Size-selective Mortality Research Articles

Size, growth, and origin-dependent mortality of juvenile Chinook salmon Oncorhynchus tshawytscha during early ocean residence

Selective mortality during early life history stages can have significant population- level consequences, yet critical periods when selective mortality occurs, the strength of selection, and under what environmental conditions can be difficult to identify. Here, we used otolith microstructure and chemistry to examine the factors potentially linked to selective mortality of juvenile fall-run Chinook salmon Oncorhynchus tshawytscha from California's Central Valley during early ocean residence. Back-calculated size and growth rates of the population were com- pared across 3 sample periods: as juveniles exited the San Francisco Bay estuary (estuary-exit), after their first month at sea (summer-ocean) and 5 mo after ocean entry (fall-ocean). We com- pared mortality dynamics during years of exceptional recruitment (addition of individuals to har- vestable population; 2000 and 2001) to a year of poor recruitment (2005). Otoliths from 2005 were also analyzed for sulfur isotopes to discern hatchery from naturally spawned stock. Significant size and growth-rate selective mortality were detected during the first month at sea in the low recruit- ment year of 2005, but not in 2000 and 2001. Individuals that were larger and growing faster dur- ing freshwater and estuarine rearing were more likely to survive to summer and fall in the low recruitment year. There was a slight, but insignificant, increase in the proportion of hatchery to naturally spawned individuals from estuary-exit to fall-ocean, suggesting that fish from neither origin were overwhelmingly favored. Our results suggest that Central Valley Chinook salmon can be subject to significant size and growth-rate selective mortality resulting in low adult abundance, and this mortality appears independent of origin.

The effect of variable winter severity on size-dependent overwinter mortality caused by acute thermal stress in juvenile red drum (Sciaenops ocellatus)

Abstract Mortality during winter can impact the population dynamics of fish at temperate latitudes. The red drum (Sciaenops ocellatus) supports valuable coastal fisheries throughout its range in the southeastern United States. At the northern edge of its distribution, severe winters may cause considerable overwinter loss and size-selective mortality among juveniles. We conducted a series of laboratory experiments to quantify overwinter survivorship of age 0 fish. To determine thermal tolerance, fish were exposed to various minima (1, 3, or 5°C) for up to 14 d. The effect of winter severity on survivorship was then evaluated by exposing fish to simulated cold-front events of varying frequency and duration. Body size was incorporated as a factor into each set of experiments. Age 0 red drum were intolerant of even brief exposure to temperatures ≤3°C and experienced mortality after prolonged exposure to 5°C. Higher frequency of simulated cold-front events impacted survivorship more than longer-event duration, and recovery time between events improved survivorship. Size-dependent mortality was only evident for fish exposed to mild and moderate winter severity conditions, with larger fish surviving longer. For juvenile red drum, severe winters may cause high mortality independent of body size, whereas size-dependent year-class restructuring may occur during milder winters.

Relative contribution of spring- and summer-spawned bluefish cohorts to the adult population: effects of size-selective winter mortality, overwinter growth, and sampling bias

Length distributions of juvenile bluefish (Pomatomus saltatrix) are bimodal, consisting of spring- and summer-spawned fish. Research during the 1990s from the northeastern United States suggested that the summer cohort contributes little to the adult population and that overwinter mortality may limit their survival. We examined length distributions of juvenile bluefish from before and after winter and found that the mean length of the summer cohort increased during winter. Based on a winter-growth experiment and temperatures from the two winters examined, changes in mean length were due to size-selective mortality for one year class and growth for the other. Despite evidence for winter mortality, summer-spawned bluefish were commonly caught at age 1. We reexamined the relative contribution of each cohort to the adult population using archived scales from North Carolina fisheries. Cohort origin of adults was determined by back-calculating length at age 1. One-third of adults consisted of summer-spawned fish, contrasting with previous research from the northeast. The differences in relative cohort abundance between the northeastern and southeastern United States arise from sized-based migration in age-1 and age-2 bluefish (the dominant age groups sampled in these studies) and the size-selective gear and sampling season in which they were collected.

Growth and size-dependent loss of newly settled bivalves in two distant regions of the Wadden Sea

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 472:141-154 (2013) - DOI: https://doi.org/10.3354/meps10011 Growth and size-dependent loss of newly settled bivalves in two distant regions of the Wadden Sea Henrike Andresen1,*, Ine Dorresteijn1, Jaap van der Meer1,2 1Royal Netherlands Institute for Sea Research (NIOZ), Den Burg (Texel) 1790 AB, The Netherlands 2Department of Theoretical Biology, Free University, Amsterdam 1081 HV, The Netherlands *Email: henrikea@gmx.de ABSTRACT: Individual growth and the timing of life history events determine the course of individual body sizes. Hence, these factors can influence size-dependent species interactions, such as predation. In the Wadden Sea, predation by crustaceans plays an important role in the survival of pre-recruit bivalves. To investigate growth rates and size-dependent loss in 2 distant regions of the Wadden Sea, in situ marking with the fluorochrome calcein was combined with cage experiments. Through marking, we obtained, for the first time, size-increment data for the fragile newly settled juveniles of the species Macoma balthica and Cerastoderma edule in the field. Growth and size-selective mortality were analysed separately, whereas hitherto both have usually been inferred from size distributions at this life stage. Fluorescent marking also revealed that growth rates were very variable among individuals. Growth was temporarily higher at Texel than at Sylt in both species. In combination with the exclosure experiments and the sampling of predators and migrating bivalves, selective loss of smaller individuals was detected in both regions and was related to predation pressure. Climate influences on phenology have potentially strong effects on species interactions, altering survival and thus recruitment and population dynamics. KEY WORDS: Individual marking · Predator-exclusion experiments · Benthic invertebrates · Soft‑bottom intertidal · Early life history · Post-settlement mortality · Fluorescent dye · Bivalvia Full text in pdf format PreviousNextCite this article as: Andresen H, Dorresteijn I, van der Meer J (2013) Growth and size-dependent loss of newly settled bivalves in two distant regions of the Wadden Sea. Mar Ecol Prog Ser 472:141-154. https://doi.org/10.3354/meps10011 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 472. Online publication date: January 09, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Growth reaction norms of domesticated, wild and hybrid Atlantic salmon families in response to differing social and physical environments

BackgroundDirectional selection for growth has resulted in the 9-10th generation of domesticated Atlantic salmon Salmo salar L. outgrowing wild salmon by a ratio of approximately 3:1 when reared under standard hatchery conditions. In the wild however, growth of domesticated and wild salmon is more similar, and seems to differ at the most by a ratio of 1.25:1. Comparative studies of quantitative traits in farmed and wild salmon are often performed by the use of common-garden experiments where salmon of all origins are reared together to avoid origin-specific environmental differences. As social interaction may influence growth, the large observed difference in growth between wild and domesticated salmon in the hatchery may not be entirely genetically based, but inflated by inter-strain competition. This study had two primary aims: (i) investigate the effect of social interaction and inter-strain competition in common-garden experiments, by comparing the relative growth of farmed, hybrid and wild salmon when reared together and separately; (ii) investigate the competitive balance between wild and farmed salmon by comparing their norm of reaction for survival and growth along an environmental gradient ranging from standard hatchery conditions to a semi-natural environment with restricted feed.ResultsThe main results of this study, which are based upon the analysis of more than 6000 juvenile salmon, can be summarised as; (i) there was no difference in relative growth between wild and farmed salmon when reared together and separately; (ii) the relative difference in body weight at termination between wild and farmed salmon decreased as mortality increased along the environmental gradient approaching natural conditions.ConclusionsThis study demonstrates that potential social interactions between wild and farmed salmon when reared communally are not likely to cause an overestimation of the genetic growth differences between them. Therefore, common-garden experiments represent a valid methodological approach to investigate genetic differences between wild and farmed salmon. As growth of surviving salmon of all origins became more similar as mortality increased along the environmental gradient approaching natural conditions, a hypothesis is presented suggesting that size-selective mortality is a possible factor reducing growth differences between these groups in the wild.

Hiding or dead? A computationally efficient model of selective fisheries mortality

100 years after Rosa Lee (1912) showed that higher mortality on faster growing fish can alter length-at-age distributions in fish populations, we present a computationally-efficient and parsimonious method for modeling size-selective mortality within a commonly-used assessment model, Stock Synthesis. Stock Synthesis allows the normal distribution of length-at-age to be partitioned into three or five overlapping platoons with slow, medium, or fast growth trajectories. The platoons are tracked separately in the model, and experience different degrees of size-selective fishing pressure and mortality, but are assumed to be unobservable except through changes in the length distribution. Simulations are used to explore this phenomenon in conjunction with dome-shaped selectivity, an alternative explanation for observing fewer than expected large fish in sampled data, but with very different implications for population productivity. For data simulated both with and without platoons, misspecification of the assumptions about growth are found to bias model results, with selectivity often incorrectly identified as the cause of fewer observations of larger fish. Trends in dome-shaped selectivity were explored as a potential diagnostic of model misspecification.

Promoting effects on reproduction increase population vulnerability of Daphnia magna

Environmental risk assessment of chemicals is based on single species tests at the individual level with single compounds. However, the protection goal is the sustainability of a population, which faces several natural stressors and mixtures of chemicals in the environment. Therefore, experiments were undertaken to quantify the combined effects of chemicals with different modes of action on Daphnia magna populations. Populations continuously exposed to dispersogen A and at abundance equilibrium were treated with a 2-d pulse of p353-nonylphenol. In previous studies, dispersogen A was shown to act as a natural info-chemical, promoting the reproduction of daphnids (higher offspring quantity) coupled with reduced offspring fitness, whereas nonylphenol in pulsed-exposure caused size-selective mortality. Dispersogen A caused accelerated population growth to maximum abundance, shifted the population structure towards smaller individuals, and increased the population sensitivity to nonylphenol. The authors showed that a positive effect observed at the individual level can be transposed to a negative effect when monitored at the population level. So far, positive effects are not addressed in environmental risk assessment, and even in higher-tier testing, population structure is not quantified. Both factors indicate a potential mismatch between protection aim and risk assessment practice.

Costs of living for juvenile Chinook salmon (Oncorhynchus tshawytscha) in an increasingly warming and invaded world

Rapid environmental change in freshwater ecosystems has created a need to understand the interactive effects of multiple stressors, with temperature and invasive predators identified as key threats to imperiled fish species. We tested the separate and interactive effects of water temperature and predation by non-native smallmouth bass ( Micropterus dolomieu ) on the lethal (mortality) and sublethal (behavior, physiology, and growth) effects for juvenile Chinook salmon ( Oncorhynchus tshawytscha ) in seminatural stream channel experiments. Over 48 h trials, there was no difference in direct predation with warmer temperatures, but significant interactive effects on sublethal responses of juvenile salmon. Warmer temperatures resulted in significantly stronger and more variable antipredator responses (surface shoaling and swimming activity), while physiological indicators (plasma glucose, plasma cortisol) suggested suppression of physiological mechanisms in response to the combined stressors. These patterns corresponded with additive negative growth in predation, temperature, and combined treatments. Our results suggest that chronic increases in temperature may not increase direct predation over short periods, but can result in significant sublethal costs with negative implications for long-term development, disease resistance, and subsequent size-selective mortality of Pacific salmon.

Stage-specific biomass overcompensation by juveniles in response to increased adult mortality in a wild fish population

Recently developed theoretical models of stage-structured consumer-resource systems have shown that stage-specific biomass overcompensation can arise in response to increased mortality rates. We parameterized a stage-structured population model to simulate the effects of increased adult mortality caused by a pathogen outbreak in the perch (Perca fluviatilis) population of Windermere (UK) in 1976. The model predicts biomass overcompensation by juveniles in response to increased adult mortality due to a shift in food-dependent growth and reproduction rates. Considering cannibalism between life stages in the model reinforces this compensatory response due to the release from predation on juveniles at high mortality rates. These model predictions are matched by our analysis of a 60-year time series of scientific monitoring of Windermere perch, which shows that the pathogen outbreak induced a strong decrease in adult biomass and a corresponding increase in juvenile biomass. Age-specific adult fecundity and size at age were higher after than before the disease outbreak, suggesting that the pathogen-induced mortality released adult perch from competition, thereby increasing somatic and reproductive growth. Higher juvenile survival after the pathogen outbreak due to a release from cannibalism likely contributed to the observed biomass overcompensation. Our findings have general implications for predicting population- and community-level responses to increased size-selective mortality caused by exploitation or disease outbreaks.

Growth- and size-selective mortality in pelagic ­larvae of a common reef fish

Larvae of most aquatic species experience high mortality in the plankton and where traits are naturally variable, mortality can be selective. We tested the hypothesis that reef fish lar- vae with faster growth rates and larger size-at-age will be more likely to survive the pelagic larval period. We examined the otolith microstructure of larval bluehead wrasse Thalassoma bifasciatum collected across the Straits of Florida to obtain daily growth rates and relative size-at-age. Using a cross-sectional approach, we compared these traits between 2 larval age groups (young larvae: 18 to 26 d old, survivors: 27 to 36 d old) to determine whether mean traits varied with age. In contrast to expectations, survivors had slower early growth and were smaller-at-age than the young lar- vae, suggesting that faster growing, larger individuals were selectively removed from the popula- tion over time. To examine the role of water masses, we estimated maximum upstream larval posi- tions. Differential current speeds across the Straits of Florida resulted in substantial overlap in back-estimated upstream locations of larvae at younger ages, suggesting that differences in early growth environments were unlikely the cause of the observed patterns. Instead, the most parsimo- nious explanation is the selective loss of fast-growing, large-at-age larvae in the plankton. Shifts in the direction of selective pressures across stages may contribute to the maintenance of high plasticity in the early life history traits of this species.

Implications of a warming eastern Bering Sea for Bristol Bay sockeye salmon

AbstractFarley, E. V., Starovoytov, A., Naydenko, S., Heintz, R., Trudel, M., Guthrie, C., Eisner, L., Guyon, J. R. 2011. Implications of a warming eastern Bering Sea for Bristol Bay sockeye salmon. – ICES Journal of Marine Science, 68: 1138–1146. Overwinter survival of Pacific salmon (Oncorhynchus sp.) is believed to be a function of size and energetic status they gain during their first summer at sea. We test this notion for Bristol Bay sockeye salmon (O. nerka), utilizing data from large-scale fisheries and oceanographic surveys conducted during mid-August to September 2002–2008 and from February to March 2009. The new data presented in this paper demonstrate size-selective mortality for Bristol Bay sockeye salmon between autumn and their first winter at sea. Differences in the seasonal energetic signatures for lipid and protein suggest that these fish are not starving, but instead the larger fish caught during winter apparently are utilizing energy stores to minimize predation. Energetic status of juvenile sockeye salmon was also strongly related to marine survival indices and years with lower energetic status apparently are a function of density-dependent processes associated with high abundance of juvenile sockeye salmon. Based on new information regarding eastern Bering Sea ecosystem productivity under a climate-warming scenario, we hypothesize that sustained increases in spring and summer sea temperatures may negatively affect energetic status of juvenile sockeye salmon, potentially resulting in increased overwinter mortality.

Harvest-induced maturation evolution under different life-history trade-offs and harvesting regimes

The potential of harvesting to induce adaptive changes in exploited populations is now increasingly recognized. While early studies predicted that elevated mortalities among larger individuals select for reduced maturation size, recent theoretical studies have shown conditions under which other, more complex evolutionary responses to size-selective mortality are expected. These new predictions are based on the assumption that, owing to the trade-off between growth and reproduction, early maturation implies reduced growth. Here we extend these findings by analyzing a model of a harvested size-structured population in continuous time, and by systematically exploring maturation evolution under all three traditionally acknowledged costs of early maturation: reduced fecundity, reduced growth, and/or increased natural mortality. We further extend this analysis to the two main types of harvest selectivity, with an individual's chance of getting harvested depending on its size and/or maturity stage. Surprisingly, we find that harvesting mature individuals not only favors late maturation when the costs of early maturation are low, but promotes early maturation when the costs of early maturation are high. To our knowledge, this study therefore is the first to show that harvesting mature individuals can induce early maturation.

Maintenance of phenotypic variation: repeatability, heritability and size-dependent processes in a wild brook trout population.

Phenotypic variation in body size can result from within-cohort variation in birth dates, among-individual growth variation and size-selective processes. We explore the relative effects of these processes on the maintenance of wide observed body size variation in stream-dwelling brook trout (Salvelinus fontinalis). Based on the analyses of multiple recaptures of individual fish, it appears that size distributions are largely determined by the maintenance of early size variation. We found no evidence for size-dependent compensatory growth (which would reduce size variation) and found no indication that size-dependent survival substantially influenced body size distributions. Depensatory growth (faster growth by larger individuals) reinforced early size variation, but was relatively strong only during the first sampling interval (age-0, fall). Maternal decisions on the timing and location of spawning could have a major influence on early, and as our results suggest, later (>age-0) size distributions. If this is the case, our estimates of heritability of body size (body length = 0.25) will be dominated by processes that generate and maintain early size differences. As a result, evolutionary responses to environmental change that are mediated by body size may be largely expressed via changes in the timing and location of reproduction.

Age and growth of non-native monkey gobyNeogobius fluviatilis(Teleostei, Gobiidae) in the River Ipeľ, Slovakia

Amongst the non-native Ponto-Caspian gobies that have invaded the Middle Danube is the monkey goby Neogobius fluviatilis (Pallas, 1814). Despite a strong specialization towards sandy substratum, revealed in a previous study, monkey goby is considered an invasive species and therefore should demonstrate great plasticity in its biological traits. The main aim of the present study was to evaluate the age and growth of a non-native population of monkey goby from the River Ipeľ in Slovakia. Six age groups (0 - V) were observed in the sample from the River Ipeľ (n = 165), with males demonstrating faster growth than females, which was reflected both in the smoothing cubic B-spline model and in the annual increment. The cubic B-spline model indicated a reverse Lee's phenomenon, especially in males, which is likely to have arisen from the size selective mortality. Growth of invading monkey goby from the River Ipeľ was slower than that reported for the species' native range. Together with other growth patterns, this may suggest (among other possible explanations) a greater allocation of resources to reproduction than to somatic growth, which is a life-history pattern typical for newly established populations of successful invaders.

Timing and selectivity of mortality in reared Atlantic cod revealed by otolith analysis

In juvenile fish production, large samples of known-aged material can be sampled at pre-determined time intervals from the same population. This enables an accurate determination of size-selective mortality by means of repeated samplings of fish and comparison of otolith size-at-age based on samples from different dates. An example is provided from an experiment with larval and juvenile Atlantic cod, Gadus morhua, where groups fed smaller sized enriched rotifers did not reveal any size-selective mortality during weaning to formulated feed, while those groups that were fed larger live natural zooplankton lost a significant fraction of the smaller-sized individuals during the same time period. This was contrary to the overall mortality which was higher among the rotifer fed groups during weaning. Part of this difference may be attributed to size differences between the groups, where larger zooplankton fed larvae were more prone to engage in cannibalistic behaviour.

Behavioural mediation of the costs and benefits of fast growth in a marine fish

Trade-offs between the costs and benefits of growth are thought to be mediated by behaviour, whereby rapid growth is associated with greater predation mortality because of increased foraging effort. We tested this hypothesis by collecting young Pomacentrus amboinensis using light traps and settling them onto patch reefs where their behaviour and survivorship were monitored for 24 h. One month later, individuals of the same cohort were collected from shallow reefs and released onto patch reefs where their behaviour and survivorship were monitored for 6 days. At settlement young fish suffered high (60% in 24 h) mortality that preferentially removed larger, faster-growing individuals. However, we could find no evidence that foraging behaviour contributed to this selective mortality. In contrast, 1 month later the same cohort underwent negative size-selective mortality where the smallest, slower-growing fish were preferentially removed by predators. Larger fish spent more time foraging, were more aggressive, swam greater distances and chased more fish than smaller individuals. Thus, consistent individual differences in behaviour contributed to patterns of mortality, but in a way that involved no apparent trade-off with growth. For P. amboinensis, consistent variation in growth may be maintained by spatial and temporal differences in the selective regime within the reef environment.

Tree Mortality following Prescribed Fire and a Storm Surge Event in Slash Pine (Pinus elliottiivar.densa) Forests in the Florida Keys, USA

In fire-dependent forests, managers are interested in predicting the consequences of prescribed burning on postfire tree mortality. We examined the effects of prescribed fire on tree mortality in Florida Keys pine forests, using a factorial design with understory type, season, and year of burn as factors. We also used logistic regression to model the effects of burn season, fire severity, and tree dimensions on individual tree mortality. Despite limited statistical power due to problems in carrying out the full suite of planned experimental burns, associations with tree and fire variables were observed. Post-fire pine tree mortality was negatively correlated with tree size and positively correlated with char height and percent crown scorch. Unlike post-fire mortality, tree mortality associated with storm surge from Hurricane Wilma was greater in the large size classes. Due to their influence on population structure and fuel dynamics, the size-selective mortality patterns following fire and storm surge have practical importance for using fire as a management tool in Florida Keys pinelands in the future, particularly when the threats to their continued existence from tropical storms and sea level rise are expected to increase.

What can size distributions within cohorts tell us about ecological processes in fish larvae?

size, prey concentrations, and temperature are shown to influence growth rates. We present a method based on cumulative size distributions (Csds) for visualizing variability of sizes within cohorts over time. Analysis of Csds revealed sizeselective mortality and variations among populations in size- and temperature-dependent growth throughout ontogeny. We found that cod larvae consistently exhibit higher growth rates than herring larvae. While cod larvae may have an advantage over herring larvae when food availability is high, herring were more able to survive at low food concentrations than cod. Cod and herring seem to represent two growth strategies: cod larvae are relatively small at hatching and a high growth rate appears to be a prerequisite for success, whereas herring larvae are initially large, but grow more slowly.

Mitigating fisheries-induced evolution in lacustrine brook charr (Salvelinus fontinalis) in southern Quebec, Canada.

Size-selective mortality caused by fishing can impose strong selection on harvested fish populations, causing evolution in important life-history traits. Understanding and predicting harvest-induced evolutionary change can help maintain sustainable fisheries. We investigate the evolutionary sustainability of alternative management regimes for lacustrine brook charr (Salvelinus fontinalis) fisheries in southern Canada and aim to optimize these regimes with respect to the competing objectives of maximizing mean annual yield and minimizing evolutionary change in maturation schedules. Using a stochastic simulation model of brook charr populations consuming a dynamic resource, we investigate how harvesting affects brook charr maturation schedules. We show that when approximately 5% to 15% of the brook charr biomass is harvested, yields are high, and harvest-induced evolutionary changes remain small. Intensive harvesting (at approximately >15% of brook charr biomass) results in high average yields and little evolutionary change only when harvesting is restricted to brook charr larger than the size at 50% maturation probability at the age of 2 years. Otherwise, intensive harvesting lowers average yield and causes evolutionary change in the maturation schedule of brook charr. Our results indicate that intermediate harvesting efforts offer an acceptable compromise between avoiding harvest-induced evolutionary change and securing high average yields.

Detecting age-structured effects in growth performance of coral reef fish juveniles

The growth performance of coral reef fish juveniles collected in different habitats is often used as a proxy for habitat quality for juveniles. However, back-calculated growth trajectories of juve- niles may be age-structured, for instance, because of potential differences in initial offspring size and/or quality or size-selective mortality. A novel approach is proposed to isolate growth performance of coral reef fish juveniles from potential age-based factors. Juveniles of Chromis viridis (Pomacentri- dae), Lethrinus genivittatus (Lethrinidae) and Siganus fuscescens (Siganidae) were collected from waters around inshore and offshore islets in a coral reef lagoon. Individual growth trajectories were back-calculated from otolith increments and compared with repeated-measures generalised linear models (RM-GLMs). Settlement marks in otoliths were used to differentiate larval and juvenile growth trajectories for each individual. For the 3 species, fish from around the offshore islet presented signifi- cantly larger size-at-age during their larval stage than those from from around the inshore islet. Juve- niles of L. genivittatus and S. fuscescens from around the offshore islet remained larger than inshore juveniles at the same age, while growth curves of C. viridis from the 2 islets crossed at settlement so that inshore fish were larger as juveniles than offshore individuals. RM-GLMs revealed that the growth trajectory was significantly age-structured for C. viridis only. These results suggest that post-settlement age may be used as a covariate in comparative analyses of larval growth in order to isolate growth per- formance from potential age-based factors.