Increase In Natural Mortality Research Articles

Carbon store dynamics in the structure of mangrove in the southern Pacific of Mexico

Mangroves are recognized as the wetlands with the most significant capacity to store carbon, this ability to store carbon is due to their high productivity and the low rates of organic matter decomposition. The study area is one of the Intensive Carbon Monitoring Sites (SMIC, initials in Spanish) in Mexico. In the southern coast of the Mexican Pacific, in the La Encrucijada Biosphere Reserve (REBIEN). The SMIC has an extension of 1x1 km. In this area there are eight conglomerates. Each conglomerate contains four circular-shaped secondary units of 400 m2. The inventory includes trees ≥ 2.5 cm of DBH (Diameter at breast height). Tree species, DBH, height, canopy diameter, the basal area and tree density. Tree biomass was quantified, and the carbon store was determined using the biomass-to-carbon conversion factor of 0.48. The carbon in standing dead trees was estimated using the methodology of Kauffman et al. (2012). The environmental parameters quantified: interstitial salinity, pH, temperature and flood level. Results R. mangle is the dominant species, conglomerate 2 presented the highest tree density and conglomerate 4 the lowest. Conglomerates 8 and 4 had the highest averages for DBH, basal area, height, and crown area, in contrast, the conglomerates 1, 5, and 2, had the lowest averages. In the study area, the fall of trees was the factor that most affected the aboveground (aerial) carbon content, which increased 6.91 and 3% respectively during the second and third year of monitoring. Natural mortality increased and affected mostly young trees 2.5 to 10 cm tall. Wood extraction remained constant during the three years of study. During the three years of study, conglomerate 6 presented the highest biomass averages with 210.4 Mgha-1 and carbon stores of 101.0 MgCha-1; in contrast, conglomerate 5 registered the lowest average with 124.9 Mgha-1 and 60.0 MgCha-1 . R. mangle presents the biggest store of carbon with 60.4 MgCha-1. This mangrove system registered an increase in natural mortality during the study period, which could be the result of the massive amounts of sediment the river has been carrying after the passage of Hurricane “Stan” in 2005.

Identifying direct and indirect associations among traits by merging phylogenetic comparative methods and structural equation models

Abstract Traits underlie organismal responses to their environment and are essential to predict community responses to environmental conditions under global change. Species differ in life‐history traits, morphometrics, diet type, reproductive characteristics and habitat utilization. Trait associations are widely analysed using phylogenetic comparative methods (PCM) to account for correlations among related species. Similarly, traits are measured for some but not all species, and missing continuous traits (e.g. growth rate) can be imputed using ‘phylogenetic trait imputation’ (PTI), based on evolutionary relatedness and trait covariance. However, PTI has not been available for categorical traits, and estimating covariance among traits without ecological constraints risks inferring implausible evolutionary mechanisms. Here, we extend previous PCM and PTI methods by (1) specifying covariance among traits as a structural equation model (SEM), and (2) incorporating associations among both continuous and categorical traits. Fitting a SEM replaces the covariance among traits with a set of linear path coefficients specifying potential evolutionary mechanisms. Estimated parameters then represent regression slopes (i.e. the average change in trait Y given an exogenous change in trait X) that can be used to calculate both direct effects (X impacts Y) and indirect effects (X impacts Z and Z impacts Y). We demonstrate phylogenetic structural‐equation mixed‐trait imputation using 33 variables representing life history, reproductive, morphological, and behavioural traits for all >32,000 described fishes worldwide. SEM coefficients suggest that one degree Celsius increase in habitat is associated with an average 3.5% increase in natural mortality (including a 1.4% indirect impact that acts via temperature effects on the growth coefficient), and an average 3.0% decrease in fecundity (via indirect impacts on maximum age and length). Cross‐validation indicates that the model explains 54%–89% of variance for withheld measurements of continuous traits and has an area under the receiver‐operator‐characteristics curve of 0.86–0.99 for categorical traits. We use imputed traits to classify all fishes into life‐history types, and confirm a phylogenetic signal in three dominant life‐history strategies in fishes. PTI using phylogenetic SEMs ensures that estimated parameters are interpretable as regression slopes, such that the inferred evolutionary relationships can be compared with long‐term evolutionary and rearing experiments.

Short-term effects of exposure to particulate matter concentrations originating from desert dust on mortality in Athens, Greece

Background & Aim: Evidence on the independent associations of source-specific particulate matter (PM) exposures with health outcomes is limited. Our aim was to investigate the effects of short-term exposure to PM≤ 10μm (PM10) concentrations originating from windblown desert dust and local sources on mortality. Methods: We investigated the effect of exposure to total PM10 concentrations, as well as estimated concentrations originating from desert dust and other sources on total mortality, between the years 2010-2019. We used Poisson regression models to estimate the association between PM10 from different sources, also controlling for possible confounding by season, meteorology, day of the week and holiday effect. Results: During the 10-year study period, 35% of days were affected by desert dust (desert dust PM10> 0μg/m3). An increment of 10μg/m3 in total PM10 concentrations was associated with a 0.52% (lag 0–1, 95% confidence interval (CI): 0.21, 0.83%) increase in natural mortality. When mutually adjusting non-desert and desert dust PM10 concentrations, a 10μg/m3 increase in non-desert was associated with a 0.45% (95% CI: 0.07% to 0.84%) increase in natural mortality. On the other hand, the effect of desert PM10 (lag 0-1 days) concentrations was non-significant (0.59%; 95% CI: -0.53% to 1.72%). Conclusions: Our findings support the hypothesis that daily exposure to traffic-related particles have more toxic effects, compared to ones originating from windblown desert dust, in Athens, Greece leading to increased mortality.

Age is not just a number-Mathematical model suggests senescence affects how fish populations respond to different fishing regimes.

Senescence is often described as an age‐dependent increase in natural mortality (known as actuarial senescence) and an age‐dependent decrease in fecundity (known as reproductive senescence), and its role in nature is still poorly understood. Based on empirical estimates of reproductive and actuarial senescence, we used mathematical simulations to explore how senescence affects the population dynamics of Coregonus albula, a small, schooling salmonid fish. Using an empirically based eco‐evolutionary model, we investigated how the presence or absence of senescence affects the eco‐evolutionary dynamics of a fish population during pristine, intensive harvest, and recovery phases. Our simulation results showed that the presence or absence of senescence affected how the population responded to the selection regime. At an individual level, gillnetting caused a larger decline in asymptotic length when senescence was present, compared to the nonsenescent population, and the opposite occurred when fishing was done by trawling. This change was accompanied by evolution toward younger age at maturity. At the population level, the change in biomass and number of fish in response to different fishery size‐selection patterns depended on the presence or absence of senescence. Since most life‐history and fisheries models ignore senescence, they may be over‐estimating reproductive capacity and under‐estimating natural mortality. Our results highlight the need to understand the combined effects of life‐history characters such as senescence and fisheries selection regime to ensure the successful management of our natural resources.

Use of Angling Gear to Evaluate the Potential Benefits of Minimum Length Limits on a Highly Mobile Midwestern Blue Catfish Population

AbstractBlue Catfish Ictalurus furcatus provide a popular sport fishery in Mark Twain Lake, Missouri, but little is known about their population demographics or exploitation by anglers. Our objectives were to (1) determine population demographics of Blue Catfish, (2) determine exploitation of Blue Catfish, and (3) determine whether new harvest regulations would improve the quality (e.g., size structure and abundance) of the fishery in Mark Twain Lake. We sampled Blue Catfish by using jug lines baited with pieces of Gizzard Shad Dorosoma cepedianum, the gear used by the majority of anglers in this reservoir. Blue Catfish grew slowly, reaching a preferred size (762 mm TL) in about 12 years. Annual mortality was estimated at 24.8% using a weighted catch curve. Using reward tags, we estimated exploitation to be between 8% and 12%, and we modeled the effects of 508‐, 610‐, and 660‐mm minimum TL limits on yield and size structure of the Blue Catfish population relative to a baseline of 381 mm TL, the smallest fish usually harvested by anglers. All three length limits increased the proportion of larger fish (≥762 mm TL) in the population without reducing yield, except for slight decreases (6% and 10%) under the 610‐ and 660‐mm limits at the highest modeled natural mortality rate. We recommend the 610‐mm limit instead of the 660‐mm limit because it allows more harvest and higher yields with increases in natural mortality and still doubles the proportion of larger fish relative to the 508‐mm limit.

Association Between Short-Term Exposure to PM2.5 and PM10 and Mortality in Susceptible Subgroups: A Multisite Case-Crossover Analysis of Individual Effect Modifiers.

We performed a multisite study to evaluate demographic and clinical conditions as potential modifiers of the particulate matter (PM)-mortality association. We selected 228,619 natural deaths of elderly persons (ages ≥65 years) that occurred in 12 Italian cities during the period 2006-2010. Individual data on causes of death, age, sex, location of death, and preexisting chronic and acute conditions from the previous 5 years' hospitalizations were collected. City-specific conditional logistic regression models were applied within the case-crossover "time-stratified" framework, followed by random-effects meta-analysis. Particulate matter less than or equal to 2.5 µm in aerodynamic diameter (PM2.5) and particulate matter less than or equal to 10 µm in aerodynamic diameter (PM10) were positively associated with natural mortality (1.05% and 0.74% increases in mortality risk for increments of 10 µg/m3 and 14.4 µg/m3, respectively), with greater effects being seen among older people, those dying out-of-hospital or during the warm season, and those affected by 2 or more chronic diseases. Limited associations were found among persons with no previous hospital admissions. Diabetes (1.98%, 95% confidence interval (CI): 0.54, 3.44) and cardiac arrhythmia (1.65%, 95% CI: 0.37, 2.95) increased risk of PM2.5-related mortality, while heart conduction disorders increased risk of mortality related to both PM2.5 (4.22%, 95% CI: 0.15, 8.46) and PM10 (4.19%, 95% CI: 0.38, 8.14). Among acute conditions, recent hospital discharge for heart failure modified the PM10-mortality association. The study found increases in natural mortality from PM exposure among people with chronic morbidity; diabetes and cardiac disorders were the main susceptibility factors.

Effects of varying natural mortality and selectivity on the assessment of eastern Baltic cod (Gadus morhua Linnaeus, 1758) stock

In 2014, the International Council for the Exploration of the Sea (ICES) was unexpectedly unable to provide an analytical assessment of eastern Baltic cod stock; factors such as data issues, assessment methodology, and the ecological situation of cod were indicated as the reasons for this failure. Some evidence suggests that the natural mortality (M) of cod could increase substantially in forthcoming years and that the selectivity could change. In this paper, age-structured and stock-production assessment models were applied to simulate the dynamics of cod stock; in the models, both constant and increasing natural mortalities were permitted. In the age-structured model, the effects of selectivity related to the cod size on the cod assessment were also analysed. In addition, stock with characteristics similar to Baltic cod stock and increasing natural mortality was generated and assessed with the age-structured model using both constant and increasing M. It was shown that models with increasing natural mortality of cod in recent years perform much better than models with constant natural mortality in terms of the distribution of residuals and retrospective patterns. The models with size-dependent selectivity did not perform better than other standard assessments. The assessment of generated stock (where natural mortality was increasing) with constant natural mortality in the assessment model showed a poor distribution of residuals and strong retrospective patterns, similar to the ICES assessment with constant M. The conducted simulations strongly suggest that the main reason for the poor recent cod assessment is the increase in natural mortality, which is not considered in the assessment methodology.

Changes in natural mortality of Atlantic cod (Gadus morhua) on Eastern Georges Bank

The assessment of Eastern Georges Bank cod with the assumption of a constant natural mortality of 0.2 has over-estimated stock productivity, resulting in a severe retrospective pattern since the late 2000s. Comparing relative exploitation rate (ratio of fishery catch at age to survey abundance indices at age) with total mortality calculated from the age distribution in surveys indicated a conflict when constant natural mortality was assumed. This inconsistency implies an increase in natural mortality since the mid-1990s. In this paper, natural mortality estimated by Virtual Population Analysis (VPA) indicated that natural mortality for this stock has increased to 0.8 since the mid-1990s for ages 6+. Potential factors contributing to this elevated natural mortality, including poor fish conditions and increased losses due to seal predation were discussed.

Desert Dust Outbreaks in Southern Europe: Contribution to Daily PM₁₀ Concentrations and Short-Term Associations with Mortality and Hospital Admissions.

Background:Evidence on the association between short-term exposure to desert dust and health outcomes is controversial.Objectives:We aimed to estimate the short-term effects of particulate matter ≤ 10 μm (PM10) on mortality and hospital admissions in 13 Southern European cities, distinguishing between PM10 originating from the desert and from other sources.Methods:We identified desert dust advection days in multiple Mediterranean areas for 2001–2010 by combining modeling tools, back-trajectories, and satellite data. For each advection day, we estimated PM10 concentrations originating from desert, and computed PM10 from other sources by difference. We fitted city-specific Poisson regression models to estimate the association between PM from different sources (desert and non-desert) and daily mortality and emergency hospitalizations. Finally, we pooled city-specific results in a random-effects meta-analysis.Results:On average, 15% of days were affected by desert dust at ground level (desert PM10 > 0 μg/m3). Most episodes occurred in spring–summer, with increasing gradient of both frequency and intensity north–south and west–east of the Mediterranean basin. We found significant associations of both PM10 concentrations with mortality. Increases of 10 μg/m3 in non-desert and desert PM10 (lag 0–1 days) were associated with increases in natural mortality of 0.55% (95% CI: 0.24, 0.87%) and 0.65% (95% CI: 0.24, 1.06%), respectively. Similar associations were estimated for cardio-respiratory mortality and hospital admissions.Conclusions:PM10 originating from the desert was positively associated with mortality and hospitalizations in Southern Europe. Policy measures should aim at reducing population exposure to anthropogenic airborne particles even in areas with large contribution from desert dust advections.Citation:Stafoggia M, Zauli-Sajani S, Pey J, Samoli E, Alessandrini E, Basagaña X, Cernigliaro A, Chiusolo M, Demaria M, Díaz J, Faustini A, Katsouyanni K, Kelessis AG, Linares C, Marchesi S, Medina S, Pandolfi P, Pérez N, Querol X, Randi G, Ranzi A, Tobias A, Forastiere F, MED-PARTICLES Study Group. 2016. Desert dust outbreaks in Southern Europe: contribution to daily PM10 concentrations and short-term associations with mortality and hospital admissions. Environ Health Perspect 124:413–419; http://dx.doi.org/10.1289/ehp.1409164

Short-term effects of particulate matter on mortality during forest fires in Southern Europe: results of the MED-PARTICLES Project

BackgroundAn association between occurrence of wildfires and mortality in the exposed population has been observed in several studies with controversial results for cause-specific mortality. In the Mediterranean area, forest fires...

Extreme increases in natural mortality prevent recovery of collapsed fish populations in a Northwest Atlantic ecosystem

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 519:165-182 (2015) - DOI: https://doi.org/10.3354/meps11012 Extreme increases in natural mortality prevent recovery of collapsed fish populations in a Northwest Atlantic ecosystem Douglas P. Swain*, Hugues P. Benoît Fisheries and Oceans Canada, Gulf Fisheries Centre, 343 Avenue de l’Université, Moncton, NB, E1C 9B6, Canada *Corresponding author: doug.swain@dfo-mpo.gc.ca ABSTRACT: Improved understanding of the dynamics of populations at low abundance is needed in the face of global biodiversity loss. We examined the dynamics of depleted demersal fish populations in the southern Gulf of St. Lawrence, Canada. Twenty years ago, a number of these populations collapsed due to overexploitation. Since then, others have declined to low abundance. Despite negligible levels of fishing mortality and strong rates of production of small juvenile fish, these populations have shown no sign of recovery and some continue to decline. Lack of recovery is due to dramatic increases in the natural mortality of larger individuals in these populations. In some of these fishes, natural mortality has risen to levels typical of high-turnover forage fishes rather than long-lived demersal fishes. We hypothesize that these high levels of mortality reflect a ‘predator pit’ or predation-driven Allee effect, resulting from the severely depleted abundance of these fishes and the high and rising abundance of their marine mammal predators, in particular grey seals. Recovery of collapsed demersal fish populations does not appear to be possible under current conditions in this ecosystem, even in the absence of fishing. Our results indicate a need for more precautionary management regimes in order to avoid population collapses that are not reversible by reducing exploitation. KEY WORDS: Allee effect · Natural mortality · Population recovery · Predation · Marine fishes Full text in pdf format Supplementary material PreviousNextCite this article as: Swain DP, Benoît HP (2015) Extreme increases in natural mortality prevent recovery of collapsed fish populations in a Northwest Atlantic ecosystem. Mar Ecol Prog Ser 519:165-182. https://doi.org/10.3354/meps11012 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 519. Online publication date: January 20, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Increased natural mortality at low abundance can generate an Allee effect in a marine fish.

Negative density-dependent regulation of population dynamics promotes population growth at low abundance and is therefore vital for recovery following depletion. Inversely, any process that reduces the compensatory density-dependence of population growth can negatively affect recovery. Here, we show that increased adult mortality at low abundance can reverse compensatory population dynamics into its opposite—a demographic Allee effect. Northwest Atlantic cod (Gadus morhua) stocks collapsed dramatically in the early 1990s and have since shown little sign of recovery. Many experienced dramatic increases in natural mortality, ostensibly attributable in some populations to increased predation by seals. Our findings show that increased natural mortality of a magnitude observed for overfished cod stocks has been more than sufficient to fundamentally alter the dynamics of density-dependent population regulation. The demographic Allee effect generated by these changes can slow down or even impede the recovery of depleted populations even in the absence of fishing.

Short-term effects of particulate air pollution on mortality in 25 cities in Italy and the role of the individual susceptibility–results of the EPIAIR2 project

Background. A surveillance system (EPIAIR) is ongoing since 2001 in Italy to monitor short-term effects of air pollution. The relationship between PM10 and mortality has been already studied for the period 2001-2005, identifying specific subgroups of population most sensitive to air pollution. Aim. To estimate the association between exposure to PM10 and cause-specific mortality, and to evaluate individual characteristics as potential effect modifiers of this association in an extended period until 2010. Methods. Data on 422,723 natural deaths, age 35+, were collected in 25 Italian cities within the period 2003-2010. Information about cause of death, socio-demographic characteristics and chronic conditions from previous hospitalizations was gathered. Cardiac, cerebrovascular and respiratory diseases were investigated. Exposure data were collected from background monitoring stations. A case-crossover analysis was conducted within each city, followed by random-effects meta-analysis. Results. We found a 0.45% (95% confidence interval [CI]:0.07-0.83) increase in natural mortality for a 10µg/m3 increase of PM10 (lag 0-1). The effect for cardiac mortality lasted several days with an increase of 0.9% (95%CI:0.12-1.68) at lag 0-5. Higher effects of PM10 on natural mortality were found among people aged 85+ years (0.95%, 95%CI:0.39-1.51, p-value of the relative effect modification (p-REM)=0.080), and during summer season (1.51%,95%CI:0.18-2.85, p-REM=0.085). Subjects with previous hospitalizations for diabetes were more susceptible to the effect of PM10 (1.02%, 95%CI:0.30-1.74, p-REM=0.089); previous hospitalizations for anemia, diseases of pulmonary circulation, and heart failure were also suggestive of an effect modification. Conclusions. A strong evidence of a short-term effect of PM10 on natural and cardiac mortality up to the fifth day has been found. Old age, warm season, and diabetes turned out to be significant effect modifiers.

Contrasting decadal trends in mortality between large and small individuals in skate populations in Atlantic Canada

Mature thorny (Amblyraja radiata), winter (Leucoraja ocellata), and smooth (Malacoraja senta) skates have declined to very low abundance in the southern Gulf of St. Lawrence (SGSL) and on the eastern Scotian Shelf (ESS). We used stage-structured state-space models to examine decadal patterns in mortality rates in these skates. Mortality at early life stages (embryos in egg cases, hatchlings, and (or) small juveniles) appeared to decrease between the 1970s and the 2000s. In contrast, estimated mortality rates increased for larger individuals over this period. Although potentially confounded in models with effects of any changes in juvenile growth, the estimated increases in mortality could not instead be attributed solely to changes in growth. Increases in the mortality of large individuals appeared to reflect increases in natural mortality, possibly due to predation by grey seals. Increases in natural mortality were not evident for skates on the neighbouring western Scotian Shelf, where grey seal abundance has remained lower. Even in the absence of fishing, recovery of skates is unlikely under current ecosystem conditions in the SGSL and on the ESS.

Effect modification of the association between short-term meteorological factors and mortality by urban heat islands in Hong Kong.

BackgroundPrior studies from around the world have indicated that very high temperatures tend to increase summertime mortality. However possible effect modification by urban micro heat islands has only been examined by a few studies in North America and Europe. This study examined whether daily mortality in micro heat island areas of Hong Kong was more sensitive to short term changes in meteorological conditions than in other areas.MethodAn urban heat island index (UHII) was calculated for each of Hong Kong’s 248 geographical tertiary planning units (TPU). Daily counts of all natural deaths among Hong Kong residents were stratified according to whether the place of residence of the decedent was in a TPU with high (above the median) or low UHII. Poisson Generalized Additive Models (GAMs) were used to estimate the association between meteorological variables and mortality while adjusting for trend, seasonality, pollutants and flu epidemics. Analyses were restricted to the hot season (June-September).ResultsMean temperatures (lags 0–4) above 29°C and low mean wind speeds (lags 0–4) were significantly associated with higher daily mortality and these associations were stronger in areas with high UHII. A 1°C rise above 29°C was associated with a 4.1% (95% confidence interval (CI): 0.7%, 7.6%) increase in natural mortality in areas with high UHII but only a 0.7% (95% CI: −2.4%, 3.9%) increase in low UHII areas. Lower mean wind speeds (5th percentile vs. 95th percentile) were associated with a 5.7% (95% CI: 2.7, 8.9) mortality increase in high UHII areas vs. a −0.3% (95% CI: −3.2%, 2.6%) change in low UHII areas.ConclusionThe results suggest that urban micro heat islands exacerbate the negative health consequences of high temperatures and low wind speeds. Urban planning measures designed to mitigate heat island effects may lessen the health effects of unfavorable summertime meteorological conditions.

Seal–cod interactions on the Eastern Scotian Shelf: Reconsideration of modelling assumptions

The cod stock on the Eastern Scotian Shelf (the 4VsW management area) collapsed in the early 1990s, and has experienced high natural mortality since then, while the abundance of grey seals foraging in the area has doubled about every seven years since the 1960s. The causes of the high cod natural mortality are not well understood, but seals are not considered to have played a significant role. This study takes a fresh look at the impact of seals on 4VsW cod abundance. Abundance trends of the Sable, Eastern Shore and Gulf seal herds which forage on the Scotian Shelf are estimated to 2020. The Sable herd is projected to stabilize at about 350,000 individuals. If their exponential growth continues, the Eastern Shore and Gulf herds could in aggregate reach about 200,000 individuals. However, density dependent processes are likely to slow population growth of these two herds sometime in the coming decade. Total annual food consumption of the three herds is estimated. In 2010, in excess of 550,000 t of fish are consumed annually by the Sable and Eastern herd, with the Gulf herd consuming about 138,000 t. The literature on seal diets is summarized and three scenarios of type II predator–prey functional response are defined. In an ADAPT analysis of the 4VsW cod stock, seals are treated as an additional fishing gear sector under two scenarios: respectively a “flat-top” and “domed” partial recruitment vectors. Model results infer that seals have contributed to increases in natural mortality since the late 1980s, and have contributed to the lack of recovery of the stock since 1993. However, predictions by the functional models are not consistent with estimate of recent increases in abundance of cod in trawl surveys. Present levels of grey seal abundance have not occurred on the Scotian Shelf since at least the 1800s.

Evolutionary and ecological feedbacks of the survival cost of reproduction.

Arguably the most fundamental of trade-offs in life-history evolution is the increase in natural mortality resulting from sexual maturity and reproduction. Despite its central importance, this increase in mortality, a survival cost, garners surprisingly little attention in fish and fisheries modeling studies. We undertook an exploratory analysis to evaluate the consequences of this omission for life-history projections. To this end, we developed a simulation approach that integrates quantitative genetics into the ecological dynamics of a fish population and parameterized the model for Atlantic cod (Gadus morhua, L.). When compared to simulations in which the mortality of immature and mature individuals is equal, the inclusion of a survival cost results in larger asymptotic body size, older age at maturity, and larger size at maturity. We also find that measures of population productivity (spawning stock biomass, recruits-per-spawner) are overestimated if the survival cost is excluded. This sensitivity of key metrics of population growth rate and reproductive capacity to the magnitude of the survival cost of reproduction underscores the need to explicitly account for this trade-off in projections of fish population responses to natural and anthropogenic environmental change, including fisheries.

Spatial structure induced by marine reserves shapes population responses to catastrophes in mathematical models

Catastrophic events such as oil spills, hypoxia, disease, and major predation events occur in marine ecosystems and affect fish populations. Previous evaluations of the performance of spatial management alternatives have not considered catastrophic events. We investigate the effects of local and global catastrophic events on populations managed with and without no-take marine reserves and with fishing mortality rates that are optimized accounting for reserves. A spatial population dynamics model is used to explore effects of large, catastrophic natural mortality events. The effects of the spatial spread, magnitude, probability of catastrophe, and persistence of a catastrophic event through time are explored. Catastrophic events affecting large spatial areas and those that persist through time have the greatest effects on population dynamics because they affect natural mortality nonlinearly, whereas the probability and magnitude of catastrophic events result in only linear increases in natural mortality. The probability of falling below 10% or 20% of unfished abundance was greatest when a no-take marine reserve was implemented with no additional fishing regulations and least when a no-take marine reserve was implemented in addition to the maintenance of optimal fishing mortality in fished areas. In the absence of implementation error, maintaining abundance across space using restrictions on fishing mortality rates, regardless of the existence of a no-take marine reserve, decreased the probability of falling below 10% or 20% of unfished abundance.

Life-history evolution and elevated natural mortality in a population of Atlantic cod (Gadus morhua).

Fisheries-induced evolution has been hypothesized to delay the recovery of collapsed fish stocks through effects on their productivity. The cod stock in the southern Gulf of St. Lawrence (SGSL) collapsed in the early 1990s and has shown no recovery since then, due mainly to high natural mortality of adult cod. Age and size at maturation of SGSL cod decreased sharply over time in cohorts produced in the 1950s and 1960s, likely reflecting an evolutionary response to intensified fishing, and have remained low since then, despite severe reductions in fishing mortality over the past 15 years. A predicted consequence of early maturation is increased natural mortality due to higher costs to reproduction. Early maturation may be a cause of increases in natural mortality of SGSL cod in the 1970s but does not appear to be related to the much larger increases since then. Instead, the current high natural mortality of SGSL cod appears to be primarily a cause, rather than a consequence, of the continued early maturation in this population, now replacing fishing mortality as the agent of selection favouring early maturity. This striking example of the failure to reverse fisheries-induced evolution by relaxing fishing pressure emphasizes the need for management strategies that minimize the chances of harvest-induced genetic change.

Modelling fishing-induced adaptations and consequences for natural mortality

When trade-offs involving predation and mortality are perturbed by human activities, behaviour and life histories are expected to change, with consequences for natural mortality rates. We present a general life history model for fish in which three common relationships link natural mortality to life history traits and behaviour. First, survival increases with body size. Second, survival declines with growth rate due to risks involved with resource acquisition and allocation. Third, fish that invest heavily in reproduction suffer from decreased survival due to costly reproductive behaviour or morphology that makes escapes from predators less successful. The model predicts increased natural mortality rate as an adaptive response to harvesting. This extends previous models that have shown that harvesting may cause smaller body size, higher growth rates, and higher investment in reproduction. The predicted increase in natural mortality is roughly half the fishing mortality over a wide range of harvest levels and parameter combinations such that fishing two fish kills three after evolutionary adaptations have taken place.