Annual Reproductive Output Research Articles

Why breed every other year? The case of albatrosses.

Albatrosses exhibit extremely low reproductive rates, each pair brooding only one egg and subsequent chick at a time. Furthermore, in several of the species, the majority of successful pairs breed only once every second year (termed 'biennial' breeding). Thus, on average, these latter species have an annual fecundity of about half an offspring per year, while other albatrosses produce an egg and chick every year. Using our 40-year bank of demographic data, we compared 12 species of albatrosses according to these two breeding strategies to examine potential causes of biennial breeding. Biennial breeding could be due to physiological constraints, larger animals breeding more slowly, or ecological constraints, more distant pelagic feeding trips being energetically costly, or both. We tested these hypotheses by looking for predicted associations between the duration of the rearing period, the distance to the oceanic feeding zone and breeding frequency. We also looked for associations of these variables with other life-history traits. Body size had a strong influence on the duration of the rearing period, but not on the distance that birds travelled to the feeding zone. Both the duration of the rearing period and distance to the feeding zone appeared to have direct influences on breeding frequency, as revealed by a path analysis, and thus both hypotheses to explain biennial breeding were supported. Finally, breeding frequency exhibited a strong trade-off with adult survival and age at maturity, indicating that slower breeders live through more breeding seasons, perhaps mitigating their lower annual reproductive output.

Microclimates and energetics of free-living box turtles, Terrapene carolina, in South Carolina.

We measured microclimate, field metabolic rates (FMRs), water flux, and activity patterns of telemetered box turtles (Terrapene carolina) in South Carolina from September 1987 to October 1988. Turtles were inactive for most of the winter and were active only sporadically during the rest of the year. Using the doubly labeled water method, we found that water flux averaged 8.8, 18.9, and 26.4 mL kg(-1) d(-1) in winter, spring, and summer/fall, respectively. FMR for the same periods averaged 0.028, 0.065, and 0.124 mL CO(2) g(-1) h(-1). Differences in FMR among seasons were significant but not between sexes. Using operative temperatures, we predicted standard and maximum metabolic rates of turtles. In winter, FMR was elevated above standard metabolic rates and close to maximum metabolic rates, whereas in spring and summer/fall, FMR fell midway between standard and maximum metabolic rates. We used a model to predict metabolic rates, geographical distribution, and potential reproductive output of box turtles across latitudes in eastern North America. Low FMR and low annual reproductive output may allow box turtles to survive and flourish in unpredictable resource environments by minimizing costs and risks, thereby maintaining greater lifetime reproductive success.

Effects of age, breeding experience, mate fidelity and site fidelity on breeding performance in a declining population of Cassin’s auklets

Summary We examined how mate and site fidelity varied with age, experience and sex, and how age, breeding experience, mate experience, site experience and sex affected annual reproductive success and lifetime reproductive output in a declining population of Cassin’s auklets (Ptychoramphusaleuticus). Our 276 study birds were 2–14 years of age, recruited at age 2–12 years, and had 0–11 years’ breeding experience, 0–8 years’ experience with the same mate and 0–11 years’ experience in the same nest box. Mate fidelity was significantly greater with increasing age in males but not females. There was also a significant negative relationship between mate fidelity and breeding density (as measured by proportion of box occupancy); i.e. the lower the breeding density the higher the incidence of breeding with the same mate. Site fidelity showed significant linear and curvilinear increases with age that were significant in females but not males. There was also a significant negative relationship between site fidelity and breeding density; i.e. the lower the breeding density the higher the incidence of breeding at the same site. Previous breeding experience had no effect on either mate fidelity or site fidelity, and both mate and site fidelity were significantly lower after a breeding season was skipped. In addition, mate fidelity was significantly lower when a site was switched and vice versa. Lifetime reproductive output increased significantly with mate fidelity but showed no relationship with site fidelity. This suggests that fitness is optimized more through mate selection than site selection and that mate fidelity is not a by‐product of site fidelity. Annual reproductive success showed a significant linear increase with age in males but not females, and a strong parabolic relationship with breeding experience that was significant in both sexes and significantly greater in males than females. These results suggest that (i) males may be more responsible for mate selection and females for site selection; (ii) improved foraging experience with age and a cost of reproduction may be more important factors in males than females; and (iii) reproductive success may be optimized by behaviour of the male rather than the female. Controlling for the age and experience terms of both parents, experience with a mate had a significant positive linear effect on annual reproductive success. This suggests that mate fidelity is adaptive in Cassin’s auklets, and that studies examining the effects of age and experience on reproductive performance should separately consider the duration of the pair bond. Controlling for all other variables, neither experience at a breeding site nor breeding density showed significant correlations with reproductive success. We suggest that reductions in food supply, which correlate with reduced breeding densities, may prevent all but the highest quality breeders (those which have already established a pair bond) from reproducing, and that the increase in quality offsets the reduction in food availability.

NEST SUCCESS IS NOT AN ADEQUATE COMPARATIVE ESTIMATE OF AVIAN REPRODUCTION

Quantifying productivity of bird populations often involves calculating nest success from samples of nests found during a breeding season. Female reproductive output (annual or lifetime) is important to demographic modeling of birds, but nest success is a poor metric to compare productivity among areas or populations. Based on preliminary review, we hypothesized that avian literature poorly distinguishes annual reproductive success from nest success regardless of species' breeding strategies, and that the influence of renesting and multiple brooding on avian productivity is infrequently recognized. We investigated the reporting of nest success, annual reproductive success, renesting, and multiple brooding in 356 articles across 81 journal-years for nine journals that frequently publish ornithological literature. We found 54% of 356 articles reporting productivity estimators used variations of nest success. However, only 10% of articles reported estimates of annual reproductive output. We found that 28% of articles reported the breeding strategy of the species studied, and 47% acknowledged renesting or multi-brooded species. There was no temporal increase in acknowledgment of species' breeding strategy, renesting, or multi-brooding. Our review indicated little distinction between nest success and annual reproductive success. To accurately support conservation evaluations, we believe use of nest success in assessing avian productivity must be critically examined.

Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata , in the north-western Mediterranean Sea

Biological aspects of the only two epi-/mesopelagic pasiphaeid species distributed along the continental marginof the north-western Mediterranean Sea were studied over the period 1991–1994. The annual reproductive cycle, reproductive output, sex and size distributions by depth (near-bottom distribution), and growth were analysed using 4156 specimens of Pasiphaeasivado (Risso, 1816) and 5491 specimens of P.multidentata Esmark, 1866. The reproductive cycle in the shallower-dwelling species, P.sivado, was nearly continuous, in contrast to the reproductive cycle in P.multidentata, which was markedly seasonal in nature. Both species exhibited similar size and population structure patterns with depth, with individual size increasing along the bottom depth gradient. The shallower-dwelling species had a significantly higher reproductive output level, in terms of the relative number of eggs, than the deeper-dwelling species. Biological trends for each of the species have been compared. The comparative results for these two congeneric species revealed that the biological parameters analysed were closely linked to their population structure. The slight differences in the depth range inhabited by each species have been considered the main environmental factor affecting the life histories of these two species. The data presented supported the hypothesis that depth exerted an important influence on the biological patterns of deep-water species, with the seasonality of reproductive processes increasing in deeper-dwelling species.

Conservation of New Zealand lizards (Lacertilia: Scincidae) by translocation of small populations

The successful removal of rodents from islands around New Zealand has enabled translocation of rare species of lizards to new sites within their former range. Four species of skinks were translocated to Korapuki Island, Mercury Islands, New Zealand: Cyclodina alani, C. oliveri, C. whitakeri and Oligosoma suteri. Responses of three of the lizard species to release were predicted using a deterministic population model then compared with the actual performance of wild populations. The population models predicted that if populations of about 30 C. whitakeri and O. suteri were released, C. whitakeri would be increasing by 7% p.a. and O. suteri would be increasing by over 35% p.a. during year five. If 20 C. alani were released, the population could be increasing by 70% p.a. during year five. Population expansions are being recorded for all three species, but observed population increases were lower than those predicted for two species, partly because of low catchability of immature lizards. The lowest rate of increase (up to 7% p.a.) was for C. whitakeri. Low rates of population expansion found in species such as C. whitakeri raise a conundrum: the rarer a species is, and the lower its annual reproductive output, the larger the number of individuals that may be required for translocations to succeed. Translocations may be regarded as successful when new populations are self-sustaining and comprised only of locally born animals. Despite the populations increasing, it may not be possible to claim success for the three intensively studied species on Korapuki Island until at least 20 years after release. This is because of longevity of the founders, which are still being caught up to 12 years after release. Extreme longevity, and hence prolonged post-release monitoring, is likely to be a recurrent problem for translocations of rare lizards.

Variation in Metabolic Rate between Populations of a Geographically Widespread Lizard

In geographically widespread ectotherms, variation in life history phenotypes may be caused by differences in maintenance metabolism of individuals. I estimated daily and annual maintenance metabolism of eastern fence lizards, Sceloporus undulatus, from two populations with markedly different life histories; lizards in South Carolina grow faster, mature earlier, and have greater annual reproductive output than lizards in New Jersey. I measured diel cycles of resting metabolic rate (RMR) at four temperatures (20 degrees, 30 degrees, 33 degrees, and 36 degrees C) during spring, summer, and fall. In all seasons, RMR increased significantly from 20 degrees to 33 degrees C but did not differ significantly between 33 degrees and 36 degrees C. Adults from New Jersey had a higher RMR than adults from South Carolina in summer and fall but not in spring. Juveniles from South Carolina had a higher RMR than juveniles from New Jersey in summer but not in spring or fall. Annual maintenance metabolism of New Jersey lizards (53.7 kJ) was greater than that of South Carolina lizards (45.8 kJ), despite the shorter duration of activity in New Jersey. I conclude that the difference in maintenance metabolism between populations contributes to the greater production by S. undulatus in South Carolina.

The effects of low levels of pollutants on the reproduction of golden eagles in western Norway.

The reproductive success of golden eagles was studied in an area in western Norway between 62 degrees and 63 degrees N between 1973 and 1999. Addled eggs were collected for analysis of chemical pollutants from nine territories; five from coastal areas and four from inland. The coastal sites had lower annual reproductive output than inland sites, and the eggs had a higher content of organochlorine compounds. There were relatively strong negative correlations between reproductive output and (1) shell thickness and (2) DDE concentration in eggs. The data indicate that the golden eagle may be a particularly sensitive species to DDE. It is proposed that the higher organochlorine content found in the eggs of coastal birds was caused by a contribution of marine birds to the diet, as opposed to inland eagles which have a prey basis consisting almost entirely of terrestrial herbivores such as grouse, mountain hare and cervids. Our data-set on reproductive output is, however, too small to establish a general relationship between DDE contamination and reproductive output in golden eagles.

Parental care of replacement clutches in common terns ( Sterna hirundo )

In long-lived seabirds with low annual reproductive output, the renesting decision after breeding failure is critical, and the parents have to weigh benefits of replacement clutches against possible future reproductive costs. In this study, we investigated factors influencing renesting decisions in common terns (Sterna hirundo) and compared aspects of breeding biology and body mass between two breeding attempts by the same pairs in each of 4 years of heavy losses due to predation. Renesting birds were characterized by early laying dates and by a high age. Among early breeders, high egg mass reduced the probability of renesting. A long relaying interval coincided with low mass of replacement eggs in one year, and short intervals with high egg mass in another. Further, egg mass decreased and relaying intervals increased the later the predation events occurred. Evidence of high levels of parental care of replacement clutches came from body mass data: female mass increase prior to egg laying was higher in the second attempt than in the first, whereas male mass was lower during the second courtship period than during the first. Male mass also affected relaying intervals and mass of replacement eggs. We conclude that common terns expend high levels of parental care of replacement clutches. Intrinsic factors related to individual quality (age, body condition) seemed most important for renesting decisions and for the degree of parental care, but foraging conditions seemed to have modifying effects.

The annual reproductive cycle and sperm storage in the bearded dragon, Pogona barbata

Pogona barbata, a large lizard that inhabits the open woodlands of eastern Australia, has been reported to have a high annual reproductive output, which is contrary to expectations for a large lizard. To better understand this anomaly, its reproductive cycle and morphology were investigated. Males were spermatogenic year round, with only a brief period of regression in January (late summer). Females were vitellogenic and gravid through August–December (spring to early summer). Two or three large clutches (14–26 eggs per clutch) were produced each breeding period. Females had two germinal beds in each ovary, and all four actively produced eggs simultaneously. Clutches overlapped such that the next clutch began vitellogenesis before the first was laid. These characteristics make this species’ reproductive output one of the largest known among lizards. Reproductive females had sperm stored in oviductal crypts, but there was no evidence of sperm storage outside the breeding period. The short-term storage of sperm by breeding females may promote sperm competition between males.

Egg Size and Annual Egg Production by Female Desert Tortoises (Gopherus agassizii): The Importance of Food Abundance, Body Size, and Date of Egg Shelling

We studied egg production in two Californian populations of desert tortoises, (Gopherus agassizii) in 1992 and 1993. One population inhabited the Desert Tortoise Research Natural Area (DTNA) in the western Mojave Desert, where most of the rain falls in the winter. The second population lived near Goffs, in the eastern Mojave, where annual precipitation is divided more evenly between winter and sum- mer. Due to El Niio conditions, heavy winter rains fell at both sites in both years (1991-1992 and 1992- 1993). Consequently, the biomass of spring annuals and annual egg production by tortoises were high in both years at both sites. There were no differences in reproductive output between years so we pooled data for both years to examine the relationship between egg-laying parameters (clutch size and frequency, annual egg production, egg size, etc.) and female size. Variation in annual egg production was due mainly to variation in clutch size, not clutch frequency. Annual egg production per female was lower at DTNA than at Goffs, because some adult females at DTNA did not produce eggs in some years. Females that did lay eggs produced the same number of eggs per year at both sites, even though females at Goffs were smaller (midline carapace length = 214 mm) than females at DTNA (MCL = 234 mm). Despite correction for these body size differences, the eggs produced at Goffs were smaller in all dimensions than eggs produced at DTNA. Smaller eggs and presumably smaller neonates may be related to the greater predictability of sum- mer rain and consequent greater food supply for emergent hatchlings at Goffs. For adult females, food supply probably limits reproduction only during drought years. How can individual females vary their annual reproductive output? Our more extensive data for DTNA tortoises showed that larger females pro- duced larger clutch sizes. In addition, by statistically removing the effects of body size we showed that larger clutches contained smaller eggs. Moreover, larger females produced eggs earlier in the year giving them a better opportunity to produce a second clutch that year. Thus, timing of first clutch was important Still, much of the variation in reproductive output was not explained. Other characteristics of individuals (e.g, age, genetics, physiological maturity, home range quality, or forage selection) may explain some of the variation in reproductive output

Experimental evidence for density dependence of reproduction in great tits

1. Density dependence of avian reproduction has often been analysed using correlations between annual mean reproductive output and population density. Experiments are necessary to prove that density is the cause of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduction. 2. This paper presents experimental evidence that reproductive decisions in great tits, Parus major L., are causally affected by breeding density. The breeding density of great tits was manipulated by providing nest‐boxes at different densities in an ecologically homogeneous area. 3. Within years the densities in the high and low density plots differed approximately 8‐fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a second brood were all lower in the high density plot. In 5 years with equal breeding densities in both parts, clutch size did not differ between the plots. The patterns found were consistent with the density effects as predicted from the non‐experimental data.

Factors affecting clutch Size and Duckling Survival in the Common Goldeneye Bucephala clangula

Female age and condition as well as nesting schedule may affect clutch size and chick survival, which both are important factors affecting the annual reproductive output. Clutch size and chick survival are often interrelated in altricial birds but in precocial species the interrelation is dubious. To study factors affecting clutch size and the survival of ducklings in the common goldeneye Bucephala clangula, we examined ringing‐recovery data, supplied by ringers, from seven intensively studied goldeneye populations from different parts of Finland. Due to large sample sizes it was possible to distinquish the effects of interrelated variables, e.g. female age and hatching date. Clutch size was significantly associated with both female age and hatching date; first‐time breeders and late breeders had small clutches. Clutch size was not associated with female condition. The recovery rate of ducklings was associated with hatching date (low in late broods) but not with female age, condition or brood size. The natal brood sizes of recruited female ducklings did not differ from annual averages, but the natal hatching date of recruited females was earlier and their mothers were older than annual averages in the populations into which the recruited females were bom. The timing of nesting was the most important factor affecting the reproductive output of common goldeneye females in a given year.

Structure et dynamique d’une population d’Agama impalearis (Sauria : Agamidae) dans les Jbilets centrales, Maroc

A population study of Agama impaleari was conducted during two consecutive years (1993-1994) in a Jujube bush site located in an arid area at the Jbilet Mountains (Western Morocco). Lizards of both sexes required approximatively one year to reach sexual maturity. One to two clutches were produced per breeding season (May-August) and clutch size was significantly correlated with maternal body size. The annual reproductive output was in average of 20 eggs per female per year and did not exhibit a significant difference between the two years of study. Population size showed important seasonal changes throughout the study period and absolute spring density estimates were of 15 and 11 ind/ha, respectively in 1993 and 1994. In the post-reproductive period (September) the population density (excluding hatchlings) markedly declined and was only of 4-5 ind/ha. After the completion of hatching period (July to early November), the densities of hatchlings were 11 and 13 ind/ha respectively in 1993 and 1994 compensing for the population losses. The annual turnover rate of the population did not exhibit a significant yearly change and was of 77 %. Adult survivorship after the reproductive activity varied between years and juvenile survival during the first year was estimated to be 35 %. Maximum longevity assessed by means of squelettochro-nology was 5 years.

Reproduction of river and lake populations of Hyridella depressa (Unionacea: Hyriidae) in New South Wales: implications for their conservation

The freshwater mussel Hyridella depressa is an important component of the macrobiota of freshwater systems of south-eastern Australia. This species has declined in river habitats but is abundant in impounded water storages. Reproduction of river and lake populations of H. depressa was investigated to document details of their gametogenic cycle and to determine when the females incubate embryos in their gill marsupia. Four populations in Lake Burragorang and two populations in the Hawkesbury-Nepean River were studied over two breeding seasons. Reproduction was documented by anatomical and histological examination of the gonads. The study sites differed in anthropogenic influence and trophic status and the influence of these factors on reproduction in H. depressa was assessed. A skewed sex ratio in favour of females was common in the populations and histology revealed that some of these females were microhermaphrodites. It appears that sexuality in H. depressa is labile with the potential for self-fertilisation by individuals that largely function as females. H. depressa is a long-term brooder with progeny present in the marsupia for up to eight months of the year from mid-winter to late summer. Across all sites brooding started in July or August and finished in January or February of the following year. In parallel with the prolonged spawning pattern, gametogenesis in H. depressa was continuous. At the eutrophic sites the gonads contained advanced gametes throughout the year. H. depressa at these sites had a high annual reproductive output with the females producing several clutches each year. In contrast, gametogenesis in mussels at the oligotrophic sites was more seasonal. At one of the oligotrophic sites reproductive failure was common and in 1996 most females failed to brood a single clutch. Implications of this research for conservation of H. depressa are discussed.

Histological analysis of the egg capsule of the ovoviviparous polystomatid monogenean, Pseudodiplorchis americanus

Transmission of Pseudodiplorchis americanus is restricted to the brief period when its host, a desert toad, enters water to spawn. The parasite accumulates its entire annual reproductive output within an elongated uterus during the 10–11-month period of host hibernation. Embryos of P. americanus, at all stages of development, are retained within the uterus which eventually becomes packed with around 150 encapsulated infective larvae. Recently formed eggs, which comprise a fertilized ovum and 2–3 vitelline cells, are closely surrounded by a primary eggshell which stains positively for acidic proteins and keratin. Initially, during passage along the proximal uterus, the egg capsule is only 60 μm in diameter, but as it passes to the distal uterus it expands to 800 μm in diameter to accommodate the growing larva. Due to chemical alterations or complete replacement of the shell, the final (secondary) egg capsule is a large sac-like structure composed of elastin. The flexible nature of this shell maximizes the numbers of infective larvae which can be stored in utero. TEM studies have revealed this capsule to be composed of multi-laminate membranes with a specialized cytoplasmic lining involved in a unique mechanism for embryo nutrition. This is the first report of an elastintype eggshell within the Monogenea.

Resource Allocation in the Intertidal Salt-Marsh Mussel Geukensia demissa in Relation to Shore Level

Ribbed mussel populations at two levels of the intertidal shore were studied for 3 yr at a salt marsh site in Jamaica Bay (New York). Length-specific dry body weight was determined monthly and shell growth was monitored between April and November of each year. Production (Pi) was measured as monthly changes in body dry weight. Annual reproductive output (PR) was estimated indirectly for mussels between 10 mm and 80 mm by determining the gain in body weight prior to spawning (Weight Gained Method). Using a second method (Basal Weight Method), monthly rates of somatic production (Pg) were determined by assuming that, as mussels grow in length, a fraction of monthly Pt is allocated to increasing body weight. The monthly production allocated to growth (Pg) was estimated based on the length-specific body weight (basal weight) in March, when body weights in these populations begin to increase due to gametogenesis. Monthly rates of reproductive output (Pr) were then estimated be substraction as the difference between monthly Pt and Pg. Total PR determined by both methods were very similar. Shell and body growth were concurrent in sexually immature mussels, but body weight preceeded shell growth in older, larger animals. Annual shell increments were correlated with annual PG. In sexually mature mussels, Pr preceeds Pg in larger mussels, that is, allocations of net production to reproduction and somatic growth diverge in time with increasing shell length. Between April and November, shell growth rates were correlated with water temperature. At the higher shore level, mussels allocated a much smaller proportion of total production to growth, exceeding the reproductive effort of the lower-shore population. Variation in mussel production between years probably is related to variability in nannoplankton biomass. Although mussels higher on the shore can reduce somatic growth in order to increase fecundity and reproductive effort, total reproductive output is much reduced as compared to the marsh edge.

Breeding success and reproductive strategies of twoAcrocephalus warblers

Several hypotheses have been proposed in the literature to account for the different responses birds show to varying degrees of nest predation. These are discussed using data on Marsh and Reed Warbler (Acrocephalus palustris, A. scirpaceus) which differ greatly in nest predation even when they share the same habitat. Breeding success was distinctly higher in Marsh than in Reed Warblers per breeding attempt (averaging 68.1% and 44.9% respectively). Different levels of egg predation and Cuckoo (Cuculus canorus) parasitism were the main reasons. During an egg laying period of about 14 weeks Reed Warblers produce many replacement clutches and second broods resulting in an average annual reproductive output of 3.8 fledged young per female. The single brooded Marsh Warbler with a laying period of 7 weeks produces 3.3 fledglings. Differences in structure and seasonality of the species' respective habitats, i. e. reedbeds in Reed Warbler and herbaceous vegetation in Marsh Warbler, contribute to the differences in breeding success and the length of breeding periods open to the two species. Marsh Warblers respond to limited breeding opportunities due to their narrow breeding season with higher clutch size, and by avoiding nest losses. They conceal nests better, breed in lower densities, and efficiently reject Cuckoo eggs. Ecological constraints may restrain the Reed Warbler to its comparably unsafe nest sites. In other open-nesting and ecologically similar species pairs, a similar higher breeding success also occurs in those species that stay for a shorter period on the breeding grounds. It appears that high breeding success is an important prerequisite for species breeding in ephemeral habitats.

Biennial Reproduction with a Fourteen-Month Pregnancy in the Gecko Hoplodactylus maculatus from Southern New Zealand

Hoplodactylus maculatus is a small, nocturnal and viviparous gecko that is widespread throughout New Zealand and represents a complex of several undescribed species. Common geckos in the Macraes-Middlemarch region of Central Otago in southern New Zealand have a biennial ovarian cycle with virtually continuous ovarian activity. Females begin vitellogenesis in autumn and ovulate the fol- lowing spring. Pregnant females carry full-term, viable embryos by the end of autumn about 7 mo after ovulation, but remain pregnant during the subsequent winter. Parturition occurs the following late spring or summer, about 14 mo after ovulation. In populations of this species complex in three other regions of New Zealand, annual ovarian cycles with a 3-5 mo pregnancy have been reported. Biennial reproduction in the Macraes-Middlemarch region is associated with colder temperatures during spring-autumn than in regions with annual reproduction. Common geckos from the Macraes-Middlemarch region also have larger body sizes and a smaller mean litter size than in two populations with annual reproductive cycles. Small litter size and biennial reproduction in the Macraes-Middlemarch region result in an annual reproductive output of about 0.85 young, comparable with the lowest value reported for any other lizard. These findings imply a long life-span in common geckos from this region.

Variation in growth and reproduction of walleye (Stizostedion vitreum) in three Manitoba lakes

Growth, lifetime egg production and biomass, and lipid accumulation in three female populations of walleye (Stizostedion vitreum) were studied. Two populations inhabit large, heavily exploited Canadian prairie lakes and the third inhabits a smaller, cooler, unexploited boreal lake in the Canadian Shield. Fish in the unexploited population grew more slowly and to a smaller asymptotic size than those in the other two populations. Although annual reproductive output per individual was less for this population, the reproductive life-span was greater. Lifetime egg production and ovary lipid biomass per recruit appeared to be similar (within 13%) for female walleye in the three different populations. The significance of energy allocation in the life history of these populations is discussed.