Diplostomiasis in the sand smelt, Atherina presbyter (Cuvier), population at Fawley Power Station

Diplostomiasis, or black spot, is a disease of teleost fish, which results from the encystment of metacercarial larvae of digenean trematodes in the skin of the fish. This parasite occurs on sand smelt, Atherina presbyter , and pout, Trisopterus luscus , at Fawley. Since the debilitating effects of disease or parasitism may be a factor influencing fish impingement, the incidence of black spot in the Fawley sand smelt population has been studied. Fish 3 months old can be infected, and an average of 9% of the population up to 2 years old is infected. There is a rapid size-correlated rise in infection in 2 + fish, associated with their migratory habit. The metacercarial morphology has been examined, and it is tentatively identified as a species of Neodiplostomum . There is a greater settlement of metacercaria on the fins and the eye of the sand smelt than on the scaled parts of the body. No seasonality or change in levels of infection over the years 1977 to 1981 were found. The incidence of the parasite has no effect on the impingement of sand smelt at Fawley; the reasons for this are discussed. Models of the changing rate of infection with migration imply that the population associated with the power station is less infected than surrounding natural populations. The infection rates of diplostomiasis in separate populations of host fish may prove useful in distinguishing the origins of individuals in a mixed impingement on the power station screens.     

Diplostomiasis in sand smelt, Atherina presbyter Cuvier, from the Fleet, Dorset and its use as a population indicator

The population of sand smelt, Atherina presbyter , breeding in the Fleet, Dorset, shows a high infection of diplostomiasis. The population was studied in 1983 to clarify aspects for this parasitic condition previously analysed for the sand smelt population at Fawley, Southampton Water. All age classes showed a higher percentage infection and mean number of metacercaria per fish than at Fawley. Analysis of postlarvae and juveniles showed that infection can occur at 1 week old, and verified the hypothesis that the scales of older fish inhibit cercarial settlement. Circumstantial evidence suggests that Potamopyrgus jenkinsi may be the first vector host for this parasite, and the densities of this species and of nesting little tern colonies would account for the differential infection between these two sand smelt populations. The increase in infection of 2 fish at Fawley cannot be attributed to mixing with the Fleet population, and the different infection levels demonstrate population isolation. High levels of infection are limited to this part of the English Channel: sand smelt samples from around the coasts of the British Isles show minimal infection rates elsewhere and suggest a southerly distribution of the parasite, away from Atlantic oceanic waters.     

Modeling possible cooling-water intake system impacts on Ohio River fish populations

To assess the possible impacts caused by cooling-water intake system entrainment and impingement losses, populations of six target fish species near power plants on the Ohio River were modeled. A Leslie matrix model was constructed to allow an evaluation of bluegill, freshwater drum, emerald shiner, gizzard shad, sauger, and white bass populations within five river pools. Site-specific information on fish abundance and length-frequency distribution was obtained from long-term Ohio River Ecological Research Program and Ohio River Sanitation Commission (ORSANCO) electrofishing monitoring programs. Entrainment and impingement data were obtained from 316(b) demonstrations previously completed at eight Ohio River power plants. The model was first run under a scenario representative of current conditions, which included fish losses due to entrainment and impingement. The model was then rerun with these losses added back into the populations, representative of what would happen if all entrainment and impingement losses were eliminated. The model was run to represent a 50-year time period, which is a typical life span for an Ohio River coal-fired power plant. Percent changes between populations modeled with and without entrainment and impingement losses in each pool were compared to the mean interannual coefficient of variation (CV), a measure of normal fish population variability. In 6 of the 22 scenarios of fish species and river pools that were evaluated (6 species x 5 river pools, minus 8 species/river pool combinations that could not be evaluated due to insufficient fish data), the projected fish population change was greater than the expected variability of the existing fish population, indicating a possible adverse environmental impact. Given the number of other variables affecting fish populations and the conservative modeling approach, which assumed 100% mortality for all entrained fish and eggs, it was concluded that the likelihood of impact was by no means assured, even in these six cases. It was concluded that in most cases, current entrainment and impingement losses at six Ohio River power plants have little or no effect at the population level.     

Optimal growth strategies under divergent predation pressure

The conditions leading to gigantism in nine‐spined sticklebacks Pungitius pungitius were analysed by modelling fish growth with the von Bertalanffy model searching for the optimal strategy when the model's growth constant and asymptotic fish size parameters are negatively related to each other. Predator‐related mortality was modelled through the increased risk of death during active foraging. The model was parameterized with empirical growth data of fish from four different populations and analysed for optimal growth strategy at different mortality levels. The growth constant and asymptotic fish size were negatively related in most populations. Optimal fish size, fitness and life span decreased with predator‐induced mortality. At low mortality, the fitness of pond populations was higher than that of sea populations. The differences disappeared at intermediate mortalities, and sea populations had slightly higher fitness at extremely high mortalities. In the scenario where all populations mature at the same age, the pond populations perform better at low mortalities and the sea populations at high mortalities. It is concluded that a trade‐off between growth constant and asymptotic fish size, together with different mortality rates, can explain a significant proportion of body size differentiation between populations. In the present case, it is a sufficient explanation of gigantism in pond P. pungitius.     

The biology of the black goby, Gobius niger L. in an English south-coast bay

A population of Gobius niger in a south-coast bay was sampled over a 12-month period. The bay received the cooling water discharge from Fawley power station. Four year-classes were recorded of which the 1+ group was found to be responsible for the bulk of egg production. The main spawning period was from April to May although some evidence of batch spawning was found. Growth was found to be faster than has been reported for populations elsewhere, possibly because of the raised water temperature. Despite higher temperatures the growing season was not extended beyond that of other comparable black goby populations, being restricted to a 4–5 months period after the main spawning period. Seasonal changes in diet were believed to be the result of the effect of water temperature on fish movement. Feeding occurred throughout the year.     

The influence of increased line-fishing mortality on the sex ratio and age of sex reversal of the venus tusk fish

The age of sex reversal of the venus tusk fish Choerodon venustus , caught by line fishing at various locations on the southern Great Barrier Reef, indicated that C. venustus is capable of modifying its life cycle in response to increased mortality. The evidence suggests Masthead Reef fish, which experience the highest mortality, underwent sex reversal at a smaller size and younger age than at the other sites. The largest female fish, sexually transitional fish and males were smaller at Masthead Reef than at the Swains Reefs or One Tree Reef at Masthead Reef. There was also considerable overlap in the size of males and females within the exploited populations indicating that sex reversal is not initiated at a particular length but may have a social cause. The sex ratio of fish was essentially the same for fish fully susceptible to line fishing in the Swains and Masthead samples. Circumstantial evidence suggested that the absence of large males in a population may initiate sex reversal, indicating the maintenance of a constant sex ratio may have a social basis.     

Indicators of AEI applied to the Delaware Estuary

We evaluated the impacts of entrainment and impingement at the Salem Generating Station on fish populations and communities in the Delaware Estuary. In the absence of an agreed-upon regulatory definition of "adverse environmental impact" (AEI), we developed three independent benchmarks of AEI based on observed or predicted changes that could threaten the sustainability of a population or the integrity of a community. Our benchmarks of AEI included: (1) disruption of the balanced indigenous community of fish in the vicinity of Salem (the "BIC" analysis); (2) a continued downward trend in the abundance of one or more susceptible fish species (the "Trends" analysis); and (3) occurrence of entrainment/impingement mortality sufficient, in combination with fishing mortality, to jeopardize the future sustainability of one or more populations (the "Stock Jeopardy" analysis). The BIC analysis utilized nearly 30 years of species presence/absence data collected in the immediate vicinity of Salem. The Trends analysis examined three independent data sets that document trends in the abundance of juvenile fish throughout the estuary over the past 20 years. The Stock Jeopardy analysis used two different assessment models to quantify potential long-term impacts of entrainment and impingement on susceptible fish populations. For one of these models, the compensatory capacities of the modeled species were quantified through meta-analysis of spawner-recruit data available for several hundred fish stocks. All three analyses indicated that the fish populations and communities of the Delaware Estuary are healthy and show no evidence of an adverse impact due to Salem. Although the specific models and analyses used at Salem are not applicable to every facility, we believe that a weight of evidence approach that evaluates multiple benchmarks of AEI using both retrospective and predictive methods is the best approach for assessing entrainment and impingement impacts at existing facilities.     

Sexual dimorphism in Odonata: age, size, and sex ratio at emergence

Males and females of many organisms differ in important life-history and behavioral characters. Following a recent optimization analysis of sexually dimorphic life histories, we employed an odonate-like parameter set to identify patterns of life history and behavior to be expected in an odonate population. The default parameter magnitudes generated a smaller body size and shorter development time for males than for females, which resulted in a male-biased sex ratio. Whether population growth was density dependent or density independent, and whether development time was fixed or flexible had major impacts on life-history features. The model generated five general predictions for odonate systems. (1) For species with fixed development times, males and females should differ more in activity level, growth and mortality rates than for species with flexible life cycles. (2) In species with fixed development times, populations at high latitude or high altitude should be more active, emerge and reproduce at smaller size and have a more male-biased sex ratio than low latitude and low altitude populations. (3) In density-dependent populations, with density dependence mediated by activity-dependent mortality, higher predation rates should increase activity levels and reduce development time in species with flexible development times. (4) For species with flexible development times, in strongly density-dependent populations with density dependence mediated by mortality, activity levels should decrease and development times should increase at high prey abundance. (5) Males should be larger at emergence relative to females, and the sex ratio at emergence should be more female-biased in territorial than in non-territorial species. Existing empirical evidence concerning these predictions is generally sparse and equivocal; focused tests are clearly needed.     

Fishery-aspects of eutrophication

Summary Eutrophication influences among others food supply, prey catchability, reproduction success, growth and mortality of fish. The first stages of eutrophication are favourable for many fish species. Proceeding eutrophication interferes so strongly with the environment that fish is brought in a vulnerable position: vegetation, which is a prerequisite for a number of species, disappears; oxygen depletion near the bottom affects fish food organisms; turbidity hampers catchability of preyfish and decomposition of dead phytoplankton or vegegation may especially at the end of the summer result in lethal oxygen contents. Hence at higher eutrophic levels the fish population surpasses the optimum and decreases again. These optima vary with the species; for the most important species the sequence from oligotrophic to hypertrophic is: 1. Coregonids and Salmondis, 2. pike, 3. roach and perch, 4. pikeperch, 5. bream. Hence the fish populations of very turbid hypertrophic waters are dominated by bream and pikeperch. This population will furthermore usually consist of smaller quantities of eel, smelt, ruffe and white bream. The total fish population in such a hypertrophic water is considerably larger than in a oligotrophic or mesotrophic lake. The vulnerability of the fish community, however, has increased too. In the hypertrophic situation the oxygen supply is the weak spot and every interfering negative influence, as for example sewage discharge, may have disastrous consequences. The many severe fish mortalities prove this to be a real danger.     

The critical swimming speed of the sand smelt (Atherina presbyter Cuvier) in relation to capture at a power station cooling water intake

Critical swimming speeds (CSS) of sand smelt, Atherina presbyter , were measured in a laboratory flume. Individuals of all age classes (0+ to III +) found in the vicinity of Fawley power station, Hampshire, were tested at temperatures covering the seasonal range. The median CSS was 2.7 body lengths per second (bl s⁻¹) at 5.8 °C, rising to 5.7 bl s⁻¹ at 18.5°C.
A two-variable (water temperature and body length) regression model was fitted to the data, and this was used to assess escape potential of fish coming into contact with the power station cooling water intake currents and the extent of possible bias in fish length data collected from this source. It is concluded that the sand smelt remains vulnerable to entrainment at the power station over its whole length range and over the full range of seasonal temperatures and that size-dependent swimming performance will not lead to significant bias in sample length distributions.     

Life history traits of Scardinius acarnanicus (Economidis, 1991) (Pisces: Cyprinidae) in two Greek lakes (Lysimachia and Trichonis)

Age, growth, reproduction and mortality of Scardinius acarnanicus, an endemic cyprinid fish from central and western Greece, were investigated in two contrasting environments, the eutrophic Lake Lysimachia and the oligotrophic to mesotrophic Lake Trichonis. Maximum ages observed were 7+ for males and 10+ for females in Lake Lysimachia and 8+ for males and females in Lake Trichonis. Scardinius acarnanicus grew allometrically (slope of fork length–somatic weight regressions >3) and relatively rapidly until age 2 or 3 years, then slowed at a time coincident with maturation. Total instantaneous mortality rates of fish from Lake Trichonis were higher than in Lake Lysimachia. Scardinius acarnanicus is a multiple spawner; monthly values of the gonadosomatic index (GSI) indicated that spawning occurred mainly in April. Mean egg diameter in Lake Lysimachia was 1.54 ± 0.2 and 1.28 ± 0.16 mm in Lake Thrichonis. In Lake Lysimachia, the population sex ratio departed markedly from 1 : 1; females were more abundant at ages >3 and the overall male : female ratio was 1 : 1.47. In Lake Trichonis, females were more abundant at ages >6; the overall male : female ratio was 1 : 1.16, not significantly different from a 1 : 1 ratio.     

Sex allocation and sex‐specific parental investment in an endangered seabird

Biased offspring sex ratio is relatively rare in birds and sex allocation can vary with environmental conditions, with the larger and more costly sex, which can be either the male or female depending on species, favoured during high food availability. Sex‐specific parental investment may lead to biased mortality and, coupled with unequal production of one sex, may result in biased adult sex ratio, with potential grave consequences on population stability. The African Penguin Spheniscus demersus, endemic to southern Africa, is an endangered monogamous seabird with bi‐parental care. Female adult African Penguins are smaller, have a higher foraging effort when breeding and higher mortality compared with adult males. In 2015, a year in which environmental conditions were favourable for breeding, African Penguin chick production on Bird Island, Algoa Bay, South Africa, was skewed towards males (1.5 males to 1 female). Males also had higher growth rates and fledging mass than females, with potentially higher post‐fledging survival. Female, but not male, parents had higher foraging effort and lower body condition with increasing number of male chicks in their brood, thereby revealing flexibility in their parental strategy, but also the costs of their investment in their current brood. The combination of male‐biased chick production and higher female mortality, possibly at the juvenile stage as a result of lower parental investment in female chicks, and/or at the adult stage as a result of higher parental investment, may contribute to a biased adult sex ratio (ASR) in this species. While further research during years of contrasting food availability is needed to confirm this trend, populations with male‐skewed ASRs have higher extinction risks and conservation strategies aiming to benefit female African Penguin might need to be developed.     

Predicting the effects of exploitation on male-first sex-changing fish

Sex change is widespread among tropical marine fishes, many of which are targeted by fisheries. Conservation concerns have been raised that sex-changing species may be particularly prone to overexploitation by size-selective fishing. In the case of male-first sex-changers, populations may become egg limited if large females are disproportionately killed. However, if males reduce the size at which they change sex in response to higher female mortality, the population may still be sufficiently productive. We develop an age-based model to explore the effects of fishing on two types of male-first sex-changing fish: one with flexibility in size-at-sex-change and one without. These effects were compared with those of non-sex-changing populations with similar life-history and population characteristics. The model predicts that if male-first sex-changers cannot respond to elevated female mortality by adjusting their size-at-sex-change, the population will be more prone to recruitment limitation and extinction than non-sex-changers. These effects will be amplified as smaller individuals become susceptible to fishing mortality. However, if size-at-sex-change is flexible, sex-changers may be as resilient to fishing as non-sex-changers. Knowledge of a species' size-at-sex-change, and the mechanisms controlling it, should be fundamental to the selection of fisheries conservation strategies.     

Limestone Treatment of Whetstone Brook, Massachusetts. II. Changes in the Brown Trout (Salmo trutta)and Brook Trout (Salvelinus fontinalis)Fishery

Density, age structure, and growth rates of wild brook trout (Salvelinus fontinalis)and brown trout (Salmo trutta)in Whetstone Brook in northcentral Massachusetts were monitored for 4 years before and 3 years during limestone treatment to mitigate acidic conditions. The population density of brook trout increased significantly during treatment. Liming did not have any significant effects on the growth rates of brook trout or brown trout. Actual survival rates of brook trout and brown trout were not calculated due to the low density of both species, but more older individuals of both species were captured during the treatment period. Fulton condition factors (an index of fish condition) increased significantly for both brook trout and brown trout during treatment. Seven-day in situ bioassays of brown trout and rainbow trout demonstrated that liming improved the chemical environment for fish in Whetstone Brook. During a pretreatment bioassay in 1987, 100% rainbow trout mortality was observed at both the control and treatment stations in Whetstone Brook. Brown trout mortality was 67% in the control station and 70% in the treatment station. The pH during the 1987 bioassay averaged 4.90 in the control station and 4.99 in the treated station. During a bioassay conducted in 1990 after treatment began, rainbow trout mortality was 100% in the control station and 0% in the treatment station. Brown trout mortality was 17% in the control station and 0% in the treatment station. The pH during the 1990 bioassay averaged 5.23 in the control station and 6.60 in the treatment station. Analysis of total aluminum in the gills of fish from the 1990 bioassay revealed higher levels in fish from the control station than in those from the treatment station.     

An agent-based modelling approach to estimate error in gyrodactylid population growth

Comparative studies of gyrodactylid monogeneans on different host species or strains rely upon the observation of growth on individual fish maintained within a common environment, summarised using maximum likelihood statistical approaches. Here we describe an agent-based model of gyrodactylid population growth, which we use to evaluate errors due to stochastic reproductive variation in such experimental studies. Parameters for the model use available fecundity and mortality data derived from previously published life tables of Gyrodactylus salaris, and use a new data set of fecundity and mortality statistics for this species on the Neva stock of Atlantic salmon, Salmo salar. Mortality data were analysed using a mark-recapture analysis software package, allowing maximum-likelihood estimation of daily survivorship and mortality. We consistently found that a constant age-specific mortality schedule was most appropriate for G. salaris in experimental datasets, with a daily survivorship of 0.84 at 13°C. This, however, gave unrealistically low population growth rates when used as parameters in the model, and a schedule of constantly increasing mortality was chosen as the best compromise for the model. The model also predicted a realistic age structure for the simulated populations, with 0.32 of the population not yet having given birth for the first time (pre-first birth). The model demonstrated that the population growth rate can be a useful parameter for comparing gyrodactylid populations when these are larger than 20-30 individuals, but that stochastic error rendered the parameter unusable in smaller populations. It also showed that the declining parasite population growth rate typically observed during the course of G. salaris infections cannot be explained through stochastic error and must therefore have a biological basis. Finally, the study showed that most gyrodactylid-host studies of this type are too small to detect subtle differences in local adaptation of gyrodactylid monogeneans between fish stocks.     

Mortality costs of sexual selection and parental care in natural populations of birds

Abstract Is the cost of reproduction different between males and females? On the one hand, males typically compete intensely for mates, thus sexual selection theory predicts higher cost of reproduction for males in species with intense male‐male competition. On the other hand, care provisioning such as incubating the eggs and raising young may also be costly, thus parental care theory predicts higher mortality for the care‐giving sex, which is often the female. We tested both hypotheses of reproductive costs using phylogenetic comparative analyses of sex‐specific adult mortality rates of 194 bird species across 41 families. First, we show that evolutionary increases in male‐male competition were associated with male‐biased mortalities. This relationship is consistent between two measures of mating competition: social mating system and testis size. Second, as predicted by the parental cost hypothesis, females have significantly higher adult mortalities (mean ± SE, 0.364 ± 0.01) than males (0.328 ± 0.01). However, the mortality cost of parental care was only detectable in males, when the influence of mating competition was statistically controlled. Taken together, our results challenge the traditional explanation of female‐biased avian mortalities, because evolutionary changes in female care were unrelated to changes in mortality bias. The interspecific variation in avian mortality bias, as we show here, is driven by males, specifically via the costs of both mating competition and parental care. We also discuss alternative hypotheses for why most birds exhibit female‐biased mortalities, whereas in mammals male‐biased mortalities predominate.     

Age, growth and mortality of black redhorse (Moxostoma duquesnei) and shorthead redhorse (M. macrolepidotum) in the Grand River, Ontario

Despite comprising a large portion of riverine fish biomass, very little demographic information is available for redhorse sucker ( Moxostoma spp.) populations in Ontario (Canada). Information is presented on size and age structure, growth, and mortality of black redhorse ( M. duquesnei ), a threatened fish species, and shorthead redhorse ( M. macrolepidotum ) in the Grand River, Ontario. Estimates of body condition, adult mortality, longevity, and maximum attained length were very similar for both species. However, the more abundant shorthead redhorse reached maturity at a smaller size and younger age and grew at a slower rate than the black redhorse. Compared to populations in more southerly parts of its range, Grand River black redhorse grew more slowly, matured later and at a larger size, reached a higher maximum length and age, and was characterized by lower adult mortality rates. Reflective of its central location in the species range, the age and length characteristics of the Grand River population of shorthead redhorse were intermediate of, and overlapped with, other sampled populations.     

Fish impingement at power stations situated along the rivers Rhine and Meuse in The Netherlands

Summary Substantial numbers of predominantly O⁺ fish were impinged on the cooling water screens of Harculo, Gelderland, Merwedehaven and Amer power stations. Low numbers were impinged at the two stations located on the river Meuse (Maas and Claus). Two possible reasons for this difference are suggested,viz. a low fish density in the river Meuse and the deep position of the cooling water intakes. In comparison with Bergum power station situated on a lake the impingement rate at the river stations was more than ten times as low.Most fish were impinged during the summer months when large numbers of juvenile fish are present in the rivers. Dominating species impinged were roach, pikeperch, bream and perch. The abundance of pikeperch in the screensamples, points to a recent increase in the population of pikeperch in the rivers. The growth pattern of O⁺ pikeperch and O⁺ roach during summer seemed very similar for power stations located at different rivers. Thirty fish species and four crustacean species were collected, among whichOrconectus limosus was quite abundant at Maas and Claus power stations.     

Patterns of sex ratio, virginity and developmental mortality in gregarious parasitoids

Theory considering sex ratio optima under ‘strict local mate competition with offspring groups produced by a single foundress’ makes a suite of predictions, one of which is a mean female bias. Treating individual offspring as discrete units, theory further predicts sex ratios to have low variance (precise sex ratio) and to equal the reciprocal of clutch size (one male per clutch). The maternal decision may be complicated by imperfect control of sex allocation, limited insemination capacity of sons and offspring developmental mortality: each can lead to virgin daughters (with zero fitness) and consequently select for less biased sex ratios. When clutches are small and/or developmental mortality is common, appreciable proportions of virgins are expected, even when control of sex allocation is perfect and the mating capacity of males is unlimited. This suite of predictions has been only partially tested. We provide further tests by examining sex ratios and developmental mortalities within and across species of locally mating parasitoids. We find a wide range of mean developmental mortalities (6–67%), but mortality distributions are consistendy overdispersed (have greater than binomial variance) and sexually differential mortality appears to be absent. Sex ratios are female biased and have low variance, but are not perfectly precise and variance is increased by mortality within species and (equivocally) across species. Sex ratios less biased than the reciprocal of clutch size are observed; probably due to a maternal response to developmental mortality in one species, and to limited insemination capacity in others. Cross species comparisons indicate that mean proportions of mortality and virginity are positively correlated. Virginity is more prevalent than predicted among species with higher mortalities but not among lower mortality species. Predicted relationships between virginity and clutch size are supported in species with lower mortalities but only partially supported when mortality rates are higher.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.