The effect of dietary arachidonic acid on growth, survival, and cortisol levels in different-age gilthead seabream larvae (Sparus auratus) exposed to handling or daily salinity change

The effect of dietary arachidonic acid (ArA) on survival, growth, and cortisol level in different-age gilthead seabream larvae exposed to handling or daily fluctuating salinity was tested. Premetamorphosing (3-19 DPH) larvae were reared in 400-1 V-tanks and fed one of three rotifer treatments containing ArA levels of 1.14, 2.11, or 3.87 mg g^-1. At 20 DPH, the larvae were divided into two groups where each larval group was randomly divided over twelve 27-1 aquaria (300 larvae per aquarium) and the stress of transfer was defined as an acute stressor. In each set of 12 aquaria, larvae were fed over 12 days three Artemia metanauplii treatments, which were tested in four aquaria per treatment, giving Artemia ArA levels of 0.59, 3.42, or 5.86 mg g ^-1 dry weight (DW). One set of 12 aquaria received seawater of constant salinity (25‰) and these larvae, exposed only to the stress of transfer, were considered as controls. The other set of 12 aquaria was supplied with seawater with a daily fluctuating salinity from 25‰ to 40‰ and back to 25‰, exposing the larvae to 12 days of salinity change. Another trial was carried out on 30 DPH metamorphosing larvae, which were similarly stocked in the aquaria and tested with the identical Artemia treatments as the premetamorphosing larvae study. A positive correlation was found between increasing dietary ArA level and survival at the end of the study in the control premetamorphosing (20-32 DPH) and metamorphosing (30-42 DPH) larvae (38%, 48.6%, and 77.2%, and 58%, 56.4%, and 90.4%, respectively). On the other hand, premetamorphosing and metamorphosing larvae exposed to salinity change, although exhibiting an increase in survival at the intermediate level of ArA, demonstrated a decrease in survival (55.3%, 60.0%, and 25.8%, and 70%, 83%, and 76%, respectively) when fed the highest ArA level. In the control metamorphosing larvae, basal cortisol (6.0, 8.2, and 11.4 ng g^-1 DW) was independent of dietary ArA while fish exposed to salinity change demonstrated distinctly higher basal cortisol levels (7.5, 15.9, and 19.8 ng g^-1 DW) that markedly increased with rising dietary ArA levels at 42 DPH. Fish exposed to salinity change and fed Artemia containing 0.59, 3.42, and 5.86 mg g^-1 ArA exhibited significantly (P<0.05) decreasing SGR values (12.15, 10.68, and 9.69, respectively) while the SGR values in the control larvae (10.23, 10.92, and 9.79, respectively) were generally stable. The results showed that dietary ArA promoted survival in fish encountering only handling stress. In contrast, repetitive salinity change altered the nature of the stress response where dietary ArA appeared to upregulate cortisol synthesis coinciding with reduced growth and increased mortality.