Accelerated growth in salmon comes at the expense of cardiac health

Key Findings of the Study

1. A study conducted across four farming sites in the Faroe Islands revealed that fast-growing Atlantic salmon exhibited significantly more severe coronary artery lesions.
2. At seawater transfer, hatchery smolts with higher growth rates showed double the prevalence of vascular damage (67% vs. 26%) compared to same-age fish with moderate growth rates.
3. In facilities with more intensive marine grow-out rates, lesions progressed more rapidly, ultimately matching the severity of the site with the poorest initial baseline.
4. Prior to harvest, 90% to 100% of the fish exhibited vascular lesions, with 14% to 28% showing arterial narrowing previously associated in trout with reduced stress tolerance.
5. While this observational design does not demonstrate direct causality, the findings suggest that maximizing growth across both production phases is not biologically benign for cardiac health.

The day begins early in the Faroe Islands. The service vessel remains moored alongside the sea cage, with pumps running and the thermal delousing loop calibrated. At first glance, the stock meets standard production benchmarks: sound biometrics, good condition factor, and consistent feed intake over recent weeks, offering no warning of health setbacks.

By day’s end, however, mortality counts exceed expectations. Fish that swam vigorously hours earlier lie lifeless at the net bottom, exhibiting neither external lesions nor visible pathological signs. The farm logs will record it as another routine “handling-related mortality”—a convenient operational label that fails to explain the underlying physiological cause.

Research published in Aquaculture suggests the root cause lies in an anatomical structure frequently overlooked during routine monitoring: the coronary microartery supplying the salmon myocardium. The study directly links this pathology to the industry’s highest-priority variable: growth rate. Findings indicate that rapidly growing salmon—both during freshwater hatchery rearing and marine grow-out—develop more severe coronary deterioration, likely compromising their resilience to operational stress. For an industry accustomed to defining efficiency through daily gain and abbreviated cycles, these findings compel an overdue reassessment of production priorities.

Why is coronary health so critical in marine cages?

Unlike many aquatic species, salmonids possess dedicated coronary arteries that supply oxygenated blood directly to their myocardium. Their evolutionary status as “marine athletes”—adapted to migrations spanning thousands of kilometers—demands a resilient cardiovascular system engineered to sustain vigorous cardiac output under extreme metabolic loads.

Within intensive aquaculture, however, the salmon heart faces severe operational stresses: high stocking densities, grading, transport, pumping, and, most critically, sea lice treatments. Each handling event drives acute surges in heart rate and oxygen consumption. If the coronary artery is constricted, the cardiac muscle suffers acute hypoxia at the exact moment demand peaks—akin to running a high-pressure pump through a collapsed hose: it withstands routine baseline flow, but fails catastrophically under peak load.

This pathological condition is clinically defined as coronary arteriosclerosis: the progressive thickening and luminal narrowing of the vessel. Unlike in humans, where cholesterol plaques predominate, in salmon this occurs via smooth muscle cell hyperplasia within the arterial tunica intima, progressively restricting the lumen.

The documented case histories cited in the study warrant serious veterinary vigilance. In 2007, an incident of over 10% mortality was documented following routine wellboat transport; necropsies revealed severe coronary pathology and myocardial necrosis consistent with infarction. Similar lesions have since been recorded with increasing frequency in post-handling losses. Correspondingly, research in rainbow trout confirms that fish with advanced vasculopathy exhibit markedly diminished tolerance to standardized crowding challenges.

The long-standing industry suspicion that science had yet to quantify

For over three decades, the industry presumed that accelerated growth exacerbated these vascular anomalies. Pioneering studies—such as a 1992 investigation on Atlantic salmon and a 2020 study on trout—suggested this correlation; however, none directly measured specific growth rates, instead relying on proxy estimates such as differing fish sizes or contrasting thermal regimes across farm sites.

This methodological gap is critical for a sector that prioritizes maximizing daily weight gains and accelerating biomass turnover, as confirming whether such intensification carries a cardiovascular cost demands rigorous empirical evidence rather than theoretical assumptions.

To address this challenge, an international consortium led by Heidi S. Mortensen of the Firum institute and the University of Gothenburg partnered with specialists from Pharmaq Analytiq, Nofima, the Swedish University of Agricultural Sciences, benchmarking firm Avrik, and the Norwegian University of Life Sciences (NMBU).

Field methodology: four farming sites, two production phases, and thousands of histological sections

The study was conducted under real-world commercial conditions rather than in a controlled laboratory setting. Between 2021 and 2022, the research team sampled apparently healthy Atlantic salmon—all from the commercial Stofnfiskur strain—across four marine farm sites in the Faroe Islands, leveraging mandatory and randomized welfare checks for sea lice monitoring.

To isolate growth rate from biological age—since older fish exhibit greater cumulative vascular wear—the sites were paired for direct comparison:

• Sites 1 and 2: Younger fish (approximately 15 months post-hatching at seawater transfer) sourced from the same operating company and sharing a single hatchery.
• Sites 3 and 4: Fish with five additional months of development (around 20 months post-hatching), originating from a different producer and reared in the same land-based facility.

Sampling was performed across two operational windows: one month post-seawater transfer and ten months later, immediately prior to harvest. Hearts were collected from each specimen, and the primary coronary tract was processed into 18 ultra-thin histological sections of 3 micrometers (µm). Using light microscopy, the team evaluated the extent of vascular involvement, tissue damage severity on a standardized scale from 0 to 5, and the degree of arterial luminal narrowing.

The investigation overcame notable methodological challenges, including substantial anatomical variability in coronary branching patterns; only 31.8% of the 220 examined fish exhibited a single unbranched artery across the evaluated segment. Consequently, strict quality-control criteria established a final analytical cohort of 153 specimens after confirming this morphology did not correlate with age, weight, or growth, while sample size at one site was limited to 20 fish due to unfasted holding conditions. Due to commercial confidentiality surrounding land-based data, hatchery development rates were calculated in grams per day, restricting the use of the Thermal Growth Coefficient (TGC) strictly to the marine grow-out cycle.

Evidence at Seawater Transfer: The Hatchery Footprint on Coronary Health

The most revealing comparative assessment emerged between Sites 3 and 4: while both cohorts shared virtually identical biological ages, they exhibited divergent development rates during the hatchery phase (a daily gain of 1.2 g/day at Site 3 versus 0.8 g/day at Site 4). Just one month post-seawater transfer, the disparity in cardiac morphology was striking:

• Higher Pathological Prevalence: Vascular lesions affected 67% of specimens from Site 3, compared to just 26% from Site 4.
• Lesion Extent: The proportion of affected arterial bed quadrupled (25.6% vs. 5.7%).
• Histological Severity: The mean severity score reached 0.8 at Site 3 versus 0.1 at Site 4, with only the former registering maximum grade 5 damage.
• Hemodynamic Compromise: In the most severe cases, luminal narrowing reached 13.5%, compared to merely 4.4% at Site 4.

Statistical analyses revealed that freshwater growth rates independently accounted for 34% of the variation in lesion extent and 31% of overall severity; this trend was reinforced across Sites 1 and 2, where fish grew at nearly identical land-based rates (1.1 and 1.0 g/day) and displayed matching arterial profiles (an identical 29% prevalence with comparable lesion grades). However, the study issues a critical caveat: slow-growing fish (Site 4) entered seawater with a condition factor of 0.9—verging on emaciation—demonstrating that while slower growth safeguarded vascular architecture, it compromised overall physiological robustness, underscoring that the production solution does not lie in unmitigated growth suppression.

Marine Phase: Accelerated Growth and Convergence in Coronary Pathology

The subsequent performance of the batches revealed a decisive trend: during the seawater stage, growth dynamics reversed, with fish from Site 4 displaying faster development (TGC of 4.2) compared to those from Site 3 (TGC of 3.5). According to Avrik’s Laksur 2.0 benchmark model—which evaluates observed weight against projected trajectories derived from historical datasets of Faroese farms between 2005 and 2022—Site 4 fish surpassed their expected biomass by 22.7%, whereas Site 3 fish fell 14.6% below expectations.

The researchers propose several explanatory hypotheses: thermal seasonality may have exerted a decisive influence, as Site 4 was stocked in May amid rising temperatures while Site 3 was stocked in December on the cusp of winter; furthermore, compensatory growth following earlier restriction might have played a role, aligning with literature documenting that salmon with rapid hatchery development exhibit lower marine performance.

Regardless of the underlying mechanism, the outcome was definitive: approaching harvest, both sites converged in final body weight and coronary artery deterioration, with 90% of fish across both sites exhibiting vascular lesions, mean severity scores of 2.5 and 3.0, and luminal narrowing of 15.6% and 16.7% in severe cases. The cohort that was initially healthier matched the compromised group precisely as it accelerated its grow-out rate; additionally, marine growth accounted for 16% of the variance in lesion extent at Sites 1 and 2, culminating in 90% to 100% coronary pathology prevalence across all four facilities prior to slaughter.

Physiological Implications: Vascular Vulnerability to Operational Stress

What constitutes the critical threshold of vascular compromise? While the study did not directly measure physiological tolerance in these salmon, it establishes a quantitative benchmark: in a 2020 study on rainbow trout, fish that failed a standardized 30-minute crowding challenge exhibited a 26% to 39% reduction in coronary luminal area. In the Faroe Islands, between 14% and 28% of the salmon evaluated prior to harvest surpassed that 26% threshold in their most severe lesion.

Under the premise that Atlantic salmon share a physiological tolerance profile comparable to that of trout, up to one in four fish would have a reduced capacity to withstand demanding operational routines—an extrapolation the authors frame with methodological caution. Furthermore, the researchers contextualize historical mortality trends across the Faroese archipelago, noting that loss rates began rising following the 2009 sea lice regulatory framework (which mandated more frequent interventions) and worsened around 2015 as chemical treatments were phased out in favor of thermal and mechanical delousing methods. The authors conclude that a synergy of factors is at play: fish with compromised coronary architecture that previously withstood milder handling now succumb to far more intensive, repeated operational stressors.

Physiological Mechanism: The Biomechanical Impact of Rapid Growth on the Vascular Bed

The most widely supported hypothesis in the scientific literature is biomechanical in nature: the main coronary artery of Atlantic salmon courses over the bulbus arteriosus, an elastic structure that dilates and contracts with each systole to damp hemodynamic pressure, meaning sustained mechanical stretching induces microtrauma in the vessel wall and triggers smooth muscle cell proliferation as reactive luminal hyperplasia.

Any condition elevating cardiac output—such as vigorous swimming, environmental stress, or an elevated metabolic rate—exacerbates this vascular wear. However, the authors underscore the methodological limitations inherent to an observational study conducted under commercial conditions, because even though cohorts were matched for genetics, hatchery origin, age, and producer, each sea site presented unique environmental conditions including hydrodynamics, thermal regimes, and stress events, necessitating controlled experimental trials to decouple growth rates from confounding operational variables.

Back to the Fjord: The Cumulative Origin of Operational Stress

Returning to the delousing treatment scenario and the fish’s apparent vitality, the study demonstrates that the mortality event did not originate on that day, but rather accumulated across months of production: it began in a hatchery that stimulated rapid weight gain and continued through a marine phase that sustained peak metabolic demand, leaving vulnerable individuals devoid of functional myocardial reserve when subjected to operational handling.

The scientific conclusion does not advocate curbing production to sub-optimal levels—since inadequate growth remains zootechnically unviable—but rather urges the salmon sector to recognize that growth rates across both phases leave a cumulative imprint on the fish’s cardiovascular system, making the balance between productivity and cardiac integrity the determining factor between unexplained operational losses and sustainable harvests.

Contact
Heidi S. Mortensen
Firum, Við Áir 11, FO-430 Hvalvík, Faroe Islands.
Email: heidi@firum.fo

Reference (open access)
Mortensen, H. S., Wisløff, H., Zena, L. A., Ekström, A., Gräns, A., Dam, R., Egholm, E., Vang, A., Johansen, I. B., & Sandblom, E. (2027). Rapid growth rates are associated with increased coronary arteriosclerosis in farmed Atlantic Salmon (Salmo salar) under commercial production conditions. Aquaculture, 626, 744394. https://doi.org/10.1016/j.aquaculture.2026.744394

Milthon Lujan

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.