Fecundity and egg size variation in North American Pacific salmon (Oncorhynchus)

We examined regional and latitudinal variation in fecundity and egg weight for five species of Pacific salmon ( Oncorhynchus ) along the Pacific coast of North America. Data were examined for 24 chum salmon, 15 pink salmon, 34 sockeye salmon, 44 chinook salmon, and 40 coho salmon populations from published sources, unpublished Canadian hatchery records, our own laboratory investigations, and other unpublished sources. Substantial regional variation in fecundity and egg weight was observed, with salmon on the Queen Charlotte Islands and Vancouver Island in British Columbia generally having lower fecundity and larger egg size than nearby mainland populations. The relative distance of freshwater migration to the spawning grounds generally had a marked effect on both fecundity and egg size, with populations spawning in the upper portions in the drainages of large rivers like the Fraser River in British Columbia having reduced fecundity and egg size compared with coastal spawning populations. Fecundity was generally higher and egg size generally lower in more northern populations of sockeye, chinook, and coho salmon compared with southern ones. We suggest that egg size tends to be lower in northern populations of some species as a result of increased fecundity due to their older ages at maturity and a limited amount of energy that can be expended on egg production.     

Genetic differentiation and the problems of conservation of masu salmon (<Emphasis Type="Italic">Oncorhynchus masou</Emphasis> Brevoort, 1856 (Pisces: Salmonidae)) populations

This study focuses on the strategy for the conservation of masu salmon, Oncorhynchus masou, in the northern part of the species range (via the masu populations in Sakhalin Oblast), based on data of its population structure. It is shown that masu populations that inhabit different rivers genetically differ from each other in allele frequencies at microsatellite markers. In the Naiba River basin, at least two genetically distinct masu populations exist: in the upper reaches and in a tributary, the Bolshoy Takoy River. The masu populations on Iturup Island significantly differ from those on Sakhalin Island; within Sakhalin, the masu salmon from the Chernaya River in the southwestern part of the island is genetically distinct from the southeastern Sakhalin and Aniva Bay populations. The genetic diversity of Iturup populations is substantially lower than that on Sakhalin, probably due to their small sizes. The measures for the conservation and recovery of masu salmon populations should be based primarily on their own genetic resources, or, in the case of a lack of spawners, on the base populations of their ecological/geographical region. In the latter case, masu populations of large rivers can be considered as base ones: for southeastern Sakhalin, this is masu salmon of the Naiba River; for Aniva Bay, this is masu salmon of the Lyutoga River. Transplantation of fish, fertilized eggs, or any other genetic material from a population that is different genetically and inhabits the waters with different ecological gradients should be strongly restricted. The formosan masu salmon from Taiwan Island is studied as an example of a strict genetic isolate.     

Individual growth and phase differentiation of lacustrine masu salmon, Oncorhynchus masou, under artificial rearing conditions

 In lacustrine masu salmon, Oncorhynchus masou, originated from anadromous fish and inhabiting an artificial lake (Shumarinai Lake), we examined the relationship between individual growth during the juvenile stage and phase differentiation under artificial rearing conditions. In females, the mean fork length of potential (subsequent) 1+ smolts and potential 1+ parr (1+ parr show fish that neither smolted nor matured until 1+ autumn) always differed after their first summer (0+ summer), the former being larger. In males, the juveniles that grew faster during their first spring became 0+ mature male parr. After their first summer, potential 1+ smolts were always larger in body length than potential 1+ mature parr and potential 1+ parr. These results were similar to previous reports of anadromous masu salmon, suggesting that the lacustrine masu salmon studied seems to have maintained the phase differentiation as in the original fish, likely because only 60 years have passed since the formation of the lacustrine population.     

Occurrence of a hybrid between endemic Miyabe charr Salvelinus malma miyabei and introduced masu salmon Oncorhynchus masou masou in the Lake Shikaribetsu system, Hokkaido, Japan

A hybrid specimen between endemic Miyabe charr Salvelinus malma miyabei and introduced masu salmon Oncorhynchus masou masou was collected in an inlet stream (Yamada Creek) of the Lake Shikaribetsu. This specimen showed intermediate external characteristics between two species and was also confirmed as a hybrid by DNA markers. The mtDNA of the specimen was identical with that of S. malma miyabei, suggesting that the mating occurred between a female Miyabe charr and a male masu salmon. Hybridization with introduced masu salmon may cause deleterious effects on S. malma miyabei.     

Effects of husbandry parameters on the life-history traits of the apple snail, Marisa cornuarietis: effects of temperature, photoperiod, and population density

Abstract. These experiments are part of a larger study designed to investigate the influence of husbandry parameters on the life history of the apple snail, Marisa cornuarietis . The overall objective of the program is to identify suitable husbandry conditions for maintaining multi-generation populations of this species in the laboratory for use in ecotoxicological testing. In this article, we focus on the effects of photoperiod, temperature, and population density on adult fecundity and juvenile growth. Increasing photoperiod from 12 to 16 h of light per day had no effect on adult fecundity or egg hatching and relatively minor effects on juvenile growth and development. Rearing snails at temperatures between 22°C and 28°C did not influence the rates of egg production or egg clutch size. However, the rates of growth and development (of eggs and juveniles) increased with increasing temperature in this range, and when temperatures were reduced to 22°C egg-hatching success was impaired. Juvenile growth and development were more sensitive to rearing density than adult fecundity traits. On the basis of the present results, we conclude that rearing individuals of M. cornuarietis at a temperature of 25°C, a photoperiod of 12L:12D, and a density of <0.8 snails L⁻¹ (with lower densities for juvenile snails) should provide favorable husbandry conditions for maintaining multi-generation populations of this species.     

The curvilinear relationship between worm length and fecundity of Teladorsagia circumcincta.

The variation among sheep in fecundity of Teladorsagia (Ostertagia) circumcincta was estimated by dividing the faecal egg count by the worm number following deliberate infection of mature Scottish Blackface lambs. Fecundity was skewed and ranged from 0 to 350 eggs per worm per day. Most animals had relatively low worm fecundities, but a small number of individuals had relatively high worm fecundities. However, as fecundity is a ratio of two imprecise estimates, extreme values may be statistical artefacts. Following both deliberate and natural infection, differences in worm fecundity were associated with differences in adult female worm length. In both infections, fecundity varied with worm length to the power 0.4. This relationship should assist the measurement of fecundity in studies of host immunity, in epidemiological modelling and in estimating the influence of density-dependent relationships.     

Fecundity of the invasive marine gastropod Crepidula fornicata near the current northern extreme of its range

The calyptraeid gastropod Crepidula fornicata is native to the eastern coast of the United States but has now become an extremely successful invader along much of the European coastline. As the northern limit of its spread is thought to be determined by an inability of adults to tolerate prolonged exposure to low winter temperatures, this study sought to compare the fecundity of females collected from two sites along the Norwegian coastline with that of females collected from Rhode Island, USA. Few other studies have compared the fecundities of marine invertebrates from invasive populations with those found in native populations. For both populations studied, fecundities increased with increasing shell length. However, contrary to expectations, size‐related fecundities were significantly higher for Norwegian females than for Rhode Island females, with Norwegian females producing larger egg capsules and a greater number of embryos per capsule, but not a greater number of egg capsules per brood. Current evidence suggests that at the northern extreme of its invaded range, the fecundity of C. fornicata is increased rather than compromised.     

Variations in fecundity over catchment scales: Implications for caddisfly populations spanning a thermal gradient

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Antagonistic effects of energy status on meal size and egg-batch size of Aedes aegypti (Diptera: Culicidae).

The effect of sugar feeding on egg-batch size in Aedes aegypti was examined in a way that would distinguish between the roles of a recent sugar meal (full vs. empty crop) and of repeated sugar feeding and digestion (high vs. low energy reserves). Egg numbers of females representing the four combinations of these characteristics (full crop/high reserves, full crop/low reserves, empty crop/high reserves, empty crop/low reserves) were counted during their first gonotrophic cycle. In addition, the sizes of their replete sugar meals and human blood meals were measured to understand the interactions between them. Results demonstrated that blood-fed mosquitoes with full crops and low energy reserves produced the fewest eggs (mean = 56.2); those with empty crops and high energy reserves produced the most eggs (mean = 84.6); and those with the opposite combinations (full/high and empty/ low) had an intermediate fecundity (mean = 75.2 and 76.9, respectively). This ranking of fecundities did not correspond to blood-meal size ranks, owing to direct and indirect effects of energy reserves on meal sizes and egg number. Full-crop females with low reserves ingested the smallest blood meals (mean = 0.62 mg) and had the lowest fecundities. Full-crop females with high reserves ingested more blood (mean = 0.82 mg) and produced more eggs. But empty-crop females with low reserves ingested the largest blood meals (mean = 1.19 mg), yet produced significantly fewer eggs than their high-reserve counterparts, which took smaller blood meals (mean = 0.99 mg). These results demonstrate extremes in the reproductive penalty of crop sugar and the reproductive reward of digested sugar. Energy reserves and an empty crop are similarly valuable in promoting fecundity at the time blood is taken.     

Maternal and interannual comparison of the ovulatory periodicity, egg production and egg quality of the batch-spawning yellowtail flounder

Estimates of the ovulatory periodicity of yellowtail flounder indicate that a 1-day interval, which predominated over all other intervals, may characterize regular ovulation patterns. Females produced a mean number of 14–22 batches in 1994 and 1995, respectively. Batch fecundities usually remained within a range of 10 000–60 000 eggs. Mean egg production increased from 549 756 eggs per female in 1994 to 1 186 881 eggs in 1995. Mean fertilization rates rose interannually from 38 to 57%, while hatching rates, tested in 1994, had a mean of 63%. Maternal variation in egg production and egg quality was large and independent of size differences among females. Some females had disrupted ovulation patterns which affected the realization of potential fecundity contained within the prespawning ovary. High interbatch variation in egg quality was not related to progressive decreases in egg diameter and dry weight over time. Batches with high survival rates appeared at random within a female's duration of ovulation.     

Size-structured Interactions between Native and Introduced Species: Can Intraguild Predation Facilitate Invasion by Stream Salmonids?

Dynamics of biological invasions may be complicated in size-structured animal populations. Differences in timing of life history events such as juvenile emergence create complex interaction webs where different life stages of native and non-native species act as predators, competitors, and prey. Stream salmonids are an ideal group for studying these phenomena because they display competition and predation in size-structured populations and have been introduced worldwide. For example, introduced rainbow trout (Oncorhynchus mykiss) are invading streams of Hokkaido Island, Japan and have caused declines in native masu salmon (O. masou) populations. However, age-0 rainbow trout emerge later than age-0 masu salmon and are smaller, which raises the question of why they are able to recruit and therefore invade in the face of a larger competitor. We conducted experiments in laboratory stream channels to test effects of increasing density of age-0 and age-1 rainbow trout on age-0 masu salmon. Age-1 rainbow trout dominated age-0 masu salmon by aggressive interference, relegating them to less favorable foraging positions downstream and reducing their foraging frequency and growth. The age-1 trout also reduced masu salmon survival by predation of about 40% of the individuals overall. In contrast, age-0 rainbow trout had little effect on age-0 masu salmon. Instead, the salmon dominated the age-0 trout by interference competition and reduced their survival by predation of 60% of the individuals. In each case, biotic interactions by the larger species on the smaller were strongly negative due to a combination of interspecific competition and intraguild predation. We predict that together these produce a positive indirect effect in the interaction chain that will allow the recruitment of rainbow trout in the face of competition and predation from age-0 masu salmon, and thereby facilitate their invasion in northern Japan.     

Comparison of the spectral sensitivity of three species of juvenile salmonids

The spectral sensitivity of chum Oncorhynchus keta , pink Oncorhynchus gorbuscha and masu Oncorhynchus masou masou salmon was measured by the optomotor reaction index in monochromatic light of 400, 440, 480, 520, 560, 600 and 620 nm using an interference filter. The reaction rate of chum salmon was highest at 520 nm but the rates of pink and masu salmon were highest at 560 nm. In addition, a high reaction rate at 400 nm was also observed in masu salmon, suggesting that masu salmon are sensitive to ultraviolet light.     

Egg size of skippers (Lepidoptera: Hesperiidae) in relation to their host specificity and to leaf toughness of host plants

The adaptive significance of egg size of skippers (Lepidoptera; Hesperiidae) in Japan was evaluated in relation to the leaf toughness of their major host grasses. The hesperids that fed on tougher grasses laid larger eggs. Hesperids that laid larger eggs were larger in body size, but lower in fecundity. They also had a wider host range. Thus, despite the lower fecundity, hesperids may benefit from large eggs by having a wider host range of larvae. Grass feeders had wider range of host plants than broadleaf feeders.     

Optimal reproductive strategies in age-structured populations of zooplankton

SUMMARY. We describe a model of zooplankton population dynamics that accounts for differences in mortality and physiology among animals of different ages or sizes. The model follows changes in numbers of individuals and changes in individual and egg biomass through time and it expresses mortality and net assimilation as functions of animal size.
We investigated the effect of egg size, age at first reproduction, and size at first reproduction on the per capita growth rates of populations growing under different conditions. In the absence of predation or when exposed to vertebrate predators that prefer large prey, populations achieve maximum growth rates when animals hatch from small eggs and reach maturity quickly at small sizes. Populations exposed to invertebrate predators that concentrate on small animals may increase r in two different ways. One way is for animals to increase juvenile survivorship by hatching from large eggs and by shortening the juvenile period. An alternative strategy is for animals to hatch from small eggs and to postpone maturity until they grow beyond the range of sizes available to their predators. Certain life history strategies maximize r if animals continue to grow after they reach maturity. By growing larger, non-primiparous females are able to hatch larger clutches and thereby increase the overall rate of population growth.
The model analysis shows how to assess age-dependent mortality rates from field data. The net rate of population increase and the age distribution of eggs together provide specific, quantitative information about mortality.     

Body size and fecundity in the waterstrider Aquarius remigis: a test of Darwin's fecundity advantage hypothesis

The general female bias in body size of animals is usually attributed to fecundity selection. While many studies have demonstrated a positive relationship between body size and fecundity, the most common interpretation of fecundity selection is that larger females have larger abdomens and can hold more eggs, yet the relationship between abdomen size and fecundity has rarely been examined. For the waterstrider, Aquarius remigis, we find a significant relationship between body size and fecundity and demonstrate that the target of fecundity selection is abdomen size. Thus, larger females have higher fecundities because they have larger abdomens and not because of their total size per se. The rate at which fecundity increases with increasing abdomen size exceeds that which would be expected due to a simple volume constraint and suggests that other factors, such as increased ability to obtain resources, may contribute to the increase in fecundity with body size. Selection intensities estimated from our data indicate that fecundity selection could be a significant selective force on both total and abdomen lengths. Previous studies have found that abdomen size increased faster than body size and thus, larger females had relatively larger abdomens. The relationship of abdomen length and thorax length in A. remigis is hypoallometric and indicates that larger females have relatively smaller abdomens. We hypothesize that this may reflect conservation of abdomen size in females developing under poor conditions. Finally, while egg size is not directly related to body size, we find a trade-off between egg size and number when female abdomen length is held constant, suggesting that selection on egg size may influence abdomen length only indirectly through its effects on fecundity.     

Modelling urban populations of the Australian White Ibis (Threskiornis molucca) to inform management

Since the 1970s, populations of the Australian White Ibis (Threskiornis molucca) have dramatically increased in many Australian urban centres. Managers of ibis are currently focusing on limiting this bird's reproductive success in order to reduce population sizes or at least halt further increases in urban areas. Here we use data on nesting success and survival for three populations of ibis around greater Sydney to develop an age-structured population model. The estimated growth rate for all populations combined was about 1.5 % per year and for individual sites were more variable at −1, −7, and 9 %. For all populations, growth rates were most sensitive (based on elasticity analyses) to the survival of adults and least sensitive to fecundity, especially of 3 year olds. Further exploration of the importance of fecundity rates, which are relatively poorly known for these populations, suggests that rates of <0.4 fledglings per nest per year is very likely to lead to a population decline (λ less than lower bound of 95 % CI). Conversely, positive population growth is nearly assured (λ greater than upper bound of 95 % CI) for fecundities of >0.7 fledgling per nest per year. The results suggest that ibis from other locations (probably their traditional breeding areas in inland Australia) have immigrated into urban environments as estimated growth rates cannot account for current population sizes. Management strategies must take these findings into account and also consider that ibis are declining in their traditional habitats to avoid exacerbating their decline at a regional scale.     

The structure of sculpin populations along a stream size gradient

Synopsis Data on spatial variation of sculpin density, growth and fecundity support the hypothesis that populations of stream fish are structured by changes in risk of predation and prey availability along a gradient in stream size. Cottus bairdi in warm streams and C. cognatus in cold streams exhibit similar patterns. Sculpins in large streams have faster individual growth rates and higher fecundities than those in small streams, but occur at lower density. The patterns appear to be persistent and suggest that predation reduces sculpin density in larger streams. Competitive release, variations in prey productivity, and local factors probably contribute to the variation in sculpin growth.     

Size and fecundity hierarchies in an herbaceous perennial

Summary Inequalities in size in populations have potentially important effects on fitness but have rarely been examined in natural populations. I measured size (number of culms) and fecundity (number of spikelets) in five populations of the grass Danthonia spicata from 1981 to 1985. The populations were in sites in a Pinus-Quercus-Populus forest in northern lower Michigan, USA comprising a secondary succession sequence. The sites had been burned in 1980, 1954, 1948, 1936, and 1911, respectively. Mean sizes and fecundities and the amount of hierarchy in size and fecundity, measured by the Gini coefficient, were compared between the YOUNG population, 1980 burn site, and the OLD populations, 1954, 1948, 1936, and 1911 burn sites. I found large differences in mean size and fecundity between the YOUNG and OLD populations with much larger individuals in the YOUNG population. No differences in size hierarchies were found in either the first year of measurement or after five years. The fecundity hierarchies showed no significant difference among the populations in the first year but after five years the YOUNG population showed a significant decrease in amount of inequality. The longterm patterns of size and fecundity hierarchies differed because fecundity was a cumulative trait while size was not. Size inequalities may not always be a good measure of fecundity inequalities. Short-term measures of inequality in perennials may not be a good indicator of long-term values. In contrast to greenhouse studies, habitat light levels did not affect size hierarchies although they did affect fecundity hierarchies.     

Evolution of marine invertebrate reproductive patterns

A simple model of the evolution of reproductive patterns in marine benthic invertebrates is presented. The aim is to discuss the dichotomous distribution of forms into those which produce a large number of small eggs with planktotrophic development, and those which produce a small number of large eggs with direct or lecithotrophic development. The fecundity of adult individuals is assumed to be inversely proportional to egg size, and the mortality of planktonic larvae is assumed to be density independent and size dependent. The growth of planktonic larvae is assumed to be sigmoid with metamorphosis occurring at a given size. The model concludes that there are at most two evolutionarily stable egg sizes. Depending on larval growth rate and death rate, the metamorphosis size, a smaller egg size, or both may be evolutionarily stable. The predictions of the model are compared to patterns observed in nature. Illustrative data are supplied mainly from prosobranch molluscs.