Grow-out of sandfish Holothuria scabra in ponds shows that co-culture with shrimp Litopenaeus stylirostris is not viable

We examined the potential for producing the large numbers of sandfish (Holothuria scabra) needed for restocking programmes by co-culturing juveniles with the shrimp Litopenaeus stylirostris in earthen ponds. Our experiments in hapas within shrimp ponds were designed to detect any deleterious effects of sandfish on shrimp, and vice versa. These experiments showed that a high stocking density of juvenile sandfish had no significant effects on growth and survival of shrimp. However, survival and growth of sandfish reared with shrimp for 3 weeks were significantly lower than for sandfish reared alone. Increased stocking density of shrimp also had a significant negative effect on survival and/or growth of sandfish. A grow-out trial of juvenile sandfish in 0.2-ha earthen ponds stocked with 20 shrimp post-larvae m^− 2, and densities of sandfish between 0.8 and 1.6 individuals m^− 2, confirmed that co-culture is not viable. All sandfish reared in co-culture were dead or moribund after a month. However, sandfish stocked alone into 0.2-ha earthen ponds survived well and grew to mean weights of ∼ 400 g within 12 months without addition of food. The grow-out trial demonstrated that there is potential for profitable pond farming of sandfish in monoculture. Further research is now needed to identify the optimal size of juveniles, stocking densities and pond management regimes.     

Characterization of water quality in shrimp ponds of different sizes and with different management regimes using multivariate statistical analysis

Coastal shrimp aquaculture in Bangladesh is mostly practiced in an agricultural unit called a Gher, which is a special type of agricultural field with elevated surrounding embankments/borders situated by the side of a river that is used to grow rice in winter and shrimp in summer. Ghers of different sizes are managed in different ways. An on-farm project was carried out to study the effects of Gher size and their related management practices on water quality, shrimp production and the economic returns of shrimp farming in the Southwest coastal region of Bangladesh. We present here the results of this project with respect to water quality as explored through factor analysis, which was applied as a way to understand the nature and extent of the effects of different variables. For the majority of the variables and factors, one-half to two-thirds of their variability was explained by changes over time (months), and a further one-quarter to one-third was explained by Gher size and associated management practices. The alternating dry/monsoon season and water management practices determined long-term and large-scale (annual) water exchange in the Ghers that mainly affected processes related to live and dead particles suspended in the water column. Processes related to decomposition and nitrification were more dependent on short-term (days) water circulation in the Ghers, which is turn was reliant on the time span between the exchanges of water in the Ghers and river. Our results show that improved management leads to higher natural and shrimp productivity in the smaller Ghers and strongly support the argument put forward by the policy makers and scientific community in Bangladesh that the smaller the Gher, the better they can be managed and the better the possibility of increased shrimp survival and production.     

Determinism of the induction of the reproductive cycle in female Eurasian perch, Perca fluviatilis: Identification of environmental cues and permissive factors

In Eurasian perch, Perca fluviatilis, gametogenesis is induced by decrease of both temperature and photoperiod. However, a multiplicity of other factors can display non-permissive or modulating effects on the induction of the reproductive cycle. Consequently, a 9-week study was carried out to determine the environmental cues and permissive factors controlling the induction of the reproductive cycle in this species. A two-level fractional factorial design 2^8-4 was used in order to identify the influent factors from 8 environmental and nutritional ones and their 28 interactions. Tested factors were: (1) amplitude of temperature decrease, (2) time, (3) kinetics and (4) amplitude of photoperiod decrease, (5) initial nutritional state, (6) handling, (7) feeding rate and (8) light spectrum. Increase of gonadosomatic index (GSIi), proportion of female in exogenous vitellogenesis (EVP), plasma 17 β-estradiol (E₂) and cortisol levels, fat consumption and food intake were evaluated.This study showed that the amplitude of the decrease of temperature and the time of photoperiod decrease are the two main environmental cues controlling the induction of the reproductive cycle in Eurasian perch. GSIi was the highest (3.8%) when females were exposed to both low amplitude of temperature decrease and precocious decrease of photophase. It was the lowest (1.1%) when high amplitude of temperature decrease and late decrease of photophase were applied. Handling in association with temperature modulated the broodstock response. 100% of the unhandled fish held at 14 °C were at the exogenous vitellogenesis stage with mean E₂ and basal cortisol levels of 1.6 ng mL^− 1 and 9 ng mL^− 1 respectively. Handled fish and fish held at 6 °C exhibited lower vitellogenesis (40-73%) associated with lower E₂ (0.6-1.1 ng mL^− 1) and higher basal cortisol levels (37-89 ng mL^− 1). No other factor nor interaction displayed a cueing or non permissive effect on the induction of the reproductive cycle in female Eurasian perch. A first schematic model is proposed to explain the factorial determinism of the induction of the reproductive cycle.