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Overfishing can affect life history traits, resulting in population collapse and oftentimes a decrease in length-at-age and maturation size in fish populations. However, little is known about the recovery mechanisms and time scales of these traits in exploited wild populations. In the study reported here, we documented long-term shifts in growth and mature size in Miyabe charr Salvelinus malma miyabei associated with a history of recreational fishing in Lake Shikaribetsu for approximately 80 years. Downsizing in the charr was observed when the charr population collapsed due to intensive recreational fishing. However, subsequent moratoriums and the introduction of fishing regulations, especially the implementation of a catch-and-release policy, during the following 10–30 years facilitated the recovery of population size, length-at-age and mature fish size. This study provides important insights into the biological changes and required recovery time scales of a heavily harvested population and supports management and conservation strategies.

Recreational angling is a popular leisure activity globally and can be a threat or useful conservation tool depending on the management strategy. Miyabe charr Salvelinus malma miyabei is an endemic fish that inhabits Lake Shikaribetsu, and a management program was established to harmonise recreational fishery and conservation. To examine the suitability of this program, population assessment was conducted with the cooperation of anglers. The population size in 2014 as assessed by tag-and-release angling was estimated to be 105,300 [95% confidence interval (CI) 37,300-178,600], much higher than the reported estimate in 1995. Further, angling mortality was estimated to be quite low. Moreover, no decreasing population trend was detected on analysing 8 years of anglers' catch data. Consequently, angling has facilitated stock assessments of Miyabe charr under the current program. This case is a good example of recreational angling acting as a conservation tool under appropriate management.

The catch forecast is important for fisheries activities. Previous research has tried to improve forecast accuracy. However the forecast accuracy does not directly correspond to the forecast benefit, and an inaccurate forecast could be more beneficial than accurate one. Herein as part of the forecast utility, predictability was evaluated using information theory. Mutual information (MI) was used as index of predictability. MI denotes a reduction in uncertainty when a forecast is taken into account. Adding this, hit ratio (HR) and relative entropy (R) were used as consistency indices. HR denotes a frequency for which the predicted values are consistent with the actual values, and R denotes the distance of the probability distribution between the actual and forecasted fishing conditions. As an application, the long-term change-ratio forecasts in 1972–2009 (n = 36), short-term change-ratio forecasts (n = 34), and short-term level forecasts (n = 33) in 2004–2009 of Pacific saury Cololabis saira fishery were evaluated. The order of MI, HR, and R varied between these forecasts, indicating that forecast predictability and consistency do not correspond. Monitoring multiple indices would improve forecasting systems.

The aim of this study was to examine the effects of stratification of the survey region on the performance of species distribution models (SDMs) described by generalized linear models or generalized additive models when estimating school abundance by using a line transect survey. True covariates that define spatial school distribution are not always obtainable explanatory variables. When the true covariates differ from explanatory variables in the model, the explanatory variables are determined to be misspecified. We evaluated the performance of SDMs in abundance estimation with misspecified covariates by using dummy datasets for which the true abundance was known. Simulated replicates of spatial distributions of a whale school and sighting data were generated from possible scenarios motivated by the spatial school distribution of Antarctic minke whales Balaenoptera bonaerensis. This distribution was obtained from the Japanese Whale Research Program under Special Permit in the Antarctic. Our results showed that the relative bias of the abundance estimators was large when covariates were misspecified and a survey region was stratified. Although stratification of the survey region is intended to produce a conventional line transect estimator with a smaller variance than that of non-stratified survey region, it also acts to increase the bias of the abundance estimate obtained from SDMs.

The yield-per-recruit (YPR) analysis has been used to assess growth-overfishing of exploited fish stocks. Since the ex-vessel price per weight widely depends on the size of the fish, the result of the YPR does not imply the revenue. The value-per-recruit (VPR) analysis, which is an economic expansion of the YPR analysis, was conducted for bighand thornyhead Sebastolobus macrochir caught off the Pacific coast of northern Honshu, Japan. The results showed that, under the current age at first capture (tc = 3), the current fishing mortality (F = 0.12) was close to optimum level. However, under the current F, optimum tc in the VPR model was 8 years and would lead to a 47% increase compared to the current revenue. The maximum YPR was obtained at tc = 6 years, but the revenue at tc = 6 only gave a 20% increase in the maximum revenue. The difference was caused by the size dependency of the ex-vessel price per weight. The VPR model would generate fisheries management plans with both economical and biological consideration and would help in making general consensus of fisheries management strategies., 成長乱獲診断に広く用いられているYPR解析を拡張し、単価のサイズ依存を考慮したVPR 解析を、東北海域で漁獲されたキチジのデータに対して行った。その結果、現状の漁獲開始年齢（tc=3歳）では、現状の漁獲係数（F=0.12）は、最適なYPRやVPRを概ね達成しているが、現状のFを維持した場合、最適なtcはVPR解析とYPR解析で、それぞれ8歳と6歳、またその時の収入の増加はそれぞれ47％と20％となり、結果の差異が明らかになった。VPR解析が資源管理における合意形成に寄与することが期待される。

To evaluate walleye pollock stock management procedures in the northern waters of the Sea of Japan, 30-year population dynamics, including uncertainties, were forecast. Errors in current stock size estimation, variability in future recruitment and changes in future fishing mortalities were incorporated. Results of virtual population analysis (VPA) from resampled catch-at-age data with bootstrap methods was used as the current stock size estimation with uncertainty. Performances of each scenario were evaluated using conservation, utilization, stability and reliability factors. Twenty-two management scenarios and continuing the current fishing mortality were evaluated. Scenarios with minor regulation changes and continuing the current fishing mortality showed poor stock conservation performances. Scenarios with minor regulation changes produced good short-term but poor long-term utilization. Stabilities were poor in continuing the current fishing mortality and fishing ban scenarios. Reliability in all scenarios after 30 years was smaller than in continuing the current fishing mortality; however, small differences among scenarios were observed. The simulation results indicated that multilateral assessment is needed to evaluate the management candidates. Uncertainty caused by recruitment variability mostly affected future population dynamics. The role of simulations in the production of effective scientific advice is discussed.

Effects of sampling errors on abundance estimates from virtual population analysis (VPA) were quantified with the bootstrap method for stock of walleye pollock in the the Sea of Japan. In the bootstrap method, individual fish measurements were resampled. A total of 1000 bootstrap samples were produced for each year from 1991 to 2001. The coefficients of variation (CV) of catch at age in 2001 ranged 6.1–33.1%. The CV of an abundance estimate in 2001 ranged 9.0–35.7%. Abundance estimates of the oldest age and the latest year, which had larger CVs than the other estimates, were sensitive to sampling errors. Effects of sampling errors became smaller when the catch at age had been accumulated over a few years. Although VPA includes various types of errors, only the sampling errors have room for improvement in reality. Quantifying the effect of sampling error on VPA estimates is essential for sound and efficient stock management, and is emphasized in this study.

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.

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.

Age-based population assessment is widely used, but there are cases where information on age or even body length of landed fish is difficult to collect. In the present study, the biomass and fishing mortality of the southern Hokkaido stock of Pacific cod Gadus macrocephalus was estimated directly from body weight composition, using weight-based virtual population analysis, from 1994 to 2000. The estimated biomass over 1 kg body weight was 5607 t in 1994 and increased to 7908 t in 2000. The increase was explained by an increase in recruitment.

A histological examination of two harbor porpoises Phocoena phocoena caught in a set net established off Usujiri, Hokkaido was conducted in order to further understand the reproductive biology of the species. In the male, a unilateral testis comprised a testicular lobe and rete testis, seminiferous tubules of the former being composed mostly of Sertoli (sustentacular) cells and a few primodial germ cells, whereas the latter contained numerous anastomosing tubules consisting of columnar epithelia. In the female, the smooth-surfaced ovary consisted mostly of primodial and secondary follicles and a few atretic follicles, although more developed Graafian follicles were not found. A highly intricate oviduct (fallopian tube) was constructed from simple high columnar cells, but did not contain AF-positive substance. Both individuals examined were considered to be infants owing to their very immature gonads.

We carried out mail and access point surveys to estimate the catch numbers, angling effort of recreational angling and the population of sockeye salmon Oncorhynchus nerka in Lake Toya in 1998 and 1999. The mail survey questions in questionnaires distributed to anglers with a recre- ational angling license included date of angling, caught fish species and size, and catch number. In the access point survey we also asked anglers if they had a license. We measured the fork length and weight of caught fish. The catch rate (catch number in a day by each angler) from both surveys showed no significant difference, suggesting no bias for the catch rate between surveys. The esti- mated total angling effort was 1760 people (1998), and 1516 people (1999). The estimated recre- ational catch was 28 889 (1998) and 5455 (1999), that is, two or three times larger than the commercial catch. The fish population was estimated by using the DeLury method as 60 262 (1998) and 10 806 (1999). The total exploitation rate was 62% (1998) and 78% (1999). The size of caught fish was much larger in 1999 than 1998 because of the difference in age composition.

SUMMARY: The ages of individual specimens of white-spotted char Salvelinus leucomaenis, collected from the Furuu River in Hokkaido, Japan, were determined from otoliths using NIH Image. We employed two methods to determine age. First, for a surface method (traditional method), the ages were made from examinations of otoliths under a microscope by experienced readers A, B and an unexperienced reader C. Second, for an auto-counting method, a computer program was used to detect the peaks from otolith images. We made a mathematical model to estimate the probability of a correct reading for each reader, using the results of the age determination and the per cent age agreement among readers. The ability to determine the age of white-spotted char depended on whether or not the reader had ever previously made age determinations. The ability of auto-counting method was below that of reader A (one of the experts), and the method was slightly superior to reader B (the other expert).

津軽海峡の鯨類に関する知見は極めて乏しい。本研究では津軽海峡の鯨類の鯨種，それらの出現頻度を明らかにするため，2003年4月から2004年11月まで函館・青森間を運行するフェリーからの目視調査を実施した。58日6,102kmの目視調査により，合計415群3,010 個体の鯨類の発見があった。発見はカマイルカが優占し，マイルカ，イシイルカ，ハンドウイルカ、ネズミイルカ、コビレゴンドウ等が発見された。カマイルカの来遊のピークは5～6月の年一回であった。