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.
Volume 13
Issue 2
Available Online: http://www.ejpau.media.pl/volume13/issue2/art-04.html


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



The paper shows the results of different types of food used in rearing and the impact of the food on the further survival and growth of brown trout fry (Salmo trutta m.  fario L.) in the wild. Rearing of larvae was conducted in three groups: 1) zooplankton-fed group, 2) chironomids-fed group and 3) pellet-fed group. During the rearing period, the pellet-fed group reached the greatest SGR value – 4.25% day-1, while the zooplankton-fed group reached 4.25% day-1. The chironomid-fed group were characterised by the lowest SGR value (1.94% day-1). In the wild, the zooplankton-fed group achieved the greatest rate of survival (60%) and high data of SGR (2.23% day-1). The zooplankton-fed group's survival and growth rate was higher than the pellet-fed group survival (38%) and SGR (1.87% day-1). In the wild, the chironomid-fed group reached the lowest rates of survival (23%), but the fork length and weight results were not different in comparison to the zooplankton-fed group. Moreover, the SGR result of this group was the highest – 2.55% day-1. We stated that rearing of brown trout larvae on live zooplankton had a positive effect on survival and growth of brown trout fry in the wild.

Key words: Salmo trutta, rearing, foraging skills, stocking.


The interest concerning the survival and growth of salmon increased during the period when the salmon population had drastically decreased [5]. At present the restitution of salmonids depends exclusively on measures undertaken by people [5,26,3]. One of the major problems concerning restitution of salmonids in natural conditions is their high mortality a few days after release [34]. The high mortality may be the result of worse adaptation by hatchery-reared fish for living in the wild. Recently, attempts have been made to improve adaptation of hatchery fish to life in natural surroundings by acclimatizing them in a hatchery or in stream in order that they may learn foraging skills [5]. The relevant studies concern the interactions between the wild and hatchery-reared fish [1], ways of finding food [32], and anti-predator skills [29]. Despite the many attempts and annual stockings, the survival of smolts is still low [1]. Survival of smolts generally does not reach 20–30% [7,39]. It is therefore necessary, to seek effective ways which will act positively on the survival of salmon larvae released into the wild.

Stocking with hatchery-reared fish is a popular way to restore salmonids. Stocking with hatchery-reared fish ensures better survival and growth than trying to restore salmon without rearing fry  [37,12,13]. However, studies have shown that hatchery fish mortality is high and at least twice as high as that of wild fish [34,12]. Perhaps this is linked to the adaptation of fish larvae to the wild and limited catching of live food. Stradmeyer and Thorpe [33] reported that salmonid fry in natural conditions feed mainly on insect larvae drifting in the stream. These insect larvae are the primary food for them [2]. Therefore, in order to adapt the fry to natural conditions it seems reasonable to feed the young fish on drifting food. It seems reasonable but it isn't so straightforward. The ability to get large quantities of insect larvae in the cold season for the hatching salmonids, or the breeding of insect larvae in a hatchery, is not possible or is associated with many difficulties. In addition, insect larvae in the tanks may fall to the bottom as a result of the lack of turbulence. It has been suggested that zooplankton has many of the necessary advantages. It can be used as a substituted food because: (1) it is readily available throughout the year, so it is available during the important hatching period of the salmonids, (2) in spite of the lack of turbulence in the tank, zooplankton actively swim non-stop in a water column which resembles a drifting motion. This motion provokes fish to attack the prey and (3) zooplankton are a valuable source of nutrients [23].

The main aim of this study was to determine the effect that rearing using live zooplankton, dropped larvae of chironomids, and pellets, would have on the survival and growth of hatchery-reared brown trout (Salmo trutta m. fario L.) in the wild. We assumed a hypothesis that the fish fed on live food during rearing reached greater survival and growth in the wild, than fish fed on pellet. We also wanted to demonstrate the possibility of using live zooplankton and chopped chironimids larvae when rearing brown trout larvae.


The experiment was performed in a closed recirculation system hatchery of the University of Szczecin's Department of General Zoology. Larvae for the experiment were from the hatchery of the Polish Angling Association in Goleniów. The 18-day old fish larvae of brown trout (Salmo trutta m. fario L.) from hatcheries were introduced into water tanks. They were given their first food on the day when they were observed coming to the water surface. At the beginning the densities in each tank were of 210 fish per 55 L water volume  and 25 cm water column height.  The temperature of the water was maintained by a cooling device. The water was kept between 11 and 15ºC. The time of rearing in all tanks was 6 weeks. This is a relatively short period of time, chosen deliberately, because as claimed by Gross [15] and Dębowski [11] long detention of the fish while rearing change the developmental strategy of a population.

The rearing was performed in three variants: A – fry fed on zooplankton (zooplankton-fed group), B – fry fed on chopped chironomids larva (chironomids-fed group) and C – fry fed on pellet dry food (pellet-fed group) (Skretting, Perla Larva Proactive 4.0; 62% protein and 11% fat). Experiments in each variant were repeated three times. The food, in the three variants, 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 an average of every 3–4 hours. For the first 8 days the fish of variant B and C were frequently given the food. They were fed every hour, until they learnt to collect food from the bottom. In the beginning they were only able to catch the food particles falling in the water.

Every day while they were being fed, and for 0.5 hour after each feeding, the behavior of each group of fish was observed. The zooplankton which was to be used as fish food was caught by net in a nearby pond. The mesh size of the net was 50 ľm. The frequency of the zooplankton collection depended on the density of the zooplankton in the pond water. When density was high, one collection was enough for a whole day of feeding. When density wasn’t high, zooplankton was collected two or three times a day. The zooplankton was transported to the hatchery and kept there in an aerated tank until used. Throughout the experiment 90–98%             of the wet mass of the zooplankton were Copepods with a prevalent contribution of Cyclops vicinus. Only in the third week were 50% of the wet mass of the zooplankton Rotifera. Frozen chironomids larvae were chopped into 0.5–1.0 mm pieces.

Every week, 50 individual fish larvae were caught from each tank. They were anaesthetized in a Propiscin solution. Fork length (LF) was recorded to the accuracy of 0.5 mm. Weight (M) was recorded to the accuracy of 0.1 mg. Once the fish were roused they were placed back into the tanks.  The condition factor (K) was calculated by K = 105 M LF-3. The specific growth ratio 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.

The fry from each variant of rearing was marked. The fry fed on zooplankton (A) had the adipose fin cut off in the second week of rearing. The fry fed on chopped chironomids larvae (B) were marked using fluorescence immersion on the second day following the completion of hatching. Marking by fluorescence was done by immersion for 3 hours in an alizarin red pigment solution at a concentration of 100 ppm. The fry fed on a pellet diet (C) were left untouched. This method is effective and safe for fish growth and survival [36,28].

On March 22, 2008 after 6 weeks of rearing 165 individuals of each group were released into the stream. The larvae were released in the upper section of this stream (Fig. 1). All groups were released together in the same place. Two days before release we caught all the fish living in the stream. Fish migrating from the lake to the upper section of the stream had no chance of moving to a stocking place because 200 meters before the mouth of the stream to the lake, was a 60 cm natural shoot. In  the section below the shoot were 33 gudgeon (Gobio gobio L.), 12 roach (Rutilus rutilus L.) and 6 perch (Perca fluviatilis L.). All perch were transferred to the lake. Above this shoot, to the place of the headwaters of the watercourse, there were no fish.

Fig. 1. Study area

For stocking, the small watercourse Bagnica was chosen (Fig. 1). This watercourse is 7 km long and ends in Lake Adamowo (buffer zone of the Drawieński National Park, NW Poland). The bottom of the watercourse is predominantly covered with gravel. The width of the watercourse  varies from 0.5 to 3 m. The water temperature in summer does not exceed 20°C. The current speed and water discharge on the stocking day were as follows: 0.6 m s-1, 1.33 m3 s-1. A stream offers a uniform habitat and a good environment for salmonids.

Fish survival was checked in the autumn in 22 October 2008. At this  time they were captured with the use of an electric fish catcher (Hans Grassl ELT60 II, Germany). The fish were caught along the whole length of the stream. To make sure that all the fish were caught, the procedure was performed three times on the same day. The fish were caught by three persons: two persons were collecting the fish, while the third person walked 50 meters behind them to check that the rest of the stunned fish were not carried downriver. No other predatory species were caught because the predatory fish had been caught prior to stocking the stream with the fry.

The statistical significance of the differences in the rate of survival, SGR, fork length and weight of reared and captured fish, was tested by the variance analysis ANOVA, and the post-hoc Scheffe test.


In the hatchery
Results of the ANOVA analysis indicate significant differences in fork length and weight of fish between different groups, for both the entire rearing period, as well as for each week  The Scheffe test shows that fork length of the zooplankton-fed group was significantly greater than the chironomids-fed group from the first to the fourth week P = 0.0037, P = 0.0012,  P = 0.0087, P = 0.0004, respectively (Fig. 2, Table 1). Additionally, the zooplankton-fed group had significantly greater fork length than the pellet-group, but only in second week P = 0.0095. In the fifth and six week the fork length of chironomids-fed group was significantly shorter than the two other groups (P < 0.0001).

Fig. 2. Mean ± SD of brown trout larvae (Salmo trutta m. fario L.) fork length (A – zooplankton-fed group, B – chironomids-fed group, C – pellet-fed during rearing

Growth of fork length LF (cm) and weight M (g) of brown trout larvae fed on zooplankton (A), chironomids larvae (B) and pellet feed (C) in last day of rearing in the hatchery and of fry in the wild. S – survival rate (%); K – condition factor; SGR – Specific Growth Rate (% day-1). Values of condition factor and SGR during rearing relate to entire period of rearing


Age of fish (days)



W ± SD



In hatchery




0.31 ± 0.15

0.2372 ± 0.0435

0.71 ± 0.08




0.24  ± 0.20

0.1020 ± 0.0267

0.68 ± 0.11




0.31 ± 0.30

0.2594 ± 0.0476

0.73 ± 0.11


In the wild




14.06 ± 2.06

35.76 ± 13.09

1.22 ± 0.08




13.39 ± 2.07

31.55 ± 13.47

1.22 ± 0.08




11.31 ± 1.29

18.20 ± 6.76

1.20 ± 0.11


The fork length, like the weight of the zooplankton-fed group, was characterised by significantly greater data results up to the fourth week of rearing (Fig. 3). In first two weeks the Scheffe test indicated that the weight of the zooplankton-fed group was significantly higher than the chironomids-fed group P = 0.0007, P = 0.0005, respectively and the pellet-diet group  0.0222, P = 0.0026, respectively. In the third and fourth week the A and C groups showed significantly greater weight than the chironomid-fed group A vs. B – P = 0.0278, B vs. C – P = 0.0437, A vs. B – P = 0.0002, B vs. C – P = 0.0034, respectively. While in the last two weeks of rearing, the weight  between these groups differed most (P < 0.0001). Ultimately, the pellet-fed group had the greatest weight.

Fig. 3. Mean ± SD of brown trout larvae (Salmo trutta m. fario L.) weight (g). A – zooplankton fed group, B – chironomids-fed group, C – pellet-fed during rearing

For the whole rearing period it was the pellet-fed group that reached the highest value of SGR (Table 1). In first two weeks, the zooplankton-fed group showed the fastest growth,. In the third week, however, the pellet-fed group had the highest value of this parameter. In the fourth week the zooplankton-fed group grew faster than the chironomid-fed group, but the difference was small.

The curve of relationship between length and weight shows, that when at the same length, the zooplankton-fed group and pellet-fed group were characterized by greater weight than the chironomid-fed group (Fig. 4). The condition index of the pellet-fed group reached the highest value in comparison with the fish in the other two groups.

The zooplankton-fed group reached the greatest survival rate – 99%, next were the pellet-fed group – 91% and the lowest was among the chironomids-fed group 77% (Table 1).

Fig. 4. Relationship between fork length and weight of reared fish. A – zooplankton-fed group, B – chironomids-fed group, C – pellet-fed group

The results of observations of fish during rearing indicate differences in the behavior of fish from different groups. The zooplankton-fed group reacted to the food on the first day of feeding. During whole period of rearing this fish group actively swam in the entire volume of the tank. The pellet-fed group, generally ate food from the bottom of the tanks, and moved mainly during feeding time. The pellet-fed group was observed actively swimming in the tank and eating the falling feed particles from bulk water on the 7–8 day of rearing. The pellet-fed group was able to completely eat the pellets from the bottom after the third week. The chironomid-fed group ate food only from the bottom and they did not react to food as it was sinking to the bottom. The chironomid-fed group did not eat all the food throughout the whole period of rearing. Among the zooplankton-fed group non-feeding individuals were not observed, whilst among the other two groups information about the non-feeding individuals were carefully noted.

In stream
After the capture of fish from the watercourse the zooplankton-fed group reached the greatest survival rate. They had a rate of up to 60% (98 individuals). The next highest was the pellet-fed group. The pellet-fed group had a survival rate of 38% (62 individuals). The lowest rate of survival was found in the chironomid-fed group. They had a survival rate of 23% (38 individuals) (Table 1).

Results of the ANOVA analysis indicates significant differences in fork length and weight between the three groups (P < 0.05). The Scheffe test shows, that fork length and weight of the zooplankton-fed group and the chironomid-fed group were significantly greater than the pellet-fed group (P < 0.0001) (Table 1). Thus, in the wild the chironomid-fed group reached greater fork length and weight than the pellet-fed group. In the wild the zooplankton-fed group reached the greatest length and weight, but these results were not significant in relation to the chironomid-fed group (P > 0.05).

The zooplankton-fed group and chironomid-fed group both reached the same, highest value for this factor (Table 1).

Fig. 5. Relationship between fork length and weight of captured fish from the stream. A – zooplankton-fed group, B – chironomids-fed group, C – pellet-fed group
The curve of relationship between fork length and weight shows, that in the wild the pellet-fed group was characterised as having the same length but this group weighed less than the others (Fig. 5).


The zooplankton-fed fish in the wild, had the best parameters of growth and survival. Their parameters for growth and survival were better than the parameters for growth and survival of the pellet-fed group. In the wild, the recorded survival rate of the pellet-fed group was lower than fish fed on live zooplankton, but it was higher than the chironomid-fed group. This may be related to the fact that the pellets were more likely caught by fish than the chopped pieces of chironomid larvae. The pellets remained in the water longer than chopped chironomids larvae, so more fish could learn to catch the pellets. The chopped chironomids larvae fell quickly to the bottom, without always catching the attention of the fish. By contrast, the live zooplankton drifting non-stop in water bulk, could provoke the fish to attack. According to Morrison [25], live zooplankton is readily accepted as food because its continuous motion stimulates the interest of the fish. It seems, therefore, that the important factor here was the time  the food was kept in the water bulk.

It is generally known that the survival of hatchery reared fish in the wild is low [34,38,40] in comparison to wild fish, However, the results of this study show that the addition of a natural element during rearing, contributed to increasing the survival of fish in the wild. The stated survival rate of the zooplankton-fed group caught from the stream was substantially higher than that stated in the literature data in the introduction. There is also evidence of the positive impact that rearing larvae salmon on a pellet diet, has on the survival increase of trout. The survival rate of hatchery-reared 0 + trout in the wild may reach 40 – 50% [39,6]. However, in most cases the result was much lower (Brown & Day, 2002). Much worse results are found when stocking trout that was not reared. These results were from seven to more than 90% [8,22,31].

It should be noted that the absence of predators in the watercourse also had an impact on the positive results of fish survival. According to Kennedy and Strange [18] survival and growth may be limited by the presence of predatory fish. In the stream, there were no predators. The possibility of fish migrating to the lake was excluded. First, 80% of the fish were caught in the section located far from the lake, and secondly, the first fish was caught at a place located               2 km before the mouth of the stream to the lake (Fig. 1).

In analyzing the growth of fish in the wild, it can be concluded that the type of food used during rearing probably influenced the results of length, weight and the condition of the fish. The zooplankton-fed group had the highest weight and length results as well as the highest survival rates. The pellet-fed group had much lower results compared to the chironomid-fed group and the zooplankton-fed group. The chironomid-fed group and the zooplankton-fed group had similar weight and length results. It can therefore be assumed that  natural food even if it was immobile, had an impact on the growth of fish in the watercourse. During rearing a small number of the chironimids-fed group had learned to eat the chopped pieces of chironomids larvae and most of these fish survived in the wild. Chironomids larvae are an important part of the diet of salmon in the wild [2,24]. In the present study the live diet used during rearing positively influenced the survival and growth of brown trout fry in the wild. A diet of chopped chironomids larvae, however, had a positive effect only on fish growth.

Based on the results of this study it can be assumed that during rearing the type of food had an influenced on the foraging skills of brown trout larvae. This in turn could have an effect on fry survival. The zooplankton-fed group learned to eat food more quickly than the pellet-fed group. According to Paszkowski and Olla [27], Lazzaro [20] and Reiriz et al. [30], the process of learning to find and identify the prey is of key significance for the future survival of the fish in the watercourse. The fish fed on pellets revealed a different type of behaviour in the watercourse: they were looking for food near the bottom or at the bottom and moved intensely only when the food was supplied. They spent less energy on finding food and they reached the highest values of body mass. However, in the watercourse the fish fed on pellets reached much poorer values of survival rate and growth parameters than the fish fed a living diet. This is probably due to an underdeveloped predatory instinct.

During the rearing period, the pellet-fed group reached the lowest growth rates in the first two weeks. It seems that this group started eating pellet intensively at the end of the first or in the second week  Strandmeyer and Thorpe [32] also stated that Atlantic salmon larvae ate pellets 2–3 days later than the larvae that ate insects. In the present study, we observed also, that SGR rate of the pellet-fed group and the chironomid-fed group was similar in the first two weeks of rearing. We suppose that during this time the growth of these fish was only possible thanks to yolk sac resorption. After this period the weight of the pellet-fed fish differed rapidly from the weight of the chironomid-fed group. The chironomid-fed group continued to grow. Until the end of rearing, the chironomid-fed group continued to show a slightly faster growth. But this slightly faster growth took into account only small quantities of fish, most probably those which had learned to prey. In the third week the rate of SGR of the zooplankton-fed group was the lowest. This could have been due to the smaller biomass of adult copepods, and the more than 50% share of small rotifers in the biomass of the zooplankton. In the other weeks large copepods formed over 90% of the biomass of zooplankton. These large copepods were more visible to the fish. Szlauer [35] fed trout fry on live zooplankton and found that the fish ate intensively during the whole rearing period. He also found that the large copepods dominated in their stomachs. In the present study, the increase in water temperature may have caused the much faster growth of fish recorded in the fifth week of rearing.

The survival of the reared fish depended on the use of food. The greatest mortality was achieved by the chironomid-fed group, and then by the pellet-fed group. No signs of disease were observed in the dead fish. They probably died of starvation. According to Goryczko [33], the visible symptoms of the irreversible starvation changes in trout larvae are the loss of mass (thinning) and a darkening of colour. As these symptoms were noted in the dead fry in our experiment, we supposed that they had died because they did not eat. A similar observation was made by Czerniawski and Czerniejewski [9] and Czerniawski et al. [10]. They noted that the biggest loss was from pellet-fed larvae of sea trout, and the lowest from larvae fed live zooplankton.

To sum up, it can be concluded that the most effective type of food in hatchery rearing of the brown trout to be used as stock, is natural live zooplankton. Most authors studying the relationship between fish reared in artificial and natural or semi-natural conditions, strongly suggest a rearing which is the most similar to the natural. This is because natural conditions positively affect the survival, behavior and physiology of fish [4,21,17,19]. However, the research of Jokikokko [16] shows behavioral differences between wild salmon and hatchery-reared salmon, even where natural elements were used. Despite this, for reasons of rational management of salmons, it is worth using natural elements during the rearing of larvae intended for stocking.


To sum up, it can be concluded that the most effective type of food in hatchery rearing of the brown trout  to be used as stock, is natural live zooplankton. The diet used during rearing should be the closest possible to the natural one in the rational management of the salmonids in view of their restitution program.


We would like to thank Rafał Pender from the West-Pomeranian District of the Polish Angling Association for his valuable help in the rearing experiment.


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Accepted for print: 8.04.2010

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

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

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

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