Does exposure to high salinity result in reduced reproduction in Salton Sea tilapia (Oreochromis mossambicus x O. urolepis hornorum)?

Thesis (M.S., Biological Sciences (Ecology, Evolution and Conservation)) -- California State University, Sacramento, 2013.Salinities in the Salton Sea, California, are currently 45 grams per liter (g l-1) and continue to rise at a rate of 0.3 g l-1 per year due to a decline of freshwater inflow and increasing evaporation rates. These elevated salinities may be problematic for resident Salton Sea tilapia (Oreochromis mossambicus x O. urolepis hornorum), members of the family Cichlidae, and considered freshwater fishes, even though some species manage to survive at extraordinarily high salinities upwards of 60 g l-1. This is remarkable given that the salinity of normal sea water is 35 g l-1. Does this capability to survive come at a cost to reproductive ability? In other words, as salinity increases, are tilapia less able to reproduce successfully? I hypothesized that as salinity increases, successful reproduction, female fecundity, and parental care (mouthbrooding) would decline. Two different experiments were conducted to determine if salinity affected reproduction. \ud \ud \ud For both experiments, tilapia that were acclimated to a salinity of 20 g l-1 in stock aquaria were randomly assigned to a 20, 40, or 60 g l-1 salinity treatment. The trials were conducted in 114 L aquaria. After the tilapia were introduced to the designated treatment, the salinity was gradually increased from 20 g l-1 to the desired salinity for the treatment.\ud Experiment 1 evaluated whether increasing salinity affected the ability of the tilapia to spawn and/or affected fecundity, which was defined as the number of eggs produced. There was a significant effect of level of salinity on the incidence of female spawning, i.e., females were more likely to spawn at lower levels of salinity (20 g l-1: four spawnings; 40 g l-1: three spawnings; 60 g l-1: no spawning). There was no significant effect of female body condition on the incidence of spawning. Level of salinity significantly affected the number of eggs spawned (20 g l-1: mean 464; 40 g l-1: mean 394), but female body condition did not. \ud Experiment 2 evaluated whether level of salinity affected the amount of parental care by the female, possibly causing her to reject the offspring during the mouthbrooding period due to salinity stress. Only the 20 and 40 g l-1 treatments were conducted. There was no significant effect of level of salinity on the incidence of female spawning (20 g l-1: three spawnings; 40 g l-1: one spawning). Of those females that spawned, in no case did the female cease mouthbrooding (i.e., reject the offspring). \ud This research has shown that even though resident Salton Sea tilapia have an extraordinarily high tolerance for salinity and were able to successfully reproduce at 40 g l-1, under the conditions of these experiments, reproduction did not occur at 60 g l-1. This suggests that with the salinity of the Salton Sea greater than 40 g l-1, the conditions in the Salton Sea are approaching the limits for successful reproduction of this species.Biological Sciences (Ecology, Evolution and Conservation