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1.
Density‐dependent processes such as growth, survival, reproduction and movement are compensatory if their rates change in response to variation in population density (or numbers) such that they result in a slowed population growth rate at high densities and promote a numerical increase of the population at low densities. Compensatory density dependence is important to fisheries management because it operates to offset the losses of individuals. While the concept of compensation is straightforward, it remains one of the most controversial issues in population dynamics. The difficulties arise when going from general concepts to specific populations. Compensation is usually quantified using some combination of spawner–recruit analysis, long‐term field monitoring or manipulative studies, and computer modelling. Problems arise because there are limitations to each of these approaches, and these limitations generally originate from the high uncertainty associated with field measurements. We offer a hierarchical approach to predicting and understanding compensation that ranges from the very general, using basic life‐history theory, to the highly site‐specific, using detailed population models. We analyse a spawner–recruit database to test the predictions about compensation and compensatory reserve that derive from a three‐endpoint life‐history framework designed for fish. We then summarise field examples of density dependence in specific processes. Selected long‐term field monitoring studies, manipulative studies and computer modelling examples are then highlighted that illustrate how density‐dependent processes led to compensatory responses at the population level. Some theoretical and empirical advances that offer hope for progress in the future on the compensation issue are discussed. We advocate an approach to compensation that involves process‐level understanding of the underlying mechanisms, life‐history theory, careful analysis of field data, and matrix and individual‐based modelling. There will always be debate if the quantification of compensation does not include some degree of understanding of the underlying mechanisms.  相似文献   

2.
The effects of biotic (density‐dependent) and environmental (flow and temperature) factors on the apparent survival, mean length and size variation of a low‐density brown trout population in the juvenile stage, that is, from their first summer (0+) to the end of the second year (1+), were determined. Apparent survival was negatively related to the age class density during the three periods (first summer, first winter and second summer). A significant interaction between the mean flow and 0+ density highlighted a gradient towards strong density dependence acting on fish loss (i.e., mortality or migration) with decreasing summer flow. Conversely, no density dependence was reported at higher mean flows. The mean length was determined by density‐dependent and density‐independent (temperature and flow) factors throughout the study period. The negative relationship between fish length and intracohort density was highly significant during the three periods. The yearling (1+) density was negatively related to 0+ fish length measured after the first summer, suggesting intercohort effects. A positive effect of temperature on fish length was observed. Mean length after the summer seasons (0+ and 1+ fish) was also positively related to mean flow. Fish size variation around the mean measured with the coefficient of variation (CV) increased with increasing 0+ densities, both at the end of the first summer and the first winter. Results suggested that density‐dependent and density‐independent factors acted jointly on apparent survival and growth with a predominance of biotic processes. We discussed the potential implications of density‐dependent regulations on growth and survival for population resilience after catastrophic events.  相似文献   

3.
Vincenzi S, Crivelli AJ, Jesensek D, De Leo GA. Detection of density‐dependent growth at two spatial scales in marble trout (Salmo marmoratus) populations.
Ecology of Freshwater Fish 2010: 19: 338–347. © 2010 John Wiley & Sons A/S Abstract – Density‐dependent body growth has often been observed in freshwater salmonid populations. Several studies suggest this compensatory pattern as a potential mechanism of population regulation. The choice of the spatial scale is important for the detection of density‐dependent growth, as study areas need to be of the appropriate size to capture the density of conspecifics actually experienced by individuals over the preceding growth period. Here, we used four marble trout (Salmo marmoratus) populations (Gatsnik, Gorska, Huda and Zakojska) living in Slovenian stream to study the relationships between early density of marble trout and subsequent body growth. As streams are divided in sectors delimited by natural barriers that prevent or strongly limit movement of individuals, we tested the relationship between early density and body size through the lifetime at two spatial scales, that is, sector level (for Gatsnik and Zakojska) and whole stream level (the four populations were pooled). Sector length in Gatsnik and Zakojska ranged from 113 to 516 m. At both sector and whole stream level, temporal data were pooled. Growth declined significantly with increasing density both at the sector and whole stream levels, and the density‐dependent relationship was described by negative power curves. However, at the sector level the density‐dependent pattern was stronger in Gatsnik, a stream in which fish could move across sectors, than in Zakojska, where upstream movement across sectors is prevented by waterfalls.  相似文献   

4.
Understanding of density‐dependent effects is key to achieving sustainable management of self‐regulating biological resources such as fish stocks. Traditionally, density‐dependent effects on population abundance in fish have been considered to occur from hatching to recruitment, based on the paradigm of proportionality between spawning stock biomass and total egg production. Here, we demonstrate how the existence of intraspecific and interspecific density dependence in egg production changes the current understanding of density‐dependent processes in the life history of fish, by disentangling density‐dependent effects on egg production and survival from egg to recruitment, using sardine (Sardinops melanostictus, Clupeidae) and anchovy (Engraulis japonicus, Engraulidae) as model species. For sardine, strong intraspecific density‐dependent effects occurred in egg production, but no density‐dependent effects occurred or if any they were weak enough to be masked by environmental factors from hatching to recruitment. In contrast, for anchovy, interspecific density‐dependent effects occurred in egg production. In the survival after hatching, anchovy experienced stronger intraspecific density‐dependent effects than currently recognized. This analysis could overturn the current understanding of density‐dependent effects in the life history, highlighting contrasts between the effects on individual quality and population abundance and between the model species. We propose to reconsider the basis of fisheries management and recruitment studies based on the revised understanding of density‐dependent effects in the life history of the respective species.  相似文献   

5.
The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying density‐ and size‐dependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical state‐space modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and sub‐arctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that size‐dependent mortality and density‐dependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.  相似文献   

6.
Habitat quality often varies substantially across space and time, producing a shifting mosaic of growth and mortality trade‐offs across watersheds. Traditional studies of juvenile habitat use have emphasised the evolution of single optimal strategies that maximise recruitment to adulthood and eventual fitness. However, linking the distribution of individual behaviours that contribute to recruitment at the population level has been elusive, particularly for highly fecund aquatic organisms. We examined juvenile habitat use within a population of sockeye salmon (Oncorhynchus nerka) that spawn in a watershed consisting of two interconnected lakes and a marine lagoon. Otolith microchemical analysis revealed that the productive headwater lake accounted for about half of juvenile growth for those individuals surviving to spawn in a single river in the upper watershed. However, 47% of adults had achieved more than half of their juvenile growth in the downstream less productive lake, and 3% of individuals migrated to the estuarine environment during their first summer and returned to freshwater to overwinter before migrating back to sea. These results describe a diversity of viable habitat‐use strategies by juvenile sockeye salmon that may buffer the population against poor conditions in any single rearing environment, reduce density‐dependent mortality and have implications for the designation of critical habitat for conservation purposes. A network of accessible alternative habitats providing trade‐offs in growth and survival may be important for long‐term viability of populations.  相似文献   

7.
Abstract White crappie, Pomoxis annularis Rafinesque, provides important sport fisheries across North America but sometimes exhibits slow growth and fails to reach sizes acceptable to anglers. Three dissimilar white crappie populations were studied to assess whether differences in growth were associated with size and age at maturity and subsequent larval production. Similar to other species, female white crappie matured at younger ages and smaller sizes and contained larger ovaries in the population with the slowest growth; this population also had the lowest annual mortality. Larval production (number m?3) in this population was intermediate to that of the other populations. Attempts to improve growth of white crappie in the slow‐growing population will probably be hindered by these individuals’ greater reproductive investment, which reduces the amount of energy available for somatic growth.  相似文献   

8.
Following the 1989 MV Exxon Valdez oil spill (EVOS) and subsequent herring population collapse in Alaska's Prince William Sound (PWS), the Pacific herring (Clupea pallasii) fishery was closed. In the 25 yr since EVOS, herring and several herring‐dependent marine bird species have failed to reach pre‐spill population levels. One hypothesis is that intense predation pressure may be inhibiting herring recovery. To inform herring modeling efforts, this study estimated marine bird consumption of juvenile and adult herring in PWS for 10 winters over an 18‐yr period (1989–90 through 2006–2007). Total estimated herring consumption by wintering marine birds averaged 2409 ± 950 t, indicating that avian consumption represents a substantial and inter‐annually variable source of herring mortality. Common murre (Uria aalge) consumed the greatest portion (16–80%) of herring in all years among marine bird species. Juvenile herring biomass consumed annually by common murre was greater than murre consumption of adult herring biomass. Time lag analyses showed that marine bird consumption of adult herring is negatively correlated with the amount of herring spawn observed in subsequent years, but such effects were not observed more than 2 yr. Our models indicate that during years in which herring recruitment is low or bird populations are particularly large, marine birds can consume up to 10% of the annual adult herring biomass. Our results highlight the importance of herring to wintering PWS birds. We propose that future management of herring stocks seeks to reduce negative impacts on marine birds that prey on herring.  相似文献   

9.
Population characteristics of gizzard shad, Dorosoma cepedianum (Lesueur), an ecologically important fish species in Midwestern and southern USA impoundments, vary widely among water bodies. Impoundment productivity is known to influence population variables (e.g., recruitment, growth, mortality, size and age composition), but the role of impoundment morphometry (e.g., surface area, depth) is unknown. Differences in population characteristics of gizzard shad were compared between large and small impoundments across an impoundment–productivity gradient (indexed by total phosphorus [TP] concentration). Populations shifted from those comprised mostly of low abundances (indexed by catch per unit effort [CPUE], fish/hr of electric fishing) of long‐lived, large adults to those comprised of high abundances of short‐lived, small adults as TP increased. However, the trends in population variables across the TP gradient were less apparent in small impoundments than in large impoundments owing to considerable variation at a given TP level. Further analysis revealed that CPUE of gizzard shad was lowest in the smallest impoundments (<33 ha surface area) regardless of TP concentrations, probably because of more frequent winter kills. Overall, population characteristics were influenced by both density‐dependent effects and impoundment characteristics. Characteristics of gizzard shad were highly variable among populations, which have important consequences for ecosystem structure and function.  相似文献   

10.
Exotic species present a great threat to native fish conservation; however, eradicating exotics is expensive and often impractical. Mechanical removal can be ineffective for eradication, but nonetheless may increase management effectiveness by identifying portions of a watershed that are strong sources of exotics. We used mechanical removal to understand processes driving exotic brown trout (Salmo trutta) populations in the Logan River, Utah. Our goals were to: (i) evaluate the demographic response of brown trout to mechanical removal, (ii) identify sources of brown trout recruitment at a watershed scale and (iii) evaluate whether mechanical removal can reduce brown trout densities. We removed brown trout from 2 km of the Logan River (4174 fish), and 5.6 km of Right Hand Fork (RHF, 15,245 fish), a low‐elevation tributary, using single‐pass electrofishing. We compared fish abundance and size distributions prior to, and after 2 years of mechanical removal. In the Logan River, immigration to the removal reach and high natural variability in fish abundances limited the response to mechanical removal. In contrast, mechanical removal in RHF resulted in a strong recruitment pulse, shifting the size distribution towards smaller fish. These results suggest that, before removal, density‐dependent mortality or emigration of juvenile fish stabilised adult populations and may have provided a source of juveniles to the main stem. Overall, in sites demonstrating strong density‐dependent population regulation, or near sources of exotics, short‐term mechanical removal has limited effects on brown trout populations but may help identify factors governing populations and inform large‐scale management of exotic species.  相似文献   

11.
We address the problem of optimal size‐selective exploitation in an age‐structured fish population model by systematically examining how density and size dependency in growth, mortality and fecundity affect optimal harvesting patterns when judged against a set of fisheries objectives. The study offers five key insights. First, while minimum‐length limits often maximize the biomass yield, exploitation using harvest slots (i.e. regulations that protect both immature and very large individuals) can generate within 95% of maximum yield; harvest slots also generally maximize the number of fish that are harvested. Second, density dependence in growth and size‐dependent mortality predict more liberal optimal size limits than those derived under assumptions of no density and size dependence. Third, strong density dependence in growth maximizes the catch of trophy fish only when modest harvest is introduced; the same holds for numbers harvested, when the stock–recruitment function follows the Ricker type. Fourth, the inclusion of size‐dependent maternal effects on fecundity or egg viability has only limited effects on optimal size limits, unless the increase in fecundity with mass (“hyperallometry”) is very large. However, large hyperallometry in fecundity shifts the optimal size limit for biomass yield from the traditional minimum‐length limit to a harvest slot. Fifth, harvest slots generally provide the best compromises among multiple objectives. We conclude that harvest slots, or more generally dome‐shaped selectivity to harvest, can outperform the standard minimum‐length selectivity. The exact configuration of optimal size limits crucially depends on objectives, local fishing pressure, the stock–recruitment function, and the density and size dependency of growth, mortality and fecundity.  相似文献   

12.
Giant clam populations have been over‐exploited throughout their range over the past decades for their meat and shells. Tridacna maxima, commonly known as the ‘small giant clam’, has remained relatively untargeted by fishers in areas where larger species occur (e.g. Tridacna squamosa), and high densities of the species are still observed on some isolated and enclosed reefs of the Central Pacific. However, it is unclear whether reported discrepancies in densities worldwide reflect differences in fishing pressure only or a combination of differences in exploitation levels and environmental forcing. We reviewed T. maxima surveys throughout its range to (i) identify patterns of density at global scale, site scale (e.g. island) and intrasite scale; (ii) discuss the influence of sampling method on density estimates; and (iii) identify the primary drivers of giant clam density along gradients of human pressure and natural forcing. We found 59 studies that reported density estimates for 172 sites across 26 countries in the Indo‐Pacific and Red Sea. At intrasite scale, densities were strongly dependent on sampling protocols and surveyed habitats. At site scale, we found close links between T. maxima density and human population per reef area, suggesting that isolated reefs where exploitation only recently started may be more vulnerable to stock collapse in the future. Density patterns were also found to vary significantly depending on reef type (e.g. atoll, island, continental coastline). We discuss how natural processes and fishing pressure may control population dynamics and densities among sites, and make recommendations for future research.  相似文献   

13.
Understanding the complex variation in patterns of density‐dependent individual growth and survival across populations is critical to adaptive fisheries management, but the extent to which this variation is caused by biological or methodological differences is unclear. Consequently, we conducted a correlational meta‐analysis of published literature to investigate the relative importance of methodological and biological predictors on the shape and strength of density‐dependent individual growth and survival in salmonids. We obtained 160 effect sizes from 75 studies of 12 species conducted between 1977 and 2019 that differed in experimental approach (sensu Ecological Monographs, 54, 187–211; 65 laboratory experiments, 60 observational field studies, and 35 field experiments). The experimental approach was the strongest factor influencing the strength of density dependence across studies: density‐dependent survival was stronger than growth in field observational studies, whereas laboratory experiments detected stronger density‐dependent growth than survival. The difference between density‐dependent growth and survival was minimal in field experiments, and between lotic and lentic habitats. The shape of density dependence (logarithmic, linear, exponential or density‐independent) could be predicted with 66.7% accuracy based solely on the experimental approach and the density gradient (highest/lowest*100) of the study. Overall, the strength and shape of density dependence were primarily influenced by methodological predictors, while biological factors (predator presence, food abundance, and species) had predictable but modest effects. For both empirical studies and adaptive fisheries management, we recommend using field experiments with a density gradient of at least 470% to detect the proper shape of the density‐dependent response, or accounting for potential biases if observational or laboratory studies are conducted.  相似文献   

14.
A paradigm of proportionality between spawning stock biomass (SSB) and total egg production (TEP) has been largely untested at multidecadal scales mainly because of difficulty in estimating annual TEP. Recently, this paradigm was directly tested for sardine (Sardinops melanostictus) and anchovy (Engraulis japonicus) at a multidecadal scale to reveal that SSB–TEP proportionality was partially distorted by intraspecific (sardine) and interspecific (anchovy) density dependence in total egg production per spawner individual (TEPPS) or unit weight (TEPPSW). In the present study, we demonstrate intraspecific density dependence in TEPPS/TEPPSW for chub mackerel (Scomber japonicus) in the Kuroshio Current system, using a proxy for TEPPS/TEPPSW, calculated from snapshot abundance data based on fishery‐independent egg surveys in combination with fishery‐dependent stock assessment data, at a multidecadal scale (38 years). TEPPS/TEPPSW exponentially declined with SSB, indicating a strong intraspecific density dependence in TEPPS/TEPPSW in chub mackerel. The observed phenomenon for chub mackerel was similar to that for sardine. Hence, intraspecific density dependence in TEPPS/TEPPSW may be a phenomenon that is generally applicable for species with a high maximum biomass and large population fluctuations. Lastly, we recommend the application of a TEP‐based framework to studies on recruitment mechanisms of fish.  相似文献   

15.
Differences in zooplankton populations in relation to climate have been explored extensively on the southeastern Bering Sea shelf, specifically in relation to recruitment of the commercially important species walleye pollock (Gadus chalcogrammus). We addressed two research questions in this study: (i) Does the relative abundance of individual copepod species life history stages differ across warm and cold periods and (ii) Do estimated secondary production rates for copepods differ across warm and cold periods? For most copepod species, warmer conditions resulted in increased abundances in May, the opposite was observed in colder conditions. Abundances of smaller‐sized copepod species did not differ significantly between the warm and cold periods, whereas abundances of larger‐sized Calanus spp. increased during the cold period during July and September. Estimated secondary production rates in the warm period were highest in May for smaller‐sized copepods; production in the cold period was dominated by the larger‐sized Calanus spp. in July and September. We hypothesize that these observed patterns are a function of temperature‐driven changes in phenology combined with shifts in size‐based trophic relationships with primary producers. Based on this hypothesis, we present a conceptual model that builds upon the Oscillating Control Hypothesis to explain how variability in copepod production links to pollock variability. Specifically, fluctuations in spring sea‐ice drive regime‐dependent copepod production over the southeastern Bering Sea, but greatest impacts to upper trophic levels are driven by cascading July/September differences in copepod production.  相似文献   

16.
Understanding ecological effects of altered stream flows is an essential objective. In a comparative field study of juvenile steelhead trout (Oncorhynchus mykiss) that compared natural‐ and reduced‐flow sites, we used mark–recapture modelling and information theory to quantify spatially and temporally explicit patterns of density, specific growth rate, survival and outmigration; and test predictions for biotic and abiotic drivers. Densities were lower in water withdrawal treatments, resulting in lower intraspecific competition and, higher specific growth rate and survival. We observed yearly differences in density and intraspecific competition, with a negative relationship between density and specific growth rate over a wide range of densities, but reductions in survival only at the highest densities. Moreover, individual variability within sites was important. At high density (sites and years), survival related negatively to body size. In contrast, when overall density was lower, specific growth rate was negatively related to body size. Lastly, individuals were more likely to outmigrate when they had larger body size, lower survival or reared in habitats with reduced flows, and these patterns appeared mediated by the intensity of intraspecific competition. Our results underscore the harsh bioenergetic conditions induced by higher temperatures and densities during summer baseflow (relative to other seasons), particularly for larger fish demanding more resources, and suggest a density‐dependent mechanism for why this period is important for regulating salmonid populations. We found that a complex combination of natural (e.g., density) and anthropogenic (e.g., withdrawal) factors affected juvenile salmon populations and life history expression in the face of altered flows.  相似文献   

17.
A paradigm of fisheries science holds that spawning stock biomass (SSB) is directly proportional to total egg production (TEP) of fish stocks. This “SSB–TEP proportionality” paradigm has been a basic premise underlying the spawner–recruitment models for fisheries management and numerous studies on recruitment mechanisms of fish. Studies on maternal effects on reproductive potential of a stock have progressed during the last few decades, leading to doubt concerning the paradigm. Nonetheless, a direct test of the paradigm at multidecadal scales has been difficult because of data limitations in the stock assessment systems worldwide. Here, we tested the paradigm for marine fish based on a novel combination of two independent 38‐year time series: fishery‐dependent stock assessment data and fishery‐independent egg survey data. Through this approach, we show that the SSB–TEP proportionality is distorted by density dependence in total egg production per spawner individual (TEPPS) or spawner unit weight (TEPPSW) at a multidecadal scale. The TEPPS/TEPPSW exponentially declined with biomass and thus was density‐dependent for Japanese sardine, a small pelagic species exhibiting a high level of population fluctuation, in the western North Pacific. By contrast, the TEPPS/TEPPSW was sardine‐density‐dependent for Japanese anchovy, another small pelagic species exhibiting a moderate level of population fluctuation well‐known for being out of phase with sardine. Our analysis revealed intraspecific (sardine) and interspecific (anchovy) density dependence in TEPPS/TEPPSW, which was previously unaccounted for in spawner–recruitment relationships. Such density‐dependent effects at the time of spawning should be considered in fisheries management and studies on recruitment mechanisms.  相似文献   

18.
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19.
The progression of external signs of Ichthyophonus infection in Pacific herring Clupea pallasii Valenciennes was highly variable and asynchronous after intraperitoneal injection with pure parasite preparations; however, external signs generally persisted through the end of the study (429 days post‐exposure). Observed signs included papules, erosions and ulcers. The prevalence of external signs plateaued 35 days post‐exposure and persisted in 73–79% of exposed individuals through the end of the first experiment (147 days post‐exposure). Among a second group of infected herring, external signs completely resolved in only 10% of the fish after 429 days. The onset of mortality preceded the appearance of external signs. Histological examination of infected skin and skeletal muscle tissues indicated an apparent affinity of the parasite for host red muscle. Host responses consisted primarily of granulomatous inflammation, fibrosis and necrosis in the skeletal muscle and other tissues. The persistence and asynchrony of external signs and host response indicated that they were neither a precursor to host mortality nor did they provide reliable metrics for hindcasting on the date of exposure. However, the long‐term persistence of clinical signs in Pacific herring may be useful in ascertaining the population‐level impacts of ichthyophoniasis in regularly observed populations.  相似文献   

20.
Mass mortality events are ubiquitous in nature and can be caused by, for example, diseases, extreme weather and human perturbations such as contamination. Despite being prevalent and rising globally, how mass mortality in early life causes population-level effects such as reduced total population biomass, is not fully explored. In particular for fish, mass mortality affecting early life may be dampened by compensatory density-dependent processes. However, due to large variations in year-class strength, potentially caused by density-independent variability in survival, the impact at the population level may be high in certain years. We quantify population-level impacts at two levels of mass mortality (50% and 99% additional mortality) during early life across 40 fish species using age-structured population dynamics models. The findings from these species-specific models are further supported by an analysis of detailed stock-specific models for three of the species. We find that population impacts are highly variable between years and species. Short-lived species that exhibit a low degree of compensatory density dependence and high interannual variation in survival experience the strongest impacts at the population level. These quantitative and general relationships allow predicting the range of potential impacts of mass mortality events on species based on their life history. This is critical considering that the frequency and severity of mass mortality events are increasing worldwide.  相似文献   

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