salar, stock management in chalk rivers

Many salmon populations in the UK are under threat, which is assumed to result from a combination of anthropogenic and natural factors. Positive management is required to bring them back to their former productivity levels. An attempt has been made to use two simple models to assess the suitability of several management approaches for use on chalk rivers. One model is based on the survival rates at each stage of the salmon life cycle and the other estimates the predation of young salmon by other fish. This work highlights the paucity of data available to calibrate even the simplest salmon management models. For those variables where data are available, the variability is very high. Despite the poor quality of input data, the outputs are close to observed numbers in the River Frome, a chalk river in Dorset, UK. The models were used to assess the effect of managing the fishery using reduced rod catches, pike culling, gravel cleaning and trout stocking on adult salmon and smolt numbers.