We can get omega-3 fatty acids from ground flaxseeds, hemp seeds, canola oil, walnuts, algae and other plant-based sources. If someone can’t get enough omega-3 or their body can’t absorb it, an algae-based DHA supplement will solve this. Eating plant sources of omega-3 is actually superior to eating fish to get DHA. This is because fish is riddled with heavy metals such as mercury and PCBs, which damage the brain and counteract the positive effects of eating the omega-3 in fish.
A study by Zeilmaker et al looked at 33 fish species and its impact on brain development concluded that “for most fish species the adverse effect of MeHg on the IQ score exceeded the beneficial effect of DHA.” Read more about why it's preferable to get omega-3 from plants.