Vitamin A supplementation enhances Senegalese sole (Solea senegalensis) early juvenile’s immunocompetence: new insights on potential underlying pathways

Senegalese sole (Solea senegalensis) has been considered since the 1990´s to be a promising flatfish species for diversifying European marine aquaculture. However, pathogen outbreaks leading to high mortality rates can impair Senegalese sole commercial production at the weaning phase. Different approaches have been shown to improve fish immunocompetence; with this in mind the objective of the work described herein was to determine whether increased levels of dietary vitamin A (VA) improve the immune response in early juveniles of Senegalese sole. For this purpose, Senegalese sole were reared and fed with Artemia metanauplii containing increased levels of VA (37,000; 44,666; 82,666 and 203,000 total VA IU Kg-1) from 6 to 60 days post-hatch (early juvenile stage). After an induced bacterial infection with a 50 % lethal dose of Photobacterium damselae subsp. damselae, survival rate, as well as underlying gene expression of specific immune markers (C1inh, C3, C9, Lgals1, Hamp, LysC, Prdx1, Steap4 and Transf) were evaluated. Results showed that fish fed higher doses of dietary VA were more resistant to the bacterial challenge. The lower mortality was found to be related with differential expression of genes involved in the complement system and iron availability. We suggest that feeding metamorphosed Senegalese sole with 203,000 total VA IU Kg-1 might be an effective, inexpensive and environmentally friendly method to improve Senegalese sole immunocompetence, thereby improving survival of juveniles and reducing economic losses.

First insight into personality traits in Northern pike (Esox lucius) larvae: a basis for behavioural studies of early life stages

Personality traits have been studied for some decades in fish species. Yet, most often, studies focused on juveniles or adults. Thus, very few studies tried to demonstrate that traits could also be found in fish larvae. In this study, we aimed at identifying personality traits in Northern pike (Exos lucius) larvae. Twenty first-feeding larvae aged 21 days post hatch (16.1 +/− 0.4 mm in total length, mean +/− SD) were used to establish personality traits with two tests: a maze and a novel object. These tests are generally used for evaluating the activity and exploration of specimens as well as their activity and boldness, respectively. The same Northern pike twenty larvae were challenged in the two tests. Their performances were measured by their activity, their exploratory behaviour and the time spent in the different arms of the maze or near the novel object. Then, we used principal component analysis (PCA) and a hierarchical ascendant classification (HAC) for analysis of each data set separately. Finally, we used PCA reduction for the maze test data to analyse the relationship between a synthetic behavioural index (PCA1) and morphometric variables. Within each test, larvae could be divided in two sub groups, which exhibited different behavioural traits, qualified as bold (n = 7 for the maze test and n = 13 for the novel object test) or shy (n = 9 for the maze test and n = 11 for the novel object test). Nevertheless, in both tests, there was a continuum of boldness/shyness. Besides, some larvae were classified differently between the two tests but 40 % of the larvae showed cross context consistency and could be qualified as bold and/or proactive individuals. This study showed that it is possible to identify personality traits of very young fish larvae of a freshwater fish species.

Nutrients limiting the Algal Growth Potential (AGP) in the Gulf of Riga, eastern Baltic Sea, in spring and early summer 1996

In May, June and July 1996, samples wcre collected along one transect greatly influenced by river discharge (eastern side of the gulf), along one transect slightly influence by river discharge (western side), at one station Iocated in the mouth of the main river (River Daugava), at one station located in the center of the Gulf and at several nearshore locations of the western side. Ratios of rnolecular concentrations of in situ dissolved ioorganic nitrogen, phosphorus and silicon, as weIl as enrichment bioassays were llsed to dctcrrnine which nutrient (s) lirnited the potential biomass of phytoplankton. Both comparison of (NO.d-N02+NJ.L): P04 (DIN: DIP) values with Redfic1d's ratio and bioassay inspection led to the sarne conclusions. Phosphorus was clearly the nutrient most limiting for the potcntial biornass of test species in nitrogen- rich waters, which occurred in mid spring, in the upper layer of the southern-eastern part of the Gulf which is greatly influenced by river discharge. In late spring, with the decrease of the total DIN reserve, nitrogen and phosphorus showed an equallimiting role. In deeper layers of this area and out of the river plume (western side and central part of the gulf), nitrogen was the limiting nutrient. In summer, whcn river discharge was the lowest, a11 DIN concentrations but one ranged between 1.6 and 2.6 µM, and the whole area was nitrogen-limited for both the cyanobacterial and the algal test strains. In 74% of the samples for which nitrogen was the limiting nutrient, phosphorus was recorded to be the second potentially limiting nutrient. In contrast, silicon never appeared as limiting the growth potential of either Microcystis aeruginosa or Phaeodactylum tricornutum; phosphorus was the limiting nutrient when DIN: Si03 values were >1 (in May), but DIN: Si03 was <1 when nitrogen was limiting (June and July). The authors conclude that the recently reported decrease of silicon loading in coastal waters and its subsequent enhanced importance in pushing the outcome of species competition towards harmful species may not yet be the most important factor for the Gulf of Riga. Iron appeared for 12% of the tests in the list of nutrients limiting the potential biomass. Tentative results also indicated that a significant fraction of the nitrogen (~,4 µg-atom N 1(-1) taken up by Microcystis aeruginosa may have been in the form of dissolved organic nitrogen (DON). It is thus also suggested tentatively that more attention be paid to these nitrients during further research in the Gulf of Riga.