Abstract:The trend towards kelp Saccharina japonica cultivation in offshore areas is becoming increasingly obvious, but there is no relevant research report on the influence of different offshore distances on the agronomic traits of kelp aquaculture. The factors influencing the growth (length, width, thickness, fresh weight, and dry weight) of kelp and the seedling drop rate were explored for different kelp cultivars ("Xunshan No.2", "Xunshan No.3", kelp "205"), offshore sea areas (from far to near, defined as offshore, middle area and inshore), and pitches of twist kelp breeding rope (80.8 mm, 65.6 mm, 61.1 mm, 53.4 mm, 48.3 mm); the mechanical properties and aging degree of the breeding rope were also investigated. The results showed that: for the same kelp cultivar in different sea areas, the length, width, and thickness properties of the blade were different. The cultivars "Xunshan 2" and "Xunshan 3" showed better economic traits in the offshore area. The fresh and dry weights of the different cultivars were higher in the offshore area, followed by the middle area, and were lowest in the inshore area; however, offshore cultivation had different effects on the fresh and dry weight enhancement of the different cultivars. Whether before or after use, the breaking strength of the special breeding rope was significantly higher than that of the traditional breeding rope (P<0.05). After using a year, the strength retention rate of the special breeding rope was 93.8%, whereas that of the traditional breeding rope was only 63.6%. The seedling drop rate of the breeding ropes with a twist pitch of approximately 61.1 mm was the lowest, and the seedling drop rate significantly increased if the pitch of twist of the breeding ropes was too large (80.8 mm) or too small (≤53.4 mm). The molecular chain of the breeding rope used in the offshore was oxidized more severely, and the degree of aging was higher. The above results showed that offshore aquaculture can improve the yield of different kelp cultivars, but different cultivars have different adaptabilities to different offshore distances; and controlling the pitch of twist of the culture rope can effectively reduce the kelp seedling drop rate. For kelp culture under offshore sea conditions, a special rope with aging resistance and a twist pitch of approximately 61.1 mm can prolong the use time and reduce the rate of seedling drop. The results of this study can guide the development of the offshore kelp culture industry.
