Showing papers in "Environmental Biology of Fishes in 1982"
TL;DR: It is demonstrated that ASR greatly enhances survival in hypoxic water, even in fish not morphologically specialized to use the surface film, and that with extreme deoxygenation fish perform ASR over 90% of the time.
Abstract: Use of the surface water for aquatic respiration (aquatic surface respiration, ASR) is one of the few alternatives to aerial respiration which allow fish to survive extreme hypoxia, yet it has received very little attention. This report examines three generalizations concerning ASR on a phylogenetically and geographically diverse range of tropical freshwater fishes. It demonstrates that ASR greatly enhances survival in hypoxic water, even in fish not morphologically specialized to use the surface film, that ASR is initiated at a distinct threshold oxygen concentration, with time spent at the surface increasing rapidly as O2 declines, and that with extreme deoxygenation fish perform ASR over 90% of the time. Ninety-four percent of the 31 species of non-air breathing fish tested showed ASR., with the threshold oxygen concentration ranging from 6 to 40 torr.
218 citations
TL;DR: Evidence of a direct rate of thyroid hormones in the regulation of migration, salmonid smoltification, oxygen consumption, and osmotic or ionic regulation although highly suggestive in a number of areas is inconclusive and requires further critical experimental evaluation.
Abstract: Recent studies of thyroid hormone function are reviewed as they relate to the environmental physiology of teleost fish. In addition, reports dealing with the apparent interdependence of thyroid gland function with that of other endocrine glands are discussed with emphasis on the interrelated endocrine response associated with changing physiological status of teleosts.
138 citations
TL;DR: Temperature is less limiting for these fishes in Florida than was previously recognized, and temperatures associated with reduction in feeding, cessation of feeding, loss of equilibrium and death were identified by decreasing water temperature 1°C day−1.
Abstract: The most important factor affecting the potential range of 14 non-native fishes in Florida appears to be their lack of tolerance to low temperatures. In this study, temperatures associated with reduction in feeding, cessation of feeding, loss of equilibrium and death were identified by decreasing water temperature 1°C day−1. Fishes tested and their mean lower lethal temperatures were: Astronotus ocellatus (12.9°C), Belonesox belizanus (9.7°C), Cichlasoma bimaculatum (8.9°C), C. cyanoguttatum (5.0°C), C. meeki (10.3°C), C. octofasciatum (8.0°C), C. trimaculatum (10.9°C), Clarias batrachus (9.8°C), Hemichromis bimaculatus (9.5°C), Hypostomus sp. (11.2°C), Tilapia aurea (6.2°C), T. mariae (11.2°C), T. melanotheron (10.3°C) and T. mossambica (9.5°C). These data indicate that temperature is less limiting for these fishes in Florida than was previously recognized.
114 citations
TL;DR: The superior ability of the flounder in maintaining OZ uptake across the gills during a reduction in water O2 tension may in part explain why the species, unlike plaice, inhabits very shallow waters with large fluctuations in dissolved oxygen.
Abstract: Gill ventilation, breathing frequency, breath volume, oxygen extraction from the ventilatory water current and oxygen uptake through the gills were measured in flounder, Platichthys flesus, and plaice, Pleuronectes platessa, at water O2 tensions ranging from 35 to 155 mm Hg at 10° C. Ventilation volumes were similar in the two species at high water O2 tension. Exposure to hypoxic water elicited a larger increase in ventilation in the flounder. The per cent extraction of O2 from water decreased slightly in both species as water O2 tension was lowered. At comparable levels of ventilation O2 extraction was higher in flounder. At the higher levels of water O2 tension, O2 uptake across the gills of flounder was stable, the critical O2 tension being between 60 and 100 mm Hg. The plaice behaved as an oxygen conformer over the entire range of O2 tensions investigated. The superior ability of the flounder in maintaining OZ uptake across the gills during a reduction in water O2 tension may in part explain why the species, unlike plaice, inhabits very shallow waters with large fluctuations in dissolved oxygen.
97 citations
TL;DR: Small size, low metabolic rate, tolerance of low oxygen conditions and reduced activity resulted in reduced demand for dissolved oxygen, and head shape, ventilation rate, vertical movement and utilization of high oxygen microzones also enhanced exploitation of low levels of dissolved oxygen.
Abstract: Three species (central mudminnow, fathead minnow and brook stickleback) survive when a northern Wisconsin lake becomes anoxic in winter. Some gas bubbles beneath the ice contained as much as 11° oxygen when the lake water contained <0.30 mg 1−1 dissolved oxygen. Experiments conducted in the field determined that gas bubbles prolonged survival of all species, especially the mudminnow and stickleback. In the laboratory, brook sticklebacks exhibited the lowest and fathead minnows the highest routine metabolic rate corrected for weight. Rate of gill ventilation of all three increased from 20 to 70 beats per minute as oxygen levels declined from 4.0 to 0.25 mg 1−1. At low oxygen levels they moved to the upper one-third of the test tanks. Small size, low metabolic rate, tolerance of low oxygen conditions and reduced activity resulted in reduced demand for dissolved oxygen. Head shape, ventilation rate, vertical movement and utilization of high oxygen microzones also enhanced exploitation of low levels of dissolved oxygen. Central mudminnows used oxygen directly from gas bubbles found under the ice.
88 citations
TL;DR: Three species of dasyatid stingrays in Southeast Asia are presumed to be endemic freshwater species or are known only from freshwater habitats: Hemantura krempfi (Chabanaud, 1923), known mainly from specimens obtained at Pnom Penh, Kampuchea, without precise indication of habitat;Himantura signifer new species, described from specimens caught in the Kapuas basin, western Borneo, and tentatively identified as occurring also in the Indragiri River, Sumatra, Perak River, western Malay Peninsula, and
Abstract: Three species of dasyatid stingrays in Southeast Asia are presumed to be endemic freshwater species or are known only from freshwater habitats:Himantura krempfi (Chabanaud, 1923), known mainly from specimens obtained at Pnom Penh, Kampuchea, without precise indication of habitat;Himantura signifer new species, described from specimens caught in the Kapuas basin, western Borneo, and tentatively identified as occurring also in the Indragiri River, Sumatra, Perak River, western Malay Peninsula, and Chao Phrya River, Thailand; and an unidentifiedDasyatis orHimantura from the Mekong River in Laos. In addition to these, an unidentifiedHimantura is reported from the Fly River basin; this is the first record of a stingray from fresh water in New Guinea.
87 citations
TL;DR: Though the fish species studied probably compete for food in the summer and fall, this competition did not account for the community structure observed, and diet breadths for all species increased as food levels declined, indicating that these fishes experienced resource depression.
Abstract: The purpose of this study was to evaluate the intensity of competition for food among 9 species of stream fishes that primarily eat aquatic invertebrates. The taxonomic and size composition, and numerical abundance of aquatic invertebrates were monitored for one year using drift and benthic samples. Diet data were obtained from stomachs of fishes captured at the same time and place that invertebrate sampling was done. Diet characteristics examined included taxonomic and size composition, number of prey per fish, and diet breadth. Drifting invertebrates were more abundant early in the year (March–June) than later (July–January). The summer-early fall scarcity of invertebrates was especially notable among those>3.6 mm long, which comprised the bulk of prey found in fish stomachs. Average prey size eaten by a fish species was positively correlated with fish mouth size, but interspecific overlap in prey size was extensive. Cyprinids as a group (5 species) ate proportionally fewer small (< 3.6 mm long) prey from July to January than did the centrarchids and stonecat. Taxonomic compositions of available invertebrates and fish diets varied markedly among sampling dates, but the use of prey taxa by fishes was not correlated with the availability of those taxa. Use of aquatic prey taxa was generally similar among fish species, but cyprinids as a group ate proportionally more terrestrial prey from July to January than did the centrarchids and stonecat. Diet breadths for all species increased as food levels declined, indicating that these fishes experienced resource depression. Food scarcity was evidently more severe for cyprinids since their stomachs contained few prey through the summer and fall relative to the centrarchids and stonecat. Though the fish species studied probably compete for food in the summer and fall, this competition did not account for the community structure observed.
78 citations
TL;DR: The use of median weights of stomach contents was found to yield more meaningful estimates of the daily ration of individual bass than those based on means, which were consistent with the observed pattern of growth and with information on maintenance rations and satiation levels.
Abstract: Sample of stomach contents collected on 10 dates from May to October, 1978, were used to describe the diet and estimate the daily ration of subadult largemouth bass (primarily age-III fish) in Lake Rebecca, Minnesota. The method of Elliott & Persson (1978) was used to estimate daily ration. Data from sources in the literature were used to quantify gastric evacuation, which was found to be adequately described by an exponential decay model. The exponent of gastric evacuation increased exponentially with temperature. Seasonal changes in the diet with respect to composition, distribution of food among stomachs, and food particle size were reflected in the seasonal pattern of growth. Weight gain and the formation of scale annuli did not occur until the diet shifted from large bluegills and insects to age-0 largemouth bass and bluegills. Estimates of daily ration ranged from almost zero in mid-May and early June to over 5% in late August. The use of median weights of stomach contents was found to yield more meaningful estimates of the daily ration of individual bass than those based on means. Estimates based on medians were consistent with the observed pattern of growth and with information on maintenance rations and satiation levels. A growth-limiting lack of suitably-sized forage fish apparently occurred in the early part of the growing season.
70 citations
TL;DR: Observations in Lake Sibaya suggest the importance of behavioral and throphic plasticity in the ability of tilapia to utilize unstable habitats.
Abstract: JuvenileTilapia mossambica Peters (2.5–10.0 cm standard length) in Lake Sibaya, South Africa, move daily from deep offshore waters to shallow (<0.5 m) littoral areas where they feed for several hours and then return to deep water. The timing of these movements varies in response to changing physical and biological features of the littoral environment. During this study (December 1973 – February 1976), lake level rose more than 1 m. At the start of the study, littoral areas visited by juvenileT. mossambica were free of vegetation and debris, but at higher lake levels trees and brush from the eroding shoreline were common in nearshore littoral waters. When the littoral zone was free of debris, juvenileT. mossambica visited nearshore waters only during daylight hours. After a 0.65 m rise in lake level and considerable accumulation of debris, these fish were abundant in the littoral zone only at night. This reversal can be attributed to efforts by juvenileT. mossambica to avoid their principal predator, the catfishClarias gariepinus. At low lake levelC. gariepinus ventured into the littoral only at night but at higher lake levels this predator was found within flooded vegetation during the day. When further increases in lake level flooded marginal grasslands, juvenileT. mossambica abandoned the littoral zone in favor of this newly created eulittoral habitat where potential fish predators did not occur. With the change in habitat, the diet changed from benthic detrital aggregate to periphyton. These observations suggest the importance of behavioral and throphic plasticity in the ability of tilapia to utilize unstable habitats.
64 citations
TL;DR: In this article, a 22 daylight submersible dives in August 1979 numerous juvenile and adult tilefish, Lopholatilus chamaeleonticeps, were observed in and around vertical burrows in the clay substrate of portions of Hudson submarine canyon in depths from 110-230m.
Abstract: During 22 daylight submersible dives in August 1979 numerous juvenile and adult tilefish, Lopholatilus chamaeleonticeps, were observed in and around vertical burrows in the clay substrate of portions of Hudson submarine canyon in depths from 110–230 m. The size and shape of the burrows varied considerably with the smallest juveniles occupying simple vertical shafts in the substrate. Larger fish were found in much larger burrows (up to 4–5 m in diameter and at least 2–3 m deep) that were funnel shaped in cross-section with the upper conical portions containing numerous smaller burrows of associated crabs. The range of burrow sizes observed suggests a regular sequence of burrow construction by tilefish and the associated crabs. Both juvenile and adult tilefish swam into the burrows head first and exited tail first. This behavior, which would preclude the possibility of ambushing prey, and evidence of predation by sharks and other tilefish, suggests that the burrow is a refuge from predators. Tilefish burrows appear to serve as a focus for biological activity. Species associated with the burrows included galatheid crabs, Cancer sp., Acanthocarpus alexandri, Homarus americanus, Heliocolenus dactylopterus and Conger oceanicus. Tilefish may play an important role in structuring outer continental shelf communities. They physically shape their environment and probably have significant biological interactions with the species that associate with their burrows.
59 citations
TL;DR: A total of 377 Pristis perotteti tagged in the Lake Nicaragua-Rio San Juan System, 214 (56.8% were recovered) were recovered as discussed by the authors.
Abstract: Of a total of 377 Pristis perotteti tagged in the Lake Nicaragua-Rio San Juan System, 214 (56.8% were recovered. Eighty were recovered at the original tagging site; four moved downstream the full length of the river; and 127 tagged at the source of the river were recovered in all parts of the lake. Only one was recovered in a different river system, 58 km down the coast from the main mouth of the Rio San Juan.
TL;DR: The damming of the Rio Bayano and the creation of a large impoundment in 1976 provides a test of the ability of members of both species trapped there to survive and to establish breeding stocks, confirming the ability to survive in fresh water for long periods.
Abstract: The Rio Bayano in eastern Panama is one of many tropical rivers in which bull sharks (Carcharhinus leucas) and largetooth sawfish (Pristis perotteti) have been known to occur. Since both species can osmoregulate in fresh water throughout life, theoretically, both could survive in landlocked situations for many years.P. perotteti reproduces in fresh water, butC. leucas ordinarily does not, so only the former would appear to have the potential for establishing a breeding stock in such a landlocked freshwater body.
TL;DR: Predator-prey data obtained in a study of feeding ecology of land-locked alewife,Alosa pseudoharengus, in Claytor Lake, Virginia, were subjected to two indices of electivity as well as a nonparametric paired-comparisons statistical test and demonstrated the importance of considering the size composition of zooplankton populations in prey preference studies.
Abstract: Predator-prey data were obtained in a study of feeding ecology of land-locked alewife,Alosa pseudoharengus, in Claytor Lake, Virginia, and subjected to two indices of electivity as well as a nonparametric paired-comparisons statistical test. Values obtained from the two indices were often contradictory with each other and the statistical test. Attributes of the statistical procedure render it more appropriate than electivity indices to describe feeding selectivity in fish populations. Results of the study also demonstrated the importance of considering the size composition of zooplankton populations in prey preference studies; measures of selection were often positive for zooplankters ≥ 1.0 mm while negative for those of the same prey group < 1.0 mm length. Because numerous problems are inherent with in situ feeding selectivity studies of planktivorous fishes, selectivity determinations, regardless of methodology, should be construed in a relative rather than absolute context.
TL;DR: Differences in life history traits are interpreted in terms of mechanisms permitting the coexistence of these closely-related territorial fish.
Abstract: The life history traits (longevity, growth, age at maturity, fecundity, egg size and length of breeding season) were compared for four sympatric sticklebacks (Gasterosteidae). Significant differences occurred for growth, longevity, age at maturity and length of breeding season, but not for egg diameters. Differences in life history traits are interpreted in terms of mechanisms permitting the coexistence of these closely-related territorial fish. Egg numbers were lowest inApeltes quadracus and highest inGasterosteus aculeatus withGasterosteus wheatlandi andPungitius pungitius having similar and intermediate fecundities.
TL;DR: The sexual structures of the populations of four Caribbean gobies were examined, finding the two hermaphroditic species live in large populations while the two gonochorists live in small groups.
Abstract: The sexual structures of the populations of four Caribbean gobies were examined. Gobiosoma multifasciatum and Coryphopterus personatus are protogynous hermaphrodites, while G. illecebrosum and G. saucrum are gonochoristic. Sex changes were induced in females of the first two species in the laboratory, but not in females of the latter two species. The two hermaphroditic species live in large populations while the two gonochorists live in small groups. The significance of these findings to models of the evolution of protogyny are discussed.
TL;DR: Morphological and geological evidence and some comparative litter characteristics suggest that life history differences among populations in isolated drainages are in part genetic.
Abstract: The life history characteristics of tule perch were compared within and among populations in three drainages with substantially different environmental conditions. Within populations there were, in general, substantial increases in brood size, size of young, brood weight, and gonadal-somatic index (GSI) with age and with size of female. In at least one population there was a trade-off between the number and size of young produced. However, in a lake where individual growth was very slow, brood weight, GSI, and other life history characters showed decreasing trends with age. Among populations in isolated drainages, female length at first reproduction and longevity varied directly and mean brood size varied inversely with environmental predictability. Morphological and geological evidence and some comparative litter characteristics suggest that life history differences among populations in isolated drainages are in part genetic.
TL;DR: The behavior of this species was similar to other studied wrasses, with the exception of a post-spawning display by males and the regular formation of multi-male aggregations in the late afternoon.
Abstract: The wrasse Coris dorsomaculata was investigated at Miyake jima, Japan. This species is sexually dichromatic and males arise from sex-changed females. Individuals were found living in harems which may be maintained through the defense of food by dominant males. The behavior of this species was similar to other studied wrasses, with the exception of a post-spawning display by males and the regular formation of multi-male aggregations in the late afternoon. Reproductive activity appeared to be affected by the warm-temperate climate at the study area. When day length was long, the spawning time coincided with outgoing tidal currents. When day length was short, the daily spawning time became fixed.
TL;DR: An experiment to identify the point of no return of fish alevins and larvae has been made using the proteolytic activity as an index using Coregonus pollan and Salmo gairdneri larvae as a control and proteolytics enzymes measured.
Abstract: An experiment to identify the point of no return of fish alevins and larvae has been made using the proteolytic activity as an index. Larvae of Coregonus pollan and Salmo gairdneri were starved or fed with zooplankton as a control and proteolytic enzymes measured. The lowest value of the total proteolytic activity coincides with the point of no return expressed in degree-days based on survival trials.
TL;DR: Slimy sculpins (Cottus cognatus) were caught in the littoral region of Toolik Lake, an arctic lake and the hypothesis that this ‘preferred’ area provided both increased prey and reduced predation was advanced.
Abstract: Slimy sculpins (Cottus cognatus) were caught in the littoral region of Toolik Lake, an arctic lake. These sculpins grew slower and lived longer (71 mm at 8+ years) than more southerly populations. Sculpin distribution along the slope of the rocky littoral zone was greatest at the 3.5 m depth and coincided with the rock-mud interface. The hypothesis that this ‘preferred’ area provided both increased prey and reduced predation was advanced. Yearly production estimate of littoral sculpins was 0.40 g · m−2 · yr−1 with a P/B ratio of 0.36. This is considerably less than estimates for more southerly populations of the same species.
TL;DR: One male was reproductively active for 73 days during which it completed five distinct reproductive cycles, and spawned with up to five females in short succession and remained with the eggs until hatching.
Abstract: The reproductive biology of one of the poorly known armoured catfishes, Loricaria uracantha, was studied in Panama. Natural nesting sites consisted of cavities in pieces of wood, open at both ends and positioned above the stream bottom, but artificial cavities of PVC pipe were readily used. Males possess bony bristles on the head and pectoral fins which contact the female prior to oviposition. Males spawned with up to five females in short succession and remained with the eggs until hatching. Following hatching many initiated another spawning cycle. One male was reproductively active for 73 days during which it completed five distinct reproductive cycles. Spawning extended throughout the year.
TL;DR: Annual production by the brook charr population in Valley Creek, Minnesota, over the five-year period 1968–1972, was closely correlated to production by its main invertebrate food, Gammarus pseudolimnaeus, which had been reduced by siltation.
Abstract: Annual production by the brook charr (Salvelinus fontinalis) population in Valley Creek, Minnesota, over the five-year period 1968–1972, was closely correlated to production by its main invertebrate food, Gammarus pseudolimnaeus, which had been reduced by siltation. Annual production was 163–191 kg ha−1 (wet weight) in 1968–1969, respectively, and then dropped to a mininum of 79 kg ha−1 in 1971, the year most seriously affected by decreased invertebrate production and siltation; year-class strength, standing stock, and total year-class (cohort) production followed approximately the same pattern. Total year-class production was highest for the 1968–1969 year classes at 211–178 kg ha−1, respectively, and lowest for the 1971 year class at 76 kg ha−1. Annual P/B ratios ranged from 1.0 to 1.9; cohort P/B ratios, for the 1968–1972 year classes, ranged from 5.6 to 7.2.
TL;DR: In this paper, a brook charr (Salvelinus fontinalis) spawned successfully on a 0.3 m thick aggregation of waterlogged sticks, woodchips and debris overlying the soft ooze bottom of a small Precambrian Shield lake.
Abstract: Brook charr (Salvelinus fontinalis) spawned successfully on a 0.3 m thick aggregation of waterlogged sticks, woodchips and debris overlying the soft ooze bottom of a small Precambrian Shield lake. Brook charr were apparently attracted to groundwater seeping up through the aggregation and utilized the tangle of various sized sticks as a spawning substrate. Eggs were deposited in late October and alevins emerged from the area in late March, periods that coincide with the conventional spawning of native brook charr on gravel areas in nearby lakes. Our observations support the contention that upwellin, water is more important than bottom type in stimulating brook charr to select a spawning site.
TL;DR: The pollutant bioaccumulation model of Norstrom et al. (1976) adequately described the observed accumulation of organic mercury in young perch from June through September 1974, but either underestimated uptake or overestimated clearance for the period October 1974–May 1975.
Abstract: Diet, growth, energy content and mercury concentrations of yearling yellow perch,Perca favescens, were determined from June 1974 through May 1975. Benthic invertebrates predominated in the diet throughout the summer with zooplankton important in June and fish in August and September. Ash, lipid, protein and energy content of the yearling yellow perch increased through the summer reaching maximal values in August, 1974. Fish collected May 15, 1975 has reduced ash, lipid., protein and energy content with an increased moisture content. Both protein and lipid were catabolized during overwintering. Seasonal variation in organic mercury concentration was not significant (p>0.05). Inorganic mercury decreased significantly (p≤0.05) in September, 1974 and May, 1975 samples, reflecting changes in diet and the shorter biological half-life of inorganic mercury. The pollutant bioaccumulation model of Norstrom et al. (1976) adequately described the observed accumulation of organic mercury in young perch from June through September 1974, but either underestimated uptake or overestimated clearance for the period October 1974–May 1975.
TL;DR: Comparison of these laboratory data with previously published field data suggest that behavioral responses to temperature and light play significant roles in determining age- and size-specific differences in seasonal depth stratification and onshore-offshore distributions in this species.
Abstract: Sixteen yearling winter flounder, tested individually for 3-day periods in electronic shuttleboxes, voluntarily occupied an 8–27°C range of temperature, with a modal final thermal preferendum of 18.5°C (mean 18.7°C, median 19°C, midpoint 17.5°C, s.d. 1.9°C, Sk + 0.33). The locomotor activity pattern of the yearling fish in the laboratory was markedly nocturnal, with mean hourly nocturnal activity exceeding mean hourly diurnal activity by a factor of 3.4. Maximum activity occurred at 0300 EST, minimum activity at 1400 EST. While activity generally increased with temperature, a local activity minimum occurred at 18.7°C, coinciding with the 24-hour mean final thermal preferendum. Comparison of these laboratory data with previously published field data suggest that behavioral responses to temperature and light play significant roles in determining age- and size-specific differences in seasonal depth stratification and onshore-offshore distributions in this species.
TL;DR: The proportional length of the prebranchial, branchial, trunk and tail regions changed throughout larval life in a pattern similar to that demonstrated in other lampreys, and body depth changed also throughout much of larvallife.
Abstract: Larval, metamorphosing and adult representatives of the southern brook lamprey,Ichthyomyzon gagei, were collected from Choclafaula Creek in Alabama to provide information on aspects of its life cycle. The proportional length of the prebranchial, branchial, trunk and tail regions changed throughout larval life in a pattern similar to that demonstrated in other lampreys. Body depth changed also throughout much of larval life. Oral disc, dorsal fin height, and eye diameter increased through all or most of metamorphosis and the adult interval. Length-frequency curves for larvae indicate a larval life of 3 1/4–4 1/4 years. Growth rate, reflected by change in total length, was constant throughout the year for each age group. Sex ratio of larvalI. gagei was 1♂: 1.02♀. Metamorphosis is initiated in early September and is completed between early February and mid March. Adult 1. gagei spawn between mid April and early May. The number of oocytes ranged from 820–2485 between 95–150 mm total length. The maximum gonadosomic ratio in males was 17.1 and 26.3% in females.
TL;DR: Development of the fish community on a submerged 16 m barge and variation in fish abundance on nearby transects were surveyed twice monthly for twenty months and a steady increase in abundance was observed for certain fishes on the barge, whereas a few species exhibited distinct seasonal variation on both the Barge and tran Sects.
Abstract: Development of the fish community on a submerged 16 m barge and variation in fish abundance on nearby transects were surveyed twice monthly for twenty months. A steady increase in abundance was observed for certain fishes on the barge, whereas a few species exhibited distinct seasonal variation on both the barge and transects. Most of the seasonal species settled between March and May. Some seasonal species appeared to be site selective in their settlement and consequently settled juveniles were clumped in their distribution. An abundance of preferred topographical features may be why settlement was relatively high at the study site and indirectly why predators became significantly (r3 = 7.67***, N = 37) more abundant at the study area during the months of maximum prey settlement. Concurrent settlement of several species during the same few months may be important because juveniles become an abundant food source to predators during those few months only. Periodic swamping of predators by abundant juvenile prey may improve the chances for individuals of rarer prey species to be overlooked and therefore be succesfully recruited.
TL;DR: It appears that the earlier cleavage stages of the estuarine teleost, Fundulus heteroclitus, are the most sensitive to mercury intoxication.
Abstract: Embryos in specific stage of the estuarine teleost, Fundulus heteroclitus, were exposed to mercuric chloride (MC) and methylmercuric chloride (MMC) under several distinct treatment conditions Four-eight cell stage eggs (0-day old) were exposed for 4 days (continuous), 2 days and one day to each mercury compound One-day old (mid-blastula), 2-day old (mid-neurula) and 5-day old (beating heart) embryos were exposed 4 days to MC and MMC Mortality for the four days immediately following the initiation of exposure was the embryonic response measured Under most exposure conditions to the 4–8 cell eggs, progressive and significant reductions in survival were observed at all concentrations above 40 and 30 μgHg++l−1 as MC and MMC, respectively Reducing the duration of exposure to 1 day most significantly increased the survival potential of the 4–8 cell eggs For all exposure treatments to the 4–8 cell eggs, significant differences in survival, between eggs exposed to MC and MMC, were determined at 40, 60 and 80 μgHg++l−1, indicating the presence of compound-dependent response differences In all cases demonstrating response differences between MC and MMC exposed embryos, survival was significantly lower following exposure to MMC Survival of embryos was progressively increased when the initiation of continuous exposure (4 days) was delayed 1, 2 and 5 days after fertilization As a result, compound-dependent response differences were progressively shifted to higher He++ concentrations For both MC and MMC, survival of 1-day old embryos exposed for 4 days was greater than that of 0-day old eggs exposed for 1 day Of the embryonic stages examined, it appears that the earlier cleavage stages are the most sensitive to mercury intoxication
TL;DR: The age structure of the population was unstable and the variable natural survival, immigration into and emigration from the study site could not be separated, and an annual growth cycle with the most rapid growth for all year classes taking place from May to early August was found.
Abstract: The numbers of trout,Salmo trutta, in Granslev », Denmark, were estimated by the removal method on 18 dates from March 1974 to March 1976. Populations density varied from 0.39 to 0.74 trout m−2 in 1974–1975 and from 0.36 to 0.59 m−2 in 1975–1976 and at all times four or five year classes were present. The age structure of the population was unstable and the variable natural survival, immigration into and emigration from the study site could not be separated. An annual growth cycle with the most rapid growth for all year classes taking place from May to early August was found. Statistically significant differences between different years occurred in the growth of the 0,I and II age groups, but no evidence of density-dependent growth was found. The biomass ranged from 35.4 to 9.5 g m−2. The total mean annual biomass was 22.8 and 14.7 g m−2 in the two years and the II group made the greatest contribution, 44 and 48%, respectively. During 1975–1976 the mean annual biomass of each year class only was about two-thirds of that in 1974–1975. Annual production in the two years was 25.7 (range 24.7–28.5) and 12.6 g m−2 (range 11.7–15.0) and the II group accounted for about 46 and 38% of the production. In addition eel,Anguilla anguilla, produced about 0.5 g m−2 yr−2. The unstable age structure of the trout population was compared with trout populations from other streams. The importance of immigration as a recruitment process in middle and lower reaches of streams and of migrations as a mechanism to optimise utilization of the total stream habitat, as well as temperature as a factor controlling the growth rate are discussed.
TL;DR: It was found that in such a bispecific group, no difference exists in agonistic acts between intra — or interspecific encounters, and this phenomenon seems to be produced by two independent mechanisms — random settling of juveniles from the plankton and predation pressure that strongly limits the re-settling of the fish.
Abstract: The two damselfish, Dascyllus aruanus and D. marginatus are coral-dwelling species, occurring in separate groups or together in the corals Stylophora pistillata and Acropora spp. The two species, however, occur together more frequently than would be expected by chance. The niches of these species overlap in space, time and food. Looking for mechanisms that allow such coexistence, it was found that in such a bispecific group, no difference exists in agonistic acts between intra — or interspecific encounters. This phenomenon seems to be produced by two independent mechanisms — random settling of juveniles from the plankton and predation pressure that strongly limits the re-settling of the fish.
TL;DR: The toxicity of zinc to rainbow trout was determined and the 72-hour median lethal concentration was found to be 2.00 mg l−1 in freshwater, hardness 7.50 mgl−1 as calcium as discussed by the authors.
Abstract: The toxicity of zinc to rainbow trout was determined and the 72 h median lethal concentration was found to be 2.00 mg l−1 in freshwater, hardness 7.50 mg l−1 as calcium. An insignificant increase in zinc concentration of internal tissues occurred in fish exposed to 1.52 mg l−1 in freshwater for 72 h. However, there was a significant uptake of zinc by gills and the body surface. Fish exposed to 10 mg l−1 zinc for 72 h in two-thirds sea water showed significant zinc uptake by liver, rectum and muscle, when compared to control fish. Drinking rate decreased from 1.43 to 0.26 ml kg−1 h−1 when zinc sulphate was added to freshwater. Trout adapted to two-thirds sea water showed no decrease in drinking, about 7 ml kg−1 h−1 when zinc was added to the water.