Fish oil fatty acids interact with the peroxisome proliferator-activated receptor (PPAR) system, which are a class of receptors (PPARα, PPARβ/δ, and PPARγ) that seem to respond to dietary lipids and similarly structured molecules. They are highly involved in the treatment of diabetes and metabolic syndrome (via the drug classes of fibrates and thiazolidinediones), with varying effects on fat mass (PPARα increases beta-oxidation of fatty acids,  while PPARγ promotes fat storage but improves insulin tolerance;  PPARδ appears to be similar to PPARα in this regard  ).
Clarke, R., Bennett, D. A., Parish, S., Verhoef, P., Dotsch-Klerk, M., Lathrop, M., Xu, P., Nordestgaard, B. G., Holm, H., Hopewell, J. C., Saleheen, D., Tanaka, T., Anand, S. S., Chambers, J. C., Kleber, M. E., Ouwehand, W. H., Yamada, Y., Elbers, C., Peters, B., Stewart, A. F., Reilly, M. M., Thorand, B., Yusuf, S., Engert, J. C., Assimes, T. L., Kooner, J., Danesh, J., Watkins, H., Samani, N. J., Collins, R., and Peto, R. Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias. 2012;9(2):e1001177. View abstract.