The effect of dietary arachidonic acid (20:4 n−6) on growth, survival and resistance to handling stress in gilthead seabream ( Sparus aurata) larvae

Abstract

The effects of high dietary docosahexaenoic acid (22:6 n−3, DHA) and varying arachidonic acid (20:4 n−6, AA) were tested on growth, survival and resistance to handling stress in 5–35 day old gilthead seabream larvae. Three enrichment treatments differing in their DHA/AA ratios were fed to rotifers ( Brachionus rotundiformis) and Artemia nauplii. The high DHA (35.9% TFA) enrichment treatment (DHA-PL) contained no AA and included lipid from the heterotrophically grown DHA-rich dinoflagellate Crypthecodinium sp. A second enrichment treatment (AADHA), selected from an earlier screening study, supplemented the high DHA enrichment treatment with an AA-rich lipid (52% TFA) from the heterotrophically grown fungus Mortierella alpina. A third enrichment treatment (ALGA) was the commercial product Algamac 2000, which is devoid of AA, but includes approximately 12.9% of TFA as docosapentaenoic acid (DPA, 22:5 n−6). Rotifers fed the DHA-PL, AADHA and ALGA treatments demonstrated a range of DHA/AA ratios (20.9, 5.6 and 10.1, respectively) as did the Artemia nauplii (25.8, 3.7 and 4.6, respectively). The enriched rotifers were fed to larvae reared in 400 l V-tanks from day 5 to day 19 post-hatching. Following this period, larvae were exposed to controlled handling stress during transfer to 27 l aquaria, where they were then fed the enriched nauplii from day 20 to day 35 post-hatching. Although larval fatty acid profiles reflected the enrichment treatments, there were no marked differences ( P>0.05) in survival and growth in 5–19 day old larvae at the end of rotifer feeding. However, the larvae fed the AA enriched rotifers prior to the handling stress of transfer to the aquaria demonstrated daily and significantly ( P<0.05) lower accumulated mortality after transfer and during Artemia feeding than larvae fed the AA-deficient (DHA-PL) and ALGA-enriched rotifers. As larvae fed the ALGA, rotifers partially retroconverted DPA to AA in their tissues, the final survival (31.0%) in these larvae was markedly better ( P<0.05) than larvae fed the DHA rotifers (17.5%), but significantly ( P<0.05) lower than larvae ingesting AADHA rotifers (42.9%). Conversely, the high-accumulated mortality in larvae fed the AA-deficient rotifers could not be corrected during the post-handling phase by feeding AA supplemented Artemia. The results suggest that dietary AA fed prior to handling stress improved survival more effectively than when fed following handling stress. These findings imply, as well, the importance of early larval nutrition on later larval and juvenile survival during crowding, grading and other handling stressors.