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The spatial stock complexity of marine fish species requires that population structure is taken into account in fisheries management. The aim of this study was to determine whether the amino acid composition (AAC) of the adult fish allows the identification of subpopulations within the stock. During a cruise in November 2003 along the entire Mediterranean coast of Spain, individuals were collected of the following pelagic species: Sardina pilchardus, Sardinella aurita, Engraulis encrasicolus, Trachurus trachurus, Trachurus mediterraneus, Scomber scombrus and Scomber colias. Individuals of S. pilchardus and E. encrasicolus were also collected from the waters of the Strait of Sicily in 2002 and 2003. The AAC of the fish eyes was seen to be species specific, and therefore, the differences in AAC among species may be based on inherited characters. Moreover, a clear differentiation was seen between the Spanish and Sicilian populations of S. pilchardus and E. encrasicolus. Furthermore, in the Spanish waters of the Mediterranean Sea, discriminant analysis revealed a substantial separation between the northern and southern subpopulations of S. pilchardus, S. aurita and E. encrasicolus. Temporal variations in AAC within species in each area were lower than the spatial variations observed Guest editors: among areas for each species, probably reflecting the influence on the AAC of the contrasting environmental characteristics of each area. Our results indicate that the ACC of the eyes in adult fish is a good tool for discriminating among subpopulations in pelagic marine fish species.

The role of mesozooplankton grazers in the development of monospecific algal blooms has often been examined in a scenario in which grazers, depending on their abilities of recognition, select against toxic species and increase grazing pressure on non-toxic species. Here, we present a different ecological scenario in which grazers may select between different strains (toxic and nontoxic) of the same species, which may coexist in similar densities in natural environments prior to bloom initiation. The calanoid copepod Acartia clausi was fed with single and mixed diets of 2 strains of the dinoflagellate Alexandrium minutum, a producer of paralytic shellfish poisoning (PSP) toxins. One strain produced high, and the other low, quantities of PSP toxins. We examined feeding strategies and estimated copepod responses based on their food selection abilities, toxic effects on maintenance physiology and fitness, and benefits produced by a toxin-dilution strategy in a mixed diet. Copepods were found to feed selectively on A. minutum strains. Diet composition had a strong effect on parameters such as food ingestion, mortality, egg hatching, and egg production. The effect on copepod mortality and egg production was greatly reduced when a mixed diet (toxic + non-toxic) was provided to the copepods. However, the negative effects on egg hatching were dose-dependent, and this parameter was not recovered by toxin-dilution mechanisms. We conclude that copepods did not effectively reject the toxic strain and that the effect of A. minutum on mortality and egg production, but not on egg hatching, is reduced by dilution mechanisms. Therefore, we suggest that feeding pressure by grazers does not appear to be an important mechanism that favors toxic over non-toxic strains prior to bloom initiation.

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Nannochloropsis gaditana was tested as functional ingredient in low fishmeal diets for gilthead seabream juveniles, and its short‐term effects were evaluated in terms of intestinal morphology, digestive function, intestinal immunological and oxidative status, and intestinal microbiota. A diet with 850 g/kg of the protein provided by plant feedstuffs and 150 g/kg provided by fishmeal was used as control, and three other diets identical to the control were supplemented with 5, 7.5 and 15 g/kg N. gaditana meal and fed to 56.6 g fish for 37 days. At the end of this period, intestinal mucosa integrity and digestive capacity (luminal enzyme activity and absorption) were not altered by dietary microalgae supplementation. Intestinal transcript levels of key markers of inflammation (IL‐1β and TNF‐α) and of key enzymes involved in redox homeostasis (CuZnSOD, MnSOD and catalase) were also similar across experimental groups. However, microbiota richness increased with dietary supplementation of microalgae, and such modulation requires further investigation, particularly regarding its potential effects on disease resistance at short and long term.

This study aimed to evaluate the effects of including microalgae Chlorella sp. or Nannochloropsis sp. in plant-based diets on antioxidant mechanisms of European sea bass (Dicentrarchus labrax) juveniles. For this purpose, three isoproteic (50%) and isolipidic (19%) diets were formulated: a practical diet, containing 15% fish meal (FM) and plant ingredients as the protein source and a mixture of fish oil and vegetable oils (40: 60) as lipid source (control diet); and two diets identical to the control but with the FM replaced by Nannochloropsis sp. or Chlorella sp. (diets Nanno and Chlo, respectively). The diets were offered to quadruplicate groups of 25 fish (initial body weight: 24 ± 1 g) for 11 weeks and then enzymatic and non-enzymatic antioxidant mechanisms and lipid oxidative biomarkers were assessed in the liver and intestine of these fish. Results showed that the antioxidant response was tissue-dependent, with the liver exhibiting lower glutathione peroxidase and glucose-6-phosphate dehydrogenase (only in Chlo group) activities, and intestine lower superoxide dismutase activity with the diets including microalgae compared to control diet. An increase of oxidized glutathione content was also observed in the intestine of fish fed the microalgae diets. Catalase and glutathione reductase activities, oxidative stress index, and total and reduced glutathione, were unaffected by dietary treatments in both tissues. Overall, the lipid peroxidation status was not compromised by the replacement of FM by microalgae.

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