Growth Rate Of Salmon Research Articles

Evaluating the suitability of nitrate-nitrogen levels for post-smolt Atlantic salmon Salmo salar production in RAS with assistance from heart rate bio-loggers

Previous onsite research determined that post-smolt Atlantic salmon Salmo salar growth, health, and welfare were unaffected by exposure to 99 mg/L nitrate-nitrogen (NO3-N) in freshwater recirculating aquaculture systems (RAS). A subsequent study was conducted to determine if higher concentrations negatively impact Atlantic salmon within a similar size range. Salmon production metrics were compared in triplicate RAS with NO3-N levels increasing from 100 to 250 mg/L (“high”) vs. 50–100 mg/L NO3-N (“low”). An average feed loading rate of 2.53 kg feed/m3 makeup water/day was maintained to facilitate natural NO3-N accumulation up to 75–100 mg/L, and sodium nitrate was continuously dosed to achieve higher concentrations. All-female diploid and all-female triploid Atlantic salmon were comingled in six replicated RAS, resulting in 227 fish/tank with an initial mean weight of 0.35 kg. Six diploid salmon with surgically implanted heart rate bio-loggers were included in each RAS. Continuously logged heart rates were matched with frequently measured NO3-N levels to determine the onset of a physiological response. At the end of the 6-month study, salmon exposed to the high and low NO3-N conditions weighed 1.84 ± 0.05 and 1.91 ± 0.02 kg, respectively (p > 0.05). Diploid and triploid salmon growth rates, feed conversion ratios, maturation prevalence, survival, fin condition, and cataract scores were unaffected (p > 0.05). However, salmon heart rates were generally higher in the high NO3-N treatment after concentrations exceeded 150 mg/L NO3-N. Higher plasma chloride, hematocrit, and hemoglobin levels (p < 0.05) consistent with an adaptive response to a stressor were also measured in salmon from the high NO3-N RAS during this period. These findings suggest that NO3-N concentrations < 150 mg/L do not affect post-smolt Atlantic salmon under similar operating conditions, including freshwater RAS without denitrification technologies and water hardness >300 mg/L as CaCO3. Additional research is required to determine if higher NO3-N concentrations are suitable for Atlantic salmon production over longer exposure periods and with naturally produced NO3-N levels imparted by reduced water exchange. Feed- and nitrogen-loading rates conducive to maintaining the tested NO3-N levels are reported.

Estimating the benefits of floodplain restoration to juvenile Chinook salmon in the upper San Francisco Estuary, United States, under future climate scenarios

Many river systems within the Central Valley of California have been disconnected from their floodplains, hypothesized to be partially responsible for declining Chinook salmon populations (Oncorhynchus tshawytscha). The primary floodplain of the system, Yolo By‐Pass (known regionally as “Yolo Bypass”), offered an opportunity to examine whether improved connectivity between the floodplain and river could limit negative climate change effects on salmon populations. Specifically, the top of the floodplain (Fremont Weir) is being modified to provide Sacramento River Chinook salmon better access to floodplain rearing habitat. We estimated restoration effects on the Yolo By‐Pass flood regime now and under future climate scenarios using flow rating curves. Additionally, we used temperature and flow‐specific effects on Chinook salmon population dynamics within the Yolo By‐Pass and Sacramento River complex to describe how the restoration project and climate change may interact to affect juvenile Chinook salmon biomass production. Our results indicate that the Fremont Weir restoration project will extend the frequency, timing, and duration of Yolo By‐Pass flooding. Our production model indicates that the modification will result in greater salmon entrainment rates into the Yolo By‐Pass, where salmon growth rates, survival rates, and biomass production were higher when compared to the Sacramento River main stem. The project appears to benefit all regional runs of Chinook salmon, which should help support life history diversity. Our results suggest that the weir modification should benefit native fish from the Central Valley that use floodplain habitat and that these benefits may be resilient to challenges created by a changing climate.

Long-Term Trends in Freshwater and Marine Growth Patterns in Three Sub-Arctic Atlantic Salmon Populations

The rapid warming of the Northern hemisphere has especially challenged the evolvability of anadromous fish species, such as Atlantic salmon (Salmo salar), which must cope with drastically different environments depending on their life-history stage. We studied the long-term trends in, and the effects of environmental factors and life-history traits on, Atlantic salmon growth rates in both freshwater and in the ocean using c. 35,000 scale samples collected across 48 years from spawners returning to three tributaries of the subarctic River Teno in the northernmost parts of Finland and Norway (70° N). The freshwater growth has decreased in all three populations and spending more than three juvenile years in freshwater before the sea migration comes at the expense of growth. On the other hand, returning mature salmon (one-sea-winter, 1SW) showed increased growth at the sea with increasing marine temperatures, which results in larger sizes at return in 1SW spawners. We did not observe such trends in growth rates in larger, two-sea-winter salmon. Here, we report the contrasting responses in Atlantic salmon growth rates to a warming climate depending on the life-history stage.

The impact of parasitic sea lice on harvest quantities and sizes of farmed salmon

Sec lice infections are recognized as a primary challenge for both salmon farming and adjacent stocks of wild salmon and sea trout, triggering strict regulations at farm sites and in larger coastal production areas. In 2017 the Norwegian government implemented the Traffic Light System (TLS), where green, yellow, and red lights imply that salmon farmers can raise, maintain, or must reduce production quantities depending on estimated sea lice-induced mortality of wild salmonids. Past research has explored the impacts of sea lice on the growth rate of salmon, indicating a possible link between sea lice numbers and harvest practices. This study evaluates the impact of sea lice on salmon farmers' harvest behavior focusing on production quantities and fish sizes while controlling for market prices of salmon and fish meal. We also investigate whether and to what extent the TLS implementation affects harvest behavior. Our empirical results indicate that farmers tend to harvest marginally faster in response to increasing levels of sea lice and slower during delousing operations. The implementation of the TLS strengthened the negative impacts of delousing operations on harvest quantities. Fish sizes at harvest are negatively associated with sea lice levels and delousing operations, regardless of the implementation of the TLS. Control variables, such as seawater temperature, salmon prices, and fish meal prices, also influence harvest quantities and fish sizes.

Riverscape heterogeneity in estimated Chinook Salmon emergence phenology and implications for size and growth

AbstractMany salmonid‐bearing rivers exhibit thermal and hydrologic heterogeneity at multiple spatial and temporal scales, but how this translates into spatiotemporal patterns of fry emergence is poorly understood. Understanding this variability is important because emergence timing determines the biophysical conditions fish first experience (e.g., temperature, flow, and food supply), thereby influencing growth opportunities and survival during this critical life stage. We predicted spring Chinook Salmon (Oncorhynchus tshawytscha) emergence phenology across four northeastern Oregon subbasins over 5–9 years using empirical spawning and temperature data. We then related interannual emergence timing estimates to juvenile salmon size and growth rates at consistent sampling locations. There were clear longitudinal patterns of predicted emergence timing in each subbasin: The shape of these patterns was consistent among years, but not among subbasins. In two subbasins, emergence occurred progressively later with distance upstream, whereas in the other two subbasins emergence was earliest at upstream sites. Within each year, median emergence dates among sites within each subbasin ranged between 44 and 58 days. This spatial variation was comparable to interannual variation, with median emergence dates for a given location in each subbasin ranging between 47 and 74 days among years. Contrary to our expectations, juvenile salmon were not larger in years with earlier emergence, owing to slower estimated spring and summer growth rates compared to years with later emergence. Despite large interannual variation in estimated emergence dates, these results suggest that other factors (e.g., stream flow, temperature, and density‐dependence) were more important than growth duration in determining juvenile salmon growth rates and size among years. We demonstrated considerable spatial and interannual variation in emergence phenology within these subbasins. Understanding how this variation translates to spatiotemporal patterns of juvenile salmon habitat use, growth, and survival has important implications for guiding restoration efforts and understanding how climate change may impact these populations.

The power of genetics: Past and future contribution of balanced genetic selection to sustainable growth and productivity of the Norwegian Atlantic salmon (Salmo salar) industry

In less than 50 years Atlantic salmon farming has become a major industry in Norway, and several attempts at predicting future growth have been made. In all food production, genetics has played an important role in production optimisation and efficiency. The benefits of genetic selection in aquaculture are less documented due to a variable production environment, long production time and challenges such as sea-lice, infectious diseases, and market fluctuations. This study investigates the impact of 50 years of balanced genetic selection on growth and productivity of the Norwegian Atlantic salmon industry. In a common garden experiment eggs from females representing present-day farmed Atlantic salmon (generation 11), were fertilised with cryo-preserved milt from males representing previous generations, and the progeny (half-sib families) were reared under the same commercial farming conditions. At the termination of this experiment, the difference in average body weight between generation 0 and half-sibs from present-day salmon was 1.5 kg. The results from the common garden experiment were used to model the contribution of genetic selection for increased salmon growth rate to the industry from generation 0 (harvested in 1975–1978) to generation 11 (harvested in 2017 – 2019) and to simulate the power of genetics for expected industry growth until 2050 (generation 24) giving four different forecast scenarios. We found that a total Norwegian production of 2.5 to 3.2 million tonnes could be achieved in 2050 solely by improving the inherent growth potential of Atlantic salmon through genetic selection. Further, the production time in seawater is expected to be reduced by 40 to 53% from 15.3 months in 2019 to a production time ranging from 9.2 to 7.2 months in 2050, respectively. Reduced production time in seawater results in reduced time at risk of diseases and other unfortunate events, and in our most progressive scenario, mortality in seawater was expected to be reduced by up to 50%. The contribution to reduced losses and increased resource utilisation, increases the sustainability of the Norwegian aquaculture industry. This study demonstrated that it is possible to significantly increase the Norwegian aquaculture production by genetic improvement, and that contribution from genetic selection for growth rate alone can close 77% of the gap of the forecasted five million tonnes in 2050. These scenarios demonstrate that genetic selection should be included as a central component when predicting future development and growth of aquaculture production, both in Norway and other salmon producing countries.

Variability in foodscapes and fish growth across a habitat mosaic: Implications for management and ecosystem restoration

Riverine ecosystems in their natural state are complex mosaics of habitats whose conditions vary across space and time as landscape features filter prevailing hydrologic forcing. Yet, through anthropogenic alteration many large river systems have become simplified through the construction of levees and dams that reduce lateral connectivity and flow variability. The extent to which shifts in habitat mosaics create conditions that support different trophic responses that manifest in differences in fish growth across the landscape remains largely untested. This is primarily due to limitations in linking habitat features, dynamic physical processes, and trophic transfer of energy to higher taxa at the landscape scale. Here, we conducted large-scale enclosure experiments across varying habitats on a fluvial floodplain as a model system to measure factors that influence habitat-specific growth rates in multiple Chinook Salmon (Oncorhynchus tshawytscha) stocks important to fisheries and of conservation concern. Using an ecosystem approach, we reveal that landscape context, water residence time, and habitat type (agricultural, wetland, river channel) result in different hot-spots of primary and secondary food production. This variation in the aquatic foodscape resulted in significant variation in salmon growth rates and ultimate size and morphology across the landscape. Floodplain habitats generally exhibited higher water residence times as highlighted by higher specific conductance, salinity, and chlorophyll-a values. Pelagic invertebrate abundance was 10 to 100 times more abundant in the off-channel habitats compared to the river channels. The average daily growth rates of the juvenile Chinook Salmon ranged from 0.15 mm day−1 and 0.01 g day−1 in the riverine habitat to 0.55 mm day−1 and 0.07 g day−1 in the off-channel habitat. These data were used to build mixed effects models that showed the influence of chlorophyll-a concentration, water temperature and pelagic invertebrate composition on fish growth across locations throughout the experiment. As landscapes become increasingly simplified there is increased risk of losing the mosaic of habitats necessary to achieve enhanced fish growth and phenotypically diverse and sustainable salmon populations. This in-situ experimental and modeling approach can be applied to other systems to develop ecosystem indicators such as habitat-specific fish growth rates to manage landscapes and processes to support resilient fish populations.

Insects in Atlantic salmon (Salmo salar) diets – comparison between full-fat, defatted, and de-chitinised meals, and oil and exoskeleton fractions

The present study investigated the effect of meals and fractions of black soldier fly larvae (BSFL; Hermetia illucens) in diets for Atlantic salmon (Salmo salar) on the physical quality of feed pellets, nutrient utilisation, and growth performance. Six extruded diets were produced: control diet (CD); full-fat BSFL meal diet (IM); defatted BSFL meal diet (DFIM); de-chitinised BSFL meal diet (DCIM); BSFL oil diet (IO) and BSFL exoskeleton diet (EX). The full-fat, defatted and de-chitinised meals replaced 15% of protein in the control diet. An eight-week study was conducted using salmon with average 28 g initial weight. The full-fat and de-chitinised meals in the diets numerically reduced pellet hardness, expansion, and water stability. The full-fat and de-chitinised meals improved growth rate of salmon, whilst defatted meal, oil and exoskeleton supported similar growth performance as the control. Feed intake and growth rate of fish fed full-fat meal diet were higher than those fed the other insect diets, but defatted meal gave a better feed conversion ratio than full-fat meal. Defatted meal, de-chitinised meal and exoskeleton reduced protein digestibility in fish, however; defatted meal increased the digested protein retention. In conclusion, use of full-fat BSFL meal improved feed intake and growth rate of salmon when replacing 15% of dietary protein. The present results suggest that less processed fullfat form of BSFL is more optimal in diets for salmon and further processing to remove lipid or exoskeleton fractions would only lead to an additional cost.

Delayed maturity does not offset negative impact afflicted by ectoparasitism in salmon

The negative effects of parasitism on host population dynamics may be mediated by plastic compensatory life-history changes in hosts. Theory predicts that hosts should shift their life-history towards early reproduction in response to virulent pathogens to maximize reproduction before death. However, for sublethal infections that affect growth, hosts whose fecundity is correlated with body size are predicted to shift towards delayed reproduction associated with larger body size and higher fecundity. This has been observed in Atlantic salmon and parasitic sea lice, via mark-recapture studies that recover mature fish from paired groups of control and parasiticide-treated smolts. We investigated whether such louse-induced changes to age at maturity can offset some of the negative effect of mortality on population growth rate in salmon using a structured population matrix model. Model results show that delayed maturity can partially compensate for reduced survival. However, this only occurs when marine survival is moderate to poor and growth conditions at sea are good. Also, the impact of delayed maturity on population growth when parameterizing the model with empirical data is negligible compared with effects of direct mortality. Our model thus suggests that management that works on minimizing the effect of sea lice from fish farms on wild salmon should focus mainly on correctly quantifying the effect of parasite-induced mortality during the smolt stage if the goal is to maximize population growth rate.

Farm to Fish: Lessons from a Multi-Year Study on Agricultural Floodplain Habitat

Large areas of California’s historic floodplain have been separated from adjacent river channels by levee construction, allowing the development of an extensive agricultural industry. Based on successful partnerships between agriculture and conservation groups to support migrating waterfowl, we examined whether seasonally flooded rice fields could be modified to provide off-channel rearing habitat for juvenile Chinook Salmon Oncorhynchus tshawytscha. During winter and spring of 2012-2017, we conducted a series of experiments in Yolo Bypass and other regions of California’s Central Valley using hatchery Chinook Salmon as a surrogate for wild Chinook Salmon, the management target for our project. Overall, we found that seasonally flooded fields are highly productive, resulting in significantly higher levels of zooplankton and high Chinook Salmon growth rates as compared to the adjacent Sacramento River. We found similar results for multiple geographical areas in the Central Valley, and in different cover types, such as non-rice crops and fallow areas. Although field substrate type did not detectably affect fish growth and survival, connectivity with upstream and downstream areas appeared to drive fish occupancy, because rearing young salmon were generally attracted to inflow in the fields, and not all of the fish successfully emigrated off the fields without efficient drainage. In general, we faced numerous logistic and environmental challenges to complete our research. For example, periodic unmanaged floods in the Yolo Bypass made it difficult to schedule and complete experiments. During severe drought conditions, we found that managed agricultural habitats produced low and variable salmon survival results, likely because of periodically high temperatures and concentrated avian predation. In addition, our project required substantial landowner time and effort to install and maintain experimental fields. Recent and future infrastructure improvements in Yolo Bypass could substantially improve options for experimental work and broaden efforts to enhance salmon habitat.

Detrital food web contributes to aquatic ecosystem productivity and rapid salmon growth in a managed floodplain.

Similar to many large river valleys globally, the Sacramento River Valley has been extensively drained and leveed, hydrologically divorcing river channels from most floodplains. Today, the former floodplain is extensively managed for agriculture. Lack of access to inundated floodplains is recognized as a significant contributing factor in the decline of native Chinook Salmon (Oncorhynchus tshawytscha). We observed differences in salmon growth rate, invertebrate density, and carbon source in food webs from three aquatic habitat types—leveed river channels, perennial drainage canals in the floodplain, and agricultural floodplain wetlands. Over 23 days (17 February to 11 March, 2016) food web structure and juvenile Chinook Salmon growth rates were studied within the three aquatic habitat types. Zooplankton densities on the floodplain wetland were 53x more abundant, on average, than in the river. Juvenile Chinook Salmon raised on the floodplain wetland grew at 0.92 mm/day, 5x faster than fish raised in the adjacent river habitat (0.18 mm/day). Two aquatic-ecosystem modeling methods were used to partition the sources of carbon (detrital or photosynthetic) within the different habitats. Both modeling approaches found that carbon in the floodplain wetland food web was sourced primarily from detrital sources through heterotrophic pathways, while carbon in the river was primarily photosynthetic and sourced from in situ autotrophic production. Hydrologic conditions typifying the ephemerally inundated floodplain—shallower depths, warmer water, longer water residence times and predominantly detrital carbon sources compared to deeper, colder, swifter water and a predominantly algal-based carbon source in the adjacent river channel—appear to facilitate the dramatically higher rates of food web production observed in the floodplain. These results suggest that hydrologic patterns associated with seasonal flooding facilitate river food webs to access floodplain carbon sources that contribute to highly productive heterotrophic energy pathways important to the production of fisheries resources.

Climate Warming, Resource Availability, and the Metabolic Meltdown of Ectotherms.

Climate warming may lower environmental resource levels, growth, and fitness of many ectotherms. In a classic experiment, Brett and colleagues documented that growth rates of salmon depended strikingly on both temperature and food levels. Here we develop a simple bioenergetic model that explores how fixed temperatures and food jointly alter the thermal sensitivity of net energy gain. The model incorporates differing thermal sensitivities of energy intake and metabolism. In qualitative agreement with Brett's results, it predicts that decreased food intake reduces growth rates, lowers optimal temperatures for growth, and lowers the highest temperatures sustaining growth (upper thermal limit). Consequently, ectotherms facing reduced food intake in warm environments should restrict activity to times when low body temperatures are biophysically feasible, but-in a warming world-that will force ectotherms to shorten activity times and thus further reduce food intake. This "metabolic meltdown" is a consequence of declining energy intake coupled with accelerating metabolic costs at high temperatures and with warming-imposed restrictions on activity. Next, we extend the model to explore how increasing mean environmental temperatures alter the thermal sensitivity of growth: when food intake is reduced, optimal temperatures and upper thermal limits for growth are lowered. We discuss our model's key assumptions and caveats as well as its relationship to a recent model for phytoplankton. Both models illustrate that the deleterious impacts of climate warming on ectotherms will be amplified if food intake is also reduced, either because warming reduces standing food resources or because it restricts foraging time.

Muscle-specific gene expression and metabolic enzyme activities in Atlantic salmon Salmo salar L. fry reared under different photoperiod regimes.

This study was conducted to characterise the muscle-specific gene expression, energy metabolism level and growth rates of Atlantic salmon Salmo salar L. reared under different photoperiod regimes. The effects of two photoperiod regimes - LD 16:8 (16 h light:8 h dark) and LD 24:0 (24 h light:0 h dark) over a period of 3 months (August to October) on growth, energy metabolism enzyme activities (cytochrome c oxidase, COX; lactate dehydrogenase, LDH; and aldolase) and the gene expression levels of myogenic regulatory factors (MRFs - MyoD1 paralogues (MyoD1a, MyoD1b, MyoD1c), Myf5, MyoG), myostatin paralogues (MSTN-1a, MSTN-1b, MSTN-2a) and the fast skeletal myosin heavy chain (MyHC) in the muscles of Atlantic salmon underyearling fry (0+) were investigated. The experiment was conducted in a fish hatchery with natural variations in water temperature. The results were compared with those obtained in salmon reared under the lighting conditions of a fish hatchery (HL, hatchery lighting). The results revealed that the fry reared under constant light (LD 24:0) grew faster and were bigger at the end of the experiment. Fishes reared within the photoperiod regime LD 16:8 had a lower growth rate. COX activity was lower in fish under the LD 16:8 regime compared with the LD 24:0 group. The LDH and aldolase enzyme activities were higher in the group with constant light in comparison to control in the beginning of September. The expression level for all of the genes studied variated during the duration of the experiment, and MyHC, MyoG, MyoD1a and Myf5 expression depended on the light regime as well. The more noticeable changes in gene expression occurred in October. The MyHC and MyoG mRNA levels increased, accompanied by MyD1c gene expression, in both groups that had additional lighting (LD 16:8 and LD24:0) at the beginning of October and were higher than the HL group. In the HL group, the elevation of MyHC and MyoG mRNA was gradual during October, but there was a sharp increase in Myf5 expression at the beginning of October. MyoD1 paralogues differently expressed during the experiment. The MyoD1a mRNA level was elevated at the end of October along with MyHC and MyoG expression, but MyoD1b and MyoD1c mRNA levels decreased along with Myf5 gene expression. The expression of MSTN paralogues were elevated with increases in MyHC and MRFs transcripts. These findings show that constant light has a positive effect on the growth rate of salmon, affecting the aerobic and anaerobic capacity in their muscles. The alterations in muscle-specific gene expression between the groups with different light indicated that the mechanisms for regulating muscle growth processes in fish depend on photoperiod duration.

Assessment of Juvenile Chinook Salmon Rearing Habitat Potential Prior to Species Reintroduction

AbstractSuccessful Pacific salmon reintroduction into rivers where they have been extirpated requires understanding the range of habitats and environmental conditions that currently exist and their ability to support target species. Chinook Salmon Oncorhynchus tshawytscha were extirpated from the San Joaquin River, California, over 60 years ago and are targeted for reintroduction into the system. To assess the remaining habitat along the longitudinal lower San Joaquin River gradient, juvenile Chinook Salmon were reared within an Alluvial Site, a Transitional Site, and two lowland sites (Lowland Cinnamon Slough and Lowland Eastside Bypass) during two rearing periods (early and late). The highest Chinook Salmon growth rates were observed during the early rearing period within the lowland sites when water temperatures were moderate and prey densities were greatest. However, high‐water events precluded a spatial comparison of growth with that of fish reared at the Alluvial and Transitional sites during the early period. In the late rearing period, the lowest and highest growth rates were observed at the Lowland Eastside Bypass (lowest prey abundance) and Lowland Cinnamon Slough (highest prey abundance), respectively. Growth rates at the Alluvial and Transitional sites were intermediate between those at the two lowland sites. Main‐channel (Alluvial and Transitional) growth rates were generally lower than those within the lowland sites, suggesting that main‐channel restoration may require targeted food web enhancement. Our results indicate that lowland floodplain sites can provide quality rearing habitat and growth benefits early in the rearing season. As temperatures increase, salmon can continue to grow if sufficient prey is available. Managers considering measures for improving floodplain access to provide juvenile Chinook Salmon rearing habitat should consider interactions between environmental conditions and prey density—and how these factors vary along the river continuum—to determine when these habitats can provide the greatest growth benefit.

Marine-entry timing and growth rates of juvenile Chum Salmon in Alaskan waters of the Chukchi and northern Bering seas

Climate change in the Arctic has implications for influences on juvenile Chum Salmon Oncorhynchus keta early life-history patterns, such as altered timing of marine entry and/or early marine growth. Sagittal otoliths were used to estimate marine entry dates and daily growth rates of juvenile Chum Salmon collected during surface trawl surveys in summers 2007, 2012, and 2013 in the Chukchi and northern Bering seas. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to discriminate between freshwater and marine sagittal growth on the otoliths, and daily growth increments were counted to determine marine-entry dates and growth rates to make temporal and regional comparisons of juvenile Chum Salmon characteristics. Marine-entry dates ranged from mid-June to mid-July, with all region and year combinations exhibiting similar characteristics in entry timing (i.e. larger individuals at the time of capture entered the marine environment earlier in the growing season than smaller individuals in the same region/year), as well as similar mean marine-entry dates. Juvenile Chum Salmon growth rates were on average 4.9% body weight per day in both regions in summers 2007 and 2012, and significantly higher (6.8% body weight per day) in the Chukchi Sea in 2013. These results suggest that juvenile Chum Salmon in the northern Bering and Chukchi seas currently exhibit consistent marine-entry timing and early marine growth rates, despite some differences in environmental conditions between regions and among years. This study also provides a baseline of early marine life-history characteristics of Chum Salmon for comparisons with future climate change studies in these regions.

Rapid, Efficient Growth Reduces Mercury Concentrations in Stream‐Dwelling Atlantic Salmon

Mercury (Hg) is a potent toxin that biomagnifies in aquatic food webs. Large fish generally have higher Hg concentrations than small fish of the same species. However, models predict that fish that grow large faster should have lower Hg concentrations than small, slow-growing fish due to somatic growth dilution (SGD). We examined the relationship between Hg concentrations and growth rate in fish using a large-scale field experiment. Atlantic salmon (Salmo salar) fry hatched under uniform initial conditions were released at eighteen sites in natural streams, collected after one growing season, and Hg concentration and growth measured. As expected for Hg accumulation from food, mercury concentrations in salmon tracked Hg concentrations in their prey. Nonetheless, large, fast-growing salmon had lower Hg concentrations than small, slow-growing salmon, consistent with SGD. While prey Hg concentration accounted for 59% of the explained variation in salmon Hg concentration across sites, salmon growth rate accounted for 38% of the explained variation independent of prey Hg concentration. A mass-balance Hg accumulation model shows that such SGD occurs when fast growth is associated with high growth efficiency. Fish growth is tremendously variable and sensitive to anthropogenic impacts, so SGD of Hg has important implications for fisheries management.

Models of the effects of marine-derived nutrients on salmon (Oncorhynchus spp.) population dynamics

Pacific salmon ( Oncorhynchus spp.) populations transfer large quantities of nutrients from their marine to their freshwater habitats. These nutrients have been shown to affect salmon populations in fresh water, including increasing basal food resources and elevating juvenile salmon growth rates and condition. The broader effects on recruitment and commercial harvests, however, are not clear. I developed and explored mathematical models of the effects of these nutrients on stock–recruitment relationships and used these models to investigate management implications. Populations strongly dependent on nutrients had lower sustained yields than those not dependent on nutrients. When nutrients strongly affected the stock–recruitment relationship, relatively low harvest rates and high escapement levels were necessary to maintain the population’s productivity. However, in some scenarios, the highest yields were obtained from small, nutrient-depleted populations. In other scenarios, the nutrient dependence had few management implications.

Competition, predation and flow rate as mediators of direct and indirect effects in a stream food chain

Using semi-natural stream channels, we estimated the effects of competition and predation exerted by juvenile and adult exotic rainbow trout (Oncorhynchus mykiss) on the diel activity pattern of juvenile native Atlantic salmon (Salmo salar), a secondary consumer. We also evaluated the direct and indirect effects of competition, predation and abiotic factors (water depth and velocity) on the growth rate of salmon, the biomass of invertebrate grazers (primary consumers) and the biomass of periphytic algae (primary producers; chlorophyll a). The presence of chemical cues emanating from adult predatory trout reduced the daily activity of juvenile Atlantic salmon. In contrast, competition imposed by juvenile rainbow trout forced Atlantic salmon to be more active during the day, even if adult rainbow trout were also present. We found no effect of either competition or of predatory cues on the growth rate of Atlantic salmon, and no evidence of indirect effects on either the biomass of invertebrates or the biomass of chlorophyll a. In contrast, we demonstrated that this food chain (fish--invertebrate grazers--periphytic algae) was under the control of a critical abiotic factor, the water velocity, and of bottom-up processes. We concluded that the exotic species directly increases the risk of predation of the native Atlantic salmon, but behavioral compensation probably limits the effects on growth rate. The competition and predation imposed by the invaders had no indirect effects on lower trophic levels. Top-down effects may have been mitigated by the dominant influence of water velocity controlling all components of the food chain and by elevated levels of primary production.

Life history changes in Atlantic salmon from the River Dee, Wales

The change in life history of Atlantic salmon (Salmo salar L) on the River Dee over the last 60 years is described. Over the last 60 years, salmon have shown a change in run timing, the majority currently entering the liver between August and October compared with prior to June. This has coincided with a change in the sea age composition, which was dominated by multi-sea winter salmon prior to the 1980s after which the proportion of 1sea-winter fish increased until they now dominate the mature population. Growth rates of salmon in fresh water remained relatively stable until the mid-1980s and then increased. By the end of the 1990s juvenile salmon were, by the end of their first and second year, respectively, ∼60 and ∼19%, on average, larger than they were between the late 1930s and mid-1980s. This has been reflected in a change in the age composition of smolts where the mean smolt age has declined from ∼2 years prior to the 1980s to ∼1.6 years in the late 1990s. There was no observed trend in post-smolt (marine) growth for salmon. Size at return for 1SW salmon appeared stable while there is some evidence of an increase in mean length of 2SW salmon at the end of the 1990s. A steady state life history model was developed which suggests an increase in the instantaneous rate of mortality by 2.9% from 1.495 year−1 in 1937/1938 to 1.538 year−1 in 1967/1969 and by 21.6% to 1.870 year−1 in 1997/1999. This is considered to explain the shift in mean age at maturity from 5.2 to 4.8 to 3.9 years for the three periods examined. There is close argeement between the observed mean age at maturity and that predicted by the model suggesting optimal lifetime reproductive success.

Growth, feed conversion and chemical composition of Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) fed diets supplemented with krill or amphipods

The effects of partial replacement of fish meal (FM) with meal made from northern krill (Thysanoessa inermis), Antarctic krill (Euphausia superba) or Arctic amphipod (Themsto libellula) as protein source in the diets for Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) on growth, feed conversion, macro-nutrient utilization, muscle chemical composition and fish welfare were studied. Six experimental diets were prepared using a low-temperature FM diet as control. The other diets included northern krill where 20, 40 or 60% of the dietary FM protein was replaced with protein from northern krill, and two diets where the FM protein was replaced with protein from Antarctic krill or Arctic amphipod at 40% protein replacement level. All diets were iso-nitrogenous and iso-caloric. Atlantic salmon grew from 410 g to approximately 1500 g during the 160 day experiment, and Atlantic halibut grew from 345 g to 500–600 g during the 150 day experiment. Inclusion of krill in the diets enhanced specific growth rate in salmon, especially during the first 100 days (P < 0.01), and in a dose–response manner in halibut for over the 150 day feeding period (P < 0.05). Feed conversion ratio did not differ between dietary treatments, and no difference was found in dry matter digestibility, protein digestibility and fish muscle composition. Good growth rates, blood parameters within normal ranges and low mortalities in all experimental treatments indicted that fish health was not affected either Atlantic salmon or Atlantic halibut fed the various zooplankton diets.