If you decide to pursue a master's degree after working in a related field, you can pursue one of two options. The non-thesis option is appropriate if you're interested only in advanced training and knowledge, but not in extensive research. If you do want to conduct extensive research, or if you want to prepare for a doctoral degree, you should select the thesis option.
When choosing your school, consider geographical location. If you're interested in freshwater fisheries science, you may want to apply to schools that are located near freshwater bodies such as rivers, lakes, and streams. Most schools offer field courses based on local resources.