Zur Dynamik exploitierter Populationen vonTisbe holothuriae (Copepoda,Harpacticoida)

In weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes, the dynamics of population development and the effect of the rate of water renewal were studied under conditions of surplus food supply, constant temperature (22° C) and salinity (30 ‰). At exploitation rates of 10, 30, 50, 70 or 90% per week, the populations maintained stable mean populations sizes. None of the populations established stationary equilibria. Reduced rates of water renewal resulted in a reduction of mean population density, in the course of which the density of adults plus copepodids was diminished to a lesser degree when low, as opposed to high, percentages of specimens were removed. The density of nauplii was distinctly reduced at all exploitation levels. The age structure shifted in favour of the adults and copepodids and was subject to higher variability at moderate exploitation rates and lowered rate of water renewal. To avoid and to remove difficulties inherent to current concepts of population dynamics, the populations were regarded as a component of a feedback control system, and an attempt has been made to describe the observed processes by analogy to technical control systems. According to not yet completed series of experiments, populations undergoing higher exploitation rates seem to compensate more succesfully for perturbations of their environment, than populations exploited at lower rates.     

On the dynamics of exploited populations ofTisbe holothuriae (Copepoda,Harpacticoida)

A total of 75 weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes were exposed to 0.06, 0.3, 1.5, 7.5 or 37.5 μg Cd⁺⁺l^?1, combined with exploitation rates of 10, 30, 50, 70 or 90% under conditions of surplus food supply at 22° C and 30‰ S. An unusually high mortality was observed for some weeks which could not be ascribed to the added amounts of Cd⁺⁺. A change in properties of supplied water is considered responsible for the unintentional perturbation which offered a further opportunity to study the effect of compensating reactions in stress situations. The results obtained clearly indicate an effect of exploitation rate on responses to detrimental influences. A superior performance has been found in populations exploited at higher rates. The findings are discussed with respect to earlier investigations on the population dynamics ofT. holothuriae.     

On the dynamics of exploited populations ofTisbe holothuriae (Copepoda,Harpacticoida)

A total of 30 weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes were exposed to 148 or 222µg Cd⁺⁺ l^–1, combined with exploitation rates of 10, 30, 50, 70 or 90% under conditions of surplus food supply at 22 °C and 30 S. During the observation period, most populations exhibited distinct U-shaped density trends. Addition of Cd⁺⁺ prolonged and reinforced the downward trends present in the initial periods of the experiments. If population densities, both in control and test populations, fell short of a certain value, the age structures shifted in favour of the nauplii. More pronounced reductions in population densities of the test populations did not result in increased elevations of the nauplius fraction. Within 23 weeks (or 20 generations), the effect of Cd⁺⁺ on population density was counteracted by an acclimation process. No relationship could be established between exploitation rate and observed effects. Erroneous assumptions on the adaptability of parametric methods had led to invalid statements on the dispersion of data published in previous papers of this series. These statements have been revised. Possible limitations of the significance of the results obtained are discussed in some detail.     

Zur Dynamik exploitierter Populationen vonTisbe holothuriae (Copepoda,Harpacticoida)

Zusammenfassung 1. An wöchentlich mit Raten von 0,10; 0,30; 0,50; 0,70 oder 0,90 exploitierten Populationen des harpacticoiden CopepodenTisbe holothuriae Humes wurde unter konstanten Bedingungen (22° C, 30 S) die Abhängigkeit der Reproduktionsleistung, des Geschlechtsverhältnisses, der Entwicklungsdauer und der Überlebenszeit von der Exploitationsrate untersucht.2. Es lassen sich drei Klassen von Eisäcken unterscheiden: (a) Eisäcke mit Entwicklung aller Eier bis zum schlüpfenden Nauplius, (b) Eisäcke mit Entwicklung nur eines Teils der Eier und (c) Eisäcke, aus denen keine Nauplien hervorgehen. Eine Abhängigkeit von der Entnahmerate besteht bei den Anteilen an Eisäcken, bei denen aus allen Eiern oder aus keinem Ei Nauplien hervorgehen.3. Der Anteil an Eisäcken, aus denen keine adulten Individuen hervorgehen, ist bei allen Entnahmeraten größer als der Anteil, aus dem keine Nauplien schlüpfen. Der Unterschied entsteht durch Eisäcke, bei denen unmittelbar nach dem Schlupf sämtliche Nauplien sterben.4. Die Zahl der pro 100 Eisäcke im Mittel produzierten Eier, Nauplien und adulten Individuen ist abhängig von dem Mittel der mittleren Populationsdichten zwischen den Entnahmen.5. Bei allen Befunden wird die durch die Exploitationsrate beeinflußte mittlere Populationsdichte zwischen den Entnahmen als die unmittelbarer wirkende Einflußgröße angesehen.6. Bei höherer Entnahmerate beziehungsweise geringer mittlerer Populationsdichte zwischen den Entnahmen werden im Mittel pro Eisack mehr Eier, Nauplien und adulte Individuen produziert als bei geringer Entnahmerate beziehungsweise höherer mittlerer Populationsdichte zwischen den Entnahmen. Es scheint die Tendenz zu bestehen, unabhängig von der Entnahmerate einen durch die Kulturbedingungen als Führungsgröße vorgegebenen Sollwert der Biomasse zu erreichen und konstant zu halten.7. Die Sterberate der Nauplien ist bei der Entnahmerate von 0,90 herabgesetzt. Das Reproduktionspotential wird allerdings selbst bei dieser Entnahmerate nur etwa zur Hälfte ausgenutzt.8. Außer bei der Entnahmerate von 0,90 werden im Mittel pro Eisack mehr Embryonen gefunden als Nauplien schlüpfen. Die relative Häufigkeit der Eier und Eisäcke, in denen keine Embryonen gefunden werden, zeigt keine Anhängigkeit von der Entnahmerate beziehungsweise der mittleren Populationsdichte zwischen den Entnahmen.9. Bei isolierten zeigt sich unmittelbar nach der Isolierung eine Abhängigkeit der Frequenz der Eisackbildung von der Entnahmerate beziehungsweise der mittleren Populationsdichte zwischen den Entnahmen. Es scheint, als erführen die Individuen durch die Populationen, aus denen sie stammen, eine Prägung, die in der Isolation allerdings bald verlorengeht.10. Das Geschlechtsverhältnis ist abhängig von der Entnahmerate beziehungsweise der mittleren Populationsdichte zwischen den Entnahmen. Bei der Entnahmerate von 0,90 überwiegt die Zahl der , bei der Entnahmerate von 0,10 die der .11. Bei isolierten Brutsätzen konnte eine Abhängigkeit der embryonalen Entwicklungsdauer von der Entnahmerate beziehungsweise der mittleren Populationsdichte zwischen den Entnahmen beobachtet werden. Eine Beeinflussung der Überlebenszeit adulter sowie der Geschwindigkeit der Entwicklung vom geschlüpften Nauplius bis zum adulten Individuum konnte bei den isolierten Brutsätzen nicht festgestellt werden.12. Das mittlere Gewicht eines Individuums im adulten oder Copepoditstadium und die mittlere Länge des adulten stehen in inverser Beziehung zur mittleren Populationsdichte zwischen den Entnahmen.

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On the dynamics of exploited populations ofTisbe holothuriae (Copepoda, Harpacticoida)III. Reproduction, sex ratio, rate of development and survival time

In weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes, effects of exploitation rate on production of eggs, embryos, nauplii and adults, as well as frequency of egg-sac formation, rate of development of embryos and nauplii and survival time of adult females were studied under conditions of surplus food supply at 22° and 30 S. Three classes of egg sacs could be distinguished: (a) Egg sacs in which all eggs developed until hatching of the nauplii, (b) egg sacs in which only part of the eggs developed, and (c) egg sacs with eggs that did not hatch at all. The fraction of egg sacs giving rise to no adult individual is larger at all exploitation rates than the fraction of egg sacs giving no nauplius. This difference is caused by the fact that some egg sacs produce nauplii, all of which die shortly after hatching. The mean number of eggs, nauplii and adults produced per egg sac depends on the mean average population densities between exploitations. In general, the mean average population densities between exploitations are considered to be a more immediate causal agent than the removal rate. At a higher removal rate or a lower density between exploitations, more eggs, nauplii and adults are produced per egg sac than at a lower removal rate or at a higher density. There seems to be a tendency to reach and to keep constant a set point of biomass, rather than population size, the culture conditions being the controlling variable. Death rate of nauplii is lowered at the exploitation rate of 0.90, and even at this highest removal rate only half of the eggs gave rise to adult individuals. Except at the exploitation rate of 0.90, more embryos per egg sac are found on the average than hatched nauplii, implying death of a certain number of embryos during development. The relative frequency of eggs and egg sacs in which no embryos could be detected shows no dependency upon the removal rate or mean population density between exploitations. In isolated females, an inverse relationship has been found between the frequency of egg-sac formation and the average population density between exploitations shortly after isolation. There seems to be some imprinting by the populations in which the specimens are living. Sex ratio is influenced by the removal rate or the average population density between exploitations in such a way that at the removal rate of 0.90 there is a surplus of males while at the rate of 0.10 a surplus of females is found. In isolated broods an inverse relationship between embryonic developmental rate and average population density between exploitation could be established. No influence of population density on rate of development of juvenile stages and survival time of adult females could be observed in equally isolated broods. The average weight of an individual in the adult or copepodid stage, and the average length of an adult female are inversely related to the average population density between exploitations.

     

On the dynamics of exploited populations ofTisbe holothuriae (Copepoda,Harpacticoidae)

A total of 90, weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes were exposed to 148, 222, 333, 500, 750 or 1125µg Cd⁺⁺ l^–1, combined with exploitation rates of 10, 30, 50, 70 or 90% under conditions of surplus food supply at 22° C and 30 S. All populations exposed to concentrations down to 500µg Cd⁺⁺ l^–1 and 3 populations (out of 15) exposed to 333µg l^–1 became extinct within the experimental period of 30 weeks. Survival time significantly depended on concentration. A recovery phase from an initially high mortality preceded eventual population extinction after adding 500µg Cd⁺⁺ l^–1. In the initial phase, higher nauplii mortality prevailed. During these experiments on acute intoxication, no relationship could be established between survival and exploitation rate. However, experiments on the effects of stepwise increases in Cd concentration (results not yet published) produced evidence of such a relationship. In spite of increased mortality, no significant numbers of dead copepods were detected in weekly samples because of their rapid decomposition and cannibalism, which depends on the amount of food available. Sampling regimes of 5 times per week yielded significant numbers of dead individuals.     

Short-term impacts of polyunsaturated aldehyde-producing diatoms on the harpacticoid copepod, Tisbe holothuriae

Diatoms that produce toxic oxylipins can be detrimental to the reproductive success of aquatic invertebrates. Despite the potential importance of these toxins in shaping aquatic ecosystems, marine studies to date have focused almost exclusively on planktonic calanoid copepods. The current work examines the response of the benthic harpacticoid copepod, Tisbe holothuriae, to direct exposure to diatom-derived oxylipins and the short-term impact of oxylipin-producing diatom diets on reproductive success. The most toxic oxylipin was the polyunsaturated aldehyde (PUA) 2E,4E-decadienal with an LD₅₀ of 9.3 μM for T. holothuriae nauplii. The least tolerant life-stage was the nauplius followed by adult males then adult females. Short-term exposure to PUA-producing diatoms (Skeletonema marinoi and Melosira nummuloides) in maternal diets had no significant impact on reproductive success compared with non-PUA-producing diets (Skeletonema costatum, Navicula hanseni, Phaeodactylum tricornutum and Tetraselmis suecica). The PUA producers had no negative impact on the survival and development of naupliar stages to adulthood. T. holothuriae expresses a higher degree of tolerance to PUA-producing diatoms than many planktonic calanoids, possibly reflecting a degree of adaptation to higher stress levels associated with the benthos. This is the first study to investigate the reproductive responses of harpacticoid copepods feeding on known PUA-producing diatoms.     

Effect of density on larval development and female productivity ofTisbe holothuriae (Copepoda,Harpacticoida) under laboratory conditions

The harpacticoid copepodTisbe holothuriae has been cultivated in the Helgoland laboratory for more than 20 years. The effects of density on the larval development and the female productivity were studied by comparing two culture systems: (1) enclosed system, and (2) running-water system. In both systems, a nutritious mixed diet ofDunaliella tertiolecta, Skeletonema costatum, and granulatedMytilus edulis was offered. Larval mortality, larval development and female productivity are found to be significantly dependent on both the population density and specificity of the culture system. Increasing density causes higher larval mortality, longer larval development time, and a reduction in female productivity. In comparison with the enclosed system, the running-water system shows decisive advantages: larval mortality is about 20% lower, the rate of larval development is about two days shorter, and there is a very high rate of nauplii production. The sex ratio exhibits high variations, but in general, there is no clear relationship between sex ratio and population density. Nevertheless, when reared in the running-water system, a relatively high percentage of females (>45%) was found at lower densities.     

Effect of crowding on nauplii production during mating time inTisbe clodiensis andT. holothuriae (Copepoda,Harpacticoida)

Tisbe clodiensis andT. holothuriae females and males were maintained at different densities, from sexual maturity to the extrusion of the first egg sac, in order to study the effect of crowding on the number of nauplii produced by the first egg sac. They were tested in pure cultures at densities of 1, 2, 4, 6 and 8 couples per 20 ml of sea water, and in mixed cultures (half of the individuals belonging to each of the two species) with 2, 4, 6 and 8 couples per 20 ml, without food limitation. At the highest crowding level, and forT. holothuriae only, the effect of daily renewal of the culture water was examined. The mean number of nauplii per female decreases with increased density. A significant linear regression of number of nauplii on number of couples was found. The regression slopes appear to be the same for both species, regardless of whether they are cultured singly or together. The daily renewal of water diminishes the effect of crowding. The possibility that a complex chemical compound, produced by the animals, which enables the latter to perceive and to react to crowding, is discussed.     

Relation between exploitation and production in experimental populations ofPoecilia reticulata

In order to make researches on the effect of exploitation upon fish populations, six populations of guppy were maintained during a period of 331 weeks under conditions of space, light, temperature, pH, and food controlled as much as possible. The six populations were divided into three groups two by two: a control group without exploitation, a 10% rate of exploitation group and a 20 and 33% rate of exploitation group. All the populations repeated an increase and a decrease in number and in biomass about once a year. The exploitation affected the age composition in the populations: in the non-exploited population adults occupied the largest part, and the more intensive the exploitation the larger the part fry occupied. The exploitation decreased the biomass of exploited populations, but productivity increased with increasing of of rate of exploitation. The relationship between adult and fry could be represented byRicker's reproduction curve. The form of the curve was changed by an intensity of exploitation. It was thought that exploitation acts as a preventer of aging of populations by removing old fish and results increasing number of fry and productivity.     

Effect of detritus supply on trophic relationships within experimental benthic food webs. I. Meiofauna-polychaete (capitella capitata (Type I) Fabricius) interactions

In a long-term (40–55 wk) microcosm experiment, the presence of the polychaete Capitella capitata (Type I) Fabricius reduced population densities and trophic transfer efficiencies (detritivore production per amount of detritus supplied) of the nematode Diplolaimella chitwoodi Gerlach at high (150 mg N · m^?2 · day^?1) detritus ration and of the harpacticoid copepod Tisbe holothuriae Humes at low (50 mg N · m^?2 · day^?1) detritus supply. The rarer nematodes Theristus ostentator Wieser & Hopper and Paracyatholaimus pesavis Wieser & Hopper fed at depth on fungal hyphae attached to polychaete fecal pellets thereby minimizing contact with the other metazoa.In a series of short-term (4-day) growth experiments, the presence of either Diplolaimella chitwoodi or Tisbe holothuriae reduced daily weight-specific growth (g increase · g worm^?1 · day^?1) of Capitella capitata on separate diets of mixed cereal and red seaweed, Gracilaria foliifera (Harvey) Taylor, detritus.Both sets of experiments suggest that Capitella capitata competes for food with Diplolaimella chitwoodi and Tisbe holothuriae. Meiofauna-polychaete interactions were dependent, in part, upon the degree of similarity among species in food requirements and habitat preferences, and may incorporate a variety of regulatory mechanisms other than competition or predation.     

Assessment of the status of wild populations of land snail (escargot) Helix pomatia L. in Moldova: the effect of exploitation

Helix pomatia L., the Roman snail, is a species faced with growing commercial interest in Moldova. Its life history characteristics (slow maturation and recruitment, high mortality among juveniles and low fecundity) along with its strong spatial aggregation, makes it especially vulnerable to exploitation. In this study, differences in density, shell size and age distribution were assessed in 7 unexploited and 10 exploited sites in the northern and central parts of Moldova. A significant impact of exploitation on snail population densities, shell size of adult snails and age distribution was revealed. Exploited sites had much lower densities than unexploited ones and in two places no live snails were found. This may suggest that exploitation is currently carried out at an unsustainable level, but additional information on the demography of populations and intensity of exploitation is required in order to make inferences regarding sustainability and long-term population management. There was a higher proportion of adult snails in exploited sites than in non-exploited, because of the collection strategies: not only adults, but also all other age groups are gathered. Bigger adult shell size in exploited sites may be related to lower population density, but further study is required to confirm this. Establishing of well-organized population monitoring systems and development of snail breeding enterprises are proposed in order to conserve the species in Moldova.     

Demographic and environmental influences on life-history traits of isolated populations of the Andean catfish Astroblepus ubidiai

In this study, we examined the influence of demographic and environmental variables on the life histories of six remnant populations of the Andean catfishAstroblepus ubidiai (Actinopterygii; Siluriform) located in isolated refuges, and tested six predictions on the relationships between age at maturity, generation time, population density and juvenile and adult growth and survival rates. The three populations inhabiting the watershed with a direct connection to a major lake (Imbakucha) exhibited later maturity, a longer generation time, higher adult survivorship, a higher adult-to-juvenile survival ratio and lower biomass density than those of the adjacent, non-lacustrine watersheds. Across all six populations, there was a strong correlation between mean age at maturity and both the adult-to-juvenile survival ratio and biomass density. Study populations also showed a pattern of inverse density-dependence associated with Allee effects. We conclude that without limitations in the availability of environmental resources, higher population density can be negatively related to age at maturity. This relationship would be expected to become positive when carrying capacity of the systems is reached. Delayed maturity in the Imbakucha populations may be an adaptive response for increasing juvenile survival under stressful conditions.     

Effects of seven diets on the population dynamics of laboratory culturedTisbe holothuriae Humes (Copepoda,Harpacticoida)

The harpacticoid copepodTisbe holothuriae was collected from Saronicos Gulf (Greece) and reared under constant laboratory conditions. In order to study the effects of food on the population dynamics, seven diets were tested: the seaweedUlva; five artificial compound feeds: the liquid Fryfood^® (Waterlife), a powder ofMytilus, yeast, soya andSpirulina, respectively; and a mixed diet consisting ofUlva and Fryfood. The life cycle parameters (mortality, sex ratio, generation time, offspring production) were measured, and the demographic variables [mean generation time (T), net reproductive rate (Ro), and intrinsic rate of natural increase (r_m)] were determined. As to their efficiency regarding population dynamics, the diets ranked as follows: (1)Ulva+Fryfood, (2),Ulva, (3) Fryfood, (4)Mytilus, (5) soya, (6) yeast, and (7)Spirulina. In this order they cause a progressive increase of both larval mortality and generation time, a progressive decrease of sex ratio, number of offspring per egg sac, number of egg sacs per female and, consequently, of Ro and r_m. The observed differences between diets were most pronounced with respect to offspring production. Of the compound diets, those containing animal extracts were more efficient than those containing vegetable materials.Ulva plays an important role in the nutrition ofT. holothuriae, favouring offspring production as well as larval survival, development and pigmentation.Ulva in combination with Fryfood led to a greater copepodid survival and offspring production. This mixed diet proved to be the most favourable for rearing the Greek population oft. holothuriae, resulting in an efficient intrinsic rate of natural increase (r_m=0.304) of the population.     

Effects of light (photoperiod,spectral composition) on the population dynamics of Tisbe holothuriae Humes (Copepods,Harpacticoida)

The harpacticoid copepod Tisbe holothuriae Humes was reared for several generations under different photoperiods (white light) and spectral quality (continuous illumination). The main life cycle parameters were measured and demographic variables were determined. Development time was retarded under green wavelengths and at photoperiods different from LD 12:12, especially under continuous white light. Total body length of females and males was shortest under LD 12:12 and longest under constant dark. Photoperiods different from LD 12:12 prolonged the interval between broods of egg sacs. Short photoperiods caused higher abortion rate, while long photoperiods retarded the maturation time of the egg sacs. Red wavelengths stimulated the abortion rate. Photoperiod LD 12:12 and blue wavelengths increased the production of offspring and the life span of females. Highest intrinsic rates of natural increase were estimated under LD 12:12 (rm =0.304) and blue wavelengths (rm = 0.254). These light conditions proved the most favourable for rearing Tisbe holothuriae.     

Shifting dynamic forces in fish stock fluctuations triggered by age truncation?

Accumulating evidence shows that environmental fluctuations and exploitation jointly affect marine fish populations, and understanding their interaction is a key issue for fisheries ecology. In particular, it has been proposed that age truncation induced by fisheries exploitation may increase the population's sensitivity to climate. In this study, we use unique long‐term abundance data for the Northeast Arctic stock of cod (Gadus morhua) and the Norwegian Spring‐Spawning stock of herring (Clupea harengus), which we analyze using techniques based on age‐structured population matrices. After identifying time periods with different age distributions in the spawning stock, we use linear models to quantify the relative effect of exploitation and temperature on the population growth rates. For the two populations, age truncation was found to be associated with an increasing importance of temperature and a relatively decreasing importance of exploitation, while the population growth rate became increasingly sensitive to recruitment variations. The results suggested that the removal of older age classes reduced the buffering capacity of the population, thereby making the population growth rate more dependent on recruitment than adult survival and increasing the effect of environmental fluctuations. Age structure appeared as a key characteristic that can affect the response of fish stocks to climate variations and its consequences may be of key importance for conservation and management.     

The ecology and production of copepods, particularly Thermocyclops hyalinus, in the tropical Lake George, Uganda

(1) Total copepod numbers per litre in Lake George, Uganda, vary slightly during the course of a year. Numbers increase during the dry seasons June-August and January-February, and decrease during the rains. This variation is more marked in the mid-lake area, where copepod numbers are higher, but the change in numbers is not more than two-fold. By comparison with populations in temperate regions the population in Lake George is stable in size and shows little seasonal variation. (2) The carnivorous cyclopoid Mesocyclops leuckarti forms only 4–21 %, by num-bers, of the total copepod population and does not occur in numbers large enough to be analysed profitably. (3) The bulk of the population is the much smaller Thermoeyclops hyalinus and analysis of this population into the main development stages of its life history shows an almost constant age structure over the lake as a whole, throughout the year. (4) The development time of the eggs, as determined previously (Burgis, 1970), is 1–5 days. This, plus mean development-times of 6 and 11 days for nauplii and copepodites respectively, has been used to determine the daily rate of recruitment from each stage of the life history to the next. The recruitment from nauplii to copepodites and from copepodites to adults are very similar and almost constant. Recruitment from eggs to nauplii is. however, mueh greater, and it thus appears that the major loss to the population is at this eariy stage in the life history. This may well be due to predation by the larvae of Chaohorus. (5) Using this recruitment rate from eggs to nauplii, the potential turnover time of population numbers was calculated to have a mean value of 4–2 days for the lake as a whole. This is similar to maximum summer turnover times found by authors such as Hall (1964) for Daphnia populations in temperate waters. (6) The biomass of individuals at various stages in their life history was deter-mined as carbon, and application of these determinations to the numerical counts was used to calculate the standing-crop biomass in the lake. With a constant population of stable age-structure it is possible to estimate the daily biomass production from the turnover time of numbers. For the population of Thermocyelops hyalinus in Lake George this gives a mean figure of 211 μg C/I/day during 1969–70. (7) Instantaneous birth rates and instantaneous rates of change in population numbers (b and r) have also been calculated for the population of Thermocyclops. The values of r show some variation at one centre site, fluctuating around zero, but when calculated for lake mean figures r is hardly different from zero throughout the year. The values of b also show fluctuations when calculated for one site but when calculated for lake mean figures show no more than two-fold variations, being higher at the beginning of the two rainy seasons. The values of b are not as high as those attained during the summer in some temperate Daplmia populations such as those described by Wright (1965). The values of ZJ tend to be higher inshore than in mid-lake areas of the lake. (8) The instantaneous mortality rate r/has been calculated from b-r. With such very small values of r, d is always very similar to b. Most, if not all, recruitment to the population is counterbalanced by loss, probably predation. (9) At one inshore site the population was analysed and counted in detailed cate-gories, over a period of 10 days. This allowed a more accurate estimation of the standing-crop biomass and thus of production. A mean value of 103 μg C/l/day was obtained for these samples. (10) The mean daily level of production, c. 211 μg C/l/day, o(Thermocyclops hyalinus in Lake George, is among the higher values of zooplankton production available for comparison most of which are from temperate regions of the world. The fact that it is applicable throughout the year gives an annual value of 77g C/m³/year which is probably high.     

Simple life-history omnivory: responses to enrichment and harvesting in systems with intraguild predation

This article analyzes the nature of top-down and bottom-up effects and alternative states in systems characterized by life-history omnivory. The analysis is based on a three-species food web with intraguild predation (IGP). The top predator population has juvenile and adult stages, which consume the basal resource and the intermediate prey, respectively; the prey consumes only the resource. The per capita reproduction of the adult predators depends on their consumption rate of prey, while the maturation rate of the juvenile predators depends on their resource consumption rate. Enriching the resource can increase or decrease the abundances of one or both of the two consumer species; an increased density is more likely in the intermediate species than in the systems where IGP is not based on stage differences. Alternative states that have or lack the predator occur frequently, particularly when the prey population is capable of reducing the resource to very low densities. These results differ from those of several other recent models of life-history omnivory. They suggest that life-history omnivory may be one of the primary reasons why exploited populations undergo sudden collapses and why collapsed populations fail to recover in spite of large reductions in the exploitation rate.     

The harpacticoid copepod Tisbe holothuriae is resistant to the insidious effects of polyunsaturated aldehyde-producing diatoms

Numerous species of diatoms liberate oxylipins including polyunsaturated aldehydes (PUAs) in response to cellular damage such as may occur during grazing. PUAs are cyto- and genotoxic and negatively disrupt reproductive processes in copepods, their principal grazers, although experimental evidence would suggest that the grazer response may be species specific. The reproduction of the benthic harpacticoid copepod Tisbe holothuriae was compared over two generations. Copepods were reared using four diet treatments: PUA-producing diatom strains Skeletonema marinoi (Adriatic Sea Isolate FE6) and Melosira nummuloides (CCAP 1048/6); and non-PUA-producing diatom strains Phaeodactylum tricornutum (CCAP 1052/A) and S. marinoi (Seasalter (Walney) Ltd). Life tables were generated for each treatment using measured reproductive parameters and the net reproductive rate (R₀) calculated. No significant differences were observed between the individual reproductive parameters of T. holothuriae fed PUA-producing diatoms compared to those fed non-PUA-producing diatoms although diets of P. tricornutum resulted in some decreases in individual reproductive parameters in the second generation. There were no significant differences in the R₀ values between the four tested diets. These observations indicate that T. holothuriae exhibits a tolerance of known PUA-producing diatom diets that has not been similarly demonstrated in pelagic calanoid copepods. Harpacticoid copepods may have a greater capacity to detoxify diatom oxylipins than their planktonic calanoid counterparts.     

Life history of an invasive and unexploited population of Nile tilapia (Oreochromis niloticus) and geographical variation across its native and non-native ranges

Although the life history traits of Nile tilapia, Oreochromis niloticus have been studied since the early 20^th century, the potential range of life history parameters in unexploited populations and geographical variability in life history traits are still poorly understood. We explored life history traits (age composition, growth rate, mortality, size, and age at maturity) of an invasive and unexploited population in the Tabaru River, Yonaguni-jima Island, southwestern Japan, through comparisons with exploited populations across the species’ global distribution. Analysis of sectioned otoliths from 307 fish revealed that growth and maximum age were sexually dimorphic (females growing less but having greater longevity). Large-scale comparisons with exploited populations revealed that the unexploited Tabaru River population had a greater life span than exploited populations in other regions, but the growth rate was in the middle of the range of observed values. Although a high variation in life history parameters was observed among populations (L _∞, K, maximum age), we found no significant variation in life history traits by latitude or between African and non-African populations. Such a combination of long life span and high variability in life history traits in response to environmental and fishing pressures may aid the success of non-native Nile tilapia in various environments.     

Growth and survival effects on maturation pattern in populations of grayling with recent common ancestors

Mortality and growth rates were shown to influence maturation patterns in five populations of grayling ( Thymallus thymallus ) in central Norway. The populations share recent common ancestors as they derive from introductions performed in 1910, and they inhabit lakes with different environmental conditions (i.e. length of growth season, lake area and fishing pressure). Mortality rate (range of Z -values: 0.36–0.77) and growth pattern varied strongly among the populations. Mortality rates were negatively associated with population mean age at maturity ( r _sp=−0.90), supporting life-history theory which predicts early maturation to be favoured under conditions with high adult mortalities. Maturation reaction norms differed significantly among the populations. Individuals from one population showed no maturation plasticity (all individuals matured at age three), whereas rapid growers were found to mature earlier than slow growers in the remaining four populations. Life-history theory is again supported as it predicts rapid growers to mature early due to high age-specific fecundity and short generation times. Given low mortality risks, slow growers are predicted to delay maturation in order to gain high first-time fecundity. In high-mortality systems all individuals are predicted to mature early. This theory is supported by the present data as populationwise maturation plasticity increased with decreasing mortality rates. In the population with no maturation plasticity the corresponding high mortality rates were probably due to high fishing pressures.