- Email: [email protected]
Silage production from salmon farm mortalities
Aquacultural Engineering 12 (1993) 37-45
Silage Production from S a l m o n Farm Mortalities K. V. Lo, P. H. Liao, C. Bullock Bio-Resource Engineering Department, University of British Columbia. 2357 Main Mall, Vancouver,British Columbia, Canada V6T 1Z4
& Y. Jones National Research Council, 3650 Wesbrook Mall, Vancouver, British Columbia, Canada, V6S 2L2
(Received 18 April 1991; revised version received 21 September 1991; accepted 3 September 1992) ABSTRACT Fish silage was made from salmon farm mortalities. Citric, formic and propionic acid, as well as combinations of these acids, were used as ensiling media. Stability, liquefaction rate and soluble nitrogen contents were studied. The silages made with 2 or 3% (w/w) of citric acid alone were unable to achieve stability, while the pH increased from 3"8 to 4"5 by day 30for the 4.5% (w/w) citric acid silage. The pH for the other acids and for the acid combinations was very stable. All of the silages liquified quickly for the first ten days, and then liquefaction &veiled off. The amount of soluble nitrogen increased for all the silages, as did the percent liquid. The results indicated that salmon farm mortalities could be used as the raw material in the production offish silage and a marketable liquid fertilizer.
INTRODUCTION Salmon farms in British Columbia currently have problems disposing of their fish mortalities. In this paper it is p r o p o s e d that the production of fish silage would serve as a good solution to their mort disposal problem. Fish silage is a liquid product derived from the mixing of whole fish, or c o m p o n e n t s of fish, with an acid. T h e acid allows the enzymes from 37 Aquacultural Engineering 0144-8609/93/S06.00 - © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
K. V. Lo, P. H. Liao, C Bullock, Y. Jones
the gut to liquify the mixture while restricting the growth of spoilage bacteria (Tatterson & Windsor, 1974). The use of different kinds of acids in the ensiling process could potentially affect the rate of liquefaction of the fish silage, the stability of the silage and the soluble nitrogen content of the silage. The stability of silage is determined by its tendency to remain below a certain pH value. Studies have shown that with the use of inorganic acids, the pH should remain below 2 (Wood, 1980). With the use of organic acids, the silage should remain stable at pH 3.5-4.0 with formic acid, and pH 4.5 with propionic acid (Gildberg & Raa, 1977). The organic acids are preferred for ensiling, since they retain their antimicrobial properties at a relatively high pH. The products resulting from ensiling would therefore not require neutralization prior to animal feeding. For the same reason, organic acids would reduce problems of corrosion of equipment and containers, and are therefore preferable. Formic acid, or a mixture of formic and mineral acids, are most commonly used in the commercial production of fish silage. However, citric acid is commonly used in the food and beverage industry to adjust the pH and enhance antioxidant additives, since it is cheaper to use than formic acid. This research therefore made the citric acid ensiling process its major focus. The use of citric acid and mixtures of citric acid, formic acid and propionic acids were examined as ensiling media for salmon mortalities.
MATERIALS AND METHODS Raw materials and the production of sUages Silage was made from salmon farm mortalities of approximately 2 kg each that had been decomposing from one to four days and then frozen at -20°C. In the ensiling process, fish is usually minced to allow complete penetration of the acid. After thawing, the whole carcasses were therefore passed through an auger-type meat grinder with disc holes of 4 mm and mixed so as to establish an homogeneous mince. Two experimental trials were conducted. In the first trial, five different silages were made in triplicate of 500 g minced fish with (i) 3% (v/w) formic acid, (ii) 1.5% (w/w) citric acid/1.5% (v/w) formic acid, (iii) 1.5% (v/w) formic acid/i-5% (v/w) propionic acid, (iv) 2% (w/w) citric acid, and (v) 3% (w/w) citric acid. In the second trial, six different silages were made: the sets (i), (ii) and (iii) of the first trial were repeated, two sets of 4.5% (w/w) citric acid and one set of 1% (w/w) citric acid/l% (v/w)
Silage Productionfrom Salmon Farm Mortalities
39
formic acid were added. The silages were stored at room temperature (22°C) in covered glass beakers.
Analyses Moisture content, ash content and pH were determined according to the standard methods (APHA, 1985). Oil content was measured by extracting the oil from oven-dried samples by Soxhlet using diethyl ether (Tatterson & Windsor, 1974). Total nitrogen was determined by using a block digester and a Technicon Auto Analyzer II as described by Schumann et al. (1973). Soluble nitrogen was determined by adding 10 ml of 20% trichloroacetic acid to 5 g of silage and mixing for 2 min. The mixture was filtered and the filtrate was analyzed as described above. Liquefaction was measured by the percentage of liquid phase after centrifuging. Ten-gram samples of silage were centrifuged at 3000 rpm for 20 min. The liquid portion and remaining mass were then determined. Data was statistically analyzed using SYSTAT (SYSTAT, 1989). All data were subjected to a multivariate analysis of variance and the differences among the various acid treatments were analyzed using multiple comparisons.
RESULTS AND DISCUSSION
Stability The stability of the silage was determined by measuring the silage's capacity to remain below a certain pH value. In the first trial, the silages made with 2 or 3% (w/w) of citric acid alone were unable to achieve stability. The pH of these citric acid silages increased immediately after the ensiling process (Fig. 1). The pH of the 2% (w/w) citric acid silage not only had greater increases in pH, but also remained higher than that of the 3% (w/w) citric acid silage by day 30. The silages using 3% (v/w) formic acid, a mixture of formic acid/citric acid and a mixture of formic acid/propionic acid maintained stability (Fig. 2). The 3% (v/w) formic acid silage gave the lowest pH value. Based on the results from the first run, it was thought citric acid could be used for ensiling without the addition of other acids. However, in order to achieve stability, a larger volume of citric acid alone would be needed than if it were mixed with other acids. Tests were therefore conducted on two sets of 4.5% (w/w) of citric acid and one set of 1%
40
K. V. Lo, P. H. Liao, C. Bullock, Y. Jones 8 7
j _ . . ~ x
6 5 k--e-'---qt---~.
~
o
G
-
o
It
3 • 1.5~ Citric/Formic o 1.5~. Propionic/Formlc
* 3 ~ Form;c 2 ~ Citric 3~, Citric
2 1
0 0
I
I
I
I
1
l
5
I0
IS
20
25
3o
Time (day)
Fig. 1.
The pH versus time for the first trial.
-
-"
-.
|
a 3Z Formic x 1.5~ Formlc/Proplonlc a 1.5~ Formlc/Cltdc • 4 . 5 ~ Citric o 4 . 5 X CltHc • 1~ Citric/Formic
I
,o
I
,5
2's
nM[ (a°y)
Fig. 2.
The pH versus time for the second trial.
Silage Production from Salmon Farm Mortalities
41
(w/w) citric acid/l% (v/w) formic acid in the second run. The results are shown in Figs 1 and 2. The pH of the 4.5% (w/w) citric acid silage increased to 4.5 from 3.8 by day 30, while the pH of the silages made with the two different acids remained steady. The pH values of the silages were monitored again at day 90. The pH of 4.5% citric acid silage had further increased to around 6"9. However, the other silages remained at identical pH values. More studies are needed to assess the stability of the silage made with citric acid. The 3% (v/w) formic acid silage had a pH value of 3.3, while that for the mixture 1% (v/w) formic acid/l% (v/w) propionic acid was 3"9. This indicated that in practice, 1% (v/w) formic acid/l% (v/w) propionic was enough for retaining complete stability of the silage. A combination of propionic acid/formic acid silage was less susceptible to microbial deterioration than the silage made with formic acid alone (Gildberg & Raa, 1977). The formic acid/citric acid silage also gave a lower pH value than did the formic acid/propionic acid silage.
Liquefaction Liquefaction results are shown in Figs 3 and 4. The chemical/physical reaction of the mince to the addition of formic acid yielded a low percentage of liquid on the first day. The other acids did not elicit this response. 7060-
50-
j..--e
v ¢,.
._o
40-
30-
2o2
• 1.5:¢ Citrlc/Formio o 1.5~ Propionio/Forrnic a 3~ Citrio
10 0 0
I
I
5
10
Fig. 3.
I
I
15 20 Time (day)
I
2
T h e liquefaction for the first trial.
~'0
K. V. Lo, P. H. Liao, C. Bullock, Y. Jones
42
70 60 50
2o o @
g
4030/fir
~ I . ~ rorrnic/.Proplo.lc o • o •
20
0
I
5
1.5~ F o r m i c / C l t r l c 4 . 5 ~ Citric 4 . 5 ~ Citric I~. Citrlc/Forrn[c
I
,o
I
2'o
3'0
TIME (cloy)
Fig. 4.
T h e liquefaction for the second trial.
All of the silages liquified quickly for the first 10 days and then levelled off. The rate of liquefaction varied among the different acid treatments. Using multiple comparison analysis, they were significantly different except for the cases of the 2% (w/w) and 3% (w/w) citric acid, the 3% (v/w) formic acid and 1.5% (v/w) formic acid/1.5% (v/w) propionic acid, the 3% (v/w) formic acid and 1% (v/w) formic acid/l% (v/w) propionic acid, and the 4.5% (w/w) citric acid and the 1% (w/w) citric acid/l% (v/w) formic acid. In both trials, the liquefaction rate for the formic acid/ propionic acid mixture was slower than that of the other silages, while the citric acid/formic acid silage had the fastest liquefaction rate among the silages.
Nitrogen and silage composition During the ensiling process, the protein is broken down by the enzymes and the nitrogen in the silage becomes more soluble. In order to study these changes, the soluble nitrogen contents were also determined (Figs 5 and 6). For all the silages, the amount of soluble nitrogen increased rapidly in the beginning, and then levelled out after 10 days. This was similar to the results for liquefaction. These results agree with those reported by other researchers (Tatterson & Windsor, 1974; Tatterson, 1982).
Silage Production from Salmon Farm Mortalities
43
70
60
50v Z
40z _e
30-
,13
20. * 3~ Formic x 2Z Citric o 3X Citric
10.
• 1.SX C l t r l e . / T o r m l ¢ o 1.5Pl P r o p l o n l e / F o r m l c
0 0
3'0 Time (day)
Fig. 5.
The soluble nitrogen for the first trial.
501 40-
v Z
-6
30-
z
_o JO
20-
J
x o • o •
O o'1
I0 "
0 0
Fig.
i
i
5
10
6.
1.5~ Formic//Pr.oplonic 1.5~ Formic/Citric 4.5~ Citric 4.5~ Citrtc I~ Cltric/Formic
i
i
15 20 TIME (day)
/
I
25
30
The soluble nitrogen for the second trial.
The correlation between soluble nitrogen as a percentage of the total nitrogen and the percent liquid (w/w) at each treatment was also calculated. The results fit a linear regression. For the 3% (v/w) formic acid silage is:
44
K.V. Lo, P. H. Liao, C. Bullock, Y. Jones Soluble nitrogen (%)= 14-6 + 0"67 (liquid %) ( R 2 --- 0"79,
n -- 22)
The composition of the fish silages was virtually identical to the raw materials. The salmon mortalities had moisture contents of 70-74%, ash contents of 3.8-3.9%, and oil contents of 3.17-3.20% (based on the dry weight). Given the poor quality of the salmon farm mortalities, the silage could not be used as a feed, although it could be used as a liquid fertilizer. The fertilizer values of the silage were around 3% nitrogen, 0.4% phosphorus and 0.3% potassium.
CONCLUSIONS The results indicate that salmon mortalities could be liquified in a much shorter time than is the current practice. They also establish that the ensiling process is very simple and that the handling of morts is straightforward. Citric acid, or a combination of citric acid with other acids, could be used effectively as acid media. The nutrient content of the silages examined remained virtually identical to that of raw salmon. It is concluded that the ensiling of mortalities could reduce on-site storage problems for British Columbia salmon farms and provide the basis of a good mort disposal/utilization system.
ACKNOWLEDGEMENTS The help in statistical analysis of data rendered by Ms Y. Gao is gratefully acknowledged.
REFERENCES APHA. ( 1985). Standard Methods for the Examination of Water and Wastewater, 16th edn American Public Health Association, Washington, DC. Gildberg, A. & Raa, J. (1977). Properties of a propionic acid/formic acid preserved silage of cod viscera. Journal Sci. FdAgric., 28, 647-53. Schumann, G. E., Stanley, M. A. & Knudsen, D. (1973). Automated total nitrogen analysis of soil and plant samples. Proc. Soil Sci. Soc. Amer., 37, 480-1. SYSTAT (1989). The System for Statistics for the PC, SYSTAT, Inc., Evanston, IL.
Silage Production from Salmon Farm Mortalities
45
Tatterson, I. N. & Windsor, M. L. (1974). Fish Silage. Journal Sci. Fd Agric., 25, 369-79. Tatterson, I. N. (1982). Fish silage -- preparation, properties and uses. Animal Feed Science and Technology, 7, 153-9. Wood, J. F. (1980). The preparation of water-stable fish feeds. 2. The potential for fish silage as a fish feed ingredient. Trop. Sci., 22,357-60.
Silage Production from S a l m o n Farm Mortalities K. V. Lo, P. H. Liao, C. Bullock Bio-Resource Engineering Department, University of British Columbia. 2357 Main Mall, Vancouver,British Columbia, Canada V6T 1Z4
& Y. Jones National Research Council, 3650 Wesbrook Mall, Vancouver, British Columbia, Canada, V6S 2L2
(Received 18 April 1991; revised version received 21 September 1991; accepted 3 September 1992) ABSTRACT Fish silage was made from salmon farm mortalities. Citric, formic and propionic acid, as well as combinations of these acids, were used as ensiling media. Stability, liquefaction rate and soluble nitrogen contents were studied. The silages made with 2 or 3% (w/w) of citric acid alone were unable to achieve stability, while the pH increased from 3"8 to 4"5 by day 30for the 4.5% (w/w) citric acid silage. The pH for the other acids and for the acid combinations was very stable. All of the silages liquified quickly for the first ten days, and then liquefaction &veiled off. The amount of soluble nitrogen increased for all the silages, as did the percent liquid. The results indicated that salmon farm mortalities could be used as the raw material in the production offish silage and a marketable liquid fertilizer.
INTRODUCTION Salmon farms in British Columbia currently have problems disposing of their fish mortalities. In this paper it is p r o p o s e d that the production of fish silage would serve as a good solution to their mort disposal problem. Fish silage is a liquid product derived from the mixing of whole fish, or c o m p o n e n t s of fish, with an acid. T h e acid allows the enzymes from 37 Aquacultural Engineering 0144-8609/93/S06.00 - © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
K. V. Lo, P. H. Liao, C Bullock, Y. Jones
the gut to liquify the mixture while restricting the growth of spoilage bacteria (Tatterson & Windsor, 1974). The use of different kinds of acids in the ensiling process could potentially affect the rate of liquefaction of the fish silage, the stability of the silage and the soluble nitrogen content of the silage. The stability of silage is determined by its tendency to remain below a certain pH value. Studies have shown that with the use of inorganic acids, the pH should remain below 2 (Wood, 1980). With the use of organic acids, the silage should remain stable at pH 3.5-4.0 with formic acid, and pH 4.5 with propionic acid (Gildberg & Raa, 1977). The organic acids are preferred for ensiling, since they retain their antimicrobial properties at a relatively high pH. The products resulting from ensiling would therefore not require neutralization prior to animal feeding. For the same reason, organic acids would reduce problems of corrosion of equipment and containers, and are therefore preferable. Formic acid, or a mixture of formic and mineral acids, are most commonly used in the commercial production of fish silage. However, citric acid is commonly used in the food and beverage industry to adjust the pH and enhance antioxidant additives, since it is cheaper to use than formic acid. This research therefore made the citric acid ensiling process its major focus. The use of citric acid and mixtures of citric acid, formic acid and propionic acids were examined as ensiling media for salmon mortalities.
MATERIALS AND METHODS Raw materials and the production of sUages Silage was made from salmon farm mortalities of approximately 2 kg each that had been decomposing from one to four days and then frozen at -20°C. In the ensiling process, fish is usually minced to allow complete penetration of the acid. After thawing, the whole carcasses were therefore passed through an auger-type meat grinder with disc holes of 4 mm and mixed so as to establish an homogeneous mince. Two experimental trials were conducted. In the first trial, five different silages were made in triplicate of 500 g minced fish with (i) 3% (v/w) formic acid, (ii) 1.5% (w/w) citric acid/1.5% (v/w) formic acid, (iii) 1.5% (v/w) formic acid/i-5% (v/w) propionic acid, (iv) 2% (w/w) citric acid, and (v) 3% (w/w) citric acid. In the second trial, six different silages were made: the sets (i), (ii) and (iii) of the first trial were repeated, two sets of 4.5% (w/w) citric acid and one set of 1% (w/w) citric acid/l% (v/w)
Silage Productionfrom Salmon Farm Mortalities
39
formic acid were added. The silages were stored at room temperature (22°C) in covered glass beakers.
Analyses Moisture content, ash content and pH were determined according to the standard methods (APHA, 1985). Oil content was measured by extracting the oil from oven-dried samples by Soxhlet using diethyl ether (Tatterson & Windsor, 1974). Total nitrogen was determined by using a block digester and a Technicon Auto Analyzer II as described by Schumann et al. (1973). Soluble nitrogen was determined by adding 10 ml of 20% trichloroacetic acid to 5 g of silage and mixing for 2 min. The mixture was filtered and the filtrate was analyzed as described above. Liquefaction was measured by the percentage of liquid phase after centrifuging. Ten-gram samples of silage were centrifuged at 3000 rpm for 20 min. The liquid portion and remaining mass were then determined. Data was statistically analyzed using SYSTAT (SYSTAT, 1989). All data were subjected to a multivariate analysis of variance and the differences among the various acid treatments were analyzed using multiple comparisons.
RESULTS AND DISCUSSION
Stability The stability of the silage was determined by measuring the silage's capacity to remain below a certain pH value. In the first trial, the silages made with 2 or 3% (w/w) of citric acid alone were unable to achieve stability. The pH of these citric acid silages increased immediately after the ensiling process (Fig. 1). The pH of the 2% (w/w) citric acid silage not only had greater increases in pH, but also remained higher than that of the 3% (w/w) citric acid silage by day 30. The silages using 3% (v/w) formic acid, a mixture of formic acid/citric acid and a mixture of formic acid/propionic acid maintained stability (Fig. 2). The 3% (v/w) formic acid silage gave the lowest pH value. Based on the results from the first run, it was thought citric acid could be used for ensiling without the addition of other acids. However, in order to achieve stability, a larger volume of citric acid alone would be needed than if it were mixed with other acids. Tests were therefore conducted on two sets of 4.5% (w/w) of citric acid and one set of 1%
40
K. V. Lo, P. H. Liao, C. Bullock, Y. Jones 8 7
j _ . . ~ x
6 5 k--e-'---qt---~.
~
o
G
-
o
It
3 • 1.5~ Citric/Formic o 1.5~. Propionic/Formlc
* 3 ~ Form;c 2 ~ Citric 3~, Citric
2 1
0 0
I
I
I
I
1
l
5
I0
IS
20
25
3o
Time (day)
Fig. 1.
The pH versus time for the first trial.
-
-"
-.
|
a 3Z Formic x 1.5~ Formlc/Proplonlc a 1.5~ Formlc/Cltdc • 4 . 5 ~ Citric o 4 . 5 X CltHc • 1~ Citric/Formic
I
,o
I
,5
2's
nM[ (a°y)
Fig. 2.
The pH versus time for the second trial.
Silage Production from Salmon Farm Mortalities
41
(w/w) citric acid/l% (v/w) formic acid in the second run. The results are shown in Figs 1 and 2. The pH of the 4.5% (w/w) citric acid silage increased to 4.5 from 3.8 by day 30, while the pH of the silages made with the two different acids remained steady. The pH values of the silages were monitored again at day 90. The pH of 4.5% citric acid silage had further increased to around 6"9. However, the other silages remained at identical pH values. More studies are needed to assess the stability of the silage made with citric acid. The 3% (v/w) formic acid silage had a pH value of 3.3, while that for the mixture 1% (v/w) formic acid/l% (v/w) propionic acid was 3"9. This indicated that in practice, 1% (v/w) formic acid/l% (v/w) propionic was enough for retaining complete stability of the silage. A combination of propionic acid/formic acid silage was less susceptible to microbial deterioration than the silage made with formic acid alone (Gildberg & Raa, 1977). The formic acid/citric acid silage also gave a lower pH value than did the formic acid/propionic acid silage.
Liquefaction Liquefaction results are shown in Figs 3 and 4. The chemical/physical reaction of the mince to the addition of formic acid yielded a low percentage of liquid on the first day. The other acids did not elicit this response. 7060-
50-
j..--e
v ¢,.
._o
40-
30-
2o2
• 1.5:¢ Citrlc/Formio o 1.5~ Propionio/Forrnic a 3~ Citrio
10 0 0
I
I
5
10
Fig. 3.
I
I
15 20 Time (day)
I
2
T h e liquefaction for the first trial.
~'0
K. V. Lo, P. H. Liao, C. Bullock, Y. Jones
42
70 60 50
2o o @
g
4030/fir
~ I . ~ rorrnic/.Proplo.lc o • o •
20
0
I
5
1.5~ F o r m i c / C l t r l c 4 . 5 ~ Citric 4 . 5 ~ Citric I~. Citrlc/Forrn[c
I
,o
I
2'o
3'0
TIME (cloy)
Fig. 4.
T h e liquefaction for the second trial.
All of the silages liquified quickly for the first 10 days and then levelled off. The rate of liquefaction varied among the different acid treatments. Using multiple comparison analysis, they were significantly different except for the cases of the 2% (w/w) and 3% (w/w) citric acid, the 3% (v/w) formic acid and 1.5% (v/w) formic acid/1.5% (v/w) propionic acid, the 3% (v/w) formic acid and 1% (v/w) formic acid/l% (v/w) propionic acid, and the 4.5% (w/w) citric acid and the 1% (w/w) citric acid/l% (v/w) formic acid. In both trials, the liquefaction rate for the formic acid/ propionic acid mixture was slower than that of the other silages, while the citric acid/formic acid silage had the fastest liquefaction rate among the silages.
Nitrogen and silage composition During the ensiling process, the protein is broken down by the enzymes and the nitrogen in the silage becomes more soluble. In order to study these changes, the soluble nitrogen contents were also determined (Figs 5 and 6). For all the silages, the amount of soluble nitrogen increased rapidly in the beginning, and then levelled out after 10 days. This was similar to the results for liquefaction. These results agree with those reported by other researchers (Tatterson & Windsor, 1974; Tatterson, 1982).
Silage Production from Salmon Farm Mortalities
43
70
60
50v Z
40z _e
30-
,13
20. * 3~ Formic x 2Z Citric o 3X Citric
10.
• 1.SX C l t r l e . / T o r m l ¢ o 1.5Pl P r o p l o n l e / F o r m l c
0 0
3'0 Time (day)
Fig. 5.
The soluble nitrogen for the first trial.
501 40-
v Z
-6
30-
z
_o JO
20-
J
x o • o •
O o'1
I0 "
0 0
Fig.
i
i
5
10
6.
1.5~ Formic//Pr.oplonic 1.5~ Formic/Citric 4.5~ Citric 4.5~ Citrtc I~ Cltric/Formic
i
i
15 20 TIME (day)
/
I
25
30
The soluble nitrogen for the second trial.
The correlation between soluble nitrogen as a percentage of the total nitrogen and the percent liquid (w/w) at each treatment was also calculated. The results fit a linear regression. For the 3% (v/w) formic acid silage is:
44
K.V. Lo, P. H. Liao, C. Bullock, Y. Jones Soluble nitrogen (%)= 14-6 + 0"67 (liquid %) ( R 2 --- 0"79,
n -- 22)
The composition of the fish silages was virtually identical to the raw materials. The salmon mortalities had moisture contents of 70-74%, ash contents of 3.8-3.9%, and oil contents of 3.17-3.20% (based on the dry weight). Given the poor quality of the salmon farm mortalities, the silage could not be used as a feed, although it could be used as a liquid fertilizer. The fertilizer values of the silage were around 3% nitrogen, 0.4% phosphorus and 0.3% potassium.
CONCLUSIONS The results indicate that salmon mortalities could be liquified in a much shorter time than is the current practice. They also establish that the ensiling process is very simple and that the handling of morts is straightforward. Citric acid, or a combination of citric acid with other acids, could be used effectively as acid media. The nutrient content of the silages examined remained virtually identical to that of raw salmon. It is concluded that the ensiling of mortalities could reduce on-site storage problems for British Columbia salmon farms and provide the basis of a good mort disposal/utilization system.
ACKNOWLEDGEMENTS The help in statistical analysis of data rendered by Ms Y. Gao is gratefully acknowledged.
REFERENCES APHA. ( 1985). Standard Methods for the Examination of Water and Wastewater, 16th edn American Public Health Association, Washington, DC. Gildberg, A. & Raa, J. (1977). Properties of a propionic acid/formic acid preserved silage of cod viscera. Journal Sci. FdAgric., 28, 647-53. Schumann, G. E., Stanley, M. A. & Knudsen, D. (1973). Automated total nitrogen analysis of soil and plant samples. Proc. Soil Sci. Soc. Amer., 37, 480-1. SYSTAT (1989). The System for Statistics for the PC, SYSTAT, Inc., Evanston, IL.
Silage Production from Salmon Farm Mortalities
45
Tatterson, I. N. & Windsor, M. L. (1974). Fish Silage. Journal Sci. Fd Agric., 25, 369-79. Tatterson, I. N. (1982). Fish silage -- preparation, properties and uses. Animal Feed Science and Technology, 7, 153-9. Wood, J. F. (1980). The preparation of water-stable fish feeds. 2. The potential for fish silage as a fish feed ingredient. Trop. Sci., 22,357-60.