In sea cage farming, fish are exposed to seasonal variations of water temperature, and these variations can differ
from one location to another. A small increase in water temperature does not only stimulate growth of the fish
(until an optimal level) but also lowers dissolved oxygen concentration in water. Dissolved oxygen may then become a rearing constraint during the production cycle if the oxygen requirement of fish is higher than the supply.
The impact of this constraint on production parameters (stocking density of cages and/or batch rotation) and
thus on economic profit of a farm will depend on both local thermal regime and growth potential of the fish. Increased growth is one of the most important traits in a breeding objective to increase production capacity and
profitability. We used a bioeconomic model of seabass reared in cages to calculate the economic value (EV) of increasing thermal growth coefficient (TGC) by selection in different conditions of average temperature (Tm) and
amplitude of temperature variation (Ta). Tm and Ta values were taken from different locations in the eastern and
western Mediterranean. Results show that increasing TGC has two consequences: (i) fast growing fish reach harvest weight earlier, which increases the number of batches that can be produced per year, and (ii) fast growing
fish have higher daily feed intake and, consequently, higher daily oxygen consumption. To balance the oxygen
demand and availability in a cage, a farmer might have to reduce the average stocking density, resulting in
fewer fish produced per batch. Consequently, EV of TGC is positive when Tm is 19.5 °C or 21 °C, when an increase
in number of batches produced compensates for the decrease in stocking density. EV of TGC is negative or null in
areas where Tm is closer to 18 °C because the increase in number of batches produced cannot compensate for the
decrease in stocking density. Our results show, for the first time, the importance of variation in ambient temperatures for breeding programs in fish.
Statement of relevance: The economic impact of improving growth rate in sea cage farming system depends on
temperature. This result is important for the development of breeding objectives maximizing economic return
in fish breeding programs.