Omega-3 promotes weight loss.

 This study investigated whether supplementation with omega-3 alone, then consumed together with a very low calorie diet (VLED), facilitated weight loss, improvements in blood lipids and positive changes to inflammatory mediators. For 4 weeks of prior supplementation, one group consumed 6 × 1 g capsules per day monounsaturated oil (placebo), the other group consumed 6 × 1 g capsules per day of fish oil each comprising 70 mg EPA and 270 mg DHA, while consuming their usual diet. Each group continued with their supplements for another 4 weeks while both groups followed a VLED regimen. At 4 weeks levels of EPA and DHA doubled in the fish oil group, with no significant changes to anthropometric measurements for either group. At 8 weeks a significant reduction in weight and BMI with a greater percentage decrease for females was seen in the fish oil group compared to placebo.

Note that the dose of 2040mg combined EPA/DHA is not achievable through diet. You'll need a high quality supplement.

Munro IA, Garg ML. Prior supplementation with long chain omega-3 polyunsaturated fatty acids promotes weight loss in obese adults: a double-blinded randomised controlled trial. Food Funct. 2013 Apr 25;4(4):650-8.