Habitat use of juvenile rockfish (Sebastiscus marmoratus) in mussel farming waters: a preliminary study

Mussel farming habitat distributed in Shengsi archipelago has been growing in scales since the last century and playing key roles as marine ranching. Fully understanding of habitat use strategies of rock fishes in mussel farming area can be important reference for design of marine ranching programs. A monthly survey targeted on juvenile rockfish Sebastiscus marmoratus was carried out from May to July, 2018 in the northern area of Zhoushan archipelago, in order to explore the habitat use characteristics of juvenile rockfish in mussel farming area, as well as its reference value for marine ranching projects. Self-designed nets were applied to collect juvenile rockfish. Inhabitant density and its spatial distribution characteristics, habitat selection strategies, food selection characteristics based on stomach contents analysis, and early life cycles based on otolith rings, were discussed. The inhabitant density for juvenile rockfish from May to July was respectively (10±6) inds/string, (7±5) inds/string and (5±5) inds/string, showing a monthly decline trend. Juvenile individuals distributed widely during early month and then moved from offshore side of mussel farming area toward inshore side till its complete disappearance in offshore sites. However, sites with the greatest densities were always located beside shoreline area. Positive correlation was detected between juvenile densities and void volume as well as macroalgae biomass from hanging mussel strings based on correlation analysis, although not at significant level. High feeding intensity lasted all three months with zero rate of empty stomachs, and was revealed by stomach content analysis. The average feeding degree was higher in mussel farming area than that in the rocky reef habitat. The juvenile rockfish mainly fed on Amphipoda represented by Caprellidae sp.and Gammaridea sp., which accounted for 99.2% together by weight among its total diet composition. Its early stage inhabited in mussel farming habitat lasted about two to three months with average daily growth ring equal to (57±12) d for juvenile rockfish. Comparison of homogeneousness and regularity on otolith rings revealed that individuals inhabited in mussel farming area showed much more consistence than those from rocky reef habitats. It indicated that hanging strings in mussel farming waters around Gouqi Island supplied ideal habitats to juvenile rockfish by offering much more stable physical environment and abundant food sources, which in turn made great contributions to its stock enhancement and population conservation effects in local areas. It is suggested that local government should pay close attention to habitat use strategies of dominant reef fishes such as rockfish by means of applying suspended structures on proper scales in local marine ranching programs.