Comparison between surface reading and cross-section methods for age determination in the flathead flounder, Korean flounder, and scale-eye plaice in the Sea of Japan, off Hokkaido

 

To establish an appropriate method for age determination of the flathead flounder（Hippoglossoides dubius）, Korean flounder （Glyptocephalus stelleri）, and scale-eye plaice（Acanthopsetta nadeshnyi）collected from the Sea of Japan, off Hokkaido, we compared the number of translucent zones on sagittal otoliths that were counted using the cross-section and surface reading methods. The translucent zones were counted by 4 researchers with experience in the observation of flatfish otoliths. For all species, we counted 1 or 2 more translucent zones by using the cross-section method than by using the surface reading method, on an average. The translucent zones formed in the otoliths of old specimens were counted more precisely by using the cross-section method than the surface method. Our results suggested that the cross-section method is more appropriate for age determination of these species.

Conversely, the consistency of the count among the readers for the cross-section method was lower than that for the surface reading method. Therefore, training for age-reading is required for precise determination of age.

 

Stock evaluation using DeLury’s method for the red snow crab in the Sea of Japan, off Southern Hokkaido

 

Experimental fishing of the red snow crab was performed off the coast of Southern Hokkaido in the Sea of Japan from the late 1970s to 2011. In this study, we have reported the stock evaluation results for the red snow crab on the basis of analysis of the fishing data. The fishing grounds were divided into 4 areas, and catching efficiency and annual stock size for each area were estimated using DeLury’s method. The estimates for annual stock size in every fishing area increased until the middle of the 2000’s and have decreased gradually since then. The exploitation rate was estimated to be 30% on an average. These results suggest that the recent TAC was appropriately decided for the level of stock size ; however, the subsequent change in stock size must be considered.

Downstream migration of chum salmon released in the Abashiri River

 

To investigate the downstream migration of chum salmon Oncorhynchus keta in the Abashiri River, eastern Hokkaido, three groups of otolith-marked juveniles were released and sampled using cast nets in the lower reach of the river in 2004 and 2005. Marked chum salmon were recaptured within 2 weeks of release, and thereafter the numbers of fish recaptured quickly decreased. Our study indicates that most of chum salmon juveniles (over 0.8g in weight) released in the Abashiri River in May migrated seaward immediately after release. Only a few of the marked fish released at the end of May were  recaptured and the river water temperature exceeded 20℃ in mid-June, indicating the survival of chum salmon released at that timing would be low unless they migrate immediately after release. The release timing in each river should be considered by examining the river and coastal water temperature around the release timing.

Changes in water quality during long-distance transportation of chum salmon juveniles

 

Changes in water quality(temperature, pH, electric conductivity (EC), dissolved oxygen (DO), and ammonia nitrogen NH4-N concentration) in fish tanks containing chum salmon juveniles were examined during long-distance transportation by car. From March through April in 2011 chum salmon juveniles (mean body weight 0.7-0.9 g, total number 860,000) were transported while supplying

oxygen from a hatchery to 4 distant rearing sites in Hokkaido, Japan. Duration of feed withdrawal for fish before transportation was one day. Densities (total fish weight per tank volume) in the fish tanks were maintained at 6.0 to 7.2%. Carrying distances were 227 to 340 km (5 to 7.1 hours by car). The water temperature was nearly constant. The pH decreased rapidly at the early stages of transportation and then remained constant. The DO increased and reached supersaturation. The concentration of NH4-N increased in proportion to the transportation time (hours) and reached 6.38 mg/l (0.0009 mg NH3 /l) 7 hours later. Moreover, EC was proportional to the content of NH4-N. The fish showed no mortality during and after transportation. These results suggest that the conditions of the long-distance transportation were appropriate for use in chum salmon juveniles and also that EC would be a useful indicator for NH4-N content.

Starvation limits of newly hatched larvae of wakasagi, Hypomesus nipponensis, and their downstream migration to Lake Abashiri

 

After hatching in the rivers flowing into Lake Abashiri, it is vital for the survival of Wakasagi（Hypomesus nipponensis）larvae to arrive at Lake Abashiri with yolk storages. The present study aims to estimate the time required for the newly hatched larvae of wakasagi to arrive at Lake Abashiri. Yolk absorption and starvation tolerance were examined in the reared yolk-sac larvae, and sampling surveys of drifting larvae were conducted at the inflowing river. Newly hatched larvae completed yolk absorption by six days after hatching at 13℃. The point-of-no-return（PNR）of yolk-sac larvae was found by the five day mark. The results of a 24 -hour sampling survey for the drifting larvae show the larvae hatched just after sunset and immediately migrated to Lake Abashiri. Judging from the yolk volume of these larvae, the time required to reach the lake was estimated to be from 0-1 day after hatching. Newly hatched wakasagi larvae are considered to have sufficient yolk volume for starvation tolerance at the time of entering the lake, even though arrival and first feeding at the lake are required within four days of hatching for survival.

Fish density in Dodoromap River, a tributary of Abashiri River, before and after construction of fish ways（Note）