Answer: 

Organizations, such as SeaChoice.org, provide useful information on which types of fish are abundant and sustainably harvested, and which stocks are threatened or poorly managed. Download their handly wallet card for a simple green - yellow- red ranking of popular fish species. 

Your public health department is a great place to start when looking for information on contaminant levels in fish and to find information on the safety of fish caught in local waters. Toronto Public Health, for example, provides guidance for pregnant women, children and families on fish consumption, listing fish according to their mercury levels and also noting which are "eco-unfriendly" because of overfishing and other sustainability concerns.