Introduction
In salmonid production, opercula shortening and deformities are a relative common (average prevalence of 7-10%), especially in the early start feeding period (Østevik et al., 2025). Potential risk factors as indicated by the industry are linked to operational and production routines such as feed allocation, tank design, water environment and current, stocking density and water temperatures. However, there is a notable lack of controlled studies specifically addressing the implications of opercular shortening and deformities on fish health and welfare.
According to Østevik et al. (2025), concerns associated with opercular abnormalities include reduced growth performance, increased susceptibility to gill-related health challenges, and diminished capacity to cope with stressors such as vaccination, handling, and environmental fluctuations. These issues may result in direct economic losses due to impaired growth and increased mortality, as well as indirect costs linked to additional management measures, including enhanced monitoring and sorting of affected fish.
To date, limited research has gone into uncovering potential biological mechanisms involved, more detailed description of shape variations observed and opercula shortening`s effect on the physiological functioning of the fish. Here we present a detailed scoring system for opercular length, relating opercular deformities to changes in the gill tissue as well as documenting the ability of opercula to improve as the fish grows. These are pivotal tools to an increased understanding of the mechanisms behind opercula shortening and the consequences for the physiology of the fish.
Research aims
This study aims, by means of applying qualitative and quantitative analysis of opercula shortening, to provide insight into the improvement potential of opercula over time. In addition, the shape of gills is analyzed in relation to the opercula shortening to evaluate how opercula deformities affect the respiratory area.
Methods
At the start of the trial, fish on average 12 grams were pit tagged with 8 mm Smartrac Glass TAGs. All individuals were photographed at start and when leaving the experiment at either approximately 30 grams or 140 grams. The total length of the trial was 7 weeks. Length and weight were recorded at the same time points and minimally 2 observers independently scored the opercula shortening based on a qualitative scoring scheme. Additionally, a selection of 48 individuals, with varying severity of opercula shortening, were sacrificed for tissue sampling (operculum, second gill arch or the entire gill basket and heart) at these two time points. The tissue samples were fixed in 10% buffered formalin. Formalin fixed gill arches were photographed.
The trial included 70 individuals with initially no visual opercula shortening and 163 individuals with opercula that had varying degrees of shortening and / or other types of deformities in opercular bone structure. The opercula shortening was evaluated based on a qualitative scoring scheme. In this scheme the operculum is divided into three different zones based on the anatomical structure of the most distal opercular bones. The severity of shortening is classified from 0 (no shortening) to 4 (more than half of the gill filaments visible) in each of the three regions. Both sides of the fish were evaluated.
The shapes of opercula and gills are currently being analyzed following the principles of geometric morphometrics using a combination of fixed anatomical landmarks and semilandmarks indicating outline curves (Webster & Sheets, 2010; Zelditch et al., 2026). A variety of anatomical landmarks on fish shape are established to which opercula landmarks can be referenced in order to visualize the extent of shortening. Analyses will focus on evaluating how different areas of the operculum are affected. Additionally, the results of the opercula shape analyses will be overlain with those of the gill arches to investigate the effect of opercula shortening on the respiratory surface of the gills.
Preliminary results and expected outcomes
Based on the results from the live scoring of the opercula shortening at the start and end of the trial, the potential for substantial improvement of the opercula has been confirmed. The group of fish selected as those with some degree of shortening of the opercula at the start (163 individuals) had on average score 3.7 (moderate to severe shortening) at both the start and 30 grams. Contrastingly, a clear improvement was seen at 140 grams with an average score of 0.6 (very slight shortening). The group of fish selected as those with no shortening at the start of the trial (70 individuals) had an average score of 0.1 (no to very slight shortening) at the start, on average 0.9 (very slight to slight shortening) at 30 grams, and 0.3 (no to very slight shortening) on average at 140 grams. When all individuals in the trial were seen as a whole the average score was 2.6 and 2.8 at the beginning and at 30 grams respectively (slight to moderate shortening) and improved over time to an average score of 0.5 (no to very slight shortening).
More detailed insights into whether each area of the operculum has similar improvement potential will be further explored by means of the morphometrics analysis currently in progress.
Based on observations made over the course of this study, and from case-by-case evaluation of fish groups at the research station, opercula shortening seems to affect the shape of the gill filaments. A quantitative assessment combining the shape analyses from both the opercula and the (second) gill arch(es) will be used to provide further insight into the extent of the effect.
Acknowledgment
This work is funded by FHF - Norwegian Seafood Research Fund ("Tidlig utvikling hos regnbueørret" project number: 901899).
References
Webster, M., & Sheets, H. D. (2010). A practical introduction to landmark-based geometric mophometrics. In J. A. a. G. Hunt (Ed.), The Paleontological Society Papers In Quantitative Methods in Paleobiology (Vol. 16, pp. 163–188). The Paleontological Society.
Zelditch, M., Swiderski, D., & Sheets, H. D. (2026). Geometric Morphometrics for Biologists: A Primer (3rd edition ed.). Academic Press.
Østevik, L., Stormoen, M., Kortner, T., Gundersen, I., Viken Kjønstad, M., Salvesen, Ø., Heggland, G., Grøndahl, A., Manji, F., Boerlage, A., & Eze, J. (2025). Gjellelokkforkortelse hos laksefisk: En kunnskapssammenstilling (Sluttrapport for FHF-prosjekt 901872).