Aquaculture 283 (2008) 134–140

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Comparison of growth, feed intake, and nutrient efficiency in a selected strain of cohosalmon (Oncorhynchus kisutch) and its source stock

Kathleen G. Neely ⁎, James M. Myers, Jeffrey J. Hard, Karl D. ShearerNational Marine Fisheries Service, Northwest Fisheries Science Center, 2727 Montlake Blvd. E. Seattle, WA 98112 USA

⁎ Corresponding author. Tel.: +1 425 347 6935x227.E-mail address: kathleen.neely@noaa.gov (K.G. Neel

0044-8486/$ – see front matter. Published by Elsevierdoi:10.1016/j.aquaculture.2008.06.038

A B S T R A C T

A R T I C L E I N F O

Article history:

We compared growth in a d Received 19 March 2008Received in revised form 25 June 2008Accepted 26 June 2008

Keywords:Coho salmonGrowthSelectionProximate compositionDigestibility

omesticated strain of coho salmon (Oncorhynchus kisutch), that had been selectedfor rapid growth over 16 generations, to that of its hatchery-origin unselected parental stock. Fish werespawned on the same date and incubated under similar conditions. First feeding fry were fed to satiation andthen were fed a commercial salmon feed at two ration levels, either to satiation or on a fixed ration from thesize at which the smallest fish could accept a 1-mm pellet (domesticated 0.65 g, hatchery 0.96 g). Thedomesticated fish outperformed unselected fish by growing faster and to a larger size, ingesting a greateramount of feed when fed to satiation, and exhibiting greater efficiency in feed conversion. When fed tosatiation, domesticated fish ingested 53% more feed and gained 78% more weight compared to unselectedfish. The selected fish also appeared to utilize dietary lipids for energy while sparing protein for growth,while unselected fish deposited dietary lipids as body fat. These results indicate that selection over 16generations for growth also resulted in changes in feed efficiency and energy allocation. Understanding themechanisms underlying improved growth will aid future selection studies by identifying multiple targets ofselection that contribute most to growth.

Published by Elsevier B.V.

1. Introduction

The number of cultured fish undergoing selective breeding isincreasing worldwide, but is still relatively small compared to farmedterrestrial animals. Gjedrem (2005) stated that breeding programs aimto improve three primary attributes of cultured animals: (1) animalwelfare through domestication to reduce stress, (2) animal productivity,and (3) product quality. Rapid growth and high feed conversionefficiency of farmed animals, including salmonids, is a key determinantof commercial farming success (Sizemore and Siegel, 1993; Fjalestadet al., 2003). Variation in growth rate in salmonids can be attributed tomany mechanisms. These include maternal effects (Heath et al., 1999),rate of embryonic development (Robison et al., 2001), stomach size(Rindorf, 2002; Grove et al., 1978), feed intake (Ogata et al., 2002;Mambrini et al., 2006), metabolic rate (Boily and Magnan, 2002),temperature and genotype (Wangila andDick,1988), domestication andbehavior (White, 1985; Robinson and Doyle, 1990; Ruzzante, 1994), anddigestion efficiency (Bendiksen et al., 2003; Menoyo et al., 2003).Understanding the mechanisms underlying changes in growth rateenable one to develop effective breeding programs and predict potentialchanges in performance.

One long running selection program with Atlantic salmon (Salmosalar)was initiated in1971byAKVAFORSK, andhas beenunder selectionfor multiple generations (Gjedrem, 2000). Thodesen et al. (1999)

y).

B.V.

compared growth of the fifth generation of the AKVAFORSK fish withthat of the source population and reported faster growth in thedomesticated stock and attributed this to higher feed intake and feedefficiency. Gjedrem (2000) reported that an 11% genetic gain pergeneration, for growth rate, has been seen in highly selected stocks ofAtlantic salmon. Because salmon have a high degree of phenotypicplasticity, environment can play a large role in genotype expression.Overall, both genetic and environmental factors are implicated invariation in many traits among conspecific strains of fish.

This study compared a domesticated strain of coho salmon(Oncorhynchus kisutch), selected for rapid growth, to its sourcepopulation, a hatchery stock from a watershed with a robust naturallyspawning population. Dømsea-Aquaseed,1 in cooperation with scien-tists from the University of Washington (UW), developed the Dømseacoho salmon strain in 1977 in Washington State (USA), from a line ofmass-selected fish developed by NOAA Fisheries in 1971 (Novotny,1975). These fish have been subjected to intensive selection for over16 generations. Northwest Fisheries Science Center (NWFSC) andUniversity of Washington scientists have used Dømsea stock cohosalmon in numerous studies since 1977 (Tave, 1989). This strain has arecorded pedigree and considerable information on its genetics andperformance has been compiled and analyzed (Myers et al.,1999, 2001).

The Dømsea coho salmon strain was bred primarily for rapid growth.Selection program goals were to produce fish with a 2-year life cycle thatwould have a large body weight at the end of the freshwater phase of

1 AquaSeed, Inc, 2301 NE Blakeley Street, Suite 102, Seattle, WA 98105-3293 USA.

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rearing, a large body weight at the end of the 8-month saltwaterproductionphase (toproduce350-gpan-sizedfish), andahighpercentageof smolts in the first year of life (Tave, 1989). Beginning in 1986, theDømsea broodstocks were retained in fresh water for culture for theirentire life cycle. Although the Dømsea strain has been a closed populationsince its inception with inbreeding rates increasing by less than 2% pergeneration on average (Myers et al., 2001), there have been significantimprovements in growth and smoltification success under aquacultureconditions relative to its source population, Skykomish stock coho salmon(Myers et al., 2001). The overarching question for our study was “whatgeneral mechanism(s) are responsible for the rapid growth rate of thedomesticated stock relative to its source population?” This studycompared feed intake, growth, nutrient and energy retention, bodycomposition and digestibility between these stocks in a controlledexperiment.

2. Methods and materials

2.1. Fish used, gamete collection and incubation

Ten pairs of mature male and female Dømsea coho salmon wereselected at random and spawned at the Dømsea-Aquaseed facility inRochester, WA on 1 December 2003. This process was repeated at theWallace River Hatchery for Skykomish coho salmon by a second teamon the same day. Kidney and liver samples were taken from allspawners, as well as ovarian fluid samples from females, for pathogenscreening. All collected gametes were packed on ice and brought tothe NWFSC, where crosses were made on the evening of 1 December2003 so that any developmental variation between the treatmentgroups due to differences in fertilization time was eliminated. Allfamilies were assigned by mating one male to one female (single-pairmatings), with a total of ten families for each group (Dømsea andSkykomish). Each family was kept in a single egg lot “isobucket” toprevent disease transmission. Due to the results of an Enzyme-LinkedImmunoSorbent Assay (ELISA) for bacterial kidney disease (BKD)obtained from the kidney tissue, seven Dømsea stock families and sixSkykomish stock families screened for infection were discarded. Allremaining families were pooled by stock after hatching.

Table 1Results of growth and digestibility experiment for both Dømsea and Skykomish fish, for bo

Satiation Ration

Dømsea Skykomish Dømsea

Start weight (g) 0.65±0.0a 0.96±0.0b 0.66±0End weight (g) 23.60±0.4a 13.84±0.4b 9.71±0Weight gain (g) 22.95±0.7a 12.88±0.3b 9.05±0SGR1 5.09±0.04a 3.81±0.3b 3.84±0Feed intake (g/fish) 17.70±0.31a 11.58±0.06b 5.14±0Feed efficiency2 1.30±0.01a 1.11±0.03b 1.76±0Proximate composition (%)Crude protein 14.3±0.15a 14.4±0.30ab 14.8±0Crude lipid 7.9±0.49a 8.7±0.31a 5.7±0Moisture 74.7±0.003b 73.7±0.003b 77.5±0Ash 2.1±0.001 2.1±0.000 2.0±0

Condition factor3 1.36±0.011a 1.28±0.010b 1.21±0DigestibilityProtein 78.42±1.02 78.98±0.681Lipid 92.71±0.80 93.38±0.49Energy 81.52±1.43 81.64±0.34

PPV4 3.45±0.15a 2.89±0.03c 4.64±0PER5 48.69±2.33a 41.67±0.67c 60.0±0Energy retention (%)6 38.2±0.37 34.9±1.31 52.2±2

Groups with the same superscript denote no significant difference (PN0.05).1SGR (specific growth rate): [(ln end weight (g)− ln start weight (g)) ·d−1] ·100.2Feed efficiency: weight gain ·dry feed fed−1.3Condition factor: k =[weight (g) ·100] · length (cm)−3.4PPV (protein productive value): % protein retained.5PER (protein efficiency ration): protein gain (g) ·protein fed (g)−1.6Energy retention: (energy gain ·energy intake−1) ·100.

2.2. Rearing experiment and sampling regime

There were four treatment groups: Dømsea ration (DR) andSkykomish ration (SR) and Dømsea satiation (DS) and Skykomishsatiation (SS). Facility constraints limited ration treatments to tworeplicate tanks for each group and satiation treatments to threereplicate tanks for each group. It was anticipated that satiationtreatments would exhibit higher variation and therefore wereallocated more replicate tanks. Treatment groups were randomlyassigned to tanks, and water depth and flow and lighting were similarfor each tank. Fish were reared under simulated natural photoperiodin recirculated fresh water maintained between 10 and 11 °C. Whenthe fish were ready to commence feeding, fish from each stock werepooled and dispersed into the ten 1.3-m diameter (~4 m3) rearingtanks. The fish were fed a BioOregon (Warrenton, OR 97416 USA)starter diet until they were large enough to take a 1-mm pellet(103 days post fertilization), at which time the Dømsea fish averaged(±1 SE) 0.62±0.0 g and the Skykomish fish averaged 0.90±0.0 g. Atthis time, fish numbers in each tank were equalized at 350 fish.Mortalities in all groups were removed upon discovery, at least oncedaily.

Fish were fed by hand to apparent satiation or at a ration based on80% of the ration recommended by the feed manufacturer. Ration ratewas based on the fish size andwater temperature. Fishwere fed hourly(0800 to 1700) each day for the first month, and thrice daily for theremainder of the study. The BioOregon diet contained 45% protein,15.5% fat, 6.0% carbohydrate, 2.0% fiber, 8.5% ash, 23.5% moisture and13.18 J energy. Pellet size was adjusted as the fish increased in sizeaccording to the manufacturer's recommendations. For the ration fish,feedwasweighed daily to the nearest 0.1 g based on the twicemonthlysize measurements. Fish were reared and handled according to thepolicies and guidelines of the University of Washington InstitutionalAnimal Care and Use Committee (IACUC Protocol #2313-09).

Initial pooled weights were measured at ponding on 17 March2004, and all tanks were sampled fortnightly from 23 March 2004until 1 June 2004. At each sampling, three bulk weights of 50 fish and35 individual paired weights and lengths were taken from each tank.Ten fish from each tank were euthanized with an overdose of tricaine

th ration and satiation treatments (means±SEM)

ANOVA

Skykomish Stock Ration Stock×Ration

.0a 0.97±0.0b b0.0001 NS NS

.5c 9.35±0.5c b0.0001 b0.0001 b0.0001

.3c 8.38±0.0c b0.0001 b0.0001 0.007

.04b 3.24±0.01c b0.0001 b0.0001 b0.0001

.00c 5.41±0.00c b0.0001 b0.0001 b0.0001

.05c 1.55±0.03d 0.0038 0.0003 NS

.46ab 15.2±0.04b 0.003 0.03 NS

.47b 7.8±0.44a 0.02 0.01 NS

.005a 75.0±0.00b 0.06 0.001 0.08

.001 2.1±0.001 NS NS NS

.015c 1.25±0.013c 0.02 b0.0001 b0.0001

NSNSNS

.13b 4.07±0.02d b0.0001 0.004 NS

.00b 54.0±0.00d 0.002 b0.0001 NS

.70 55.5±0.95 NS 0.001 NS

Fig. 1. Growth in body weight over time. Data are weights in g (±1 SEM), based on tankmeans, with n=3 for satiation fish, and n=2 for ration fish. Tank means were based onthree pooled samples of 50 fish. Dømsea ration, DR; Dømsea satiation, DS; Skykomishration, SR; Skykomish satiation, SS.

Fig. 2. Qualitative comparisons of body size and shape for Dømsea and Skykomish cohosalmon stocks and feeding treatments (satiation and restricted ration). Each fish wassampled on 1 June 2004 (80 days since first feeding). The fish shown are generallyrepresentative of their respective experimental groups.

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methanesulfonate (MS-222) placed in bags and frozen for proximateanalysis and for bomb calorimetry.

At the conclusion of the feeding study (71 days post-ponding), fishfrom the satiation treatments were fasted for two days and then fed tosatiation with feed that had been repelleted with 100 ppm yttriumoxide, Y2O3 as an inert marker. After three days of feeding, fish wereanesthetized with MS-222 and were manually stripped of fecesaccording to Austreng (1978). This process was repeated again oneweek later. Feces were pooled by tank and collection date, then freezedried and stored at −80 °F until analyzed.

2.3. Chemical analyses

All analyses were conducted at the NOAA Fisheries Lab in Seattle.Fish tissue samples were finely chopped and dried at 105 °C todetermine moisture content. Dried samples were finely groundwith apestle and mortar and stored in a dissector. Lipid was determinedusing a Buchi 810 Soxhlet with dichloromethane as the solvent, ashwas determined by combustion at 550 °C using a muffle furnace, andprotein was measured using a Leco Model 602-000-300 nitrogenanalyzer. Feed and fecal analysis was identical to that of the dried fish;however, due to the small size of the samples, fecal lipids wereanalyzed on a Leco FA-100 lipid analyzer. Preparation of samples foryttrium analysis followed Shearer (1984). Analyses were performedusing an Optima 300 ICP-OES (Perkin Elmer, North Kingston, RI 02852USA) and prepared standards (Ultra Scientific, Wellesley, MA 02481USA). Energy was measured using a Parr Instrument Model 1266isoperibol calorimeter (Parr Instrument Company, Moline, IL 61265-9984 USA). Formulas used to compute nutrition metrics were fromFulton (1902), Maynard and Loosli (1969), and Steffens (1989).

The ANOVA and regressional analyses were conducted usingStatView (Adept Scientific, Inc., 257 Great Road, Acton, MA 01720USA). A Student–Newman–Keuls test was used to detect differencesamong means, and probabilities of 0.05 were considered significant.An a priori power analysiswas done for all samplings to ensure that thesample size was adequate to detect treatment and stock differences(statistical power at least 0.8).

3. Results

3.1. Growth

Mortality was less than 1% and did not vary significantly (PN0.05)among the treatments. When fed to satiation, Dømsea stock fish grewsignificantly (Pb0.01) larger, and at the end of the experiment Dømseasatiation-fed fish averaged 23.6 (±1 SE) ±0.04 g in weight, comparedto a mean of 13.84±0.04 g for Skykomish satiation-fed fish (Table 1,Figs. 1 and 2). The groups that were fed a size-dependent ration didnot differ significantly in weight and length at the end of theexperiment despite the fact that the Dømsea fish were initially 31%smaller than the Skykomish fish (Pb0.05). Specific growth rates (SGR)were relatively high for all treatments and ranged from 3.2 to 5.1, withthe Dømsea satiation group exhibiting the highest growth rate and theSkykomish ration group the lowest growth rate (Table 1). The percentreduction in Total Sum of Squares (TSS) for the SGR indicated that48.8% of the variance was due to stock differences and 44.4% of thevariance was due to treatment differences. There were significant(Pb0.05) differences between stocks and treatments, in addition to ahighly significant (Pb0.01) stock by treatment interaction (Table 1).

3.2. Feed intake and efficiency

When fed to satiation, Dømsea fish consumed more feed onaverage than did Skykomish fish (17.7 g/fish versus 11.6 g/fish,respectively; Table 1). Dømsea fish had a significantly higher(Pb0.05) feed efficiency than the Skykomish fish under both feedingregimens (1.47 g fish/g feed versus 1.27 g fish/g feed, respectively)based on two-way ANOVA (Table 1). In addition the ration fish hadsignificantly higher (Pb0.05) feed efficiency (gain/feed) than thesatiation fish (1.52 g fish/g feed versus 1.21 g fish/feed, respectively).

Fig. 3. Percent whole body lipid versus average fish weight. As fish increased in size,Skykomish fish in both treatments exhibited significantly more whole body fat than theDømsea fish in the corresponding treatment.

137K.G. Neely et al. / Aquaculture 283 (2008) 134–140

3.3. Proximate composition and condition factor

The ration fish had slightly higher (Pb0.05) concentrations ofwhole body moisture and protein but lower lipid (Table 1, Fig. 3) thanthe satiation fish. Regression analysis showed an inverse correlationbetween whole body lipid levels versus whole body moisture (Fig. 4).There were no significant differences (PN0.05) in whole body ashbetween treatments or stock. Dømsea satiation fish had a significantlyhigher (Pb0.05) condition factor than did Skykomish satiation fish.Both stocks had significantly higher (Pb0.05) condition factors (a ratioof fish girth to length) when fed to satiation than when fed acontrolled ration. The ration treatment fish showed no significantdifference (PN0.05) in condition factor between stocks.

3.4. Digestibility, PPV and PER, and energy retention

There were no significant differences in the digestibility of lipid,protein or energy between the Dømsea and the Skykomish satiationgroups (Table 1). Productive protein value (PPV) and protein efficiencyration (PER) were significantly higher (Pb0.05) in the ration fish (bothstocks) and in the Dømsea stock relative to the Skykomish stock.Energy retentionwas higher (Pb0.05) in the ration fish but there wereno significant stock differences (Table 1). The ANCOVA (Fig. 5) showedthat the fish allocated protein at different rates over time and between

Fig. 4. Regression analysis of whole body moisture versus whole body lipid for bothstocks and both treatments. There was no interaction effect between treatments andstocks therefore groups are not individually distinguished. There was a highly negativecorrelation between whole body moisture and whole body lipid levels.

stocks and treatments. An ANCOVA of body burden protein datasuggested that the Dømsea satiation fish appeared to be sequesteringhigher amounts of body protein than the Skykomish satiation fish.

4. Discussion

4.1. Growth, FE, body composition

After 16 generations of selection for rapid growth, Dømsea cohosalmon outperformed their founding population in several key aspects ofgrowth, including growth rate, food consumption, and feed assimilationefficiency. More importantly, we detected differences in several under-lying physiological parameters, and focused on identifying those factorscontributing to the improved growth exhibited by the Dømsea fish. Wefound that the Dømsea fish grew larger faster due to three primaryprocesses: increased ingestion, higher feed efficiency, and protein sparing.

Dømsea fish, when fed to satiation, ingested far greater amounts offeed and grew considerably faster than did fish from the sourcepopulation. The groups that were fed a size-dependent ration did notdiffer significantly in size at the end of the experiment, despite the factthat the Dømsea fish were initially significantly smaller. Regardless ofintake, the Dømsea stock exhibited greater feed efficiency than theSkykomish stock. Correlations between growth and feed efficiency havebeen documented in other species (Li et al., 1998; Grisdale-Helland andHelland, 1998). Salmonids have shown to be especially plastic in theirphenotypic expression of growth andmaturation (Tymchuk and Devlin,2005; Aubin-Horth et al., 2005; Wright, 2007), and because of this, it isquestionable whether the domesticated fish are simply exhibitingvariances in their genotypeor are indeedbeingdramaticallymodifiedbydomestication. In addition, Dømsea fish appeared to exhibit proteinsparing, a process that reflects greater efficiency of incorporating feedprotein into tissue. These processes have been observed in otherdomesticated animals. For example, Small (2005) reported that growthin catfish was positively correlated with increased feed intake, and thispattern has also been seen in sheep (Cammack et al., 2005), swine(Woltmann et al., 1992), rabbits (Ozimba and Lukefahr, 1991), and mice(Lin et al., 1979). Experiments such as this one allow for greaterunderstanding of the mechanisms behind certain desirable character-istics, such as rapid growth and feed efficiency.

Feed efficiency is of economic importance to fish culturists sincestrains that put on more weight per kilogram of feed cost less to grow;feed costs can constitute a considerable portion of overall expenses inaquacultureoperations. In this study the ration restrictedfishhadahigherfeed efficiency than the satiation-fed fish in both stocks; however, theDømsea stock exhibited significantly higher feed efficiency than theSkykomish fish under both feeding regimens. Grisdale-Helland and

Fig. 5. The ANCOVA of mount of whole body burden protein in Dømsea and Skykomishcoho salmon, both stocks and feeding treatments (satiation and restricted ration).

Fig. 6. Partitioning analysis of energy, crude lipid, crude protein, and mass betweenDømsea and Skykomish coho salmon, both stocks and feeding treatments (satiation andrestricted ration). Each graph represents how each stock assimilated an identicalamount of feed consumed (broken into four components: energy, crude lipid, crudeprotein, and mass).

138 K.G. Neely et al. / Aquaculture 283 (2008) 134–140

Helland (1998) found a positive genetic correlation between growth andfeed efficiency in Atlantic salmon (S. salar), and it has been demonstratedthat growth rate has a strong genetic basis in captive salmonids (Gjedrem,1976; Su et al., 1996). It has also been shown that feed efficiency is bothspecies- and size-dependent in salmonids (Azevedo et al., 2004) and thatthe feed efficiency of a particular animal can change over its lifetime(Steffens, 1989). Zoccarato et al. (1994) also found that the best feedefficiency occurs when fish are fed below satiation. Additionally,improving feed efficiency can lead to a decrease in waste products inthe environment (Hillestad et al., 1999).

Fish fed to satiation generally have higher condition factors (CF)thanfish fed a restricted ration if allfish are the same length (Johanssonet al., 1995), a pattern also observed in our study. There was a strongstock by treatment interaction for condition factor (CF, Table 1). Amongthe satiation fish, the Dømsea satiation group had the highestcondition factor (1.36±0.011, compared with 1.28±0.010 in theSkykomish satiation group), while fish ration treatments showed nosignificant differences in CF.With theDømsea satiationfish therewas anoticeable change in body shape that was not seen in either theSkykomish satiation fish or in the ration groups for either stock (Fig. 2).Although theDømsea satiationfish had a higher condition factor, otherdomestication factors (e.g., increased FE) appear to have more thancompensated for the loss of a wild-type streamlined shape and theincrease in drag when in swimming. The fact the Dømsea satiation fishexhibited such rapid growthwhilemaintaining a high CF is contrary tostudies that show that salmonids with a stout body shape often incurhigher swimming costs than more streamlined wild salmonids (Boilyand Magnan, 2002; Petrell and Jones, 2000). Wang et al. (2004) alsoproposed that selecting for fish that had a more streamlined bodyshape (and therefore reduced drag) could improve feed conversion,thus allowing for greater somatic growth with less feed.

4.2. Body composition, energy retention, digestibility

Comparisons of fish proximate composition: moisture, protein,lipid (fat), carbohydrate, and ash reveal how fish in the varioustreatment groups utilized the feed differently. Specifically,whether thefish is primarily retaining lipids and using protein for energy (Lee andPutnam,1973), or if thefish is utilizing feed-based lipids for energy, andprotein is being used for somatic growth (in which case a greaterportion of the body weight gain is moisture). We also saw a negativecorrelation betweenbodymoisture and lipid in our experiment (Fig. 4).There was no stock by treatment interaction and both stocks underboth treatments showed a negative correlation between percentwhole bodymoisture and lipid (r=−0.946). The ration fish had slightlyhigher concentrations of whole body protein and moisture but lowerlipid than the satiation fish. Thewhole body content of water and lipidare generally negatively correlated in fish (Shearer, 1994). In this studyDømsea stock fish had significantly higher moisture content than theSkykomish stock fish, implicating that the Dømsea fish were utilizinglipid formetabolism. An alternate explanation is behavioral differencesbetween the Dømsea and Skykomish fish. Although we observed nodifferences in behavior between the treatments and the stocks, we didnot quantify possible differences in behavior. While there arealternative explanations for the selected fish having less whole bodylipid in relation to body size, our experiment alludes to protein sparingas a mechanism for rapid growth in the Dømsea stock.

When fish store lipid, they release excess water to maintain ahydrodynamic shape; fish that are using lipid for metabolism tend tohave higher moisture contents and allocate protein to somatic growth(Ogata et al., 2002; Gjedrem, 2005). When examining the nutrientpartitioning of the two stocks (Fig. 3) the Dømsea fish had significantlymore protein gain andwater gain than the Skykomishfish, (the Dømseafish also showed significantly lessmetabolic loss) (Fig. 6). TheSkykomish(unselected) fish appeared to use protein for metabolism to a greaterextent. These results imply that the faster growth of the domesticated

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Dømseafishwasdue, inpart, to the sparingof protein for growth and theuse of lipid formetabolism. The sparingof protein by fat has been seen inAtlantic salmon (Johnsen et al., 1993). Grisdale-Helland and Helland(1997a,b) found that protein sparing occurred but only when dietscontaining 9–10 or 17–18% starch were used was the effect significantwithout compromising overall weight gain. Protein is the mostexpensive component of feed, therefore it is important that the fishmaximize utilization of the protein source for growth and utilizes thenon-protein components of the feed to satisfy energy requirements (DeSilva and Anderson,1994). The conclusion that fish can spare protein forgrowth has been seen in studies with yellowtail (Seriola quinqueradiata)(Shimeno et al., 1985) and Atlantic salmon (Johnsen et al., 1993;Grisdale-Helland and Helland, 1997a,b). In many instances, the amountof protein sparing is dependent on the amount of fat in the feed (amongother factors). Lee et al. (2002) reported that an increase of dietary lipidproduced a protein sparing effect in juvenile rockfish (Sebastes schlegeli).Other investigators, such as Azevedo et al. (2004), concluded that fishwere able to compensate for a lower dietary amino acid supply byimproving protein utilization.

Energy retention is the amount of energy from food that is retainedfor somatic growth (De Silva and Anderson, 1994). Energy retentioncan be increased either through reduced maintenance costs or byretaining consumed energy at levels above maintenance (Lin et al.,1979). In our study, energy retention was higher in the ration fish(both stocks) versus to satiation fish (both stocks). The probable causeof this effect was that the satiation fish were larger than the ration fishduring much of the study which resulted in more energy beingutilized (Azevedo et al., 2004). Domestication can lead to a less activeanimal that uses less energy or allocates more energy for somaticgrowth than its non-domesticated counterparts (Fleming et al., 2002).In contrast, wild fish, when held in captivity, are typically more activethan domesticated fish (Hoar et al., 1979; Holm and Fernoe, 1986).Increased metabolism (as affected by all types of activity, inherentgenetic factors, and environment) tends to reduce somatic growth.Fish that are constantly swimming at high rates of speed or display“burst swimming” utilize energy at a faster rate than fish that swim ata slower speed (Puckett and Dill, 1984; White, 1985). Additionally, fishthat have been under domestication for generations are not as easilystressed as wild fish under captive culture (Hoar et al., 1979; Gadagkar,1998; Kincaid, 1994; Simpkins et al., 2003).

Many factors can affect digestibility in fish, and numerous studieshave evaluated the apparent digestibility of feed by salmonids (Cooket al., 2000; Bendiksen et al., 2003). Analysis of fecal material showedno significant difference in digestibility of dry matter, lipid, protein orenergy between the Dømsea and the Skykomish satiation groups.These results may be due to the high digestibility of the feed used andthere may be a stock difference under different feeding regimens.

4.3. Future research

Future research with these stocks should focus on four avenues ofinvestigation to further characterize the mechanisms limiting rapidgrowth and its response to selection: 1) controlled growth experi-ments to quantify growth in relation to feed efficiency and bodycomposition throughout the entire (commercial) life cycle, 2)evaluating the effects of different feeds, and feed formulations, 3)looking further at the protein sparing abilities of Dømsea fish usingalternate methods such as respirometry and 4) genetic investigationsof growth and correlated traits using informative molecular markers.Analyzing the feed efficiency and body composition throughout theentire life cycle would allow for specific feed formulations to be usedat certain periods in the life cycle to maximize the growth of theDømsea fish. Future studies that increase or decrease the amount of fatin feed to Dømsea fish could ameliorate concerns that the fish are notactually sparing protein, but simply utilizing the feed better. Geneticimprovements that promote improved growth (especially on lower

protein diets) and high feed conversion efficiency are increasing inimportance to the culture of food animals. Finally, knowing theunderlying genetic parameters for rapid growth in salmon stocks canassist substantially in improving the performance of cultured stocks(Kinghorn, 1983). To that end, identification of quantitative trait loci(QTL) would help the culturist in improving commercially desirabletraits such as feed efficiency (Kamler and Kato, 1983; Kristjansson andVollestad, 1996; Pakkasmaa and Jones, 2002) through marker-assistedselection. Through the use of genetic marker technology and selectivebreeding regimes, domestic lines of salmonids may incorporate evenhigher feed efficiencies and faster growth than presently seen.

Acknowledgements

The authors wish to thank Aquaseed, Inc. and the Wallace RiverHatchery for providing animals. We also wish to thank Dr. MarkPetterson for pathogen screening and Brad Gadberry and Paul Parkins(NWFSC hatchery staff).

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