Quantifying larval dispersal portfolio in seabass nurseries using otolith chemical signatures

The temporal asynchronies in larvae production from different spawning areas are fundamental components for ensuring stability and resilience of marine metapopulations. Such a concept, named portfolio effect, supposes that diversifying larval dispersal histories should minimize the risk of recruitment failure by increasing the probability that at least some larvae successfully settle in nursery. Here, we used a reconstructive approach based on otolith chemistry to quantify the larval dispersal portfolio of the European seabass, Dicentrarchus labrax, across six estuarine nursery areas of the northeast Atlantic Ocean. The analysis of natal and trajectory signatures indicated that larvae hatch in distinct environments and then dispersed in water masses featured by contrasting chemical signatures. While some trace elements appeared affected by temporal changes (Mn and Sr), others varied spatially during the larval stage but remained poorly affected by temporal fluctuation and fish physiology (Ba, Cu, Rb and Zn). We then proposed two diversity metrics based on richness and variations of chemical signatures among populations to reflect spatio-temporal diversity in natal origins and larval trajectories (i.e., estimates of dispersal portfolio). Along the French coast, the diversity estimates were maximum in nurseries located at proximity of offshore spawning sites and featured by complex offshore hydrodynamic contexts, such as the Mont St-Michel bay. Finally, our findings indicate that the dispersal portfolio was positively related with the local abundance of seabass juveniles, supporting the assumption that heterogeneity in dispersal history contributes to promote recruitment success in nurseries.

Planning hydrological restoration of coastal wetlands: Key model considerations and solutions

The hydrological restoration of coastal wetlands is an emerging approach for mitigating and adapting to climate change and enhancing ecosystem services such as improved water quality and biodiversity. This paper synthesises current knowledge on selecting appropriate modelling approaches for hydrological restoration projects. The selection of a modelling approach is based on project-specific factors, such as costs, risks, and uncertainties, and aligns with the overall project objectives. We provide guidance on model selection, emphasising the use of simpler and less expensive modelling approaches when appropriate, and identifying situations when models may not be required for project managers to make informed decisions. This paper recognises and supports the widespread use of hydrological restoration in coastal wetlands by bridging the gap between hydrological science and restoration practices. It underscores the significance of project objectives, budget, and available data and offers decision-making frameworks, such as decision trees, to aid in matching modelling methods with specific project outcomes.

Intraseasonal predictability of natural phytoplankton population dynamics

It is difficult to make skillful predictions about the future dynamics of marine phytoplankton populations. Here, we use a 22-year time series of monthly average abundances for 198 phytoplankton taxa from Station L4 in the Western English Channel (1992-2014) to test whether and how aggregating phytoplankton into multi-species assemblages can improve predictability of their temporal dynamics. Using a non-parametric framework to assess predictability, we demonstrate that the prediction skill is significantly affected by how species data are grouped into assemblages, the presence of noise, and stochastic behavior within species. Overall, we find that predictability one month into the future increases when species are aggregated together into assemblages with more species, compared with the predictability of individual taxa. However, predictability within dinoflagellates and larger phytoplankton (>12 μm cell radius) is low overall and does not increase by aggregating similar species together. High variability in the data, due to observational error (noise) or stochasticity in population growth rates, reduces the predictability of individual species more than the predictability of assemblages. These findings show that there is greater potential for univariate prediction of species assemblages or whole-community metrics, such as total chlorophyll or biomass, than for the individual dynamics of phytoplankton species.

Environmental risk in an age of biotic impoverishment

The science underpinning biodiversity's importance to human well-being seems to be taken up little by environmental decision makers. Since the 1950s, ecological, evolutionary and environmental research has pointed to the importance of biodiversity as a significant factor influencing the stability and functioning of population, community, eco- and Earth-systems and the environmental services they provide. Despite its prominence and the tremendous contributions to our understanding of the natural world, this field of research, which we term 'bio-functional ecology', seems not to have had the impact it should. Biotic impoverishment, the loss of biodiversity across all scales and across all taxa, continues to worsen. We suggest that redirecting ecology's emphasis on ecological stability to a focus on environmental risk could help bring bio-functional ecology research more into the environmental arena. Rather than managing biodiversity as an agent of ecological stability, biodiversity could be managed as a natural capital asset in a portfolio of social, human, produced and financial capital assets. This would allow using portfolio theory to identify options for minimizing environmental risk while ensuring human well-being. In this essay, we argue that environmental risk more accurately captures people's motivation to preserve and manage biodiversity than does ecological stability. This redirection from stability to risk may provide greater clarity for decision makers and people in general as to why biodiversity is fundamentally linked to human well-being. In doing so, we can help curb the currently unabated spread of biotic impoverishment across the biosphere.

Synchrony erodes spatial portfolios of an anadromous fish and alters availability for resource users

Environmental forces can create spatially synchronous dynamics among nearby populations. However, increased climate variability, driven by anthropogenic climate change, will likely enhance synchrony among spatially disparate populations. Population synchrony may lead to greater fluctuations in abundance, but the consequences of population synchrony across multiple scales of biological organization, including impacts to putative competitors, dependent predators or human communities, are rarely considered in this context. Chinook salmon Oncorhynchus tshawytscha stocks distribute across the Northeast Pacific, creating spatially variable portfolios that support large ocean fisheries and marine mammal predators, such as killer whales Orcinus orca. We rely on a multi-population model that simulates Chinook salmon ocean distribution and abundance to understand spatial portfolios, or variability in abundance within and among ocean distribution regions, of Chinook salmon stocks across 17 ocean regions from Southeast Alaska to California. We found the expected positive correlation between the number of stocks in an ocean region and spatial portfolio strength; however, increased demographic synchrony eroded Chinook salmon spatial portfolios in the ocean. Moreover, we observed decreased resource availability within ocean fishery management jurisdictions but not within killer whale summer habitat. We found a strong portfolio effect across both Southern Resident and Northern Resident killer whale habitats that was relatively unaffected by increased demographic synchrony, likely a result of the large spatial area included in these habitats. However, within the areas of smaller fishing management jurisdictions we found a weakening of Chinook salmon portfolios and increased but inconsistent likelihood of low abundance years as demographic synchrony increased. We suggest that management and conservation actions that reduce spatial synchrony can enhance short-term ecosystem resilience by promoting the stabilizing effect multiple stocks have on aggregate Chinook salmon populations and overall resource availability.

Production and outmigration of young-of-year northern pike Esox lucius from natural and modified waterways connected to Lower Green Bay, Wisconsin

Production and outmigration of young-of-year (YOY) northern pike from natal sites in Lower Green Bay, WI, USA, were documented over three consecutive years (2013-2015). We tested the hypothesis that spawning success and outmigration characteristics of YOY northern pike would vary among natural and anthropogenically modified habitats. Sixteen focal study locations were surveyed, including a restored natural wetland, agricultural drainage ditches, a flooded forested wetland and several unimpounded tributaries. We collected 1469 YOY northern pike with most individuals (N = 1163) originating from a flooded forested wetland on the east shore. Most sites produced YOY in all years (range N = 2-1145 individuals among study years). Outmigration ranged between 1 and 40 days during 2013-2015. Greater production and extended outmigration times occurred at most sites in 2014 (range 17-40 days) when the region experienced a late spring with heavy precipitation. In contrast, the lowest production and shortest outmigration period occurred at most sites in 2015 (range 14-23 days) when environmental conditions reflected regional averages. Outmigration began nearly 3 weeks earlier in 2015 (5/8) than in other study years (8 June 2013 and 25 May 2014). Total length (TL) of outmigrating northern pike ranged between 17 and 138 mm. Total length of YOY was significantly different among sites in 2013 and 2014, with the smallest fish (17 mm TL) outmigrating from agricultural ditches in both years. There were no significant variations in size among sites in 2015 (range 21-95 mm TL). Our results indicate significant variation in YOY northern pike outmigration characteristics within Lower Green Bay that may reflect the interplay between adult spawning site selection and annual weather patterns. Our findings highlight the importance of quantifying overlooked habitats in regions of mixed development.

Mixed-stock analysis using Rapture genotyping to evaluate stock-specific exploitation of a walleye population despite weak genetic structure

Mixed-stock analyses using genetic markers have informed fisheries management in cases where strong genetic differentiation occurs among local spawning populations, yet many fisheries are supported by multiple, weakly differentiated stocks. Freshwater fisheries exemplify this problem, with many populations supported by multiple stocks of young evolutionary age and isolated across small spatial scales. Consequently, attempts to conduct genetic mixed-stock analyses of inland fisheries have often been unsuccessful. Advances in genomic sequencing offer the ability to discriminate among populations with weak population structure, providing the necessary resolution to conduct mixed-stock assignment among previously indistinguishable stocks. We used genomic data to conduct a mixed-stock analysis of eastern Lake Erie's commercial and recreational walleye (Sander vitreus) fisheries and estimate the relative harvest of weakly differentiated stocks (pairwise F ST < 0.01). Using RAD-capture (Rapture), we sequenced and genotyped individuals from western and eastern basin local spawning stocks at 12,081 loci with 95% reassignment accuracy, which was not possible in the past using microsatellite markers. A baseline assessment of 395 walleye from 11 spawning stocks identified three reporting groups and refined previous assessments of gene flow among walleye stocks. Genetic assignment of 1,075 walleye harvested in eastern Lake Erie's recreational and commercial fisheries indicated that western basin stocks constituted the majority of harvest during the peak walleye fishing season (July-September), whereas eastern basin individuals comprised much of the early season harvest (May-June). Clear spatial structure in harvest composition existed; catches in more easterly sites contained more individuals of eastern basin origin than did more westerly sites. Our study provides important stock contribution estimates for Lake Erie fishery management and demonstrates the utility of genomic data to facilitate mixed-stock analysis in exploited fish populations having weak population structure or limited existing genetic resources.

Ecological implications of purple sea urchin (Heliocidaris crassispina, Agassiz, 1864) enhancement on the coastal benthic food web: evidence from stable isotope analysis

A responsible approach to marine stock enhancement is an effective approach to restore fishery resources. While the release strategy of target species has been well investigated, the impacts on local ecological equilibrium and habitat qualities have only been poorly considered. In the present study, we evaluated how the macro-benthic food web in Daya Bay was affected by purple sea urchin (Heliocidaris crassispina (Agassiz, 1864) stock enhancement using stable isotope analyses (δ¹³C and δ¹⁵N). Our results indicated that the distribution of local species and trophic diversity were influenced to a certain degree by release of purple sea urchins and changes in the feeding habit of the urchins were observed in line with food abundance, which seasonally varied. When food is abundant, the main food source of sea urchins was microphytobenthos and no significant differences were observed among sites; significant differences in the diet of purple sea urchins were detected when food is less abundant. These results suggested that optimization of the release strategy should include information on seasonal productivity of local recipient sites, food web structure and feeding habits of released species. Such information is essential for building a responsible release approach to maximize production enhancement.

Fishing in hot waters threatens phenotypic diversity

In Focus: Morrongiello, J. R., Sweetman, P. C., & Thresher, R. E. (2019). Fishing constrains phenotypic responses of marine fish to climate variability. Journal of Animal Ecology, 88, 1645-1656. Forces of unnatural selection, such as climate change and harvest, are rarely studied in concert, yet hold the great potential to act synergistically on individual performance, susceptibility to harvest, tolerance to warming temperatures, and ultimately population persistence and resilience. In this paper, Morrongiello et al. (2019) used long-term monitoring of a site-attached temperate reef fish, the purple wrasse (Notolabrus fucicola), to test novel predictions about how fisheries management and climate variability could alter individual growth rates and thermal reaction norms within and across stocks. Otolith growth increments were collected from three south-east Australian populations between 1980 and 1999, pre- and post-harvest, throughout an intensive warming spell. Using hierarchical models to partition variation in growth within and between individuals and populations, Morrongiello et al. detected increased average growth rate with warming, a release from density dependence post-harvest, and a fishing-by-warming interaction that decreased diversity in thermal growth reaction norms because large individuals that tend to better tolerate warm temperatures were effectively culled from the population. This study outlines the importance of determining which phenotypes are more resilient to increasing temperatures, how fisheries should manage for them, and how such collective knowledge could help preserve and even promote resilience of managed populations to increasing temperatures in ecosystems threatened by climate change.

Spatial heterogeneity contributes more to portfolio effects than species variability in bottom-associated marine fishes

Variance of community abundance will be reduced relative to its theoretical maximum whenever population densities fluctuate asynchronously. Fishing communities and mobile predators can switch among fish species and/or fishing locations with asynchronous dynamics, thereby buffering against variable resource densities (termed 'portfolio effects', PEs). However, whether variation among species or locations represent the dominant contributor to PE remains relatively unexplored. Here, we apply a spatio-temporal model to multidecadal time series (1982-2015) for 20 bottom-associated fishes in seven marine ecosystems. For each ecosystem, we compute the reduction in variance over time in total biomass relative to its theoretical maximum if species and locations were perfectly correlated (total PE). We also compute the reduction in variance due to asynchrony among species at each location (species PE) or the reduction due to asynchrony among locations for each species (spatial PE). We specifically compute total, species and spatial PE in 10-year moving windows to detect changes over time. Our analyses revealed that spatial PE are stronger than species PE in six of seven ecosystems, and that ecosystems where species PE is constant over time can exhibit shifts in locations that strongly contribute to PE. We therefore recommend that spatial and total PE be monitored as ecosystem indicators representing risk exposure for human and natural consumers.