Developmental response to a seasonal time constraint: the effects of photoperiod on reproduction in the grasshopper Romalea microptera

Abstract.  1. Some organisms respond adaptively to seasonal time constraints by altering development time to life-history transitions (e.g. metamorphosis, oviposition). Such life-history changes may have costs (e.g. reduced fecundity, mass, offspring quality).
2. The hypothesis that a northern population of the grasshopper Romalea microptera (Beauvois) would show adaptive plasticity in oviposition timing in response to seasonal time constraints was tested by manipulating photoperiod to simulate the middle of the active season (Long photoperiod), the end of the active season (Short photoperiod), and seasonal change (photoperiod Declining from long to short). Females received either high or low food rations. Short or Declining photoperiod were predicted to induce early oviposition with costs of reduced egg number, post-oviposition mass, or egg size, particularly in low-food females.
3. Effects of food ration and photoperiod, but not interaction, were significant in failure time analysis of age at oviposition. mancova on age at oviposition, egg number, and post-oviposition mass yielded similar effects. The multivariate effect of photoperiod resulted primarily from reduced time to oviposition in Short or Declining photoperiod. No costs in egg number or post-oviposition mass were associated with this photoperiod-induced reduction in time to oviposition. The multivariate effect of food ration resulted mainly from lower egg number with low food. Food ration affected egg size, but photoperiod and interaction did not. In all cases, Short and Declining photoperiod produced similar effects.
4. In its northern range, R. microptera accelerates reproduction in response to seasonal constraints, a response that may be adaptive. How R. microptera avoids costs associated with this reduced pre-oviposition period remains unknown.     

The effect of a seasonal time constraint on development time, body size, condition, and morph determination in the horned beetle Allomyrina dichotoma L. (Coleoptera: Scarabaeidae)

Abstract.  1. In horned beetles selection favours males that adjust their investment in horn development in relation to cues that predict adult body size. Here it is shown that in the Japanese horned beetle, Allomyrina dichotoma . There is a significant discontinuity in the horn length body size allometry. This can be described as a linear relationship that is shifted towards an increased horn length to body length ratio in males with horns longer than 16 mm.
2. Larval nutrition explains morph determination in A. dichotoma . However, unlike other species, variation in larval nutrition was the result of a seasonal time constraint that limits the time available for feeding prior to the onset of winter diapause.
3. Even when eggs were reared with an ad libitum food supply, minor morphs were still observed. Individuals that were oviposited later in the season had less time to feed, shorter development times, eclosed as smaller individuals and, in the case of males, were more likely to be hornless. Major morphs, minor morphs, and females all reduced their body size in response to seasonal time constraints in the same way. However, males that were laid later in the season had faster development times than females laid at the same time, but showed no reduction in their size relative to females, suggesting seasonal time constraints increase growth rates in males but not in females.
4. No evidence was found that seasonal time constraints resulted in a reduction of size-corrected fat reserves at eclosion, or that minor morphs gained any developmental advantage by reducing investment in horn length.     

Effect of prey quantity and temperature on nest demography of social wasps

Abstract. 1. To determine the effect of prey quantity and temperature on nest demography of social wasps ( Polistes fuscatus ), field experiments were conducted, in which wasps were provided with a low quantity of caterpillars (approximately nest subsistence level) or high quantity (three times as much). In addition, in the third year, the nest boxes were modified to be relatively cool (white and insulated) or warm (black).
2. In 1997 and 1998, high-food nests had a high proportion of cells containing developing offspring, produced more offspring, and had disproportionately more female offspring compared with low-food nests.
3. In 1999, the cool and warm nest boxes exhibited a daily average difference of 1.3 °C and a maximum difference of 6.9 °C. The warm high-food nests produced more offspring than any other treatment. The cool high-food nests produced similar numbers of offspring as the cool low-food and warm low-food treatments.
4. As indicated by a surplus of uncapped cells, foundresses of low-food nests, which would have had the egg-laying potential to produce many offspring, apparently sacrificed eggs to provide a food supplement for the developing offspring. This pattern plus more time observed foraging suggests considerable expenditure of energy by foundresses, in response to the low supply of food.
5. Importantly, these results indicate that an interactive effect of prey quantity and temperature on offspring production occurred, which may reflect the different effects on developmental rate and growth rate at cool versus warm nest temperatures.     

Variability in the life history of the water strider Gerris lacustris (Heteroptera: Gerridae) across small spatial scales

Abstract.  1. Variability in life-history traits of the common pond skater Gerris lacustris (L.) (Heteroptera: Gerridae) was investigated on a small geographical scale in Bavaria, Germany (≈ 49 °N). In an area of approximately 3.5 × 3.0 km, voltinism and wing length data of G. lacustris were recorded from 65 ponds located either in an open field or surrounded by forest.
2. Life history differed significantly between populations in the two habitat types. Gerris lacustris showed a bivoltine life-cycle in most of the field ponds with a high proportion of long-winged individuals in the diapause generation and a large increase of the short-winged morph in the midsummer generation. In contrast, almost all populations inhabiting forest ponds were univoltine. Only a few forest populations produced any second generation larvae. The long-winged morph dominated during the whole season in forest ponds.
3. The appearance of first-instar larvae was delayed by 3–4 weeks in forest ponds compared with field ponds. Presumably, low temperatures led to this reduction in the developmental rate of eggs and larvae. Consequently, 'field larvae' of the first generation developed under increasing daylight conditions (triggering direct reproduction and brachyptery) but 'forest larvae' developed under a shortening photoperiod (triggering diapause and macroptery).
4. Offspring raised from 'forest parents' in the laboratory showed the same variability in life history when reared in a cool (16.9 ± 2.4 °C) or warm (23.1 ± 1.8 °C) temperature regime.
5. Given the small spatial scale of the study, it is concluded that the variability in the life history of G. lacustris has a strong phenotypic component.     

Sex and Season Affect Individual-Level Diet Variation in the Neotropical Marsupial Gracilinanus microtarsus (Didelphidae)

Individual-level diet variation was investigated in Gracilinanus microtarsus , an insectivorous marsupial whose diet is affected by sex and season in the highly seasonal Brazilian Cerrado. To measure individual-level diet variation, the diets of individual males and females were compared to that of their population in the warm-wet and cool-dry seasons using the proportional similarity index (PS _i ). This index varies from 1 (complete overlap between the individual i  's and population's diets) toward 0 (decreasing overlap). Mean PS _i values were computed as a measure of the degree of interindividual diet variation (the larger the mean PS _i value, the smaller the variation among individuals' diets). Interindividual diet variation among females was similar between seasons, whereas among males it decreased from the warm-wet to the cool-dry season. Diet variation among males is probably reduced in the cool-dry season because of constraints on food consumption generated by interactions between endogenous (physiological needs associated with high rates of body mass growth) and exogenous (food limitation) factors.     

Ecophysiological diversity of alpine populations of Gammarus lacustris in relation to environmental calcium

1. The contribution of environmental calcium to ecophysiological diversity among populations of Gammarus lacustris was investigated by comparing five alpine lakes in France.
2. Calcium concentrations in the water varied with substratum lithology, ranging from 6 mg L^–1 in lakes on crystalline (igneous) rocks to 32.7 mg L^–1 in lakes on limestone, and up to 248 mg L^–1 in a lake with a gypsum environment.
3. The duration of the moult cycle and mortality rate were compared between populations by means of field and in situ laboratory and translocation experiments on adult males. Animals from water with low calcium concentrations showed a significantly longer moult cycle and higher mortality rate than those from water with high calcium concentrations. A significant increase in the duration of the moult cycle was observed in animals translocated to water of lower calcium concentration and vice versa.
4. These results suggest the existence of different physiological races of G. lacustris with an ecophysiological plasticity of calcium metabolism narrower than that of river-dwelling gammarids such as G. fossarum. This may be a consequence of geographic isolation among lacustrine populations together with the relatively stable environmental parameters in lakes.     

Genetic evidence for `leaky' cohorts in the semivoltine stonefly Peltoperla tarteri (Plecoptera: Peltoperlidae)

1. Genetic techniques are being used increasingly to address questions about dispersal and gene flow of freshwater invertebrates. However, population genetic structure can be affected by factors other than dispersal. Many stream insects have long life cycles that result in the simultaneous existence of multiple cohorts throughout the larval development period. If larval development is fixed, successive cohorts may be reproductively isolated and, as a result, genetically distinct. In such cases, significant levels of genetic differentiation between cohorts could confound estimates of dispersal based on population genetic structure.
2.  Peltoperla tarteri is a stonefly that can be abundant in Appalachian headwater streams. Although P. tarteri is univoltine at the type locality (Big Paint Hollow, WV, U.S.A.), the study populations in southwestern Virginia, U.S.A., were semivoltine. This semivoltine life cycle results in the simultaneous existence of multiple cohorts with the potential for significant genetic differentiation among them.
3. Levels of genetic differentiation among P. tarteri cohorts were analysed with mitochondrial DNA (mtDNA) sequence data from the non-coding origin of replication or `control' region from 93 individuals from two successive cohorts (collected in 1998 and 1999).
4. Analysis of molecular variance ( AMOVA ) indicated no genetic differentiation among cohorts ( F _ST=0.0), and gene flow among cohorts was very high ( Nm =∞).
5. High levels of gene flow among cohorts suggest that larval development of P. tarteri is not fixed. Gene flow among cohorts most likely occurs as a result of a cohort split in which some individuals complete development in one or three years instead of two.     

Sexual size dimorphism in the two spot ladybird beetle Adalia bipunctata: developmental mechanism and its consequences for mating

Abstract  1. The literature on ladybirds indicates that males are consistently smaller than females but take the same length of time to complete their development. Rearing Adalia bipunctata at 20 and 25 °C confirmed that protandry cannot account for sexual size dimorphism in this species, nor can a difference in egg size.
2. Female larvae consumed more food and had a higher relative growth rate in the fourth instar than did male larvae.
3. When food is limited, small males appear to be more successful at mating than are large males.
4. To account for these results, it is hypothesised that the gonads of male larvae compete more strongly with the soma for resources and that this reduces the growth potential of the soma of male larvae relative to that of female larvae. The greater mating success of small males when food is limited supports the eat or mate hypothesis, which predicts that when food is limited small males will spend less time feeding and more time mating than will large males.     

Conservation implications of genetic variation between spatially and temporally distinct colonies of the endangered damselfly Coenagrion mercuriale

Abstract.  1. Good conservation management is underpinned by a thorough understanding of species' historical and contemporary dispersal capabilities along with the possible adaptive or neutral processes behind any spatio-temporal genetic structuring. These issues are investigated with respect to the rare damselfly Coenagrion mercuriale (Charpentier) – the only odonate species currently listed in the U.K.'s Biodiversity Action Plan – in east Devon where its distribution has become fragmented.
2. The two east Devon C. mercuriale populations, only 3.5 km apart, have accumulated strong differences in frequencies of alleles at 14 microsatellite loci as a consequence of poor adult dispersal and drift. There is no contemporary migration between sites.
3. A genetic signature of population decline at both sites corresponds with known demographic reductions. Coenagrion mercuriale in east Devon are now significantly genetically less diverse than those from a population stronghold in the Itchen Valley.
4. Colonies would benefit from improved connectivity between areas and possibly by a transfer of individuals from other ecologically similar areas.
5. Because C. mercuriale has a semivoltine life cycle throughout the U.K., the possibility that alternate-year cohorts are reproductively isolated is explored. Genetic differentiation among cohorts is an order of magnitude less than between sites, suggesting that some larvae delay their development into adults for a year and recruit to a different cohort.
6. To our knowledge, this is the first study to document migration and gene flow between alternate-year cohorts in a species of odonate. From a conservation standpoint, the cohorts do not require separate management.     

Phenotypic variation in male and worker encapsulation response in the bumblebee Bombus terrestris

Abstract.  1. Life-history traits such as immunity are often characterised by the presence of large phenotypic variation, but it often remains unclear how and why this variation is maintained by selection.
2. Here an annual social insect, the bumblebee Bombus terrestris , was used to study variation in encapsulation response of males and workers. Bumblebees are a suitable system to study offspring immunity because they are host to a broad variety of different parasites. Bumblebee males, in particular, have a long lifespan compared with other social insect males and their immunity should therefore be an important element for colony reproductive success.
3. Encapsulation response, which was used here as a measurement for the generalised immune defence capacity of an individual, was found to be a highly variable trait. High levels of worker response correlated with low levels of colony parasitism rates.
4. Encapsulation response was found to be (a) lower in males compared with sister workers, and (b) lower in late-produced cohorts compared with early ones.
5. In colonies with delayed sexual reproduction, males had a lower encapsulation response. Thus, investments into immunity seemed reduced in later male cohorts and those eclosing later in the season, perhaps because males had a shorter expected remaining time to acquire matings. The results presented add further evidence that immune defence is a key variable defining colony fitness in social insects.     

Plasticity of Noddy Parents and Offspring to Sea-Surface Temperature Anomalies

Behavioral and/or developmental plasticity is crucial for resisting the impacts of environmental stressors. We investigated the plasticity of adult foraging behavior and chick development in an offshore foraging seabird, the black noddy (Anous minutus), during two breeding seasons. The first season had anomalously high sea-surface temperatures and ‘low’ prey availability, while the second was a season of below average sea-surface temperatures and ‘normal’ food availability. During the second season, supplementary feeding of chicks was used to manipulate offspring nutritional status in order to mimic conditions of high prey availability. When sea-surface temperatures were hotter than average, provisioning rates were significantly and negatively impacted at the day-to-day scale. Adults fed chicks during this low-food season smaller meals but at the same rate as chicks in the unfed treatment the following season. Supplementary feeding of chicks during the second season also resulted in delivery of smaller meals by adults, but did not influence feeding rate. Chick begging and parental responses to cessation of food supplementation suggested smaller meals fed to artificially supplemented chicks resulted from a decrease in chick demands associated with satiation, rather than adult behavioral responses to chick condition. During periods of low prey abundance, chicks maintained structural growth while sacrificing body condition and were unable to take advantage of periods of high prey abundance by increasing growth rates. These results suggest that this species expresses limited plasticity in provisioning behavior and offspring development. Consequently, responses to future changes in sea-surface temperature and other environmental variation may be limited.     

Can grazing sheep compensate for a daily foraging time constraint?

1. Theoretical studies of large herbivore foraging assume that total daily grazing time is a key constraint on daily intake and diet choice. We experimentally tested this assumption and investigated the effects of food availability on the ability of grazing sheep to compensate for restriction of available daily grazing time.
2. Foraging behaviour, intake and diet digestibility by sheep, were measured on grass pastures in a replicated 2 × 2 factorial experiment, in which overnight access to pasture was varied (restricted overnight and continuous access) on two sward heights (5·5 and 3·0 cm), representing high and low food availability.
3. Regardless of food availability, the overnight-restricted sheep fed for almost all of the available grazing time by grazing for fewer, longer foraging bouts, but still had much shorter total daily grazing time than the continuous access sheep.
4. In response to overnight penning, the sheep had a significantly higher instantaneous rate of intake achieved mainly via larger bites. The continuous access sheep were hence not maximizing their short-term rate of intake, whilst grazing according to the daily schedule considered normal for sheep.
5. The behavioural responses to overnight food restriction were able to counteract the reduction in daily grazing time only where food availability was high. In contrast on short swards overnight grazing restriction led to a reduction in total daily intake. We suggest that the interactions between the factors considered as constraints on foraging behaviour of herbivores are, as yet, only poorly quantified.     

Influence of adult nutrition on the relationship between body size and reproductive parameters in a parasitoid wasp

Abstract.  1. An important constraint upon life-history evolution in parasitoids is the limit imposed by body size on allocation of limited metabolic resources to different fitness-related physiological functions such as reproduction and survival.
2. The influence of adult nutrition on reproductive and maintenance variables was studied in the synovigenic ectoparasitoid Mastrus ridibundus , and it was determined whether resource allocation to these different functions depends on body size.
3. Over the course of adult life there was a positive relationship between body size and the number of mature eggs in adult females both in the presence and absence of food. However, only in the presence of food did egg maturation rates increase significantly with body size. Starved wasps produced significantly smaller eggs than fed ones, which has not been documented before. Moreover, starved wasps produced fewer offspring than fed wasps, and attacked fewer hosts.
4. The availability of food had a major effect on longevity, with fed females living about 10 times longer than starved ones. There was also a positive relationship between body size and longevity. In starved wasps, this relationship was the same both in the presence and absence of hosts, but in fed wasps there was a positive relationship between body size and longevity in the absence of hosts only. Allocation to initial eggs relative to lifetime progeny production did not decline with body size.
5. The data reveal that in M. ridibundus the trade-off between maintenance and reproduction varies with life expectancy.     

Effects of dietary nicotine on the development of an insect herbivore, its parasitoid and secondary hyperparasitoid over four trophic levels

Abstract.  1. Allelochemicals in herbivore diet are known to affect the development of higher trophic levels, such as parasitoids and predators.
2. This study examines how differing levels of nicotine affects the development of a herbivore, its parasitoid and secondary hyperparasitoid over four trophic levels. Separate cohorts of the herbivore, Manduca sexta , were fed on artificial diets containing 0.0, 0.1, and 0.5% wet weights of nicotine. Some of the larvae in each cohort were separately parasitised in the first (L1) and third (L3) instars by the gregarious endoparasitoid, Cotesia congregata . Newly emerged parasitoid cocoons were, in turn, parasitised by the hyperparasitoid, Lysibia nana .
3. Pupal mass in M . sexta was negatively correlated with nicotine concentrations in the artificial diet, although larval development time was unaffected.
4. Hyperparasitoid survival was highest when there were low levels of nicotine in the diet of M . sexta . Cocoon mass in C . congregata and adult mass in L . nana were mostly affected by nicotine levels in host diet when L1 M. sexta larvae were parasitised. The effects were slightly stronger on L . nana than on C . congregata , indicating the presence of both qualitative and quantitative effects of nicotine concentration on both species.
5. The results suggest that allelochemicals in herbivore diet can have both direct and indirect effects on the performance of higher trophic levels. However, in multitrophic interactions these effects can vary with the stage of the herbivore attacked by the primary parasitoid, as well as with the strategy employed by the herbivore to deal with plant phytotoxins.     

Predation risk perception and food scarcity induce alterations of life-cycle traits of the mosquito Culex pipiens

Abstract.  1. Predators may affect prey populations by direct consumption, and by inducing defensive reactions of prey to the predation risk. Food scarcity frequently has effects on the inducible defences of prey, but no consistent pattern of food–predation risk interaction is known.
2. In this study the combined effect of food shortage and predation-risk perception in larvae of the mosquito Culex pipiens was investigated. Water exposed to the aquatic predator bug Notonecta glauca was used as a source of predation intimidation. Mosquito larvae were reared in three different media containing either no predator cues or the cues of N. glauca that had been fed on either C. pipiens larvae or on Daphnia magna . Food was provided in favourable or limited amount for these set-ups.
3. The results showed that chemical cues from the predators fed with prey's conspecifics caused a decreased survival, delayed pre-imaginal development, and reduction in body size of emerged mosquitoes, whereas chemical cues from predators fed with D. magna caused only delayed development. Food scarcity significantly exacerbates the negative effect of the predator cues on pre-imaginal development of C. pipiens . Effects of the cues on larval development and body size of imagoes are significantly stronger for females than for males.
4. The present study suggests that when food is limited, predators can affect population dynamics of prey not only by direct predation, but also by inducing lethal and sublethal effects due to perception of risk imposed by chemical cues. To understand the effects of predators on mosquito population dynamics, environmental parameters such as food deficiency should be considered.     

The role of food and colony size in sexual offspring production in a social insect: an experiment

Abstract.  1. Large colonies of ants are known to have a higher propensity for sexual offspring production, probably owing to their high capacity to exploit food resources.
2. The effects of food supplementation on the propensity for sexual offspring production, and whether it is linked with colony size, were investigated in an environment with poor resources (clear-cut areas).
3. Large colony size was associated with a higher propensity for sexual offspring production in food-supplemented colonies, whereas in non-supplemented control colonies an association with colony size was not found.
4. The results demonstrate that large colonies seem to have a higher capacity to exploit supplemented food. In addition, the production of sexual offspring was apparently limited by food availability in clear-cuts, especially for large colonies.     

GEOGRAPHIC VARIATION IN LIFE-HISTORY TRAITS OF THE ANT LION,MYRMELEON IMMACULATUS: EVOLUTIONARY IMPLICATIONS OF BERGMANN'S RULE

In eastern North America, body size of the larval ant lion Myrmeleon immaculatus increases from south to north, following Bergmann's rule. We used a common-garden experiment and a reciprocal-transplant experiment to evaluate the effects of food and temperature on ant lion growth, body size, and survivorship. In the laboratory common-garden experiment, first-instar larvae from two southern (Georgia, South Carolina) and two northern (Connecticut, Rhode Island) populations were reared in incubators under high- and low-food and high- and low-temperature regimes. For all populations, high food increased final body mass and growth rate and decreased development time. Growth rates were higher at low temperatures, but temperature did not affect larval or adult body mass. Survivorship was highest in high-food and low-temperature treatments. Across all food and temperature treatments, northern populations exhibited a larger final body mass, shorter development time, faster growth rate, and greater survivorship than did southern populations. Results were similar for a field reciprocal-transplant experiment of third-instar larvae between populations in Connecticut and Oklahoma: Connecticut larvae grew faster than Oklahoma larvae, regardless of transplant site. Conversely, larvae transplanted to Oklahoma grew faster than larvae transplanted to Connecticut, regardless of population source. These results suggest that variation in food availability, not temperature, may account for differences in growth and body size of northern and southern ant lions. Although northern larvae grew faster and reached a larger body size in both experiments, northern environments should suppress growth because of reduced food availability and a limited growing season. This study provides the first example of countergradient selection causing Bergmann's rule in an ectotherm.     

Ontogenetic changes in the association between antipredator responses and growth variables

Abstract.  1. An organism's growth parameters are expected to depend on environmental constraints, such as predation risk and food supply. However, antipredator responses, food intake, and thus growth of an animal may be mediated by behavioural traits, which are likely to differ among developmental stages. In this study, it was investigated how the relationship between growth and behavioural antipredator responses changes during ontogeny in the time-constrained dragonfly species Libellula depressa , and which factors influenced specific behavioural decisions at different points in ontogeny.
2. The results revealed that behavioural strategies differed between larval developmental sages, depending on associations between larval growth, food supply, and predation risk. Early in ontogeny, faster development was correlated with high larval activity and high food supply. This resulted in high activity levels under high food conditions irrespectively of predator presence, and under low food supply in predator absence only. In the intermediate stage of development, all larvae displayed a high activity level, which was correlated in general with fast development. However, growth later in ontogeny was not only influenced by the activity level, but also by predator presence and food supply, with larvae reared under high food supply and/or in presence of predators attaining a higher final mass. Thus, not only the way in which larval growth parameters and behaviour are related changed during development, but also whether the factors influenced larval growth and behaviour. Once the larvae reached the ultimate stage of development, in which they overwinter, behavioural patterns observed were consistent with model predictions.
3. It is advocated that behavioural plasticity of prey organisms in different developmental stages should be analysed in the context of associated growth variables.     

ADAPTIVE PHENOTYPIC PLASTICITY IN GROWTH,DEVELOPMENT, AND BODY SIZE IN THE YELLOW DUNG FLY

Life-history theory predicts that age and size at maturity of organisms should be influenced by time and food constraints on development. This study investigated phenotypic plasticity in growth, development, body size, and diapause in the yellow dung fly, Scathophaga stercoraria. Full-sib families were allowed to develop under predator-free field conditions. The time before the onset of winter was varied and each brood was split into three environments differing in the amount of dung a set number of larvae had as a resource. When resources were abundant and competition was minimal, individuals of both sexes grew to larger body sizes, took longer time to mature, and were able to increase their growth rates to attain large body sizes despite shorter effective development periods later in the season. In contrast, limited larval resources and strong competition constrained individuals to mature earlier at a smaller adult size, and growth rates could not be increased but were at least maintained. This outcome is predicted by only two life-history optimality models, which treat mortality due to long development periods and mortality due to fast growth as independent. Elevated preadult mortality indicated physiological costs of fast growth independent of predation. When larval resources were limited, mortality increased with heritable variation in development time for males, and toward the end of the season mortality increased as larval resources became more abundant because this induced longer development periods. Sexual and fecundity selection favoring large body size in this species is thus opposed by larval viability selection favoring slower growth in general and shorter development periods when time and resources are limited; this overall combination of selective pressures is presumably shaping the reaction norms obtained here. Flexible growth rates are facilitated by low genetic correlations between development time and body size, a possible consequence of selection for plasticity. Heritable variation was evident in all traits investigated, as well as in phenotypic plasticity of these traits (genotype X interactions). This is possibly maintained by unpredictable spatiotemporal variation in dung abundance, competition, and hence selection.     

Reproductive traits of stream-dwelling brown trout Salmo trutta in contrasting neighbouring rivers of central Spain

1. Reproductive traits were studied in seven wild populations of resident brown trout in Spain. We examined whether growth, and certain environmental conditions such as water temperature or food abundance, could explain interpopulation variation in the reproductive characters.
2. The results indicated that nearby populations subjected to a wide variation in environmental conditions exhibited a similar reproductive performance.
3. Age distributions and sex ratios were not significantly different among populations. Age ranged from 0+ to 4+ years but the populations were dominated by the 0+ to 2+ groups. In all rivers females matured at age 2+ as opposed to age 1+ and 2+ for males. Mean length at the end of the growth period differed significantly among populations. However, annual growth rate was similar among rivers and was not significantly correlated with either biomass of the benthos or water temperature during the growth season.
4. Reproductive effort, body condition, fecundity and egg size did not correlate significantly with either the abundance of benthic invertebrates or water temperature. Once the effect of body length on both egg size and number was removed, a significant negative correlation was found between these two traits among populations.