ABSTRACT

High quality marine lipid (ML) sources are expensive and unavailable at times. Hence, cost effective alternatives are needed for aquafeeds. This research assessed the potential for using canola oil (CO) in diets for chinook (Experiment 1) and Atlantic (Experiment 2) salmon. In Experiment 1 (28-weeks), we found that CO could comprise 51% of the total dietary lipid (17.5%) by replacement of marine lipid (ML) without compromising growth, feed intake, feed efficiency and percent survival of chinook salmon (mean initial weight, 81.4-90.2g) in ambient temperature (7.8-13.5°C) sea water. Fillets from fish fed the ML-rich basal diet, however, had markedly higher levels (%) of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (n-3 HUFAs) and lower percentages of (n-6) fatty acids than found in those from fish fed the diet with CO. Conversion of two of four replicate groups fed the diets with supplemental ML or CO to an EPA and DHA-enriched diet between week 16 and 28, revealed that the ML profile could largely be reinstated in the latter fish. In Experiment 2 (16-weeks), we noted that CO (crude super-degummed product) could comprise 47% of the lipid (28%) in grower diets for Atlantic salmon (mean initial weight, 145.2-181.3g) in ambient temperature (8.0-10.5°C) sea water without adversely affecting growth, feed utilization, and percent survival. Flesh levels of n-3 HUFAs, however, were decreased and those of (n-6) fatty acids were increased as the dietary CO level was raised. Regardless of diet treatment, the absolute amounts of n-3 HUFAs (mg/100g of fillet) were near values reported for wild salmon. Interestingly, a direct relationship was found between dietary CO level and swimming performance. CO is thus an excellent cost effective partial substitute for ML in salmon diets.

INTRODUCTION

Marine fish oils have been used traditionally in salmon diets to provide essential fatty acids of the omega-3 (n-3) family as well as readily available non-protein energy and other benefits. The global supplies of fish oils, however, have remained relatively static over the last decade (average, 1.18 million tonnes per year) even though more demands are being placed upon these valuable commodities for expanding aquaculture feed markets and to some extent, for direct use in the human diet to prevent an array of health-related problems (Higgs et al. 1995; Fish Farming International, 1997). By the year 2010, it has been forecasted that all of the

global supply of fish oil could be utilized in aquafeeds. The preceding trend has been accompanied by increased prices for premium quality marine lipids (ML) especially within and just after El Niño years and the maximum value noted for ML to date occurred in the middle of 1998 (US $ 790/tonne). Indeed, the prices for ML have exceeded those of vegetable oils such as canola oil (CO) from the middle of 1997 until the latter part of 1998. It should be mentioned further that the cost of production of salmon can be near their market value (Higgs, 1997) and that feed represents the single largest operational expense (35-60%). Moreover, the lipid fraction in high-energy extruded grower diets (40% protein and 33% lipid) for salmon presently accounts for about 33% of their cost. This percentage will undoubtedly increase in the future unless decreased reliance is placed upon ML to furnish most of the dietary non-protein energy needs.

In this report, we describe some of the main findings from two studies that were conducted at the Fisheries and Oceans, Canada, West Vancouver Laboratory (WVL). The main goal of this research was to explore the potential for including CO as a partial replacement for ML in the diets of chinook (Oncorhynchus tshawytscha) and Atlantic (Salmo salar) salmon held in ambient sea water. An additional objective in the chinook salmon study (Experiment 1), was to determine whether fish previously fed the diet supplemented with CO for 16 weeks could reinstate the “marine type” of fatty acid composition in their flesh by consuming an (n-3)-enriched diet over a 12-week period. Also, a second objective in the Atlantic salmon study (Experiment 2), was to determine whether progressive replacement of the supplemental ML with CO would influence their swimming performance.

MATERIALS AND METHODS

Experiment 1

Phase 1 (week 0 - 16) Quadruplicate groups of 35 - 40 chinook salmon (initial mean weight, 81.4 - 90.2 g), held in 1100 L fibreglass tanks that were supplied with running, aerated, ambient sea water, were each given one of four steam pelleted dry diets that were equivalent in protein (crude protein ranged from 49.2 -52.1% of dry matter) and lipid (lipid varied from 16.8 - 17.6% of dry matter) content using a randomized block design. The test diets were manufactured at the WVL and they were identical in ingredient and proximate composition except for the sources of supplemental lipid (9% of dry diet) that were employed. Two of the four sources were sardine oil (SO; high in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are collectively termed n-3 HUFAs, but low in omega-6 (n-6) fatty acids), and CO (rich in linolenic acid, the parent fatty acid of the (n-3) series, (n-6) fatty acids and oleic acid), while the other two sources that will not be considered further were corn oil and pork lard. All groups were fed their prescribed diets by hand two to three times daily to satiation. Measurements of fish growth (weight gain (WG) and specific growth rate (SGR)), feed efficiency (FE), percent survival (% S) and percentages of n-3 HUFAs and (n-6) fatty acids in the muscle lipid were taken.

Phase 2 (week 16 - 28) This part of the study was undertaken to determine how rapidly reduced levels of n-3 HUFAs and higher levels of (n-6) fatty acids in the flesh of fish fed the diet supplemented with CO (diet 2) could be transformed to those characteristic of fish fed the diet with SO (diet 1) and wild salmon (high in levels of n-3 HUFAs and low in (n-6) fatty acids). Accordingly, two of the four replicate groups of fish were maintained on their phase 1 diets for an additional 12 weeks, whereas the other two groups in each case were transferred to an (n-3)-enriched diet (diet 3). The latter was identical in ingredient composition to the phase 1 diets except for the source of supplemental lipid (EPA/DHA marine lipid concentrate (EPA-CHOL-750), provided by Capsule Technology Group Inc., Windsor, Ontario). Selected percentages (% of total fatty acids) of (n-6), (n-3), and n-3 HUFAs in diet 1 were respectively, 5.69, 36.1 and 29.2. Corresponding values in diets 2 and 3 were 16.2, 18.1, and 10.1, and 4.80, 47.8, and 42.9. Ratios of n-3 to n-6 fatty acids in diets 1, 2 and 3 were respectively, 6.34, 1.12 and 9.96. The feeding protocol and performance parameters were as described for phase 1. Arcsine square root transformations of percentage data were conducted to achieve homogeneity of variance.

During the study, water temperatures ranged from 7.8-13.5 °C , dissolved oxygen levels exceeded 80% of saturation and a natural photoperiod was provided.

Experiment 2

Feeding trial Triplicate groups of 38 - 41 Atlantic salmon (initial mean weight, 145.2 - 181.3 g) held in 1100 L fibreglass tanks that were supplied with running, aerated, ambient sea water, were each given one of four isonitrogenous (~36% digestible protein on a dry weight basis) and isoenergetic (~18.8 MJ of digestible energy/kg dry diet) diets using a randomized block design. All of the test extruded dry pelleted diets were manufactured by Moore-Clark, and they were identical in ingredient and proximate composition except for the sources of supplemental lipid (25% of diet) that were employed. These were as follows: diet 1, 25% menhaden oil (MO); diet 2, 20.75% MO and 4.25% CO; diet 3, 16.5% MO and 8.5% CO; diet 4, 12.25% MO and 12.75% CO. The CO used in this study was a crude (undegummed) product that was supplied by CanAmera Foods, Wainright, Alberta. CO comprised either 0, 17, 34 or 51% of the supplemental dietary lipid or 0, 15.5, 31.2, or 47.0% of the total dietary lipid content (~28% on an air-dry basis).

Fish were fed their prescribed diet twice daily to satiation (except for 24 hr before each weighing time) and records of daily feed intake and mortality were maintained. Throughout the 112-day study the fish were subjected to a natural photoperiod, and water temperature, dissolved oxygen and salinity varied between 8 - 10.5 °C, 9 - 10.5 ppm and 28 - 30 ‰, respectively.

The effect of diet treatment on fish performance was assessed by determining WG, SGR, FE, %S and percentages of n-3 HUFAs and (n-6) fatty acids in the muscle lipids. Muscle lipid compositions on day 112 were determined using the methods of Silver et al (1993). Arcsine square root transformations of percentage data were conducted to achieve homogeneity of variance.

Swimming study At the end of the feeding trial, 16 salmon from each dietary treatment were transported to the Department of Zoology, University of British Columbia where they were held in circular fibreglass tanks supplied with running 12 °C sea water. After 7 days of acclimation, during which time, the fish were fed their prescribed diets at 1% of body weight/day, exercise performance was measured on 5 - 7 fish of similar size from each dietary group (mean weights of diet groups varied between 377 and 444 g). Thereafter, individual fish were placed in a modified Brett-type respirometer filled with sea water and forced to swim for 2 hr at 0.5 body lengths/second (Bl/sec). At the end of this period, the respirometer was sealed and oxygen consumption was measured using a computerized data acquisition system. Five fish were then taken through a series of 0.5 Bl/sec increments in swimming speed, every 30 min, until exhaustion. Maximum swimming speeds (U_crit ) were calculated as described by Brett (1964).

RESULTS

Experiment 1

Complete replacement of the supplemental SO in diet 1 by CO did not significantly influence chinook WG, SGR, FE or %S, but this treatment did significantly reduce percentages of n-3 HUFAs and raise (n-6) fatty acids in the flesh lipids (Table 1). Groups of chinook salmon fed the (n-3)-enriched diet in phase 2 had almost identical percentages of n-3 HUFAs in their flesh lipids on day 196, regardless of whether they received diets supplemented with SO or CO in phase 1. This treatment, however, did not fully reduce the percentages of (n-6) fatty acids in the muscle of fish previously fed the diet with CO to the levels noted in fish fed the diet with SO during the study.

Table 1. Mean weight gains (WG), specific growth rates (SGR), feed efficiencies (FE), and percent survivals (%S) of chinook salmon fed the test diets for 112 days, and percentages of n-3 HUFAs, and (n-6) fatty acids in the muscle lipids (% of total fatty acids) on day 112 (end of phase 1) and 196 (end of phase 2).

^1/ The data for all parameters (n = 4 (phase 1) or 2 (phase 2) means per diet treatment) were analyzed by randomized block ANOVA and Tukey’s test (P = 0.05) when appropriate. Within a column, diet groups with the same superscript letter were not significantly different. Each mean for n-3 HUFAs and (n-6) fatty acids was comprised of 3 (phase 1) or 4-5 (phase 2) fish. ^2/ SGR = [(ln final body weight - ln initial body weight)/days]∙100; ^3/ FE = WG/dry feed intake (day 0 - 112).

Experiment 2

Replacement of ≤ 51% of the supplemental ML in a high-energy grower diet for Atlantic salmon by CO (CO provided 47% of the dietary lipid content) did not significantly influence growth, FE or %S (Table 2). Percentages of n-3 HUFAs and (n-6) fatty acids in the muscle lipids, however, were negatively and positively correlated, respectively with the dietary level of CO. However, it is noteworthy that the absolute concentrations of n-3 HUFAs in the muscle (mg/100 g) of the Atlantic salmon on day 112 fell within the range previously reported for farmed and wild Atlantic salmon regardless of diet treatment. Interestingly, significant differences in U_crit were found, and fish fed diet 1 had significantly lower swimming speed than those fed diet 4. Salmon fed diets 2 and 3 had intermediate swimming speeds.

CONCLUSIONS

CO is an excellent source of supplemental dietary lipid for chinook and Atlantic salmon in sea water. Diets containing CO, however, must have adequate levels of n-3 HUFAs for growth and health of the fish and for high flesh quality. Finishing diets enriched with EPA and DHA can restore the “marine type” of fatty acid profile in salmon previously fed diets containing CO. Also, enrichment of salmon diets with CO enhances the maximum swimming speed of salmon in sea water. This finding may have application for improving the quality (%S) of salmon smolts released into the natural environment.

Table 2. Mean weight gains (WG), specific growth rates (SGR), feed efficiencies (FE), and percent survivals (%S) of Atlantic salmon fed the test diets for 112 days, terminal percentages of n-3 HUFAs, and (n-6) fatty acids in the muscle lipids (% of total fatty acids) and maximum swimming speeds (U_crit; body lengths/second) of the fish in relation to diet treatment.^1/

^1/ The data for all parameters (n = 3/ diet treatment) except U_crit, were analyzed by randomized block ANOVA and Tukey’s test (p = 0.05) when appropriate. Within a column, diet groups with the same superscript letter were not significantly different. U_crit data were analyzed by One-way ANOVA;^2/ SGR = [(ln final body weight - ln initial body weight)/days]∙100; ^3/ FE = WG /dry feed intake (day 0 - 112).

ACKNOWLEDGMENTS

We thank Moore-Clark (Canada) Inc., and the National Research Council of Canada for fiscal support.

REFERENCES

1. Brett, J.R. 1964. The respiratory metabolism and swimming performance of young sockeye salmon. Journal of the Fisheries Research Board of Canada 21:1183-1226.

2. Fish Farming International. 1997. Future world fish supplies should be enough for farm needs. 24: 14-16.

3. Higgs, D.A., J.S. Macdonald, C.D. Levings, and B.S. Dosanjh. 1995. Nutrition and feeding habits in relation to life history stage. In: Physiological Ecology of Pacific Salmon. (Edited by C. Groot, L. Margolis and W.C. Clarke). pp. 159-315. UBC Press, Vancouver, BC.

4. Higgs, D.A. 1997. Nutritional strategies for cost effective salmon production. In: Proceedings of the First Korea-Canada Joint Symposium in Aquatic Biosciences, October 16 1997, Pukyong National University, Institute of Fisheries Science, Pukyong National University, 67-91.

5. Silver, G.R., D.A. Higgs, B.S. Dosanjh, B.A. McKeown, G. Deacon and D. French. 1993. Effect of dietary protein to lipid ratio on growth and chemical composition of chinook salmon (Oncorhynchus tshawytscha) in sea water. In Fish Nutrition in Practice (Edited by S.J. Kaushik, and P. Luquet). pp. 459-468. IV^th International Symposium on Fish Nutrition and Feeding, INRA, Paris.