Sex differences in the energetic content of reproductive tissues in the Blackeye Goby,Rhinogobiops nicholsii

Differential energetic investment in reproduction between the sexes has been a driving a force of life history theory and sexual selection. However, reproductive costs between the sexes have often been based on morphology, such as gonad mass and gonadosomatic indices (GSI), and few have directly measured the energy content of gonadal tissues in relation to GSI. Using the blackeye goby, Rhinogobiops nicholsii, we measured the energetic content of whole gonadal tissues, specifically testes, ovaries and associated reproductive tissues using oxygen bomb calorimetry. The energy content per gram unit of gonadal tissues was generally predictive of GSI, indicating that GSI is a reasonable measure of energetic costs. Interestingly, although females had greater gonadal mass, GSI and energy content per gram than males, the sex difference in energy content per mass unit was only 13 %, suggesting that gross indices such as gonadal mass or GSI may overestimate energetic costs where instead the cost difference in a unit gram of gonadal tissues between the sexes is smaller than often predicted. This study also demonstrates that although the cost of ovaries is greater than testes, males’ investment in reproductive tissue can be considerable, which is consistent with the often inflated reproductive success for males in haremic mating systems.     

Do sexual dimorphisms in reproductive allocation and new shoot biomass increase with an increase of altitude? A case of the shrub willow Salix reinii (Salicaceae)

Based on the general tendency for females of dioecious plants to pay higher reproductive cost than males, it has been predicted that females should have much more reduced reproductive outputs and diminished vegetative production than males in energy-limited habitats. Nevertheless, this prediction has rarely been directly investigated. We investigated altitudinal changes in reproductive biomass and shoot production, normalized by plant size, for females and males of a shrub willow, Salix reinii, on Mt. Hakkoda, northeast Japan. Females maintained higher reproductive biomass than males at all altitudes; however, reproductive allocation for both sexes tended to decrease at a similar rate with an increase in altitude. Moreover, females vegetatively produced at the same rate as males at all altitudes. These findings suggest that females have a mechanism to compensate for the extra investment in reproduction irrespective of a changing environment. Shoot production did not change with altitude, suggesting that S. reinii gave priority to vegetative investment at the cost of reproductive output at higher altitudes. Inconsistent with general predictions, females did not respond more sensitively than males to severe environmental conditions in either reproductive allocation or shoot production, despite much higher resource investment in reproduction.     

Reproductive effort and costs of reproduction do not explain female-biased sex ratios in the moss Pseudocalliergon trifarium (Amblystegiaceae)

A fundamental assumption in life-history theory is that reproduction is costly. Higher reproductive investment for fruits than for flowers may result in larger costs of reproduction in females than in males, which is often used to explain male-skewed sex ratios in unisexual seed plants. In contrast, bryophytes have predominantly female-biased sex ratios, suggested to be a product of a higher average cost of sexual reproduction in males. Empirical evidence to support this notion is largely lacking. We investigated sex-specific reproductive effort and costs in the unisexual moss Pseudocalliergon trifarium that has a female-dominated expressed sex ratio and rarely produces sporophytes. Annual vegetative segment mass did not differ among male, female, and non-expressing individuals, indicating that there was no threshold-size for sex expression. Mean and annual mass of sexual branches were higher in females than in males, but branch number per segment did not differ between sexes. Prefertilization reproductive effort for females was significantly greater (11.2%) than for males (8.6%). No cost for sexual branch production in terms of reduced relative vegetative growth or decreased investment in reproductive structures in consecutive years was detected. A higher realized reproductive cost in males cannot explain the unbalanced sex ratio in the study species.     

The long shadow of senescence: age impacts survival and territory defense in loons

Senescence, increased mortality that occurs among animals of advanced age, impacts behavior and ecology in many avian species. We investigated actuarial, reproductive, and behavioral senescence using capture, marking, and resighting data from a 26‐year study of common loons Gavia immer. Territorial residents of both sexes exhibited high annual survival (0.94) until their mid 20s, at which point survival fell to 0.76 and 0.77 in males and females, respectively. Sexual symmetry in actuarial senescence is somewhat surprising in this species, because males make a substantially greater investment in territory defense and chick‐rearing and because males engage in lethal contests for territory ownership. Survival of displaced breeders (0.80) was lower than that of territorial residents in both young and old individuals. Old males and females also experienced slightly higher annual probability of eviction (0.16 for males; 0.17 for females) than prime‐aged breeders (0.13 for both sexes), indicating senescence in territory defense. Prime‐aged males reclaimed territories at a high rate (0.49), in contrast to females of the same age (0.33). However, old males resettled with success (0.35) similar to old females (0.31), suggesting that males decline in competitive ability as they age. Nonetheless males, but not females, showed an apparent increase in breeding success over the entire lifetime, a possible indication that very old males make a terminal investment in reproductive output at the cost of survival.     

Sex differences in parental care: Gametic investment,sexual selection,and social environment

Male and female parents often provide different type and amount of care to their offspring. Three major drivers have been proposed to explain parental sex roles: (1) differential gametic investment by males and females that precipitates into sex difference in care, (2) different intensity of sexual selection acting on males and females, and (3) biased social environment that facilitates the more common sex to provide more care. Here, we provide the most comprehensive assessment of these hypotheses using detailed parental care data from 792 bird species covering 126 families. We found no evidence for the gametic investment hypothesis: neither gamete sizes nor gamete production by males relative to females was related to sex difference in parental care. However, sexual selection correlated with parental sex roles, because the male share in care relative to female decreased with both extra‐pair paternity and frequency of male polygamy. Parental sex roles were also related to social environment, because male parental care increased with male‐biased adult sex ratios (ASRs). Taken together, our results are consistent with recent theories suggesting that gametic investment is not tied to parental sex roles, and highlight the importance of both sexual selection and ASR in influencing parental sex roles.     

The evolution of gonad expenditure and gonadosomatic index (GSI) in male and female broadcast‐spawning invertebrates

Sedentary broadcast‐spawning marine invertebrates, which release both eggs and sperm into the water for fertilization, are of special interest for sexual selection studies. They provide unique insight into the early stages of the evolutionary succession leading to the often‐intense operation of both pre‐ and post‐mating sexual selection in mobile gonochorists. Since they are sessile or only weakly mobile, adults can interact only to a limited extent with other adults and with their own fertilized offspring. They are consequently subject mainly to selection on gamete production and gamete success, and so high gonad expenditure is expected in both sexes. We review literature on gonadosomatic index (GSI; the proportion of body tissue devoted to gamete production) of gonochoristic broadcast spawners, which we use as a proxy for gonad expenditure. We show that such taxa most often have a high GSI that is approximately equal in both sexes. When GSI is asymmetric, female GSI usually exceeds male GSI, at least in echinoderms (the majority of species recorded). Intriguingly, though, higher male GSI also occurs in some species and appears more common than female‐biased GSI in certain orders of gastropod molluscs. Our limited data also suggest that higher male GSI may be the prevalent pattern in sperm casters (where only males release gametes). We explore how selection might have shaped these patterns using game theoretic models for gonad expenditure that consider possible trade‐offs with (i) somatic maintenance or (ii) growth, while also considering sperm competition, sperm limitation, and polyspermy. Our models of the trade‐off between somatic tissue (which increases survival) and gonad (which increases reproductive success) predict that GSI should be equal for the two sexes when sperm competition is intense, as is probably common in broadcast spawners due to synchronous spawning in aggregations. Higher female GSI occurs under low sperm competition. Sperm limitation appears unlikely to alter these conclusions qualitatively, but can also act as a force to keep male GSI high, and close to that of females. Polyspermy can act to reduce male GSI. Higher male than female GSI is predicted to be less common (as observed in the data), but can occur when ova/ovaries are sufficiently more resource‐intensive to produce than sperm/testes, for which some evidence exists. We also show that sex‐specific trade‐offs between gonads and growth can generate different life‐history strategies for males and females, with males beginning reproduction earlier. This could lead to apparently higher male GSI in empirical studies if immature females are included in calculations of mean GSI. The existence of higher male GSI nonetheless remains somewhat problematic and requires further investigation. When sperm limitation is low, we suggest that the natural logarithm of the male/female GSI ratio may be a suitable index for sperm competition level in broadcast spawners, and that this may also be considered as an index for internally fertilizing taxa.     

Biomass and nutrient allocation in a neotropical dioecious palm

Summary The sexes of Chamaedorea ernesti-augusti are largely undifferentiated in the distribution of biomass, nitrogen, phosphorus, potassium, and total non-structural carbohydrates, among leaves and stems. Males bear more inflorescences that are cheaper except in nitrogen, but most females bear greater annual energetic and nutritional burdens due to seed production. The ratio of vegetative to reproductive biomass is 3.5 for males but only 1.2 for females on a per module basis.     

Indiscriminate females and choosy males: within- and between-species variation in Drosophila

Abstract The classic view of choosy, passive females and indiscriminate, competitive males gained theoretical foundations with parental investment theory. When females invest more in offspring than males, parental investment theory says that selection operates so that females discriminate among males for mates (i.e., females are choosy and passive) and males are indiscriminate (i.e., males are profligate and competitive). Here we report tests of predictions using Drosophila pseudoobscura and D. melanogaster , with typical asymmetry in gamete sizes (females > males), and in D. hydei with far less asymmetry in gamete size. Experimental observations revealed that the labels "choosy, passive females" and "profligate, indiscriminate males" did not capture the variation within and between species in premating behavior. In each of the species some females were as active in approaching males (or more so) than males in approaching females, and some males were as discriminating (or more so) than females. In pairs focal males and females responded differently to opposite-sex than to same-sex conspecifics. Drosophila hydei were less sex-role stereotyped than the other two species consistent with parental investment theory. However, D. pseudoobscura females approached males more often than did D. melanogaster females, and male D. hydei approached females as often as males of the other two species, both results inconsistent with parental investment theory. Male D. pseudoobscura and D. hydei were more likely to approach males in same-sex pairs than male D. melanogaster , inconsistent with parental investment theory.     

The energy costs of sexual dimorphism in mole-rats are morphological not behavioural

Different reproductive strategies of males and females may lead to the evolution of differences in their energetic costs of reproduction, overall energetic requirements and physiological performances. Sexual dimorphism is often associated with costly behaviours (e.g. large males might have a competitive advantage in fighting, which is energetically expensive). However, few studies of mammals have directly compared the energy costs of reproductive activities between sexes. We compared the daily energy expenditure (DEE) and resting metabolic rate (RMR) of males and females of two species of mole-rat, Bathyergus janetta and Georychus capensis (the former is sexually dimorphic in body size and the latter is not) during a period of intense digging when males seek females. We hypothesized that large body size might be indicative of greater digging or fighting capabilities, and hence greater mass-independent DEE values in males of the sexually dimorphic species. In contrast to this prediction, although absolute values of DEE were greater in B. janetta males, mass-independent values were not. No differences were apparent between sexes in G. capensis. By comparison, although RMR values were greater in B. janetta than G. capensis, no differences were apparent between the sexes for either species. The energy cost of dimorphism is most likely to be the cost of maintenance of a large body size, and not the cost of behaviours performed when an individual is large.     

Sex‐specific winter distribution in a sexually dimorphic shorebird is explained by resource partitioning

Sexual size dimorphism (SSD) implies correlated differences in energetic requirements and feeding opportunities, such that sexes will face different trade‐offs in habitat selection. In seasonal migrants, this could result in a differential spatial distribution across the wintering range. To identify the ecological causes of sexual spatial segregation, we studied a sexually dimorphic shorebird, the bar‐tailed godwit Limosa lapponica, in which females have a larger body and a longer bill than males. With respect to the trade‐offs that these migratory shorebirds experience in their choice of wintering area, northern and colder wintering sites have the benefit of being closer to the Arctic breeding grounds. According to Bergmann's rule, the larger females should incur lower energetic costs per unit of body mass over males, helping them to winter in the cold. However, as the sexes have rather different bill lengths, differences in sex‐specific wintering sites could also be due to the vertical distribution of their buried prey, that is, resource partitioning. Here, in a comparison between six main intertidal wintering areas across the entire winter range of the lapponica subspecies in northwest Europe, we show that the percentage of females between sites was not correlated with the cost of wintering, but was positively correlated with the biomass in the bottom layer and negatively with the biomass in the top layer. We conclude that resource partitioning, rather than relative expenditure advantages, best explains the differential spatial distribution of male and female bar‐tailed godwits across northwest Europe.     

Regulation of gonad development and respiratory metabolism associated with food availability and reproductive diapause in the rice bug Leptocorisa chinensis

Food has an influence on many life history traits related to dormancy in insects. In our previous study with the rice bug Leptocorisa chinensis (Dallas) (Hemiptera: Alydidae), diapausing females transferred to conditions physically favorable for promoting the gonad development required food intake to resume gonad development, whereas males did not. These differences in response to food between males and females lead to two questions: (1) Was diapause in the starved females completed? (2) Were the starved males that resumed gonad development at the same physiological status as fed males? We tested these questions with two physiological indicators: gonad status and respiratory rate. Results indicate that starved females are able to complete diapause, but show depressed respiration relative to well-fed insects in diapause. Similar to females, starved males that resumed postdiapause gonad development also had depressed respiratory rate, and hence physiological status is presumed to be different between starved and fed individuals. In this study, it was also found that the photoperiodic signal is storable, whereas the food signal acts directly. The adaptive significance of regulation of gonad development and respiratory metabolism in relation to phenology of suitable host plant and reproductive strategy is discussed in both sexes.     

The Sicker Sex: Understanding Male Biases in Parasitic Infection,Resource Allocation and Fitness

The “sicker sex” idea summarizes our knowledge of sex biases in parasite burden and immune ability whereby males fare worse than females. The theoretical basis of this is that because males invest more on mating effort than females, the former pay the costs by having a weaker immune system and thus being more susceptible to parasites. Females, conversely, have a greater parental investment. Here we tested the following: a) whether both sexes differ in their ability to defend against parasites using a natural host-parasite system; b) the differences in resource allocation conflict between mating effort and parental investment traits between sexes; and, c) effect of parasitism on survival for both sexes. We used a number of insect damselfly species as study subjects. For (a), we quantified gregarine and mite parasites, and experimentally manipulated gregarine levels in both sexes during adult ontogeny. For (b), first, we manipulated food during adult ontogeny and recorded thoracic fat gain (a proxy of mating effort) and abdominal weight (a proxy of parental investment) in both sexes. Secondly for (b), we manipulated food and gregarine levels in both sexes when adults were about to become sexually mature, and recorded gregarine number. For (c), we infected male and female adults of different ages and measured their survival. Males consistently showed more parasites than females apparently due to an increased resource allocation to fat production in males. Conversely, females invested more on abdominal weight. These differences were independent of how much food/infecting parasites were provided. The cost of this was that males had more parasites and reduced survival than females. Our results provide a resource allocation mechanism for understanding sexual differences in parasite defense as well as survival consequences for each sex.     

Sexual differences in biomass and nutrient allocation of first-year Silene dioica plants

Reproductive and somatic biomass, nitrogen (N), and phosphorus (P) pools were compared between females and males in 1st-year plants of Silene dioica. We estimated irretrievable resources allocated to seeds, pollen, flowers, and unrecovered summer leaf investment by collecting plant parts at abscission throughout the season. At the end of the season, we determined resources lost through senescent stems and autumn leaf turnover and resources stored in perennial roots and overwintering buds. Sexual differences in allocation patterns depended on the resource used for comparison, and whether absolute or proportional resource pools were assessed. Total resource pools in terms of biomass and N were similar for females and males. However, male plants acquired relatively more P. The proportional reproductive investment, i.e., reproductive effort, was similar for males and females in terms of biomass and N. In terms of P, male reproductive effort was higher. There was no difference between sexes in the proportional and relative biomass allocated to perennial roots and overwintering buds. However, in terms of absolute and relative N allocation to below-ground parts, females had larger reserves than males. Females, moreover, had a larger proportion of their P in below-ground parts. However, as male total P pools were larger, absolute P reserves did not differ between sexes. The high reproductive effort and N depletion of below-ground parts in males resulted largely from higher flower production compared to females. In females, seeds were the major component of reproductive effort. These results show that if biomass and nutrient allocation are assessed in parallel for dioecious plants, we obtain a more complete view of their sexual differences. Received: 07 May 1998 / Accepted: 30 October 1998     

Determinants of biased sex ratios and inter-sex costs of reproduction in dioecious tropical forest trees

Estimates of the sex ratio and cost of reproduction in plant populations have implications for resource use by animals, reserve design, and mechanisms of species coexistence, but may be biased unless all potentially reproductive individuals are censused over several flowering seasons. To investigate mechanisms maintaining dioecy in tropical forest trees, we recorded the flowering activity, sexual expression, and reproductive effort of all 2209 potentially reproductive individuals within 16 species of Myristicaceae over 4 years on a large forest plot in Amazonian Ecuador. Female trees invested >10 times more biomass than males in total reproduction. Flowering sex ratios were male-biased in four species in ≥1 year, and cumulative 4-year sex ratios were male-biased in two species and for the whole family, but different mechanisms were responsible for this in different species. Annual growth rates were equivalent for both sexes, implying that females can compensate for their greater reproductive investment. There was no strict spatial segregation of the sexes, but females were more often associated with specific habitats than males. We conclude that male-biased sex ratios are not manifested uniformly even after exhaustive sampling and that the mechanisms balancing the higher cost of female reproduction are extremely variable.     

Female butterflies modulate investment in reproduction and flight in response to monsoon‐driven migrations

Migratory species may display striking phenotypic plasticity during individual lifetimes. This may include differential investment in body parts and functions, differential resource use and allocation, and behavioural changes between migratory and non‐migratory phases. While migration‐related phenotypic changes are well‐reported, their underlying mechanisms are usually poorly understood. Here we compare individuals from migratory (reproductive diapause) and non‐migratory (reproductive) phases of closely related aposematic butterfly species to study how sexual dimorphism and migratory behaviour underlie significant morphological tradeoffs, and propose a plausible scenario to explain the migration‐related phenotypic plasticity observed in females of migratory species. We found that female butterflies invested significantly more in their abdominal mass compared to males irrespective of their migratory phase, and underwent a clear shift in their body mass allocation after the switch from the reproductive diapause phase to the reproductive phase. In reproductive phase, females invested much more in reproductive tissue. This switch occurred as a result of increased abdominal mass (i.e. reproductive tissue mass) without significant reduction in the thoracic mass (i.e. flight muscle mass). Migratory males, however, were not significantly different from non‐migratory males in terms of relative investment in flight and reproductive tissues. These patterns were consistent between migratory and non‐migratory aposematic species within and across clades. While migratory habits may influence the physiology and behaviour of both sexes, long‐distance migration affected female morphology much more markedly compared to that of males. These results show the sex‐specific nature of adaptations to migratory behaviour, and suggest that seemingly disparate life‐history traits such as aposematism and migration may have similar influences on the lifetime energetic investments of insects.     

A physiological marker for quantifying differential reproductive investment between the sexes in Yellow-legged gulls (Larus michahellis)

Asymmetry between males and females in the energy they invest initially in reproduction has resulted in the evolution of differing reproductive strategies (caring females vs. competitive males). However, parental care in many birds is shared by both sexes suggesting that male energy expenditure in agonistic behaviors and courtship feeding might compensate female costs of clutch production. Here, we tested the hypothesis that initial investment in reproduction by both sexes in the Yellow-legged Gull (Larus michahellis), a species with biparental care, is similar from a physiological perspective. In this income breeder, female and male reproductive investment during early breeding can be ultimately related to muscular activity (local foraging effort required for clutch production in females and courtship feeding and agonistic behaviors in the case of males). Thus, we evaluated sex-specific patterns of creatine kinase (CK, IU/L) levels in plasma, an indicator of physical effort associated with muscular activity dependent behaviors, through incubation as a reflection of the physiological response of both sexes to the reproductive investment they made up to clutch completion. Raw levels of CK were related to plasma levels of total proteins (TP, g/dL) to account for the differential physiological state of individuals when sampled (i.e. differential dehydratation). Female costs of clutch production were associated with post-laying levels of CK/TP. We grouped females according to their relative investment in clutch production: < 15.8%, 15.8 to 17.3% and > 17.3% of field metabolic rate; which showed increasing values of CK/TP (24.6, 53.1 and 66.0 IU/g, respectively). Moreover, we found similar CK/TP trends throughout incubation for both sexes (CK/TP = 50.2− [3.3 × days from laying]) suggesting similar physiological responses to reproductive effort and, therefore, analogous sex-specific initial investment. Thus, male investment in agonistic behaviors and courtship feeding apparently equaled female investment in clutch production. The use of CK measurements is revealed as a useful approach to investigating overall investment in reproduction for both sexes, providing further insights into our comprehension of reproductive strategies in seabirds.     

Body mass dynamics during incubation and duration of parental care in Pacific Dunlins Calidris alpina pacifica: a test of the differential parental capacity hypothesis

Breeding is energetically expensive and individuals face a trade‐off between current and future breeding investment. Due to their production of large eggs, female birds are thought to have substantially higher initial energetic investments than males, which decrease the female's offspring rearing capacity. The differential parental capacity hypothesis argues that this large initial investment limits the ability of female shorebirds to provide extended parental care, which can ultimately lead to offspring desertion. This hypothesis predicts that (1) during early incubation females will be in poorer condition than males, (2) both sexes will lose condition during incubation, but the decline in females will be slower than the decline in males and (3) there will be a positive relationship between female condition and the duration of maternal brood care. These predictions were tested using data on body mass adjusted for body size (as a proxy for condition) and parental care from Pacific Dunlins Calidris alpina pacifica nesting on the Yukon Kuskokwim Delta, Alaska. None of the predictions received support: females were heavier than males in early incubation, the overall pattern during incubation was that males gained mass while female mass remained relatively constant, and there was no relationship between female mass and maternal brood care duration. These results suggest that the factors influencing parental care decisions are more complex than a parent simply caring until it is physiologically unable to do so.     

Sex differences in the onset of seasonal reproductive quiescence in hamsters

Day length is the primary cue used by many mammals to restrict reproduction to favourable spring and summer months, but it is unknown for any mammal whether the seasonal loss of fertility begins at the same time and occurs at the same rate in females and males; nor it established whether the termination of mating behaviour in males and females coincides with the loss of fertility. We speculated that females, owing to their greater energetic investment in reproduction, are the limiting sex in terminating offspring production in short days (SDs). Oestrous cycles and production of young were monitored in Syrian hamsters (Mesocricetus auratus) transferred from long days (LDs) to SDs. Females were mated to LD males after three to eight weeks of SD treatment; in a parallel experiment, males housed in SDs were mated to LD females. After five and eight weeks in SDs, at least twice as many males as females were fertile. Both males and females continued to copulate for several weeks after becoming infertile. The onset of seasonal infertility occurs earlier in females than males and the decline in fertility precedes the seasonal loss of mating behaviour in both sexes.     

Fat is sexy for females but not males: the influence of body reserves on reproduction in snakes (Vipera aspis)

Reproduction is energetically expensive for both sexes, but the magnitude of expenditure and its relationship to reproductive success differ fundamentally between males and females. Males allocate relatively little to gamete production and, thus, can reproduce successfully with only minor energy investment. In contrast, females of many species experience high fecundity-independent costs of reproduction (such as migration to nesting sites), so they need to amass substantial energy reserves before initiating reproductive activity. Thus, we expect that the relationship between energy reserves and the intensity of reproductive behavior involves a threshold effect in females, but a gradual (or no) effect in males. We tested this prediction using captive vipers (Vipera aspis), dividing both males and females into groups of high versus low body condition. Snakes from each group were placed together and observed for reproductive behavior; sex-steroid levels were also measured. As predicted, females in below-average body condition had very low estradiol levels and did not show sexual receptivity, whereas males of all body condition indices had significant testosterone levels and displayed active courtship. Testosterone levels and courtship intensity increased gradually (i.e., no step function) with body condition in males, but high estradiol levels and sexual receptivity were seen only in females with body reserves above a critical threshold.     

The population density, growth rate and production of roach Rutilus mtilus (L.) in Tjeukemeer, The Netherlands

A method of estimating the population density of roach in Tjeukemeer (21.3 km²) using 20 529 introduced fin-clipped fish is described. Fyke nets proved to be an effective method of sampling the population for marked fish during the spawning season. A total of 20 277 roach were processed during the recapturing period. The population density of roach (⋝ 14 cm) was estimated to be 1 246 458. The growth rate of roach in the lake although relatively poor (von Bertalanffy's L ∞ for males and females, 22 and 26 cm respectively) for the species was similar to that recorded in three other Friesian lakes. The mortality rate of males was higher than that of females. Logarithmic length-weight regression analyses showed that the value of the coefficient varied both within and between the sexes, that of females being higher (range 3.03–3.375) than that of males (range 2.76–3.254). Seasonal changes in the size of the coefficient were due to a disproportionate loss of gonad weight in larger fish. The fecundity of the population was comparatively high for the species.
The total production of the population was estimated to be 95 hg ha⁻¹ of which 39 kg ha⁻¹ was contributed by fry. In older (⋝ IV) fish the production of females (12 kg ha⁻¹) exceeded that (2 kg ha⁻¹) of males, due to differences in their growth and mortality rates. The relatively poor performance of roach in Tjeukemeer, in terms of biomass and production was due to a scarcity of zoobenthos and competition from other species offish. There is no evidence either from this study or the literature that the productive potential of roach in lakes is high, even though macrophytes and detritus can be consumed in significant amounts.