{"created":"2024-04-08T04:46:47.032288+00:00","id":2001661,"links":{},"metadata":{"_buckets":{"deposit":"99696638-c20c-4ac3-aebc-e592872862ae"},"_deposit":{"created_by":10,"id":"2001661","owners":[10],"pid":{"revision_id":0,"type":"depid","value":"2001661"},"status":"published"},"_oai":{"id":"oai:fra.repo.nii.ac.jp:02001661","sets":["12:14:1716950587014"]},"author_link":[],"control_number":"2001661","item_10002_biblio_info_7":{"attribute_name":"書誌情報","attribute_value_mlt":[{"bibliographicIssueDates":{"bibliographicIssueDate":"1996-03","bibliographicIssueDateType":"Issued"},"bibliographicNumberOfPages":"144","bibliographicPageEnd":"144","bibliographicPageStart":"1","bibliographicVolumeNumber":"60","bibliographic_titles":[{"bibliographic_title":"北海道区水産研究所研究報告","bibliographic_titleLang":"ja"},{"bibliographic_title":"Bulletin of the Hokkaido National Fisheries Research Institute","bibliographic_titleLang":"en"}]}]},"item_10002_description_5":{"attribute_name":"抄録","attribute_value_mlt":[{"subitem_description":"Oyashio Region is one of the most productive fishing grounds of the world. Not only boreal fish, temperate and subtropical species also migrate into this region from southern areas during warm seasons from early summer to autumn. Most of them are known to be massively productive fishes and are planktivores. This suggests that in the Oyashio Region production of the lower trophic levels supports bulk of the fisheries production. In this study I illustrate, at first, the seasonal variability of zooplankton biomass as a measure of food condition of the pelagic fish along with its environmental factors such as phytoplankton biomass and nutrient concentrations in the Oyashio Region. Secondly, I demonstrate the diel feeding rhythm of copepods, the most dominant group of the zooplankton assemblage in this region, which enhances the efficiency of energy and material transfer from primary production to fish. I also discuss about adaptive significance of the diel feeding rhythms in life history strategy of copepods themselves. Seasonal variability in the productivity of lower trophic levels in the Oyashio Region In order to investigate the seasonal changes of plankton biomasses of both phyto- and zooplankton and their environmental factors in the Oyashio Region, total of 25 cruises were carried out along two fixed lines off Akkeshi Bay, on the eastern-south coast of Hokkaido, i.e., A-line and WO-line, during the period from January 1990 to October 1993. During winter, nitrate was supplied to the surface mixed layer and phytoplankton biomass increased rapidly and formed spring blooms in April or May. The spring bloom was terminated by depletion of nitrate in the surface layer by early summer. Level of phytoplankton biomass during the bloom was higher in the northern part of the survey area where more nitrate was supplied in winter than in the southern part. Macrozooplankton biomass increased simultaneously with phytoplankton increase but decreased slowly and gradually well after the termination of the spring bloom of phytoplankton. These trends of seasonal change in biomass of plankton in the Oyashio Region was basically similar to that in the North Atlantic rather than in Alaska Bay in the eastern subarctic Pacific. However, level of zooplankton biomass was higher in the Oyashio Region than in the North Atlantic and Alaska Bay as well. Zooplankton/phytoplankton ratios in carbon weight in the Oyashio Region were lower than 1 during winter and spring. After the termination of spring bloom, the ratios increased abruptly reaching 9 and then gradually decreased. This indicates that organic matter produced by phytoplankton during spring bloom period is taken rapidly by and stored long in zooplankton after the spring bloom. This plays an essential part of the roles of zooplankton as food of the massively producing planktivorous fishes which migrate into the Oyashio Region during warm seasons when the phytoplankton biomass is not large. Diel feeding rhythm of neritic copepods Due to frequent nutrient supply from the bottom and/or land to the euphotic layer, chlorophyll concentration is usually higher in the coastal areas than in oceanic areas. Feeding behavior of neritic copepods adapting to such a food-rich environments may therefore be different from that of oceanic copepods adapting to food-poor environment. I describe the diel feeding rhythms of the neritic and the oceanic copepods separately here. Ingestion rate of copepods was determined in this study by the so-called gut fluorescence method. In this method, ingestion rate of copepod (I) is represented by the following equation, I=1/1-D kG (1) where k means gut evacuation rate constant, G means gut pigment contents and D means pigment destruction rate to non-fluorescent matter in gut of a copepod. Gut pigment contents were measured 7 to 11 times a day to determine the diel feeding rhythm. The neritic copepods almost always showed a diel feeding rhythm except for Paracalanus sp. males. The rhythm was observed over wider ranges of temperature form -1.4 to 14˚C, and chlorophyll a concentration from 0.3 to 9.0 mg m-3, which were identical to the annual ranges in coastal waters around Hokkaido. Diel feeding rhythm was also detected even under the sea ice, where temperature and chlorophyll concentration were at the lowest end of copepod habitat and light intensity was very low due to absorption and reflection by sea ice and snow. These show that the diel feeding rhythm is a fundamental and essential behavior of neritic copepods. Increase of ingestion rate was always observed around the dusk and usually kept higher through night than in day. However, the decrease was observed 3-4 hours before the sunrise in some observations. These results indicate that the increase in ingestion rate is triggered by the light stimulus but, on the other hand, the decrease is controlled by not only the light stimulus but also other factors, probably by an endogenous rhythm. Timing of the increase and decrease in ingestion rate was different with species. Increase in ingestion rate of Acartia spp. occurred earlier than one of Pseudocalanus spp. On the other hand, decrease in ingestion rate of Acartia spp. occurred later than one of Pseudocalanus spp. Ratio of average ingestion rate during nighttime to one during daytime was always higher than unity with one exception, i.e., 2.3, 2.1, and 1.8 for Acartia omorii females, A. omorii males and Paracalanus sp. females, respectively. However, there were no significant correlations between this ratio and food concentration. Previous authors suggested that diel rhythm was a consequent behavior of the diel vertical migration. In this study, however, diel feeding rhythm was observed for not only migratory copepods but also non-migratory copepods which stayed near-bottom layer throughout the day. Diel feeding rhythm of oceanic copepods Diel feeding rhythm of oceanic copepods was investigated during pre-blooming, blooming and post-blooming periods in the Oyashio Current Region. Neocalanus spp. were the dominant species during the investigations and they always showed basically similar diel rhythms in feeding behaviors to neritic copepods. Diel feeding rhythm was maintained even when amount of daily ingestion could not be sufficient to meet daily requirement for metabolism. Except for Neocalanus cristatus CV, the increase and decrease in ingestion rate were observed after the sunset and before the sunrise, respectively. On the other hand, N. cristatus CV increased their ingestion rate before sunset and decreased after sunrise. The ratios of average ingestion rate during nighttime to one during daytime were quite variable among species, stages, and observations (1.4 -10). However, a tendency that the ratios are higher for oceanic copepods than neritic copepods was clear. Gut pigment analysis for individual copepods was carried out for N. cristatus CV during blooming periods. In nighttime, gut pigment contents of all individuals examined were higher than the maximum contents at midday. Average gut pigment contents in nighttime were always significantly higher than one in daytime. Coefficients of variations of the averages at midnight and midday were lower than those at dusk and dawn. Timing on the increase of ingestion rate was variable by individuals. Neither significant relationship was observed between gut pigment contents and prosome length of copepods. Based on the results obtained grazing pressure of dominant copepods on phytoplankton could be calculated to be 6.9% and 10.5% of standing stocks in preblooming and post-blooming periods, respectively. Mathematical model of the diel feeding rhythm In order to analyze the diel pattern of ingestion rate, a mathematical model was developed. Diel pattern in gut pigment contents was reconstructed by a model based on the two assumptions, that 1) ingestion rate switches two times a day increasing at dusk (Tss+ß) by factor F and decreasing at dawn (Tsr+α) by factor 1/F, 2) gut evacuation rate constant (k) is invariable throughout the day. On these assumptions, gut pigment contents (G) can be represented by the following equations, dG/dt = I* - kGt (Tsr+α≦t