Figure 1 - uploaded by Jan Ohlberger
Content may be subject to copyright.
Source publication
A persistent debate in population ecology concerns the relative importance of environmental stochasticity and density dependence in determining variability in adult year-class strength, which contributes to future reproduction as well as potential yield in exploited populations. Apart from the strength of the processes, the timing of density regula...
Contexts in source publication
Context 1
... in the Barents Sea. By late summer the 0-group fish (pelagic juveniles) are distributed over large areas of the Barents Sea, where they settle to the bottom in late autumn (demersal juveniles). The fish remain in the Barents Sea until they reach maturity and start their annual migrations to the spawning grounds, typically at 6-8 years old [14]. Fig. 1 shows the general life cycle and ecology of NEA cod and the observational data used in this study. Over the past 65 years, total stock biomass of NEA cod has fluctuated between 1 and 4 million tons, and annual fishery landings of 0.2-1.3 million tons have been reported ...
Context 2
... for the posterior distributions. In addition to these formal diagnostics we used informal criteria to ensure model convergence: chains were initialized with different starting values to address problems related to meta-stability in parameter space, trace and density plots were used to detect improper mixing and asymptotic behavior of chains ( Fig. S1 in File S1). Furthermore, autocorrelation in the chains was consistently below 0.2 for all parameters (Fig. S2 in File S1), and the posterior samples generally showed low cross-correlations, with the notable exception of the age-effects in fishing mortality (Fig. S3 in File ...
Context 3
... age-specific fishing mortality shows a smooth hump-shaped relationship, with the lowest average mortality at age 4 and the highest average mortality at age 8-9 (Fig. S6 in File S1). The year- effect in fishing mortality, which represents gradual changes in effort over time, shows a temporal trend that reflects the historic changes in fishing pressure (Fig. S7 in File S1). Fishing mortality slightly increased until the late 1980s, then dropped significantly within a few years, returned to previous levels in the ...
Context 4
... fishing mortality shows a smooth hump-shaped relationship, with the lowest average mortality at age 4 and the highest average mortality at age 8-9 (Fig. S6 in File S1). The year- effect in fishing mortality, which represents gradual changes in effort over time, shows a temporal trend that reflects the historic changes in fishing pressure (Fig. S7 in File S1). Fishing mortality slightly increased until the late 1980s, then dropped significantly within a few years, returned to previous levels in the late 1990s, and has been declining continuously ever since. The resulting temporal dynamics in spawning biomass as predicted by the model (Fig. 5A) are in close correspondence with estimates ...
Context 5
... observation errors show a U-shaped relationship, with the lowest observation error on 6-year-old cod ( Fig. S9 in File S1) and the highest observation error on the 0- group. The catchability estimates for the survey suggest that surveyability of age-classes 1-2 increased after 1993, and that surveyability continuously declines from age 3 to 9 (Fig. S10 in File S1). All parameter estimates are presented in Table S1 in File ...
Context 6
... S1: Posterior medians and 95% credible intervals for all model parameters. Figure S1: Posterior distributions of all model parameters. Figure S2: Temporal autocorrelation of all model parameters. ...
Citations
... To evaluate the efficiency of fishing reduction after a MM event in the early stages of life, we employ a 'what if' scenario approach where we simulate MM events in the past followed by different types of fishing reductions. For this purpose, we used a life cycle model that was previously developed by Ohlberger et al. (2014) and was fitted to data from both scientific surveys and commercial landings of NEA cod. Specifically, the model was fitted using survey indices of cod eggs, larvae, 0-group fish, and age classes 1-9, as well as commercial landings of age classes 4-12, covering a total period of 54 years . ...
... To consider the thermal conditions in the Barents Sea, temperature records from the Kola meridian transect were used. A more complete description of the model is provided in Appendix S1 (see also Ohlberger et al., 2014 for details). The present study uses a modelling approach, as such no animal ethical approval was required. ...
... Within the range of observed temperatures during the period covered by the model, there is a positive relationship between temperature and larval survival (Ohlberger et al., 2014). We can hypothesize that with higher temperatures, the recovery after a catastrophic event will be faster. ...
Mass mortality (MM) events affecting early life stages of fish can have strong and long‐term consequences for population abundance and demography as well as the economic activity supported by exploited stocks. Adaptive fishery management may help mitigate economic impacts and ensure sustainable resource use following a MM event.
Using a state‐space life cycle model, we simulated ‘what‐if’ scenarios of MM on Northeast Arctic cod (Gadus morhua) eggs and larvae. We compared the expected catches, total stock biomass (TSB), and interannual variability in catches over a period of 10 years after the simulated disturbance. We further evaluated a range of management mitigation strategies, namely reductions in fishing mortality of varying duration (1–10 years) and intensity (no fishing reduction to full ban).
A large range of reductions in fishing led to an increase in expected catches over 10 years compared to no reductions, especially when applied immediately after the perturbation and when the cod population was characterized by a high mean age and high TSB. Severe fishing reductions can increase catches substantially but are associated with high interannual variability. Fishing reductions of moderate intensity applied between 1 and 4 years would allow to increase catches with only a slight increase in interannual variability.
Our findings demonstrate the potential benefits of an adaptive approach to fisheries management and highlight that mitigation actions may ensure the sustainable exploitation of fish stocks in the wake of unexpected disturbances.
Synthesis and application. Mass mortality events during early life stages of fish can potentially have substantial and long‐term effects on the population. Mitigation is more efficient when the affected population has a diverse age structure and when the mitigation strategy is applied immediately after the perturbation. Severe reduction in fishing mortality is an efficient measure to increase the expected average catch but is associated with high interannual variability. Fishing reduction of moderate intensity applied during 1–4 years after the event also increases the expected average catch with only slightly higher interannual variability in catches.
... Climateor fisheries-induced spatial shifts in recruitment may, therefore, have consequences for population dynamics. Furthermore, variations in survival in the climate change scenario are on the same order of magnitude as the interannual variations that have been estimated for juvenile survival of NEA cod (Ohlberger et al., 2014). ...
Climate change and harvesting result in temporal and spatial changes and variability in spawning, and thus in offspring ambient drift conditions. As a result, variable survival of offspring and thereby in recruitment are expected. This is especially true for species with long reproduction migration as is the case for some Atlantic cod stocks. We utilize biophysical model simulations to analyze survival from spawning until age 1 resulting from different scenarios of spatial and temporal changes in spawning. We find that survival is 1.5–2 times higher when spawning is shifted southwards as compared to northerly shifts. In general, survival is more sensitive to shifts in spawning location than in spawning time. Early spawning is only favourable if spawning is concurrently shifted farther north. A future spawning scenario with a northward shift in spawning grounds beyond what has been observed historically suggests reduced offspring survival and increased sensitivity to the timing of spawning.
... Demographic information, for example data collected by mark-recapture studies, is essential in order to assess extinction risk in small or declining populations (Bonebrake et al., 2010). In this context, there is a history of debate surrounding the contribution of multiple environmental and density-dependent processes which act on population vital rates such as survival, recruitment and fecundity (Andrewartha & Birch, 1954;Leirs et al., 1997;Nowicki et al., 2009;Ohlberger, Rogers & Stenseth, 2014). However, ecological theory generally recognises population density dependence as an integral process that often has a role in regulating population abundance (Henderson & Magurran, 2014). ...
The impact of factors such as density dependence, food availability and weather are known to be important for predicting population change in a wide range of species. However, a challenge in ecology is understanding the contributory and interactive role of these drivers on populations. This is necessary to design effective conservation and management strategies. Using data from long‐term studies of five hazel dormouse Muscardinus avellanarius populations in Europe, we tested the relationship between population density and weather and their impact on demographic rates. We used an integrated population modelling approach, estimating age‐specific overwinter survival, annual population growth and fecundity rates. We found strong negative effects of population density, precipitation and winter temperature on population growth rates. This suggests that warmer and wetter weather negatively affects dormouse survival for both adults and juveniles, but we found subtle differences in these effects between age classes. We also identified an interaction between weather measures and population density on age‐specific survival, possibly as a result of weather impacts during hibernation. Although we found low winter temperature was positively associated with population growth, we found evidence consistent with density dependence. We discuss our results in the context of woodland habitat conservation management.
... Density-dependent juvenile mortality has been reported in insects (Bradshaw & Holzapfel 1986), bivalves (Andresen et al. 2014), a large number of fish species (Myers & Cadigan 1993;Kimmerer et al. 2000;Holbrook & Schmitt 2002;Craig et al. 2007;Minto et al. 2008;Hazlerigg et al. 2012;Ohlberger et al. 2014), birds (Elliott GP 1996;Armstrong et al. 2002;Lok et al. 2013;Fay et al. 2015;Saether et al. 2016), and mammals (McCullough 1979;Choquenot 1991;Clutton-Brock et al. 1991;Gaillard et al. 1997;Singer et al. 1997;Mduma et al. 1999). In insect populations, the emphasis of density regulation lies in the larval stage where biotic interactions, including processes causing delayed density-dependence, predominate (Haukioja et al. 1988;Ruohomäki 1994;Bylund 1995;Tammaru et al. 1996). ...
... We found a positive but relatively weak link between NEA cod productivity and temperature, as reported previously (Ohlberger et al., 2014;Ottersen & Loeng, 2000;Stige et al., 2010). Higher temperatures likely increase growth and survival during early life stages. ...
Long‐term changes in the age and size structure of animal populations are well documented, yet their impacts on population productivity are poorly understood. Fishery exploitation can be a major driver of changes in population age–size structure because fisheries significantly increase mortality and often selectively remove larger and older fish. Climate change is another potential driver of shifts in the demographic structure of fish populations. Northeast Arctic (NEA) cod is the largest population of Atlantic cod (Gadus morhua) and one of the world's most important commercial fish stocks. This population has experienced considerable changes in population age–size structure over the past century, largely in response to fishing. In this study, we investigate whether changes in spawner age structure have affected population productivity in NEA cod, measured as recruits per spawning stock biomass, over the past 75 years. We find evidence that shifts in age structure toward younger spawners negatively affect population productivity, implying higher recruitment success when the spawning stock is composed of older individuals. The positive effect of an older spawning stock is likely linked to maternal effects and higher reproductive output of larger females. Our results indicate a threefold difference in productivity between the youngest and oldest spawning stock that has been observed since the 1950s. Further, our results suggest a positive effect of environmental temperature and a negative effect of intraspecific cannibalism by older juveniles on population productivity, which partly masked the effect of spawner age structure unless accounted for in the model. Collectively, these findings emphasize the importance of population age structure for the productivity of fish populations and suggest that harvest‐induced demographic changes can have negative feedbacks for fisheries that lead to a younger spawning stock. Incorporating demographic data into harvest strategies could thus facilitate sustainable fishery management.
... It is critical to understand the causes of commercially exploited fish population distribution [1,2], which could change due to density-independent and density-dependent processes [3,4]. Density-dependent changes are related to changes in predation intensity [5,6], food availability [7,8], or variation in habitat temperature [9,10]. ...
... However, most models of exploited population dynamics assume that density-dependent regulation only affects early life processes [13]. For example, Ohlberger et al. [4] found that the juvenile life stage of Atlantic cod (Gadus morhua) is compensatory, and that adult cod cannibalism affects the survival of age-0 cod. Andersen et al. [14] showed that habitat size determines density-dependent regulation and can occur early in large habitats. ...
... Age-0 and -1 fish could seek refuge in neritic habitats, which would allow them to increase survival by decreasing the risk of predation [31,60], particularly the predation of hake by their ichthyophagous congeners [32]. The negative relationship between the abundance of juveniles at ages 0 and 1 and older adults (7+), shows the potential pressure cannibalism could indirectly exert on younger ages [4,30,31]. These relationships suggest that removing adults by fishing (overexploitation, illegal capture, and discarding) [24,26] could expand the spatial distribution of juveniles in the stock, such as the higher occurrence of juveniles after 2003. ...
The abundance of juvenile fish changes due to endogenous processes, and determining the functional relationships among conspecifics is essential for fisheries’ management. The hake (Merluccius gayi) is an overexploited demersal fish widely distributed in Chile, from 23°39′ S to 47°00′ S in shallow and deep water over the continental shelf and shelf break. We studied the spatiotemporal distribution of hake juveniles (from ages 0 and 1), emphasizing endogenous relationships among juveniles and adults. The abundance per age data were obtained from bottom trawl cruises carried out in the austral winter between 1997 and 2018. Generalized additive models showed a similar spatiotemporal pattern for ages between 0 and 1, and negative effects of adult hake aged seven and older on the abundance of the young generation. Regarding the changes in juvenile abundance, the residual deviance of selected models explained 75.9% (for the age 0) and 95.3% (for the age 1) of the null deviance, revealing a significant increase in juvenile abundance from 2002 to 2007 and subsequent abundance stability at higher levels. Furthermore, the expansion in the abundance of juveniles after 2002 was favored by the low abundance of older adult hake, most which are able to cannibalize young hake. Our results highlight the importance of endogenous factors in the spatial distribution of Chilean hake juveniles to identify nurseries or juvenile areas free of potential cannibal adults.
... abundance can be proportional to recruitment and density dependence does not always ensure persistence, recruitment variation alone cannot regulate a population (Armsworth, 2002;Chesson, 1998). When in the life cycle those density-dependent regulatory processes occur, and the mechanisms involved, will also influence population responses to extreme events driven by climate change (Jaatinen et al., 2021;Ohlberger et al., 2014;Okamoto et al., 2016). Moreover, failure to simultaneously consider densityindependent stochastic variation and density-dependent feedback will limit understanding of the effects of environmental change on populations (Gamelon et al., 2017). ...
... According to theory, for population regulation to occur, negative density feedback must be temporal (i.e. within each population) rather than among populations, and affect local populations (Turchin, 1995 be determined by when in the life cycle regulatory processes occur relative to stochastic influences and will drive population variability (Ohlberger et al., 2014). That negative feedback occurred in the longest life stage and was associated with the key resource for individual growth in our study, would work to maximise the compensatory capacity of the populations. ...
In organisms with complex life cycles, the various stages occupy different habitats creating demographically open populations. The dynamics of these populations will depend on the occurrence and timing of stochastic influences relative to demographic density dependence, but understanding of these fundamentals, especially in the face of climate warming, has been hampered by the difficulty of empirical studies.
Using a logically feasible organism, we conducted a replicated density‐perturbation experiment to manipulate late‐instar larvae of nine populations of a stream caddisfly, Zelandopsyche ingens, and measured the resulting abundance over 2 years covering the complete life cycle of one cohort to evaluate influences on dynamics.
Negative density feedback occurred in the larval stage, and was sufficiently strong to counteract variation in abundance due to manipulation of larval density, adult caddis dispersal in the terrestrial environment as well as downstream drift of newly hatched and older larvae in the current. This supports theory indicating regulation of open populations must involve density dependence in local populations sufficient to offset variability associated with dispersal, especially during recruitment, and pinpoints the occurrence to late in the larval life cycle and driven by food resource abundance.
There were large variations in adult, egg mass and early instar abundance that were not related to abundance in the previous stage, or the manipulation, pointing to large stochastic influences. Thus, the results also highlight the complementary nature of stochastic and deterministic influences on open populations. Such density dependence will enhance population persistence in situations where variable dispersal and transitioning between life stages frequently creates mismatches between abundance and the local availability of resources, such as might become more common with climate warming.
... In this study, I analyze the interactions between haddock and cod as a two-species study system since the shift of the interaction type through age is identified ( Figure 1). First, I quantify the variation of the strength of interactions between the two species by developing a state-space model (Aeberhard et al., 2018;Cadigan & Marshall, 2016;Millar & Meyer, 2000;Ohlberger et al., 2014), which includes interactions among life history stages of two harvested interacting species that accounts for observation errors. The model also allows analyzing the sensitivity of the two species to variation in the strength of the species interactions due to variation in harvest and climate. ...
... This kind of model enables separate ecological process variation (i.e., stochastic process) and observation errors (de Valpine & Hastings, 2002). This study is based on state-space models that describes individual species life cycle models (Millar & Meyer, 2000;Ohlberger et al., 2014). For a trade-off in model complexity, I extended and corrected the haddock life cycle model of Patin et al. (2015) by removing the residual variability in fishing mortality that was independent of year and age (i.e., reflected by τ W ), and fixed the age-specific mortality for haddock older than 4 years. ...
... Spawning stock biomass Sh y of haddock (h) was estimated as a function age-specific abundances Nh a from age a to A, weight-at-age Wh a,y and the probability of being mature Ph a,y , The mean number of juveniles was estimated using fecundity estimates in number of eggs per gram and mortality from eggs to 0-group. Based on Ohlberger et al. (2014), the number of juveniles of haddock Nh 0,y depends on temperature and is expressed as an exponential relationship such as, where 2 is the process error variance, and m e is the mortality during the period from egg to 0-group survey; then the mean fecundity φ is expressed such as, ...
Climate change and harvesting can affect the ecosystems' functioning by altering the population dynamics and interactions among species. Knowing how species interact is essential for better understanding potentially unintended consequences of harvest on multiple species in ecosystems. I analyzed how stage-specific interactions between two harvested competitors, the haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua), living in the Barents Sea affect the outcome of changes in the harvest of the two species. Using state-space models that account for observation errors and stochasticity in the population dynamics, I run different harvesting scenarios and track population-level responses of both species. The increasing temperature elevated the number of larvae of haddock but did not significantly influence the older age-classes. The nature of the interactions between both species shifted from predator-prey to competition around age-2 to -3. Increased cod fishing mortality, which led to decreasing abundance of cod, was associated with an increasing overall abundance of haddock, which suggests compensatory dynamics of both species. From a stage-specific approach, I show that a change in the abundance in one species may propagate to other species, threatening the exploited species' recovery. Thus, this study demonstrates that considering interactions among life history stages of harvested species is essential to enhance species' co-existence in harvested ecosystems. The approach developed in this study steps forward the analyses of effects of harvest and climate in multi-species systems by considering the comprehension of complex ecological processes to facilitate the sustainable use of natural resources.
... Our results synthesized the effects of both abiotic and biotic environmental factors on polar cod, a fish species with a close relationship to sea ice cover, in addition to being a common prey for a large variety of Arctic top predators (Welch et al. 1992). Statespace models have already been used in the Barents Sea ecosystem to study ecological dynamics, including studies of predator−prey interactions (Stige et al. 2018, effects of mass mortality events (Langangen et al. 2017) and the effects of density-dependence and stochastic ecosystem processes on a fish population (Ohlberger et al. 2014). Our approach illustrates the use of the state-space framework to separate and estimate the effects of multiple environmental factors on a population at an age-resolved level and simulate consequences of change in environmental conditions on the population, while taking into account uncertainties linked to environmental effects. ...
... The change from a strong effect on recruitment at age 1 to a weaker effect on TSB and SSB suggests that changes in recruitment at age 1 caused by abiotic factors are dampened during later stages and are unlikely to be responsible for major changes in TSB and SSB as suggested previously (Huserbråten et al. 2019). Reduction of the environmental effects may be a result of density dependence between age groups (Ohlberger et al. 2014, Langangen et al. 2017) as evidenced by the slight but nonetheless compensatory density-dependent effects on survival at ages 2 to 4. ...
Climate change has large effects on population dynamics of fish species in high latitude ecosystems. Arctic fish stocks experience multiple pressures with changing abiotic living conditions and increased competition and predation from boreal species. However, there are many unknowns regarding how environmental change influences the dynamics of those populations. Here, we focused on the Barents Sea polar cod, a pan-Arctic zooplanktivorous key fish species physiologically and ecologically adapted to the presence of sea ice. We developed an age-resolved Bayesian state-space model of the dynamics of polar cod based on 30 yr of survey data (1986-2015). Using this model, we quantified how inter-annual changes in abundance were associated with abiotic variables (temperature and sea ice cover) and biotic variables (prey biomasses and a predation index). Using the model output, we used a hindcast scenario approach to investigate to which degree the observed variations in total population size were related to the abiotic or biotic variables. Our results showed that variation in abundance of young polar cod (ages 0 and 1) was best explained by abiotic variables while variation in the older age groups (ages 3 and 4) was best explained by predation. Hindcast scenarios showed that the abiotic variables had a more evident effect than predation on population dynamics, but none of the variables we considered could explain the drastic population decline observed in recent years. Our work shows the advantages of studying age-specific responses as a stepping stone to understand changes at the population level.
... several Bering Sea crab fisheries; Szuwalski et al., 2020;Zheng and Kruse, 2000). Further, several studies have linked stock declines to environmental change (Audzijonyte et al., 2013;Benoît et al., 2011;Szuwalski et al., 2020), demonstrating the importance of accounting for environmental factors in population assessments Crone et al., 2019;Ohlberger et al., 2014;Sguotti et al., 2018). ...
Identification of key drivers of population dynamics and prediction of rates of population recovery following stock decline are crucial aspects of fisheries management. The abundance of a blue swimmer crab population (Portunus armatus) in Cockburn Sound, Western Australia, which once supported the largest commercial fishery for this short-lived species in the State, declined markedly and has remained low despite two commercial fishery closures. This study employed state-space biomass dynamics models to explore evidence for potential factors contributing to the lack of stock recovery, including fishing pressure, changes in primary production (using chlorophyll-a as a proxy) and depensatory stock-recruitment dynamics. Likelihood ratio test results indicated better statistical fits for models with production functions modified to account for chlorophyll-a or both depensation and chlorophyll-a. Models incorporating both depensation and chlorophyll-a provided the most biologically-feasible results (e.g. estimated intrinsic increase, r, not near zero) and the estimated biomass trajectories were less uncertain. For all models, estimated annual harvest rates peaked in the late 1990s, prior to the first major stock decline, and again in 2009−12, when the fishery was briefly re-opened with tight management restrictions. Results for models including both depensation and chlorophyll-a indicated that stock productivity is positively-related to chlorophyll-a. Thus, below-average chlorophyll-a concentrations in Cockburn Sound in recent years, in combination with some form(s) of depensation (e.g. associated with predation pressure), may be preventing stock recovery. Despite a sustained period of very limited recreational fishing and no commercial fishing, stock recovery appears highly uncertain under current environmental conditions. The results of this study highlight the value of incorporating environmental data and alternative stock-recruitment assumptions when fitting production models to explore key factors influencing population dynamics.
