Date of Award


Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)


Marine Biology


Nick Brown

Second Committee Member

Ian Bricknell

Third Committee Member

Linda Kling


One of the challenges for commercial production of Atlantic cod (Gadus morhua) is the sometimes high prevalence of cataracts, which affect growth, survival, and ultimately profitability. We investigated two specific factors that have been implicated in cataract formation in farmed fish; histidine deficiency in the diet and dissolved carbon dioxide levels. Triplicate groups of cod were fed diets containing one of three histidine levels (Low = 1.7%, Mid = 2.3%, and High = 2.9% HIS as protein) and exposed to one of three carbon dioxide levels (Low = 7.00 ± 1.13ppm; Mid = 12.20 ± 0.79ppm; High = 19.66 ± 1.82ppm) in a 3x3 factorial design for 5 months. Weight (g), length (mm), cataract prevalence, and cataract severity were monitored every four weeks. Blood glucose, lactate, and pH, as well as pH of the aqueous humor, were measured at the start, midpoint, and end of the trial. Results show that increasing CO2 levels significantly affected growth, feed conversion, cataract prevalence, cataract severity, eye pH, blood pH, glucose, and lactate values. Histidine did not have a significant effect on any of the measured parameters in cod, which suggests that histidine does not help prevent cataract formation, as demonstrated with salmon. We also analyzed calcium using inductively coupled plasma atomic emission spectroscopy and found that calcium levels in the lens dramatically increase with cataract severity. A histological analysis was performed throughout the experiment to monitor and describe changes within the eye as cataracts progress.

Our findings implicate high dissolved carbon dioxide as a causative agent of cataract formation in Atlantic cod by altering the acid-base equilibrium within the eye, and suggest the optimum range for good growth and health is lower than previously recommended industry standards. We also determined that cataracts pose a significant economic impact on cod production. Further focus on carbon dioxide and its removal should help alleviate the problem of cataracts in Atlantic cod aquaculture and help commercial growers become more efficient producers in the state of Maine.