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1.
Five years of field, laboratory, and numerical modelling studies demonstrated ecosystem‐level mechanisms influencing the mortality of juvenile pink salmon and Pacific herring. Both species are prey for other fishes, seabirds, and marine mammals in Prince William Sound. We identified critical time‐space linkages between the juvenile stages of pink salmon and herring rearing in shallow‐water nursery areas and seasonally varying ocean state, the availability of appropriate zooplankton forage, and the kinds and numbers of predators. These relationships defined unique habitat dependencies for juveniles whose survivals were strongly linked to growth rates, energy reserves, and seasonal trophic sheltering from predators. We found that juvenile herring were subject to substantial starvation losses during a winter period of plankton diminishment, and that predation on juvenile pink salmon was closely linked to the availability of alternative prey for fish and bird predators. Our collaborative study further revealed that juvenile pink salmon and age‐0 herring exploit very different portions of the annual production cycle. Juvenile pink salmon targeted the cool‐water, early spring plankton bloom dominated by diatoms and large calanoid copepods, whereas young‐of‐the‐year juvenile herring were dependent on warmer conditions occurring later in the postbloom summer and fall when zooplankton was composed of smaller calanoids and a diversity of other taxa. The synopsis of our studies presented in this volume speaks to contemporary issues facing investigators of fish ecosystems, including juvenile fishes, and offers new insight into problems of bottom‐up and top‐down control. In aggregate, our results point to the importance of seeking mechanistic rather than correlative understandings of complex natural systems.  相似文献   

2.
Hydrography and tidal circulation are described for two deep fjords (Whale Bay and Eaglek Bay) and two shallow bays (Simpson Bay and Zaikof Bay) used as nursery habitat by juvenile Pacific herring (Clupea pallasi) in Prince William Sound, Alaska from October 1995 to March 1998. The seasonal hydrography varied markedly among the four locations due to the interaction of various factors including local climate, hydrology, advection of allochthonous glacial water, and vertical mixing from winds and tides. The fjords exhibited strong haline stratification in the summer that persisted into winter. In contrast, the bays exhibited brief periods of weak to strong stratification that dissipated early in the autumn. The timing of peak freshwater input also varied among locations depending on the maximum size and elevations of watersheds and the extent of precipitation stored within alpine snow and ice fields. In late winter (March), surface water in the fjords was relatively cold (1–3 °C) and slightly fresh (29–30 psu) in comparison to the bays (3.5–5.5 °C and 31–31.5 psu). The subsurface water (40–80 m) was warmer in the fjords (5–7 °C), whereas the bays exhibited vertically uniform temperature and salinity (T/S) properties. Tidal currents were typically highest near the mouth of most basins (35–150 cm s?1) and exhibited horizontal and vertical shear in the summer and autumn. During these seasons, these flows created baroclinic currents within most basins, anticyclonic eddies at Simpson and Zaikof bays, and inflow of allochthonous glacial water at Whale Bay. Local climate and watershed hydrology of individual bays have a strong influence on water temperatures and haline stratification, which potentially affect production of phytoplankton, zooplankton, and growth and survival of age‐0 herring. Additionally, variability in transport of Gulf of Alaska derived carbon sources into nursery bays is dependent on geographical location and the physical features of each basin, such as maximum depths, presence or absence of sills, and spatial patterns in the winds and currents.  相似文献   

3.
From 1994 to 1998, a multidisciplinary ecosystem study (the Sound Ecosystem Assessment) examined the primary physical and biological factors that influence the production of pink salmon and Pacific herring in Prince William Sound (PWS), species that experienced population declines after the 1989 Exxon‐Valdez oil spill. Three physical processes are described that influence ecosystem processes: surface layer stratification; upper layer (<100 m) circulation; and exchange between PWS and the northern Gulf of Alaska (NGOA). Stratification formed first in the PWS nearshore regions in March, and subsequently in the northern central basin in April, primarily due to freshening. The northern stratified layer and the associated zonal density front persisted at least through June, but year‐to‐year differences occurred. In spring and summer, circulation in central PWS could be either cyclonic or anticyclonic. Drifter trajectories often linked central PWS to a nearshore bay or fjord, or one bay or fjord to another. September was characterized by a cyclonic circulation and isopycnal doming in central PWS, with little year‐to‐year variability. At Hinchinbrook Entrance, a main connection with the Gulf of Alaska, alternating inflows and outflows occurred in spring over all depths. In summer through early autumn (1995), in the absence of predominantly westward winds, the dominant exchange pattern was outflow above about 150 m and inflow below. In summer through early autumn (1996–98), there was also surface (<20 m) inflow at Montague Strait (the other main entrance). Northward transport at Hinchinbrook Entrance was maximum in late autumn through winter, with inflow above ~150 m and outflow below. Westward wind events over the shelf associated with the weather cycle drove inflow events at both Hinchinbrook Entrance and Montague Strait that may result in transport of zooplankton important to the PWS ecosystem.  相似文献   

4.
Following the 1989 MV Exxon Valdez oil spill (EVOS) and subsequent herring population collapse in Alaska's Prince William Sound (PWS), the Pacific herring (Clupea pallasii) fishery was closed. In the 25 yr since EVOS, herring and several herring‐dependent marine bird species have failed to reach pre‐spill population levels. One hypothesis is that intense predation pressure may be inhibiting herring recovery. To inform herring modeling efforts, this study estimated marine bird consumption of juvenile and adult herring in PWS for 10 winters over an 18‐yr period (1989–90 through 2006–2007). Total estimated herring consumption by wintering marine birds averaged 2409 ± 950 t, indicating that avian consumption represents a substantial and inter‐annually variable source of herring mortality. Common murre (Uria aalge) consumed the greatest portion (16–80%) of herring in all years among marine bird species. Juvenile herring biomass consumed annually by common murre was greater than murre consumption of adult herring biomass. Time lag analyses showed that marine bird consumption of adult herring is negatively correlated with the amount of herring spawn observed in subsequent years, but such effects were not observed more than 2 yr. Our models indicate that during years in which herring recruitment is low or bird populations are particularly large, marine birds can consume up to 10% of the annual adult herring biomass. Our results highlight the importance of herring to wintering PWS birds. We propose that future management of herring stocks seeks to reduce negative impacts on marine birds that prey on herring.  相似文献   

5.
The protistan parasite Ichthyophonus occurred in populations of Pacific herring Clupea pallasii Valenciennes throughout coastal areas of the NE Pacific, ranging from Puget Sound, WA north to the Gulf of Alaska, AK. Infection prevalence in local Pacific herring stocks varied seasonally and annually, and a general pattern of increasing prevalence with host size and/or age persisted throughout the NE Pacific. An exception to this zoographic pattern occurred among a group of juvenile, age 1+ year Pacific herring from Cordova Harbor, AK in June 2010, which demonstrated an unusually high infection prevalence of 35%. Reasons for this anomaly were hypothesized to involve anthropogenic influences that resulted in locally elevated infection pressures. Interannual declines in infection prevalence from some populations (e.g. Lower Cook Inlet, AK; from 20–32% in 2007 to 0–3% during 2009–13) or from the largest size cohorts of other populations (e.g. Sitka Sound, AK; from 62.5% in 2007 to 19.6% in 2013) were likely a reflection of selective mortality among the infected cohorts. All available information for Ichthyophonus in the NE Pacific, including broad geographic range, low host specificity and presence in archived Pacific herring tissue samples dating to the 1980s, indicate a long‐standing host–pathogen relationship.  相似文献   

6.
The winter is often considered as a survival bottleneck for stream‐living fish. Juvenile salmonids generally become less active during this period, and while food intake continues to some extent, growth rates are typically low. Here we present the results of an over‐winter field experiment where energy levels were manipulated in late autumn. Three groups of juvenile (age 1+) brown trout, from an anadromous population, were monitored with respect to over‐winter growth rate and survival (as indicated by recapture rates). Two groups were fed either high (HR), or low (LR) food rations in the laboratory for a month (October); the third group remained in the stream (STR). Over‐winter growth rates were relatively low in all groups, and no growth compensation could be detected. Compared to HR and LR, STR fish had higher recapture rates after winter, indicating that laboratory housing may have affected the subsequent stream survival negatively. Comparing the two laboratory‐housed groups, the LR group reached similar condition as the HR group in early spring, without indications of differences in survival. However, the initiation rate of body silvering (indicating initiation of smoltification) was lower in the LR group. Thus, it appears that food restriction during late autumn affect the onset of smoltification in juvenile brown trout. The results support previous laboratory studies indicating that salmonids modify their over‐winter foraging behaviour to avoid too low energy levels at the end of winter. This modification appears to delay smoltification, but may not necessarily be costly in terms of over‐winter mortality.  相似文献   

7.
A three‐dimensional, primitive‐equation ocean circulation model was applied to Prince William Sound, Alaska (3D‐PWS circulation model), under forcing of an ocean tide, freshwater runoff, surface heat flux, Alaska Coastal Current (ACC) throughflow (inflow/outflow), and daily (synoptic), spatially varying winds. The 3D structures and seasonal cycles of the circulation patterns, temperature, salinity (density), and mixed layer are examined. Freshwater runoff significantly contributes to the basin‐scale cyclonic circulation, which was not addressed in the previous simulations. Two typical circulation regimes, cyclonic and anticyclonic, characterize the complex flow patterns that depend on the intensities of the ACC thoughflow, freshwater discharge, and the synoptic wind. The spring (April–May) circulation pattern is characterized by a weak (maximum current 0.1 ms?1) anticyclonic flow in the central Sound, while the autumn (September–October) circulation is dominated by a basin‐scale, cyclonic gyre (maximum current 0.2 ms?1) due to the increase of the ACC throughflow and the maximum freshwater influence. During the summer, the circulation includes the cyclonic and anticyclonic gyres. During the winter, the circulation pattern is controlled by the basin‐scale cyclonic gyre and surface drift driven by the strong north‐easterly (south‐westward) wind forcing. The seasonal cycles of temperature (T) and salinity (S) vs. depth compare well with the observations. The simulated spring and autumn surface circulation patterns compare qualitatively well with the towed ADCP (acoustic Doppler current profilers) flow patterns and dynamic height patterns in the central Sound. An application of this model to zooplankton overwintering is discussed.  相似文献   

8.
Drift of propagules occurs within many populations inhabiting flow fields. This affects the number of propagules that rejoin their source population (recruitment) and plays a role in adaptive spatial redistribution. We focus on the cause and consequence of interannual variation in geographic distribution of population density among five cohorts of young‐of‐the‐year (age‐0) juvenile walleye pollock Gadus chalcogrammus in the western Gulf of Alaska (GOA). The coastal GOA is a wind‐driven advective system. Walleye pollock spawn during spring and their eggs and larvae drift southwestward; by late summer, age‐0 juveniles are variously distributed over the shelf. We found that high population densities of age‐0 juveniles (ca. 6 months old) near the southwestward exit of the Alaska Coastal Current from the GOA corresponded with high abundance of larvae from the major spawning area upstream, but did not translate into high abundance at older ages. Further, offshore and upwelling‐favorable winds were associated with the high downstream abundance and presumed export. In contrast, downwelling‐favorable (northeasterly) wind during and shortly after spawning (April–May) was associated with high recruitment at age 1. Finally, we found that recruitment also increased with apparent retention of age‐0 juveniles in favorable habitat upstream near the main spawning area. We hypothesize that wind‐related retention in superior upstream habitat favors recruitment. Our results argue for including wind‐driven transport in future walleye pollock recruitment models. We encourage more work on the juvenile stage of marine fishes aimed at understanding how transport and species‐specific habitat suitability interact to affect population response to large‐scale forcing.  相似文献   

9.
Nursery areas for juvenile fishes are often important for determining recruitment in marine populations by providing habitats that can maximize growth and thereby minimize mortality. Pacific ocean perch (POP, Sebastes alutus) have an extended juvenile period where they inhabit rocky nursery habitats. We examined POP nursery areas to link growth potential to recruitment. Juvenile POP were captured from nursery areas in 2004 and 2008, and estimated growth rates ranged from ?0.19 to 0.60 g day?1 based on differences in size between June and August. Predicted growth rates from a bioenergetics model ranged from 0.05 to 0.49 g day?1 and were not significantly different than observed. Substrate preferences and the distribution of their preferred habitats were utilized to predict the extent of juvenile POP nursery habitat in the Gulf of Alaska. Based on densities of fish observed on underwater video transects and the spatial extent of nursery areas, we predicted 278 and 290 million juvenile POP were produced in 2004 and 2008. Growth potential for juvenile POP was reconstructed using the bioenergetics model, spring zooplankton bloom timing and duration and bottom water temperature for 1982–2008. When a single outlying recruitment year in 1986 was removed, growth potential experienced by juvenile POP in nursery areas was significantly correlated to the recruitment time‐series from the stock assessment, explaining ～30% of the variability. This research highlights the potential to predict recruitment using habitat‐based methods and provides a potential mechanism for explaining some of the POP recruitment variability observed for this population.  相似文献   

10.
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11.
Biomass and duration of the over-wintering period of the Rügen spring spawning herring stock (RHS) in the Sound (ICES Subdivison 23) were investigated as well as possible hydrographical factors affecting relative distribution and triggering southwards migration towards the spawning grounds. Monitoring was performed during 27 surveys over a 6-year period (1993–1998). Abundance of 45–165 000 t in August–February, 5–60 000 t in March–May, and <2000 t in June–July was found. This indicates a longer over-wintering period than hitherto assumed. The year classes 1988, 1991 and 1994 were relatively strong occurring in the Sound from age 2 (winter-rings) and abundance in autumn 1993 and 1996–1997 was relatively high. Relative year class strengths are in accordance with variations in larval indices at the spawning grounds. Decreasing abundance in late spring in the Sound was concordant with observed peaks in commercial landings and (subsequent) peak larval indices at the spawning grounds. Larger size groups seem to migrate southwards in spring before smaller herring size groups. Peak herring densities occur in the northern Sound in autumn and they do not concentrate near the southern Drogden threshold in spring before southwards spawning migration. Highest densities were found from 8 to 22 m below sea surface, and just below or in the halocline in areas with stratified water column. Indications of inflows to the Baltic triggering southwards migration can be observed at some occasions, however, these results are not conclusive because of relatively few biomass estimates in 1996–1998. The identified patterns in herring occurrence agree with some conclusions on migration routes for RHS from previously reported tagging studies, but the present investigations indicate prolonged over-wintering in the Sound. This report quantifies the abundance and distribution of herring in the Sound over the year on basis of repeated investigations, and investigates possible influence of hydrographical factors on distribution and migration.  相似文献   

12.
《水生生物资源》2003,16(3):247-253
The Pacific herring Clupea pallasi population in Prince William Sound (PWS), Alaska, is both a valuable commercial resource and an important forage species for marine fish and wildlife. Historically, the herring were managed by a combination of age-structured models and egg deposition estimates. When these methods predicted a large return for spring 1993 that failed to materialize, we began surveying with echointegration–purse seine methods. After a decade of acoustic surveys, we show the new approach yields highly precise biomass estimates, which are consistent with historical measures of the miles of beach spawning. When compared, we show the traditional methods overestimated stock biomass, which resulted in harvest rates approaching 40%. In contrast, the acoustic methods are most likely to underestimate biomass. Since the acoustic estimates can be quickly obtained, we recommend their use to set harvest quotas for the fishery in the spring just prior to harvest. The shift from the traditional preseason to inseason management practices for herring in PWS is consistent with the Precautionary Principle by the fact that protection of the spawning population does not rely on the ability of science to predict how the population is changing. Furthermore, synoptic infrared measurements on our night-time acoustic surveys revealed herring to be the most important winter forage to marine birds and wildlife in PWS, including the endangered Steller sea lion Eumetopias jubatus. Given the importance of forage to marine birds and wildlife in the North Pacific during the extended winter conditions (October–March), the implementation of inseason management for herring using echointegration–purse seine techniques may be the most effective method to restore depressed populations of marine birds and mammals in the North Pacific.  相似文献   

13.
We studied the relationships between the planktonic food base and feeding patterns of juvenile mid-summer/early autumn Eurasian perch Perca fluviatilis L., a common predatory freshwater fish in large parts of Europe and Asia. The feeding of 0+ perch was studied during summer and autumn in littoral habitats of seven lakes with different environmental conditions – four Latvian (Auciema, Riebinu, Vārzgūnes, Laukezers) and three Estonian (Kaiavere, Prossa and Akste) lakes. Simultaneously, the abundance, biomass and structure of zooplankton communities were examined. We focused on the littoral areas because many studies in lakes suggest that littoral habitats are particularly important for 0+ fish growth and survival. We were interested in the question: can the diet and growth of 0+ perch be explained by zooplankton community structure? We also presumed that if the amount of zooplankton is low, more benthic invertebrates will be consumed by 0+ perch. Opposite to expectations, we found that zooplankton always counted for over 90% of diet biomass in perch. There were also clear correlations between the zooplankton biomass in a given lake, the zooplankton biomass in 0+ perch stomachs, and the fish growth rate. The study also suggested that nutrient enrichment can positively impact the 0+ perch feeding conditions in lakes.  相似文献   

14.
Fish stocks vary in abundance. The causes behind the fluctuations may be difficult to determine, especially ones caused by natural fluctuations, but long‐term data series may provide indications of the mechanisms. Assessments show that the recruitment to the Norwegian spring‐spawning herring (Clupea harengus, Clupeidae) has remained low since 2004, a year which produced the last really rich year‐class. Long time‐series of estimated recruitment and mean winter temperature in the ocean showed a significant positive correlation for the period 1921–2004. Here, we show that this positive correlation did not continue from 2005 onwards as the winter temperature increased to high levels while herring recruitment decreased and has remained low. The density of zooplankton in the drift route of the herring larvae dropped significantly after 2004, and their centre of gravity shifted northwards. There may currently be heavy predation on the larvae by Atlanic mackerel (Scomber scombrus, Scombridae), and top‐down regulation is suggested to hamper successful recruitment. Our analysis indicates that the presence of food and overlap with high food concentrations are likely important regulators of survival in herring larvae. The findings may be important for future management and planning of fisheries of this stock because recruitment failure may continue if temperature remains high and food abundance remains low.  相似文献   

15.
This study examines the relationships between first year growth of juvenile Prince William Sound herring, temperature and their food. We present time series of herring first year growth, determined from scale measurements as a proxy for herring length, water temperature and indices of multiple trophic levels of plankton obtained from Continuous Plankton Recorder (CPR) sampling on the adjacent Gulf of Alaska shelf. We show that there was a significant correlation between herring growth and water temperature, when the three warmest years were excluded (the mean July and August temperatures were greater than 12.5°C in 1989, 2004 and 2005). There were also strong, significant relationships between the abundance of appropriately sized (for first‐feeding herring) planktonic prey groups and herring growth. First year herring growth was greater in years with higher abundances of diatoms, microzooplankton and small mesozooplankton but not related to variability in abundance of larger mesozooplankton (such as euphausiids and large copepods). Furthermore, the strong interannual relationship between diatoms and herring growth held true even in the warmest years where the relationship between temperature and growth broke down. We also found seasonal timing and abundance changes in the plankton in warm years that would make the prey more abundant during the summer months immediately after metamorphosis of the herring larvae. We thus conclude that young‐of‐the‐year herring may grow better in warm years because the timing of key prey is a better match for their first feeding.  相似文献   

16.
Variation in growth and body size during critical life history stages can have important implications for life history schedules and survivorship. For Pacific herring (Clupea pallasii), there is still debate as to whether juvenile body size is governed by density‐dependent or ‐independent processes and few have evaluated whether the relative importance of either process shifts over the course of early ontogeny. We used a unique data set consisting of seasonal measurements of abundance, body size, and spatial distribution within a semi‐enclosed basin of Puget Sound (Washington State, U.S.A.) to measure the relative importance of temperature and cohort abundance on body size at distinct time periods, and evaluated whether density‐dependent habitat shifts might be responsible for density‐dependent growth. Over the 9 years of sampling (2001–2010) midsummer body size was positively related to temperatures experienced during the egg/yolk sac and larval stages and unrelated to cohort abundance. However, fall body size was negatively correlated with abundance and uncorrelated with both midsummer body size and temperature, indicating a shift from density‐independent to density‐dependent control over the course of the growing season. Thus, density‐dependent effects may supplant density‐independent effects exhibited early in herring life history. Our data on spatial distributions of herring and their zooplankton prey indicate that density‐dependent reductions in growth may be explained by density‐dependent habitat shifts that lead to reduce overlap of herring with zooplankton. Evidence of density‐dependent growth in marine fish populations is often attributed to exploitative competition, but our results suggest that these patterns may partly be mediated by density‐dependent distribution expansions in to prey‐poor habitat.  相似文献   

17.
Offshore‐ and deepwater‐spawning flatfish species face the problem of transport of their planktonic stages to shallow juvenile nursery grounds that are often far shoreward in bays or estuaries. We compare life history attributes of four offshore‐spawning flatfish species in the Gulf of Alaska: Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), rex sole (Glyptocephalus zachirus) and Dover sole (Microstomus pacificus) to examine how their larvae get from a spawning location at the edge or beyond the continental shelf to specific inshore nursery zones. We utilize historical records of survey catches of different life stages to characterize the stage‐specific changes in distribution of spawning, planktonic stages and juvenile nursery areas. We infer transport mechanisms based on the shifts in distribution of the life stages and in comparison with local physical oceanography. This comparison provides insight into the different mechanisms marine species may use to solve the common ‘problem’ of planktonic drift and juvenile settlement.  相似文献   

18.
Our collaborative work focused on understanding the system of mechanisms influencing the mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound, Alaska. Coordinated field studies, data analysis and numerical modelling projects were used to identify and explain the mechanisms and their roles in juvenile mortality. In particular, project studies addressed the identification of major fish and bird predators consuming juvenile salmon and the evaluation of three hypotheses linking these losses to (i) alternative prey for predators (prey‐switching hypothesis); (ii) salmon foraging behaviour (refuge‐dispersion hypothesis); and (iii) salmon size and growth (size‐refuge hypothesis). Two facultative planktivorous fishes, Pacific herring (Clupea pallasi) and walleye pollock (Theragra chalcogramma), probably consumed the most juvenile pink salmon each year, although other gadids were also important. Our prey‐switching hypothesis was supported by data indicating that herring and pollock switched to alternative nekton prey, including juvenile salmon, when the biomass of large copepods declined below about 0.2 g m?3. Model simulations were consistent with these findings, but simulations suggested that a June pteropod bloom also sheltered juvenile salmon from predation. Our refuge‐dispersion hypothesis was supported by data indicating a five‐fold increase in predation losses of juvenile salmon when salmon dispersed from nearshore habitats as the biomass of large copepods declined. Our size‐refuge hypothesis was supported by data indicating that size‐ and growth‐dependent vulnerabilities of salmon to predators were a function of predator and prey sizes and the timing of predation events. Our model simulations offered support for the efficacy of representing ecological processes affecting juvenile fishes as systems of coupled evolution equations representing both spatial distribution and physiological status. Simulations wherein model dimensionality was limited through construction of composite trophic groups reproduced the dominant patterns in salmon survival data. In our study, these composite trophic groups were six key zooplankton taxonomic groups, two categories of adult pelagic fishes, and from six to 12 groups for tagged hatchery‐reared juvenile salmon. Model simulations also suggested the importance of salmon density and predator size as important factors modifying the predation process.  相似文献   

19.
The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38–46 cm) and the winter–spring cohorts (ML range: 16–28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different‐sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter–spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food‐rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food‐rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer.  相似文献   

20.
The upper‐layer net‐zooplankton community in Prince William Sound, Alaska, is characterized by strong seasonality. Abundance and wet‐weight biomass in the upper 50 m drop to fewer than 100 individuals and 10 mg m?3 in February before rebounding to 5000 individuals and 600 mg m?3 in June. Copepods dominate in all months, but are augmented by other prominent taxa, particularly pteropods and larvaceans during the late spring, summer and fall. The small copepods Pseudocalanus, Acartia and Oithona are common. Though much less abundant, larger calanoids like Neocalanus, Calanus and Metridia contribute substantially to the biomass in spring and early summer. Meroplankters like barnacle nauplii are also occasionally very abundant. Neocalanus, Calanus and Pseudocalanus all exhibit ontogenetic vertical displacement of populations when stage 5 copepodites (C5) leave the surface in late May and early June for deep water. This seasonality has implications for food‐webs supporting juvenile pink salmon (Oncorhynchus gorbuscha) and Pacific herring (Clupea pallasi) in Prince William Sound.  相似文献   

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