Atlantic gilthead seabream show growth edge over Mediterranean cousins in 15-month cage trial

Gilthead seabream is one of the pillars of European aquaculture, a high-value marine fish farmed intensively across the Mediterranean basin for decades. Yet despite its commercial importance, surprisingly few controlled trials have asked a deceptively simple question: do fish of Atlantic origin and fish of Mediterranean origin actually perform differently when raised side by side under identical commercial conditions? A new 15-month cage experiment conducted in Çandarlı Bay, İzmir, Türkiye, and published in Acta Ichthyologica et Piscatoria, set out to answer exactly that, comparing growth, feed efficiency, survival, and genetic diversity between the two lineages. The results point to a consistent, tantalizing edge for Atlantic fish, but with a statistical caveat that the researchers themselves are careful to emphasize.

The experimental design was unusually rigorous for a commercial-scale comparison. Juvenile seabream from a commercial hatchery in İzmir were divided into two groups based on geographic origin: one of Atlantic Ocean ancestry and one from the Aegean–Mediterranean region. Roughly 80,000 fish were placed in each of six net cages, three replicate cages per group, each cage twelve meters in diameter and depth. Initial mean body weights were nearly identical, at 4.24 grams for the Atlantic group and 4.28 grams for the Mediterranean group, ensuring that any later divergence would stem from biology rather than starting conditions. The trial ran for 457 days, spanning more than a full year of seasonal temperature swings in the bay.

Feeding followed a standard commercial protocol. The fish received a gilthead seabream diet produced by a commercial manufacturer, with pellet sizes ranging from one to six millimeters as the fish grew. Crude protein and lipid ratios shifted from 52:18 in the smallest pellets to 42:20 in the largest, matching the changing nutritional demands of growing juveniles. Fish were fed three times daily, with feeding rates adjusted according to fish size and water temperature. Water quality remained within optimal ranges throughout: mean temperature was 19.53 degrees Celsius, dissolved oxygen averaged 6.54 milligrams per liter, salinity 37.49 parts per thousand, and pH 8.15. These stable conditions minimized environmental confounding, a critical consideration when comparing genetically distinct stocks.

Monthly sampling was extensive. Four hundred fish per cage, 1,200 per group per event, were anesthetized with phenoxyethanol, weighed to the nearest 0.01 gram, and measured for total length. Fifty fish per cage were dissected monthly to determine hepatosomatic and viscerosomatic indices, which measure the relative sizes of the liver and visceral organs against total body weight. Liver samples were fixed, sectioned at five micrometers, and stained with Sudan Black to visualize lipid accumulation. The sampling design was grounded in a power analysis targeting the detection of a ten percent difference in mean body weight with eighty percent statistical power, a level of rigor rarely seen in commercial aquaculture trials.

By the end of the experiment, the Atlantic group had reached a mean final weight of 415.32 grams, compared with 374.03 grams for the Mediterranean group. The Atlantic fish also showed a higher specific growth rate, 0.89 versus 0.74 percent per day, and a better feed conversion ratio, 1.79 versus 1.92, meaning they converted feed into body mass more efficiently. Survival rates were statistically indistinguishable, at 88.0 percent for Atlantic and 85.5 percent for Mediterranean fish. Condition factor, hepatosomatic index, and viscerosomatic index showed no meaningful differences between groups, suggesting that overall body shape and organ proportions were similar even as growth rates diverged.

Here lies the study’s most important methodological lesson. The initial analyses, conducted with conventional analysis of variance, suggested statistically significant differences in final weight, specific growth rate, and feed conversion ratio, all with p-values below 0.05. However, when the researchers applied a False Discovery Rate correction using the Benjamini–Hochberg procedure, designed to account for the many comparisons made across multiple traits and time points, none of these differences remained statistically significant. Adjusted p-values were 0.151 for final weight, 0.294 for specific growth rate, and 0.112 for feed conversion ratio. The authors are transparent about this: the consistent direction of the trends may reflect genuine biological differences, but random variation cannot be ruled out, and the findings should be treated as indicative rather than definitive.

Histological examination of liver tissue added a subtle but intriguing dimension. By the end of the trial, the Mediterranean group showed more pronounced lipid accumulation in liver sections than the Atlantic group, though the difference was not statistically significant in the physiological indices. Hepatic steatosis, or fat accumulation in the liver, is a well-documented phenomenon in farmed gilthead seabream, typically linked to dietary lipid levels and energy balance. Because both groups received identical feed, the researchers suggest that the observed differences may reflect not only dietary effects but also interactions between genetic background and lipid metabolism, a possibility that could have important implications for feed formulation and fish health management in commercial operations.

The genetic analysis revealed a rich picture of population structure. Using five microsatellite loci and RAPD-PCR markers, the researchers found high within-population diversity in both groups. The number of alleles per locus ranged from six to sixteen in the Mediterranean population and eight to fourteen in the Atlantic population, with expected heterozygosity values consistently high across loci, ranging from 0.672 to 0.903 in the Mediterranean group and 0.710 to 0.880 in the Atlantic group. Pairwise genetic differentiation, measured by F_ST values, ranged from –0.010 to 0.045, indicating low-to-moderate but statistically relevant structuring between the two origins. Both populations were in Hardy–Weinberg equilibrium, suggesting genetic stability in these farmed stocks.

RAPD analysis reinforced the picture of distinct but not deeply divergent lineages. The mean within-population genetic similarity index was 0.583 for the Atlantic group and 0.662 for the Mediterranean group, while the between-population similarity was 0.499, confirming that individuals within each group were more genetically similar to each other than to members of the other group. The genetic distance between populations was 0.354, notably higher than the 0.245 reported in earlier Turkish broodstock comparisons, underscoring the influence of broader geographic separation and environmental adaptation on genetic structuring. Notably, the Atlantic group exhibited higher genetic variability overall, a trait often associated with greater adaptability, disease resistance, and growth efficiency.

The study’s conclusions are measured but forward-looking. Atlantic-origin seabream showed a consistent trend toward better growth performance and feed conversion under the tested conditions, and they carried higher genetic variability, making them potentially attractive candidates for broodstock selection. However, the lack of statistical significance after multiple-testing correction means these findings remain preliminary. The researchers call for multi-environmental studies, higher-resolution genomic tools such as SNP panels and whole-genome sequencing, and multi-generational comparisons to determine whether the observed trends reflect true genetic advantages relevant to aquaculture. For an industry increasingly focused on sustainability, feed efficiency, and selective breeding, this work provides a valuable foundation, even as it reminds us that in biology, as in statistics, the most interesting answers often lie just beyond the reach of a single experiment.

Subject of Research: Atlantic gilthead seabream show growth edge over Mediterranean cousins in 15-month cage trial

Article Title: Growth, feed efficiency, and genetic diversity in two farmed populations of gilthead seabream, Sparus aurata ( Actinopterygii , Eupercaria , Sparidae ), of Atlantic and Mediterranean origin

Article References: Sert, C. S., Korkut, A. Y., Gamsiz, K., Karahan, B., Özcan-Gökçek, E., Gurkan, S., Engin, S., Tolon, M. T., Karacalar, U., & Tokaç, A. (2026). Growth, feed efficiency, and genetic diversity in two farmed populations of gilthead seabream, Sparus aurata (Actinopterygii, Eupercaria, Sparidae), of Atlantic and Mediterranean origin. Acta Ichthyologica et Piscatoria, 56, 223-235. https://doi.org/10.3897/aiep.56.188432

Image Credits: AI Generated

DOI: 10.3897/aiep.56.188432

Keywords: aquaculture, gilthead seabream, genetics, growth performance, feed efficiency, Sparus aurata, selective breeding, population genetics, Growth, feed, efficiency, genetic