Reexamination of the maturity rate of Ishikari stock of the sailfin sandfish Arctoscopus japonicus at the age of one

 

The maturity rate of the Ishikari stock of the sandfish Arctoscopus japonicus is believed to be 100%. This study re-examined the maturity rate of this stock of fish aged 1 year to determine whether any deviations from 100% maturity could be detected. The mean values of body length of age-one fish were estimated for 2005–2008 year-classes by otolith back-calculation analysis of fish that were sampled at the age of 2 years. The maturity rate of the 2006 year-class of age-one fish was estimated 64% in females because of slower growth. These results indicate that maturity rates are not always close to 100%, particularly in females, and that the possibility of lower rates should be considered carefully when evaluating the spawning biomass of this stock.

Characteristics of population dynamics in Ishikari stock of the sailfin sandfish Arctoscopus japonicus

 

The sandfish Arctoscopus japonicus is an important stock in the Sea of Japan, off Hokkaido. In this study, annual changes in the population size were estimated on the basis of VPA. The population size of the Ishikari stock increased gradually in the late 1990s because of high recruitment in the 1997 and 1999 year-classes. The spawning stock biomass recovered more than 200 tons in 2000; consequently, the 2001 year-class had the highest level of recruitment since 1995. However, the survival rate of juveniles, as indicated by the RPS values, decreased in the early 2000s, resulting in a decline in the population size and catch. The RPS values of this stock may be strongly affected by the water temperature in the Ishikari Bay during the early summer season.

Maturation and age of the male Japanese common squid, Todarodes pacificus, from the Japan Sea off Hokkaido and the waters around the Tsugaru Strait

 

The relationship between maturation and age for males (in days) of the Japanese common squid Todarodes pacificus was investigated using counts of statolith daily growth increments. Squid were collected from the Japan Sea off Hokkaido and the waters around the Tsugaru Strait in 1991-2003 and ranged from 165 to 339 days in age. The maturity rate increased with age, and 50% maturity was estimated to occur at approximately 255 days (eight and half months). The youngest mature squid and oldest immature squid were found to be 220 and 297 days old, respectively. Therefore, the age at maturation for male T. pacificus spanned over 2 months.

Proof test of the rearing technique based on DHA requirement during larval period to prevent the morphological abnormalities in brown sole Pseudopleuronectes herzensteini

 

The purpose of this study was to evaluate the suitability of a rearing technique and to prevent morphological abnormalities in juveniles of the brown sole Pseudopleuronectes herzensteini by fulfilling their DHA requirements during the larval period. Treatments of enriched larval food included 4 different rotifer preparations and only 1 preparation of Artemia nauplii with a commercial product. Larvae at 15 days after hatching (D developmental stage) were fed for 11 days with rotifers that were treated with different materials and enriched-Artemia. During developmental stages F-I, larvae were fed solely with enriched-Artemia. The incidence of normal morphology and survival rate in larvae fed with rotifers containing enough DHA to meet the DHA requirements during the D-E stages were significantly higher than those in larvae fed with rotifers containing insufficient DHA. Therefore, these results confirmed that this rearing technique, based on providing enough DHA to meet requirements of larval brown sole during the D-E stages, successfully reduced morphological abnormalities to approximately 10%, yielding a normal morphology rate of 90-94%. This study also indicated that the rearing technique may be used to prevent morphological abnormalities in juvenile brown sole regardless of the enrichment material used, since both commercial product and DHA oil were effective.

Availability of sea star (Asterias amurensis) spicules as a marine biological filter

 

To elucidate the possibility of using spicules from the sea star Asterias amurensis as a marine biological filter, changes in water quality were examined in experimental tanks containing coral sand versus sea star spicules after adding NH₄Cl. In addition, to test the effectiveness of sea star-spicule filtration for fish cultivation, the black rockfish Sebastes schlegeli was reared for 40 days, and water was circulated through sea star filters under the 3 stocking densities of 10, 20, and 40 fish per tank. In both tanks containing sea star spicule and those containing coral sand, ammonium nitrogen (NH₄-N) concentration decreased to 0 mg/L 19–20 days after the addition of NH₄Cl. Nitrite (NO₂-N) concentration increased and reached a maximum at 20–24 days and thereafter dropped to 0 mg/L at 45–53 days in tanks containing sea star spicules and at 44–48 days in tanks containing coral sand. In the stocking density of 10 and 20 fish per tank, NH₄-N and NO₂-N were maintained at less than 0.5 mg/L. Final body weight of fish in tanks with the stocking density of 40 fish per tank was significantly lower than that of fish from tanks with lower stocking densities. These results suggest that the nitrification ability of sea star spicules is similar to that of coral sand and that sea star spicules could be useful for filtration during fish cultivation.

Age composition of chum salmon returning to rivers on the Sea of Japan side of Hokkaido

 

Numbers of chum salmon Oncorhynchus keta returning to Hokkaido have been at a historically high level since the 1990s; however, recent return rates in the Sea of Japan area fluctuated within a low level. To assess the difference in the return rates of juveniles released in each river, we began to assess the age compositions of chum salmon returning to rivers in the Sea of Japan area. Scales of chum salmon in 10 rivers and 14 rivers were sampled in 2009 and 2010, respectively, and ages were determined. Similar trends were generally observed among the age compositions of chum salmon captured in rivers in the same regions in both years. In 2009, rivers in the same regions generally clustered together in the cluster analysis. Post-release survivals of hatchery-reared juveniles are different among rivers, hatcheries, and fish quality. This study indicates environmental factors, e.g. coastal sea water temperature, would affect the survivals of hatchery-reared chum salmon commonly within each region of the Sea of Japan area.

Evidence for existence of wild population of pink salmon in the Tohoro River system flowing into Nemuro Strait, eastern Hokkaido, Japan（Short Paper）

 

The homing migration behavior of the pink salmon Oncorhynchus gorbuscha was investigated in the Tohoro River system in eastern Hokkaido. Juvenile pink salmon were released into the Tohoro River after their otoliths were marked with alizarin complexone (ALC). For this study, the number of returned living adult pink salmon and their post-reproduction carcasses were visually counted, and fork lengths of the carcasses were measured. Otoliths were collected from the carcasses in the Tohoro River, where the marked juvenile fish had been released and from the Sakura River, its tributary. It was concluded that the marked fish selectively returned to the Tohoro River and that unmarked fish selectively returned to the Sakura River. Furthermore, unmarked fish were significantly smaller in body size than marked fish. These results therefore suggest that the unmarked fish were a wild population that had been reproducing independent of the released fish.

Distribution of naturally spawning chum salmon populations in Hokkaido (Note)