Theme D: Developing optimal release strategies
45. DETERMINING OPTIMAL RELEASE HABITAT FOR BLACK ROCKFISH: EXAMINING GROWTH RATE, FEEDING CONDITION AND RECAPTURE RATE
Byung Sun Chin, Masahiro Nakagawa, Tsutomu Noda, Yoh Yamashita
Maizuru Fisheries Research Station, Kyoto University, Maizuru, Kyoto 625-0086, Japan
hkhachin@kais.kyoto-u.ac.jp
The black rockfish, Sebastes schlegelii, is a viviparous inhabitant of shallow coastal waters (< 100 m deep) in Korea, Japan and northern China. This species comprises one of the most commercially important Sebastes coastal fisheries in these eastern Asian countries and appears to be an appropriate model for studying stock enhancement strategies because of key biological traits such as rapid growth and limited migration. In this study, field surveys (wild fish ecology and release-recapture trials) were conducted to assess the efficiency of stocking based on the ecological characteristics of this species. To assess the suitability of variable environments as nursery grounds for black rockfish juveniles, growth rate and feeding condition of wild juveniles were examined in two nursery sites, Akamae (Stn. 1) and Hanoki (Stn. 2), from 2004 to 2006. Stn. 1 is characterized by brackish mud/sand sediment with sea grass vegetation and Stn. 2 by thick sea grass beds. Growth rate of wild juveniles was higher at Stn. 1 relative to Stn. 2. Stomach contents of juveniles at Stn. 1 were comprised mainly of mysids and large gammarids, Ampithoe spp., while juveniles at Stn. 2 foraged mainly on small gammarids. The abundance of mysids and large gammarids present at Stn. 1 supported the high growth rate of juveniles.
In addition, experimental releases were conducted to examine the recapture potential of each habitat. Small size hatchery fishes (40 mm in TL) were released at Stn. 2 in 2004 and 2005, at Stn. 1 in 2006 and 2007, and at Shirahama (Stn. 3) every year (2004 to 2007). Stn. 3 was located in the middle part of the bay where large sizes (100mm in TL) hatchery fish were released from 1999. As of June 2010, the recapture rate (market return rate) of hatchery fish released at Stn. 1 was 5.0% in the 2006 group and 8.0% in the 2007 group. Return rate of fish released at Stn. 3 ranged from 0.8 to 2.9% (2004-2007) while those at Stn. 2 were 3.0% (2004) and 1.1 % (2005). Landings of black rockfish at the Miyako Fish Market have increased approximately 2.5 times after the onset of stockings from 1999. The efficiency (economic return rate; market sales/hatchery costs) of hatchery fish released at Stn. 1 was estimated to be 1.25, which is comparable to that of fish released at Stn. 3 at a size of 95 mm TL (0.98). We conclude that habitats with an abundance of mysids and large gammarids bear higher potential as successful nursery grounds for black rockfish because growth rate and food quality in these areas. The higher return rates and economic return rate for fish released at Stn. 1 also supports the notion that these brackish shallow habitats constitute the most appropriate release site for black rockfish. These results indicated the possibility of reducing size at release to less than half, which implies a strong reduction of hatchery costs if juveniles are released in appropriate sites and may also have implications for the stocking strategy of other Sebastes species.
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