Plankton-feeding Fish Research Articles

SPATIOTEMPORAL VARIABILITY OF DAPHNIA CRISTATA SARS, 1862 AND DIAPHANOSOMA BRACHYURUM (LIÉVIN, 1848) POPULATIONS IN LAKE LESCHOVOYE IN 2016–2020

The article presents results on the spatiotemporal variability of two zooplankton species – Daphnia cristata and Diaphanosoma brachyurum, in Lake Leschovoye. Surveys were conducted from June to September in 2016–2020. According to monitoring data from this period, the abundance of these species varied widely over time. Specific surveys at six sites in July–August 2019–2020 revealed also high spatial variation of the species’ abundances. Compared to the other species, D. cristata dominated in the lake with a 71 % share in Cladocera abundance. In 2020, however, D. brachyurum abundance was relatively high. In this period, its share in the cladoceran zooplankton reached 31 % and was twice that of D. cristata that year. A positive effect of water transparency on D. brachyurum abundance was observed. The study found no significant effect of other limnological parameters (gas conditions, pH, dissolved organic matter concentration, water color and turbidity) on the species. Another factor potentially influencing the species abundance is interspecific competition for food, as the range of particles consumed by the species is similar. Alongside competition, predation of plankton-feeding fishes and organic matter input with runoff from managed meadows can affect the abundance of the species. The effect of the runoff, nonetheless, can be dual. In some years, large amounts of allochthonous organic matter can lead to suppression of both species by clogging their filtration system. In other periods, runoff can lower water transparency thereby reducing the hunting efficiency of plankton-feeding fishes and leading to an increase in D. brachyurum abundance.

ワカサギ中の放射性セシウム濃度の体重依存性と経年変化

The accident of the Fukushima Dai-ichi Nuclear Power Plant in March, 2011, released large amounts of radiocesium into the atmosphere, and contaminated the environment of Gunma Prefecture in eastern Japan. In particular, 640 Bq kg–1-wet of radiocesium concentration was found in wakasagi (Hypomesus nipponensis) in Lake Onuma on Mt. Akagi in August, 2011. Thus, to elucidate the body-size effect in weight-dependent and dynamics of radiocesium in the ecosystem of Lake Onuma, we determined the age of wakasagi, the body-weight class of the radiocesium concentration in wakasagi and the effective ecological half-life (Teff) of radiocesium in wakasagi 0+ from 2012 to 2016. The body-size effect was found for the 137Cs concentration of wakasagi fished from 2012 to 2015, i.e., the concentration of 137Cs increased with the increase in its body weight. On the other hand, no body size effect was found in 2016. This result may be caused by the following two factors: the 137Cs concentration of the lake water reached a steady state after May 2014; wakasagi is a small plankton-feeding fish, while it is known that larger piscivorous fishes show the strong body size effect. Teff of the 137Cs concentration in wakasagi 0+ consists of two components, fast- and slow-term ones, and the decay rate of the 137Cs concentration in wakasagi 0+ was greatly reduced. As stated above, the radiocesium contamination in Lake Onuma has still been lasting; we are thus continuing our monitoring studies.

Use of stable isotopes to understand food webs in Macao wetlands

In this study, components of the food-web in Macao wetlands were quantified using stable isotope ratio techniques based on carbon and nitrogen values. The δ13C and δ15N values of particulate organic matter (δ13CPOM and δ15NPOM, respectively) ranged from −30.64 ± 1.0 to −28.1 ± 0.7 ‰, and from −1.11 ± 0.8 to 3.98 ± 0.7 ‰, respectively. The δ13C values of consumer species ranged from −33.94 to −16.92 ‰, showing a wide range from lower values in a freshwater lake and inner bay to higher values in a mangrove forest. The distinct dietary habits of consumer species and the location-specific food source composition were the main factors affecting the δ13C values. The consumer 15N-isotope enrichment values suggested that there were three trophic levels; primary, secondary, and tertiary. The primary consumer trophic level was represented by freshwater herbivorous gastropods, filter-feeding bivalves, and plankton-feeding fish, with a mean δ15N value of 5.052 ‰. The secondary consumer level included four deposit-feeding fish species distributed in Fai Chi Kei Bay and deposit-feeding gastropods in the Lotus Flower Bridge flat, with a mean δ15N value of 6.794 ‰. The tertiary consumers group consisted of four crab species, one shrimp species, and four fish species in the Lotus Flower Bridge Flat, with a mean δ15N value of 13.473 ‰. Their diet mainly comprised organic debris, bottom fauna, and rotten animal tissues. This study confirms the applicability of the isotopic approach in food web studies.

Changes in weight-at-length and size-at-age of mature Northeast Atlantic mackerel (Scomber scombrus) from 1984 to 2013: effects of mackerel stock size and herring (Clupea harengus) stock size

Abstract Weight-at-length and length-/weight-at-age were analysed for mature 3- to 8-year-old Northeast Atlantic mackerel (Scomber scombrus; n= 26 084) collected annually in autumn (September and October) at the end of the annual feeding season during 1984–2013 in the northern North Sea. The age range represented 92% of the mackerel stock size (age 3+). During the most recent decade, mackerel length- and weight-at-age continually declined. In 2013, the average mackerel was 3.7 cm shorter and weighed 175 g less than the average individual in 2002. Individual weight-at-length, demonstrating annual summer feeding success, continually declined during the most recent 5 years, whereas somatic growth of cohorts aged 3–8 continually declined for the last 11 of 25 cohorts investigated. Growth of the latest cohort was 34% of the maximum cohort growth recorded. Both weight-at-length and cohort growth were negatively affected by mackerel stock size and Norwegian spring-spawning herring (Clupea harengus) stock size (weight-at-length: r2 = 0.89; growth (length): r2 = 0.68; growth (weight): r2 = 0.78), while temperature was not significant. Conspecific density-dependence was most likely mediated via intensified competition associated with greater mackerel density. Negative effects of herring were likely mediated by exploitative competition for shared food resources rather than direct competition due to limited spatio-temporal overlap between mackerel and herring during the feeding season. Herring begin their seasonal feeding migration at least a month before mackerel; therefore, herring consumption influences prey availability for the later-arriving mackerel. Record low mackerel growth and negative effects of mackerel and herring stock size suggest that the carrying capacity of the Norwegian Sea and adjacent areas for plankton-feeding fish stocks have been reached. However, compounding effects of a less productive Norwegian Sea during the 30-year period cannot be excluded.

Productivity and biomass of fishes in the California Current Large Marine Ecosystem: Comparison of fishery-dependent and -independent time series

Commercial landings are often the only data available to assess the status of marine fish populations, but may reflect economics, technology and management policy in addition to stock size. In the California Current Large Marine Ecosystem (CCLME), the availability of CalCOFI ichthyoplankton data enabled us to compare commercial landings with a fishery-independent time series from 1951 to 2011. Fishery landings for the CCLME generally increased over this period, peaking about 1990 and displaying no clear subsequent trend. However, the abundance of ichthyoplankton in the southern CCLME has declined by ~70% since about 1970, based on the decline of taxa with cool-water affinities, apparently related to ocean warming and reduced productivity, and of mesopelagic taxa since the 1990s in response to declining midwater oxygen concentrations. These results highlight the importance of sustained fishery-independent time series at a time of secular climate change.Calibration of the larval fish abundance time series with stock assessments and acoustic/trawl surveys provided a proxy time series of spawning stock biomass for epi- and mesopelagic planktivores since 1951. We estimate that mesopelagic fishes comprised ~83% of the biomass of plankton-feeding fishes in the southern CCLME, but their prey consumption appears approximately equivalent to that of epipelagic planktivores because of the greater metabolic requirements of epipelagic fishes. The “wasp-waist” paradigm for eastern boundary currents needs to be re-assessed based on revised assessments for the abundance of mesopelagic fishes.

Physico-Chemical Quality and Plankton Density of Water in Duck-Fish Production Systems

This study was conducted to evaluate the effect of duck manure and spilled duck feed on water quality and production of Oreochromis niloticus in an integrated system with local duck breeds. Fish with an average body weight of 20.17±1.28g, were stocked into experimental ponds (area 72m 2 each) at a stocking density of 5 fish/m 2 in duplicate. Treatment 1 (T1) was integrated with 12 Mallard ducks (Anas platyrhynchos ) while Treatment 2 (T2) was integrated with 12 Muscovy ducks ( Cairina moschata ). Treatment 3 (T3) was used as control, without integration. Fish in T3 was fed compounded feed of 30% crude protein content three times a day while those in T1 and T2 fed on duck manure and spilled duck feed (15% crude protein content). Water quality parameters of the fish ponds, growth parameters of fish and ducks were monitored. After a 12-week period, mean weight gain of fish were 140.68, 122.11 and 157.19g in T1, T2 and T3 respectively while percentage survival was highest in T3 and lowest in T2. The levels of physicochemical parameters were generally favourable for fish production and dissolved oxygen (DO) was higher in the integrated treatments probably due to duck activity in the ponds. Plankton analyses shows that duck manure engenders the growth of plankton and this favours the growth of plankton-feeding fish species like O. niloticus . Mallard ducks are recommended for the duck-fish system since they performed better than Muscovy ducks both in survivability and in ability to stimulate physico-chemical and biological processes that engender fish growth. Original Research Article

Simulation of radioactive cesium transfer in the southern Fukushima coastal biota using a dynamic food chain transfer model

The Fukushima Dai-ichi Nuclear Power Plant (1F NPP) accident occurred on 11 March 2011. The accident introduced 137Cs into the coastal waters which was subsequently transferred to the local coastal biota thereby elevating the concentration of this radionuclide in coastal organisms. In this study, the radioactive cesium levels in coastal biota from the southern Fukushima area were simulated using a dynamic biological compartment model. The simulation derived the possible maximum radioactive cesium levels in organisms, indicating that the maximum 137Cs concentrations in invertebrates, benthic fish and predator fish occurred during late April, late May and late July, respectively in the studied area where the source was mainly the direct leakage of 137Cs effluent from the 1F NPP. The delay of a 137Cs increase in fish was explained by the gradual food chain transfer of 137Cs introduced to the ecosystem from the initial contamination of the seawater. The model also provided the degree of radionuclide depuration in organisms, and it demonstrated the latest start of the decontamination phase in benthic fish. The ecological half-lives, derived both from model simulation and observation, were 1–4 months in invertebrates, and 2–9 months in plankton feeding fish and coastal predator fish from the studied area. In contrast, it was not possible to similarly calculate these parameters in benthic fish because of an unidentified additional radionuclide source which was deduced from the biological compartment model. To adequately reconstruct the in-situ depuration of radiocesium in benthic fish in the natural ecosystem, a contamination source associated with the bottom sediments is necessary.

ZOOPLANKTON DIVERSITY OF AMBADI RESERVOIR TALUKA KINWAT, MAHARASHTRA

Plankton is most essential for many fishes as food. The growth of plankton feeding fishes mostly depends on plankton dynamics of the water body.In the complex chain of community metabolism, the different trophic strata, and a fraction of which ends up as fish flesh. Therefore, structure of different fish food biotic communities assumes greater significance to reservoir fish management. Ambadi reservoir in Kinwat Taluka of Nanded District of Maharashtra. represents 17 species of zooplankton.These include 6 species of rotifera, 4 species of copepoda, 6 species of cladocera and 1 species of ostracoda.

Prediction and comparative characteristic of the biological production of reservoirs on the Zeya and Bureya Rivers

A mass balance model is applied to assess the biotic energy flux in ecosystems of the operating Zeiskoe and Bureiskoe reservoirs and the anticipated Lower Zeiskoe and Lower Bureiskoe Reservoirs. A forecast of the mean annual biomass and annual production of phytoplankton, macrophytes, epiphytes, and phytobenthos (primary producers), bacterioplankton, bacteriobenthos (decomposers), “peaceful” and predatory zooplankton, zoobenthos, plankton-feeding, benthos-feeding, and predatory fish (consumers).

Deep sea megafaunal assemblages off the northern Iberian slope related to environmental factors

Four deep sea fish and invertebrate assemblages from the northern Spanish slope were identified and related to geographic and bathymetric factors. The Penas Cape comprises a boundary area splitting the more productive, wider and sandy western shelf from the less productive, narrow and muddy eastern part. The four faunal assemblages described were a shelf-slope transition assemblage (SST, 400-500 m); two upper slope assemblages (500-650 m), one corresponding to the western area of Cape Penas and one to the eastern area; and a middle slope assemblage (650-750 m). The SST was dominated by plankton-feeding fish and was hence less diverse than the other strata. This dominance was more evident west of the Penas Cape, where upwelling was stronger. Regarding invertebrates, plankton-feeding fish and benthos-feeding invertebrates dominated the SST, whereas zooplankton-feeding and deposit-feeding invertebrates dominated the middle slope. The western assemblages were richer in species as a consequence of a more complex food web, especially of guilds related to water column production: plankton-feeding fish and filter- and zooplankton-feeding invertebrates.

Zooplankton and forage fish species off Peru: Large-scale bottom-up forcing and local-scale depletion

The Humboldt Current System, like all upwelling systems, has dramatic quantities of plankton-feeding fish, which suggested that their population dynamics may ‘drive’ or ‘control’ ecosystem dynamics. With this in mind we analysed the relationship between forage fish populations and their main prey, zooplankton populations. Our study combined a zooplankton sampling program (1961–2005) with simultaneous acoustic observations on fish from 40 pelagic surveys (1983–2005) conducted by the Peruvian Marine Research Institute (IMARPE) and landing statistics for anchoveta ( Engraulis ringens) and sardine ( Sardinops sagax) along the Peruvian coast from 1961 to 2005. The multi-year trend of anchoveta population abundance varied consistently with zooplankton biovolume trend, suggesting bottom-up control on anchovy at the population scale (since oceanographic conditions and phytoplankton production support the changes in zooplankton abundance). For a finer-scale analysis (km) we statistically modelled zooplankton biovolume as a function of geographical (latitude and distance from the 200-m isobath), environmental (sea surface temperature), temporal (year, month and time-of-day) and biological (acoustic anchovy and sardine biomass within 5 km of each zooplankton sample) covariates over all survey using both classification and regression trees (CART) and generalized additive models (GAM). CART showed local anchoveta density to have the strongest effect on zooplankton biovolume, with significantly reduced levels of biovolume for higher neighbourhood anchoveta biomass. Additionally, zooplankton biovolume was higher offshore than on the shelf. GAM results corroborated the CART findings, also showing a clear diel effect on zooplankton biovolume, probably due to diel migration or daytime net avoidance. Apparently, the observed multi-year population scale bottom-up control is not inconsistent with local depletion of zooplankton when anchoveta are locally abundant, since the latter effect was observed over a wide range of overall anchoveta abundance.

Trophic relationships of the spinner dolphin at Fernando de Noronha Archipelago, SW Atlantic

We present an overview of predator-prey and other trophic relationships of spinner dolphins ( Stenella longirostris ) around Fernando de Noronha Archipelago, off northeastern Brazil, with use of original data and a brief review of data published elsewhere. Records were made while snorkelling among the dolphins in daytime. Individual fish pursuit and coordinated school herding were the two hunting tactics recorded. Three small prey types (oceanic squids, fishes, and prawns) were recovered from vomits collected in situ and from stomachs of two stranded dolphins. In their turn, spinners were preyed on by the cookiecutter sharks (Dalatiidae) as evidenced by round and crater-like wounds and circular scars. Additionally, the dolphins were preyed on by large sharks (Lamnidae, Carcharhinidae), as evidenced by crescent-shaped wounds and scars. Unidentified fishes bit fin pieces, as evidenced by variably-shaped marks. On the other hand, dolphins’ particulate faeces, vomits and live roundworms were eaten by plankton-feeding fishes. Thus, the trophic role of the spinner dolphins of Fernando de Noronha may be summarised as that of : 1) a predator of small oceanic squids, fishes, and prawns; 2) a prey for the small, piece-eating cookiecutter sharks and other unidentified fishes; 3) a prey for large sharks able to kill a dolphin; and 4) a particulate food supplier for plankton-eating fishes.

Tracing Mississippi River influences in estuarine food webs of coastal Louisiana

The Breton Sound estuary in southern Louisiana receives large amounts of Mississippi River water via a controlled diversion structure at the upstream end of the estuary. We used stable isotopes to trace spatial and seasonal responses of the downstream food web to winter and spring introductions of river water. Analysis of delta13C, delta15N, and delta34S in the common local consumers such as grass shrimp (Palaemonetes sp.), barnacles (Balanus sp.), and small plankton-feeding fish (bay anchovies, Anchoa mitchilli) showed that the diversion was associated with two of the five major source regimes that were supporting food webs: a river regime near the diversion and a river-influenced productive marsh regime farther away from the diversion. Mixing models identified a third river-influenced source regime at the marine end of the estuary where major natural discharge from the Bird's Foot Delta wraps around into estuarine waters. The remaining two source regimes represented typical estuarine conditions: local freshwater sources especially from precipitation and a brackish source regime representing higher salinity marine influences. Overall, the Mississippi River diversion accounted for 75% of food web support in the upper estuary and 25% in the middle estuary, with influence strongest along known flow pathways and closest to the diversion. Isotopes also traced seasonal changes in river contributions, and indicated increased plant community productivity along the major flow path of diversion water. In the Breton Sound estuary, bottom-up forcing of food webs is strongly linked to river introductions and discharge, occurring in spatial and temporal patterns predictable from known river input regimes and known hydrologic circulation patterns.

Relationships between rocky-reef fish assemblages, the sea urchin Diadema antillarum and macroalgae throughout the Canarian Archipelago

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 278:157-169 (2004) - doi:10.3354/meps278157 Relationships between rocky-reef fish assemblages, the sea urchin Diadema antillarum and macroalgae throughout the Canarian Archipelago Fernando Tuya1,*, Arturo Boyra1, Pablo Sanchez-Jerez2, Carmen Barbera1, Ricardo J. Haroun1 1Department of Biology, Faculty of Marine Sciences, Campus Tafira, University of Las Palmas de Gran Canaria, 35017 Las Palmas, Canary Islands, Spain 2Marine Biology Laboratory, University of Alicante, POB 99, 03080 Alicante, Spain *Email: fernando@canariasporunacostaviva.org ABSTRACT: In situ visual surveys using a hierarchical sampling design were carried out at 36 sublittoral rocky locations along the central-east Atlantic Canarian Archipelago to find relationships among (1) benthic primary producers, (2) the demographic structure of the herbivorous sea urchin Diadema antillarum Phillipi and (3) the trophic structure of coastal fish communities. Our correlation approach displayed a relationship between the lack of large macroinvertebrate-eating predatory fish and the increase in density of sea urchins, in addition to a decrease in fish richness. In contrast, increases in fast-growing plankton-feeding fish species were detected. The size structure of D. antillarum is dominated by small-to-intermediate sized sea urchins in environments with a high density of individuals, whereas low sea urchin density locations are characterized by the dominance of large sized individuals. The physical complexity of the substrate seems to play an important role in determining the local patchiness of D. antillarum. Finally, a non-linear decrease in the percentage of fleshy macroalgal cover with increasing density of D. antillarum was observed. We therefore propose D. antillarum as a key herbivorous species, which plays an important role in determining the structure of shallow, hard-substratum, infralittoral benthic communities throughout the Canary Islands. KEY WORDS: Urchin-fish interactions · Trophic cascades · Diadema antillarum · Sea urchins · Fish assemblages · Macroalgae · Canary Islands Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 278. Online publication date: September 06, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Domoic acid in planktivorous fish in relation to toxic Pseudo-nitzschia cell densities

In at least two mass mortality events in Monterey Bay, California, planktivorous fish were implicated as vectors of the neurotoxin domoic acid (DA) from diatoms to sea birds and marine mammals. Whereas the transfer of DA from planktivorous fish to piscivorous predators has been well established, the relationship between toxin levels in plankton-feeding fish and the regional abundance of DA-producing diatoms has not been documented. Here we present data from an extensive field study in which cell densities of toxic Pseudo-nitzschia species and DA levels in anchovies and sardines were measured from samples collected weekly throughout Monterey Bay from 8 October 1999 to 18 October 2000. Four distinct blooms were documented with cell densities ranging from 3.2×103 to 5.0×105 cells l–1. DA was detected in fish viscera samples whenever toxic diatom densities reached ≥103 cells l–1 in surface waters, suggesting that anchovies and sardines regularly consume toxic diatoms when present. Fish contained DA levels above the regulatory limit (20 µg DA g–1 whole fish) only when toxic cell densities exceeded 104 cells l–1. DA was only detected in fish when toxic diatom species were also present in the water, suggesting that the toxin is quickly depurated and that fish are only dangerous vectors during the bloom period. Anchovies appear to be more potent vectors than sardines as they consistently contained more DA than sardines collected simultaneously. Maximum DA levels detected in fish were 1,815 µg DA g–1 in anchovy and 728 µg DA g–1 in sardine viscera samples. In fish with high viscera levels of DA, corresponding body tissues contained 0.2–2.2 µg DA g–1 (0.2±0.1% of the viscera level), suggesting that DA is not accumulated in edible body tissues to levels that threaten human consumers. Results from this study suggest that anchovies may be a valuable indicator species for assessing the risk of DA intoxication to piscivorous sea birds and mammals during toxic Pseudo-nitzschia blooms in Monterey Bay.

Effects of Menhaden Predation on Plankton Populations in Narragansett Bay, Rhode Island

The Atlantic menhaden,Brevoortia tyrannus, is an abundant plankton-feeding fish that undertakes extensive seasonal migrations, moving from overwintering locations offshore south of Cape Hatteras to the mid-Atlantic Bight and New England inshore waters during spring and summer. A bioenergetic model, based on field and laboratory studies, shows that when large numbers of menhaden enter Narragansett Bay, Rhode Island, during spring and early summer, they significantly influence plankton populations through size-selective grazing and nutrient regeneration. A population biomass of 9.1×106 kg of menhaden feeding for 12 h each day in the upper bay would result, in a substantial reduction of the instantaneous growth rate of the >20-μm phytoplankton. Instantaneous growth rates of zooplankton would be negative if the same population of menhaden was present, resulting in a reduction in the biomass of zooplankton. Given the ambient phytoplankton and zooplankton populations, menhaden could achieve the seasonal growth measured in Narragansett Bay during 1976 by feeding on average about 5 h d−1. Daily nitrogen excretion rates of the 9.1×106 kg menhaden population were 56.4% of the mean standing stock of ammonia-N in the upper bay. Because menhaden travel in schools their effects are likely to be intense but strongly localized, increasing spatial heterogeneity in the ecosystem. When the fish migrate southward in the fall they transfer between 3.3% and 6.2% of the nitrogen exported annually from the bay.

Trophic Status of Phewa, Begnas and Rupa Lakes in Pokhara Valley, Nepal:Past, Present and Future

Harnessing the natural resources is one of the basis of natural economy in developing countries. The wise use of such resources is very important to sustain the balance between immediate benefits and maintenance of the ecosystem. In Phewa, Begnas and Rupa lakes of Pokhara Valley, plankton feeding fish farming in net cage,enclosure and open water stocking is one of the effective example of natural resources utilisation which sustains a number of households in surrounding lakes for economic activities. These lakes are also used for drinking water, hydroelectricity, irrigation and recreation etc. However, the understanding of trophic status of the lakes is very important for long term sustainable use of the lakes in harmony with human activities. Here, we present the trophic status of three lakes of Pokhara Valley and discuss the impacts of human and natural activities on the trophic status of the lake. The study shows that heavy rain fall in the valley during monsoon is one of the strongest natural forces which flush out the accumulated nutrients from the lakes and migrate the eutrophication processes. Recommendations for sustainable use of lake water have also been discussed.;Harnessing the natural resources is one of the basis of natural economy in developing countries. The wise use of such resources is very important to sustain the balance between immediate benefits and maintenance of the ecosystem. In Phewa, Begnas and Rupa lakes of Pokhara Valley, plankton feeding fish farming in net cage,enclosure and open water stocking is one of the effective example of natural resources utilisation which sustains a number of households in surrounding lakes for economic activities. These lakes are also used for drinking water, hydroelectricity, irrigation and recreation etc. However, the understanding of trophic status of the lakes is very important for long term sustainable use of the lakes in harmony with human activities. Here, we present the trophic status of three lakes of Pokhara Valley and discuss the impacts of human and natural activities on the trophic status of the lake. The study shows that heavy rain fall in the valley during monsoon is one of the strongest natural forces which flush out the accumulated nutrients from the lakes and migrate the eutrophication processes. Recommendations for sustainable use of lake water have also been discussed.

河口湖における魚類相の変遷とそれをとりまく環境

This lake was mainly stocked with Carassius cuvieri and Hypomesus by the Kawaguchiko Fishermen's Association until the 1980s. However, since Micropterus salmoides salmoides were confirmed to exist in the lake in 1973, by early 1985 Hypomesus were no longer caught. This condition continued for a short period ; then, in the 1990s, the fish species composition showed a great change in the lake. Micropterus salmoides salmoides and Salmo (Oncorhynchus) mykiss became the main catch. As a result, 370, 000 anglers visited Lake Kawaguchi in 1996, against 35, 000 in 1978. The total number of anglers who visit the lake is increasing rapidly. At present, Lake Kawaguchi has become the most famous lake for fly fishing in Japan. In order to respond to the demand of so many anglers, the Kawaguchiko Fishermen's Association stocks the lake with many species of carnivorous fish. Moreover, they annually stock an abundance of small plankton-feeding fish which in turn become the food of carnivorous fish. Owing to the above circumstances, Lake Kawaguchi has become like an artificial fishing pond, with disturbing effects on the lake ecosystem. The ecological impact on other aquatic organisms has become a cause for concern.

Trophic Response of Fishes to Habitat Variability in Coastal Seagrass Systems

Shallow coastal areas of the northeast Gulf of Mexico are physically unstable in terms of short—term, seasonal, and year—to—year changes in temperature, salinity, nutrient concentration, and other water quality features. A 9—yr comparison was made of two estuaries, one polluted and one in the natural state, to determine the response of fish assemblages to habitat alteration in space and time. During the study period, extreme natural habitat changes due to storm water runoff and low winter temperatures were superimposed over water quality changes (increased color, turbidity, nutrients; reduced dissolved oxygen) associated with release of pulp mill effluents. Various grassbed fishes followed regular seasonal, age—specific feeding patterns, which did not change substantially in terms of qualitative food composition in the unpolluted estuary over a 7—yr period of observation. Such feeding behavior helped to explain temporally conservative cycles of relative abundance despite extreme (natural) habitat change. Anthropogenous habitat alterations, though seemingly slight, were associated with reductions in benthic macrophyte distribution, enhanced phytoplankton productivity, and changes in the relative dominance and numerical abundance of associated fish assemblages. Grassbed species were replaced by plankton—feeding fishes, and disruption of feeding habits of various species was apparent in the affected area relative to the unpolluted estuary. Those fishes dependent on specific benthic food organisms altered their feeding habits during the years of pollution in the affected estuary. Subsequent water quality improvement over time was associated with shifts in the age—specific dietary patterns of various species toward those observed in the unaffected estuary, although such recovery varied from species to species according to habitat utilization and trophic needs. From these results, it is clear that a relatively complex coastal seagrass system exposed to periodic, extreme natural disturbance is relatively resilient to such changes in terms of relative dominance and food web structure. However, apparently slight water quality changes due to pollution, which are outside the evolutionary experience of the biotic components, can cause serious disruptions of the basic habitat structure, energy flow, and community composition of the grassbed assemblages at various levels of biological organization.

Effects of the Establishment of Mysis relicta on the Macrozooplankton of a Large Lake

The effects of the introduction of opossum shrimp Mysis relicta on the macrozooplankton of Pend Oreille Lake, Idaho were examined from recent and historic zooplankton data. Establishment of M. relicta was associated with a temporal displacement in the seasonal occurrence of Daphnia and Bosmina species and a change in the species composition of Daphnia present. The amount of interaction between cladocerans and mysids appeared to be related to the vertical distribution of each. The cladocerans appeared and increased in abundance as the mysids became seasonally isolated from near-surface strata by increasing thermal stratification and transparency. A common negative response of cladocerans to mysid introduction is supported by the literature. Such introductions may result in the reduction of food available for kokanee Oncorhynchus nerka and other plankton-feeding fish.