Individual Variation In Growth Research Articles

Fluctuating selection on reproductive timing in Digitalis purpurea

Despite recent, strong interest in the modelling of monocarpic perennial flowering strategies, little is known about how variation in demographic rates affects selection on optimal timing of flowering. Temporal variation may yield fluctuating selective pressures, or, if individuals experience time trends, selection for phenotypic plasticity. Here we report the results of a 3‐year study in a large field population of the facultative biennial herb Digitalis purpurea, where we use field data on size‐dependent growth, survival and fecundity to parameterize an existing optimisation model. We compare results from models using either deterministic or individually varying demographic rates to address the degree of fluctuating selection on the flowering strategy. In addition, we explore whether recent growing conditions influence the size‐specific liability to flower. Model results differed widely between years; immediate onset of reproduction was predicted in 1999, strongly delayed reproduction in 2000. This reflected large differences in both growth and survival rates between years. Observed flowering sizes also varied between years, but were larger in 1999 than in 2000, contrary to model predictions. Incorporating individual variation in growth increased predicted optimal flowering sizes compared to models using deterministic growth, whereas the inclusion of individual survival variation had little effect. There was no significant effect of recent growth rate on flowering probability. Taken together, these results indicate highly fluctuating selection on the flowering strategy in D. purpurea, but no evidence of adaptive plasticity in response to current growing conditions. Fluctuating selection may contribute to maintain genetic variation for threshold size for flowering, and may partly explain the large within‐season size‐variation in flowering individuals found in natural populations of D. purpurea.

COUNTERINTUITIVE DENSITY-DEPENDENT GROWTH IN A LONG-LIVED VERTEBRATE AFTER REMOVAL OF NEST PREDATORS

Examining the phenotypic and genetic underpinnings of life-history variation in long-lived organisms is central to the study of life-history evolution. Juvenile growth and survival are often density dependent in reptiles, and theory predicts the evolution of slow growth in response to low resources (resource-limiting hypothesis), such as under densely populated conditions. However, rapid growth is predicted when exceeding some critical body size reduces the risk of mortality (mortality hypothesis). Here we present results of paired, large-scale, five-year field experiments to identify causes of variation in individual growth and survival rates of an Australian turtle (Emydura macquarii) prior to maturity. To distinguish between these competing hypotheses, we reduced nest predators in two populations and retained a control population to create variation in juvenile density by altering recruitment levels. We also conducted a complementary split-clutch field-transplant experiment to explore the impact of incubation temperature (25 degrees or 30 degrees C), nest predator level (low or high), and clutch size on juvenile growth and survival. Juveniles in high-recruitment (predator removal) populations were not resource limited, growing more rapidly than young turtles in the control populations. Our experiments also revealed a remarkably long-term impact of the thermal conditions experienced during embryonic development on growth of turtles prior to maturity. Moreover, this thermal effect was manifested in turtles approaching maturity, rather than in turtles closer to hatching, and was dependent on population density in the post-hatching rearing environment. This apparent phenotypic plasticity in growth complements our observation of a strong, positive genetic correlation between individual body size in the experimental and control populations over the first five years of life (rG - +0.77). Thus, these Australian pleurodiran turtles have the impressive capacity to acclimate plastically to major demographic perturbations and enjoy the longer-term potential to evolve adaptively to maintain viability.

Using competition and light estimates to predict diameter and height growth of naturally regenerated beech seedlings growing under changing canopy conditions

In a naturally regenerated beech (Fagus sylvatica L.) stand in northeastern France, five regeneration patches were selected with different degrees of canopy closure; the percentage of above-canopy light (PACL) ranged between 5 and 45 per cent. The canopy was removed by a storm 2 years later. In order to analyse the immediate and delayed effects of PACL on seedling diameter and height growth, the effects of the light conditions prevailing before canopy opening on seedling growth measured before and after canopy opening were tested. The effects of local density were examined using neighbourhood analysis. Four different competition indices (CIs) and neighbourhood radii ranging from 10 to 200 cm were tested. Models including PACL and CI accounted for between 56 and 64 per cent of the variation in individual seedling annual diameter or height growth. Local density had a strong negative influence on diameter growth and a much smaller influence on height growth. PACL was positively correlated with diameter and height growth before canopy opening. Delayed effects of PACL on height growth were observed just after canopy opening, but disappeared after 2 years. No delayed effects of PACL on diameter growth after canopy opening were observed.

The consistency of individual differences in growth, feed efficiency and feeding behaviour in African catfish Clarias gariepinus (Burchell 1822) housed individually

Individual variation in growth, feed intake and feeding behaviour has been previously recognized in several fish species. However, there is a lack of information regarding the consistency of such individual differences, which is important to understand the probability of a certain individual trait to be inherent, i.e. genetically linked. The goal of this study is to quantify the consistency of individual differences in growth, feed intake/efficiency and feeding behaviour in African catfish Clarias gariepinus (Burchell 1822). Forty-eight juvenile fish (58.9±0.4 g) were housed individually for 47 days and weighed every second week. The consistency of growth, feed efficiency (residual feed intake (RFI)) and feeding behaviour (total feeding time (TFT)) was determined using repeatability estimates. Fish exhibited pronounced individual variation in growth (CV=52.8%), feed intake (34.3%) and in the TFT (>100%). The repeatability estimates were 0.55 for growth, 0.70 for feed intake, 0.49 for RFI and 0.81 for TFT. Individual differences in growth were mainly explained by individual differences in feed intake (80%). Individual differences in feeding behaviour contributed to explain differences in feed intake by affecting the RFI. With increasing TFT, the maintenance levels also increased suggesting that slow eaters were less efficient in feed/energy utilization. The results of this study indicate that individual differences in growth, feed intake/efficiency and feeding behaviour are consistent over time and therefore probably inherent. Moreover, this study may have implications on the use of feeding behaviour as a predictor of feed efficiency in juveniles of African catfish.

Reversibilitas kalsifikasi tulang akibat kekurangan protein pre dan post natal (Reversibility of bone calcification on pre and post natal protein deficiency)

The growth and development play an important role in orthodontics mainly in bone, because it can determine the maturity of the bone. Bone maturity evaluation is very important in orthodontic treatment, because there are many individual variations in growth and development such as time, duration and velocity of the growth. Nutritional status during pregnancy and infant period will influence the growth and the development of bone. Protein diet is an important factor, which will determine the optimal calcification during bone growth and development stages. Bone calcification, in orthodontics, can be used to estimate the bone maturity for diagnosis and treatment planning. The purpose of this study was to recognize ones ability to surpass calcium and phosphor deficiency because of pre and postnatal protein deficiency. There were three groups of samples of Rattus norvegicus rats. The first group was the control group with standard diet, the second was the infant group with pre and postnatal protein deficiency, and the third group was young rat at weaning age with pre and postnatal protein deficiency supplemented with enough protein in the diet. Bone calcification stage was analyzed: 1) Histologically by measuring epiphyseal width on right femur; 2) by measuring calcium and phosphor concentration on left femur with Spectrophotometry Atomic Absorption and spectroscopy ultra light visible. The data were analyzed by one way ANOVA continued by t test. The result showed that: 1) there was significant (p < 0.01) epiphyseal width difference between group I and II, I and III (p < 0,01) but there was not significant difference between group II & III (p > 0.05); 2) there was significant calcium and phosphor concentration on bone between group I, II and III (p < 0.01). It was concluded that bone calcification damage because of pre and post natal protein deficiency was an irreversible process. Protein supplement after bone calcification could not restore the condition.

Age estimation of the wild population of Japanese mantis shrimp Oratosquilla oratoria (Crustacea: Stomatopoda) in Tokyo Bay, Japan, using lipofuscin as an age marker

In this study, the age composition of the Japanese mantis shrimp, Oratosquilla oratoria, in Tokyo Bay, central Japan, was investigated using lipofuscin, an autofluorescent pigment, as an age marker. Lipofuscin in a histological section of protocerebral bridge cell mass (PBCM) in the brain was identified by confocal microscopy, and its concentration was quantified by image analysis. Modal analysis of the lipofuscin concentration showed four regularly-spaced modes that could each be regarded as a distinct age group. This implied a constant lipofuscin accumulation in PBCM at a 6.5×10−2% volume fraction per year; it also implied the existence of individuals that are at least 4 years old. The lipofuscin concentration was found to be a more appropriate index than body length for estimating the age of O. oratoria, because the modal analysis on the body-length histogram failed to detect apparent age groups. This was probably a result of the declining growth rate with age and the individual variations in growth through molting leading to considerable overlap in the size between different age groups. The lipofuscin analysis suggested that fast-growing individuals in each cohort are recruited to the fishery, and most individuals attain the exploitable size between 2 and 3 years of age.

An integrated growth study for North Atlantic albacore (Thunnus alalunga Bonn. 1788)

Abstract The growth of North Atlantic albacore (Thunnus alalunga) was studied using three methods. Spines collected during a fishing season (n = 761) were read and used to estimate growth parameters. Additional growth estimates were obtained by applying MULTIFAN to quarterly size frequency distributions of total international catch for the period 1990–1999 as well as using tag-recapture data (n = 314). In the case of spines and tagging, models that allowed for individual variability in growth were evaluated. In the analysis with size frequency distributions, a model that allowed for seasonal growth and age-dependent standard deviation around the mean lengths-at-age was used. Growth estimates were consistent among the different methods, and a combined model based on ageing derived from spines and tagging data is proposed as the most comprehensive descriptor of growth of North Atlantic albacore.

Piscivory in Juvenile Walleyes: Relative Importance of Prey Species, Timing of Spawning of Prey Fish, and Density on Growth and Survival

We examined the effect of the timing of spawning by prey fish and the species of prey fish on the growth and survival of juvenile walleye Stizostedion vitreum. We expected that age-0 walleyes would grow more in ponds when stocked about the same time as the spawning of gizzard shad Dorosoma cepedianum than when stocked about 6 weeks after spawning. We found, however, that the timing of larval gizzard shad presence did not affect walleye growth or survival. Also contrary to our expectations, walleyes from ponds with bluegill Lepomis macrochirus grew more (by 9% in total length and 38% in mass) than those from ponds with gizzard shad. However, the density of gizzard shad was lower than that of bluegills, particularly after the first few weeks of the experiment. Bluegills remained within the gape limits of walleyes throughout the experiment, whereas gizzard shad outgrew their vulnerability to predation. Late-spawned gizzard shad remained vulnerable to walleye predation only slightly longer than did those from early spawned ponds. The survival of age-0 walleyes was unaffected by water temperature, the timing of spawning by prey fish, prey species, or density. Our results suggest that even though walleyes typically grow faster in systems with gizzard shad as prey than in those with bluegills, juvenile walleyes can still grow well in bluegill-dominated systems. In some systems, high densities of a less beneficial prey (e.g., abundant bluegills rather than less abundant gizzard shad) may overcome the growth differences of juvenile walleyes that would be expected based on the differences in prey species alone. A wide variation in individual walleye growth in our ponds, even within prey treatments, indicated the complexity of interactions among the factors affecting juvenile walleye growth.

Individual growth variation and its relationship with survival in juvenile Pacific oysters,Crassostrea gigas(Thunberg)

In order to study individual growth variability and its relationship with survival in juvenile Crassostrea gigas, parental oysters were sampled at four sites located along the French Atlantic coast and bred under controlled hatchery conditions. Four groups of larvae were obtained by crossing five males and five females from each of the four sites, and a fifth group by crossing these 20 males and 20 females together in a pool. Larvae were reared under conditions allowing the maintenance of a maximum variability of size and gave five experimental groups. Oysters were individually monitored for growth and survival from 3 to 10 months after fertilization. The individual growth performances were relatively stable over time and no noticeable compensation for growth occurred. Early growth rate was a very good predictor of size later in life: 66% of variation in the live weight at 10 months could be explained by variation in the initial growth rate calculated between 3 and 4 months. A significant group effect was observed on survival and on growth rate. Mortality mostly occurred between 3 and 5 months and appeared to affect the slow-growing animals more. However, two groups can be distinguished among those which died during the experimental period, one which showed a decrease in weight and the other whose growth was similar to surviving oysters. These results are discussed in the light of usual oyster farming practices and selective breeding.

Individual variations in growth, food intake and activity in juvenile Chinese sturgeon Acipenser sinensis Gray

Journal of Applied IchthyologyVolume 18, Issue 4-6 p. 695-698 Individual variations in growth, food intake and activity in juvenile Chinese sturgeon Acipenser sinensis Gray X. Qian, X. Qian State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorY. Cui, Y. Cui State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorS. Xie, S. Xie State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorW. Lei, W. Lei State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorX. Zhu, X. Zhu State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorB. Xiong, B. Xiong State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorY. Yang, Y. Yang State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this author X. Qian, X. Qian State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorY. Cui, Y. Cui State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorS. Xie, S. Xie State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorW. Lei, W. Lei State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorX. Zhu, X. Zhu State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorB. Xiong, B. Xiong State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this authorY. Yang, Y. Yang State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, the Chinese Academy of Sciences; Wuhan, Hubei, P. R. China; Fisheries College, Huazhong Agricultural University, Wuhan, Hubei, P. R. ChinaSearch for more papers by this author First published: 17 December 2002 https://doi.org/10.1046/j.1439-0426.2002.00400.xCitations: 25AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume18, Issue4-6December 2002Pages 695-698 RelatedInformation

Long‐term variation in piscivory in a brown trout population: effect of changes in available prey organisms

Abstract – The piscivorous behaviour in a brown trout (Salmo trutta L.) population was studied in four discrete periods over seven decades (1917–94) in the hydroelectric reservoir Tunhovdfjord in Norway established in 1919. Piscivorous brown trout were extremely scarce prior to the introduction of two fish species Arctic charr (Salvelinus alpinus L.) and European minnow (Phoxinus phoxinus L.) in the 1920s. Brown trout started eating minnow at 17 cm and Arctic charr at 22 cm of length. In the 1950s, the brown trout predated extensively (60% of analysed trout) on Arctic charr and minnow. During the next four decades, the incidence of piscivorous brown trout declined to 15%, whereas the frequency of brown trout eating Arctic charr remained constant at 10%. The growth pattern, expressed as back‐calculated length, demonstrated similarity in three periods (1920s, 1960s and 1990s) and improved growth in the 1950s. The improvement was addressed the impoundment of a reservoir upstream. We did not find any marked change in growth rate due to piscivority, but coefficient of variance of back‐calculated lengths indicated significant variation in individual growth in age group ≥6 years from 1950 onwards. We accredit this variation to the rise of piscivorous brown trout.

Modelling individual growth and competition in plant populations: growth curves of Chenopodium album at two densities

Summary We modelled the growth in estimated biomass of individuals in experimental populations of Chenopodium album grown at two densities and measured sequentially nine times over 128 days. Competition is modelled by coupling individual growth equations and, within the population, the growth rate of a plant at any point in time is a function of its size to the power a, a measure of the degree of size‐asymmetry of competition. The growth of individuals in these crowded populations was significantly better fit by a Richards growth model than by models with one fewer parameter (e.g. logistic or Gompertz growth models). The additional parameter determines the location of the inflection point and provides great flexibility in fitting growth curves. Density had a significant effect on this parameter. At the higher density, the maximum size that plants achieved was decreased and the exponential phase of growth was reduced. The estimate of the size‐asymmetry parameter a was always greater than 1 and it increased significantly at the higher density. Growth was reduced and the number of very small individuals increased at the higher density, although a few plants still achieved a large size. Our approach combines several recent innovations in the modelling of stand development, including (a) modelling of individual growth with biologically meaningful growth models and (b) modelling the relationship between size and growth of individuals within the population. Sequential, non‐destructive data on the growth of individuals over time, in combination with modern statistical computing techniques, can lead to major advances in the modelling of plant population development, providing powerful tools for exploring variation in individual growth and for testing a wide range of hypotheses.

An improved method for estimating individual growth variability in fish, and the correlation between von Bertalanffy growth parameters

A new method for estimating individual variability in the von Bertalanffy growth parameters of fish species is presented. The method uses a nonlinear random effects model, which explicitly assumes that an individual's growth parameters represent samples from a multivariate population of growth parameters characteristic of a species or population. The method was applied to backcalculated length-at-age data from the tropical emperor, Lethrinus mahsena. Individual growth parameter variability estimates were compared with those derived using the current "standard" method, which characterizes the joint distribution of growth parameter estimates obtained by independently fitting a growth curve to each individual data set. Estimates of mean von Bertalanffy growth parameters from the two methods were similar. However, estimated growth parameter variances were much higher using the standard method. Using the random effects model, the estimated correlation between population mean values of L[Formula: see text] and K was –0.52 or –0.42, depending on the marginal distribution assumed for K. The latter estimate had a 95% posterior credibility interval of –0.62 to –0.17. These represent the first reliable estimate of this correlation and confirm the view that these parameters are negatively correlated in fish populations; however, the absolute correlation value is somewhat lower than has been assumed.

Age Structure and Growth of the Dusky Damselfish, Stegastes Fuscus, From Tamandaré Reefs, Pernambuco, Brazil

Age structure and growth of Stegastes fuscus were investigated during the present study. Individuals were collected monthly by divers using nets and spears in the reefs off Tamandare, Pernambuco, Brazil. Otoliths were removed from 346 individuals and sectioned transversally to determine individual age. Otolith sections showed a clear pattern of opaque and translucent bands. Opaque bands were counted and initially attributed to the age of individual fish. To determine the periodicity of band formation, a validation experiment was performed, in which individuals (n = 8) were injected with tetracycline (50 mg kg-1) and kept in the laboratory for periods of three months to one year. Results of the validation experiment showed that one opaque band is deposited per year. The age of individuals collected from January to December 1995 ranged between 0 and 15 years. Analysis of margins of monthly collected individuals suggested that opaque bands are deposited during the dry season, between September and March. Readings of tetracycline-marked otoliths corroborated these results. The length-at-age curve yielded a growth constant (K) of 0.19 y-1. The high variability of the length-at-age data indicated high variability in individual growth. Length–frequency data were also used to obtain growth parameters. The growth constant, as determined with these data was 0.6 y-1 and differed markedly from K values determined from length-at-age data. It was concluded that length-frequency data are not suitable to determine growth of this species. The present study showed that otoliths are a reliable structure for aging S. fuscus, which showed to be a relatively long-lived and slow-growing tropical reef species.

Effect of Individual Growth Variation on Stock Assessment and Management

Effect of Individual Growth Variation on Stock Assessment and Management

The growth history of anadromous white‐spotted charr in northern Japan: a comparison between river and sea life

Anadromous white‐spotted charrSalvelinus leucomaenisin a natural river in northern Japan migrated to the sea mainly at age 3+ years. Sea‐run charr grew faster than parr, particularly as first time migrants. Sea‐run charr repeated seaward migration up to three times; the observed maximum age was 5+ years old. The likelihood ratio test identified significant differences in asymptotic size and individual growth variability between parr and sea‐run charr. Sea‐run charr had an asymptotic size (L∞) twice as large as parr, while the growth parameter (K) was similar between sea‐run and parr. Growth variability was higher for parr than for sea‐run charr.

Estimating growth and mortality of juvenile winter flounder, Pseudopleuronectes americanus, with a length-based model

This study quantifies the combined effects of density and environmental factors on young-of-the-year (YOY) winter flounder (Pseudopleuronectes americanus) in Narragansett Bay, Rhode Island. We used a length-based model to estimate growth and mortality rates from June to October each year from 1988 to 1998. In this model, mortality and growth rates are decreasing functions of length and there is variability in individual growth. Maximum-likelihood methods were used to fit the model to length-frequency data collected by the Rhode Island Division of Fish and Wildlife beach-seine survey in Narragansett Bay during the study years. The monthly mortality rate was density dependent and was positively related to temperature. Growth rate was negatively related to density. There was a significant decline in YOY winter flounder abundance during the period of study. The most recent year of the study, 1998, had the lowest density, lowest mortality, low summer temperature, and high growth rate. Thus, growth and mortality during the juvenile stage do not appear to be limiting the recovery of this depleted winter flounder population.

Typology of individual growth in sea bass ( Dicentrarchus labrax)

The individual growth variability of passive integrated transponder tagged sea bass was studied using data sets from two different experiments. In experiment 1 ( n = 485), fish submitted to different photoperiod regimes were held in fourteen groups of individual weight of 88 ± 13 g (mean ± SD). In experiment 2 ( n = 748, initial weight 243 ± 30 g) fish were held in fifteen groups and had either free or restricted access to diets with three lipid levels. After adjustment for treatment and tank effects, individual growth curves were analysed using multivariate analysis (principal component analysis and clustering) and were modelled using the summary statistics technique. Different growth profiles where characterized. All of them appeared to be curvilinear. They differed in their level (initial and final weight), slope (slope, specific growth weight, gain) and especially the ratio of males, which showed sexual growth dimorphism. The fish with similar initial weight proved to have very different growth performances, regardless of the treatment effect. Within the same sex, part of the variability between the growth profiles could be explained by differences in the social interactions and in the genetic potential of growth among individuals.

Individual variation in habitat use and growth of male and female brown trout

We examined variation in growth and habitat use of individually PIT‐tagged brown trout Salmo trutta in three stream enclosures, each divided into a fine substrate, deep pool habitat and a coarse substrate, shallow habitat. Habitat use and movements of individual fish were monitored continually by placing PIT detectors between habitats. All fish were measured and weighed biweekly over a three month period. There was no significant relationship between habitat use and initial body size, nor was there a consistent relationship between habitat use and densities of benthic macroinvertebrates or abundance of drifting invertebrates in the two habitats. Most habitat changes occurred at night, with activity peaks just prior to sunrise and after sunset. Trout used pools more at night than during the day. Within any given day, diurnal and nocturnal habitat use of individual fish varied little, with variation greater at night than during the day. Partial habitat segregation by sex was observed; only males used pools extensively during daytime, whereas males and females used riffles. Growth rate was positively related to use of pools during daytime but not at night. Growth rate was also affected by enclosure, with growth rates being highest in the most downstream enclosure, which had the deepest pool (mean of 42 cm) and lowest in the most upstream enclosure, which had the shallowest pool (mean of 28 cm). A complete exchange of trout between the most upstream and downstream enclosure indicated that the enclosure effect was due to physical differences and not to individual fish differences between enclosures. The effect appears to have been caused by differences in depth as daytime use of pools was correlated with the area of the pool ≥35 cm deep, and production of trout biomass per enclosure was directly related to mean pool depth. Our results suggest that there is a relationship between habitat use and growth of individuals that is independent of body size, but that this relationship is influenced by sex of the fish and by the physical characteristics of the environment. Further, the data indicate that short‐term behavioral decisions on habitat use by brown trout have a potential effect on longer‐term individual fitness through growth rates.

Biostatistical implications of individual variability in growth in rainbow trout and Atlantic salmon

The relationship between specific growth rate and initial body weight in fish was analysed by multivariate analysis using data sets from growth studies on rainbow trout (initial weight between 10 and 30 g, n=546) and Atlantic salmon (initial weight between 10 and 20 g, n=355). We demonstrate that individuals can be pooled into different classes according to their growth characteristics. Classes with similar initial mean weight can have very different mean specific growth rate. Indeed, we found the mean specific growth rate ratio between classes with similar initial mean weight to be between 1.5 and 5.8 in rainbow trout and between 1.2 and 1.9 in Atlantic salmon. We conclude that the constitution of replicates with homogeneous initial weight is not assurance of homogeneous specific growth rate among replicates. The consequences of such variability in growth on experimental designs and power of analysis-of-variance models are discussed.