Stocking density effects on growth and stress response of juvenile turbot(Scophthalmus maximus) reared in land-based recirculating aquaculture system

Stocking density is widely recognized as a critical factor in aquaculture and a potential source of long-term stress.The influence of stocking density on growth and stress response of juvenile turbot（Scophthalmus maximus, ~3–75g, initial to final weight） was examined in fish held under low（LD, ~0.21–5.31 kg/m^2, initial to final density）,medium（MD, ~0.42–10.81 kg/m^2） and high stocking density（HD, ~0.63–14.27 kg/m^2） for 120 days in a recirculating aquaculture system（RAS）. In this trial, the growth curve for weight of juvenile turbot in RAS, all fitted by the Schnute model. No significant difference was found in growth performance among the three densities until at the final sampling（Day 120）. The final weight and body weight increase（BWI） in the HD group were significantly lower than in other groups（P〈0.05, weight：（75.83±2.49） g,（75.39±2.08） g,（65.72±2.86） g and BWI：（2 436.12±28.10）%,（2 421.29±4.64）%,（2 097.88±20.99）% in LD, MD and HD groups, respectively）. Similarly, the specific growth rate（SGR）, feed conversion ratio（FCR） and coefficient of variation for weight（CV_w） were adversely affected by high stocking density（P〈0.05）. However, there was no difference in survival and Fulton＇s condition factor（K） of turbot among the different groups. Physiological analyses demonstrated a clear increase in the plasma cortisol level and an obvious decrease in growth hormone（GH） concentration in the HD group on Day120（P〈0.05）. There was no significant effect of stocking density on plasma glucose, Cl– and protein levels. All these findings would provide a reference for selecting the optimal stocking density of juvenile turbot in RAS.

Calculation of stress intensity factor in two-dimensional cracks by strain energy density factor procedure

In order to calculate the stress intensity factor(SIF) of crack tips in two-dimensional cracks from the viewpoint of strain energy density, a procedure to use the strain energy density factor to calculate the SIF is proposed. In this paper, the procedure is presented to calculate the SIF of crack tips in mode I cracks, mode II cracks and I+II mixed mode cracks. Meanwhile, the results are compared to those calculated by traditional approaches or other approaches based on strain energy density and verified by theoretical solutions. Furthermore, the effect of mesh density near the crack tip is discussed, and the proper location where the strain energy density factor is calculated is also studied. The results show that the SIF calculated by this procedure is close to not only those calculated by other approaches but also the theoretical solutions, thus it is capable of achieving accurate results.Besides, the mesh density around the crack tip should meet such requirements that, in the circular area created, the first layer of singular elements should have a radius about 0.05 mm and each element has a circumferential directional meshing angle to be15°–20°. Furthermore, for a single element around the crack tip, the strain energy density factor is suggested to be calculated in the location where half of the sector element's radius from the crack tip.

A New Fracture Criterion for a Crack under Combined Mode Loading

A new fracture criterion was proposed. The physical explanation of the criterion is that crack will propagate when the minimum strain energy density in iso hoop stress curve reach a critical strength of the material considered. The resulting curve of critical fracture of mixed mode cracks shows that the present fracture is efficient and more accurate than the previous criteria.