Extinction is a real risk of overfishing. Worm et al. (2006) projects the collapse of all presently fished taxa by the mid-21st century if current trends continue. Collapse is defined as a decline of 90% from normal levels.
Heino, M. & Godo, O.R. (2002). Fisheries-Induced Selection Pressures in The Context of Sustainable Fisheries. Bulletin of Marine Science, 70(2), 639-656.
Worm, B., Barbier, E.B., Beaumont, N., Duffy, J.E., Folke, C., Halpern, B.S., Jackson, J.B.C., Lotze, H.K., Micheli, F., Palumbi, S.R., Sala, E., Selkoe, K.A., Stachowicz, J.J. &Watson, R. (2006). Impacts of Biodiversity Loss on Ocean Ecosystem Services. Science, 314 (5800): 787-790.