% Age fish
  fishage( 2:d+1, 2:d+1 ) = fishage( 2:d+1, 2:d+1 ) .*fish(2:d+1,2:d+1) + ...
    fish( 2:d+1, 2:d+1 );
  if (dbug == 1), disp('aged fish'), keyboard, end
