Moderate stocking density does not influence the behavioural and physiological responses of rainbow trout (Oncorhynchus mykiss) in organic aquaculture

Welfare in farmed fish got particular attention during the last decades from both governmental and public sides. In aquaculture context, welfare concerns are mainly related to handling procedures, water quality and stoking densities. In Europe, authorities had to clarify the threshold limits of stocking densities to maintain fish good welfare, including for organics aquaculture through the EC regulation 710/2009. However, effects of stocking density on fish welfare are complex and sometimes contradictory. Moreover, there is a lack of knowledge about the impact of density on fish welfare in organic aquaculture. Thus, the aim of the study is to asses welfare state of rainbow trout (Oncorhynchus mykiss) at two initial stocking densities (low density, LD: 12 kg/m³ and high density, HD: 17 kg/m³) fed using organic feed by combining the monitoring of growth performances, behaviour (swimming activity) and physiological indicators (i.e. cortisol, glucose, lactate, hematocrit, red blood cellule count and lysozyme). At the end of experiment, the stocking density reached 21 kg/m³ and 30 kg/m³ for the LD and HD respectively. Overall, growth performances, swimming activity and level of physiological indicators of stress and welfare were similar between HD and LD over the experiment duration. To conclude, we observed no alteration of fish welfare between the two stocking densities monitored. This study suggests that a final stocking density of 30 kg/m³ can be considered for organic aquaculture of rainbow trout respecting welfare.     

Response to domestication and selection for growth in the European sea bass (Dicentrarchus labrax) in separate and mixed tanks

Selective breeding of European sea bass (Dicentrarchus labrax) receives a growing interest, as the estimated heritability of growth is medium to high. In this study, we compared the offspring of four groups of sea bass sires, mated with the same wild dams: wild (W), first generation of domestication (D), first generation of mass selection for length (M), first generation of PROSPER-like selection for length (P). The comparison was done both in replicated tanks (separate rearing) and in mixed tanks (mixed rearing) where sire origins were recovered by genotyping of eight microsatellite markers. Weight, length and growth rate were measured from day 238 post-fertilization (69 g mean weight) to day 611 post-fertilization (390 g mean weight). Both in mixed and separate tanks, both selected groups (P, M) were larger than unselected groups (W, D). No difference was seen at any time between W and D, nor between M and P. The selection response estimate on weight was larger in mixed tanks when compared to separate tanks (+ 42% in mixed tanks, + 23% in separate tanks at day 611), yielding realized heritability estimates of 0.60 and 0.34, respectively, and confirming the excellent potential of the species for growth improvement through selective breeding. Both selection response and the amplification effect between mixed and separate tanks decreased as rearing density increased. Our hypothesis is that selection response is magnified by competition in mixed tanks, while sub-optimal rearing conditions lower the observed selection response, more in separate tanks (where selected thus larger fish are at a higher density than unselected ones) than in mixed tanks (where all fish experience the same density effects).     

In Vitro and In Vivo Feedstuff Digestibility for Snook,Centropomus undecimalis,Juveniles

The degree of hydrolysis (DH) of the ingredients was evaluated for Centropomus undecimalis juveniles. The in vitro experiment included wheat gluten (WG), corn gluten (CG), soybean meal (SBM) and pretreated phytase (SBM + phytase), soy protein concentrate (SPC), canola meal (CAN) and pretreated phytase (CAN + phytase), poultry byproduct meal (PBPM), chicken meal (CHKM), dried whey (DW), Pota meal (PM: mix of giant squid, fish soluble protein concentrate, crustacean meal, and fish oil), and Protiblend (PTB: aquatic and render mix). The highest acidic DH occurred with PTB (0.38 ± 0.06), CHKM (0.33 ± 0.3), and PBPM (0.25 ± 0.03). In the alkaline condition, PTB (1.6 ± 0.17 and 0.98 ± 0.05 for pyloric caeca and intestine, respectively) and CG (1.04 ± 0.4 and 0.75 ± 0.2 for caeca and intestine, respectively) provided the highest DH values. In vitro digestibility demonstrated that PTB was the highest (85.3%) followed by PBPM (51.4%), CAN + phytase (47.6%), CG (45.1%), and CHKM (46.5%). The in vivo experiment concerned the WG, CHKM, PBPM, PM, and PTB diets plus a reference diet with fishmeal (Ref diet). The total %DH was different (P < 0.05) with the lowest values for the WG diet (0.34 ± 0.09%) and Ref diet (0.34 ± 0.15). Free amino acid released during digestion was displayed for these diets and a bifactorial analysis produced no difference (P > 0.05). The apparent digestive coefficients ranged from 89.8 to 92.9% for protein and from 68 to 71.4% for energy.     

Masculinization of the Native Bay Snook,Petenia splendida,Using Oral Administration of the Synthetic Steroid 17α‐methyltestosterone

The possibility of inducing sex reversal in bay snook, Petenia splendida, was evaluated to generate a protocol aimed at producing single‐sex populations (100% males) and improving farming conditions for this species. This popular staple food is a cichlid fish native to Central America, with aquaculture potential. We evaluated oral administration of the synthetic steroid 17α‐methyltestosterone (MT) to determine optimal dosage (30, 45, and 60 mg/kg of food) and days of feeding (30, 45, and 60) to attain efficient masculinization. The statistical analysis indicates a highly significant effect of both factors as well as for the interaction (P < 0.01). We registered significant differences in growth at the end of the experiment; fish fed with 60 mg of MT for 60 d reached the largest size, while fish fed with no MT were smaller. No significant differences were observed for survival (P > 0.05), ranging from 55.7 to 75.3% in the treatments. We recommend the use of 60 mg/kg of MT for 30 d for achieving successful sex reversal. This study provides valuable information supporting the inclusion of new species in aquaculture, reducing time and costs associated with sex reversal previously proposed for the species.     

Availability of minerals in rendered meat and bone meal for Nile tilapia: Preliminary observations

A digestibility trial was conducted with 180 Nile tilapia (32.65 ± 4.52 g) fed a reference diet containing 350 g/kg crude protein and 4,500 kcal/kg gross energy. Five different meat and bone meals (MBMs) were evaluated. The test diets compound by 70% of reference diet and 30% of each MBM were elaborated and classified according to ash content: MBM‐430, MBM‐400, MBM‐370, MBM‐340 and MBM‐310. The apparent availability coefficient (AAC) of minerals and the apparent digestibility coefficient (ADC) of dry matter (DM) were subjected to one‐way analysis of variance (p < .05). Differences among ADC and AAC were determined by Tukey's multiple range test (p < .05). When appropriate, a polynomial regression analysis was performed. Linear correlation analyses between MBM chemical composition and the AAC and ADC were performed (p < .01). Higher values of ADC_DM, AAC_Ca and AAC_P were observed in MBM‐310, while low values were observed in MBM‐430. Quadratic effects of ash, P and Ca contents on AAC_Ca, AAC_P and ADC_DM were observed. The AAC_Ca, AAC_P and ADC_DM indicated a negative correlation with ash, P and Ca contents. MBM‐310 exhibited the highest mineral AAC, which could be used as a mineral source for Nile tilapia.