The older that such fish become, the more mercury they may have absorbed. Anything that eats these fish within the food chain also consumes the higher level of mercury that the fish have accumulated. This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher
The goal, then, when it comes to having a boy is to increase the likelihood that a Y chromosome sperm reaches the egg first. Many medical researchers believe that Y chromosome sperm are faster swimmers, but that X chromosome sperm have more endurance.
There are also fertility drugs that can increase the likelihood of more than one baby being conceived. For example, using Clomid the likelihood of having twins is 10%. Other fertility treatments, like IVF can also result in twin or higher order multiple pregnancies. How Twins Are Made.