Project Results: Developing Solutions for Larval Mass Mortality and Stunted Growth at the HOST Park
Increasing frequencies of larval mortality events have resulted in major production losses and supply instability for some of the hatcheries at HOST (Hawaii Ocean Science and Technology) Park. Because larvae are a vulnerable early life history stage for most marine animals, water quality is critical to the success of the industry. In response to their own challenges with water quality, shellfish breeding and genetics company, Pacific Hybreed, conducted a 4-mo project, in collaboration with Hedros Bio, to identify causes of larval mortality and to develop practical mitigation strategies. Hedros Bio’s specialization in microbiological technology and the development of bacteriophage-based solutions in aquaculture was pivotal in reaching the projects goals.
Mortality events in shellfish hatcheries have been often associated with elevated loads of bacterial pathogens, particularly Vibrio species, that persist in seawater. To identify the pathogenic bacteria present at HOST Park, the group conducted a thorough sampling across multiple points of the seawater system and a range of biological materials. By culturing samples on selective media, several bacterial genera were identified including Vibrio, Pseudomonas, Aeromonas, and Shewanella. Further efforts to differentiate species and strains isolated 27 bacterial strains, with 18 isolates identified as Vibrio alginolyticus and three as Vibrio vulnificus. To assess the virulence of these specific strains, laboratory challenge experiments were conducted to infect larvae, in which two isolates caused significant larval mortality (>60%) within 48–96 hours. These mortality rates were comparable to a known pathogenic reference strain. These results identified these two Vibrio strains as the likely contributors to larval losses observed at the hatchery, causing significant mortality within 48 hours, even at concentrations lower than those detected during larval rearing operations.
Following the identification of the virulent strains, three mitigation strategies were employed to control the presence of pathogenic bacteria; an ultrafiltration system, bacteriophage cocktails, and probiotic treatments. Ultrafiltration is designed to remove particles much smaller than those removed from standard filtration systems used in most shellfish hatcheries. Typical mechanical filtration removes particles larger than 200 to 1000 nanometers (in diameter or shortest axis). In contrast, the ultrafiltration system is designed to remove particles larger than 10 nanometers, which in principle will remove bacteria before seawater is added to larval tanks. This test showed that the system, after iterative refinements, with continuous seawater flow-through and frequent cleaning, increased larval survival in some cases by over 60%.
The second approach entailed the use of bacteriophages - a promising solution in aquaculture to specifically target pathogenic bacteria. In this project, each pathogenic bacterial isolate was screened against a library of known bacteriophages. Different phage cocktails were formulated and tested based on laboratory efficacy. These cocktails showed lytic activities against the highly pathogenic V. alginolyticus and V. vulnificus.
The third method used probiotic supplementation in algal and larval cultures. The addition of Pseudoalteromonas increased larval survival by 2-fold across multiple trials compared to larval cultures not receiving the probiotic, with the greatest benefit observed during early larval stages. Probiotics are a commonly used mitigation strategy against Vibrio, and the data collected support the beneficial effects.
Implications for Hatchery Management
Standard-filtered seawater characterizations consistently show bacterial loads orders of magnitude higher than shellfish hatcheries in other regions, indicating that this high bacterial load is likely a key factor in water quality issues. Mechanical filtration combined with the addition of beneficial biological materials showed significant improvements in larval rearing growth and survival. The study improved the understanding of pathogen dynamics in the HOST Park seawater by identifying key bacterial species and their sources at different seasonal time points. Effective mechanical filtration to remove harmful bacteria, combined with beneficial biological inputs, significantly increased larval growth and survival.
Ultrafiltration and probiotic supplementation emerged as reliable, scalable, and cost-effective solutions against the identified pathogens linked to larval mortality. Bacteriophage therapy shows promise as a targeted, environmentally responsible approach to pathogen control and may complement existing hatchery practices.
Overall, integrating pathogen monitoring with targeted microbial management strategies can increase hatchery resilience, reduce larval mortality, and improve production reliability. These findings support the development of sustainable, antibiotic-free approaches for shellfish aquaculture.