Ecological Applications, 22(3), 2012, pp. 804–816 Ó 2012 by the Ecological Society of America Quantifying and comparing size selectivity among Alaskan sockeye salmon fisheries NEALA W. KENDALL 1 AND THOMAS P. QUINN School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, Washington 98195 USA Abstract. Quantifying long-term size-selective harvest patterns is necessary for under- standing the potential evolutionary effects on exploited species. The comparison of fishery selection patterns on the same species subject to different gear types, in different areas, and over multi-decadal periods can reveal the factors influencing selection. In this study we quantified and compared size-selective harvest by nine Alaskan sockeye salmon (Oncorhynchus nerka) fisheries to understand overall patterns. We calculated length-specific linear selection differentials (the difference in average length of fish before vs. after fishing), which are produced by different combinations of exploitation rates and length-selectivity values, and nonlinear standardized differentials, describing disruptive selection, across all years for each fishery. Selection differentials varied among years, but larger fish were caught in 73% of years for males and 84% of years for females, leaving smaller fish to spawn. Disruptive selection was observed on female and male fish in 84% and 92% of years, respectively. Linear selection was stronger on females than males in 77% of years examined, and disruptive selection was stronger on males in 71% of years. Selection pressure was influenced by a combination of factors under and beyond management control; analyses using mixed-effects models indicated that fisheries were less size selective in years when fish were larger than average and had lower exploitation rates. The observed harvest of larger than average sockeye salmon is consistent with the hypothesis that size-selective fishing contributes to decreasing age and length at maturation trends over time, but temporal variability in selection and strong disruptive selection suggests that the overall directional pressure is weaker than is often assumed in evolutionary models. Key words: age and length at maturation; Alaskan sockeye salmon; fisheries-induced evolution; fishery selection; harvest-induced selection; harvest selection; life-history evolution; linear mixed-effects models; Oncorhynchus nerka; selection differentials. INTRODUCTION Life-history traits of wild animals and plants can be strongly influenced by a variety of anthropogenic activities (Allendorf and Hard 2009, Darimont et al. 2009). Differential mortality patterns from harvest of wild populations can have significant ecological effects, including reductions in density with associated increases in growth and decreases in mean ages and lengths of individuals (Policansky 1993, Trippel 1995, Hutchings 2004). Harvest is often size selective (Todd and Larkin 1971, Sinclair et al. 2002, Coltman et al. 2003, Carlson et al. 2007, Mooney and McGraw 2007, Kendall et al. 2009), in part because hunters, fishers, and collectors target larger than average individuals (Darimont et al. 2009) and harvesting gear often selectively removes individuals with respect to length (Hamley 1975, Millar and Fryer 1999). This can alter the distribution of age and length at maturation among individuals surviving to reproduce (Law 2000, Coltman et al. 2003, Swain et al. 2007, Allendorf et al. 2008). Maintenance of diverse life- history traits and a broad distribution of ages and lengths at maturation can enhance population produc- tivity and sustainability (Hilborn et al. 2003, Schindler et al. 2010). Numerous studies have also emphasized the importance of older, larger individuals for population stability and sustainability (Birkeland and Dayton 2005, Law 2007, Hsieh et al. 2010). Accurately understanding the selective pressures generated by harvest and factors influencing size selectivity is therefore important for maintaining the phenotypic diversity and productivity of exploited wild populations. Fisheries scientists and managers have expressed concerns about adverse ecological and evolutionary effects of selective harvest for over a century (Rutter 1904, Smith 1920, Miller 1957, Handford et al. 1977, Ricker 1981, Allendorf and Hard 2009). Fishery selection may lead to genetic changes in life-history traits (Allendorf et al. 2008), which may be harder to reverse than changes associated only with phenotypic plasticity (Law 2000, Dieckmann and Heino 2007, Dunlop et al. 2009). Selective harvest is generally thought to result in adaptive demographic shifts toward smaller fish and decreased age and length at maturation Manuscript received 30 June 2011; accepted 7 October 2011; final version received 21 November 2011. Corresponding Editor: K. Stokesbury. 1 E-mail: kendalln@uw.edu 804