Can we adjust a marine cyclopoid copepod to freshwater?—First step towards a ‘universal’ live feed product for fish and shrimp larvae

The copepod Apocyclops royi tolerates very low salinity while being highly capable of synthesizing n-3 fatty acids based on PUFA-poor diets. The feasibility of culturing A. royi in freshwater fed with baker's yeast was evaluated. The copepods were adjusted for 8 days by decreasing stepwise from salinity 20 to freshwater diluting with deionized water. Initially, the cultures exhibited adequate survival and reproduction. However, after 10 days in freshwater, they stopped reproducing presumably due to the lack of vital ions in the medium. Therefore, before continuing the freshwater cultures, we tested the survival of a batch of salinity 1 pre-acclimated adults and copepodites in various types of freshwater: deionized, bottled ‘mineral’ and tap water for 48 h. All copepods died in deionized water, but a significantly higher survival, 29% ± 11%, was found in mineral water versus 11% ± 8% in tap water. Hence, we continued culturing the copepods in freshwater using ‘Maglekilde spring’ water as diluter. The copepods regained and kept reproducing until day 71 where the experiment was terminated. After a 48 h freshwater versus salinity 20 exposure, nauplii survival was significantly less in freshwater 25 ± 12 versus 79 ± 20 (±STD) at salinity 20 but not different in mean body length. For the first time, we conducted NMR metabolomics revealing a large decrease in glycine betaine and a large increase in lactate in the copepods when challenged by freshwater versus salinity 20. There is obviously a ‘price to pay’ when culturing not only the copepods in freshwater, but also perspectives in developing a ‘universal’ A. royi live-feed product.     

Direct contact between Pseudo-nitzschia?multiseries and bacteria is necessary for the diatom to produce a high level of domoic acid

Pseudo-nitzschia multiseries loses most of its ability to produce domoic acid when it is cultured axenically. However, domoic acid production recovers when bacteria from the original culture are reintroduced to axenic cultures, indicating a bacterial association with domoic acid production in this species. In order to determine the role of bacteria in domoic acid production, an axenic strain prepared from a nonaxenic strain of P. multiseries was inoculated into media in cellophane tubes, which were then placed in a bottle containing the original nonaxenic culture. Both strains showed characteristic domoic acid production similar to that previously observed. Domoic acid level in the cells of the axenic strain in the tubes was much smaller than that of the cells outside the tubes. These results show that direct contact between living bacteria and P. multiseries is necessary for producing high levels of domoic acid in P. multiseries.     

Tub gurnard Chelidonichthys lucerna L.: a new fish species suitable for farming? First answers evaluating the growth of juveniles reared at different stocking densities,welfare and fillet quality

A trial was conducted to evaluate the growth performance and survival of Chelidonichthys lucerna. A total of 13 352 180‐days old juveniles (5.5 ± 2 g; 5 ± 1 cm) were reared at two different densities (A‐EXP = 68 fish m^?3; B‐FFA = 15 fish m^?3) in eight tanks (four tanks per group) for 360 days. The welfare status and meat quality of fish were evaluated for the A‐EXP and B‐FFA groups in comparison with wild‐caught fishery gurnard (C‐WID). The survival rate was high for both A‐EXP (79%) and B‐FFA (93.5%). B‐FFA fish had the highest specific growth rate (1.16 vs. 1.07; P < 0.05), and were heavier than A‐EXP fish (321 ± 40 g vs .239 ± 44 g; P < 0.01). Rearing conditions did not affect blood metabolites, except for glucose concentrations, which were higher in C‐WID (P < 0.05). The meat quality traits showed that reared groups were fatter (2.8–3.2%) than C‐WID (0.94%); total n‐3 fatty acids (19.02–19.26%) were lower in reared groups than C‐WID (29.99%); and EPA + DHA were similar in all groups (15.1–16.61% vs. 27.99%). Despite the good growth and survival, the final mean weight was below that requested by the market (400–500 g). Future research efforts should focus on reducing the feed conversion rate (3:1).