Size selectivity of a trammel net for oval squid Sepioteuthis lessoniana

  The size selectivity of a trammel net for herded oval squid Sepioteuthis lessoniana in Tateyama Bay, Chiba Prefecture, was estimated by comparison between the mantle length frequency distributions of oval squid caught by a trammel net and by a set net. The measured mesh sizes of the inner net of the trammel net and of the final section of the set net were 85.3 and 11.3 mm, respectively. In the trammel net fishery where oval squid are herded into the net, most of the oval squid are caught in the bag‐shaped inner net. Hence, the logistic function was employed for the size selectivity curve  of  the  trammel  net.  The  ‘share  each  length's  catch  total’ (SELECT)  model  was  implemented for  the  estimation  of  the  selectivity  curve.  The  size  selectivity   r ( l )  of  the  trammel  net  for  the  oval squid  was  expressed  as  a  logistic  function  of  the  mantle  length   l :   r ( l ) = exp(−18.57 + 0.88  l )/[1 + exp(−18.57 + 0.88  l )]. From these logistic parameter estimates, the 50% selection mantle length and selection range ( L 75 – L 25 ) were calculated as 21.07 and 2.49 cm, respectively. The selection probability of oval squid whose mantle girth was equivalent to the mesh perimeter of the inner net was 0.09. Accordingly, oval squid of a girth smaller than the mesh perimeter were likely to pass through the mesh to escape from the net.