Introduction
The greater amberjack (Seriola dumerili) is a species with high potential for the aquaculture. However, the industrial production is still negligible due to several bottlenecks, among which the absence of reliable reproduction is one of the most important. Agonists of gonadotropin-releasing hormone (GnRHa) have been used to overcome the reproductive dysfunctions in several species. The present study shows the effects of an effective hormonal spawning induction method using GnRHa delivery systems on hematological and plasma biochemical parameters in order to assess the physiological condition and stress indices in greater amberjack broodstock (F1 generation).
Materials and methods
A group of 15 greater amberjack broodstock born in captivity (average weight of 18.5±9.2 kg) was maintained in an outdoor covered raceway tank of 500 m3 with continuous water supply under natural photoperiod in the facilities of the Instituto Español de Oceanografía in Canary Islands (Spain). Broodstock (7 males, 7 females and 1 undetermined) were tagged with passive integrated transponders (PIT tags). Fish were treated with an Ethylene-Vinyl acetate (EVAc) GnRHa implant (Mylonas and Zohar, 2001) loaded with Des-Gly10, D-Ala6-Pro-NEth9-mGnRHa (H-4070, Bachem, Switzerland) in May, June and July (~50 µg GnRHa kg-1 body weight). At the time of GnRHa implantation, selected females were in advanced vitellogenesis and males were producing intratesticular sperm. All broodstock were sampled four times during 2015 spawning season (May, June, July and September). Blood was collected from the caudal vessels using heparinized needles. Plasma samples were separated after centrifugation and stored at -80°C until analysis Haematological parameters were estimated from fresh samples of blood. Total erythrocytes and leucocytes were determined by counting using a Neubauer haemocytometer. Hematocrit count was carried out by capillary diffusion and centrifugation. Plasma levels of protein, triglycerides, cholesterol, glucose, lactate and enzymes (GPT, GOT, alkaline phosphatase, cholinesterase and amylase) were measured by enzymatic colorimetric assays (Biosystems, Spain). Plasma concentrations of sodium and potassium were determined using standard spectrophotometric assays (Spinreact, Spain). Plasma cortisol level was analyzed by radioimmunoassay using ELISA kits (Arbor Assay, Michigan, USA).
Results
Spawning of greater amberjack started 24-48 hours after each hormonal treatment and a total of 52 spawnings were obtained during a period of 72 days. During the first period (May to June) the eggs were collected almost daily (29 spawnings in 31 days).
The measured hematological and biochemical parameters along the spawning season are shown in Table I. Both hematological and biochemical parameters obtained in the present experiment are considered to be within the normal range for greater amberjack, compared to those of the previous findings (Dawood et al., 2015). All blood parameters studied remained constant along the study and only erythrocytes, protein, cholesterol and hematocrit were lower at the end of the spawning season.
During chronic stress in fish culture, there are often characteristically high circulating levels of cortisol. In the present study, no significant differences (P < 0.05) in cortisol levels were observed along spawning season (Table 1) although a trend to diminish was observed at the end of the spawning season (September). The primary stress response in fish is known to further trigger and lead to sequential secondary responses (e.g., increases in glucose, lactate, decreases in plasma sodium and potassium). In this study, no differences were found in glucose and lactate, however sodium showed lower values at the end of the spawning season.
Conclusions
The absence of significant changes in hematological and biochemical parameters suggests that the physiological condition of F1 greater amberjack broodstock seems largely unaffected by repeated treatment with implants of gonadotropin-releasing hormone agonist (GnRHa).
References
Dawood, M. A. O., S. Koshio, M. Ishikawa and S. Yokoyama. 2015. BioMed Research International
Mylonas and Zohar. 2001. Rev. Fish Biol. Fish., 10: 463-491.
Acknowledgments
This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration (KBBE-2013-07 single stage, GA 603121, DIVERSIFY).