1999 年 36 巻 2 号 p. 175-182
Algae are important as food resources for abalone and sea urchins・ Artificial structures have been placed so as to keep drift algae or kelp food supplies in the abalone and sea urchin fishing grounds of Northem Japan, which were frequently subject to a lack of food. To examine the capability of framed structures to keep algae, field observations were carried out using an underwater video system. Two types of structures were tested : a 12m long, 30cm high iron-bar fence and 1.5m high rectangular cages consisting of concrete frames and gratings. The fence placed on a relatively flat bottom could catch drift kelp, amounting to about lkg wet wt. per m length. The blocked drift algae, however, did not remain several days because they could pass over the fence even at the significant velocity amplitude of wave-induced oscillatory flows of < 8cm/s. The rectangular cages failed to hold entire plants or blades of kelp Eisenia bicyclis when the significant velocity amplitude exceeded 22 to 32cm/s. Based on the observation results, a model was developed for describing a stochastic process of flushing algae out of the cage by random waves.