Experimental evaluation of the effect of water velocity on the development of string-attached kelp seedlings (Laminariales) with implications for hatchery and nursery production

Abstract Kelp sea farming is performed using seedlings (early stage of sporophyte development) attached to string, known as seedling string, which are produced from an indoor culture under controlled environmental conditions. However, very little is known about the influence of water movement and the optimal growth level of early kelp sporophytes on string. Thus, the present study evaluated the development of string-attached kelp seedlings (Undaria pinnatifida) simultaneously exposed to simulated seawater velocities in an indoor tank. The experiment was conducted using string that had seedlings approximately 2 mm in length, whose growth was measured after 28 and 56 days of culture. Seawater velocity level was found to significantly influence both the rates of increase in total biomass (dry weight) and in length of kelp seedlings on string. Seedling growth rates increased up to a water velocity range of 15.8–17.1 cm s−1 beyond which it declined. This pattern is similar for the two time periods explored during the experiment, although both growth rates were significantly higher during the first 28 days of culture than after 56 days of culture. This difference is related to the increase in kelp seedling size throughout the experiment, and this increase could affect the requirements and/or optimal environmental conditions for their growth. In addition, comparatively, the rate of total biomass increase for seedlings was significantly higher than the rate of length increase for seedlings, indicating seawater velocity favours greater biomass productivity over length increase. Based on these results, suitable seawater velocities are recommended that would be the most productive for kelp seedling growth on strings in hatchery and nursery phases.