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Closer consideration of euphausiids needed

Posted by jmlogan on 30 Mar 2012 at 17:52 GMT

In Figure 5, I would be curious to see how the SIAR outputs change if you add krill as another source guild. Both stomach content and isotope data have supported the importance of krill as a diet source for small bluefin tuna in the nearby Bay of Biscay (see Logan et al. Marine Biology 158 (1): 73-85), and this group would seem like a more likely lower trophic level prey source for bluefin and swordfish than gelatinous prey. Also, all of the fish species identified in Figure 5 as potential gelatinovores have nitrogen isotope values of ~ 10-11‰ while the 2 identified gelatinovore specialists (oceanic loggerheads and sunfish) have values of 6.7 and 7.7 ‰, which would place them a full trophic level lower assuming typical diet-tissue discrimination factors of ~ 3‰.

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RE: Closer consideration of euphausiids needed

alvarezdequevedo replied to jmlogan on 25 Apr 2012 at 13:21 GMT

Dear John Logan,

Thank you for your comments. Although krill is certainly consumed by tuna in other regions, there is no published hard evidence indicating any relevant consumption of krill in the western Mediterranean. Conversely, stomach contents analysis revealed krill as a major prey for other pelagic species in the western Mediterranean, and hence there is no reason to believe that krill has been overlooked in stomach contents analysis of blue fin tuna in the Mediterranean. Thus, there was no reason to include krill in SIAR in our study. Nevertheless, when krill is included in the analysis (not shown on the manuscript), the output of SIAR reveals both krill and jellyplankton as the major potential prey. The point is that even including krill, jellyplankton still stands as a more relevant prey than fish or squid.

Regarding the ratios of the stable isotopes of nitrogen, carapace scutes were used for loggerheads and muscle for fishes. As they have different fractionation factors, the reader should compare the figures once corrected for fractionation, i.e., the stable isotope ratios in diet. These figures are in turn quite similar for loggerheads (7.3‰) and for fish thought to consume jelly plankton (7.8‰ and 7.5‰ for tuna). The only major difference is about sunfish, with a diet more depleted in 15N (4.3‰) than any other possible jelly plankton consumer. This is probably because of a negligible consumption of nekton, and the possible consumption of the jellyfish Cotylorhiza tuberculata, although this species was not included in the SIAR results shown in the paper.

No competing interests declared.