concentration of methyl mercury in fish

and

number of meals safely consumed per month

which is the dependent and independent
does the dependent go on the y or x-axis

The "dependent variable" represents the output, response, or effect. It's what's being measured by an experiment.

The "independent variable(s)" represent the input(s), controls(s), or cause(s). It's what's varied to determine if and how it changes what's being measured by the experiment.