Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.
2009
Volume 12
Issue 4
Topic:
Fisheries
ELECTRONIC
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
Czerniawski R. , Domagała J. , Pilecka-Rapacz M. 2009. REARING OF SEA TROUT FRY (SALMO TRUTTA TRUTTA L.) – AS POTENTIAL STOCKING MATERIAL, WITH LIVING ZOOPLANKTON AND DRY PREPARED FOOD, EJPAU 12(4), #14.
Available Online: http://www.ejpau.media.pl/volume12/issue4/art-14.html

REARING OF SEA TROUT FRY (SALMO TRUTTA TRUTTA L.) – AS POTENTIAL STOCKING MATERIAL, WITH LIVING ZOOPLANKTON AND DRY PREPARED FOOD

Robert Czerniawski, Józef Domagała, Małgorzata Pilecka-Rapacz
Department of General Zoology, University of Szczecin, Szczecin, Poland

 

ABSTRACT

The study was performed in three variants with the fry fed only on zooplankton, on mixed diet of zooplankton and dry food and only on dry food. The best parameters of growth were obtained for the fry fed on mixed diet and they were significantly bigger from those obtained for the fry fed on zooplankton. The survival of all variants was very good, however fry fed only on dry prepared diet survived in smallest amount. Fry fed on living zooplankton developed the behaviour typical of predators, observed already from the second day of feeding, that is fast attack to prey, in contrary to fry fed on dry prepared diet. These fish collected food mainly from the bottom of the tank and showed increased mobility only at the moment of food supply.

Key words: hatchery rearing, fry, Salmo trutta trutta, stocking, survival.

INTRODUCTION

In the last few years the population of salmonid fish has increased thanks to intense yearly stocking of hatchery produced fish [2]. The economic effect of the stocking is however limited by a number of factors: (a) low survival of fry, (b) problems with fish migration, (c) sudden changes in the natural environment and (d) the amount of smolts released. The measures aimed at increasing the survival include the rearing on prepared diets or in small watercourses [12,13,37] have not brought satisfactory results. It seems justified to expect that fry rearing under controlled conditions on living diet could increase the survival of salmonid fish. Some authors suggest that rearing of hatch or fry, in particular predatory species, on living zooplankton could be an important factor increasing survival rate [9,16,36]. According to many authors zooplankton is an excellent nutrition for many fish species, in many aspects it is comparable to dry artificial food [10,11,22]. Szlauer and Winnicki [36] indicate that the stocking success is greater with the fry fed on zooplankton than with the fry not fed or fed on artificial food and explain this effect by getting the fry into the habit of feeding on natural food. It implies that the main problem of fish rearing is to teach the fry to find food. Goryczko [14,15] reports that the lack of artificial food during rearing of trout fry leads to inhibition of learning to take food (foraging) and even if later the food was available in abundance the fry would die of starvation after using up all reserves. Although salmonids hatch in cold and clean waters of brooks and rivers, where zooplankton densities are extremely low, the contact with living zooplankton may develop in salmonid fish the hunting instinct which may directly increase the fish survival in the natural environment.

Recently much attention has been paid to fully ecological food [38], which zooplankton certainly belongs to. Much effort is directed to increase the success of stocking, which critically depends on the type of food provided. Dry artificial food is excellent for fish farming as permits getting fish of desired size and mass in a relatively short time of the controlled rearing cycle. However, this type of diet does not seem sufficient for feeding fish to be released into natural water reservoirs. Living zooplankton can be an important element of diet of salmonid fish larvae or fry, especially in the aspect of the restitution efforts of this group of fish.

The study was aimed at checking the possibility of using zooplankton for rearing of fish larvae and fry and observation of the behaviour of fry fed on living zooplankton and dry prepared food.

MATERIAL AND METHODS

The experiment was performed in a closed recirculation system. 200 ind. of 14-day old sea trout Salmo trutta trutta L. larvae from hatch were introduced into each of 9 tanks. Larvae came from the hatchery of Polish Angling Association in Goleniów. They were given the first food after absorption of 2/3 of the content of the yolk sacks and when observed to come to the water surface [15]. The time of rearing in all 9 tanks was 4 weeks with three replicates for each group. At the beginning densities applied in each tank were 5.7 fish l-3, the water volume was 35 dm3, and the height of the water column was 25 cm. The surface of tank bottom was 0.15 m2. Temperature of the water was maintained by a cooling device between 11 and 13ºC (due to technical difficulties we could not lower the temperature more). Once a week 1/3 of the water volume was exchanged. In entire rearing period mean value ± SD of ammonium (N-NH3) was 0.006 mg l-1 ± 0.003, dissolved oxygen 8.20 mg l-1 ± 0.51 and pH 8.09 ± 0.38.

The rearing was performed in three variants: A – fry fed on zooplankton, B – fry fed on zooplankton and dry food (Skretting, Perla Larva Proaktive 4.0, 62% protein and 11% fat) and C – fry fed on dry prepared food. The food, both the dry prepared food and zooplankton, was given ad libitum. The fish could feed ad libitum only during the day. Starting from 7 a.m. to 5 p.m. the fish had the food supplied on average every 3–4 hours. For the first 8 days the fish of variant B i C were given the dry food even every hour, till the time they learnt to collect food from the bottom. In the beginning they were only able to catch the food particles falling in the bulk water. Fish in every variant all the time (during the day) had food. Every week 30 individuals were caught from each tank, fish was subjected to the anaesthesia in the solution of Propiscin. Impurities were removed from containers every day. Fork length (LF) was recorded to the nearest 0.5 mm and weighed (M) to the nearest 0.1 mg, later fish were placed back into the tanks. The condition factor (K) was calculated by K = 102 M LF-3. The specific growth rate was found from the formula: SGR = (lnMF – lnMI 102) t-1, where: lnMF – the natural logarithm of the final mass; lnMI – the natural logarithm of the initial mass; t – time (days) between lnMF and lnMI.

Zooplankton to be given as food was caught in the nearby pond characterised by a high degree of eutrophication by a net of the mesh size of 50 ľm (no control of the food quality). The frequency of zooplankton collection depended on its density in the pond water. When its density was high, one collection was enough for the whole day of feeding; otherwise the zooplankton was collected two or three times a day. The zooplankton was transported to the hatchery and kept there in aerated tank till used. Once a week qualitative and quantitative composition of zooplankton was appreciated. In the first and second weeks of rearing 99% of the wet mass of the zooplankton contributed Rotifers, in the third week 98% of the wet mass of the zooplankton were Copepods with prevalent contribution of Cyclops vicinus Uljanin, 1875 in the fourth week 96% of the wet mass were the adult individuals of Daphnia pulex Müller, 1785.

The statistical significance of the differences was tested by the variance analysis ANOVA, and the post-hoc test was Tuckey test for samples of different abundance.

RESULTS

Rearing of sea trout fry
The survival of fry in tank C, fed only on dry prepared diet was 99%. The survival of the fry in other tanks was 100%. The specific growth rate SGR was calculated for the four weeks of the experiment. The fish fed on zooplankton grew on average by 3.5% day-1, the fish fed on the mixed diet grew on average by 4.6% day-1, and those fed on prepared diet – by 4.4% day-1 (Table 1).

Table 1. The specific growth rate SGR (% day-1) of sea trout larvae fed on zooplankton (A), dry feed (B) and dry feed plus zooplankton (C)

Variant

Date of research

30 March

6 April

13 April

24 April

Entire period

A

2.8

3.6

2.8

6.1

3.5

B

4.7

4.2

2.3

6.3

4.6

C

4.2

4.5

1.9

6.2

4.4

On the first day the mean fork length of the fry was 24.4 mm and the mean mass was 0.0896g. The increase in the length and mass were measured each week of rearing (Table 2). Results of the statistical analysis indicate significant differences in the mass of the fish in each week of the growth and over the whole period of the experiment (Table 3). The post-hoc test has shown a significantly lower mass of the fish fed with zooplankton (A) than those of the fish fed with mixed food (B) and dry food (C) in each week of the experiment. After the first week the differences were significant but the value of P was the lowest (A vs. B – 0.0023 and A vs. C – 0.0271). In the other weeks P = 0.0001 for A vs. B and A vs. C, and in the whole period of the experiment P < 0.0001.

Table 2. Growth of fork length LF (mm) and mass M (g) of sea trout larvae fed on zooplankton (A), dry feed (B) and dry feed plus zooplankton (C) during rearing

Date of
research

Temp.
(ºC)

R

D

LF

M

Range

Mean

Mean
±S.D.

Range

Mean

 Mean
±S.D.

23 March

10.0

   

20.00–26.72

24.44

 

0.045–0.125

0.090

 

30 March

11.0

A 1

5.7

25.54–27.84

26.46

26.43±0.71

0.091–0.142

0.108

0.109±0.018

A 2

5.7

25.49–27.61

26.65

0.090–0.140

0.111

A 3

5.7

25.49–27.30

26.16

0.093–0.130

0.108

B 1

5.7

25.19–28.74

26.88

26.88±0.97

0.106–0.160

0.123

0.124±0.014

B 2

5.7

25.74–28.51

26.85

0.108–0.159

0.124

B 3

5.7

25.75–28.24

26.91

0.109–0.153

0.126

C 1

5.7

24.36–28.96

26.72

26.82±0.88

0.080–0.177

0.117

0.121±0.017

C 2

5.7

26.12–28.50

27.02

0.111–0.158

0.126

C 3

5.7

25.64–27.72

26.68

0.108–0.134

0.119

6 April

11.0

A 1

5.7

24.98–27.85

26.52

26.52±0.81

0.083–0.178

0.115

0.115±0.017

A 2

5.7

24.89–27.82

26.54

0.101–0.128

0.115

A 3

5.7

25.00–27.74

26.51

0.010–0.125

0.116

B 1

5.7

27.17–30.44

28.98

28.92±0.98

0.139–0.192

0.168

0.168±0.017

B 2

5.7

27.71–30.33

29.03

0.136–0.194

0.167

B 3

5.7

27.06–30.35

28.77

0.149–0.195

0.168

C 1

5.7

27.12–30.30

28.67

28.74±1.11

0.142–0.190

0.166

0.166±0.017

C 2

5.7

27.04–30.43

28.73

0.141–0.188

0.166

C 3

5.7

27.33–30.13

28.79

0.139–0.183

0.165

13 April

12.0

A 1

5.7

26.22–29.31

27.37

27.47±0.94

0.120–0.181

0.140

0.140±0.018

A 2

5.7

27.16–29.20

27.39

0.119–0.182

0.140

A 3

5.7

26.42–29.14

27.65

0.121–0.170

0.141

B 1

5.7

28.12–32.51

30.47

30.43±1.29

0.149–0.246

0.195

0.196±0.030

B 2

5.7

28.01–31.82

30.47

0.147–0.245

0.196

B 3

5.7

28.13–31.63

30.37

0.157–0.221

0.199

C 1

5.6

28.30–32.07

29.91

29.86±1.36

0.159–0.255

0.191

0.190±0.026

C 2

5.7

28.38–31.41

29.87

0.160–0.232

0.188

C 3

5.6

27.18–31.53

29.85

0.165–0.232

0.191

24 April

12.0

A 1

5.7

29.85–36.17

32.49

32.29±1.76

0.210–0.399

0.278

0.275±0.046

A 2

5.7

29.42–35.27

31.96

0.217–0.333

0.268

A 3

5.7

29.96–35.28

32.42

0.215–0.380

0.278

B 1

5.7

34.80–40.86

36.92

36.99±1.81

0.322–0.630

0.400

0.396±0.070

B 2

5.7

34.53–39.01

37.04

0.331–0.500

0.390

B 3

5.7

34.18–39.21

36.99

0.323–0.499

0.398

C 1

5.6

33.95–37.55

35.80

35.90±1.30

0.287–0.489

0.373

0.375±0.046

C 2

5.7

34.13–38.11

36.17

0.335–0.408

0.379

C 3

5.6

34.34–37.99

35.77

0.297–0.488

0.373

S.D. – standard deviation; R – replicates; D – fish density in each tank (Fish L-1).

Besides the differences in the mass of the fish, also significant differences in the length of the fish in each week of growth except the first one were observed. In the whole period of the experiment the differences in the length of the fish were also significant (Table 3). The post-hoc test a significantly smaller length of the fish fed with zooplankton (A) than those fed with mixed food (B) and fodder (C), starting from the second week. After the second and third week P = 0.0001 for A vs. B and A vs. C. After the fourth week the fish grown in all variants of feeding differed significantly in the length A vs. B (P = 0.0001), A vs. C (P = 0.0001) and B vs. C (P = 0.0316), with the fish from variant C having greater mass than those grown in variant B. For the whole period of the experiment the post-hoc test results revealed significantly smaller length of the fish fed with zooplankton relative to the length of those grown in the other variants P < 0.0001.

Table 3. The differences between three variants of reared fish in fork length (LF), mass (M) and condition index (K) ( ANOVA)
 

Date of research

F

P

LF

30 March

2.47

0.0901

6 April

55.51

2.8924-16

13 April

50.37

2.9390-15

24 April

67.67

1.8762-18

Entire period

19.47

9.0015-9

M

30 March

6.56

0.0022

6 April

89.69

7.2792-22

13 April

44.53

4.8159-14

24 April

41.45

2.2682-14

Entire period

14.17

1.1990-6

K

30 March

7.27

0,0012

6 April

14.78

2.9784-6

13 April

3.72

0.0279

24 April

2.85

0.0634

Entire period

5.90

0.0029

The condition factor was lowest for the fry fed on zooplankton (A) in the first and second weeks, however, after the third week of rearing the condition factor of the fry from tank A was the same as that for the fry from tank C (fed on dry artificial food) (Table 4). The condition of the fish grown in different variants showed significant differences in each week of the growth except the fourth week and in the whole period of the experiment (Table 3). However, in contrast to the results concerning the length and mass of the fish, the post-hoc test revealed significantly smaller value for the fish fed with zooplankton after the first and second weeks and a greater value after the third and fourth weeks. After the first week the condition of the fish grown in variant A was statistically significantly different from that of the fish grown in variants B (P = 0.0026) and C (P = 0.0393), after the second week this difference increased (P = 0.0001). After the third week the fish grown in variant A had statistically significantly better condition than those from variant B (P = 0.0208), while after the fourth week no statistically significant differences were observed, which suggests that the condition of the fish fed with zooplankton was similar to that of the fish from the other variants. However, when taking into regard the whole period of the experiment, the condition of the fish fed with zooplankton was statistically significantly worse than that of the fish fed with fodder (P = 0.0026) and mixed food (P = 0.0393).

Table 4. Condition factor (K) of sea trout larvae fed on zooplankton (A), dry feed (B) and dry feed plus zooplankton (C)

Variant

Date of research

30 March

6 April

13 April

24 April

Entire period

A

0.75

0.78

0.85

0.98

0.67

B

0.81

0.86

0.85

0.92

0.71

C

0.79

0.86

0.87

0.96

0.70

The greatest increase in mass and length was observed for the fish fed on mixed diet, this increase was a bit lower for the fry fed on the dry food and the lowest for the fry fed on zooplankton. However, the fish fed on zooplankton revealed development of strategies typical of predators; that is the selection of the zooplankton according to the size, identification of the prey and capturing it after a short chase or a rapid attack. The fry fed on the prepared dry food or dry food and zooplankton did not show this type of behaviour, usually they collected food from the bottom of the tank. The fish from tanks B and C were mainly near the bottom of the tanks and were moving faster and in a more abrupt way only at the moment of food introduction. Observation of the fish fed on the mixed diet proved that only a few of the fish were in the upper layers of water and fed on the zooplankton and dry food, while the majority were near the bottom and ate mainly the dry prepared food. The fish fed only on zooplankton were observed to take food from the first day of its introduction, which is important for fish farming. The fish fed only on prepared diet for the first few days ate only the particles of the food floating in the water and were not interested in the food on the water surface and at the bottom of the tank. Moreover, among the fish fed on zooplankton the so called starvation forms resembling pins were not observed at all. Among the fry fed on dry prepared food there were individuals that took food rarely and developed much slower than the rest.

DISCUSSION

In the period of rearing, the fish fed only on zooplankton gained mass at the lowest rate and were in the first two weeks characterised by the lowest SGR values. This result can be related to the type of the living zooplankton available; at that time it contained mainly Rotifers. Only after the second week of rearing the gain in mass of the fish from variant A increased, which was related to the change in the zooplankton species available to larger and easier visible species of Daphnia sp. and Copepoda, make an excellent source of nutrients for the fish. Zooplankton is a valuable source of proteins, amino acids, fat and enzymes [25,27,29], and the level of proteins in some Cladocerans reaches from 54 to 65% [20]. According to Yurkowski and Tabachek [40] the level of amino acids in Daphnia pulex and Diaptomus sp. is sometimes even higher than that required for fry rearing. Poczyczyński [30], Marmulla and Rosch [23] claim that the nutritive value of artificial food is much higher than that of zooplankton. The artificial food provides all the components needed for fast growth in a relatively short time [3,31]. After all fish from the variant A were smallest and lightest what is most probably associated with the better nutritious composition of modern dry food. However, according to Wolska-Neja and Neja [39], in respect of nutritive value determining the rearing success the artificial food is much better but the natural food although of lower energetic value has some specific worth. The activity of digestive enzymes of the larvae is lower than that of adult fish of the same species and the deficiency of the own proteolytic enzymes is partly compensated by the enzymes of the zooplankton [30]. Some authors who observe relations between fish kept in artificial, natural or semi-natural conditions, suggest rather a natural rearing which influence positively on survival, behaviour and physiology of juvenile salmon [17,24].

The freshwater zooplankton has been rarely or not at all used in the rearing of the salmonids, which is well understood, as in cold watercourses there is practically no zooplankton and in the season of salmonids rearing (winter) it is difficult to get. Zooplankton starts occurring in large amounts from the spring months [8,35]. However, as follows from the results of our study, the contact with living fresh water zooplankton was the main factor ensuring the stocking success. Although the main diet of young salmonids is made of benthos invertebrates [1], according to some authors the fish from this family also gladly feed on zooplankton. Besides the benthos components dominant in their diet, the salmonids also eat the cryophilic zooplankton species [32,33]. Szlauer [34] who fed the rainbow trout fry on zooplankton reported intense foraging throughout the time of the experiment. She also reported that in the alimentary tracts of the fry the typically cryophilic C. vicinus and Cyclops kolensis Lilljeborg, 1901 were dominant by mass, which proved the food selectivity of the fish. Even adult salmonids living in lakes, in contrast to the river species, gladly feed on zooplankton (which is related to its availability), although the main component of their diet is nekton Jorgensen et al. [18] Kahilainen and Lehtonen [19], Nilssen and Waervagen [28].

As mentioned above, in our experiment the fry from variant A was fed on zooplankton obtained from natural water reservoirs in the period of early spring. It should be mentioned that at that time the zooplankton is clean, without algae, whose later effect on the zooplankton quality is significant [34]. For the salmonids commonly known as difficult in rearing, the early spring algae-free zooplankton seems recommendable. In the period of rearing in the three variants, the survival of the fry was very high. The high survival can be attributed to the fully controlled and optimum for lake trout rearing conditions. The starving forms not taking food were only a few. According to Goryczko [15], the visible symptoms of the irreversible starvation changes in trout larvae (which can assumed also for lake trout) are the loss of mass (thinning) and darkening of the colour. As these symptoms were noted in the dead fry in our experiment, we suppose that they died because of not taking food.

In this type fish breeding the problem is to provide zooplankton, whose collection is tedious and time-consuming. We have found in literature a simple way of mass collection of zooplankton realised by fixing a net across the river at its outflow so that it would keep the zooplankton carried out from the lake [8,35]. Because of a considerable distance between hatchery and the river outflows from the lakes, we decided to catching of zooplankton in nearby pond.

The fry rearing on zooplankton could correct probably the survival of fish in watercourses. According to literature, introduction of fry fed sea trout on dry prepared food into watercourse enhances the chances of its survival in the first year of life to 45% [5,37], while the survival of sea trout from the release of reared fry to smolting period can reach 9–11% [6]. Cunjak and Therrien [4] noted also the low survival (9%) in winter fry of wild atlantic salmon. Much worse results are obtained from release of unfed fry. The death rate of one-year old sea trout coming from the fry introduced to small watercourses can reach even 99% [7]. The introduction of unfed fry with partly absorbed yolk sac can be one of the reasons for high death rate as the yolk sac is relatively heavy and does not permit resistance to water current, high mobility and effective search for hiding places [37].

Perhaps, feeding the fry with living zooplankton in the period of rearing can influence on survival and condition factor of fish in natural conditions. The fry fed on living zooplankton developed the behaviour typical of predators, observed already from the second day of feeding, that is fast attack to prey [9,21]. The contact with living and moving food must have initiated development of the hunting instinct, often critical for survival in the natural habitat. The behaviour of the fry fed on prepared dry food was different; the fish collected food mainly from the bottom of the tank and showed increased mobility only at the moment of food supply. Their energy cost related to taking food was probably low, which resulted in a fast increase in the body mass. Fry fed on mixed diet in the period of rearing behaved similarly as those fed on dry food but their survival was greater. Thus, it can be supposed that the presence of zooplankton and sporadic feeding on it despite the availability of much favoured dry food was the factor increasing the survival of this group. Additionally, Morrison [26] thinks that zooplankton is willingly eaten by fish because is still moving.

CONCLUSIONS

To get the optimum results the rearing must be performed in fully controlled conditions, with the use of devices for aeration, cooling and inducing circulation of the water and the fry should be fed ad libitum. The zooplankton was a sufficient source of nutrition for the fry to reach the mass and body length optimum for stocking. The most important advantage of using this type of food was the fact that it stimulated development of the hunting instinct in the fry, starting already from the first days of foraging. The development of this instinct can be crucial factor determining the success of stocking.

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Accepted for print: 4.11.2009


Robert Czerniawski
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24
email: czerniawski@univ.szczecin.pl

Józef Domagała
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24
email: jozef.domagala@univ.szczecin.pl

Małgorzata Pilecka-Rapacz
Department of General Zoology, University of Szczecin, Szczecin, Poland
Z. Felczaka 3C
71-412 Szczecin
Poland
phone: +48 91 444 16 24

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