Jaw deformity

A properly formed head and jaw are essential for effective feeding and respiration through the buccal pump. Most jaw deformities tend to occur during early life stages and, like vertebral deformities, are a valuable indicator of prior and ongoing management practices. Jaw deformities, such as jaw length discrepancies, curved jaws, or pugheadedness can prevent normal feeding and breathing. Jaw deformities must be differentiated from snout damage.

Level 0	Level 1	Level 2	Level 3
Normal jaw length and alignment	Minor jaw deformity or misalignment, visible only with close inspection	Clear jaw deformity or misalignment that will not affect the fish’s ability to eat and grow normally	Severe jaw deformity or misalignment that may affect respiration and/or the fish’s ability to eat
Healthy rainbow trout with no jaw deformity⁴⁵	Rainbow trout with minor lower jaw deformity⁴⁶	Rainbow trout with clear lower jaw deformity⁴⁷	Rainbow trout with severe lower jaw deformity⁴⁸

Protocol application

Assessment requires handling the fish for close inspection of jaw alignment and skull shape. Very minor deformities are only visible with close inspection and cannot be detected in-water.

Calculate the distribution of scores across the thresholds within the sample.

Scoring notes

Assess jaw shortening and pugheadedness separately for the most accurate results by checking for jaw length and alignment, and then checking for any compressed skull shape.
Validation: This scoring system has been adapted from the Laksvel scoring system for Atlantic salmon.

Key evidence

Source	Summary of source	Key result
Godoy et al., 2023	Experimental study assessing jaw bone structure and osteogenic markers in normal and mandibular-deformed rainbow trout smolts (< 400 g) in freshwater commercial farms. 5 normal and 5 mandibular-deformed fish were compared.	Deformed jaws showed lower calcium and phosphorus content, indicating mechanically weak and/or painful jaws that limit function during feeding. Deformed fish showed poorer nutritional status, with lower plasma total protein and globulin levels and markedly reduced visceral fat, indicating a chronic energy deficit due to impaired feed intake. Deformed fish also had higher blood glucose and an altered mineral profile, suggesting physiological stress.
Näslund and Jawad, 2021	Literature review assessing the causes and impacts of pugheadedness in multiple species of fish, including rainbow trout.	Pugheadedness is linked with exophthalmos and is more likely in more deformed individuals. In severe cases, this can expose the anterior of the eyeball. Since the bones in the jaw may be severely deformed, this can restrict complete opening or closure of the mouth, potentially reducing feeding efficiency and, in turn, growth.

Supporting evidence

Cano, Irene, John Worswick, Brian Mulhearn, Matt Green, Stephen W. Feist, and Morag Clinton. “Cranial Mandibular Fibrosis Syndrome in Adult Farmed Rainbow Trout Oncorhynchus mykiss.” Pathogens 10, no. 5 (April 30, 2021): 542–42. https://doi.org/10.3390/pathogens10050542.
Godoy, Karina, Cristian Sandoval, Claudio Vásquez, Carlos Manterola-Barroso, Barbara Toledo, Joel Calfuleo, Carolina Beltrán, et al. “Osteogenic and Microstructural Characterization in Normal versus Deformed Jaws of Rainbow Trout (Oncorhynchus mykiss) from Freshwater.” Frontiers in Marine Science 10 (November 24, 2023). https://doi.org/10.3389/fmars.2023.1301449.
Näslund, Joacim, and Laith A. Jawad. “Pugheadedness in Fishes.” Reviews in Fisheries Science & Aquaculture 30, no. 3 (August 11, 2021): 306–29. https://doi.org/10.1080/23308249.2021.1957772.
Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. “Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages.” Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.

Nilsson, Jonatan. 2026. Healthy rainbow trout, jaw deformity score 0. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout with minor lower jaw deformity, score 1. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout with clear lower jaw deformity, score 2. Photograph. Personal communication, Institute of Marine Research.
Nilsson, Jonatan. 2026. Rainbow trout with severe lower jaw deformity, score 3. Photograph. Personal communication, Institute of Marine Research.