Introduction
Environmental enrichment is used to promote fish welfare and, in some cases, to reduce detrimental characteristics that fish develop in captive conditions. Senegalese sole is a promising species for European aquaculture, above all in the South of Europe, for its good growth rate and high market price (Morais et al. 2016). However, Senegalese sole shows a reproductive dysfunction related to the lack of reproductive behaviour which does not allow to close the cycle in captivity. Several approaches have been studied to increase knowledge and try to solve this disorder. The problem remains unsolved, and the production is based on breeders caught in nature. Recently, it has been observed that urine is the communication vehicle for reproduction in Senegalese sole and its potency variates depending on fish maturity and sex (Fatsini et al. 2017). Our hypothesis was that conditioning the sole to natural environmental factors in early life stages may help to develop healthier fish and future potential breeders. Therefore, the present study aimed to assess the effect of using a substrate (sand) as an environmental enrichment in the stress response and chemical communication of pre-pubertal Senegalese Sole (Solea senegalensis).
Material and methods
The present study was divided in two parts. The first part was carried out to monitor the evaluation of the stress response in sole established in different environmental conditions. For this purpose, a total of 1500 sole (~10g; 8 months) were established in 6 outdoor tanks in the experimental station of IPMA (Olhão, Portugal) maintaining natural temperature and photoperiod according to each season. The bottom of 3 tanks were covered by 2 cm of sand and the other 3 remained without sand (fiberglass). These fish were maintained in the same conditions for two years and 4 samplings (every 6 months) were conducted to collect blood samples (n=20 fish per treatment and sampling) to evaluate cortisol in plasma by ELISA. Biometric parameters were also registered. In the last sampling a total of 72 pre-pubertal fish (n=36 per treatment) were transported to Ramalhete station (Faro, Portugal) to conduct the second part of this study. This part was performed to observe the influence of the environment in chemical communication in sole using urine from sole reared with and without sand. The fish were divided in 6 tanks and 3 tanks had the same origin sand in the bottom to mimic the original conditions. Urine samples were collected from 24 fish (n=12 fish per treatment) and 4 different pools were made per sex and environment. A total of 22 sole (7 females from sand and 5 from fiberglass; 5 males from sand and 5 from fiberglass) were used as receivers to perform electro-olfactogram (EOG) to evaluate the olfactory sensitivity and potency for smelling the same urine pools. The EOG were performed in different months to avoid seasonal factor.
Results and discussion
Significant differences (P < 0.05) were observed in cortisol plasma levels of Senegalese sole reared with and without sand during the 1st, 2nd and 3rd samplings where the fish maintained in sand obtained higher levels of cortisol in plasma than sole without sand. This could be associated with the stress response to mechanical agitation in the moment of catching the fish (Bates et al. 2014). However, all blood samples were collected in the first 4 minutes after catching the fish to try to avoid the rising of basal levels, suggesting that fish reared with sand were more sensitive to stress disturbance. It was noted that sole reared in sand presented less locomotor activity than fish reared without sand, which might decrease the baseline levels of several physiological parameters including cortisol. In terms of biometric data, sole reared with sand grew faster than sole reared without sand. These results coincided with other species reared using substrates as environmental enrichment (Bates et al. 2014).
In relation with chemical communication, no differences were observed in the potency of urine from females and males reared under different conditions; however, differences were observed in the olfactory sensitivity between females and males reared with and without sand smelling males’ urine. Intriguingly, the olfactory sensitivity from females reared with sand to urine was higher than females reared in fiberglass. However, this situation might not be conclusive because females presented high variability in olfactory responses including within the same condition (Fatsini et al. 2017). In the case of males, it was observed the opposite, males reared without sand had higher olfactory sensitivity than males reared with sand. Besides, the olfactory responses from males reared without sand are described by a three-parameter Hill curve, while responses from males reared with sand are better described by a linear regression. These results might be explained by the fact that males are smelling different compounds or the olfactory epithelium receptors have a different location changing the perception of the stimuli. There is a lack information about the influence of environment and chemical communication in fish, just to observe the implication of pollutants. However, it has been observed that the environment is very important, and it is completely linked to chemical communication in crayfish (Orconectes rusticus) where depending on the environment the behaviour, urine release and chemical signals during social interactions change (Bergman et al. 2006). Nonetheless, more studies are needed to understand the implications of these results in chemical communication and the use of sand as environmental enrichment for the correct development and maturation of F1 breeders.
References
Bates, LC., Boucher, MA., Shrimpton, JM. (2014) Effect on temperature and substrate on whole body cortisol and size larval white sturgeon (Acipenser transmontanus). J. App. Ichthyol. 30, 1259-1263.
Bergman, DA., Redman, CN., Fero, KC., Simon JL., Moore, PA. (2006). The impact of flow on chemical communication strategies and fight dynamics of crayfish. Mar. Freshw. Behav. Physiol. 39, 245-258.
Fatsini, E., Carazo, I., Chauvigné, F., Manchado, M., Cerdà, J., Hubbard, P., Duncan, NJ. (2017). Olfactory sensitivity of the marine flatfish Solea senegalensis to conspecific body fluids. JEB, 220, 2057-2065.
Morais, S., Aragão, C., Cabrita, E., Conceição, L., Constenla, M., Costas, B., et al. (2016). New developments and biological insights into the farming of Solea senegalensis reinforcing its aquaculture potential. Rev. Aquaculture, 8, 227-263.
Acknowledgements
This study was funded by ReproF1 Project (Programa Operacional Mar2020, MAR-16-02-01-FMP-0059), CONDISOLE (CeiMar funds, CEIJ-005 awarded EF), ODORACID Project (PTDC/BIA-BMA/30262/2017), and Portuguese national funds (FCT-Foundation for Science and Technology) through project UIDB/04326/2020.