The farming of gilthead seabream (Sparus aurata) is a fundamental pillar of mariculture in the Mediterranean and other regions. However, producers face a constant and costly challenge: high mortality rates during the first few weeks of larval life. These often sudden and massive events represent one of the greatest bottlenecks for the sustainability and profitability of the sector.
A recent scientific study by researchers from Cairo University investigated a commercial hatchery to trace the origin of these mortalities and propose an effective defense strategy. The research, published in Aquaculture International, offers a clear roadmap by identifying the culprits and validating control strategies that combine therapy and, more importantly, prevention.
The challenge of larval rearing: A period of high vulnerability
The first few weeks post-hatching are critical for gilthead seabream. Their immune system is still immature and highly dependent on environmental conditions and feed quality. In this scenario, bacterial infections, especially those caused by bacteria of the Vibrio genus, are a constant threat that can trigger catastrophic mortalities.
The study focused on a real case at a private hatchery in Ismailia, Egypt, where significant mortalities were recorded in seabream larvae at 14, 28, and 40 days post-hatching (DPH). Clinically, affected larvae showed paleness, lethargy, and erratic, slow swimming, although some died without any apparent signs.
The path of infection: An epidemiological tracing approach
To understand how pathogens reached the culture tanks, the researchers implemented an epidemiological tracing approach. This detective-like method involved sampling all critical components within the hatchery:
- Culture water: Sourced from an artesian well and treated with UV.
- Seabream eggs: Before and after disinfection processes.
- Live feed: Cultures of microalgae (Nannochloropsis oculata and Tetraselmis sp.), rotifers (Brachionus plicatilis), and brine shrimp (Artemia salina) used to feed the larvae were analyzed.
- Seabream larvae: Samples were taken from larvae at different stages and on different diets.
Using bacteriological culture techniques, biochemical tests, and molecular analysis (PCR and 16S rRNA gene sequencing), the scientific team was able to precisely isolate and identify the infectious agents.
Live feed as the primary vector
The analysis revealed the presence of two Vibrio species: Vibrio alginolyticus and Vibrio vulnificus. However, one of them stood out as the main antagonist in this story.
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Vibrio alginolyticus: The dominant pathogen
The most conclusive finding of the study was that V. alginolyticus was the most commonly recovered species and was present throughout almost the entire production chain. It was isolated from seawater samples, eggs, algae cultures, rotifers, and brine shrimp.
Crucially, the bacterium was also found in the larvae that were fed with this contaminated live feed chain. In contrast, V. vulnificus was only detected in the brine shrimp culture. This confirms a suspicion held by many professionals: live feed, despite being nutritionally indispensable, can act as a “Trojan horse,” introducing and accumulating pathogens from the environment and transferring them directly to vulnerable fish larvae.
The internal damage: What the eye can’t see
The study did not just scratch the surface. Histopathological examination of the infected larvae revealed severe internal damage. The tissues of the liver and kidneys showed extensive degeneration and necrosis, indicating widespread organ failure. These lesions are consistent with the pathogenic power of the toxins (such as proteases and hemolysins) released by these bacteria, causing tissue destruction that rapidly leads to death.
Effective control strategies: From therapy to prevention
Identifying the problem is only half the battle. The most valuable part of the study lies in the application and validation of control strategies that proved to be highly effective.
Emergency therapeutic treatment
Once vibriosis was confirmed, an emergency treatment was implemented. Antibiotic susceptibility tests (antibiogram) showed that both Vibrio species were susceptible to florfenicol but resistant to ampicillin.
Based on this, an oral treatment with florfenicol (30 mg/kg of biomass) was administered in the feed for 10 consecutive days. Simultaneously, the tank water was disinfected by alternating between Betadine® and hydrogen peroxide. This dual strategy successfully stopped the mortalities completely by the end of the treatment.
A comprehensive and preventive biosecurity plan
Treatment is a reactive solution, but prevention is the key to sustainability. The study proposed and validated a proactive biosecurity plan focused on the identified critical control points:
- Egg disinfection: Researchers implemented an egg “hardening” and disinfection protocol for newly fertilized eggs using Betadine® (125 mg/L) and erythromycin (13.2 mg/L). This measure successfully eliminated V. alginolyticus from the egg surface, cutting off a potential vertical transmission route.
- Live feed disinfection and enrichment: Just before being fed to the larvae, rotifers and brine shrimp were rinsed with a dose of erythromycin. Additionally, β-glucan (0.5 g/L) was added to the live feed culture media. Both actions managed to eliminate Vibrio growth on culture plates while enriching the feed with a potent immunostimulant.
- Larval fortification: The powdered diet of the larvae was regularly supplemented with a commercial probiotic (Sanolife PRO-F®), vitamin C, organic selenium, and organic zinc. This combination aims to strengthen the larvae’s immune system and improve their intestinal health to better resist infections.
Conclusions
This epidemiological study underscores a fundamental lesson for any marine fish hatchery: biosecurity is not an option; it is a necessity. Pathogen tracing demonstrates that live feed and eggs are the primary pathways for Vibrio to enter the larval stages.
Implementing a strict biosecurity plan—including the systematic disinfection of eggs and live feed, along with the use of immunostimulants and probiotics to bolster the larvae’s natural defenses—is the most effective strategy to prevent mass mortalities, improve survival rates, and ultimately, ensure the economic viability of gilthead seabream production.
Contact
Alaa Eldin Eissa
Department of Aquatic Animal Medicine and Management, Faculty of Veterinary Medicine, Cairo University
Giza, 12211, Egypt
Email: aeissa2005@gmail.com
Reference (open access)
Eissa, A.E., Ziltne, R.E., Edrees, A. et al. Epidemiological tracking and control strategies of early mortalities in hatchery-reared gilthead seabream (Sparus aurata) larvae. Aquacult Int 33, 468 (2025). https://doi.org/10.1007/s10499-025-02148-9
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
