The effects of growth hormone and sexual reversal on growth of the Nile Tilapia (Oreochromis niloticus)

Transcription

1 Aquaculture America, New Orleans, Louisiana 2011 The effects of growth hormone and sexual reversal on growth of the Nile Tilapia (Oreochromis niloticus) Cindy M. Robles-Basto, Marco A. Liñán-Cabello* & Alfredo Mena-Herrera Aquaculture/Biotechnology, FACIMAR, Colima University, Km Carretera Manzanillo-Barra de Navidad, Manzanillo, Colima, México; Tel/Fax: ;

2 ABSTRACT The use of hormone aggregates in aquaculture enhances weight gain. However, few studies have examined the effects of added hormones with respect to optimization of the growth of aquatic organisms. The present study employed two batches of juvenile ( 5.0 cm in length) tilapia, Oreochromis niloticus. The fish were randomly assigned to three different treatments, and a control group, and three replicates were run for each treatment. The treatments were a) sexually reversed fish with added growth hormone (GH) (RGH); b) sexually reversed fish without GH (R); c) fish that were not sexually reversed with added GH (NGH); and, d) fish that were not sexually reversed without added GH (N). GH was injected at a dose of mg g -1 body weight. Fish were cultured over 45 days and morphometric parameters were evaluated during this time. RGH fish showed the best growth, suggesting a possible interaction between 17 α- methyltestosterone (17-MT) and injected GH.

3 Introduction Tilapia is widely cultivated, and commercial fish production will increase in the future. The fish show physiological and immunological tolerance to a wide range of environmental conditions, efficiently use low-protein diets, and are highly resistant to stress and infections. Fish gender can be manipulated by cultivation with the androgen (17-MT). The precise mechanism of hormonal action remains controversial. Fish become masculinized, and an anabolic effect of the hormone is evident. This enhances overall growth and protein synthesis, particularly that of myofibrillar proteins, resulting in a greater muscle mass gain by males compared to females. However, the hormone affects the expression of secondary sexual characteristics including the appearance of the testes and ovaries, providing evidence of effective sex reversal (Arboleda- Obregon 2005). Some authors (Phelps & Popma 2000) claim that the use of an androgen at high doses causes atrophy of secondary sexual characteristics, possibly via an interaction with other compounds, and natural recruitment in fish stocks is thus inhibited (Kristensen et al. 2005).

4 Currently, tilapia culture systems successfully exploit sexual reversal or manipulation to control reproduction. However, the effect of hormones on the time required for fish to grow to a commercial weight has not been studied (Zhou et al. 2009). Somatotropin, or growth hormone (GH), is known to enhance transport of amino acids across cell membranes, and also induces the release of fatty acids from adipose tissue, thus increasing fatty acid concentration in bodily fluids. Furthermore, all tissues are stimulated to convert fatty acids into acetyl coenzyme A (acetyl-coa) and this material is next used as an energy source, in preference to carbohydrates and proteins. GH stimulates fat utilization and protein anabolism, thus increasing lean mass. GH gene manipulation has been employed to study interactions between GH expression and that of other hormones (Hiroyuki et al. 2003), to characterize and understand paracrine-autocrine interactions, and, more recently, to clarify the role of GH in the immune system (Yada 2007). A few studies on GH addition to Nile tilapia cultures have appeared (Fragoso et al. 1999). As added hormones can optimize growth, we assessed the effects of GH on the growth of sexually reversed tilapia over time, and interactions of GH with 17-MT.

5 Materials and Methods

6 Not sexually reversed Tilapia Sexually reversed Tilapia Growth Hormone (Humatrope ), + dose (5 µg per g of fish R RGH NGH N

7 Quarantine period (15 days) T 0... T 15.. T T 45 days To Single intramuscular dose (5 µg per g of fish), vehicle for dissolution of mineral oil Initial weight 4.3 ± 0.25 g in fiberglass tanks of 2 m diameter (1000 L) 17-MT, 98 % (Chemical Lab ARGENT, INC) Pellet (Belenes, Mexico) with 45% protein Renewal of % of the volume of water were made weekly. Temperature of 27±3 oc. Morphometric Samples (To, T15, T 30 and T45)

8 Results

9 Average weights were obtained at different times (T 0, T 15, T 30, and T 45 ) (Fig. 1 and 2). No significant between-group differences were evident at T 0. At T 15, significant differences among treatments were observed, and RGH fish were heavier than those receiving other treatments, with the lowest weight being noted in the R group. By T 30, fish in the two GH groups were heavier, on average, than were other fish, and the RGH fish were heaviest. Fish in the R and N treatment groups did not differ significantly in weight (P<0.05). At T 45, weights averaged greater in the NGH and RGH treatment groups, and were not significantly different. Other groups were of lower weight (P<0.05). RGH fish showed the best weight gain, followed by NGH fish, whereas fish that did not receive GH were of lower average weight. Figure 1 shows that the time interval over which weight gain was maximal ran from T 30 to T 45. In this interval, RGH fish gained g. The lowest weight gain was in the period between T 0 and T 15, during which R fish gained 4.34 g.

10 70.00 Weight (g) a a a a a b c c a b c d a b a b RGH R NGH N Figure 1. Mean weight values of fish subjected to different treatments, over time. Letters show data equal in variance but significantly different (P<0.05; all data are means ± standard errors).

11 Fish mortality was lower if gender reversal was absent (thus in the NGH and N groups; 1 and 4 deaths, respectively) whereas 32 fish (21.4%) in the R group died. The weight gain of RGH fish was 28-fold; the figure for control fish was 17-fold (Table 1). RGH fish showed the greatest rate of weight gain, at 1.23 g day -1. NGH fish gained 1.08 g day -1, and N and R fish gained less weight. Using the terms of the Von Bertalanffy model, W obs initially remained below W cal in RGH fish (Fig. 3-a), but, by day 75 (T 30 ), W obs exceeded W cal. At the end of the experiment, W obs was much greater than W cal. Figure 3-b shows that, in N fish, W obs always remained below W cal.

12 Weight (g) RGH R NGH N Time (days) Figure 2. Mean growth of fish injected of fish injected with the different treatments during the experiment.

13 Weight (g) Weight (g) Weight (g) Weight (g) a) c) b) d) Age Age Figure 3: Growth curves based on the Von Bertalanffy model: a) W obs and W cal for RGH; b) W obs and W cal for N; c) W obs and W cal for RGH; d) W obs and W cal for N treatments.

14 Table 1. Initial and final weights, percentage weight gains, growth rates, survival levels, and weight gains per day, of tilapia cultured under different conditions. Treatment Initial weight (g) Final weight (g) Percentage weight gain (%) Survival (%) Weight/gain (g/day) RGH , R , NGH , N ,

15 Discussion At the beginning of our experiment, we observed that sexually reversed fish were slightly lower in average weight compared to fish that were not sexually reversed. However, GH injection resulted in a greater weight gain of sexually reversed fish (the RGH group), and the difference was significant at both T 15 and T 30 (P<0.05 for both comparisons). In this regard, it is important to note that several authors have proposed an interaction between the somatotroph and gonadotroph hormonal axes, represented by 17-MT and GH (Muñoz-Cueto 2005, Zhou et al. 2005). Thus, if tissue growth is to be affected, receptor levels must be upregulated. However, molecular confirmation of this idea is lacking. It is known that steroids such as 17-MT act as growth promoters in carp ( Kuwaye et al. 1993). O. mossambicus tilapia cultured in either fresh or sea water gained more weight when MT was administered (Kuwaye et al. 1993, Ron et al. 1995). We thus suggest that the weight gain observed between T 15 and T 30 possibly indicates a synergistic action of MT and GH, as indicated also by various reports showing that GH acting on sexually reversed fish increased weight gain, compared to controls ( McAndrew & Majumdar 1989). A well-controlled experiment is required to explore interactions at the molecular level between MT and GH during tilapia growth.

16 Growth-promoting mechanisms are initiated in the neuroendocrine system and the modulators interact with hormones of the hypothalamus, pituitary, and gonads (Muñoz-Cueto 2005). This process involves expression of GH and sexual hormones, such as testosterone in males and estradiol in females. The effect of GH in our experiment was most evident from T 15 showing that growth was enhanced in both RGH and NGH fish. Also, RGH fish had the highest weight gain and food intake (Figure 1, Table 1). In the experiment of Fragoso et al (1999), GH was injected intramuscularly and weight gain by tilapia that was not sexually reversed began to fall in week 6. In our experiment, both sexually reversed and non-reversed fish treated with GH had greater average length and weight gains than did fish not treated with the hormone. Use of the Von Bertalanffy model corroborated these data (Figure 2). We noted that, prior to GH administration, sexually reversed fish showed less weight gain than did fish that were not sexually reversed. Thus, the effect of GH was evident to day 90 of the model, corresponding to the 6-7 week period during which RGH fish crossed the W cal line. In the same period, W obs of N fish was almost 10 g below W cal.

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