Weight-based yield per recruitment and spawning-biomass per recruitment analyses of Pacific cod Gadus macrocephalus off the Pacific coast of southern Hokkaido, Japan

The present study assessed the stock state of Pacific cod Gadus macrocephalus caught off the coast of southern Hokkaido, Japan. Weight-based yield per recruitment (YPR) and spawning-biomass per recruitment (SPR) analyses were used for this assessment. The current fishing mortality (average from 1998 to 2000) was 0.65 and weight at first capture was 0.5 kg bodyweight. Under these fishing pressures, the YPR of Pacific cod in southern Hokkaido was 1.06 kg/recruitment and percentage of SPR (%SPR) was 6.9%. The %SPR was lower than the critical limit at 20%SPR. The main reason that values of both YPR and %SPR were not optimum, would be that the weight at first capture was too small. Raising the weight at first capture was thought to be a better strategy from the biological viewpoint, and reducing fishing mortality to 0.3 would be the next alternative strategy from the fisheries management viewpoint

High pressure growth and electron transport properties of superconducting SmFeAsO1-xHx single crystals

We report the single crystal growth and characterization of the highest Tc iron-based superconductor SmFeAsO1-xHx. Some sub-millimeter-sized crystals were grown using the mixture flux of Na3As + 3NaH + As at 3.0 GPa and 1473 K. The chemical composition analyses confirmed 10% substitution of hydrogen for the oxygen site (x = 0.10), however, the structural analyses suggested that the obtained crystal forms a multi-domain structure. By using the FIB technique we fabricated the single domain SmFeAsO0.9H0.10 crystal with the Tc of 42 K, and revealed the metallic conduction in in-plane (rhoab), while semiconducting in the out-of-plane (rhoc). From the in-plane Hall coefficient measurements, we confirmed that the dominant carrier of SmFeAsO0.9H0.10 crystal is an electron, and the hydride ion occupied at the site of the oxygen ion effectively supplies a carrier electron per iron following the equation: O2- = H- + e-.Comment: 4 figures, 2 table

個体ベース生活史モデルに基づく放流による天然個体群の近交係数増加の推移について

個体ベース生活史モデルを用いて，種苗放流が天然個体群に与える遺伝的影響を検討した。モデルでは，自然死亡，漁獲死亡，放流個体数等を考慮し，放流個体数，種苗生産時の親魚数，親魚の性比，漁獲死亡及び親魚の由来による遺伝的影響の差違を比較した。親魚数を雄 10 雌90，雄5 雌5 としたとき，雄 50 世代目における近交係数は，親魚数を雄50雌50としたときに比べそれぞれ5倍および27倍高くなった。親魚に天然個体を用いる場合は，過度の漁獲死亡係数が遺伝的多様性に影響を与えることが示された。We assessed the genetic impact of stocking on natural populations by using individual-based life-cycle models. We constructed models that used include density-dependent mortality at the early life stage, natural mortality, fishing mortality, and the number of released fish. We varied these variables by using random numbers. We focused on the genetic impact of the number of released fish, the number of parent fish producing seedlings that were released, the sex ratio of the parent fish, fishing mortality, and the methods to select the parents of the seedlings. When brood stock size in hatchery is 5 male and 5 female, is the coefficient of inbreeding is about 27 times comparing more when 50 male and 50 female at the 50th generation, and the coefficient of inbreeding increases about five times at the 50th generation, when the sex ratio of the parents changes from 50 males and 50 females to 10 males and 90 females. When parents are selected from wild populations at random, extreme fishing mortality may also reduce genetic diversity

A Simulation Method for Predicting Future Population Dynamics with Uncertainty

Unfortunately, most of the stocks are damaged by natural disasters or human activities and rebuilding the stock are urgent requirements. Until now, in the stock management, some policies are commonly believed to valid but have not been confirmed. A simulation model of the population dynamics incorporating uncertainty has been already made by the authors. The typical believes of the stock management, “Ban rebuild stocks”, “Large mesh rebuild stocks” were confirmed by the simulation model. Six stocks were randomly chosen mainly from the data of ICES for applying this simulation model. For applying the management policy of 1 year ban, 5 to 6 stocks had not significant effect compare with no regulation after around 5 years of the opening of the fishery. For 3 of 6 stocks, the effect of the enlarged the mesh size was not detected. In conclusion, the widely believed policies were valid for only a part of stocks. Thus the effects of the management policies should be confirmed before conducting and applying to stocks

Conflicts Between Recreational and Commercial Fishing in Lake Toya, Hokkaido, Japan

Recently, the number of leisure fishers has increased in Japan. Recreational anglers have caught larger numbers of fishes than commercial fishermen in some species. Formulating an appropriate management strategy that considers recreational and commercial sectors is important. Sockeye salmon (Oncorhynchus nerka) has been caught in Lake Toya, Hokkaido, Japan, by both recreational and commercial fishers. The number of sockeye salmon caught in Lake Toya is decreasing and population management and regulation must be considered. While the number of sockeye salmon by commercial catches is reported, the number of fish caught by recreational fishery is not available, which is why an appropriate management strategy has not been made until now. In this study, we made a mail survey and access point survey, from 1998 to 2003, to estimate the catch numbers, the fishing effort of recreational anglers and the population of sockeye salmon in Lake Toya. The mail survey questions in questionnaires distributed to anglers with a recreational angling license included date of angling, caught fish species and size, and catch number. In the access point survey we also asked anglers having a license or not. The fish population was estimated using the DeLury method. The estimated total angling effort was 1,400-1,800 people in 1998-2002. The estimated recreational catch was 28,889 in 1998 and rapidly decreasing. The recreational catch was two or three times larger than the commercial catch. The total exploitation rate was over 50%.Keywords: recreational fishing, Population estimation, Fisheries Economics, Lake Toya, Angler survey, Inland fishery, Management of Multiple Use Conflict

北海道苫小牧市の海岸でストランドしたコククジラ(Eschrichtius robustus)の外部寄生虫と着生生物について

Postmortem examination was performed on a female western gray whale (Eschrichtius robustus) stranded at Tomakomai-shi, Hokkaido, Japan in August, 2007, and amphipod whale lice (Cyamus scammoni) and cirripedian epizoites (Cryptolepas rachianecti) were obtained. The histopathological investigation of the epizoites was given. 2007年8月、北海道苫小牧市の海岸でストランド（座礁）したコククジラ(Eschrichtius robustus) 雌1個体の外部寄生虫クジラジラミ類（Cyamus scammoni）と着生生物フジツボ類(Cryptolepas rachianecti)について、日本では千葉に続き二例目の記録をした。また、C. rachianectiの着生部位における組織病理学的検索も実施した