Larval striped bass (M. saxatilis), tissue docosahexaenoic (DHA) and arachidonic (AA) acids levels were modulated through dietary enrichments and the effect on growth and survivorship examined. Mean growth was significantly greater in larvae enriched with AA than in larvae enriched with DHA (F-value for AA treatment was 20.5 versus only 5.1 for the DHA treatment). Dietary treatment did not have a significant effect on larval survivorship (56.0±2.4%, p > 0.05). When challenged with hypersaline (25 psu) immersion, DHA enriched larvae survived better than AA enriched larvae, but larvae with body tissue levels of 15.4 mg AA g−1 and 7.2–15.4 mg DHA g−1 dry weight provided maximal survivorship to the challenge. Elevated levels of body tissue AA was generally associated with elevated levels of whole body cortisol. On the other hand, increasing levels of DHA mainly affected the kinetics of cortisol increase to hypersaline exposure. Larvae injected intraperitoneally with formalin fixed Staphylococcus aureus responded by altering the proportion of lymphocytes, monocytes and neutrophils in perpherial blood. Lymphocytes, which accounted for the largest percentage of white blood cells (over 70%), decreased in all challenged larvae during the first 6 hours post injection then returned to pre-challenge levels after 44 hours. Conversely, the relative proportion of monocytes and neutrophils rose from 14% and 2% up to 28% and 6% of the total circulating leucocytes, respectively. The largest increase occurred in larvae fed a moderate level of both DHA and AA.
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