Hachinohe Branch, Tohoku National Fisheries Research Institute, Fisheries Research Agency, Hachinohe, Aomori 031-0841, Japan
The total catch of bighand thornyhead Sebastolobus macrochir by offshore trawlers in the Tohoku area waters off northern Japan decreased from 2,773 tons in 1971 to 291 tons in 2002, suggesting that the level of the biomass might have been low in recent years. In the present study, 702 bottom-trawling tows were carried out in the autumns of 1996-2004 in the Tohoku area and the annual biomass level was examined using the area-swept method. A population size index for each age class was calculated using an age-length key obtained from otolith readings. The biomass index remained at 1.5-1.8 thousand tons in 1996-1998 but increased to 2.5-3.2 thousand tons in 1999-2004. In 2001-2004, the recruitment levels of 2-year-old fish were more than three times as high as those in 1996-2000. Fish size at maturity was examined by histological observations of ovaries and the spawning biomass was calculated. Values of recruitment per spawning biomass (RPS) for 1997-1998 year classes ranged from 4.5 to 5.4 inds/kg and these values were much lower than those for 1999-2002 year classes (18.2-23.6 inds/kg). These results suggest that the increase of the biomass came from the high recruitment success of the year classes during 1999-2002.