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Dietary proteinand energy utiiizationby juvenile american eel (anguillsmsfrata) sean m tibbetts

Dietary Protein and Energy Utiiization by
Juvenile American Eel (Anguills msfrata)

Sean M. Tibbetts

Subrnitted in partial fulfillment of the requirements
for the degree of Master of Science
at
The Nova Scotia Agriwltural College, Truro, Nova Scotia
and Dalhousie University. Halifax. Nova Scotia

Q Copyright by Sean M. Tibbetts, 1999


1+1

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This thesis is dedicated to my niece
Kathleen Rose Dehmel born December 12, 1996

iv.



2.1.7
2.1.8
2.1.9
2.1.10
2.7.11

Vitamins...................................................................... 18
Minerals...................................................................... -18
Feed Attractants......................................................... -19
Nutrient Requirements of Arneriwn Eel..................... -20
Rationale and Objectives............................................ -20

2.2

Materials and Methods...................................................................... 20
2.2.1
Sample Preparation.................................................... -20
2.2.2
Chernical Analysis............. .
.
.
.
.............................. 21

2.3

Results and Discussion.................................................................... -22

Chapter 3

Optimum Dietary Crude Protein Requirement

3.1

Literature Review.............................................................................. -26
3.1- 1
Structure and Classification of Protein and
Amino Acids................................................................. -26
3.1.2
Dietary Protein and Essential Amino Acids.................28
3.1-3
American Eel Culture Conditions and Diets................ 33
3.1.4
Rationale and Objectives............................................ -34

3.2

Materials and Methods...................................................................... 35
3.2.1
-rimental
Diets..................................................... -35
Growth Performance.................................................... 37
3.2.2
3.2.3
Nutrient Digestibiiity ................................................... 39
3.2.4
Nutrient Retention...................................................... -40
3.2.5
Statistical Analysis...................................................... -40

3.3

Results and Discussion..................................................................... 42
3.3.1
Growth Performance.................................................... 42
3.3.2
Nutrient Digestibility .................................................. -45
3.3.3
Nutrient Retention...................................................... -47

3.4

General Discussion.........................................................................

-49

Conclusion.......................................................................................

-50

Chapter 4

4.1

Optimum Dietary Protein:Energy Ratio
Literature Review.............................................................................. -51
Dietary Energy............................................................ -51
4.1 -1
4.1.2
Bioenergetics.............................................................. -52
vi.



List of Tables
Table 1:

Proximate composition of commercial diets used to feed eels in
Atlantic Canada and comparison to known requirements................... 23

Table 2:

Name. abbreviation and classification of the twenty amino acids.. ....27

Table 3:

Essential amino acid requirements of juvenile Japanese eel........... -31

Table 4:

Estimated dietary protein requirements for maximal growth
of juvenile fish. ................................................................................... .32

Table 5:

Composition of experimental diets used to determine optimum
dietary protein requirement of juvenile American eel........................ -36

Table 6:

Average weight, growth rate and feed conversion of Arnerican
eel fed diets containing graded levels of protein for 84 days.............43

Table 7:

Apparent digestibility of organic matter, protein and gross energy
in the diets containing graded levels of protein.................................. 45

Table 8:

Body composition of American eel fed the five diets
antaining graded levels of protein after 84 days............................... 48

Table 9:

Carcass protein. lipid and gross energy gain of Arnerican
eel fed the five diets containing graded levels of protein
-48
after 84 days......................................................................................

Table 10:

Reported optimum protein:energy ratio for various fish.. ...................59

Table 11:

Composition of experimental diets used to detemine
optimum dietary protein:energy ratio for juvenile American eel..........64

Table 12:

Average weight, growth rate and feed conversion of American
eel fed diets containing varying proteimenergy ratios
for 84 days.......................................................................................... 69

Table 13:

Apparent digestibility of organic matter, protein, gross energy,
lipid and phosphorous in the five diets containing
varying protein:energy ratios.. ........................................................... .71

viii.


Table 14:

Phosphate (PO. excretion rates of juvenile Arnerican
eel fed the five diets containing varying protein:energy ratios..........-74

Table 15:

Body composition of American eel fed the five diets
containing varying protein:energy ratios af'ter84 days....................... 75

Table 16:

Carcass protein. lipid and gross energy gain of American eel
fed the five diets containing varying protein:energy ratios
after 84 days.................................. ..................................................... 76


List of Figures
Figure 1:

Basic chernical structure of an amino acid......................... .
.
........... -27

Figure 2:

Condensation reaction of two amino acids......................................... 28

Figure 3:

Effect of dietary protein level on growth of American eel at
various sarnpling times during the experiment.................................. -43

Figure 4:

Partitioningof dietary energy in food consumed by fish.................... 53

Figure 5:

Effect of dietary protein:energy ratio on growth of American eel
at various sampling times dunng the expriment.............................. -68


List of Appendices
Appendix 1: Deviations from the method of Suzuki and Early (1991)
for determination of chrornic oxide content in
feeds and faeces............................................................................... -82
Appendix 2: Chernical composition of diets and faecal samples for the
digestibility trial (protein requirement experiment)........................... .83
Appendix 3: Deviations from the method of AOAC (1984) for determination
of phosphorous content in feeds and faeces.. ................................... -84
Appendix 4: Chernical composition of diets and faecal samples for the
digestibility trial (protein:energy ratio experiment)........................... -85
Appendix 5: R2values for data analysis for the protein and
proteimenergy trials.. ......................................................................... -86

xi.


ABSTRACT
Two experiments were conducted to evaluate the nutrient requirements of
juvenile American eel (Anguilla mstrata). The growth trials ernployed fifteen aquaria
(40L), set up as a randomized block design replicated three times using five diets
fed for 84 days. The digestibility trials employed ten aquaria (60-120L), set up as
a randomized block design replicated twice using five diets containing chromium
oxide (Cr203)fed until adequate faecal sample was obtained. Al1 trials used
freshwater at 22°C. To detemine the optimum dietary crude protein requirement,
six hundred and seventy-five elvers, initial rnean weight 8.110.07 g, were fed to
satiation with diets containing 35, 39, 43, 47 and 51% protein. Highest mean
weight gain (Pc0.05) and specific growth rate (Pc0.05) were obtained when dietary
protein was 47% and 51% with average values of 13.5 g and 1.16%-day-',
respectively. Values for 35, 39 and 43% protein were significantly lower with
averages of 10.4 g and 0.98%=day'', respectively. Optimum (Pconversion ratio of 1.17 g feed-g gain" was achieved when feeding 47% protein and
was significantly lower than other diets which averaged 1.44 g feed-g gain".
Digestibilities of protein and energy were similar among the 39, 43, 47 and 51%
protein diets with mean protein digestibility of 90.7% and mean energy digestibility
of 90.3%. Highest carcass protein gain (Pe0.01) was achieved when feeding 47%
and 51% protein each with an average value of 2.2 g-fish", compared to the other
three diets at an average of 1.5 g-fish''. Highest carcass lipid gain (P<0.01) of 1.8
g-fish-'was achieved when feeding 47% protein and was higher than al1 other diets
which averaged 1.3 g-fish-'. Little difference in energy gain were observed with an
average value of 120.9 KJ-fish". The optimum level of dietary protein for juvenile
American eel is approximately 47%. To determine the optimum ratio of dietary
pr0tein:digestible energy, eleven hundred and twenty-five elvers, initial mean
weight 8.1iO.07g, were fed to satiation diets containing 44: 19, 44:20, 44:21, 48: 19
and 48:20% cnide protein:MJ digestible energy. Protein:energy ratio had no
significant (P~0.05)effect on mean weight gain and specific growth rate. Feed
conversion ratio was poorer with the 44: 19 diet at 1.25 g feed-g gain" as compared
to the other four diets at 1.O5 g feedog gain", however, not statistically. Digestibility
of the 44: 19, 48: 19 and 48:20 diets were significantly (Pc0.05) higher for organic
matter, energy and lipid with average digestibility coefficients of 87.1 O h , 90.0% and
92.3%, respectively as compared to 85.0%, 87.8% and 90.3Oh for the other two
diets. Proteimenergy ratio had no significant (P>O.OS) effect on phosphate
exuetion rate and it was estimated to be 0.05 g PO,=-kg fish".day". Protein:energy
ratio had no significant (P>0.05) effect on protein gain, lipid gain and energy gain
with average values of 1.9 g-fish", 1.5 g-fish1and 127.6 KJ-fisk', respectively. The
optimum protein:energy ratio for juvenile Arnerican eel is approximately 23 g DP-MJ
DE-'.

xii.


ACKNOWLEDGMENTS
I would like to express my deepest thanks to the numerous people who have
been instrumental in helping me camplete this thesis.
1 am grateful to my CO-supervisors,Dr. Santosh P. Lall and Dr. Derek M.
Anderson for accepting me as a graduate student. Special thanks to Santosh Lall
for his encouragement and for sharing with me his infinite wisdom in fish nutrition,
ration formulation and fish feed technology. Special thanks to Derek Anderson for
his one-onone attention, enthusiasm, encouragement and guidance.

Thanks also go to my research wmmittee, Dr. Jim Duston and Dr. Dick
Peterson for their support and efforts dufing the initial experimental design,
construction of the aquaria facility and their helpful wmments and advice with this
project.
Thanks goes to Dr. Tessama Astatkie for his invaluable consultation with me
on the various experimental designs and statistical analysis.
A special thanks to Mrs. Margie Hartling for her many hours of assistance
with laboratory analysis, her patience with me while leaming how to operate the
instruments in the nutrition laboratory and especially for her friendship over these
past few years.

Thanks goes to the numerous technicians and other students that have
helped me over the years with aquaria construction and maintenance, mixing and
pelleting diets, feeding and weighing fish. To name a few: Chris Giles, Bob Keith,
Stacey Buchanan, Cheryl Hebb, Lisa Peters, Kara Irving, Paul Maclsaac, Randy
Peach, Blair Ettinger, Katherine Benedict and Jennifer Collins.
Thanks goes to my family for putting up with me over the years and have
kept on loving me and supporting my endeavors. Thanks Mom for guiding me
through al1 the lessons in life and teaching me how to believe in myself. Thanks
Dad for sharing with me your passion of the natural world. I am a part of science
because of you. Sincere thanks to Laura, Kathleen and the rest of the clan.
This work was supported by a grant from the CanaddNova Scotia
Aquaculture Cooperative Agreement and a grant from the Department of Fisheries
and Oceans.

xiii.


Chapter 1

General Introduction
1.1

Overview of Aquaculture
1.1.1 Global

The once flourishing traditional fishery now offers limited opportunity for

economic growth due ta poor management resulting in over-exploitation of
numerous wmmercially valuable species. The demand for fish and fish-probucts
continues to increase and is e>Q>ededto mach 120 million tonnes by the year 2000
(Department of Fisheries and Oceans (DFO), 1995). As the total wild caught
fishery declines, total fanned-fish production must rise to meet this high demand.

The value of world aquaculture production has grown from $12 billion ( U S ) in
1984 (Food and Agriculture Organization (FAO), 1992) to $32.5 billion (U.S.) in
1992 (FAO, 1994). The growth in aquaculture production in the late 1980's and

early 1990's was primarily attributed to Asia, Europe and South Arnerica faming a

-

small number of aquatic organisms namely shrimp, prawn, carp, salmon, trout,
oysters and mussels. In 1992, 18.6% of al1 fish and seafood consumed globally
was produced through aquaculture. This number is expected to surpass 25% by

the year 2000 (Ratafia, 1995).

1.1.2 Canada

Canada is following the international trend away from the wild fishery
towards aquaculture. The govemments of Canada and the provinces recognize


2

aquaculture as a potential growth industry. In 1992, Canada was ranked 2gmin
ternis of total aquaculture production at 30,853 tonnes worth $146 million (Boghen,
1995). In 1993 Canada ranked 27'" with a total production of 50,375 tonnes worth
$289 mi!lion (DFO, 1995). In 1991 Canadian aquaculture provided jobs for 5,180

-

people some 2,825 in the production sector and 2,355 in the supplies and services
sector. By the year 2000 the number of jobs is e w e d to reach 12,225 with 8,125
in the production sector and 4,lW in the supplies and services sector (DFO, 1995).
On a global scale, Canada is a relatively small player in the aquaculture
industry accounting for approximately 0.2% of the world's total (Boghen, 1995).
However, the potential for growth of Canadian aquaculture is great, given that it has
an abundance of natural resources ideally suited to aquaculture. In addition,
Canadians have acquired internationally recognized technical and management
expertise and have developed state-of-the-art facilities for the production of high
quality cultured fish and shellfish.

Canada's geographical setting is also

advantageous as we have easy access to the vast Pacific Rim and North Arnerican
fish and seafood markets. Despite relatively small aquaculture production, Canada

has the potential to bewme a world leader (DFO, 1995).
In 1993, the Canadian aquaculture industry was worth $289 million and it
marked the end of the competitive phase (DFO, 1995) which is characterized by a
leveling off of production tonnage. In order for Canada to reach its full potential of
being a world leader in aquaculture, collective strategies must be devised to allow

Canadian aquaculture to successfully forge beyond the competitive phase. One


3

such strategy presently employed in Nova Scotia and New Brunswick is to turn its

attention from the more traditional culture of salmonids to culture of alternative fish
species.
When investigating the aquaculture potential of an altemate species, several
criteria are considered - including market demand, production technology, market
dimensions and cost of production (Hill, 1992).

Through an econornic

diversification agreement between the federal governrnent and the governments of

New Brunswick and Nova Scotia, six altemate finfish species have been identified
as potentially valuable for aquaculture.

They include striped bass (Morone

saxatilis) and American eel (Anguilla rostrata) for freshwater culture and others
(winter flounder, Pleumnectes amenanus; yellowtail flounder, Limanda fenuginea;
Atlantic halibut, Hippaglossus hippoglossus; haddock, Melanogrammus aeglefinus)
for marine culture.
The specie focused on in this thesis is American eel. Little is known about
its needs in ternis of husbandry and nutritional management of this specie.

1.2

Biology of American €el (Anguilla mstnite)
1.2.1 Taxonomy and Distribution
The American eel belongs to the class Osteichthyes (bony fish) and the

order of Anguillifomes. AnguiIliformes are unique among other eels in that they are
catadromous; meaning that they hatch in salt water, complete growth in freshwater
and retum to the salt water to reproduce and, thereafter, die. American eels are


4

found widely distributed over North and South America. Its northern limits are
southem Greenland, Newfoundland and Labrador and, to the south. the Gulf coast
of Mexico, as far as Tampico, Panama, the Greater and Lesser Antilles (Cuba,

Jamaica, Puerto Rico, St. Croix. St. Vincent, Dominica and Grenada), and off the
South American mainland as far as Guyana, extending eastward to Bermuda
(Degani and Gallagher, 1995).
Since this research will utilize nutritional information from European eel
(Anguilla anguilla) and Japanese eel (Anguillajaponica), it is important to review the

similarities and differences between the species. Japanese eel is considered the
largest of the three species with the 116 vertebrae. The European eel has 115
vertebrae and Arnerican eel is considered the smallest with 107 vertebrae.
Approximate maximum size of females is the same for al1 three species at 6 kg and
125 un. Distribution is different between the three species with Japanese eel found

in Japan and China, European eel found on the west coast of Europe, North Africa
and lceland and Amen'can eel on the east coast of the United States, Canada and

Greenland. It is thought that these three species have descended from a common
ancestor and apart from the few differenœs noted above there are little differences
between species (Ege, 1939).

1.2.2

Life Cycle

Little was known about the life-cycle of the Arnerican eel until 1922 when the

Danish scientist Johannes Schmidt discovered that al1 eels of Europe and America


5

spawn in the Sargasso Sea. The main breeding area is located between 20" and
30" latitude and 60" and 78" longitude, near Florida and south of Bermuda

(Schmidt, 1932). From November to January, sexually mature eels migrate towards
the Sargasso Sea following a path of increasing water temperature and salinity,

which are higher there in winter than any other place in the Atlantic Ocean.

Sexually mature males and females spawn at depths of 13004 600 feet at 15-16 OC
and 36-37 ppt salinity. It is thought that females produce 7-13 million eggs each
and both males and females die after spawning (Usui, 1991).

By February, the surviving eggs hatch and 5 mm, clear, willow-leaf shaped

leptocephali larvae emerge, rise ta the surface and are carried for the following year
by the ocean current in a north-west direction (Landau, 1992). Studies by Otake

et. al. (1993) indicate that larvae may feed on detrital aggregates less than 2Opm
in diameter and faecal pellets of zooplankton. By winter, leptocephali of the

American eel reach the Grand Banks, achieve a maximum size of 60-65 mm and
begin to metamorphose into the second larval stage. This metamorphosis is main!y
due to a IOSS
of body water. This is the first stage that the larvae begin to resernble

the body shape of the adults, however they are transparent due to lack of pigment
and are known as glass eels (Degani and Gallagher, 1995).
Migration to fresh water is apparently guided by the olfactory senses
(Creutzberg, 1957; Hain, 1975). During this migration a greyish-black pigment

appears ail over the body and they are now known as elvers. After 2 or 3 years of
feeding on small fish, insects, fish eggs, detritus, snails, Worms and other


6

invertebrates in the fresh and brackish water of coastal rivers and estuaries they
attain a yellowish-green color on the dorsal side and are known as yellow eels.

1.2.3 Sexual Maturation
After 5-10 years, yellow eels reach sexual maturation characterized by a
glistening layer undemeath the skin causing the whole body to have a metallic
silvery sheen especially on the undenide and an enlargement of the eyes (Landau.

1992). Sexually mature eels are known as silver eels weighing approximately 150
g and 500 g for males and fernales, respectively. This sexual dimorphism in size
is wmmon in many fishes. In this silver stage the fat content is greater than in the
yellow stage at 25-28% and is believed to be the key factor inducing sexual
maturation (Larsson et al., 1990). At the final stage of development, mature eels
begin downstream migration to salt water and subsequently to the Sargasso Sea

to spawn.

1.2.4 Digestive System

The digestive system of the eel can be examined as two separate, however,
not independent parts. The first being the alimentary tract that begins with the
mouth and ends with the anus. Contained within are the buccal cavity, pharynx,
esophagus, stornach and intestine. The second part of the digestive system is the
digestive glands and includes the pancreas and the Iiver.
Both the upper and lower jaws of the buccal cavity house several small and


7

well-developed teeth which are acrodental that are used to hold prey -(Hepher,

1988). Contained within the mucous membrane of the buccal cavity are thousands
of rnuwsal cells and gustatory ceils (taste buds) which aid in the smooth passage
of food and taste, respectively (Sinha, 1975). Unlike, in higher animals, there is no

salivary secretion wntaining amylase (Halver, 1989), therefore we can expect that
if eels digest carbohydrates, it does not begin in the buccal cavity.

The pharynx is the srnall segment just posterior to the buccal cavity. It is
surrounded by five gill arches on either side and is supported by the branchial
bones. The fAh gill arch contains minute pharyngeal teeth that are very sharp and
may be used to cwsh and grind food as was observed for cornmon carp and

goldfish (King, 1975). There are no enzymes excreted in the pharyngeal area
(Haiver, 1989), so very little digestion occurs in this portion of the alimentary tract.
The esophagus is light orange in color and extends posterior frorn the
pharynx to the stomach. The esophagus contains numerous mucosal cells. The
secretions of which aid in transport of ingested food (Hepher, 1988), but do not
possess any enzyme-like properties (Halver, 1989; Hirji, 1983;Sis et al., 1979)
There are often siight arnounts of acid reaction in the esophagus due to
regurgitation of gastric fluid (Halver, 1989). The esophagus of the eel also piays
a significant role in osmoregulation (Clarke and Witcomb, 1980).

The stornach is Y-shaped and c m be divided into two separate parts or
arms. The shortest arm leads from the esophagus and is connected to the small
intestine. This a m is known as the pyloric part or pylonis. The other arrn leads


from the esophagus, is much longer and ends blind. This arm is known as the
cardiac part or corpus. The gastric epithelium of the stomach contains three kinds
of cells: 1) oxyntic cells, 2) endocrine cells and 3) surface mucous cells. These

cells produce pepsin, HCI, gastrin, somatostatin, pancreatic polypeptide,
sialomucins, sulfomucins and neutral muwsubstances (Halver, 1989) that al1 aid
in digestion.

As with most animais, the intestine of the eel is differentiated into three sub-

sections:

1. duodenum
2. small intestine
3. large intestine

The duodenum is the sub-sedion of the srnail intestine that immediately follows the
stomach and receives the bile duct. Next is the srnaIl intestine, which is slightly
wider than the duodenum and is highly ciliated. Digestive wmpounds and enzymes
found here include those enzymes secreted by the liver and pancreas (trypsin,
amylase, lipase), sucrase, maltase, invertase, gut micro-fiora and the enzymes
present in the food itself.
The third sedion is the large intestine which is wider and is smoother due to
less cilia and contains more goblet cells than the small intestine. Digestive

processes are less pronounced in this portion of the intestine.
The pancreas is about two-thirds the length of the stomach and contains

exocrine glands that lead to a single duct which on entering the intestine divides
into three small ducts. At the point where it emerges from the wall of the intestine
it foms several minute ductules which excrete pancreatic juices into the intestine.


9

These pancreatic juices being, for the most part, trypsin, amylase and lipase
(Halver, 1989).
The liver is an elongated gland in the abdomen which surrounds the
esophagus, stomach and part of the small intestine. It is reddish-brown in color and
the left lobe is larger than the right. Like the pancreas, the liver secretes digestive

enzymes (namely trypsin, amylase and lipase) through small hepatic lobes into the
intestine (Halver, 1989).

1.3

Eel Culture
1.3.1 Global Market Status
Three species of eels are comrnercially important: Japanese eel (Anguilla

japonica), European eel (Anguilla anguilla) and American eel (Anguilla rostrata)
(Lovell, 1989). Total global eel production from aquaculture and capture fishery in
1992 was 186,741 tonnes and worth an estimated $2 billion annually (Lazur, 1997).

Eels have been cultured in Japan since 1894, and in recent years
aquaculture production has been about 33,636 tonnes per year, accounting for
approximately one-third of total world production (Usui, 1991). Japan is the world's
largest market for eels with an estimated consumer demand of 96,364 tonnes in
1994 (Lazur, 1997). Rapid industrializationdestroyed or occluded much of the eels'

habitat in Japanese waters. Along with fishing pressures, habitat destruction
resulted in severe depletion of elver seedstocks. Japanese eel culturists began

importing elven from Korea, Taiwan, France and the United States in 1973. Prices


1O

were $1500-kg-' or $0.55 per live elver (Shang. 1974). This new market-potential
was attractive to many countries and hence, triggered a global eel industry.

Taiwan and Korea started eel culture in 1968 and produced approxirnately
27,273 tonnes in 1985, most of which was exported to Japan (Lazur. 1997). An
average market weight of 150 to 200 grams is preferred in the Asian market for the
preparation of kabayaki. the prefened eel dish in Japan (Lazur, 1997).
A second regional market for eels is Europe. The demand for eels in

Gerrnany, the Netherlands, France, Denrnark, Sweden and England is
approximately 13.636 tonnes annually. The major producers of eel in Europe are
Italy, France. Denmark and Poland with 8,182 tonnes of production annually. The
average market weight preferred in Europe is higher than that of Asia at 125 to
1000 grams. The increased size is due to the fact that srnoked eel is the preferred
product (Lazur, 1997).

1.3.2 American Eel Culture Potential
Although wild Arnerican eels are in large supply, North America contributes
less than 1% of total eel production (Jessop, 1995). However, as the demand for
eel food products continues to increase in Asia and Europe, and since captive
spawning has not been successful to date, the local supply of elver seedstock is
expected to decrease. The demand for elven is rising to a level that New

Brunswick and Nova Scotia now recognize American eel culture and sale on the
European and Asian markets as a viable and profitable enterprise.


Il

A large population of glass eels and elvers enter eastem Canada's rivers

and estuaries every year. In 1996, nine licences were issued by the Department
of Fisheries and Oceans to capture glass eels and elvers (Stevens, 1997). Total

landings have increased from 26 kg in 1989 to 3,238 kg in 1995 (Jessop, 1996).
Many of these were sold directfy to eel culturists in Europe andlor Asia while others
were acquired by commercial farms in Atlantic Canada. Jessop (1996)reported

that a large stock of juvenile Arnerican eel exists in Nova Scotia and the Bay of
Fundy area. The growing demand from Europe and Asia provides a high market
value ($100-kg") for juveniles weighing approxirnately 5 grams (Reiss, personal
communication). A properly managed, moderate-sale fishery for glass eels and
elvers for Nova Scotia and New Brunswick aquaculture can exist without any
negative effects on existing stocks. It is predicted that 50 tonnes of eel worth
$300,000 will be produced in Atlantic Canada by the year 2000 (DFO, 1995).

1.3.3 Nutritional Constraints
The current feed formulation of Arnerican eel diets is based on the
requirements established for Japanese eel and salrnonids. Extrapolating nutritional
research frorn an established commercially important species to a potential new
species is common and certainly aids in the initial diet development. However, to
develop economical feed fonnulae and efficient conversion of feed to flesh,
information on the nutrient requirements of this species at various stages of
developrnent is essential.


12

The most desirable protein source in tems of amino acid content and

organoleptic properties in commercial American eel feeds is herring meal. Thus,
it

is logical to begin studying American eel nutrition by quantifying the dietary

protein requirement. However, dietary energy concentration has a profound effect
on how well protein is utilized by the fish so it must also be exarnined. The aim of
this thesis is to evafuate: 1) the composition of diets cumntly being fed to American

eels 2) the optimum dietary m d e protein requirement of juvenile American eel and
3) the optimum dietary cnide protein to digestible energy ratio. It was anticipated

that these data would provide some preliminary information to determine the level
of dietary crude protein per megajoule of digestible energy that yields optimum
growth performance, feed conversion efficiency, nutrient digestibility and nitrogen
retention of juvenile American eel reared on a practical fishmeai based diet-


Chapter 2

Cornparison of Cornmercially Available Feeds
2.1

Literature Review
1

1 Feeding Fish vs. Feeding Terrestrial Animals

The nutn'ents required by fish are similar to those of terrestrial animals in that
they all require protein, vitamins, rninerals and energy sources for growth,
reproduction and other normal physiological functions. However, there are sorne
notable differences in those required as dietary additions for fish. Key differences
outlined by Lovell(1989) are as follows: (a) energy requirements are lower for fish
than for wam-blooded animals, giving fish a higher dietary protein to energy ratio,

(b)fish require some lipids that wam-blooded animals do not, such as omega-3 (n3) series fatty acids for some species and sterols for crustaceans, (c) fish have the

ability to absorb soluble minerals from the water which minimizes the dietary need
for some minerals and (d) fish have limited ability to synthesize ascorbic acid and
must depend upon dietary sources. In addition, most fish have limited ability to
utilize carbohydrate in cornparison with terrestrial animals (NRC. 1993).

2.1.2

Feeding Practices

Traditionally, cultured eels have been raised on raw fish fed either whole or
minced and frozen or fresh. Daily feeding rates on wet feeds were approximately

7-15% of body weight for large eels depending on water temperature and fish size
(Matsui, 1979) and 2030% of body weight for juveniles (Lovell, 1989). A number
13


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