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A simple method for the measurement of daily feed intake of groups of fish in tanks
Aquaculture Aquaculture 139 (1996) 157-163
Technical paper
A simple method for the measurement of daily feed intake of groups of fish in tanks S. J. Helland *, B. Grisdale-Helland, AKVAFORSK,
Institute
S. Nerland
of Aquaculture Research Ltd., 6600 Sunndals@ra, Norway Accepted 2 August 1995
Abstract A simple and inexpensive method for determining daily feed intake of groups of fish in tanks is described. The method is based on the collection of waste feed from the effluent water and consists of an effective drainage system and a wire mesh collector. This technique is dependent upon the use of feed with good physical stability. The daily feed intake of fish in a tank is calculated by the difference between the amount fed and the amount of waste feed collected (corrected for leaching losses). The system can be combined with any type of feeding method in which sinking pellets are used. It allows accurate intake measurements for research purposes or can be used in commercial operations for adjusting feeding level without knowledge about biomass in the tank or water temperature. Use of the method has revealed large variations in daily feed intake of groups of Atlantic salmon fed continuously. Keywords:
Feeding and nutrition-fish;
Feed intake; Techniques-feeding
and nutrition; Fish
1. Introduction
In growth trials, it is recommended that fish be fed ad libitum or to apparent satiation (Gropp and Tacon, 1994), so that all fish in the group have adequate access to feed. However, these feeding techniques result in waste feed. Demand feeders have been tested as a means of allowing the fish to self-regulate feed availability, but this method requires an adaptation period (Boujard and Leatherland, 1992), can also result in waste feed (Alanat%, 1992) and a dominance hierarchy can be established which may differentially affect feed intake of the individuals in the group (Brannk and Alar&$ 1993). Computer controlled feeders which respond to the presence of waste feed (Juell, 1991; Blyth and Purser, * Corresponding author. Tel.: 47-716 92557; fax: 47-716 90292 0044.8486/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDlOO44-8486(95)01145-5
158
S.J. Helland et al. /Aquaculture
139 (1996) 157-163
1993) help to reduce the problem of waste, but are affected by the trade-off between the amount of waste feed allowed and the possibility of restricting the feeding level. When feeding in excess, feed intake can be estimated through measurements of the gut content of individual fish (directly or indirectly) or measurement of the amount of waste feed. The former technique consists of the use of feed labeled with isotopes (Storebakken et al., 198 I ) or X-ray opaque particles (Talbot and Higgins, 1983) and is useful for periodic measurements of feed intake and feeding behavior of individuals during a trial (Jobling, 1993). A disadvantage of this technique is that the fish are disturbed when measurements are taken. In trials where the objective is optimal growth, measurements of the total intake of the fish in a tank can be done using a combination of accurate automatic feeders and daily measurements of waste feed from each tank. This paper describes a simple and inexpensive technique for the collection of waste feed from tanks. This technique can be combined with any feeding method using sinking pellets to calculate feed intake of groups of fish.
2. Materials and methods The waste feed collection system consists of an effective drainage system and a wire mesh collector, and is dependent upon the use of feed with good physical stability. 2.1. Drainage system Effective removal of particles from the tank is necessary for this system to function. This can be controlled by the shape of the tank itself or through manipulation of the water current. A circular tank with a conical bottom gives excellent removal. In flat-bottomed tanks, removal of particles can be improved (Tvinnereim, 1990) by adjusting the speed of the current by manipulation of the water flow or the area of the openings in the water inlet pipe (I, Fig. 1) . The design of the effluent water pipe system is also important. Effective movement of particles through the system is dependent on adequate water speed and the maintenance of laminar flow. Turbulence is created when water is forced through a 90” bend and particles can be trapped behind the bend. It is necessary therefore, to replace these with bends that are less sharp. Flexible hose can also be used instead of pipes with sharp bends. Frequently, drains consisting of a rectangular depression (outlet sink) are used in fish farming. The sink is covered by a grid in the middle of the tank and a drainage pipe is attached to the side of the sink. This traditional drainage construction does not perform well. The surface area of the sink is too large and insufficient suction is generated by the drainage pipe. The result is poor removal of particles and a build-up of fungal growth in and above the outlet sink. This system can, at a low cost, be replaced by a PVC plate attached flush with the bottom of the tank. A drainage hole is cut in the middle of the plate and the effluent pipe is attached. 2.2. Waste feed collector The waste feed collector (C, Fig. 1) is a box made from stainless steel wire mesh. It is placed in a holder equipped with drainage, such as a funnel, and held in place using wire
S.J. Helland et al. /AquacuIture 139 (1996) 157-163
159
Fig. I. Tank and waste feed collection system showing water inlet pipe (I), pinch valve (P) connected to one of the two effluent water pipes, waste feed collector (C) and drainage funnel (F).
hooks. The size of the collector depends upon the biomass in the tank. The optimum mesh size depends upon the size of the feed pellets and the water flow. The mesh should allow good drainage of water and prevent the waste feed from being washed away. Mesh sizes used for different pellet sizes are given in Table 1. Movement ofpellets on the collector is affected by the amount and size of the pellets and the amount of feces present. Incomplete movement of the pellets out of the effluent water stream results in leaching of some waste feed which is accounted for by using a recovery test (see below >. 2.3. Physical quality of pellets The physical quality of the pellets has to be good enough to withstand the mechanical stress caused by the flow of water over them and the procedure used for separation of the Table I Measurements
and mesh sizes of waste feed collectors Water fIow(l rnin-‘)
~W&
Collector size (cm) Length
Width
Height
16 27
13 13
4 7
Mesh threads ( mm)
Mesh openings ( mm
0.45 0.70
0.90 1.84 3.23
&lFF-lUSS {kh’i
S-20 Pellet size (mm) I.5 2.5-4 >4
5 IO
1.oo
)
160
S..I. Helland et al. /Aquaculture
139 (1996) 157-163
waste feed and feces. Although objective methods for quantifying pellet stability are available (e.g. Melcion et al., 199 1) , this is best evaluated in situ by placing a sample of pellets on the collector under the conditions of the experiment (standardized water flow and temperature). A check of the stability of the pellets at certain times will indicate how often collections of waste feed must be made. With stable pellets, collections of waste feed can be done once per day. Pellets with only moderate stability need to be collected several times per day when the fish are fed continuously. It may also be advantageous to feed meals (using automatic feeders) and collect the waste feed shortly afterwards. This will decrease the time that the waste pellets are subjected to the water stream and reduce the amount of contamination of the pellets with feces which subsequently need to be removed. The collection of pellets with only moderate stability may also be aided by diverting the water stream from the pellets in periods between feedings. This requires the use of two outlet pipes attached with a Y joint. A pinch valve (P, Fig. I) which is controlled by air pressure and maintains laminar flow, is attached to one pipe. The valve is electronically controlled to close when the fish are not being fed. This directs the effluent water to a different spot on the collector, resulting in separation of most of the feces from the waste feed and less leaching of the pellets. 2.4. Measurements
of waste feed
Two methods for measuring waste feed are available. The first technique involves weighing the waste feed, which is then corrected for loss of dry matter. The second technique consists of counting waste pellets and calculating intake based on average pellet weight. 2.4.1. Procedure for weighing waste feed At approximately the same time each day measurements of waste feed are made. The material collected on the mesh consists of a mixture of feed and feces. The mixture is held under the water stream and is carefully moved about to wash away feces. Alternatively, the pellets can be physically separated from feces using a tool such as a metal spatula. The waste pellets are then quantitatively weighed and pooled and frozen in separate containers for each tank. At the end of the experiment, the waste feed for each tank is analyzed for dry matter content (drying at 105°C overnight}. After correcting for percentage recovery of dry matter of waste feed (see below), the total weight of feed eaten by the fish during the trial can be calculated by difference. Daily approximations of feed intake can be done using an estimate for dry matter content of the waste feed (obtained in the recovery test, see below). Air - dry feed eaten (g) =
(A XA,,I
100) - ( Wx W,&R) AD,/ 100
where A is weight of air-dry feed (g), ADM is dry matter content of air-dry feed ( %) , W is weight of waste feed collected (g), W,, is dry matter content of waste feed (%), and R is recovery of dry matter of waste feed ( %) (see below) _ Each feed must be tested for recovery of dry matter under the environmental conditions (standardized water temperature, flow rate and current speed) to be used in the experiment.
S.J. Hellandet
al. /Aquaculture
139 (1996) 157-163
161
A sample of air-dry feed is accurately weighed and placed on the feeder of a tank without fish. A feed sample is saved for dry matter analysis. The feeder should be set to distribute the feed over the time that will be used in the experiment (e.g. 20 h feeding per day). After 24 h, the waste pellets are quantitatively removed from the collector, duplicating as much as possible the procedure used under normal collections; the waste pellets are then weighed accurately. A check should be made of the tank and the feeder to ensure that all feed has come through the system. Samples of the air-dry and waste feeds are then analyzed for dry matter content and percentage recovery of dry matter calculated as follows Recovery
(%) = 100~
WX W,, AX&W
where symbols are as defined above. 2.4.2. Counting waste pellets This technique consists simply of counting the number of waste pellets collected and calculating intake based on average pellet weight. It is assumed that the fish eat the pellets independent of any variation in pellet size. However, it is important that pellet size is uniform to minimize the error arising from use of average weight. 2.5. Method illustration The use of the waste feed collection system is illustrated using data from three replicate tanks of Atlantic salmon obtained during a 20 day growth trial. The mean start weight of the fish was 80 g and the tank biomass was 2 kg. The water temperature was 9°C. The fish were fed using automatic feeders which supplied feed every 5 min during the 20 h light period. Feed waste was collected once per day and weighed. Daily feed intake was approximated every third day and the feeding level was adjusted to allow lO-15% feed waste. All tanks were fed at the same level. After determining the dry matter content of the waste feed at the end of the trial, daily feed intake of the fish in each tank was recalculated.
3. Results and discussion The daily feed intake of the three tanks of Atlantic salmon is shown in Fig. 2. The actual percentage daily waste ranged from 1 to 44%, with a mean of 12%. Daily feed intake in individual tanks varied by up to 58% between subsequent days (mean + SE absolute difference between days, 13 + 2%). It is likely that intake was restricted from days 8 to 14 by the feeding lever, as indicated by the large increase in feed intake when the feeding level was incrkased to 35 g. During this period, the mean percentage overfeeding was 9%. These data indicate that with this feeding regime, approximately 15% feed waste is necessary to maintain maximum intake. These data also show that there is less day-to-day variation in feed intake when the feeding level is slightly restricted. Because of the large day-to-day variation, data from measurements over several days should be used before adjusting feeding level.
162
S.J. Helland et al. /Aquaculture
139 (1996) I-57-163
35
30
25
20
15
0
.
2
tank 1
4
6
8
+ tank2
10
12
Days on test * tank3
14
16
18
20
--- - feeding level
Fig. 2. Feeding level and daily feed intake of three replicate tanks of Atlantic salmon.
The advent of extruded feeds with high stability in water makes this system of waste feed collection simple and inexpensive. However, because the quality of extruded pellets varies with composition and processing parameters (Kazamzadeh, 1990; Oliveira et al., 1992)) a recovery test must be done for each feed. In the past 5 years, this waste feed collection technique has been used with over 50 different extruded feeds with good results. Using the parameters listed in Table 1, recovery of 60-85% of the dry matter of the feed can be expected during 24 h collections. It has not been possible to use this method accurately with cold-pelleted feeds. With this technique, the amount of feed eaten by a group of fish is estimated by the difference between the amount fed and corrected measurements of waste feed. An overestimation of the amount of waste feed, such as will occur if feces are included in the waste which is weighed, will result in an underestimation of the amount of feed eaten. Similarly, the amount ofwaste feed will be underestimated if excess amounts of waste feed are washed away during removal of feces. It is important therefore, that the procedures used are duplicated as much as possible in the recovery test. The magnitude of error in the determination of feed intake is dependent upon the level of overfeeding and the percentage recovery of waste feed dry matter. Overfeeding by 15% for example, will result in a 15% error in the estimation of feed intake if no waste feed is collected. Recovery of 80% of the waste feed (assuming no correction for dry matter loss), will reduce the error to 3%. If the recovery of dry matter is taken into account, but there is a 10% error in the actual amount collected, then the error in estimated intake will be as low
S.J. Helland erai. /Aquaculture
139 (1996) 157-163
163
as 1.5%. This calculation demonstrates that with moderate overfeeding, the influence of an error in the recovery of waste feed is marginal. The system can be combined with any type of feeding method in which sinking pellets are used. The technique allows accurate intake measurements for research purposes or can be used in commercial operations for adjusting feeding level. This is of particular interest when the biomass in the tank is unknown and when the water temperature is not constant. Daily observations of waste feed enable determinations of the appetite of the fish without disturbing the fish and corrections can be made to the feeding level. In practical farming, it should only be necessary to monitor a few tanks. However, as a tool for observation of the health status of individual tanks of fish, collection from each tank is necessary.
References Alan&& A., 1992. Demand feeding as a self-regulating feeding system for rainbow trout (Oncor+chus mykiss) in net-pens. Aquaculture, 108: 347-356. Blyth, P.J. and Purser, G.J., 1993. Detection of feeding rhythms in sea caged Atlantic salmon using new feeder technology. In: H. Reinertsen, L.A. Dahle, L. Jorgensen and K. Tvinnereim (Editors), Fish Farming Technology. Balkema, Rotterdam, pp. 209-216. Boujard, T. and Leatherland. J-F., 1992. Demand-feeding behaviour and die1 pattern of feeding activity in Oncot-&n&us mykiss held under different photoperiod regimes. J. Fish Biol., 40: 535-544. Brj_nnBs, E. and Alar&i, A., 1993. Monitoring the feeding activity of individual fish with a demand feeding system. J. Fish Biol., 42: 209-215. Gropp, J.M. and Tacon, A.G.J. (Editors), 1994. Report of the EIFAC Workshop on Methodology for Determination of Nutrient Requirements in Fish, Eichenau, Germany, 29 June-l July 1993. EIFAC Occas. Pap. No. 29. FAO. Rome, 92 pp. Jobling, M.. 1993. Bioenergetics: feed intake and energy partitioning. In: J.C. Rankin and F.B. Jensen (Editors), Fish Ecophysiology. Chapman and Hall, London, pp. 14. Juell, J.-E., 1991. Hydroacoustic detection of food waste-A method to estimate maximum food intake of fish populations in sea cages. Aquacult. Eng., IO: 207-217. Kazamzadeh, M., 1990. Fish feed extrusion technology: The case for the twin screw. Feed International, February 1990: 22-25. Melcion, J.-P., Biboulot, B. and Le Coz, Y., 1991. Mesure de la stabilite a I’eau. Rev. Alimentation, 451: 40-42. Oliveira, M.A., MBller-Hoist, S.. H&and, H. and Rosenlund, G., 1992. The effects of process parameters on expansion of extruded fish feeds. In: J.L. Kokini, C.-T. Hoand M.V. Karwe (Editors), FoodExtrusion Science and Technology. Marcel Dekker, New York, pp. 669-676. Storebakken; T., Austreng, E. and Steenberg, K., 1981. A method for determination of feed intake in salmonids using radioactive isotopes. Aquaculture, 24: 133-142. Talbot, C. and Higgins, P.J., 1983. A radiographic method for feeding studies on fish using metallic iron powder as a marker. J. Fish Biol., 23: 21 I-220. Tvinnereim. K.. 1990. Hydraulisk utforming og drift av lukkede oppdrettsenheter for laksefisk. Rep. STF60 A90044_ Norwegian Hydrotechnicat Laboratory, Trondheim, 57 pp.
Technical paper
A simple method for the measurement of daily feed intake of groups of fish in tanks S. J. Helland *, B. Grisdale-Helland, AKVAFORSK,
Institute
S. Nerland
of Aquaculture Research Ltd., 6600 Sunndals@ra, Norway Accepted 2 August 1995
Abstract A simple and inexpensive method for determining daily feed intake of groups of fish in tanks is described. The method is based on the collection of waste feed from the effluent water and consists of an effective drainage system and a wire mesh collector. This technique is dependent upon the use of feed with good physical stability. The daily feed intake of fish in a tank is calculated by the difference between the amount fed and the amount of waste feed collected (corrected for leaching losses). The system can be combined with any type of feeding method in which sinking pellets are used. It allows accurate intake measurements for research purposes or can be used in commercial operations for adjusting feeding level without knowledge about biomass in the tank or water temperature. Use of the method has revealed large variations in daily feed intake of groups of Atlantic salmon fed continuously. Keywords:
Feeding and nutrition-fish;
Feed intake; Techniques-feeding
and nutrition; Fish
1. Introduction
In growth trials, it is recommended that fish be fed ad libitum or to apparent satiation (Gropp and Tacon, 1994), so that all fish in the group have adequate access to feed. However, these feeding techniques result in waste feed. Demand feeders have been tested as a means of allowing the fish to self-regulate feed availability, but this method requires an adaptation period (Boujard and Leatherland, 1992), can also result in waste feed (Alanat%, 1992) and a dominance hierarchy can be established which may differentially affect feed intake of the individuals in the group (Brannk and Alar&$ 1993). Computer controlled feeders which respond to the presence of waste feed (Juell, 1991; Blyth and Purser, * Corresponding author. Tel.: 47-716 92557; fax: 47-716 90292 0044.8486/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDlOO44-8486(95)01145-5
158
S.J. Helland et al. /Aquaculture
139 (1996) 157-163
1993) help to reduce the problem of waste, but are affected by the trade-off between the amount of waste feed allowed and the possibility of restricting the feeding level. When feeding in excess, feed intake can be estimated through measurements of the gut content of individual fish (directly or indirectly) or measurement of the amount of waste feed. The former technique consists of the use of feed labeled with isotopes (Storebakken et al., 198 I ) or X-ray opaque particles (Talbot and Higgins, 1983) and is useful for periodic measurements of feed intake and feeding behavior of individuals during a trial (Jobling, 1993). A disadvantage of this technique is that the fish are disturbed when measurements are taken. In trials where the objective is optimal growth, measurements of the total intake of the fish in a tank can be done using a combination of accurate automatic feeders and daily measurements of waste feed from each tank. This paper describes a simple and inexpensive technique for the collection of waste feed from tanks. This technique can be combined with any feeding method using sinking pellets to calculate feed intake of groups of fish.
2. Materials and methods The waste feed collection system consists of an effective drainage system and a wire mesh collector, and is dependent upon the use of feed with good physical stability. 2.1. Drainage system Effective removal of particles from the tank is necessary for this system to function. This can be controlled by the shape of the tank itself or through manipulation of the water current. A circular tank with a conical bottom gives excellent removal. In flat-bottomed tanks, removal of particles can be improved (Tvinnereim, 1990) by adjusting the speed of the current by manipulation of the water flow or the area of the openings in the water inlet pipe (I, Fig. 1) . The design of the effluent water pipe system is also important. Effective movement of particles through the system is dependent on adequate water speed and the maintenance of laminar flow. Turbulence is created when water is forced through a 90” bend and particles can be trapped behind the bend. It is necessary therefore, to replace these with bends that are less sharp. Flexible hose can also be used instead of pipes with sharp bends. Frequently, drains consisting of a rectangular depression (outlet sink) are used in fish farming. The sink is covered by a grid in the middle of the tank and a drainage pipe is attached to the side of the sink. This traditional drainage construction does not perform well. The surface area of the sink is too large and insufficient suction is generated by the drainage pipe. The result is poor removal of particles and a build-up of fungal growth in and above the outlet sink. This system can, at a low cost, be replaced by a PVC plate attached flush with the bottom of the tank. A drainage hole is cut in the middle of the plate and the effluent pipe is attached. 2.2. Waste feed collector The waste feed collector (C, Fig. 1) is a box made from stainless steel wire mesh. It is placed in a holder equipped with drainage, such as a funnel, and held in place using wire
S.J. Helland et al. /AquacuIture 139 (1996) 157-163
159
Fig. I. Tank and waste feed collection system showing water inlet pipe (I), pinch valve (P) connected to one of the two effluent water pipes, waste feed collector (C) and drainage funnel (F).
hooks. The size of the collector depends upon the biomass in the tank. The optimum mesh size depends upon the size of the feed pellets and the water flow. The mesh should allow good drainage of water and prevent the waste feed from being washed away. Mesh sizes used for different pellet sizes are given in Table 1. Movement ofpellets on the collector is affected by the amount and size of the pellets and the amount of feces present. Incomplete movement of the pellets out of the effluent water stream results in leaching of some waste feed which is accounted for by using a recovery test (see below >. 2.3. Physical quality of pellets The physical quality of the pellets has to be good enough to withstand the mechanical stress caused by the flow of water over them and the procedure used for separation of the Table I Measurements
and mesh sizes of waste feed collectors Water fIow(l rnin-‘)
~W&
Collector size (cm) Length
Width
Height
16 27
13 13
4 7
Mesh threads ( mm)
Mesh openings ( mm
0.45 0.70
0.90 1.84 3.23
&lFF-lUSS {kh’i
S-20 Pellet size (mm) I.5 2.5-4 >4
5 IO
1.oo
)
160
S..I. Helland et al. /Aquaculture
139 (1996) 157-163
waste feed and feces. Although objective methods for quantifying pellet stability are available (e.g. Melcion et al., 199 1) , this is best evaluated in situ by placing a sample of pellets on the collector under the conditions of the experiment (standardized water flow and temperature). A check of the stability of the pellets at certain times will indicate how often collections of waste feed must be made. With stable pellets, collections of waste feed can be done once per day. Pellets with only moderate stability need to be collected several times per day when the fish are fed continuously. It may also be advantageous to feed meals (using automatic feeders) and collect the waste feed shortly afterwards. This will decrease the time that the waste pellets are subjected to the water stream and reduce the amount of contamination of the pellets with feces which subsequently need to be removed. The collection of pellets with only moderate stability may also be aided by diverting the water stream from the pellets in periods between feedings. This requires the use of two outlet pipes attached with a Y joint. A pinch valve (P, Fig. I) which is controlled by air pressure and maintains laminar flow, is attached to one pipe. The valve is electronically controlled to close when the fish are not being fed. This directs the effluent water to a different spot on the collector, resulting in separation of most of the feces from the waste feed and less leaching of the pellets. 2.4. Measurements
of waste feed
Two methods for measuring waste feed are available. The first technique involves weighing the waste feed, which is then corrected for loss of dry matter. The second technique consists of counting waste pellets and calculating intake based on average pellet weight. 2.4.1. Procedure for weighing waste feed At approximately the same time each day measurements of waste feed are made. The material collected on the mesh consists of a mixture of feed and feces. The mixture is held under the water stream and is carefully moved about to wash away feces. Alternatively, the pellets can be physically separated from feces using a tool such as a metal spatula. The waste pellets are then quantitatively weighed and pooled and frozen in separate containers for each tank. At the end of the experiment, the waste feed for each tank is analyzed for dry matter content (drying at 105°C overnight}. After correcting for percentage recovery of dry matter of waste feed (see below), the total weight of feed eaten by the fish during the trial can be calculated by difference. Daily approximations of feed intake can be done using an estimate for dry matter content of the waste feed (obtained in the recovery test, see below). Air - dry feed eaten (g) =
(A XA,,I
100) - ( Wx W,&R) AD,/ 100
where A is weight of air-dry feed (g), ADM is dry matter content of air-dry feed ( %) , W is weight of waste feed collected (g), W,, is dry matter content of waste feed (%), and R is recovery of dry matter of waste feed ( %) (see below) _ Each feed must be tested for recovery of dry matter under the environmental conditions (standardized water temperature, flow rate and current speed) to be used in the experiment.
S.J. Hellandet
al. /Aquaculture
139 (1996) 157-163
161
A sample of air-dry feed is accurately weighed and placed on the feeder of a tank without fish. A feed sample is saved for dry matter analysis. The feeder should be set to distribute the feed over the time that will be used in the experiment (e.g. 20 h feeding per day). After 24 h, the waste pellets are quantitatively removed from the collector, duplicating as much as possible the procedure used under normal collections; the waste pellets are then weighed accurately. A check should be made of the tank and the feeder to ensure that all feed has come through the system. Samples of the air-dry and waste feeds are then analyzed for dry matter content and percentage recovery of dry matter calculated as follows Recovery
(%) = 100~
WX W,, AX&W
where symbols are as defined above. 2.4.2. Counting waste pellets This technique consists simply of counting the number of waste pellets collected and calculating intake based on average pellet weight. It is assumed that the fish eat the pellets independent of any variation in pellet size. However, it is important that pellet size is uniform to minimize the error arising from use of average weight. 2.5. Method illustration The use of the waste feed collection system is illustrated using data from three replicate tanks of Atlantic salmon obtained during a 20 day growth trial. The mean start weight of the fish was 80 g and the tank biomass was 2 kg. The water temperature was 9°C. The fish were fed using automatic feeders which supplied feed every 5 min during the 20 h light period. Feed waste was collected once per day and weighed. Daily feed intake was approximated every third day and the feeding level was adjusted to allow lO-15% feed waste. All tanks were fed at the same level. After determining the dry matter content of the waste feed at the end of the trial, daily feed intake of the fish in each tank was recalculated.
3. Results and discussion The daily feed intake of the three tanks of Atlantic salmon is shown in Fig. 2. The actual percentage daily waste ranged from 1 to 44%, with a mean of 12%. Daily feed intake in individual tanks varied by up to 58% between subsequent days (mean + SE absolute difference between days, 13 + 2%). It is likely that intake was restricted from days 8 to 14 by the feeding lever, as indicated by the large increase in feed intake when the feeding level was incrkased to 35 g. During this period, the mean percentage overfeeding was 9%. These data indicate that with this feeding regime, approximately 15% feed waste is necessary to maintain maximum intake. These data also show that there is less day-to-day variation in feed intake when the feeding level is slightly restricted. Because of the large day-to-day variation, data from measurements over several days should be used before adjusting feeding level.
162
S.J. Helland et al. /Aquaculture
139 (1996) I-57-163
35
30
25
20
15
0
.
2
tank 1
4
6
8
+ tank2
10
12
Days on test * tank3
14
16
18
20
--- - feeding level
Fig. 2. Feeding level and daily feed intake of three replicate tanks of Atlantic salmon.
The advent of extruded feeds with high stability in water makes this system of waste feed collection simple and inexpensive. However, because the quality of extruded pellets varies with composition and processing parameters (Kazamzadeh, 1990; Oliveira et al., 1992)) a recovery test must be done for each feed. In the past 5 years, this waste feed collection technique has been used with over 50 different extruded feeds with good results. Using the parameters listed in Table 1, recovery of 60-85% of the dry matter of the feed can be expected during 24 h collections. It has not been possible to use this method accurately with cold-pelleted feeds. With this technique, the amount of feed eaten by a group of fish is estimated by the difference between the amount fed and corrected measurements of waste feed. An overestimation of the amount of waste feed, such as will occur if feces are included in the waste which is weighed, will result in an underestimation of the amount of feed eaten. Similarly, the amount ofwaste feed will be underestimated if excess amounts of waste feed are washed away during removal of feces. It is important therefore, that the procedures used are duplicated as much as possible in the recovery test. The magnitude of error in the determination of feed intake is dependent upon the level of overfeeding and the percentage recovery of waste feed dry matter. Overfeeding by 15% for example, will result in a 15% error in the estimation of feed intake if no waste feed is collected. Recovery of 80% of the waste feed (assuming no correction for dry matter loss), will reduce the error to 3%. If the recovery of dry matter is taken into account, but there is a 10% error in the actual amount collected, then the error in estimated intake will be as low
S.J. Helland erai. /Aquaculture
139 (1996) 157-163
163
as 1.5%. This calculation demonstrates that with moderate overfeeding, the influence of an error in the recovery of waste feed is marginal. The system can be combined with any type of feeding method in which sinking pellets are used. The technique allows accurate intake measurements for research purposes or can be used in commercial operations for adjusting feeding level. This is of particular interest when the biomass in the tank is unknown and when the water temperature is not constant. Daily observations of waste feed enable determinations of the appetite of the fish without disturbing the fish and corrections can be made to the feeding level. In practical farming, it should only be necessary to monitor a few tanks. However, as a tool for observation of the health status of individual tanks of fish, collection from each tank is necessary.
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