1. Results and Discussion
Weaning European glass eels (Anguilla anguilla) with plant protein based diets
Mansour Torfi Mozanzadeha, Enric Gisbertb
a South Iranian Aquaculture Research Center, Ahwaz, Iran.
b IRTA, Centro de San Carlos de la Rápita, San Carlos de la Rápita, Spain.
Introduction
Eel culture is an important aquaculture activity in several European countries with a production of 4,921 MT in 2014 (FAO, 2017). Eel culture is also a common practice in several Asian countries, such as China, Taiwan and Japan, and to a smaller degree in North America and Australia. Eel production is based on catching and rearing wild-caught juvenile glass eels during their onshore migration from the spawning grounds, which is certainly unsustainable under a long term scenario.
Eel farmers have access to pelleted feeds formulated specifically for the European eel; nevertheless, they still face the problem of the limited feed acceptance; some eels do not become accustomed to the pelleted feed, so they lose weight or grow too slowly to have any economic value for the farmers. Recent improvements in diet formulation have successfully proven the feasibility of weaning glass eels into compound diets (Hirt-Chabbert et al., 2012; Aquac. Nutr. 18: ). In this context and considering that feeding costs (50-70% of total production costs) (Rana et al., 2009; FAO Technical Paper 541), it is of need to reduce feed manufacture costs. During last decades, Fish meal (FM) has become one of the most expensive raw materials in fish feeds and is the main protein source especially for carnivorous fish species (Tacon and Metian, 2008; Aquaculture 285, 146–158); thus, it is of extreme importance try to find nutritional alternatives to diets with high levels of FM inclusion.
Objectives
In the present study, authors wanted to test whether it was feasible to wean the glass eel stage of European eel with compound diets in which FM replaced (50 and 75%) by blends of plant protein sources (corn gluten, wheat gluten, soy bean meal and soy protein concentrate). The success of this process was evaluated in terms of growth performance, survival and elver condition assessed by the maturity of the digestive function.
Materials and methods
Ingredients, % FM
PP50
PP75
Fishmeal 70 LT
32.0
16.0
8.0
CPSP
5.0
Soy protein concentrate
0.0
7.0
Wheat Gluten
6.9
10.5
Corn gluten
Soybean meal 48
6.0
Rapeseed meal
5.3
Sunflower meal
Wheat meal
16.5
12.6
11.0
Pea starch
12.5
Fish oil
11.3
12.0
13.1
Vit & Min Premix
1.5
Soy lecithin
1.0
Binder
Antioxidant
0.2
Dicalcium phosphate
1.7
3.0
4.0
L-Lysine
0.4
0.7
DL-Methionine
0.3
Total
100.0
Experimental procedures were conducted in compliance with the Guidelines of the European Union Council (86/609/EU) for the use of laboratory animal. Wild glass eels (n = 5,000; 180 ± 51 mg in wet body weight, BW) were used and this study and acclimated to IRTA-SCR facilities for 2 weeks (20.1 ± 0.1 ºC, 6.7 ± 0.3 ppm O2, 1.3 ± 0.3 ‰; RAS units – IRTAmar®). Glass eels were distributed into 16 tanks (100 L) at an initial density of 200 glass eels per tank. Glass eels were progressively weaned into three different compound diets (see Table, diets manufactured by SPAROS Lda), whereas one group was maintained with the natural food regime (Artemia + hake roe). Weaning lasted for 20 days: natural food was progressively replaced by the compound diet (100/0, 75/0, 50/50, 25/75) every four days, whereas glass eels were completely weaned into experimental diets at day 25. Each treatment had four replicates, the trial lasted for 90 days.
At the end of the feeding trial, all elvers were individually counted and their BW measured (n = 100 per tank). A subsample (n = 20 fish per tank) was used for measuring the intestinal maturity level (alkaline phosphatase and leucine-alanine peptidase ratio) and activity of the main pancreatic digestive enzymes (Gisbert et al. (2009; Aquaculture 287: ). Levels of lipid peroxidation and oxidative stress enzymes were measured (Solé et al. 2004; Fish Physiol. Biochem. 30: 57–66).
Results and Discussion
Diets were offered ad-libitum and feed palatability was not adversely affected by the level of PP inclusion in diets (glass eels ate voraciously all compound diets), which might be because of the supplementation of crystalline AA. However, feed efficiency parameters could not be measured due to technical issues.
Eel survival was significantly affected by the dietary treatment (ANOVA, P < 0.05). In particular, the survival rate of glass eels fed hake roe + Artemia nauplii was 67.5 ± 3.2%, this value was significantly higher than those obtained in glass eels weaned into the FM diet (45.8 ± 5.3%). In addition, glass eels weaned into the PP50 and PP75 diets showed the lowest survival rates (31.1 ± 7.4% and 27.8 ± 9.9%, respectively) among all of the dietary treatments considered. a a a a a a ab a a b b b
SGR
1.8 ±0.2
SGR
1.8 ±0.4
SGR
1.7 ±0.1
SGR
1.1 ±0.2
At the end of the trial, there were not statistically significant differences in terms of BW and SGR between glass eels fed compound diets with different levels of FM substitution (0, 50 and 75%) with PP sources, whereas their BW values were higher (almost the double) than those found in the group fed hake roe and Artemia nauplii for 90 days (ANOVA, P < 0.05; Fig.1). These differences in BW were inversely correlated to differences in survival, since fish fed the hake roe + Artemia nauplii showed a larger frequency of smaller individuals in comparison to those fed the inert diets (R = ; P < 0.05), indicating that those glass eels that were not able to adapt to the inert diets did die, whereas this did not happen in fish fed the natural diet.
The level of maturation of the intestinal function was affected by the diet, fish fed the inert diets showed higher AP/LAP values than those of the natural diet (ANOVA, P < 0.05). The lower AP/LAP ratio found in glass eels fed the natural food might be attributed to the higher size dispersion in this dietary treatment, as glass eels fed this diet showed lesser mortality and also a lower size in BW (higher frequency of small individuals). Regarding pancreatic enzymes, the activity of total alkaline proteases followed the same trend that intestinal enzymes (ANOVA, P < 0.05), although what it was really interesting is that fish fed PP50 and PP75 performed as well as those fed the FM diet in terms of digestive activity. No differences were found in terms of amylase, lipase and pepsin activity among groups.
No differences in the levels of lipid peroxidation and activity of oxidative stress enzymes (CAT, SOD, GPX, GR, GST) were found between groups (ANOVA; P > 0.05), which indicated that inert diets did not affect the oxidative stress status of elvers.
Weaning glass eels with a compound diet with 50 and 75% FM replacement with plant protein sources (blend of corn gluten, wheat gluten, soy bean meal and soy protein concentrate) did not affect their growth in comparison to a FM base diet nor their digestive function. However, glass eel survival was reduced in comparison to the experimental group fed the natural food (hake roe + Artemia nauplii), whereas similar survival rates were obtained between FM, PP50 and PP75 diets, indicated further research is needed in order to improve compound diet formulation in order to perform as good as a natural diet in term of fish survival.
Conclusions