Do delivery rate and pellet size affect growth rate in Atlantic salmon (Salmo salar L.) raised under semi-commercial farming conditions?

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1 Aquaculture 224 (2003) Do delivery rate and pellet size affect growth rate in Atlantic salmon (Salmo salar L.) raised under semi-commercial farming conditions? Jason Bailey a, *, Anders Alanärä a, Viv Crampton b a Department of Aquaculture, Swedish University of Agricultural Sciences (SLU), S Umeå, Sweden b EWOS Innovation, N-4335 Dirdal, Norway Received 18 November 2002; received in revised form 10 February 2003; accepted 12 February 2003 Abstract How the daily feed ration is presented is an important aspect of feed management. In order to evaluate the effects of delivery rate and pellet size, Atlantic salmon (Salmo salar L.) were raised in small ( m), semi-commercial cages and were fed using 14 combinations of pellet sizes ( g) and delivery rates ( gmin 1 ( pelletsfish 1 min 1 )). Treatment groups were compared using the growth rate (Thermal Unit Growth Coefficient, TGC). No significant linear effect on growth due to pellet size or delivery rate could be detected. For pellet size, however, indications point to a non-linear relationship with growth rate. Salmon grew well (mean TGC ( F S.D.) = 2.68 F 0.49) irrespective of treatment indicating a high level of plasticity in the salmon s ability to adjust to changes in pellet size and delivery rates within the ranges used and under the conditions tested here. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Feeding intensity; Feed delivery rate; Pellet size; Feed management; Salmo salar; Atlantic salmon 1. Introduction The number of feed pellets that an individual fish can catch is related to its swimming activity, which is, in turn, partially temperature dependent (Brett, 1969; Alanärä, 1994; Tang and Boisclair, 1995). Bailey and Alanärä (2003) found that rainbow trout (Oncorhynchus * Corresponding author. Tel.: ; fax: address: Jason.Bailey@vabr.slu.se (J. Bailey) /03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 80 J. Bailey et al. / Aquaculture 224 (2003) mykiss (Walbaum)) could catch larger numbers of pellets in a single feed portion as temperatures increased from 5 to 15 jc under laboratory conditions, and they suggested that this change is likely mediated by changes in the fish s activity at different temperatures. Elliott (1975) found that the feeding rate of brown trout (Salmo trutta L.) increased rapidly between 3.8 and 6.8 jc, remaining relatively stable between 6.8 and 19.3 jc, before decreasing at temperatures above 19.3 jc. Changes in the feeding and swimming activity of salmonids at different temperatures has been presented by others (Webb, 1978; Rimmer et al., 1985; Wootton, 1991; Tang and Boisclair, 1995; Simpson et al., 1996) with the greatest change in activity occurring between 5 and 10 jc. In aquaculture, the feed delivery rate should therefore be taken into account when calculating a feeding regime. The size of feed pellets and the rate at which they are delivered may affect the amount of feed an individual fish can ingest over a period of time. Pellets of sub-optimal size or pellets that are delivered at a high rate may cause wastage, as fish may be unable to catch large numbers of pellets before they sink through the net pen. Salmon farmers use many different sizes of feed pellets during the growout period, and each time the size is changed, new calculations for the optimal number of pellets per fish and delivery must be done. It would therefore save both time and money if fewer sizes of pelleted feed can be used for larger parts of the growout period. The relationship between pellet size, feed delivery rate, and growth rate were examined in Atlantic salmon, Salmo salar L., held under small-scale commercial production conditions. 2. Materials and methods The study was conducted at Lønningdal (near Bergen) in Norway during two trials of 147 and 221 days each under ambient temperatures. Control groups were fed using pellet sizes and at delivery rates considered normal (Table 1) for trials at the research station where the work was conducted (EWOS Innovation, Lønningdal, Norway) and will hereafter be referred to as control sizes and rates, respectively. All other groups were fed using pellet weights and delivery rates that were from moderately to extremely different from usual practice (Tables 1 and 2). Automatic, timer-controlled point source feeders in the centre of the cage were used during one meal per day starting in the morning and mealtimes lasted approximately 2 h Table 1 Combinations of delivery rate and pellet size used in the study given as a percentage of the control Pellet size (% of control size) Delivery rate (% of control rate) Numbers in each cell indicate the number of treatment replicates. Pellet sizes ranged from 0.17 to 2.86 g for Trial 1 and between and 1.56 g for Trial 2.

3 J. Bailey et al. / Aquaculture 224 (2003) Table 2 Combinations of delivery rate and pellet size used in the study given as a percentage of the control Pellet size (% of control size) Delivery rate (% of control rate) Numbers in each cell indicate mean delivery rates in relation to the pellet size and number of fish (pelletsfish 1 min 1 ) during the delivery of a single feed portion. Control delivery rate (100%) was 330 gmin 1. Control pellet size (100%) increased during the study as the fish grew in order to keep ratios of pellet size to fish weight relatively constant. The amount of feed in each portion was kept constant in all treatment groups. and 20 min per day. During this meal, pellets were dispensed in the form of 34 portions, each lasting from 4.8 to 122 s. Each portion delivery was followed by a pause for 240 s minus the time required to deliver the portion, giving pauses of between 118 and s between portions from the slowest to the fastest feed delivery rates, respectively. The amount of feed in each portion was the same for all treatments. Mean delivery rates used to deliver the feed are given in Table 2. These rates represent the delivery speed during the periodic distribution of a single feed portion and are not representative of the feeding intensity (see Juell et al., 1994) during the entire meal. Before the experiment started, fish were acclimated to the different pellet sizes by gradually adding the new pellet size to the existing one at 10% per day so that after a period of 10 days, the fish were being fed only the new size pellets. Fish that were being adapted to a pellet two sizes bigger/smaller than the control were first given one size difference closer to the target size (0.5 control if the target size is 0.25 control or 2 control if target size is 4 control) for 10 days (as above), then they were adapted again to the new size after another 10 days. The same procedure was used to acclimatise fish to delivery rates. During the first trial, 16 groups of approximately 250 Atlantic salmon, with an initial mean weight of 1.25 kg, were grown in m net pens and were fed at delivery rates and with pellet sizes as outlined in Tables 1 and 2. Each trial was subdivided into three periods. A random sample of 50 fish was weighed four times during each trial, at the beginning of the trial, at the end of the trial, and on two occasions in-between. During weighing, fish were anaesthetized using 500 mgl 1 ethyl m-aminobenzoate methanesulphonate and were immediately returned to their original groups. The trial began on 02/ 07/98 and ended on 01/12/98. The mean weight of the fish at the end of the trial was approximately 5 kg. Water temperature was recorded every day at 1-m intervals from 1- to 5-m depths, and daily temperatures were recorded as a mean of these five measurements. Starting daily feed rations were calculated using recommendations from the feed manufacturer (EWOS Innovation) for fish of this size and ranged throughout the experiment between approximately 0.7% and 1.3% per day. This was adjusted daily using a liftup system (LiftUp Akva, Eikelandsheiane N-5640) and growth estimations from the software, FarmControl (EWOS Innovation). Fish in the control groups were fed using a feedback loop, and feeding stopped when about 5% of the daily feed appeared in the lift-up

4 82 J. Bailey et al. / Aquaculture 224 (2003) collector. All groups were fed using the ration calculated using this system for the control groups (i.e., Control delivery rates and pellet sizes for fish of this size). In this way, overfeeding was kept below 5%. The second trial was conducted the following year in the same manner as for Trial 1, but for a slightly longer time and with fish of a smaller start weight. Each of the 16 groups contained 450 fish with a starting mean weight of 0.5 kg and a final mean weight of 3.0 kg. The second trial began on 29/01/99 and ended on 07/09/99. Two intermediate weighings were performed on 15/03/99 and 16/06/99. Feed Conversion Ratio (FCR) and Thermal Unit Growth Coefficient (TGC; modified from Cho, 1992) were calculated according to the following formulae: FCR ¼ ðfþ ð1þ ðgþ 2 3 W 1 3 D W 1 3 o TGC ¼! 6 4 X D ð2þ 5 T i i¼1 where F is the amount of feed delivered to a group (g), G is the biomass gain of that group (g), W D is the fish weight (g) after a period of time (D), W o is the initial weight (g), and T is the mean daily temperature (jc). In order to test for any significant effect of trial and treatment on the TGC, general linear model (GLM) regressions analysis were used in a stepwise fashion. In testing for a Fig. 1. Second-order relationship between pellet size and thermal unit growth coefficient (TGC) for Atlantic salmon (S. salar L.) held in net pens. Values are means for both trials.

5 J. Bailey et al. / Aquaculture 224 (2003) Table 3 Percent deviation in TGC from control groups ( c ) for each treatment over the study period Pellet size (% of control size) Delivery rate (% of control rate) c Cells containing negative values indicate groups that grew that percentage slower than the control, while cells containing positive values indicate groups growing faster than the control. Mean TGC ( F S.D.) for the control groups was 2.68 F trial effect, the TGC was used as the dependent variable with trial and treatment as the dependent variables, along with their interaction term. In testing the effect of pellet size and delivery rate, the TGC was used as the dependent variable with pellet size and delivery rate as the independent variables, along with their interaction. All statistics were performed using Systat version 10 (SPSS Science, Chicago, IL, USA). 3. Results No significant effect on the TGC due to period for Year 1 (GLM, p = 0.410) or Year 2 (GLM, p = 0.169) was found. In addition, no significant effect on the TGC due to trial was found (GLM, p = 0.814) so the data from both trials were pooled. Neither pellet size nor delivery rate significantly affected growth rate in Atlantic salmon held in commercial sea Table 4 Mean growth rate (TGC) and feed conversion ratio (FCR) for all treatments Treatment Pellet size (% of control size) Delivery rate (% of control rate) TGC ( F S.D.) FCR ( F S.D.) F F F F F F F F F F F F F F F F F F F F F F F F F F F F 0.22 Pellet sizes and delivery rates are presented as a percentage of the control. Control delivery rate (100%) was 330 gmin 1. Control pellet size (100%) increased during the study as the fish grew in order to keep ratios of pellet size to fish weight relatively constant.

6 84 J. Bailey et al. / Aquaculture 224 (2003) Fig. 2. Average weight of representative treatment and control groups at the beginning and end of each period over both study trials. For clarity, only extreme groups are shown along with the control groups. Each line is a representative treatment combination (i.e., Group 12 (see Table 4) = 0.25R 0.25S = 1/4 control feed delivery rate and 1/4 control pellet size). All groups grew equally well irrespective of treatment. Final mean fish weights between treatments were not significantly different from one another for Trial 1 (ANOVA F = 2.071, p = 0.372) or Trial 2 (ANOVA F = 1.544, p = 0.461). The temperature (jc) over the study period is shown using a dotted line.

7 J. Bailey et al. / Aquaculture 224 (2003) cages and fed using short portions lasting from 4.8 to 122 s. Three-dimensional scatterplots of delivery rate and pellet size against the TGC failed to reveal obvious additive patterns of these variables on the TGC (figures not shown). The interaction term (Delivery Rate Pellet Size) in the multiple regression was not significant (GLM, p = 0.571), and no combined significance of the two variables produced any effects on growth rate (GLM, p = 0.953). Because the interaction term of the multiple regression was not significant, delivery rate and pellet size were examined separately. The significance of delivery rate on the TGC was examined using pellet size as a covariable, and the effect of pellet size on the TGC was examined using delivery rate as a covariable. Neither delivery rate ( p = 0.908) nor pellet size ( p = 0.830) significantly affected the TGC. There was, however, some indication of a small effect of pellet size on the mean TGC for both trials when a second-order plot was drawn of this data (Fig. 1). Percent deviation in growth rate (TGC) of treatment groups from the control is shown in Table 3. Differences from the control were low in all cases. Percent deviation ranged from 10.5% to 11.6%. All groups grew similarly and all had relatively low FCR values irrespective of treatment (Table 4). Final mean fish weights between treatments were not significantly different from one another neither for Trial 1 (ANOVA F = 2.071, p = 0.372) nor Trial 2 (ANOVA F = 1.544, p = 0.461). A plot of average weight for each treatment at the beginning and end of each period revealed slopes that were essentially identical across treatment groups (Fig. 2). 4. Discussion Growth rate of Atlantic salmon raised under semi-commercial conditions in sea cages is not adversely affected by delivery rate within the ranges and under the conditions tested here. Neither delivery rate nor pellet size could be used to predict growth rate in Atlantic salmon over long growout periods. However, there are indications that a non-linear relationship may exist between growth rate and pellet size as illustrated in Fig. 1. In addition, the combination of large pellet sizes and slow delivery rates appear to grow slightly slower than control groups (Table 3), but this difference was not significant. Point source feeders are not widely used on today s commercial farms. However, the authors feel that spreading of feed would have achieved similar results because point source feeding should have enhanced the effects of delivery rates extremes. The results shown here demonstrate the extraordinary ability of Atlantic salmon raised in sea cages to adjust to changes in feed presentation. The ability to adjust to changes in feed presentation has also been shown by Sveier and Lied (1998), where Atlantic salmon grew equally well when fed with meal lengths of between 1 and 22 h per day. In a previous study, Bailey and Alanärä (2003) suggest that rainbow trout fed using very large portion sizes showed poor FCR values when compared to trout fed using smaller portion sizes. Part of the explanation for a lack of significance of delivery rate in the present study may be due to the use of equal portion sizes between treatment groups. At the highest rate of delivery, the time taken to deliver a portion was 4.8 s, followed by a pause of 235 s, whereas, at the lowest delivery rate, the portion was delivered in 122 s, this

8 86 J. Bailey et al. / Aquaculture 224 (2003) time with a 118-s pause. These regimes may therefore have allowed the salmon to catch much of the feed in the water column before the start of the next portion delivery and may indicate that the size of, pause between, and the time required to deliver the portion (which are inherently connected to the delivery rate) may be equally as important as the delivery rate per se. The portion size used in the present study was probably too small to create feed waste at the highest delivery rates. At some extreme, pellet size will obviously have an effect on fish performance, and there are indications of this effect presented here. For example, a pellet that is larger than the gape width of the fish or is so large that handling time becomes a limiting factor in the fish s ability to ingest enough pellets to maintain good growth will clearly have adverse effects. Only a slight and insignificant effect on growth could be seen within the range of sizes tested here, even though the size was 1/4 to 4 times that which was recommended by the industry. Smith et al. (1995) examined fish feeding behaviour at a commercial Atlantic salmon farm in Scotland. In concurrence with Stradmeyer et al. (1988), they found that adult salmon showed a more immediate response to larger pellets but that these were more likely to be rejected than pellets of a shorter length. Smith et al. (1995) also found that pellets slightly smaller than the normal commercial size were eaten at the fastest rate, thereby indicating that the salmon are perhaps adjusting their feeding behaviour to compensate for a smaller feed pellet size. For Arctic charr reared under hatchery conditions, the optimal pellet size has been suggested to be around 2% of the fish s length (Linnér and Brännäs, 1994). Others have suggested that for a range of fish species, the optimal feed size appears to be 25 50% of the mouth width (Wankowski, 1979; Tabachek, 1988). It appears that fish can adjust to reasonable deviations from the optimal pellet size with no significantly negative effects on growth. A weakness of the present study, which may have masked the effects of pellet size and led to results contradictory to previous work, is the long growing periods. Pellet sizes that were very large for a particular fish size at the beginning of a Period may have become only slightly oversized by the end of that same period due to fish growth. Fish reared in commercial aquaculture operations have customarily been fed at a relatively low intensity over prolonged periods (Alanärä et al., 2001). This little-andoften strategy has partly been the result of technical limitations in some of the feeding systems and partly because many farmers think that intense feeding will result in feed waste. In the present study, fish were fed one meal per day of an approximately 2-h duration, divided into 34 portions, with a pause of between 2 and 4 min between each delivery. Such a feeding regime can be considered to be intense, although the inclusion of pauses between deliveries ensured that pellets were not available in the water column during the entire meal, but that each portion contained a distinct pause. Other studies have shown that feeding fish using one to four meals per day under high intensity have resulted in good growth rates in salmonids (Elliott, 1975; Grayton and Beamish, 1977; Jobling, 1983; Juell et al., 1994), and Cho (1990) suggested that one to two meals per day is sufficient for optimal growth in rainbow trout weighing over 200 g. The traditional strategy described above as a little-and-often feeding regime is probably associated with increased swimming activity and energy expenditures (Alanärä, 1992; Johansen and Jobling, 1998), whereas the main positive effect of a large-and-seldom regime is the inclusion of longer non-feeding periods during the rest of the day. The

9 J. Bailey et al. / Aquaculture 224 (2003) growth rates for salmon in the current study may support the idea of one meal per day being sufficient although care should be taken, as this parameter was not tested in the current experiment. However, the TGC for all treatment groups ranged between 2.54 and 2.81, and this can be considered normal for Atlantic salmon raised in sea cages (Alanärä et al., 2001). Assuming, according to the above, that the number or length of meals decrease, the rate at which feed is supplied must increase accordingly. Because salmon have shown to be extremely flexible in adopting their feeding rate to the rate of delivery, the fastest delivery rate that still produces little waste should be chosen in order to fulfil this requirement. In addition, the faster the supply rate the better in avoiding large CV values for growth, because slow rates may allow feed monopolisation by more competitive individuals (Grant, 1993). References Alanärä, A., Demand feeding as a self-regulating feeding system for rainbow trout (Oncorhynchus mykiss) in net-pens. Aquaculture 108, Alanärä, A., The effect of temperature, dietary energy content and reward level on the demand feeding activity of rainbow trout (Oncorhynchus mykiss). Aquaculture 126, Alanärä, A., Kadri, S., Paspatis, M., Feeding management. In: Houlihan, D.F., Boujard, T., Jobling, M. (Eds.), Feed Intake in Fish. Blackwell, Oxford, pp Bailey, J., Alanärä, A., Energy requirements and feeding behaviour of salmonids in culture. Ph.D. Thesis, Paper IV. Bailey, J. and Alanärä, A. Effect of feed portion size on growth of rainbow trout, Oncorhynchus mykiss (Walbaum), reared at different temperatures. Brett, J.R., Temperature and fish. Chesapeake Science 10, Cho, C.Y., Fish nutrition, feeds, and feeding with special emphasis on salmonid aquaculture. Food Reviews International 6, Cho, C.Y., Feeding system for rainbow trout and other salmonids with reference to current estimates of energy and protein requirements. Aquaculture 100, Elliott, J.M., Number of meals in a day, maximum weight of food consumed in a day and maximum rate of feeding for brown trout, Salmo trutta L. Freshwater Biology 5, Grant, J.W.A., Whether or not to defend? The influence of resource distribution. Marine Behaviour and Physiology 23, Grayton, B.D., Beamish, F.W.H., Effects of feeding frequency on food intake, growth and body composition of rainbow trout (Salmo gairdneri). Aquaculture 11, Jobling, M., Effect of feeding frequency on food intake and growth of Arctic charr, Salvelinus alpinus L. Journal of Fish Biology 23, Johansen, S.-J.S., Jobling, M., The influence of feeding regime on growth and slaughter traits of cagereared Atlantic salmon. Aquaculture International 6, Juell, J.E., Bjordal, Å., Fernö, A., Huse, I., Effect of feeding intensity on food intake and growth of Atlantic salmon, Salmo salar L., in sea cages. Aquaculture and Fisheries Management 25, Linnér, J., Brännäs, E., Behavioral response to commercial food of different sizes and self-initiated food size selection by Arctic char. Transactions of the American Fisheries Society 123, Rimmer, D.M., Saunders, R.L., Paim, U., Effects of temperature and season on the position holding performance of juvenile Atlantic salmon (Salmo salar). Canadian Journal of Zoology 63, Simpson, A.L., Metcalfe, N.B., Huntingford, F.A., Thorpe, J.E., Pronounced seasonal differences in appetite in Atlantic salmon parr, Salmo salar: effects of nutritional state and life history strategy. Functional Ecology 10, Smith, I.P., Metcalfe, N.B., Huntingford, F.A., The effect of pellet size dimensions on feeding responses by Atlantic salmon (Salmo salar L.) in a marine net pen. Aquaculture 130,

10 88 J. Bailey et al. / Aquaculture 224 (2003) Stradmeyer, L., Metcalf, N.B., Thorpe, J.E., Effect of food pellet shape and texture on the feeding response of juvenile Atlantic salmon. Aquaculture 73, Sveier, H., Lied, E., The effect of feeding regime on growth, feed utilisation and weight dispersion in large Atlantic salmon (Salmo salar) reared in seawater. Aquaculture 165, Tabachek, J.L., The effect of feed particle size on the growth and feed efficiency of Arctic charr (Salvelinus alpinus L). Aquaculture 71, Tang, M., Boisclair, D., Relationship between respiration rate of juvenile brook trout (Salvelinus fontinalis), water temperature, and swimming characteristics. Canadian Journal of Fisheries and Aquatic Science 52, Wankowski, J.W.J., Morphological limitations, prey size selectivity, and growth response of juvenile Atlantic salmon, Salmo salar. Journal of Fish Biology 14, Webb, P.W., Temperature effects on acceleration of rainbow trout, Salmo gairdneri. Journal of the Fisheries Research Board of Canada 35, Wootton, R.J., Feeding. In: Wootton, R.J. (Ed.), Ecology of Teleost Fishes. Chapman & Hall, London, pp