Variation In Recruitment Research Articles

Evidence of post-larval growth-selective mortality in Atlantic mackerel revealed by the collection of young-of-the-year juveniles ingested by the northern gannet

The growth-survival paradigm predicts that year-class strength is determined by growth-dependent mortality during the larval stage. In Atlantic mackerel Scomber scombrus, the possibility that strong growth-dependent mortality extends into the early juvenile stage has not previously been tested because of the difficulty in sampling young-of-the-year (YOY) juveniles. The present study determined the timing of the ‘endpoint’ during the early ontogeny, when growth-selective mortality decreases and recruitment is set. We relied on regurgitations from one of the main predators of mackerel, the northern gannet Morus bassanus, as a source of YOY juveniles. Early growth trajectories of YOY mackerel were reconstructed from the otolith microstructure and were compared to those of 1-yr-old (OYO) juveniles from the same cohort for the year classes of 2015 and 2017. In both cohorts, the early growth trajectory of OYO fish was faster than that of YOY juveniles, indicating that growth-selective mortality extended beyond the larval stage. For the 2017 cohort, the comparison of larval growth trajectories between 2-mo-old YOY, 3mo-old YOY and OYO juveniles indicated that strong selection for fast growth persisted until the pre-wintering period, but that winter mortality likely did not play an important role in shaping year-class strength. These findings suggest that in Atlantic mackerel, the endpoint when the relative strength of cohorts is fixed occurs at the age of 3 mo. These results highlight the importance of considering growth-dependent mortality processes occurring beyond the larval stage to obtain a better understanding of causes of recruitment variability.

Survival during the pre-fledging period rather than during post-fledging drives variation in local recruitment of an endangered migratory shorebird, the Southern Dunlin Calidris alpina schinzii

The declines in wet-grassland breeding shorebird populations are considered to mainly result from changes in reproduction. While there is plenty of information on nest survival, little reliable information exists on local recruitment due to confounding effects of permanent emigration. Furthermore, few studies have been able to study the roles of pre- and post-fledging survival on local recruitment. Therefore, it is unclear whether local recruitment of young reflects conditions at the breeding sites or at non-breeding sites. We studied an isolated population of the endangered Southern Dunlin (Calidris alpina schinzii) breeding on the west coast of Sweden to examine (1) brood survival (probability of at least one chick fledging) by following broods fates and (2) local recruitment (survival from hatching to 1 year old) using capture-recapture data. We then examined how much of the annual variation in juvenile survival was explained by variation in brood survival. Brood survival was on average 0.58 (annual range 0.08–1.00) and explained 64% of variation in annual local recruitment. Still local recruitment was rather high for a shorebird (0.17, SE = 0.023), which reflects the isolated nature of the study population. Our results suggest that local recruitment seems to be mainly constrained by chick survival during the pre-fledging period. Therefore, management of breeding sites leading to increased brood survival, e.g., reducing predation on chicks, should have strong impacts on local recruitment and local population growth.

Spatial and Temporal Influences of Nearshore Hydrography on Fish Assemblages Associated with Energy Platforms in the Northern Gulf of Mexico

In 2013–2014, remote video and diver surveys documented fish assemblages around 150 small oil platforms in nearshore federal waters off the entire Louisiana coast (≤ 18 m depth). Results were used to evaluate ecological processes driving differences in fish abundance and assemblages associated with platforms. The nearshore zone was characterized by high spatial and temporal environmental heterogeneity. Surveys documented 55 species of platform-associated fishes. Twenty-nine species were partially or wholly represented by young-of-the-year (YOY) or age 1–2 juveniles, including red, gray, and lane snapper (Lutjanus campechanus, Lutjanus griseus, and Lutjanus synagris), greater amberjack (Seriola dumerili), and gag grouper (Mycteroperca microlepis). Assemblages were compared among three coastal regions with different hydrography due to interactions between river discharge and bathymetry. Assemblage composition near platforms varied in a region × year interaction associated with inter-annual differences in river discharge and coastal distribution of Sargassum drifts. The probability of YOY L. campechanus occurrence increased with bottom DO saturation (1.40–124.3%) from 0.15 to 0.72. The probability of YOY L. synagris and M. microlepis respectively decreased and increased with depth (5.61–16.76 m) from 0.81 to 0.05 and from 0.03 to 0.68. The results show that fish assemblages around platforms in the nearshore zone experienced substantial inter-regional and inter-annual differences that were driven by hydrographic and recruitment variability. Platforms also provided suitable reef habitat for juvenile fishes in areas that experience widespread bottom water hypoxia and large freshwater inflows, highlighting the importance of nearshore platforms as nursery locations for juvenile fishes that represent a variety of early life-history strategies.

Interannual variability of diet composition and prey preference of larval redfish (Sebastes spp.) in the Gulf of St. Lawrence

Abstract Spatiotemporal overlap between fish larvae and their planktonic prey is an important source of recruitment variability. Over the past decade, one species of redfish, Sebastes mentella, from the Gulf of St. Lawrence (GSL) produced multiple strong cohorts following decades of low recruitment, which has generated strong interest in identifying potential drivers of larval survival. The present study provides the first detailed, multi-year assessment of larval redfish (Sebastes spp.) trophodynamics. Interannual variability in larval redfish diet composition and prey selectivity was assessed using high-resolution prey identification of larval gut contents and in situ prey fields. Eggs from the calanoid copepod Calanus finmarchicus represented the most frequently consumed prey in 3 of the 4 collection years, and contributed the largest proportion of carbon ingested by redfish larvae in all years. The high consumption of C. finmarchicus eggs by larvae, combined with evidence of positive selection for this taxon in some years, supports the hypothesis of a strong trophic link between larval redfish and a key calanoid copepod in the GSL ecosystem. Our results indicate that future efforts investigating GSL redfish recruitment processes should consider environment-driven variability in the reproductive phenology and abundance of C. finmarchicus.

Environmental conditions modify density‐dependent salmonid recruitment: Insights into the 2016 recruitment crash in Wales

Abstract Understanding the effects of density‐dependent and density‐independent factors on recruitment is often inhibited by difficulties quantifying their relative contributions in highly variable recruitment data. Use of data‐driven statistical methods with data that include one or more extreme recruitment events could help overcome these difficulties. Juvenile Atlantic salmon and trout abundances in Wales have declined over the last 2 decades, and 2016 was a notably poor recruitment year in rivers around southern Europe, including England and Wales. The 2016 recruitment crash coincided with extreme winter weather conditions, leading to speculation that unusually warm temperatures and high flows adversely affect salmonid recruitment and caused the 2016 crash, although this remains untested. We developed data‐driven statistical models to: (1) describe juvenile salmonid recruitment from density‐dependent and density‐independent factors; and (2) assess whether the density‐independent factors probably contributed to the 2016 salmon recruitment crash. We compiled salmon and trout young‐of‐year juvenile abundances from electrofishing surveys, egg deposition estimates and river flow and air temperature data from 2001–2017 for seven Welsh rivers, broadly representative of rivers around Wales. We used river flow and air temperature data to derive ecologically and behaviourally meaningful density‐independent explanatory variables. Salmonid recruitment in Wales was best described using density‐dependent and density‐independent factors, especially for salmon: after accounting for a concave relationship with egg deposition, salmon juvenile abundance was reduced under (1) warmer spawning temperatures that might inhibit spawning, and (2) higher flood frequencies during pre‐emergence and emergence that might washout eggs or alevins. Results were less clear for trout, perhaps because they are behaviourally more plastic. Our findings provide empirical support for general and predictable effects of temperature and flow during spawning and emergence on salmonid—especially salmon—recruitment in Wales. Furthermore, we suggest that the 2016 salmon recruitment crash was caused—in part—by particularly inclement spawning and emergence conditions, which could be more common under future climate change. Our findings suggest that future salmonid stock assessment models could include the effects of density‐independent variables on recruitment to improve their predictive power.

Long term variation of sardine Sardina pilchardus spawning along the Atlantic coast of northwest Africa (21–26°N): characterization and spatiotemporal variability in spawning habitat

Small pelagic fish such as sardine show strong recruitment variability often associated with environmental changes influencing the spawning process and ultimately, affecting population dynamics. Sardine (Sardina pilchardus, Walbaum 1792) is one of the most exploited pelagic species along the northwest African coast. The main spawning occurs during the cold season (autumn–winter). A time-series autumn–winter surveys extending from 1994 to 2015 sampled sardine eggs, along the southern area of the Moroccan Atlantic coast (26°N–21°N) were analyzed. The present work focuses on examining the inter-annual variability of the spawning habitat by analyzing the spatial-temporal variability of sardine egg distribution and density extracted from the data collected over the period 1994–2015. Generalized additive models (GAM) were used to detect the relationships between the sardine distribution, expressed as egg density and the presence or absence data and relevant hydrobiological environmental variables, such as salinity, temperature and zooplankton biomass. The generalized additive models showed significant relationships between the environment variables (SST, SSS and Zooplankton biomass) and sardine density, but not with sardine presence. Given that the study area is characterized by high mesoscale features and significant upwelling activities, the variability of upwelling processes could explain the changes of spawning ground position and thermal window.

Use of a Restrictive Regulation to Manage Walleyes in a New Glacial Lake in South Dakota

AbstractAbove‐average precipitation in the 1990s resulted in many new fisheries across northeastern South Dakota and elsewhere in the Upper Midwest. Walleye Sander vitreus populations in these new fisheries can be extremely vulnerable to angler harvest. In an attempt to maintain a Walleye fishery in Reetz Lake, South Dakota, a new lake, a restrictive regulation of one Walleye ≥711 mm TL per day was implemented when the lake opened to public fishing (October 2001). To evaluate the regulation, we monitored the estimated number of Walleyes ≥508 mm, Walleye gill‐net catch per unit of effort (CPUE) of stock‐length fish (≥250 mm), size structure, condition, length at age at capture (ages 2–5), mortality, recruitment, and angler use during the period 2001–2019. The estimated number of Wall‐eyes ≥508 mm, gill‐net CPUE, and proportional size distribution of preferred‐length fish (PSD‐P; ≥510 mm) trended higher across years. Relative weight (Wr) for the quality–preferred (380–509 mm) and preferred–memorable (510–629 mm) length‐groups and the length at age at capture for ages 4 and 5 trended lower. Walleyes <510 mm were in better condition than larger fish throughout the study. Estimated total mortality was 62% in 2004 but substantially declined in subsequent years before ranging from 6% to 21% for 2010–2019. The coefficient of variation of age‐3 gill‐net CPUE by cohort was 93%, indicating variable recruitment. Decreasing trends in Wr and growth were not surprising, as Walleye CPUE increased through time. Lengths at age remained at or above statewide and national averages. The values for number of angler‐hours and Walleye catch rates were high during the summer months when most anglers targeted Walleye. The regulation that restricted harvest at Reetz Lake to one Walleye ≥711 mm allowed an abundant Walleye population to establish and was successful at maintaining the fishery in terms of abundance, size structure, and angler use.

The role of microsite sunlight environment on growth, architecture, and resource allocation in dominant Acacia tree seedlings, in Serengeti, East Africa

Seedling establishment is a critical life history stage for savanna tree recruitment due to variability in resource availability. While tree–grass competition for water is recognized as an important driver of tree seedling mortality, the importance of sunlight exposure on tree seedling performance has received little attention in savanna ecosystems despite variable seedling light environments caused by heterogeneity in biomass of the grass canopy. We studied the seasonal sunlight micro-environment for two dominant East African tree species (Acacia = Vachellia) robusta (Burch) and A. tortilis (Forssk) under natural field conditions. In the Serengeti National Park, Tanzania, A. robusta trees occur in tall grasslands of the north (shady) and A. tortilis in the southern short grasslands (less shaded). We also designed a greenhouse experiment to quantify sunlight effects on seedling growth, architecture, and resource allocation traits. In the field, A. robusta seedlings were associated with lower understorey sunlight during the wet season compared to A. tortilis, with this trend switching during the dry season. In the greenhouse experiments, under low sunlight (25% radiation), A. robusta gained height faster than A. tortilis and self-shading among canopy leaves was evident in A. tortilis but not A. robusta. Biomass allocation to leaves, stems, and roots differed between species under different light environments suggesting phenotypic plasticity in response to variable light availability. Our study suggests that microsite light variability should be incorporated in models of the spatial and temporal variability of savanna tree recruitment.

Dispersal of juvenile cod (Gadus morhua L.) in Icelandic waters

Knowledge of dispersal and migration of juvenile fish is essential for understanding the underlying processes responsible for population structure and recruitment variation. We explored the dispersal patterns of juvenile cod in waters around Iceland from putative nursery grounds to provide insights into their population structure. About 6000 juvenile cod were tagged at five inshore locations in 1993, 1997 and 2002. Of these, 767 juveniles were recaptured after 27–3297 days at liberty. The recaptures were weighted using standardized fishing effort and revealed high fidelity of juvenile cod to their putative nursery grounds. Cod tagged west of Iceland dispersed southwards to the main spawning grounds, south and southwest of Iceland, at the time of maturation. In contrast, juveniles tagged northwest of Iceland demonstrated no affiliation with the main spawning grounds. However, the juveniles tagged north of Iceland dispersed over a large geographical area and co-occurred in the same areas as those from areas northwest and west of Iceland during their third year at liberty. Different dispersal patterns for cod tagged at different locations indicated different life history strategies of Icelandic cod with limited mixing of juveniles followed by variable migration patterns towards spawning, supporting earlier studies showing the complexity of the population dynamics of the Icelandic cod.

Red king crab larval advection in Bristol Bay: Implications for recruitment variability

AbstractRecruitment variability is poorly understood for Bering Sea crab stocks. The nearshore area in southwest Bristol Bay (Alaska, USA) is hypothesized as having historically (i.e., prior to ~ 1980) been the most important spawning ground for Bristol Bay red king crab (Paralithodes camtschaticus) because post‐larvae are thought to have been most likely to reach optimal settlement habitat along the Alaska Peninsula when hatched from this area as part of an endless‐belt reproductive strategy. We coupled a biophysical and oceanographic circulation model to test this hypothesis, investigate larval connectivity of more recent female spatial distributions, and evaluate the importance of climate variability on larval advection trajectories. Predicted settlement success varied through changes in larval pelagic duration and oceanographic circulation patterns: Shorter advective distance was associated with warmer conditions, causing higher rates of local retention relative to cold conditions. Contrary to earlier models, most larvae hatched in southwest Bristol Bay were advected offshore away from good habitat, whereas larvae hatched in central and nearshore Bristol Bay were retained in or advected to good habitat along the Alaska Peninsula. Our results suggest contemporary spatial distributions can supply settlement‐competent larvae to nurseries along the Alaska Peninsula and that under certain conditions, larvae may reach the Pribilof Islands when hatched from southwest Bristol Bay. Our study informs the role of environmental variability on larval transport and provides context within which to structure future investigations of recruitment mechanisms.

Oceanographic features and limited dispersal shape the population genetic structure of the vase sponge Ircinia campana in the Greater Caribbean

Understanding population genetic structure can help us to infer dispersal patterns, predict population resilience and design effective management strategies. For sessile species with limited dispersal, this is especially pertinent because genetic diversity and connectivity are key aspects of their resilience to environmental stressors. Here, we describe the population structure of Ircinia campana, a common Caribbean sponge subject to mass mortalities and disease. Microsatellites were used to genotype 440 individuals from 19 sites throughout the Greater Caribbean. We found strong genetic structure across the region, and significant isolation by distance across the Lesser Antilles, highlighting the influence of limited larval dispersal. We also observed spatial genetic structure patterns congruent with oceanography. This includes evidence of connectivity between sponges in the Florida Keys and the southeast coast of the United States (>700 km away) where the oceanographic environment is dominated by the strong Florida Current. Conversely, the population in southern Belize was strongly differentiated from all other sites, consistent with the presence of dispersal-limiting oceanographic features, including the Gulf of Honduras gyre. At smaller spatial scales (<100 km), sites showed heterogeneous patterns of low-level but significant genetic differentiation (chaotic genetic patchiness), indicative of temporal variability in recruitment or local selective pressures. Genetic diversity was similar across sites, but there was evidence of a genetic bottleneck at one site in Florida where past mass mortalities have occurred. These findings underscore the relationship between regional oceanography and weak larval dispersal in explaining population genetic patterns, and could inform conservation management of the species.

Linking Antarctic krill larval supply and recruitment along the Antarctic Peninsula.

Antarctic krill (Euphausia superba) larval production and overwinter survival drive recruitment variability, which in turn determines abundance trends. The Antarctic Peninsula has been described as a recruitment hot spot and as a potentially important source region for larval and juvenile krill dispersal. However, there has been no analysis to spatially resolve regional-scale krill population dynamics across life stages. We assessed spatiotemporal patterns in krill demography using two decades of austral summer data collected along the North and West Antarctic Peninsula since 1993. We identified persistent spatial segregation in the summer distribution of euphausiid larvae (E. superba plus other species), which were concentrated in oceanic waters along the continental slope, and E. superba recruits, which were concentrated in shelf and coastal waters. Mature females of E. superba were more abundant over the continental shelf than the slope or coast. Euphausiid larval abundance was relatively localized and weakly correlated between the North and West Antarctic Peninsula, while E. superba recruitment was generally synchronized throughout the entire region. Euphausiid larval abundance along the West Antarctic Peninsula slope explained E. superba recruitment in shelf and coastal waters the next year. Given the localized nature of krill productivity, it is critical to evaluate the connectivity between upstream and downstream areas of the Antarctic Peninsula and beyond. Krill fishery catch distributions and population projections in the context of a changing climate should account for ontogenetic habitat partitioning, regional population connectivity, and highly variable recruitment.

Quantifying the relative importance of stock level, river temperature and discharge on the abundance of juvenile Atlantic salmon (Salmo salar)

AbstractFisheries scientists need to understand the relationships between river temperature, discharge and production of juvenile salmonids to inform evidence‐based management and regulation of rivers and to understand the potential effects of climate change. These relationships can be determined by characterising interannual variability in abundance and environmental conditions from long‐term monitoring data and assessing their inter‐relationships. Two major challenges are (1) the requirement to separate the relative effects of stock level and environment which both affect interannual variability in abundance and (2) obtaining long‐term environmental time‐series that do not suffer from temporal biases. This study built on recent advances in hydrological, river‐temperature and juvenile salmonid modelling to investigate the influence of temperature and discharge on interannual variability in Atlantic salmon (Salmo salar) fry (age: 0 years) and parr (age: 1 year) production. The study used a unique long‐term dataset (&gt;50 years) with detailed age‐differentiated census data collected for multiple life‐stages. The study shows that most of the interannual variability in recruitment was explained by stock level. Discharge had a comparatively small effect on fry recruitment, but a greater effect than artificial stocking. Discharge had no discernible effect on parr recruitment. Temperature had no effect on recruitment of either life‐stage. This study suggests that salmon are well adapted to current environmental variability in natural upland rivers in Scotland, but reductions in discharge during spawning and emergence could negatively affect fry recruitment with consequences for regulation of river flows. The study highlights the importance of high‐quality census data for accurately determining the effects of environmental variability on recruitment.

A multicenter paper-based and web-based system for collecting patient-reported outcome measures in patients undergoing local treatment for prostate cancer: first experiences

PurposeTo give an overview of the multicenter Prostate Cancer Outcomes (PCO) study, involving paper-based and web-based collection of patient-reported outcome measures (PROM) in patients undergoing local treatment for prostate cancer in certified centers in Germany. The PCO study is part of the larger Movember-funded TrueNTH Global Registry. The article reports on the study’s design and provides a brief progress report after the first 2 years of data collection.MethodsProstate cancer centers (PCCs) certified according to German Cancer Society requirements were invited to participate in collecting patient-reported information on symptoms and function before and at least once (at 12 months) after treatment. The data were matched with disease and treatment information. This report describes progress in patient inclusion, response rate, and variations between centers relative to online/paper use, and also data quality, including recruitment variations relative to treatment in the first participating PCCs.ResultsPCC participation increased over time; 44 centers had transferred data for 3094 patients at the time of this report. Patient recruitment varied widely across centers. Recruitment was highest among patients undergoing radical prostatectomy. The completeness of the data was good, except for comorbidity information.ConclusionsThe PCO study benefits from a quality improvement system first established over 10 years ago, requiring collection and harmonization of a predefined clinical dataset across centers. Nevertheless, establishing a PROM routine requires substantial effort on the part of providers and constant monitoring in order to achieve high-quality data. The findings reported here may be useful for guiding implementation in similar initiatives.

Resampling 25 years later reveals fewer species but higher abundance of juvenile fishes in a Caribbean mangrove bay

The prop roots of mangroves play an important role as habitat for juvenile fishes. Revisit studies (&gt;10 yrs apart) provide rare and valuable insights into the structure and stability of these systems in the face of escalating pressure from coastal development, climate change, and fisheries. We compared assemblages of juvenile fishes in the mangroves of a Caribbean estuary from 1991 to 1993 and 2018 to 2019 using trap collections to quantify changes and identify their potential drivers. Although the environmental and physical properties of the mangrove landscape were similar between the two periods, there were significant changes in many aspects of the fish assemblage. Compared to 25 yrs earlier, overall fish abundance (catch per unit effort, CPUE) was 3 to 7 times higher but overall biodiversity was 30% lower at the species level and 50% lower at the genus level. Taxa with dramatic changes in CPUE were mojarras (Gerreidae, 84% lower CPUE in 2018–2019), snappers (17 times higher CPUE), puffers (Tetraodontidae, 14 times higher CPUE), and moray eels (&gt;200 times higher CPUE). Traps in 2019 also captured the first documented invasive Indo-Pacific swimming crab (Charybdis helerii) in the US Virgin Islands. The observed changes in the fish assemblage may be due to natural variations in recruitment, environmental influences, and/or hurricane disturbance, but for some species likely relate to declines in adult reef fish populations in the region.

Fecundity and spawning potential ratio of shoal bass Micropterus cataractae in the lower Flint River, Georgia, USA

Shoal bass Micropterus cataractae are endemic to the Apalachicola River system in Alabama, Georgia, and Florida, USA. Despite maintaining popular fisheries, little is known about their biology and ecology. Fecundity of this species was estimated from the lower Flint River, Georgia, to better understand egg production of this species and relation with length and weight. Further, these data were used to estimate the spawning potential ratio (SPR) of this population under the current 305-mm minimum-length limit (MLL) and a more restrictive 381-mm MLL. An age-structured model was used to examine changes in yield and SPR across a range of exploitation at three levels of natural mortality. Fecundity of shoal bass ranged from 909−35,502 oocytes with a mean estimate of 10,424 ± 1089 (SD), and increased with length (r2 = 0.46; P < 0.0001), weight (r2 = 0.51; P < 0.0001), and age (r2 = 0.59; P < 0.0001). Thus, nonlethal measures of length or weight of field-collected fish may be used to predict the potential of each fish to contribute a large number of eggs to a hatchery operation. Regression equations derived in this study suggested that hatchery operations may be streamlined by utilizing fish at least 457 mm in length or 1482 g in weight to maximize number of oocytes collected per fish, as these fish should have fecundities higher than the median. Population dynamics modeling predicted that at the lowest estimate of natural mortality, growth overfishing would occur once exploitation exceeded 30 % under the 305-mm MLL, but not under the 381-mm MLL. There was little difference between MLLs at higher levels of natural mortality. At all three levels of natural mortality, the 381-mm MLL did a better job at protecting SPR than the 305-mm MLL, which only approached the 0.20 level at exploitations higher than 60 %. When adjusted for recruitment variability at expected levels of exploitation, SPR at the lowest level of natural mortality approached the 0.20 level during periods of poor recruitment under the 305-mm MLL. Given that the critical level of SPR is unknown for shoal bass, managers should protect this important fishery by adopting a more stringent regulation.

Addressing trait selection patterns in temporary ponds in response to wildfire disturbance and seasonal succession.

Mediterranean ecosystems are increasingly threatened by disturbances such as wildfires. These disturbances are expected to shift the selective pressures that determine trait-dependent community assembly. In addition, the stochasticity in species assembly may decrease because of the introduction of strong selection regimes or may increase because of random variation in recruitment. However, these changes in the selection profile and stochasticity in disturbed communities have seldom been evaluated. We examined the relative roles of wildfire disturbance, local conditions and successional dynamics on the assembly of aquatic macroinvertebrate communities. We used the theory of community assembly by trait selection (CATS) to identify traits under selection and to estimate their dependence on wildfire disturbance and environmental gradients. We took advantage of a natural wildfire that partially burned a Mediterranean system of temporary ponds, which were surveyed before and after the wildfire, creating a natural before-after-control-impact design. Before the wildfire, the burned and unburned ponds did not show differences in the selected traits. After the wildfire event, species with larger body sizes and scrapers were favoured in the burned ponds, while collectors and active dispersers were underrepresented. Nonetheless, local environmental conditions and successional dynamics had greater relevance in the selection of traits than the wildfire. This suggests that assembly mechanisms were largely determined by seasonal successional changes regardless of wildfire disturbance. Finally, the relevance of the analysed traits diminished during the hydroperiod, suggesting more stochastic assemblages and/or a replacement in the set of selected traits. Despite the prominent role of seasonal succession over wildfire, this disturbance was associated with a change in the selection strength over specific traits related with feeding strategies and species life histories. Both hydroperiod and wildfire highlighted a strong role of trait-mediated processes (i.e. niche assembly). Therefore, the predicted increase in the frequency and intensity of wildfires is likely to result in community functional shifts. Furthermore, stochasticity was also important for community assembly, particularly from the middle towards the end of the hydroperiod. Our results evidenced the strong relevance of successional changes in trait-mediated assembly mechanisms and its interplay with wildfire disturbance in temporary pond communities.

From past to future: understanding and accounting for recruitment variability of Australia’s redleg banana prawn (Penaeus indicus) fishery

Abstract Understanding recruitment variability in marine fisheries has benefits for the stock management and dependent fishers’ ability to plan their income. Here, we overview past and new research on the complex recruitment dynamics of redleg banana prawns Penaeus indicus in Australia’s Joseph Bonaparte Gulf to assess themes dating back to the time of Hjort and identify new challenges. During 2015 and 2016, redleg prawn catches and catch per unit effort decreased to anomalously low levels, suggesting a substantial decrease in prawn biomass. We hypothesized that low catches could be explained by temporary drops in sea level and rainfall potentially reducing the ability of postlarvae to reach their nursery ground. We contend that very bad prawn catch years may be predicted using two variables that are possible drivers of recruitment—the January Southern Oscillation Index and the combined January to February cumulative rainfall. However, due to challenges in verifying and defining such environmental relationships for inclusion in a stock assessment, we propose development of a harvest strategy framework to support management recommendations. Our study highlights the increasing role of anthropogenic climate change in exaggerating the impacts of environmental drivers on recruitment processes and the need to also focus on multidisciplinary research.

Incorporating harvest–population diversity trade-offs into harvest policy analyses of salmon management in large river basins

Accounting for population diversity can be critical to the sustainable management of mixed-stock fisheries because harvest rates that can be sustained by productive populations may come at the cost of overfishing less productive ones. While these harvest–diversity trade-offs are well-recognized, their consequences for harvest policy performance are not often explicitly evaluated in contemporary fisheries management. We use closed-loop simulations to evaluate the ability of alternative harvest policies to meet population diversity and fishery objectives for one of the largest subsistence Chinook salmon (Oncorhynchus tshawytscha) fisheries in the world (Kuskokwim River Basin in western Alaska). We found clear evidence of population diversity that resulted in asymmetric trade-offs among fishery and conservation objectives whereby policies that forgo relatively small amounts of harvest result in relatively large increases in equitable access to Chinook and elimination of risk of weak stock extirpation. The performance of alternative harvest policies, and the magnitude of trade-offs, were sensitive to regime shifts and uncertainty in the drivers of recruitment variation. However, we found that harvest policies that prioritized meeting minimum subsistence needs were unlikely to jeopardize long-term sustainability.

Modeling Larval American Shad Recruitment in a Large River

AbstractClimate change is altering the spatial and temporal patterns of temperature and discharge in rivers, which is expected to have implications for the life stages of anadromous fish using those rivers. We developed an individual‐based model to track American Shad Alosa sapidissima offspring within a coarse template of spatially and temporally variable habitat conditions defined by a combination of temperature, river velocity, and prey availability models. We simulated spawning at each river kilometer along a 142‐km reach of the Connecticut River on each day (April 1–August 31) to understand how spawning date and location drive larval recruitment differentially across years and decades (1993–2002 and 2007–2016). For both temperature and flow, interannual variation was large in comparison to interdecadal differences. Variation in simulated recruitment was best explained by a combination of season‐specific spawning temperature and location along the course of the river. The greatest potential recruitment occurred during years in which June temperatures were relatively high. In years when June and July were warmer than average, maximum recruitment resulted from spawning taking place at the upstream portion of the modeled reach. Model scenarios (stationary or passive‐drift larvae; and dams or no dams) had predictable effects. We assumed that the pools above dams had negative impacts on eggs and yolk‐sac larvae that may have been deposited there. Allowing eggs and larvae to drift passively with the current reduced spatial differences in recruitment success among spawning sites relative to stationary eggs and larvae. Our results demonstrate the importance of spatiotemporal environmental heterogeneity for producing positive recruitment over the long term. In addition, our results suggest the importance of successful passage of spawners to historical spawning sites in the Connecticut River upstream of Vernon Dam, especially as conditions shift with climate change.