Effect of changing temperature on the metabolic performance and life history of Nile Tilapia - Oreochromis Niloticus (Linnaeus, 1758)

Abstract

Tropical fresh water fishes are believed to be vulnerable to the impacts of climate warming on the basis of their physiological history, that is having a narrow thermal window and a low acclimation capacity. In this study, a static respirometry and ramping methods were used to assess the sensitivity and vulnerability of Nile tilapia, Oreochromis niloticus from Lake Victoria, a fish of both ecological and commercial importance. Fitness related traits (growth, condition, fecundity and size at 50% maturity) of Nile tilapia were measured after 4-months of rearing period in six outdoor concrete tanks (hot and cool). Metabolic performance (Maximum Metabolic Rate (MMR), Standard Metabolic Rate (SMR) and Aerobic Scope (AS)) was measured on acutely exposed and 3-weeks laboratory acclimated Nile Tilapia to a range of temperature treatments (25, 29 and 33○C). Critical Thermal Maxima (CTMax) was used to assess shifts in thermal tolerance and acclimation capacity which was measured following acute exposures and a 3-days laboratory acclimation to a range of temperature treatments (25, 29 and 33○C). Nile tilapia growth was higher in the hot rearing conditions than in the cool rearing conditions (t(4) = 4.365, P= 0.012). The length-weight relationship for the hot-reared fish exhibited negative allometric growth (b<3). Relative condition (K) was not affected by temperature (t(4) -0.886, p=0.426) and Nile tilapia from hot and cool temperature regimes was found to be in good condition (K>1) . After four months of rearing, Nile tilapia in the cool conditions had not yet sexually matured but in fish reared in hot conditions, Length at 50 % maturity in females was found to be 13.6 cm and in males, it was 15.2 cm. In acutely-exposed fish, SMR, MMR, AS and CTmax increased with temperature and were highest at 33°C. Nile tilapia reared under hot conditions generally had lower SMR during both acute exposures and after 3-weeks of acclimation. MMR for the 3-week acclimated fish from the hot rearing tanks increased with experimental temperatures and then plateaued at 33°C. For acutely-exposed fish, there was a variation in AS across experimental temperatures (F=79.004, p<0.001), but no differences among rearing temperatures (F=1.414, p=0.239). However, in the thermally acclimated fish, AS in hot-reared fish raised above those of the cool-reared fish and a visual comparison of acutely-exposed versus acclimated fish showed that fish from the hot rearing conditions appear to have higher AS after three weeks of acclimation. This therefore indicates that Nile tilapia are able to acclimate to higher temperatures outside their natural environmental ranges. CTmax increased with temperature for both acutely exposed and 3-day acclimated fish and was significantly higher in the hot-reared fish than the cool- reared fish. CTmax was found to be higher in the 3-day acclimated fish than in the acutely exposed fish thus indicating that a longer acclimation period to higher temperatures in the laboratory had a direct effect on CTmax. Results from this study do not agree with the assumptions that tropical species are highly vulnerable to climate warming and have a very low acclimation capacity. The study also demonstrates that Nile tilapia is not vulnerable to climate warming impacts given its high thermal plasticity in growth rate, fecundity, and its flexibility in metabolic traits and CTmax over multiple temporal scales in a short period of 3- days of acclimation.