Probabilistic Maturation Reaction Norms Research Articles

A multi-population approach supports common patterns in marine growth and maturation decision in Atlantic salmon (Salmo salar L.) from southern Europe.

This study provides a regional picture of long-term changes in Atlantic salmon growth at the southern edge of their distribution, using a multi-population approach spanning 49 years and five populations. We provide empirical evidence of salmon life history being influenced by a combination of common signals in the marine environment and population-specific signals. We identified an abrupt decline in growth from 1976 and a more recent decline after 2005. As these declines have also been recorded in northern European populations, our study significantly expands a pattern of declining marine growth to include southern European populations, thereby revealing a large-scale synchrony in marine growth patterns for almost five decades. Growth increments during their sea sojourn were characterized by distinct temporal dynamics. At a coarse temporal resolution, growth during the first winter at sea seemed to gradually improve over the study period. However, the analysis of finer seasonal growth patterns revealed ecological bottlenecks of salmon life histories at sea in time and space. Our study reinforces existing evidence of an impact of early marine growth on maturation decision, with small-sized individuals at the end of the first summer at sea being more likely to delay maturation. However, each population was characterized by a specific probabilistic maturation reaction norm, and a local component of growth at sea in which some populations have better growth in some years might further amplify differences in maturation rate. Differences between populations were smaller than those between sexes, suggesting that the sex-specific growth threshold for maturation is a well-conserved evolutionary phenomenon in salmon. Finally, our results illustrate that although most of the gain in length occurs during the first summer at sea, the temporal variability in body length at return is buffered against the decrease in post-smolt growth conditions. The intricate combination of growth over successive seasons, and its interplay with the maturation decision, could be regulating body length by maintaining diversity in early growth trajectories, life histories, and the composition of salmon populations.

Decades of warming alters maturation and reproductive investment in fish

AbstractHow does warming affect maturation and reproductive investment in ectotherms? Younger age and smaller size at maturation, as well as altered reproduction processes, have been found in a few species subjected to elevated temperatures. These observations, however, come from studies that do not distinguish effects of warming on maturation from those on growth, are also restricted to single generation responses to warming, or have additional stressors besides warming in the study system. Here, we study warming effects on maturation and reproductive investment in wild, unexploited fish populations using a whole‐ecosystem heating experiment. The experiment is conducted on Eurasian perch (Perca fluviatilis) in a heated and control area (with >5°C temperature difference) in the Baltic Sea. We compare female perch size at maturation using estimated probabilistic maturation reaction norms (PMRNs) and the gonado‐somatic index over 17 years of heating, spanning approximately five to eight perch generations. Using the PMRN approach, we show that warming has substantial effects on maturation size independent of warming‐induced changes in body growth. We found that young fish mature at a smaller size and invest more in developing their gonads in the heated population than in the unheated population. Our findings suggest that warming effects on reproductive investment may initially compensate for the cost of warming‐induced decrease in maturation size caused by the trade‐off between early maturation and size‐dependent fecundity. After multiple additional generations of warming, maturation and reproduction traits in perch differed from those in the first generations following the onset of warming, which suggests that warming‐induced evolution may have occurred. Our study is particularly relevant in the context of climate change because of the unusually large temperature difference between the areas and the fact that the heating occurred on an ecosystem level. We call for experimental studies resolving mechanisms of trait responses to warming across generations, complemented with genomic analyses, to aid understanding of organisms' long‐term responses to climate change.

Stock-specific variation in the probability of precocious male maturation in hatchery Chinook salmon (Oncorhynchus tshawytscha)

Age of maturation in many salmonid species is phenotypically plastic and dependent on exceeding a genetically set threshold in growth, often described as a probabilistic maturation reaction norm (PMRN). Hatchery supplementation programs for Chinook salmon (Oncorhynchus tshawytscha) in the Pacific Northwest US have been developed to minimize differences between hatchery and wild fish by integrating natural-origin adults into broodstock, potentially affecting PMRNs. We raised fish from 10 Chinook salmon stocks with variable levels of natural-origin integration in a common garden environment to explore potential genetic variation in PMRNs for precocious male maturation as age 2 minijacks. Proportion minijacks varied ≈10-fold (0.043 to 0.443) and the PMRN WP50 (predicted weight at 50% maturation) varied by ≈18 g (24.1 to 41.7 g). The propensity for minijack maturation was generally higher in stocks with higher levels of natural-origin integration. These findings demonstrate the effect of genotype by environment interactions on life history of salmonids and the need for stock-specific tailoring of rearing regimes to regulate differences between hatchery and wild fish, when wild fish are used in broodstocks.

Maturation reaction norm evolution under varying conditions of eco-evolutionary change

Probabilistic maturation reaction norms (PMRNs) are commonly used to infer evolution of maturation age and size in wild fish stocks, but how well estimates from phenotypic data actually reflect underlying genotypes is debated. We used an eco-genetic model of a commercially harvested freshwater fish to simulate populations undergoing various levels of fisheries-induced evolution and density-dependent feedback and evaluated effects on the estimation of PMRNs. We estimated PMRNs from phenotypic data sampled from simulated populations (age, length, and maturation status of individuals), as is done for wild stocks, and compared estimates with the known maturation genotypes of individuals in the simulated population. PMRN estimates were robust to changes in the strength of density-dependent growth and high levels of fisheries-induced evolution. However, our ability to detect slower rates of evolution was limited, especially when individuals matured within a narrow range of ages. Furthermore, low numbers of immature or mature individuals within a given age class limited our ability to robustly estimate midpoints for more than one or two ages, despite our large sample sizes (n = 200 randomly sampled individuals of each age per year), highlighting a challenge with this approach. Nonetheless, this study suggests that the widely applied method of estimating PMRNs from readily available phenotypic data to detect underlying evolution of maturation schedule is robust to some key factors that vary in wild populations.

Determinism of Temporal Variability in Size at Maturation of Sardine Sardina pilchardus in the Bay of Biscay

Age and size at maturation appear as key parameters governing the dynamics of a population as they affect growth rate, fecundity, and survival. The expression of such life history traits is determined by genetic make-up and modulated by environmental factors mainly through phenotypic plasticity. Moreover, fishing, besides decreasing population size and changing demographic composition can alter allelic frequencies through fisheries-induced evolution by selecting for some particular traits. In the Bay of Biscay, a decreasing trend in both sardine body condition and size-at-age has recently been pointed out at the population level. The Probabilistic Maturation Reaction Norm (PMRN) approach was applied to help disentangle phenotypic plasticity and genetic changes. Based on the analysis of sardine spawning seasonality, PMRN was estimated by considering body condition as additional life-history state variable to predict the onset of maturation. The resulting PMRN was then used to investigate temporal trends in reaction norm midpoints to test whether changes in length at maturation can be explained by plastic and/or evolutionary adaptive change. Overall, our results emphasize for the first time that including sardine body condition as explanatory variable improves predictions of maturation probability. We found that better individual condition increases maturation probability. The assessment of temporal changes in length at maturation confirms the low plasticity in this trait for a species maturing mostly at age-1 and advocates for the use of a monthly time scale when investigating PMRNs for this species. Beside environmental variables included in this analysis (water temperature, chlorophyll-a, and population biomass) that only show a weak correlation with PMRN midpoints, our results reveal no evidence for recent fisheries-induced evolution in the sardine stock of the Bay of Biscay. They suggest that the short-term variability in length at maturation is strongly dependent upon individual growth which is likely driven by environmental factors. For sardine fisheries management, our study highlights the need to consider both the length-composition data and the seasonality within a stock assessment model. Finally, we discuss the fact that considering individual growth trajectories should improve our understanding of the relationship between environmental variability and changes in maturation for sardine.

Long-term changes in the maturation and growth of Cape Cod/Gulf of Maine yellowtail flounder Limanda ferruginea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 633:169-180 (2020) - DOI: https://doi.org/10.3354/meps13175 Long-term changes in the maturation and growth of Cape Cod/Gulf of Maine yellowtail flounder Limanda ferruginea Gregory R. DeCelles1,*, Tiffany Vidal1,2 1Massachusetts Division of Marine Fisheries, Resource Assessment and Monitoring Program, New Bedford, MA 02744, USA 2The Pacific Community, BP D5, 98848 Noumea, New Caledonia *Corresponding author: gregory.decelles@mass.gov ABSTRACT: Yellowtail flounder Limanda ferruginea in the Gulf of Maine have been harvested by commercial fisheries since the 1930s, and the most recent stock assessment estimated that from 1985 to 2016 fishing mortality rates exceeded the fishing mortality reference point (F40%) in all but one year. The objective of this study was to understand changes in the maturation and growth of Cape Cod/Gulf of Maine yellowtail flounder by examining fisheries-independent data from cohorts produced over a 38 yr period (1976 to 2014). Maturity ogives demonstrated that the length and age at 50% maturity declined for cohorts throughout that time period. Probabilistic maturation reaction norms (PMRNs), which describe the probability that an immature individual will become mature at a certain age and size class, demonstrated a shift towards maturation at younger ages and smaller sizes. For age 3 female and male yellowtail flounder, the size at which the probability of becoming mature was 0.5 declined from 30.8 and 26.0 cm to 23.9 and 16.4 cm, respectively. Using von Bertalanffy growth curves, a decrease in asymptotic length (L×) and an increase in the growth rate (K) was estimated for both sexes. Taken together, these life history changes are suggestive of fisheries-induced evolution. However, although the PMRN approach is designed to account for phenotypic plasticity associated with growth and survival, further work is needed to determine whether other factors (e.g. temperature and body condition) could be responsible for the changes in maturation observed in this study. KEY WORDS: PMRN · Limanda ferruginea · Fisheries-induced evolution · Maturation · Von Bertalanffy Full text in pdf format PreviousNextCite this article as: DeCelles GR, Vidal T (2020) Long-term changes in the maturation and growth of Cape Cod/Gulf of Maine yellowtail flounder Limanda ferruginea. Mar Ecol Prog Ser 633:169-180. https://doi.org/10.3354/meps13175 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 633. Online publication date: January 09, 2020 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2020 Inter-Research.

Population density and temperature correlate with long-term trends in somatic growth rates and maturation schedules of herring and sprat.

We examine long-term trends in the average growth rates and maturation schedules of herring and sprat populations using survey data collected from the North Sea and west of Scotland since the 1960s and 1980s respectively. Otolith age data and maturity data are used to calculate time series of mean lengths at age, von Bertalanffy growth parameters, and probabilistic maturation reaction norms. As the growth and maturation of fish is known to be influenced by temperature and stock abundances, we account for these variables using Generalised Additive Models. Each of the herring populations displayed either steady declines in mean length across multiple age groups, or declines in length followed years later by some recovery. Depending on region, lengths at age of sprat increased or decreased over time. Varying temporal trends in maturation propensity at age and length were observed across herring populations. Many of the trends in growth rate and maturation were correlated to population abundance and/or temperature. In general, abundance is shown to be negatively correlated to growth rates in herring and sprat, and positively correlated with maturation propensity in herring. Temperature is also shown to be correlated to growth and maturation, and although the effect is consistent within species, the temperature effects differ between herring and sprat. This study provides detailed information about long-term trends in growth and maturation, which is lacking for some of these pelagic stocks, especially in the west of Scotland.

Reaction norm analysis reveals rapid shifts toward delayed maturation in harvested Lake Erie yellow perch (Perca flavescens).

Harvested marine fish stocks often show a rapid and substantial decline in the age and size at maturation. Such changes can arise from multiple processes including fisheries‐induced evolution, phenotypic plasticity, and responses to environmental factors other than harvest. The relative importance of these processes could differ systematically between marine and freshwater systems. We tested for temporal shifts in the mean and within‐cohort variability of age‐ and size‐based maturation probabilities of female yellow perch (Perca flavescens Mitchill) from four management units (MUs) in Lake Erie. Lake Erie yellow perch have been commercially harvested for more than a century, and age and size at maturation have varied since sampling began in the 1980s. Our analysis compared probabilistic maturation reaction norms (PMRNs) for cohorts when abundance was lower and harvest higher (1993–1998) to cohorts when abundance was higher and harvest lower (2005–2010). PMRNs have been used in previous studies to detect signs of evolutionary change in response to harvest. Maturation size threshold increased between the early and late cohorts, and the increases were statistically significant for the youngest age in the western MU1 and for older ages in the eastern MU3. Maturation envelope widths, a measure of the variability in maturation among individuals in a cohort, also increased between early and late cohorts in the western MUs where harvest was highest. The highest rates of change in size at maturation for a given age were as large or larger than rates reported for harvested marine fishes where declines in age and size at maturation have been observed. Contrary to the general observation of earlier maturation evolving in harvested stocks, female yellow perch in Lake Erie may be rapidly evolving delayed maturation since harvest was relaxed in the late 1990s, providing a rare example of possible evolutionary recovery.

Changing maturation reaction norms and the effects of growth history in Alaskan Chinook salmon

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 595:187-202 (2018) - DOI: https://doi.org/10.3354/meps12564 Changing maturation reaction norms and the effects of growth history in Alaskan Chinook salmon Jared E. Siegel1,*, Milo D. Adkison2, Megan V. McPhee2 1South Fork Research, Inc., 44842 SE 145th St., North Bend, WA 98045, USA 2College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, 17101 Point Lena Loop Road, Juneau, AK 99801, USA *Corresponding author: derajlegeis@gmail.com ABSTRACT: We explored the nature of declines in size and age at maturity in 2 populations of Chinook salmon Oncorhynchus tshawytscha in western Alaska, USA, using multidimensional probabilistic maturation reaction norms (PMRNs) accounting for growth history. Individual growth histories informed by retrospective scale analysis were used to construct PMRNs and to describe the relative influence of different life-history periods on age at maturity. Similar results were found in both populations. Models accounting for growth history uniformly outperformed size-at-age models, suggesting the importance of growth history for the determination of maturation. The second year of marine growth was found to have a disproportionate influence on the age at maturity in both sexes. Males tended to grow more than females in length during the second year at sea, possibly as a consequence of females storing more energy in preparation for the high cost of female gonad development. Finally, we found that growth thresholds for maturation have shown a long-term decline in both sexes. This suggests that declines in the average age at maturity of western Alaskan Chinook salmon may have been caused in part by adaptation to environmental or fisheries-induced selection. KEY WORDS: Oncorhynchus tshawytschka · Size at age · PMRN · Age at maturity · Life history · Plasticity · Evolution · Retrospective scale analysis Full text in pdf format Supplementary material PreviousNextCite this article as: Siegel JE, Adkison MD, McPhee MV (2018) Changing maturation reaction norms and the effects of growth history in Alaskan Chinook salmon. Mar Ecol Prog Ser 595:187-202. https://doi.org/10.3354/meps12564 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 595. Online publication date: May 14, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Adaptive and plastic variation in growth and maturation of the cutlassfish Trichiurus japonicus in the subtropical Pacific Ocean

We investigate d va ria- t ion in growth and maturation in response to 1) spatial variation in climate and 2) exploitation of the subtropical cutlassfish Trichiurus japonicus , an important fishery species whose population ecology i s virtually unknown . Individuals of this cutlassfish species were sampled monthly at 2 primary landing sites: Kengfang, in northeast (NE) Taiwan, and Tsukuan, in southwest (SW ) Taiwan, during 2013–2015. Habitat temperatures were about 1–4°C lower at the NE site t han at the SW site, and the length at age of adult fish had a n inverse pattern with temperature (NE lengths > SW lengths at age). The probabilistic maturation reaction norms did not differ significantly between the 2 areas, but ages and lengths a t maturation were higher for males from NE than for males from SW. Differences in asymptotic lengths (NE>SW) and growth coefficients (NE<SW), tog eth- er with narrower length distribution at the SW site than at the NE site, indicate potentially different mortality between the areas (NE<SW). These results indicate plastic variation in maturation and potentially adaptive variation in growth for this species that are related to the differences in temperature and mortality between the areas, although reliable proxies for mortality are lacking. The observed life-history variation was consistent with the tem perature–size rule that organisms tend to grow faster, mature earlier, and reach smal ler asymptotic sizes at warmer temperatures and may indicate an adaptive divergence of cutlassfish populati

Sex-based trade-offs among growth, mortality, and maturation in Great Lakes yellow perch stocks

Trade-offs among growth, mortality, and reproduction form the basis of life history theory but may vary among populations owing to local ecological conditions. We examined life history trade-offs driving variation in maturation among 13 yellow perch (Perca flavescens) stocks in the Great Lakes using sex-specific age and length at 50% maturity (A50 and L50, respectively) and probabilistic maturation reaction norm midpoints (Lp50,a). Both sexes exhibited positive correlations between growth and mortality, and faster-growing stocks were mature at younger ages but larger sizes. Male and female A50 and L50 were positively correlated among stocks, but Lp50,a estimates were negatively correlated among stocks, indicating stocks that matured at large sizes for a given age in females matured at smaller age-specific sizes in males. Female Lp50,a estimates were negatively related to growth and mortality, while male Lp50,a estimates were positively related to growth. These results suggest that (i) sex-based life history trade-offs sometimes act to differentially structure maturation schedules in males and females and (ii) males may be less responsive to changes in mortality than females.

A deterministic eco-genetic model for the short-term evolution of exploited fish stocks

Eco-genetic models describe the contemporary evolution of quantitative genetic traits by integrating the key ecological aspects with the modes of inheritance. Because of the flexibility in incorporating physiological, ecological, and genetic detail, eco-genetic models are typically individual-based and stochastic. Here we propose a deterministic eco-genetic population model to study the evolution of maturation schedules in exploited fish stocks. It is the first deterministic model addressing the evolution of the probabilistic maturation reaction norm (PMRN)—the genetic trait recognized to control maturation in fish species. The PMRN is the probability of interseasonal maturation as a function of age and size of juvenile individuals. Being independent of the growth trajectory followed by the individual, it can be considered a non-plastic phenotype, under limited change in temperature and climate. We consider the continuous age and size structures of the fish stock and explicitly represent foraging on a single food resource and mating under 1:1 sex ratio. Inheritance follows a single-locus-two-alleles Mendelian rule to limit the number of genotypes. We adapt the standard escalator boxcar train method for the simulation of physiologically structured population models to the case of two-dimensional age distributions—mature individuals being also distributed according to the age at maturation. The model simulations qualitatively reproduce empirical trends, specifically the genotypic redistribution toward early-maturing types under increasing exploitation and the eventual collapse of the stock. Interestingly, increased effort targeted at large sizes seems to erode the genetic diversity, an effect missed by stochastic models that might serve as warning to fishery managers and policy makers.

Rapidly shifting maturation schedules following reduced commercial harvest in a freshwater fish.

Size-selective harvest of fish stocks can lead to maturation at smaller sizes and younger ages, which may depress stock productivity and recovery. Such changes in maturation may be very slow to reverse, even following complete fisheries closures. We evaluated temporal trends in maturation of five Great Lakes stocks of yellow perch (Perca flavescens Mitchill) using indices that attempt to disentangle plastic and evolutionary changes in maturation: age at 50% maturity and probabilistic maturation reaction norms (PMRNs). Four populations were fished commercially throughout the time series, while the Lake Michigan fishery was closed following a stock collapse. We documented rapid increases in PMRNs of the Lake Michigan stock coincident with the commercial fishery closure. Saginaw Bay and Lake Huron PMRNs also increased following reduced harvest, while Lake Erie populations were continuously fished and showed little change. The rapid response of maturation may have been enhanced by the short generation time of yellow perch and potential gene flow between northern and southern Lake Michigan, in addition to potential reverse adaptation following the fishing moratorium. These results suggest that some fish stocks may retain the ability to recover from fisheries-induced life history shifts following fishing moratoria.

Non-random sampling for reproductive status induces bias in probabilistic maturation reaction norm midpoints

The midpoint of the probabilistic maturation reaction norm (PMRN), which is the age-specific size at which the probability of maturing is 50%, has been used as an index of age and size at maturation of fish populations. The most important data requirement for the PMRN method is random sampling for reproductive status (mature or immature) at each age or size class. When age at first maturation varies between sexes, randomness of sampling for reproductive status can be evaluated by the sex ratio at the age of first maturity of the precocious sex (i.e., if the sex ratio at the age of first maturity of the precocious sex [sex of which age at first maturation is younger than the other] is biased, the randomness of sampling for reproductive status is not ensured). We examined the effect of the randomness of sampling for reproductive status on PMRN midpoints of precocious males in 10 tributaries containing two salmonid species. The sex ratio (the proportion of males) of fish at the age of first maturity varied among tributaries. The PMRN midpoints for males decreased with an increase in the sex ratio in both species. The biased sex ratio was thought to be caused by different movement patterns of precocious mature males and immature males/females. Our results emphasize that non-random sampling for reproductive status greatly affects estimated PMRN midpoint values.

Fishery-induced changes to age and length dependent maturation schedules of three demersal fish species in the Firth of Clyde

Probabilistic maturation reaction norms (PMRNs) were used to investigate the maturation schedules of cod, haddock and whiting in the Firth of Clyde to determine if typical lengths at maturation have changed significantly since 1986. Some potential sources of growth-independent plasticity were accounted for by including sea-surface temperature and abundance variables in the analysis. The PMRNs of the Clyde populations were compared with those from the wider west coast, in conjunction with regional differences in the fishery, to assess whether fishing may have been driving the observed trends of decreasing lengths at maturation. The lengths at which haddock, whiting and female cod were likely to mature decreased significantly during 1986–2009, with rates of change being particularly rapid in the Clyde. It was not possible to estimate PMRNs for male cod due to limited data. Trends in temperature and abundance were shown to have only marginal affects upon PMRN positions, so temporal trends in maturation schedules appear to have been due to a combination of plastic responses to other environmental variables and/or fishing. Regional differences in fishing intensity and the size-selectivity of the fisheries suggest that the decreases in lengths at maturation have been at least partially due to fishing. The importance and scale of the Clyde Nephrops fishery increased as demersal landings declined, and the majority of demersal fish landings have come from Nephrops bycatch since about 2005 when the demersal fishery ceased. Since it appears as though fishing may have caused increasingly early maturation, and a substantial Nephrops fishery continues to operate in the Clyde, reversal of these changes is likely to take a long time – particularly if there is an evolutionary component to the trends. If size-selective fishing has contributed to the lowered abundance of large fish by encouraging maturation at increasingly small lengths, then large fish may remain uncommon in the Clyde until the observed trends in maturation lengths reverse.

Probabilistic maturation reaction norm trends reveal decreased size and age at maturation in an intensively harvested stock of pikeperch Sander lucioperca

Abstract Reports of fishing-induced evolution in the maturation schedules of commercially exploited marine fishes are numerous and are alarming. Here, we analysed the probabilistic maturation reaction norms for a freshwater fish species, the pikeperch ( Sander lucioperca ), to assess whether the maturation patterns of cohorts 1993–2006 responded to intensive coastal commercial and recreational fishing in the Archipelago Sea area of the Baltic Sea. Lp 50 values estimated using individual data from catch samples revealed that maturation had shifted towards an earlier age and smaller size. Lengths-at-age showed variation, but no linear trends, whereas the condition factor decreased for most of the age groups. Cross-correlation analysis of Lp 50 values with variables describing stock size (potential social effects on maturation) and water temperature revealed no obvious growth-independent effects on the maturation trends. This suggests that fishing, in line with the life-history theory, was the most important driving factor favouring early maturation. These results suggest that fishing-induced shifts in the maturation patterns of fish are nearly ubiquitous and not limited to commercially harvested marine species. This calls for increased awareness of fishing-induced life-history changes also in coastal and inland fisheries management.

Bayesian fitting of probabilistic maturation reaction norms to population-level data

Probabilistic maturation reaction norms (PMRNs) are an important tool for studying fisheries-induced evolution and environmental effects on life history. To date there has been no way to fit a PMRN to population-level fisheries data; instead individual-level data must be used. This limits the stocks and time periods that can be studied.We introduce a Bayesian method for fitting PMRNs to population-level data. The method is verified against both an existing result and simulated data, and applied to historical Barents Sea cod data which combines observations of population-level variation in age, size and maturity status from Russia and Norway.The method shows a clear and rapid trend towards greater probability of maturation at smaller lengths in the Barents Sea cod.The new model fitting algorithm allows us to study historic changes in life history despite the lack of individual-level data seen in much long term data. Access to more data will aid the study of evolutionary hypotheses in a wide range of organisms.

Probabilistic maturation reaction norms assessed from mark-recaptures of wild fish in their natural habitat.

Reaction norms are a valuable tool in evolutionary biology. Lately, the probabilistic maturation reaction norm approach, describing probabilities of maturing at combinations of age and body size, has been much applied for testing whether phenotypic changes in exploited populations of fish are mainly plastic or involving an evolutionary component. However, due to typical field data limitations, with imperfect knowledge about individual life histories, this demographic method still needs to be assessed. Using 13 years of direct mark–recapture observations on individual growth and maturation in an intensively sampled population of brown trout (Salmo trutta), we show that the probabilistic maturation reaction norm approach may perform well even if the assumption of equal survival of juvenile and maturing fish does not hold. Earlier studies have pointed out that growth effects may confound the interpretation of shifts in maturation reaction norms, because this method in its basic form deals with body size rather than growth. In our case, however, we found that juvenile body size, rather than annual growth, was more strongly associated with maturation. Viewed against earlier studies, our results also underscore the challenges of generalizing life-history patterns among species and populations.

Ecological and evolutionary patterns of freshwater maturation in Pacific and Atlantic salmonines

Reproductive tactics and migratory strat- egies in Pacific and Atlantic salmonines are inextri- cably linked through the effects of migration (or lack thereof) on age and size at maturity. In this review, we focus on the ecological and evolutionary patterns of freshwater maturation in salmonines, a key process resulting in the diversification of their life histories. We demonstrate that the energetics of maturation and reproduction provides a unifying theme for under- standing both the proximate and ultimate causes of variation in reproductive schedules among species, populations, and the sexes. We use probabilistic maturation reaction norms to illustrate how variation in individual condition, in terms of body size, growth rate, and lipid storage, influences the timing of maturation. This useful framework integrates both genetic and environmental contributions to condi- tional strategies for maturation and, in doing so, demonstrates how flexible life histories can be both heritable and subject to strong environmental influ- ences. We review evidence that the propensity for freshwater maturation in partially anadromous species is predictable across environmental gradients at geo- graphic and local spatial scales. We note that growth is commonly associated with the propensity for fresh- water maturation, but that life-history responses to changes in growth caused by temperature may be strikingly different than changes caused by differences in food availability. We conclude by exploring how contemporary management actions can constrain or

Temporal trends in age and size at maturation of four North Sea gadid species: cod, haddock, whiting and Norway pout

Younger ages and smaller sizes at maturation have been observed in commercial fish stocks over the last century. We establish that age and length at 50% proportion mature (i.e. the proportion of mature individuals in a population or the probability that an individual is mature) decreased from the 1970s to the 2000s in North Sea cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus, but not in Norway pout Trisopterus esmarkii. The potential contributions of demography, phenotypic plasticity and evolution to these trends were assessed. First, maturation trends were extricated from demographic effects and growth-dependent plasticity by estimating probabilistic maturation reaction norms (PMRNs). PMRN midpoints have significantly shifted downwards at most ages for cod, haddock and whiting, but not for Norway pout. Second, increased temperature and food abundance, loosened trophic competition and relaxed social pressure may also trigger growth-independent plasticity in maturation. Principal component regression of PMRN midpoints on annual estimates of relevant environmental vari- ables exhibiting a temporal trend suggest that, despite some evidence of environmental effects, PMRN trends were mostly independent of growth-independent plasticity in haddock, whiting and male cod, but not in female cod. According to these findings, evolution of maturation, potentially in response to fishing, is plausible in haddock, whiting and male cod, but unlikely for Norway pout, and does not explain trends in female cod maturation. In agreement with life-history theory, the maturation response was larger in fast-growing, late- and large-maturing species exhibiting moderate reproductive effort.