Weight-based virtual population analysis of Pacific cod Gadus macrocephalus off the Pacific coast of southern Hokkaido, Japan

ABSTRACT:   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.     

Interdecadal decrease in potential fishing areas for Pacific saury off the southeastern coast of Hokkaido,Japan

We examined interdecadal variations in potential fishing grounds for the Pacific saury, Cololabis saira, off the southeastern coast of Hokkaido, Japan, for the early fishing season of August–September. We applied linear trend analysis to a time series of several oceanographic variables for 1993–2014. Trends in the appearance frequency of sea surface temperatures (SST) of 12–18 °C during August–September indicated an interdecadal reduction in the potential fishing area off the Hokkaido coast. There were localized significant increases of both SST and the sea level anomaly along the Kuril–Kamchatka Trench off the Hokkaido coast. The localized trends coincided with decadal decreases of satellite‐based chlorophyll concentrations after 2002. Analysis of mesoscale eddies revealed that the localized trends were related to clockwise (counterclockwise) eddies that appeared more frequently (less frequently) near the trench in recent years. Moreover, the Oyashio transport on the slope between the coast and the trench decreased significantly and completely disappeared in some recent summers. Interactions between the Oyashio and mesoscale eddies resulted in an interdecadal shift in the cold‐water intrusion along the main stream of the Oyashio from the along‐slope direction to the offshore direction in the upstream region of the Hokkaido coast, and large areas of favorable potential fishing grounds near the Hokkaido coast have disappeared in recent years.     

Identifying potential habitat distribution of the neon flying squid (Ommastrephes bartramii) off the eastern coast of Japan in winter

Habitat suitability index (HSI) models were applied to identify the potential habitat distribution of the neon flying squid (Ommastrephes bartramii) off the eastern coast of Japan during winter. We used an ocean reanalysis product, a satellite‐derived dataset, and commercial fisheries data during 2003–2008 to develop the HSI models, and illustrated the characteristics of the ocean environments at the fishing ground of the neon flying squid, focusing on a typical fishing ground formation event in 2006. The estimated HSI fields of the neon flying squid using three‐dimensional (3D) ocean environmental parameters showed a clear relationship between the squid habitat and the edge of a warm core ring south of the Oyashio water; this is considered a key characteristic of fishing ground formation, as noted in Sugimoto and Tameishi (Deep‐Sea Research, 39, 1992 and S183). This result suggests that mixing of the warm and nutrient‐poor Kuroshio water and the cold and nutrient‐rich Oyashio water at the edge of the ring could provide favorable conditions for the foraging of the neon flying squid. The warm water condition in the subsurface layers could be a further advantage to the formation of a stable fishing ground for the neon flying squid. Comparison of the Akaike Information Criteria among a satellite‐data‐based model, a reanalysis‐based model using the same parameters as the satellite‐based model, and a reanalysis‐based model using 3D ocean environmental parameters, showed an apparent improvement in the performance of the reanalysis‐based model using the 3D parameters, reproducing realistic features of the squid fishing ground during the winter of 2006.     

Abiotic and biotic factors affecting recruitment variability of walleye pollock (Theragra chalcogramma) off the Pacific coast of Hokkaido,Japan

Abiotic and biotic factors affecting the recruitment variability of the Japanese Pacific stock (JPS) of walleye pollock (Theragra chalcogramma) were examined using a bivariate regression and multivariate combined model. Of the abiotic variables around Funka Bay (spawning ground), February sea surface temperature (SST) and wind direction index showed significant bivariate relationships with recruitment. February SST was positively related to recruitment, suggesting that warmer water temperature in February favors JPS recruitment. On the other hand, the relationship between February wind direction index and recruitment predicts high JPS recruitment under predominant northwest winds in February. For the biotic variables in the Doto area (nursery ground), significant and negative bivariate relationships with recruitment were observed for catch per unit effort of Kamchatka flounder (Atheresthes evermanni), Pacific cod (Gadus macrocephalus), and walleye pollock, implying an important impact of predation by these groundfishes on JPS recruitment. The overall model incorporating these abiotic and biotic factors successfully reproduced the variability in JPS recruitment. Temperature and wind conditions around the spawning ground along with predator condition in the nursery ground appear to play a dominant role in the recruitment dynamics of JPS. Based on these results and prior knowledge, we propose a new hypothesis to explain the processes controlling JPS recruitment.     

Characteristics of a Pacific herring Clupea pallasii spawning bed off Minedomari, Hokkaido, Japan

ABSTRACT:   The spawning bed selection of herring Clupea pallasii off Minedomari, Atsuta, on the west coast of Hokkaido, Japan, is discussed in relation to the topography of the rocky shore and wave conditions during the spawning season. From 1998 to 2003, herring have spawned their eggs mainly on the leaves of the seagrass Phyllospadix iwatensis Makino in almost the same site on the rocky shore off Minedomari in the Atsuta area. The site is connected to a valley-like feature offshore. Wave conditions were surveyed at Minedomari during the spawning season in 2000 and the wave height was estimated from 1998 to 2003, except for in 2000, using the correlation of wave height between Minedomari and Ishikari Bay New Port, approximately 18 km south-west of Minedomari. Herring spawned under calm conditions, during which the wave height was approximately 0.5 m off Atusta and Aoshima and was 0.18–1.28 m off Minedomari. The distribution of water particle velocity on the sea bottom surface as a result of ocean waves off the Minedomari area, which was estimated based on the wave height and the topography of the coast, suggested herring could swim easily into the shallower area along the valley-like feature off Minedomari. Therefore, topographical features are thought to be one of the reasons why herring have used Minedomari as a spawning bed. Additionally, seepage of freshwater from the bottom, which was observed in this area, could also be the reason why herring spawn in Minedomari repeatedly.     

Distribution patterns of five pleuronectid species on the continental slope off the Pacific coast of northern Honshu,Japan

Information on the distributions of demersal and benthic fishes is fundamental for stock assessment and management. Spatiotemporal changes in the distribution patterns of five pleuronectid species (flathead flounder Hippoglossoides dubius, Kamchatka flounder Atheresthes evermanni, roughscale sole Clidoderma asperrimum, slime flounder Microstomus achne, and Korean flounder Glyptocephalus stelleri) off the Pacific coast of northern Honshu, Japan, at depths of 150–900 m, were examined using a generalized linear mixed model (GLMM). Densities of flathead and Korean flounder were highest in the southernmost area, where the fish were small. The body lengths of both of these species increased from 2003 to 2008, suggesting that an abundant year class was recruited in 2003. The density of Kamchatka flounder was highest in the northern area. In roughscale sole and slime flounder, there were no distinctive annual and latitudinal trends in the density distributions. The density distribution of Korean flounder was bimodal; the peaks were at depths of 210 and 410 m. The body length increased as the depth increased from 150 to 410 m, and then decreased from 410 to 550 m. Moreover, “bigger–deeper” trends were observed in flathead, Kamchatka and slime flounder.     

Variability in timing and magnitude of spring bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan

The spring bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the spring phytoplankton bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense spring bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of spring blooms in the Oyashio and the Coastal Water Masses. The spring blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.     

Feeding habits of Baird's beaked whale Berardius bairdii, in the western North Pacific and Sea of Okhotsk off Japan

ABSTRACT:   We examined the stomach contents of 26 Baird's beaked whales caught off the coast of Japan by small-type coastal whalers. The main prey for these whales was rat-tails and hakes in the western North Pacific. Pollock and squids were also important food in the whales collected from the southern Sea of Okhotsk. The prey species found in the stomachs of the whales were almost identical to those caught in bottom-trawl nets at depths greater than about 1000 m in the western North Pacific, which suggests that the Baird's beaked whale forages for prey at depths of about 1000 m or more. Baird's beaked whales in the western North Pacific migrate to waters of 1000–3000 m in depth, where demersal fish are abundant. This implies that Baird's beaked whales migrate to waters where demersal fish, especially rat-tails and hakes, are abundant. Although there is limited information on the feeding habits of ziphiid whales, they are generally thought to prefer squid. The present data suggest that demersal fish are also important prey for ziphiid whales.     

A spatial model of population dynamics of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan

We constructed a numerical model reproducing the transport, survival and individual growth of the early life stages of Japanese sardine, Sardinops melanostictus, off the Pacific coast of Japan during 1978–93. The causes of early life stage mortality, including the influence of the effects of the spatial relationship between the spawning grounds and the Kuroshio on the mortality rate, were investigated. Survival and transport from egg stage to 60 days after spawning were modelled daily in a 1 × 1 degree mesh cell and individual growth in the period was modelled in each region (Kuroshio, Inshore, Offshore and Transition regions). Individual growth and survival from 60 to 180 days after spawning were modelled daily in the Transition region. Environmental data were taken from outside the model system. Our simulation indicates that survival variability in the larval stage (5–25 mm in standard length) is the key factor in determining the year‐class strength. The simulation revealed that strong year classes occurred with good survival in the spawning ground and whilst entrained in the Kuroshio current being transported to the main feeding grounds in the Transition region. The simulation also indicated that survival rates in 1988–93 were low in the Inshore, Kuroshio and Offshore regions, which depressed the year‐class strength during that period.     

Occurrence of Pacific bluefin tuna Thunnus orientalis larvae off the Pacific coast of Tohoku area,northeastern Japan: Possibility of the discovery of the third spawning ground

Recently, based on the histological studies it is suggested that Pacific bluefin tuna (PBF) spawns in the Kuroshio–Oyashio transition, off the Pacific coast of northeastern Japan, in addition to the previously identified two spawning areas of the northwestern Pacific around the Nansei Islands and the Sea of Japan. Distributional surveys for PBF larvae have not been conducted in the Kuroshio–Oyashio transition, and thus, we conducted PBF larval surveys at twenty stations in this area in early August 2018 to obtain evidence for the spawning of PBF. Twelve PBF larvae (3.9–7.2 mm in body length) were collected at six stations where sea surface temperature ranged from 27.2 to 28.2°C. Otolith microstructure analysis indicated that age of these larvae ranged from 4 to 11 days after hatching and the larvae hatched in late July. Growth of the collected larvae was comparable to those in the other two spawning grounds. Therefore, PBF spawns, hatches, and at least survives to the postflexion stage, 11 days after hatching in the Kuroshio–Oyashio transition. This fact potentially has a large impact on recruitment processes of PBF if they survive to recruitment in this third spawning ground.     

Interannual changes in the timing of walleye pollock spawning migration and their impacts on the gillnet fishery in the southwestern Pacific coast of Hokkaido,Donan area,Japan

The southwestern Pacific coast of Hokkaido is the main spawning ground for the Japanese Pacific stock of walleye pollock Theragra chalcogramma. A commercial gillnet fishery targeting spawning adult pollock in this area mainly operates from October to January to coincide with the migration of adult pollock from the feeding ground. Given the results of acoustic surveys, and changes in the proportion of the monthly total catch that was monthly walleye pollock caught by the commercial gillnet fishery, it is thought that the timing of walleye pollock spawning migration to the Donan area varies among years and that the pollock catch of the gillnet fishery clearly reflects changes in pollock abundance in this area. A time series of interannual variability in catch data from 1980 to 2005 suggested that adult pollock migrated and concentrated on their spawning ground later in the 1980s and after 2000 than in the 1990s. Such decadal-scale shifts are presumably caused by climatic changes (e.g., in water temperature) in the Oyashio region. These shifts affect the gillnet fishery through differences in monthly unit prices of pollock and changes in the formation of fishing grounds. These scientific findings can aid the establishment of rules for more efficient walleye pollock resource management under the total allowable catch system.     

Weight-based yield per recruitment and spawning-biomass per recruitment analyses of pacific cod <Emphasis Type="Italic">Gadus macrocephalus</Emphasis> off the Pacific coast of southern Hokkaido,Japan

ABSTRACT:   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.     

Decadal changes in the diet of northern fur seal (Callorhinus ursinus) migrating off the Pacific coast of northeastern Japan

Decadal changes in northern fur seal (Callorhinus ursinus) diet were examined based on the stomach contents data collected off the Pacific coast of northeastern Japan from January to April, 1953–1988. Seventeen families of fish and seven families of squid were identified from the stomach contents. Dominant prey species in terms of percentage of occurrence and wet weight were Japanese sardine (Sardinops melanostictus), chub mackerel (Scomber japonicus), and myctophid fishes. Demersal fishes, sparkling enope squid (Watasenia scintillans), and oceanic squids were also preyed on at low incidences. Decadal‐scale diet composition of northern fur seals revealed shifts in the significance of Japanese sardine and chub mackerel in parallel with the decadal alternation in the dominance of these species within the pelagic fish community off the Pacific coast of northeastern Japan. These results suggest that northern fur seals can use a variety of prey resources in this wintering area by switching the diet according to the distribution and abundance of prey species.