Variation In Recruitment Research Articles

Quantification of the full lifecycle bioenergetics of a large mammal in the high Arctic

Energy is a critical driver of animal life-history traits and Darwinian fitness, with direct implications for population dynamics. In the Arctic, extreme seasonal climatic conditions dictate energy flow and physiology of resident species. Consequently, vital rates of animal populations in the Arctic are tightly linked to climatic factors influencing food availability and quality. The muskox (Ovibos moschatus) is a capital breeder and the largest herbivore in the Arctic. Seasonal changes in energy intake influence body reserve dynamics of this species, impacting inter-annual and spatial variations in survival and recruitment. Yet, little is known about the full lifecycle bioenergetics of this and other Arctic species, and the impact of extreme seasonal conditions on life-history traits have never been quantified. By integrating energetic and physiological processes throughout the full lifecycle of individuals, we developed an individual-based model using dynamic energy budget theory (DEB-IBM) of the muskox that covers all aspects of mammalian life-history. We parameterized a baseline muskox model using data from a well-studied captive research herd, and then explored its application to a wild population in the high Arctic to assess the life-history implications of seasonal food availability. Our baseline DEB-IBM, which for the first time explicitly characterized fetal development using DEB theory, successfully predicted life-history traits as well as patterns of growth, body condition, feeding, metabolic rates, and reproduction of captive muskoxen. Adaptation of the model to the wild population required re-parameterization of only three variables that controlled seasonal food availability and upregulated assimilation. The wild model successfully captured inter-annual costs of reproduction, such that lactating females were in poorer body condition than females without calves. Arctic seasonal food limitations reduced fecundity in wild muskoxen by 42% compared to annual reproduction in well-fed captive individuals. We have developed a mechanistic energy-based framework to study life-history processes of muskoxen but the model is broadly applicable also for other mammals in the Arctic. Our study quantifies and highlights the strong interplay between energetics and individual fitness components, opening the door to future applications of the model in assessments of population dynamics and climate change impacts.

Microgeographic variation in recruitment under adult trees: arrival of new genotypes or perpetuation of the existing ones?

Investigating spatial variation in the relative importance of sexual reproduction and clonal propagation is critical to obtain more accurate estimates of future effective population sizes and genetic diversity, as well as to identify ecological correlates of clonality. We combined a stratified sampling scheme with microsatellite genetic analyses to estimate variation in the proportion of sexual versus clonal recruits among saplings in five populations of the tree Pyrus bourgaeana. Using a likelihood framework, we identified clones among the genotypes analysed and examined variation among populations regarding the proportion of saplings coming from clonal propagation. We also examined the relationship between the relative abundance of clonal shoots across the studied populations and their herbivory levels. Our results revealed that one third of the saplings examined (N=225 saplings) had a probability above 0.9 of being clones of nearby (<10m) trees, with the ratio between clonal propagation and sexual recruitment varying up to eight-fold among populations. A small portion of these putative clonal shoots reached sexual maturity. Relative abundance of clonal shoots did not significantly relate to the herbivory by ungulates. Our results call into question optimistic expectations of previous studies reporting sufficient levels of recruitment under parental trees without animal seed dispersal services. Nevertheless, given that some of these clonal shoots reach sexual maturity, clonal propagation can ultimately facilitate the long-term persistence of populations during adverse periods (e.g. environmental stress, impoverished pollinator communities, seed dispersal limitation).

Patterns of acoustical activity of bats prior to and 10 years after WNS on Fort Drum Army Installation, New York

Previous acoustic surveys, netting, and count data have shown that overall bat activity patterns have shifted among most species between pre- and post-white-nose syndrome (WNS) years in much of North America where WNS has occurred. However, the significance of these changes is based on the species-specific susceptibility to WNS. We used acoustically recorded echolocation passes obtained at Fort Drum, New York to describe changes in bat activity pre-WNS (2004–2007) to post-WNS (2008–2018). We examined seasonal and yearly changes in bat activity as they relate to the presence of WNS at hibernacula near (<25 km) Fort Drum. A priori, we expected that overall activity for communal hibernating species would be less in years following WNS, and migratory bats or those hibernating bats that are less affected by WNS would show no response or a positive response, due to niche relaxation/competitive release. Our results indicated both an overall and seasonal decrease in activity for Myotis spp. post-WNS. For WNS-susceptible species, our results reflect the high level of mortality in regional winter hibernacula post-WNS and possibly variable reproductive effort and recruitment thereafter. Although migratory bats did show increases in post-WNS activity throughout the summer, we found little evidence that community displacement was occurring on a nightly level by any species. The continuous spread of WNS across North America has had strong negative effects on bat populations of affected species, and our research identifies how individual species (both impacted and non-impacted) respond to WNS.

Population Characteristics of Co-Managed White Bass and Hybrid Striped Bass in Lake McConaughy, Nebraska

White Bass (Morone chrysops) and Hybrid Striped Bass (M. saxatillis × M. chrysops) populations often coexist in the same waterbody and are known to achieve different lengths, have differing reproductive success, and provide different opportunities for anglers. However, comparative population dynamics from systems where Moronids are managed with the same regulation is often lacking. This study aimed to assess the recruitment, mortality, and growth of these species from seasonal samples collected at Lake McConaughy in 2015 and fall 2016. White Bass demonstrated highly variable recruitment in Lake McConaughy despite stocking efforts (mean recruitment variability index = 0.157). Hybrid Striped Bass year class strength was also variable, but RVI was not calculated as the number of missing year classes exceed the number of present year classes. Total annual mortality estimates for Hybrid Striped Bass (39.7%) and White Bass (41.4%) were consistent with values reported in other populations, but both species were observed to achieve uncommon longevity. Both species exhibited sexual size dimorphism, and growth was moderate and consistent with average values presented for North American populations. This study provides insight into the population dynamics of two species with trophy potential managed by the same regulation in Lake McConaughy. Differences in growth patterns indicate the current regulation affects each species differently, and managers may be able to utilize this to inform decisions about management of coexisting Moronid populations.

Multiscale variability in coral recruitment in the Mascarene Islands: From centimetric to geographical scale.

Coral recruitment refers to the processes allowing maintenance and renewal of coral communities. Recruitment success is therefore indispensable for coral reef recovery after disturbances. Recruitment processes are governed by a variety of factors occurring at all spatial and temporal scales, from centimetres to hundreds of kilometres. In the present context of rising disturbances, it is thus of major importance to better understand the relative importance of different scales in this variation, and when possible, the factors associated with these scales. Multiscale spatio-temporal variability of scleractinian coral recruitment was investigated at two of the Mascarene Islands: Reunion and Rodrigues. Recruitment rates and taxonomic composition were examined during three consecutive six-month periods from regional to micro-local scales (i.e. from hundreds of kilometres to few centimetres) and between two protection levels (no-take zones and general protection zones). Very low recruitment rates were observed. Rodrigues displayed lower recruitment rates than Reunion. Recruit assemblage was dominated by Pocilloporidae (77.9%), followed by Acroporidae (9.9%) and Poritidae (5.2%). No protection effect was identified on coral recruitment, despite differences in recruitment rates among sites within islands. Recruits were patchily distributed within sites but no aggregative effect was detected, i.e. the preferentially colonised tiles were not spatially grouped. Recruits settled mainly on the sides of the tiles, especially at Rodrigues, which could be attributed to the high concentration of suspended matter. The variability of recruitment patterns at various spatial scales emphasises the importance of micro- to macro-local variations of the environment in the dynamics and maintenance of coral populations. High temporal variability was also detected, between seasons and years, which may be related to the early 2016 bleaching event at Rodrigues. The low recruitment rates and the absence of protection effect raise questions about the potential for recovery from disturbances of coral reefs in the Mascarene Islands.

Food availability before aestivation governs growth and winter reproductive potential in the capital breeding fish, Ammodytes japonicus.

Capital breeders develop gametes by using energy that was stored before the spawning season. Energy is allocated to growth and reproduction, and limited food availability affects the balance of energy allocation, especially in fish that mature within a year, such as western sand lance (Ammodytes japonicus). This species aestivates without feeding until winter spawning and utilize energy stores that were accumulated prior to aestivation for maturation and spawning. This study aimed to evaluate the growth, energy storage, maturation rate, and reproduction of A. japonicus in response to food availability before aestivation. We conducted laboratory experiments in which young-of-the-year A. japonicus were fed at rates of 4% and 1% of their body weight per day; assigned as high and low ration groups, respectively. In June, body length was found to be significantly larger in the high ration group than in the low ration group, but the somatic condition did not differ significantly between the groups. Maturation rates and average fecundities were 1.0 and 6297 in the high ration group and 0.8 and 2251 in the low ration group, respectively. These results indicate that food availability before aestivation strongly governs the reproductive potential of A. japonicus, and suggest the involvement of mechanisms in the inter-annual recruitment variation in sand lance species.

Reconstruction of Atlantic herring (Clupea harengus) recruitment in the North Sea for the past 455 years based on the δ13C from annual shell increments of the ocean quahog (Arctica islandica)

AbstractUnderstanding the recruitment variability of the Atlantic herring North Sea stock remains a key objective of stock assessment and management. Although many efforts have been undertaken linking climatic and stock dynamic factors to herring recruitment, no major attempt has been made to estimate recruitment levels before the 20th century. Here, we present a novel annually resolved, absolutely dated herring recruitment reconstruction, derived from stable carbon isotope geochemistry (δ13C), from ocean quahog shells from the Fladen Ground (northern North Sea). Our age model is based on a growth increment chronology obtained from fourteen shells. Ten of these were micromilled at annual resolution for δ13C analysis. Our results indicate that the anthropogenically driven relative depletion of 13C, the oceanic Suess effect (oSE), became evident in the northern North Sea in the 1850s. We calculated a regression line between the oSE‐detrended δ13C results (δ13CṠ) and diatom abundance in the North Sea, the regression being mediated by the effect of phytoplankton on the δ13C of the ambient dissolved inorganic carbon. We used this regression to build an equation mediated by a nutritional link to reconstruct herring recruitment using δ13CṠ. The reconstruction suggests that there were five extended episodes of low‐recruitment levels before the 20th century. These results are supported by measured recruitment estimates and historical fish catch and export documentation. This work demonstrates that molluscan sclerochronological records can contribute to the investigation of ecological baselines and ecosystem functioning impacted by anthropogenic activity with implications for conservation and stock management.

Settlement and recruitment pattern variability of the mussel Mytilus galloprovincialis Lmk. from SE Bay of Biscay (Basque Country)

The annual settlement and recruitment patterns of Mytilus galloprovincialis Lmk. were assessed using collector ropes deployed at five sites along the Basque coast inhabited by wild mussel populations. The study was carried out over two periods of ten months each in three estuarine and two coastal sites, and enabled the discovery of a potential relationship between observed primary settlement and the spawning period of wild mussel populations, shedding light on the recruitment of these populations. Primary settlers were present throughout the year with highest abundances from early summer to fall. A spatial settlement pattern was also observed, with eastern estuaries showing higher settlement rates than western ones and coastal sites showing intermediate settlement rates. These spatial and temporal settlement variations could be partially explained by seasonal fluctuations in environmental factors, such as temperature and nutrients, as well as by spawning differences observed at these estuarine and coastal sites. Some inter-annual variability of settlement and recruitment was observed, with higher settlement rates in the second studied year. This inter-annual variability was consistent with the inter-annual variability of the environmental factors. The recruitment of juvenile mussels was shown to be more determined by post-settlement processes than by initial settlement abundance. Indeed, there was a substantial loss of post-larvae during fall and winter, which was due to these post-settlement processes. Thus, it is advised that ropes, which were deployed in spring and collected in winter, to be collected earlier to avoid such losses. Little space is available (i.e., due to user conflict) on the Basque Coast. Despite these limitations, these sites, particularly Bidasoa estuary, could be suitable for future seed gathering for the mussel aquaculture industry in the Basque Country.

Haplotype and biometric patterns in loggerhead turtles from the Adriatic foraging ground

The Adriatic Sea is an important foraging area for the loggerhead turtle Caretta caretta. Due to the increase of anthropogenic impacts, a growing number of individuals are captured as by-catch, found stranded or drifting at sea. Turtles mortality within the Adriatic Sea can affect Mediterranean populations, increasing the risk of losing genetic diversity. Biometric data on 93 loggerheads from three Adriatic areas (northwestern, northeastern and southern Adriatic) and patterns of sequence polymorphism of the mtDNA Control Region long fragment were used to investigate the sizes/age distribution of the collected individuals and to assess their geographic origins by using a Bayesian Mixed Stock Analysis (MSA). The biometric data revealed that collected turtles belonged to a wide range of sizes, although juveniles and subadults were the most abundant classes. Five haplotypes were detected. Two are endemic of Mediterranean rookeries and one is a rare haplotype found once in a Floridian rookery and in two Mediterranean nests. The other two haplotypes were found both in Atlantic and Mediterranean populations. The Mixed Stock Analysis revealed the presence of individuals from Greek and Turkish rookeries, this latter group was found in the southern Adriatic. Individuals from Libya and Cyprus rookeries were also found in the Adriatic stock. Current patterns, juvenile natal homing and imprinting of ontogenetic habitats could be potential drivers of the stock composition in the three Adriatic areas. Variations of turtle migration patterns, recruitment and mortality may drive differential contributions of rookeries to the foraging grounds. In this study, a substructuring within the Adriatic basin is hypothesized, consisting of differential distribution of stocks among three areas. However, these results might be biased by the uneven sample size among areas. Further studies with consistent sample sizes are recommended to quantify potential genetic differentiation within the Adriatic Sea. Nevertheless, present results recommend preservation action to be taken both at the nesting sites and in the foraging grounds were genetically isolated Caretta caretta populations, originating from different Mediterranean rookeries, mix together.

Global forage fish recruitment dynamics: A comparison of methods, time-variation, and reverse causality

The population dynamics of forage fish are often ‘boom or bust’, and variation in recruitment may be a contributing factor to changes in abundance. Here we applied several methods for identifying stock recruit relationships (SRR) to 52 forage fish stocks: a time-invariant Ricker model and two time-varying methods (dynamic linear models and regime-based models). A positive relationship between spawning biomass and recruitment existed for 28 stocks, with 14 of those being best fit by a Ricker curve. Time-varying models were preferred for 26 stocks over a time-invariant Ricker curve. We also showed through simulation that spurious, yet significant stock-recruit relationships can be generated if recruitment was environmentally driven, the lag between recruitment and spawning biomass was small, and autocorrelation in recruitment was high—a combination of characteristics that many forage fish stocks display. Based on our results, observed time-invariant SRRs can result from a reversed causality of ‘boom and bust’ dynamics in situations in which life histories are short (i.e., changes in recruitment are driving changes in spawning biomass). ‘Hidden’ SRRs are also possible because of time-variation in SRR parameters. Understanding when each of these potential failings of fitting SRRs can occur is a key challenge in forage fish management.

Spatial Variability in Size Structure, Growth, and Recruitment of Spotted Seatrout among Six Florida Estuaries

AbstractSpotted Seatrout Cynoscion nebulosus in Florida, USA, are managed under four geographical regions, including Apalachicola Bay and Cedar Key (northwest region), Tampa Bay and Charlotte Harbor (southwest region), northeast Florida (northeast region), and the northern Indian River Lagoon (southeast region). Two of these management regions are composed of more than one major estuary system (northwest and southwest regions). However, previous life history research suggests that the management regions do not accurately reflect the unique biological populations of Spotted Seatrout in Florida. Our objective was to determine (1) whether there is significant spatial variation in size structure, the age–length relationship, and recruitment patterns among populations of Spotted Seatrout in Florida; and (2) whether there is agreement in these life history parameters among estuary populations comprising individual management regions. We used long‐term fishery‐independent and fishery‐dependent data sets on recruitment, age, and size structure among six estuaries and the four management regions. Young‐of‐the‐year recruitment patterns and the length‐at‐age relationship over all ages differed significantly among estuarine populations and within management regions. Additionally, all estuaries differed significantly in length distributions of Spotted Seatrout caught by the recreational fishery. Our results highlight the need to understand the relative contributions of estuary‐specific data to regional assessment models if the four‐region boundary scheme for Florida is continued. Moreover, as variation in growth and recruitment among estuaries likely reflects estuary‐specific environmental conditions, we suggest that environmental variables be considered in future assessments and management of this recreationally valuable species.

The importance of fire season when managing threatened plant species: A long-term case-study of a rare Leucopogon species (Ericaceae)

Implemented burns are a primary source of fire in natural systems and occur outside of the wildfire season. However, the impacts of fire season shift on native plant species are rarely studied. Understanding fire season effects are particularly important for managing threatened species, which are often the focus of managed burns. To assess the impacts of fire seasonality and identify potential limiting traits, I studied the threatened Leucopogon exolasius and two common congeners, all of which persist via fire-driven population dynamics. All species were monitored over a 16 year period to assess seedling survival, growth and primary juvenile period after fire. For L. exolasius and the common L. esquamatus, comparisons of survival, growth and maturation were made after winter and summer fires, to assess the effects of season of burn. A key difference was found in primary juvenile period, which was exceptionally long for L. exolasius (>11 years for 80% of the population to flower) compared to the common congeners (3.2–7.57 years). Seasonal seed dormancy mechanisms meant that winter fires delayed emergence, leading to increases in primary juvenile period for both species. A long primary juvenile period may limit L. exolasius population persistence because plants are more likely to be killed by subsequent fire before maturation, while seasonal dormancy cues is a trait that would exacerbate the effects of this interval squeeze. In fire-prone systems, fire frequency is the key factor assumed to drive persistence, however, interactions with fire season can influence recruitment success. There are scant data on recruitment variation in response to fire seasonality, a factor that may have broad implications for rare and common species with seasonal germination requirements.

Rocky reef fish community composition remains stable throughout seasons and El Niño/La Niña events in the southern Gulf of California

The main effects of El Niño and La Niña on fish are increases in mortality rates, abundance and recruitment variation, and expansion/restriction in the range of distribution and spawning areas. Potentially, the interactions with seasonal effects can further exacerbate or dampen fluctuations in these parameters. The objective of this work, therefore, was to characterize the possible differences in the reef fish community structure in two seasons of the year (warm/cold) and between years (2010/2012) following El Niño-La Niña events. There were no significant differences in mean species richness between years. Furthermore, although more than twice the number of individuals was observed in the year after La Niña (2012) compared to the year following El Niño (2010), there were no significant differences with respect to mean abundance per transect. The total number of species recorded was slightly higher in the cold season, with 68 species across both years. In contrast, the total abundance was higher in the warm season. However, differences between seasons were neither significant for mean species richness nor for mean abundance. Neither season nor year exhibited a significant effect on community composition. The cold season in the Gulf of California is characterized by intense winds which generate a greater mixed layer; this, in turn, increases productivity, with cascading effects on higher trophic levels. In contrast, the presence of nitrogen-fixing picocyanobacteria which increase the levels of primary production may be a factor leading to maintaining a similar fish abundance in the warm season. Fluctuations between seasons and years of study were observed for some individual species, which may be related to oceanic productivity and reproductive periods. The results obtained in this work indicate that seasonality and climatic phenomena such El Niño and La Niña do not have a marked effect on community attributes of rocky reef fish in the southern Gulf of California, which apparently form a seasonally stable community, despite some fluctuations in individual species.

In vivo force-length and activation dynamics of two distal rat hindlimb muscles in relation to gait and grade.

Muscle function changes to meet the varying mechanical demands of locomotion across different gait and grade conditions. A muscle's work output is determined by time-varying patterns of neuromuscular activation, muscle force and muscle length change, but how these patterns change under different conditions in small animals is not well defined. Here, we report the first integrated in vivo force-length and activation patterns in rats, a commonly used small animal model, to evaluate the dynamics of two distal hindlimb muscles (medial gastrocnemius and plantaris) across a range of gait (walk, trot and gallop) and grade (level and incline) conditions. We use these data to explore how the pattern of force production, muscle activation and muscle length changes across conditions in a small quadrupedal mammal. As hypothesized, we found that the rat muscles show limited fascicle strains during active force generation in stance across gaits and grades, indicating that these distal rat muscles generate force economically but perform little work, similar to patterns observed in larger animals during level locomotion. Additionally, given differences in fiber type composition and variation in motor unit recruitment across the gait and grade conditions examined here for these muscles, the in vivo force-length behavior and neuromuscular activation data reported here can be used to validate improved two-element Hill-type muscle models.

Stock structure of Trachurus picturatus from the NE Atlantic and Mediterranean Sea using microsatellite DNA markers

Stock structure of Trachurus picturatus from the NE Atlantic and Mediterranean Sea using microsatellite DNA markers

Physical-biological interactions in the ocean: case studies from the northern Canary Current Upwelling Ecosystem

Physical-biological interactions in the ocean: case studies from the northern Canary Current Upwelling Ecosystem

Regional and interannual recruitment variation of the stalked barnacle Pollicipes pollicipes in SW Europe by using a new artificial substratum

Regional and interannual recruitment variation of the stalked barnacle Pollicipes pollicipes in SW Europe by using a new artificial substratum

Temporal variation in invertebrate recruitment on an Eastern Pacific coral reef

The recruitment of marine invertebrate larvae to coral reef ecosystems is critical for the continued productivity of these highly biodiverse habitats. Despite such importance, little is known about temporal variation patterns of invertebrate larval recruitment in coral reefs ecosystems, aside from studies focused only on reef corals themselves. Given this knowledge deficiency, recruitment onto dead coral branches was monitored over a one-year period in the Central Mexican Pacific, and recruit assemblage and density both changed over time. In particular, recruitment was higher in the warmer months, possibly due to reproductive processes and/or increases in metabolic rates associated with high temperatures and seasonal/local events such as upwellings that increase the food resources available for the recruits which just before the warm season, when the recruitment is more more prevalent during the warm season. The most common invertebrate recruits were malacostracans, ostracods, gastropods, maxillopods, and polychaetes, and foraminifers were commonly observed, as well. Future works in this region will seek to understand whether recruit density and diversity change in response to elevated temperatures associated with global climate change.

Modeling temporal variation in recruitment in fisheries stock assessment: A review of theory and practice

Recruitment is one of the most variable biological processes driving fisheries population dynamics and the characteristics and drivers of the variation differ among stocks. Understanding, describing (modeling), estimating, and predicting (forecasting) this variation is essential to appropriate stock assessment and fisheries management. Most modern stock assessments estimate annual variation in recruitment, particularly those that include age or size composition data. Changes over time are often partitioned into those related to the spawning biomass due to density dependence (the stock-recruitment relationship) and those related to other factors such as environmental conditions. Several methods have been used to model recruitment inside stock assessment models, and they differ by how recruitment is represented and how statistical inference is conducted. Virtual population analyses make few statistical assumptions about recruitment and instead calculate recruitment as the sum of the mortality-adjusted catches of a cohort while surplus production models generally imply a stock-recruitment relationship in their production function. Integrated statistical stock assessment models have modelled recruitment using a stock-recruitment relationship, autocorrelation, a function of covariates, regime shifts, or a combination of these. Statistical inference has typically been conducted using state-space (hierarchical) models, whether using Bayesian or maximum likelihood methods, or inferences are approximated using penalized likelihood approaches, which may not be statistically reliable. All these methods have specific issues that need to be addressed and there are tradeoffs in their use. Future temporal variation also needs to be considered when providing management advice. We review the theory and practice of modelling temporal variation in recruitment in fisheries stock assessment, provide advice on good practices, and recommend important research.

Somatic growth contributes to population variation in marine fishes.

Understanding population fluctuations is a major goal of population ecology. In unpredictable marine environments, population variation is thought to be caused primarily by varying survival rates through a critical early life-history stage. However, there is increasing evidence that somatic growth variation is common and causes population fluctuations. We examine the relative effects of empirically validated variability in somatic growth and recruitment on two response metrics across eight different life-history archetypes of marine fish. We evaluate how much variation is propagated into mature biomass (MB), a proxy for population resilience, and population production, a measure of population rebuilding capacity. Production is defined as the biomass produced by the stock above what is needed to sustain the population at a constant level. We used empirical estimates of reproductive success and somatic growth rate, coupled with a population model, to evaluate the relative role of both types of variation in population fluctuations. The effects of this variation on population production and MB were examined across three variation scenarios, in which somatic growth only, reproduction only or both processes varied temporally. We also examined three levels of age truncation to explore whether modified population age structure altered these dynamics. The contribution of somatic growth to biomass variability exceeded that of recruitment for some species (2/8), while in others (5/8 species), recruitment variation was more influential. When population production was examined, somatic growth variation contributed more to population variation for three species. The relative importance of the two processes was not clearly correlated with key life-history traits (i.e., growth and mortality rates), but instead was determined by time-series characteristics of growth and recruitment variation. Increasing age truncation slightly increased the relative effect of recruitment variation on MB variation for three species. These results suggest somatic growth variation can be as important as early life-history survival in driving population fluctuations in some marine fish species. This analysis provides a counterexample to the commonly held assumption of many marine population dynamics models: That population variability is induced primarily through variation in reproductive success.