Date of Award

Spring 2013

Project Type

Thesis

Program or Major

Zoology

Degree Name

Master of Science

First Advisor

David Berlinsky

Abstract

Black sea bass (Centropristis striata) have been the focus of research as an aquaculture species for several years due to their high consumer demand and limited seasonable availability. As protogynous hermaphrodites, black sea bass initially develop as females in the wild, and undergo sex reversal between 2 and 6 years of age. Previous studies demonstrated that in captivity, however, a significant number of fish differentiate initially as males, and sex reversal is hastened in females. Full control of reproduction is required for successful commercial culture. Since captive environments pose several sources of stress upon the cultured species, this research investigates stress and effects upon sex differentiation and sex change in black sea bass. The first study measured production of the stress hormone cortisol relative to stocking density, a common source of stress in the culture environment. Black sea bass juveniles were held in high (3.66g fish/L), medium (2.56g fish/L), low (1.4g fish/L) densities, in triplicate for 84 days, after which blood was collected for cortisol radioimmunoassay. Fish in the highest density treatment were found to have significantly (p=0.0003Wilcoxon/Kruskal-Wallis Test) lower plasma cortisol concentrations (0.419ng/ml) than those in the medium (3.243ng/ml) and low-density (3.232ng/m1) groups. Higher cortisol levels may have resulted from fish maintaining and defending territories in the lower density treatments. The second study investigates the process of sex change in adult black sea bass and interactions caused by either an acute physical stressor or exogenously administered cortisol. A preliminary trial measured plasma cortisol at 6 time points (0, 1, 6, 12, 24, 48 hours) after an acute stressor or exogenous application of cortisol through an infused feed. Both trials showed plasma cortisol peaks at 6 hours post feed/stressor. To assess effects upon sex change adult fish in triplicate tanks were fed a cortisol-infused diet (100 mg/kg feed), exposed to an acute stressor (net chasing, 30 seconds), or fed a control diet and not exposed to an acute stressor (control). After 84 days, blood was drawn for cortisol radioimmunoassay and sex was assessed by ovarian biopsy, abdominal massage or gonadal histology. A greater number of females in the cortisol-fed (30%) and chased (28%) treatments underwent sex change compared to those in the control groups (11%), but treatment effects were not significant (p=0.5089 One-Way ANOVA) due to small sample sizes. Although not statistically significant, these results would still be functionally significant in an aquaculture setting. The third study investigated the effect of exogenously administered cortisol upon sex differentiation in juvenile black sea bass. Fish (n=270) were distributed among 9 151L aquaria (30 fish per tank) and fed a cortisol infused diet (control: 0 mg /kg, high: 100 mg /kg, low :10 mg/kg) daily to apparent satiation. After 175, 189 and 217 days in treatment individuals were euthanized and gonads removed for histology to determine sex. Fish which differentiated as female represented 51.85%, 79.19%, and 17.84% of the control, low and high treatments respectively (p=0.0007, One-Way Analysis of Variance, Tukey's Test). Fully differentiated spermatozoa or spermiation was detected in 36.4%, 16.7% and 56.5% of the control, low and high treatments fish, respectively. The low treatment average length (13.64cm) and weight (39.93g) was significantly higher (Length: p=<0.0001, Weight: p=0.0002, One-Way Analysis of Variance, Tukey's Test). The precocious puberty and affected growth and may be linked to previously unperceived stressors in the culture environment. This research will increase our understanding of the physiological processes of black sea bass and help to advance successful commercial propagation.

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