13. Survival of Walleye Pollock in Early Life Stages in Funka Bay and the Surrounding Vicinity in Hokkaido

Ⅱ. Reproduction and Recruitment of Keystone Species, and Ecosystem Studie

Feeding habits of Pacific cod larvae and juveniles in Mutsu Bay, Japan

The diets and prey widths of Pacific cod (Gadus macrocephalus) larvae and juveniles were examined in Mutsu Bay. In early March, 11 yolk-sac larvae were caught in midwater of the bay mouth. Five of 11 larvae captured had not opened their mouths, and the remaining six had not fed. Post-larvae ≤ 7mm in total length (TL) fed on copepod nauplii, particularly Pseudocalanus nauplii. Prey items of post-larvae >7mm TL were chiefly crustacean eggs, and Pseudocalanus spp. copepodites and adults. From April to June, the main food organisms of cod juveniles were calanoid copepods, and changed from Pseudocalanus spp. to Calanus pacificus, Acartia clausi, and Centropages abdominalis. In addition, cod juveniles opportunistically fed on large-sized food organisms, namely, reptant megalopae, planktonic gastropods, and natant zoeae. In July, cod juveniles mainly fed on benthic prey items, such as gammarid amphipods and fish. It seems probable that these large-sized food organisms play an important role as alternative prey for cod juveniles when they change their main food from calanoid copepods to benthic prey items

Distribution and migration of adult walleye pollock off Hiyama, southwestern Hokkaido

International Symposium on the Biology and Management of Walleye Pollock. 14-16 November 1988. Anchorage, Alaska, USA

Long-term shifts in the growth and maturation size of Miyabe charr Salvelinus malma miyabei

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.北海道然別湖に生息するミヤベイワナの成長解析を行い、約80年にわたる漁獲圧と成長や成熟魚サイズの変化の関係について検討した。その結果、過剰な遊漁による漁獲圧の上昇により、ミヤベイワナの成長が鈍化し成熟魚が小型化したと考えられた。しかし、禁漁や遊漁規制の導入により、現在は乱獲状態にあった時期と比べて資源が回復し、約10年から30年で個体の成長と成熟魚サイズが回復したことが示唆された

Vertical distribution and prey of walleye pollock in the northern Japan Sea

Vertical distributions of adult walleye pollock Theragra chalcogramma and oceanographic conditions were examined in the northern Japan Sea. In the daytime, most walleye pollock were distributed from 150 to 250m depth in April and from 400 to 500m depth in October. Between 150 and 500m depth, temperatures in October (0.4-8.8℃) were similar to those in April (0.5-5.4℃), salinity levels were fairly uniform in both months (34.0-34.2 PSU), and dissolved oxygen concentrations were 4.6-6.1ml/l in April and 4.7-6.4ml/l in October. The main foods of walleye pollock in April were the amphipod Themisto japonica, the euphausiid Thysanoessa longipes, and the chaetognath Sagitta elegans. Daytime weighted mean depths of T. longipes were significantly greater in October than in April. Biomass of T. longipes in the habitat of walleye pollock was significantly greater than other layers in both months. The seasonal change in vertical distribution of walleye pollock is presumably related to food availability of T. longipes

Age and growth of brown sole Pleuronectes herzensteini in the coastal waters of western Aomori Prefecture, Japan

Age and growth of brown sole Pleuronectes herzensteini collected from the coastal waters of western Aomori Prefecture from September 1989 to September 1990 were determined from otolith analysis. Observation of the otolith margin verified that annuli (outer margins of the translucent zone) were mainly produced between February and March. This period was associated with the spawning season. Growth of brown sole was expressed by the von Bertalanffy asymptotic growth function as TLt=255.2(1-exp^[-0.463(t-0.267)]) for males and TLt=350.2(1-exp^[-0.308(t-0.300)]) for females, where TLt, is the total length in mm and t is age in years. It was found that the growth rate of brown sole in this study area is very high as compared with the more southern population in the Niigata region. In the northern Japan Sea, there is a tendency that the growth rate in the north is higher in length at comparable ages. It is possible that the growth of brown sole in the northern Japan Sea is closely related with bottom water temperature

Annual variation in otolith increment widths of walleye pollock (Gadus chalcogrammus) larvae in Funka Bay, Hokkaido, Japan

To clarify relationships between year-class strength and larval growth of walleye pollock (Gadus chalcogrammus), and oceanographic conditions in the Pacific stock off Hokkaido and Tohoku, Japan, we undertook CTD observations and investigated larval densities, larval otolith increment widths, and larval prey densities (of copepod nauplii) of the 2008, 2009, 2010, and 2011 year classes in Funka Bay. Oyashio Coastal Water (OCW) flowed into the bay in late February in 2008, 2010, and 2011, and mean water temperatures decreased to 1.9–3.1 °C in March. OCW was not observed in 2009, and it was warm in late February (≥ 3.4 °C). Increment widths of lapillar otoliths during the yolk-sac stage were wide in 2009 and 2011, medium in 2010, and narrow in 2008. Increment widths during the first-feeding stage tended to become wider as the hatch month progressed, and the annual variation during the first-feeding stage was larger than that of the yolk-sac stage. The densities of the primary food for the larvae were high in 2008 when larval increment widths were narrowest, so the effect of prey abundance on larval growth appeared to be small. The ranking of the larval abundance in March was nearly coincident with that of the increment width during the larval stage. We therefore suggest that larval growth rate is associated with mortality rate, and that the growth–mortality hypothesis may be applicable to walleye pollock in Funka Bay. Feeding success under warm water conditions may be an important factor that contributes towards high growth rates