trap nets in the bays. Afterthe eggs hatch, the fry are 

 reared until April in an effort to reduce the mortality 

 of fry which they estimate as 509f in the streams plus 

 a significant mortality during the month which the 

 fry spend in shallow bays before going to the sea. 



The eggs used in the experiment at Yamada were 

 hatched on 22 February 1971 and began feeding on 8 

 April. They were held at the hatchery at Miyako Bay 

 until 10 May because the holding tanks at Yamada 

 had not been completed. After transfer, the salmon 

 were held in fresh water at Yamada until 28 May 

 when the seawater system was completed. Transfer 

 to salt water was accomplished in about 24 hr with no 

 significant mortality. 



Because of the delay in completing the floating net 

 enclosures the small salmon were held in seawater in 

 tanks ashore until August when the temperature in 

 the tanks reached 25.8°C. They were then trans- 

 ferred to the sea. but even there temperatures 

 reached 24.4°C. These temperatures were too high 

 and the mean survival was only 42%, although 

 individual lots had survivals as high as 90%. 



According to Chikara lioka, who is in charge of 

 this Iwate Prefectural Experiment Station, they will 

 normally rear the salmon in fresh water until June or 

 July and transfer them into floating net pens before 

 midsummer. Even so. it appears likely there will be 

 significant mortalities from diseases such as Vibrio 

 since surface water temperatures usually reach a 

 maximum of 22°C according to lioka. Furthermore, 

 the thermocline is deep in this area, according to 

 lioka's measurements, and the maximum tempera- 

 ture difference between the surface and a depth of 60 

 m is less than 4°C. In midsummer there appears to be 

 only \°C difference between the surface and bottom 

 at a depth of 15-25 m where the salmon culture pens 

 are anchored. 



National Marine Fisheries Service experiments 

 with salmon culture in Puget Sound, Wash., have 

 indicated increased mortalities due to i'ihrio when 

 scauatcr temperatures exceed 15^C. On this basis it 

 appears likely that the high summer temperatures at 

 \'amada Bay will bo a limiting factor to salmon cul- 

 ture imless methods can be developed for preventing 

 or reducing Vibrio infections. 



Three kinds of net enclosures were being tested by 

 lioka and his associates. One type was a floating net 

 pen similar to those used for the culture of trout or 

 yellowtail. but with some refinements in the design. 

 These enclosures were about 6 or 8 meters square 

 and about 3 m deep with nearly 1 m of the sides above 

 the surface of the water. The lops were covered w ith 



a coarse mesh to exclude birds and the sides and 

 bottoms were made of about 8 mm square mesh. 

 These enclosures contained 4,000 young salmon 

 which by October had reached a length of 20-25 cm. 

 Another similar pen contained 300 2-yr old salmon 

 which appeared to be about 40-50 cm in length. lioka 

 reported that these floating surface pens were satis- 

 factory except that the nets had to be changed every 

 month because of heavy fouling by seaweeds. 



The second type of enclosure was a middepth pen 

 which was located at a depth of 10 m to the top of the 

 net. The enclosure was 10 meters square and 7 m 

 with a top and bottom. Even at this depth the heavy 

 growth of seaweed required changing the net about 

 every 3 mo. 



The third type of enclosure was a large octagonal 

 net 55 m in diameter which extended from the sur- 

 face to the bottom and was anchored in position. The 

 net was supported by a series of floats at the surface 

 and held in contact with the bottom by a lead line. 



A horizontal section of fine mesh at the surface 

 extended inward about 3 m from the vertical walls 

 and the inner border was supported by net floats 

 placed about 4 inches apart. lioka had found that the 

 fish made no attempt to jump over the inner row of 

 floats and indeed found refuge and shade under the 

 floating section of the net which rapidly became 

 fouled with seaweeds. 



Feeding was accomplished by rowing a boat over 

 the top of the net into the center of the pen where the 

 food was thrown into the water. 



Predation by diving water fowl was prevented by 

 stretching twisted bright-colored plastic tape. I Vi to 

 2 cm wide, across the enclosure at intervals by tying 

 it to vertical supports so that it was about I m above 

 the surface of the water. The apparent movement of 

 this tape caused by the wind seemed to scare the 

 birds away. The same system is used to keep birds 

 away from the rice fields. 



The young salmon were fed a dry pellet food that 

 had been developed for feeding rainbow trout in the 

 sea. and this was found to be fairly satisfactory. The 

 fish were fed 3 times a day. and the pellets were 

 mixed with water before feeding. The Japanese sci- 

 entists stated it is important to mix the food with 

 fresh water so that the fish have a source of fresh 

 water to replace that which the> would normally 

 receive by eating other fish. 



The objective of the Yamada Experimental Sta- 

 tion is to develop methods for commercial salnmn 

 culture soanumberofexperimentsarein progress or 

 planned. Experiments by Koganezawa et al.. re- 



120 



