Interannual change in biological traits and exploitation rate of Decapterus maruadsi in Beibu Gulf

We estimated the interannual change (size composition, growth, mortality and maturity in biological traits) of Decapterus maruadsi in Beibu Gulf based on the bottom trawls survey data during 1992−1993 (1992), 1998−1999 (1998), 2006, 2007, 2008−2009 (2009), 2010 and 2011−2013 (2012). Results show that: 1) The dominant body length groups decreased from 130−170 mm in 1992 to 110−140 mm in 2007, then showed an increasing trend, to 120−130 mm and 140 −170 mm in 2012. 2) The average body length decreased from 160.23 mm in 1992 to 139 mm in 2007, then increased to 149 mm in 2012. 3) The body length at 50% sexual maturity (L₅₀) and age at 50% sexual maturity (A₅₀) presented a similar trend with the average body length, decreasing from 180.16 mm and 1.860 in 1998 to 174.91 mm and 0.888 in 2007, respectively, then increased to 180.87 mm and 2.538 in 2012, respectively. 4) However, the growth coefficient (K) decreased gradually after increasing from 0.39 in 1992 to 0.72 in 2007, then decreased gradually to 0.5 in 2012. 5) The total mortality coefficient (Z) has been increased since the 1990s, reaching the maximum of 2.77 in 2009, then decreased to 1.68 in 2012. The variation trend of the exploitation rate (E) was similar with Z, and the E in 2010 was similar with that in early 1990s.