Effect of earthworms as replacement for trash fish and rice field prawns on growth and survival rate of marble goby (Oxyeleotris marmoratus) and Tra catfish (Pangasius hypophthalmus)

Nguyen Huu Yen Nhi, T R Preston*, Brian Ogle** and Torbjorn Lundh**

An Giang University, 25 Vo Thi Sau St., Long Xuyen City, Vietnam
nhynhi@agu.edu.vn
* TOSOLY, AA 48 Socorro, Colombia.
** Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences,
P.O. Box 7024, 750 07, Uppsala, Sweden.

Abstract

Two experiments were carried out in the research farm of An Giang University. The first experiment was a study on the growth performance of Marble goby and Tra catfish fed diets of trash fish and rice field prawns replaced (on an iso-nitrogenous basis) with 0, 25, 50, 75 and 100% of frozen earthworms (Perionyx excavates).

The weight gains of Marble goby and Tra catfish, and the survival rate in the Marble Goby, decreased markedly with curvilinear trends as the proportion of frozen earthworms in the diet was increased (Experiment 1). The Tra catfish appeared to adapt better than the Marble goby to the frozen worms.  The second experiment, which was designed to test the hypothesis  that the poor growth rates with frozen earthworms was due to the low palatability of the earthworms after being frozen, confirmed the negative effects of this method of conservation. Growth rates were 4 and 2 times greater for Marble goby and Tra catfish, respectively,  when they were fed fresh rather than frozen earthworms.  Survival rate was 100% on the fresh earthworm diet.

Keywords: Aquaculture, feed conversion, fresh and frozen earthworms

Introduction

The population of Vietnam in 2009 was reported as 85.79 million.  http://www.vnnewstime.com/tin-doi-song/vietnam%E2%80%99s-population-hit-858-million/ The need for food for the people is increasing. Fish is the most important source of protein in the national diet, so aquaculture is an important sector in Vietnamese agriculture. Fish are also an important component of the export trade, bringing valuable foreign exchange for the country.

Freshwater aquaculture systems have developed rapidly in Vietnam during recent years. An important feature of these systems is the use of trash fish harvested from coastal areas and also from freshwater surfaces as a feed for culturing fish of economic importance. There are other demands for trash fish, as livestock feed, to make fish sauce and also as human food.  As the availability of this natural resource is finite, the increase in demand is resulting in an increase in the price.

An alternative to the use of small shrimp that is now receiving increasing attention is the earthworm. A recent study by Phan Phuong Loan et al (2009) showed that the zigzag eel fish grew faster when fed on earthworms than when fed small shrimp. Earthworms can be produced by many methods, but especially interesting is the use of organic matter from agricultural activities (Tian et al 1997; García and Fragoso 2003), animal waste (Edwards et al 1978) and kitchen waste (Luu Huu Manh et al 2009). In a companion paper (Nguyen Huu Yen Nhi 2010) it was shown that the earthworm Perionyx excavates grew well on both cattle and buffalo manure.

Presently in Vietnam, the fish species Marble goby and Catfish have especially high value. Marble goby (Oxyeleotris marmorata) is commonly cultured in cages in rivers and reservoirs, ponds, and coves in Vietnam (Vu Cam Luong et al 2005). This species is considered as a delicacy in some Asian countries and fetches a high price (> US$17/kg) because of its tender meat and tasty flavor (Cheah et al 1994). However, marble goby culture has decreased due mainly to lack of seed supply and disease outbreaks, with especially high mortality during the larval stages. Marble goby is a “sit-and-wait” predator (Nguyen Phu Hoa and Yang Yi 2007). However, its feeding behavior has not been fully understood, especially at the young stage. Marble goby larvae that preyed on live food had better survival rate and growth rate than larvae fed with artificial food (Cheah et al 1994). The use of live food for nursing fingerlings might be the best way to improve seed production in this species.

Tra catfish (Pangasius hypophthalmus), a member of the Family Pangasidae, has high economic value, and has become one of the important species in Vietnam and other countries in the South East Asia region. This fish is raised in ponds, cages and fence culture (Chau Thi Da et al 2007). However, farmers have changed from the cage culture to pond culture because the cost for the cage system is higher and the fish easily become diseased. The fingerlings of Pangasius hypophthalmus are almost entirely produced by artificial rearing because of the reduced supply of natural fingerlings from the Mekong River (Van Zalinge et al 2002).

On the basis of the above reports, it was decided to study the role of earthworms in the raising of fingerlings of Marble Goby and Tra catfish, as an alternative to the traditional method of using trash fish and rice field prawns.

The underlying hypothesis of the study was that earthworms would support growth and survival rates in fingerling Marble goby and Tra catfish comparable with use of trash fish and rice field prawns

Experiment 1: Replacing trash fish and rice field prawns with earthworms in diets for Mable goby and Tra catfish

Materials and methods

Location and climate

The experiment was conducted from August 2009 to October 2009 in the experimental farm of An Giang University, Long Xuyen City, An Giang province. The climate in this area is tropical monsoon, with a rainy season between May and October and a dry season from November to April. The mean air temperature is 27°C. Average annual rainfall is 1400 -1500 mm.

Treatments and design

The treatments (Table 1) arranged as a 2*5 factorial with 3 replications were:

·        Fish species: marble goby and tra catfish

·        Protein source: different proportions of earthworms (0; 25; 50; 75; 100% on crude protein basis) replacing trash fish and rice field prawns (Table 2).

A completely randomized design (Table 2) was used.

The experimental period was 13 weeks.  The first week was for adaptation of the fish to the new diets. Data were recorded over the following 12 weeks.

Fingerlings

Marble goby (Photo 1) and Tra catfish (Photo 2) of around 3-5cm in length (1-2 g) were bought in My Thanh hatchery center, Long Xuyen City. They were kept separately in composite tanks and fed with trash fish (Photo 3) and rice field prawns (Photo 4) mixed with rice bran prior to the experiment. The stocking density was 100 fish/m³. Thus the 30 tanks (Photo 5), each 0.5 m³, had 50 fish in each tank.

Photo 1. Marble goby (Oxyeleotris marmoratus)	Photo 2. Tra catfish (Pangasius hypophthalmus)
Photo 3. Rice field prawn (Macrobrachium lanchesteri)	Photo 4. Trash fish
Photo 5. Plastic bins used in the experiment

Management and feeding

The earthworms were purchased from a commercial farm in Long Xuyen City. Rice field prawns and trash fish were bought in a local market. The earthworms, trash fish and rice field prawns were minced and mixed well with rice bran and squid oil (Table 3).

The feeds supplied 50% protein in the diet DM and were fed at 8% of the fish body weight (DM basis). They were given two times per day at 7:00 and 17:00h. The residual feed and fish excreta in the tanks were removed by siphoning, and fresh water was added before feeding in the morning. The residues of feed and feces were filtered and collected separately for Marble goby and Tra catfish, then stored in a freezer (-18⁰C) until the end of the experiment, when the pooled samples were analyzed for ash, DM and crude protein (CP).

Measurements

Feeds offered were recorded daily and samples collected twice a week for chemical analysis. Before starting the experiment, 100 Marble goby and 100 Tra catfish were randomly chosen to measure the length and weight. At the end of the experiment, the weight, length and numbers of all the fish were recorded.

The following water environment factors were measured every 2 weeks.

·        Temperature, pH and dissolved oxygen were measured in the morning and afternoon.

·      Samples of the water were taken and kept in a refrigerator until analysis for total ammonia nitrogen (TAN) and NO₂^-.

Specific growth rate (SGR) was calculated as

SGR (%/day) = 100 [Ln(w_f) – Ln(w_i)]/T ….(i)

where:                

   W_f: Final weight (g)

    W_i: Initial weight (g)

     T: Number of experimental days

Daily weight gain (DWG) was calculated as:

DWG (g/day) = (w_f - w_i)/T ……………….(ii)

Feed conversion ratio (FCR) was calculated as:

FCR = Feed DM offered/weight gain of fish …..(iii)

Survival rate (SR%) was calculated as:

SR (%) = 100[number of fish harvested/initial number of fish] …….(iv)

Chemical analysis

Feed samples were analyzed for DM, nitrogen (N), ether extract, and organic matter (OM) according to the procedures of AOAC (1990).

 Statistical analysis

The data for feed intake, growth rate and feed conversion were analyzed using the General Linear Model (GLM) of the ANOVA program with the Tukey pair-wise comparison in Minitab software (Minitab release 13.3, 2000). Sources of variation were: feed, species, feed*species interaction and error.

Results and discussion

Water quality

The water quality in all treatments was within the suitable range for the normal growth of fish (Boyd 1990) and was not affected by the experimental treatments (Tables 4 and 5),

Chemical composition of the dietary ingredients

The earthworms had a lower DM content than the trash fish and rice field prawns (Table 6). The CP content was about 15% lower in the earthworms than in the trash fish and rice field prawns.  Ether extract in the earthworms (11.1% in DM) was almost 40% higher than in the trash fish and prawns.

The DM content of the diets decreased linearly as the proportion of earthworms was increased (Table 7). This can be a disadvantage in terms of the ability of the fish to consume the feed, which breaks up easily.

Growth performance

The weight gain of both species, and the survival rate in the Marble goby, decreased markedly with curvilinear trends as the proportion of earthworms in the diet was increased (Tables 8 and 9; Figures 1 and 2). The FCR showed similar negative trends as the earthworm content of the diet was increased.

The Tra catfish were more tolerant of the high earthworm diet as the reduction in growth rate on the 100% earthworm diet was only 36% less than the growth rate on the control diet. The comparable figure for the Marble goby was 73%. Survival rates also differed: on the 100% earthworm diet it was 100% for the Tra catfish but only 63% for the Marble goby.

Figure 1. Effect of the percentage of earthworms in the diet on the daily weight gain of Marble goby

Figure 2. Effect of the percentage of earthworms in the diet on the daily weight gain of Tra catfish

The different response of the two fish species to the level of earthworms in the diet probably reflects the different eating habits. Marble goby is a pure carnivore (Siriwong et al 2009), and prefers a moving prey, whereas the Tra catfish is an omnivore. If the earthworms had been fed alive (and moving) it is possible the response of both species would not have been different.  Support for this explanation is provided by the experiences of Pereira and Gomes (1995) who offered frozen earthworms (Eisenia foetida) to replace commercial feed for rainbow trout. The feeding of earthworms depressed the growth rate. In the report of Stafford and Tacon (1984), there was no loss in trout performance when 10% worm meal protein replaced fish meal protein, but at higher levels (50 and 100% protein replacement) a decline in fish performance occurred.

Changes in weight:length ratio

Both species of fish showed an increase in the weight:length ratio as the experiment progressed (Tables 10 and 11). By the end of the experiment, the negative effect of the earthworm content of the diet was more marked in the Tra catfish (60% reduction) than in the Marble goby (42% reduction).

The percentage of offered feed (Table 12) that was consumed was higher for the Tra catfish (93.3%) than for Marble goby (88.9%). The higher content of ether extract in the residues and feces from the Marble goby also indicates that these contained higher proportions of earthworms, which are richer in ether extract than the trash fish and rice field prawns (Table 6). These figures are only approximate, as feces were mixed with feed residues. However, they support the conclusion from the growth and survival data that the Tra catfish adapted better to the earthworm diets than the Marble goby.

Experiment 2. Fresh compared with frozen earthworms as feed for Marble goby and Tra catfish

This experiment was planned to test the hypothesis, derived from the conclusions of Experiment 1, that Marble goby and Tra catfish prefer to eat live (fresh) rather than dead (frozen) earthworms.

Location

The experiment took place in the dry season, from December 2009 to February 2010, and was conducted in the same location as experiment 1.

Treatments and design

The treatments, arranged as a 2*2 factorial with 3 replications in a completely randomized design (CRD) (Tables 13 and 14), were:

·        Fish species: Marble goby and Tra catfish

·        Protein source: fresh and frozen earthworms

The experimental period lasted for 9 weeks.  The first week of the experiment was for adaptation of the fish to the new diets, followed by a recording period of 8 weeks.

Experimental procedures

Marble goby (about 4 g in weight and 5.8 cm in length) and Tra catfish (13 g and 10 cm) were bought in My Thanh hatchery center, Long Xuyen City. The stocking density was 100 fish/m³. Twelve composite tanks (0.1 m³/each) had 10 fish in each tank (Photo 6).

The earthworms were purchased from a commercial farm in Can Tho City. One part was kept in the freezer to become frozen earthworms, which were thawed before feeding. Another part was kept in a closed shelter together with cattle manure to make sure the earthworms remained alive until feeding time, when they were washed prior to offering them to the fish. The earthworms were fed ad libitum two times per day at 7:00 and 17:00h. The residual feed and fish excreta were removed daily by siphoning prior to feeding in the morning. Fresh water was added each 2 days. The residues of feed and feces were filtered and stored in a freezer (-18⁰C) until the end of the experiment when they were pooled by species and analyzed for ash, DM and CP.

Measurements

The amounts of earthworms offered were recorded daily. Samples were collected every week for chemical analysis. Before starting the experiment, all fish were measured for length and weighed. At the end of the experiment, the weight, length and numbers of all the fish were recorded.  Calculations of growth rates and feed conversion were the same as in Experiment 1.

Chemical analysis

This followed the same procedure as in Experiment 1, both for feed (the earthworms), feed residues and feces, and water quality.

Statistical analysis

The data for feed intake, growth rate and feed conversion were analyzed using the General Linear Model (GLM) of the ANOVA program, with the Tukey pair-wise comparison in Minitab software (Minitab release 13.3, 2000). Sources of variation were: feed, species, feed*species interaction and error.

Results and discussion

Data for water quality (Table 15) showed no differences among the treatments and that all criteria were in the acceptable range for fresh water fish culture (Boyd 1990).

Frozen earthworms contained less DM and less CP in the DM than the fresh worms (Table 16). The content of ether extract was lower than was recorded in Experiment 1.

The daily weight gain of Marble goby eating fresh earthworms was four times higher than when the fish were fed frozen earthworm (Table 15; Figure 3). For the Tra catfish the weight gains were two times on fresh compared with frozen worms. Growth rates on the frozen earthworms were lower for the Tra fish in Experiment 2 (0.64 g/day) than in Experiment 1 (1.80 g/day). The opposite occurred with the Marble goby, for which the growth rate was 0.14 g/day in Experiment 2 compared with 0.1 g/day in Experiment 1. However, there were major differences in initial weights, which were 13 and 3.6 g for Tra catfish and Marble goby in Experiment 2 compared with 1.19 and 0.8 g in Experiment 1.

There was no mortality in both fish species when fed fresh earthworms and very little (7 and 3% for Marble Goby and Tra catfish, respectively) on frozen worms.

These results confirm the hypothesis that both Marble goby and Tra catfish grow better when fed fresh compared with frozen earthworms.

Figure 3. Effect of fresh and frozen earthworms on the weight gain of Marble goby and Tra catfish

On the fresh earthworm diet, the Marble goby increased their weight:length ratio 82% during the 8 week experiment, compared with 40% for the Tra catfish (Table 18). Comparable figures on frozen worms were 27 and 20%, respectively.

The residual feed and feces accounted for 25.1% of feed offered to the Marble goby but only 5.9% in the case of the Tra catfish (Table 19). The higher value of the CP and lower ash content in the residues from the Marble goby indicate that feed residues were a greater proportion of the total waste for this species compared with the Tra catfish.

Conclusions

·        The weight gains of Marble goby and Tra catfish, and the survival rate in the Marble Goby, decreased markedly with curvilinear trends as the proportion of frozen earthworms in the diet was increased (Experiment 1).

·        The negative effects of feeding frozen earthworms was confirmed in Experiment 2, when growth rates were 4 and 2 times greater for Marble goby and Tra catfish fed fresh versus frozen earthworms,  respectively.

Acknowledgements

The senior author wishes to acknowledge the Swedish International Development Authority (Sida) for their financial support of this study, through the MEKARN regional project. Thanks are given to the Aquaculture Department, Agriculture and Natural Resources Faculty, An Giang University, Vietnam, for allowing use of their facilities. In particular, the undergraduate students are greatly appreciated for their excellent assistance during the course of the experiment.

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Received 15 October 2010; Accepted 23 October 2010; Published 1 November 2010

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