Fish Recruitment Research Articles

Spawning stock biomass modulation of environment–recruitment relationship in a marginal spring spawning herring (Clupea harengus membras) population

Scientific interest in the dynamics of fish recruitment dates back to the beginning of the 20th century. Since then, several studies have shown that the environment may have a stronger effect on recruitment (R) compared to that of the spawning stock biomass (SSB). By combining a suite of methods designed to detect the nonlinear, nonstationary and interactive relationships, we have re-evaluated the potential drivers and their interactions responsible for the multiannual dynamics of the recruitment dynamics of the Gulf of Riga (Baltic Sea) spring spawning herring (Clupea harengus membras) population at the longest time-span to date (1958–2015) allowing coverage of variable ecosystem conditions. R was affected significantly by prey density and the severity of the first winter. Although SSB was not a good predictor of R, adding interaction with SSB significantly improved the model, hence the effect of the two environmental variables on R was modulated by SSB. While temporal changes in the environment–R relationship were generally gradual, several abrupt changes were evident in the strength of these relationships. In addition, nonstationary, linear and nonlinear relationships were observed.

Fish Community Changes Associated with Bullhead Removals in Four Northern Wisconsin Lakes

AbstractBullheads Ameiurus spp. are found throughout much of the United States and are infrequently studied species. Although limited information has been published on the Black Bullhead A. melas and Yellow Bullhead A. natalis, it has been shown that bullheads can dominate the fish biomass in some north temperate U.S. lakes, resulting in a fish community exclusive of Walleye Sander vitreus and Yellow Perch Perca flavescens. Recently, recruitment and abundances of popular coolwater sport fishes, such as Walleye and Yellow Perch, have been declining in some northern Wisconsin lakes. These observed declines, coupled with high bullhead densities, led fisheries biologists to conduct whole‐lake bullhead removals on four northern Wisconsin lakes. Removal of Black and Yellow bullheads from these lakes resulted in substantial changes in the fish communities. The CPUE of naturally reproduced age‐0 Walleye increased in Lake Metonga and Patten Lake following the removal of bullheads. Additionally, survival of stocked Walleye increased in Crane and Pickerel lakes after bullhead removals, resulting in higher CPUE of age‐0 and age‐1 Walleye. Subsequently, adult Walleye abundance has increased or remained steady in all bullhead removal lakes. Significant declines in abundance of Bluegill Lepomis macrochirus and increases in abundance of Yellow Perch and Black Crappie Pomoxis nigromaculatus were also observed after bullhead removals, further illustrating a shift in fish community composition. Our observations suggest that when at high densities, bullheads can play a significant role in structuring fish communities and the removal of bullheads can increase recruitment, survival, and abundance of sport fishes like Walleye and Yellow Perch.

Forecasting fish recruitment in age‐structured population models

AbstractRecruitment in age‐structured stock assessment models can be forecasted using a variety of algorithms to provide advice on the anticipated consequences of different possible management actions. Selecting one method over another usually involves some subjectivity, yet can be consequential to the provision of advice. Extensive case‐specific testing is not always feasible. We evaluated the forecast skill in 3‐, 5‐ and 10‐year forecasts of 16 recruitment forecasting methods under various circumstances to provide a broad evaluation and general guidelines on the reliability of forecasts. We used 31 operating models based on existing stock assessment models applied to a diversity of stocks with empirical data, which we show to be generally representative of assessed stocks worldwide. Although no single best‐performing method could be identified, we found that time‐series methods were most likely to perform poorly. Both forecast skill across all methods and forecast sensitivity to the selected method were linked to the properties of the stock or assessment: age at maturity and recruitment autocorrelation in 3‐year forecasts and previous long‐term recruitment variability in 10‐year forecasts. In some situations, all forecasting methods resulted in systematic over‐ or underestimation of spawning stock biomass. The simulation approach employed here to assess forecast performance, rooted directly in the predictions of existing stock assessment models, can be a complementary tool to existing simulation approaches which generate alternative sets of population dynamics or observations and we discussed the advantages and limitations.

Ecosystem change as a driver of fish recruitment dynamics: A case study of two Lake Erie yellow perch populations

Abstract Although previous research has identified ways in which environmental conditions influence population processes in many ecosystems, these relationships often weaken or break down with the inclusion of additional years of data, for reasons that remain unclear. To better understand if and how the relationships between historically important environmental drivers of yellow perch (Perca flavescens) recruitment varied during 1969–2018 in two Lake Erie basins characterised by contrasting productivity, we used dynamic linear modelling, which allows for nonstationary relationships between predictor and response variables to be quantified. Our analyses revealed that abiotic conditions during egg and larval development (i.e. winter temperature, springtime river inflows, and thermal conditions) were important in the productive west basin, whereas a combination of abiotic (i.e. winter temperature and river inflows) and biotic (i.e. small‐bodied and large‐bodied predators) conditions were important in the less‐productive central basin. While the importance of most environmental factors to yellow perch recruitment remained static through time, the importance of river inflows changed through time in the west and central basins. Large‐bodied predators (i.e. walleye, Sander vitreus) also varied in their importance through time in the central basin. These changes in recruitment drivers were independent of temporal changes to the environmental conditions themselves, coinciding instead with the timing of major shifts in Lake Erie's nutrient status. Our results demonstrate that ecosystem change can cause the relationships between environmental conditions and population processes (e.g. recruitment) of aquatic organisms to vary through time and space. They also highlight the value of periodically revisiting their applicability, especially if these relationships are used to help manage or sustain populations in ecosystems experiencing human‐driven environmental change.

Synergistic Effects of Rooted Aquatic Vegetation and Drift Wrack on Ecosystem Multifunctionality

Ecosystem multifunctionality is an increasingly popular concept used to approximate multifaceted ecosystem functioning, which in turn may help advance ecosystem-based management. However, while experimental studies have shown a positive effect of diversity on multifunctionality, observational studies from natural systems—particularly aquatic—are scarce. Here, we tested the relative importance of species richness and cover of rooted aquatic vegetation, as well as cover of the loose-lying form of the macroalgae bladderwrack (Fucus vesiculosus), for ecosystem multifunctionality in shallow bays along the western Baltic Sea coast. We estimated multifunctionality based on four indicators of functions that support ecosystem services: recruitment of large predatory fish, grazer biomass, inverted ‘nuisance’ algal biomass, and water clarity. Piecewise path analysis showed that multifunctionality was driven by high cover of rooted aquatic vegetation and bladderwrack, particularly when the two co-occurred. This synergistic effect was nearly three times as strong as a negative effect of land-derived nitrogen loading. Species richness of aquatic vegetation indirectly benefitted multifunctionality by increasing vegetation cover. Meanwhile, high bladderwrack cover tended to decrease vegetation species richness, indicating that bladderwrack has both positive and negative effects on multifunctionality. We conclude that managing for dense and diverse vegetation assemblages may mitigate effects of anthropogenic pressures (for example, eutrophication) and support healthy coastal ecosystems that provide a range of benefits. To balance the exploitation of coastal ecosystems and maintain their multiple processes and services, management therefore needs to go beyond estimation of vegetation cover and consider the diversity and functional types of aquatic vegetation.

Zooplankton ecological traits maximize the trophic transfer efficiency of the Humboldt Current upwelling system

Calanid copepods of the genera Calanus and Calanoides are key components of zooplankton communities in upwelling systems. Here, we compare the life-history traits of Calanus chilensis from the Humboldt Current Systems (HCS) off northern Peru and its counterpart Calanoides natalis from the northern Benguela Current System (BCS) off Namibia. A comprehensive data set of the distribution and abundance patterns of these species along extensive horizontal and vertical scales is presented. C. chilensis from the HCS was almost exclusively restricted to the surface layer (50–0 m) above the oxygen minimum zone (OMZ), whereas C. natalis from the BCS inhabited the entire water column down to 800 m performing ontogenetic vertical migration (OVM) through the OMZ. Resting stages of C. natalis at depth accumulated high amounts of lipid (30–60% of dry mass, DM), whereas C. chilensis did not rely on lipid reserves. These findings confirm that the life cycle of C. chilensis does not include OVM with diapause at depth. Surprisingly, the regional distribution of C. chilensis secondary production extended much further offshore (>200 km from the coast) than is typical of other coastal upwelling systems. Deviating environmental conditions forced the two key calanid species to develop specific, but different life strategies for HCS and BCS. Compacted biomass concentrations of C. chilensis in the surface layer from the shelf (≤3 g DM m−2) to offshore waters (≤1.5 g DM m−2) facilitate easy and efficient foraging by predators such as juvenile Peruvian anchovies. In contrast, a large fraction of the C. natalis biomass occurs within the OMZ and is thus out of reach for hypoxia-sensitive predators. Calanoid copepods (e.g. C. chilensis) play a crucial role as important prey for growth and recruitment of small pelagic fish. Thus, the compacted biomass and high productivity of C. chilensis at the surface derived from its adaptive life-history traits (no OVM) may explain the superior trophic transfer efficiency and hence enormous fisheries yield of the HCS compared to the BCS.

Contrasting links between growth and survival in the early life stages of two flatfish species

Understanding the causes of variation in recruitment of marine fishes has been a central goal over the last century. However, the relationship between year class strength and larval growth and survival is poorly known for most species. The main goal of this study was to investigate the relationships between larval traits and year class strength of two flatfish species in an nursery estuary. We examined the effects of the variation in the early life characteristics (pelagic larval growth, pelagic larval duration, size-at-hatching and size-at-settlement) on post-settlement survival of flounder Platichthys flesus and common sole Solea solea in the Mondego estuary (Portugal) over a 5-year period (2011–2015). The two flatfish species showed distinct larval traits variation during the study period, mainly related with location of the spawning ground and intrinsic biologic patterns. Flounder was able to delay metamorphosis while gathering energy and/or waiting for suitable environmental cues from estuarine origins while sole take advantage of better environmental conditions due to a later hatching. Early fast growth were strongly related with subsequent growth, affecting larval survival. Both growth and larval traits variation during the studied period were strongly related with temperature. This study highlights the importance of early growth for fish survival and contributes with key information on fish early life stages that will support a better management of fish stocks.

Assessment of ecological status of the lagoon of Bora-Bora Island (French Polynesia)

In the South Pacific, an important island has been forgotten and neglected from monitoring and research programs on marine biodiversity: Bora-Bora, often called the ‘Pearl of the Pacific’. To fill this gap in our knowledge, we have analyzed the only two datasets available to date on the biodiversity of Bora-Bora’s reefs: Reef Checks’ program, conducted on 18 sites inside the lagoon in 2006 and 2018; and Biodiversity’s monitoring program, conducted on 31 lagoon sites in 2019. Both monitoring programs showed that the corals in Bora-Bora lagoon are in good health. The living coral cover has been stable on all recorded geomorphological zones between 2006 and 2019. In contrast, there was a strong decrease in the density of all macro-invertebrate species recorded (i.e. giant clams, sea-urchins, sea-cucumbers, sea-snails). For giant clams, three human-related causes of extraction were identified: collecting activities of tourists, local inhabitants and occasional major predation by large groups of people visiting from other islands for local festivities. Lastly, both monitoring programs showed that all commercial families (i.e. Acanthuridae, Scaridae, Serranidae) increased in density over time. This may be because the larval recruitment of fish seems to be sufficient to offset the loss of adults from local fishing. Overall, our monitoring programs, conducted on coral, macro-invertebrates and fish, suggest that the luxury tourism industry developed at Bora-Bora contributed to the maximization of the economic and social benefits of coral reefs while preserving Bora-Bora’s natural sites, ecosystems and the socio-cultural balance.

New perspectives on the role of cleaning symbiosis in the possible transmission of fish diseases

For the last seven decades, cleaning symbiosis in the marine environment has been a research field of intrigue. There is substantial evidence that, by removing undesired items from their client fishes, cleaner organisms have positive ecosystem effects. These include increased fish recruitment, abundance and enhanced fish growth. However, the intimate association and high frequency of interactions between cleaners and clients potentially facilitates pathogen transmission and disease spread. In this review, we identify knowledge gaps and develop novel hypotheses on the interrelationship between parasites, hosts and the environment (disease triangle concept), with a particular emphasis on the potential role of cleaner organisms as hosts and/or transmitters of parasites. Despite evidence supporting the positive effects of cleaner organisms, we propose the cleaners as transmitters hypothesis; that some parasites may benefit from facilitated transmission to cleaners during cleaning interactions, or may use cleaner organisms as transmitters to infect a wider diversity and number of hosts. This cost of cleaning interactions has not been previously accounted for in cleaning theory. We also propose the parasite hotspot hypothesis; that parasite infection pressure may be higher around cleaning stations, thus presenting a conundrum for the infected client with respect to cleaning frequency and duration. The impact of a changing environment, particularly climate stressors on cleaners’ performance and clients’ cleaning demand are only beginning to be explored. It can be expected that cleaners, hosts/clients, and parasites will be impacted in different ways by anthropogenic changes which may disrupt the long-term stability of cleaning symbiosis.

Effects of point source discharges on common bully (Gobiomorphus cotidianus) along the Waikato River, New Zealand

ABSTRACT The common bully (Gobiomorphus cotidianus) was used as a bioindicator to assess the impacts of geothermal, bleached kraft mill effluent (BKME), sewage, and thermal discharges in the Waikato River, Northern New Zealand. A range of morphological and physiological parameters were measured to assess fitness in fish sampled upstream and downstream from these point sources. Isotopic signatures gave evidence that fish were resident at sites and showed a gradient corresponding to site-specific sources of carbon and nitrogen along the river. There were changes in the size range and age structure of the populations but, no physiological responses were detected. Limited impacts were observed on common bully health at individual discharge sites, or downstream where the river showed gradual deterioration in water quality, particularly nutrient enrichment, and increased turbidity. Despite the gradual deterioration in water quality no concomitant cumulative impacts were observed in common bully. Responses were largely local in nature, responding to point-source discharges. Population responses at the geothermal and BKME discharge sites warrant further study to investigate possible effects on fish recruitment.

Effect of water level and water quality on small‐sized and juvenile fish assemblages in the littoral zones of the Zambezi/Chobe floodplain

AbstractThe effect of flooding phase and water quality on the littoral fish species of the Zambezi/Chobe floodplain was assessed between March 2017 and February 2018. Using the similarity percentage analyses (SIMPER), a 61.9% dissimilarity was detected in fish assemblages among the four flooding phases of the Zambezi/Chobe floodplain (rising, peak, recession and low flooding phase). Cichlids (63.1%) were the most dominant taxa during the peak flooding phase while cyprinids were dominant during the rising (40.7%), recession (45.3%) and low flooding phases (40.9%), respectively. Juveniles of the two commercially important species, Oreochromis andersonii and Coptodon rendalli, were among the four abundant species across the four flooding phases of the Zambezi/Chobe floodplain. Higher densities of these two species were in synchrony with the peak flooding phase, characterised by high levels of dissolved oxygen (5.7 ± 1.0 mg/L), neutral pH (7.9) and moderate water temperature (27.6 ± 1.1°C). During the flood recession phase, a severe decline in juvenile of the commercially important species (O. andersonii and C. rendalli) coincided with fish migration off the littoral zone into the main river at a mean size between 70 and 80 mm SL. Generally, the observed variations in the fish species composition during the different stages of flooding might reflect the influence of water quality and water volume to sustain large fish assemblages on the littoral zone of the Zambezi/Chobe floodplain. Any change in the natural flow and reduction in water volume of the Zambezi/Chobe floodplain might negatively affect the recruitment of juvenile fish into adult populations.

Characterizing Fish–Habitat Associations through Paired Fisheries‐Independent and In Situ Habitat Assessments

AbstractEstuarine habitat has a significant influence on the growth, recruitment, and survival of fish and invertebrate species, and understanding habitat dynamics can improve fisheries management. However, the cost and effort associated with habitat monitoring can be limiting, and many large‐scale fisheries‐independent monitoring programs do not capture habitat and biological data concurrently. Consequently, fisheries and their associated habitats are often managed separately. We designed a pilot study to assess the utility and effectiveness of incorporating rapid in situ habitat characterization into a statewide fisheries management program. The occurrence of six focal species (Red Drum Sciaenops ocellatus, Spotted Seatrout Cynoscion nebulosus, blue crab Callinectes sapidus, Atlantic Croaker Micropogonias undulatus, brown shrimp Farfantepenaeus aztecus, and white shrimp Litopenaeus setiferus) and the overall faunal community composition were compared across estuarine habitats to assess the ability of this method to accurately depict fish–habitat associations. Focal species’ occurrence and community composition was significantly correlated with shoreline vegetation, seagrass presence, presence of mangroves, substrate type, and heterogeneity in vegetated habitat. This study demonstrates an effective approach to integrating habitat into fisheries‐independent monitoring in pursuit of an ecosystem‐based approach to fisheries management.

Reduced Reproductive Success of Western Baltic Herring (Clupea harengus) as a Response to Warming Winters

Shallow estuaries, bays, and lagoons are generally considered hot spots of ocean productivity that often adjust rapidly to seasonal variations in atmospheric temperatures. During spring when biological reproductive processes begin in the temperate zones, regional climate variability can be immense and uncovering a non-linear biological response, such as fish recruitment to changing temperature regimes might be challenging. Using herring as a paradigm for a response of coastal spring productivity to regional climate drivers, we demonstrated how the annual timing of spawning periods can significantly affect the reproductive success of spring-spawning herring (Clupea harengus) in the western Baltic Sea. An investigation of spawning phenology in consecutive years indicated a temperature threshold range of 3.5–4.5°C triggering initial spawning in the coastal zone. Based on this finding, we analyzed the timing of larval hatching peaks, larval survival and recruitment to the adult population relative to multi-decadal time-series of seasonal sea-surface temperatures. The results revealed that the late seasonal onset of cold periods the corresponding elongation of the period where larvae hatch from the eggs and early larval hatching peaks significantly reduced larval production in a coastal nursery area and finally lead to a reduced abundance of juveniles in the entire distribution area. Using a combination of field research and time series analysis, we presented precedence for shifting regional winter regimes providing a present-day stressor to reproductive capacity of a central component of the coastal food web.

Impact of environmental changes on the fishery of motorized and non-motorized sub-sectors of the upwelling zone of Kerala, southeastern Arabian sea

The marine waters of Kerala state along the southwest coast of India are among the most productive upwelling regions in the southeastern Arabian Sea. Analyses were made to assess the impact of environmental changes on the marine fish catch using motorized and non-motorized craft operating off the coast. The catch per unit effort (CPUE) of total landings, OBGN (outboard gillnet), OBTN (outboard trawlnet), NM (non-motorized) crafts, as well as the catch of 5 major fish resources Indian mackerel, lesser sardines, Stolephorus spp., scads, and penaeid prawns were obtained. Sea surface temperature (SST), chlorophyll, salinity, rainfall, sea level anomaly, multivariate ENSO index, dipole mode index, local temperature anomaly, degree heating month (DHM), degree cooling month (DCM) and surface currents were used as variables to investigate the relationship between environment and fishery. Principal component analysis was used to select the minimum data set and GAM (Generalized additive model) was used to detect the influence of the environment on the fisheries. The adjusted R2 value of the GAM was 0.68, taking the CPUE of the total landings as the response variable. For the case of the selected fish resources, the adjusted R2 value of the GAM was 0.73 for penaeid prawns, followed by Stolephorus spp. (0.44), scads (0.33), lesser sardines (0.31), and Indian mackerel (0.25). Considering the CPUE as the response variable for the GAM also returned high adjusted R2 values for fishing method (OBGN = 0.59, OBTN = 0.53 and NM = 0.31). SST, chlorophyll, rainfall, salinity, yearly DCM and surface currents all had significant influences on the marine fisheries along the Kerala coast. Lower SST, high salinity and rainfall promote the fisheries of both small pelagics and large pelagic groups like scads. Along with the negative relationship between SST and fishery, the present analyses also indicate a positive relationship with yearly DCM, which clearly indicates that the warming of coastal waters has a negative impact on fisheries along the Kerala coast. From these results, we find that ocean currents can play a crucial role in fish recruitment in the coastal waters of Kerala. Primary production, the key factor for marine fishery production, was found to be controlled by environmental factors like SST and currents. These results point to the vulnerability of the small-scale fisheries sector of the tropical upwelling zone to increasing SST and climate change.

Using a larval growth index to detect the environment-recruitment relationships and its linkage with basin-scale climate variability: A case study for Japanese anchovy (Engraulis japonicus) in the Yellow Sea

Increasing evidence has suggested that climate variability has a vital impact on the stock fluctuation of many small pelagic fishes by regulating their recruitment. However, most recruitment-related studies have focused on large spawning/nursing grounds at annual or seasonal scales, while few have considered the impact of short-term environmental variations during the vulnerable passive-transport period. In this study, a larval growth index based on a Lagrangian particle-tracking model coupled with a hydrodynamic model that focused on the early growth and passive transport of the SPF was developed to identify the key spawning/nursing period and grounds accommodating fish recruitment. Taking Japanese anchovy (Engraulis japonicus) in the Yellow Sea as an example, the key spawning grounds and time for its recruitment was identified and the impact of sea-surface temperature (SST) and climate variability on its recruitment was then investigated and discussed. Significant positive (negative) correlations were identified between the recruitment index and the mean growth index (mean water temperature) along the tracking path of eggs or larvae released in the Bohai Sea and the southwestern Yellow Sea during July (1987–2004), implying that these were the key spawning grounds and period for recruitment success. The multi-year tracking path of released particles indicated that both the southwestern Yellow Sea and the central Bohai Sea may be the key nursing grounds. Meanwhile, the SST in July was found to be associated with basin-scale atmosphere–ocean interactions. A potential “atmosphere–ocean-recruitment” influence pattern for Japanese anchovy in the Yellow and Bohai Seas was thus proposed, in which higher (lower) temperature during the key spawning/nursing period induced by atmospheric variation might diminish (promote) recruitment success by limiting the growth and survival of anchovy larvae. The larval growth index developed in this study can be widely applied to other small pelagic fishes in other waters and provide implications for climate-induced recruitment studies.

Spatial and seasonal comparisons of growth of wild and stocked juvenile lake trout in Lake Champlain

After 42 years of stocking in Lake Champlain, recruitment of wild juvenile lake trout (Salvelinus namaycush) was first observed in 2015. Abundance of wild lake trout juveniles was spatially heterogeneous. Recruitment of wild fish to age-1 and subsequent survival are likely related to growth including overwinter growth. We hypothesized that growth potential or growth-related mortality of wild and stocked fish may explain spatial differences in abundance. We collected juvenile (age-0 to 3) lake trout by bottom trawling in the central, north, and south Main Lake every 2–4 weeks during the ice-free season, 2015–2018. The percentage of wild juveniles increased from 27.8% of the total catch in 2015 to 65.7% in 2018. Rates of growth in length and change in condition were compared in wild versus stocked lake trout, among sampling areas, and between seasons (sampling season relative to winter). Wild juveniles grew equally or faster in length than stocked juveniles at the same age, but changed more slowly in condition. There was a higher percentage of wild juveniles in the central sampling area than the north and south, but no differences in growth among sampling areas. Wild and stocked fish grew in length over winter, but most cohorts (6 of 7) maintained or increased condition. Results indicate high growth potential of wild juvenile lake trout and progress toward population restoration.

Human proximity suppresses fish recruitment by altering mangrove-associated odour cues

Human-driven threats to coastal marine communities could potentially affect chemically mediated behaviours that have evolved to facilitate crucial ecological processes. Chemical cues and their importance remain inadequately understood in marine systems, but cues from coastal vegetation can provide sensory information guiding aquatic animals to key resources or habitats. In the tropics, mangroves are a ubiquitous component of healthy coastal ecosystems, associated with a range of habitats from river mouths to coral reefs. Because mangrove leaf litter is a predictable cue to coastal habitats, chemical information from mangrove leaves could provide a source of settlement cues for coastal fishes, drawing larvae towards shallow benthic habitats or inducing settlement. In choice assays, juvenile fishes from the Caribbean (Belize) and Indo-Pacific (Fiji) were attracted to cues from mangroves leaves and were more attracted to cues from mangroves distant from human settlement. In the field, experimental reefs supplemented with mangrove leaves grown away from humans attracted more fish recruits from a greater diversity of species than reefs supplemented with leaves grown near humans. Together, this suggests that human use of coastal areas alters natural chemical cues, negatively affecting the behavioural responses of larval fishes and potentially suppressing recruitment. Overall, our findings highlight the critical links that exist between marine and terrestrial habitats, and the importance of considering these in the broader conservation and management of coastal ecosystems.

Data and R Code for 'Human proximity suppresses fish recruitment by altering mangrove-associated odour cues'

Data and R Code for 'Human proximity suppresses fish recruitment by altering mangrove-associated odour cues'

Using Freshwater Bivalves (Corbicula Fluminea) to Alleviate Harmful Effects of Small-Sized Crucian Carp (Carassius Carassius) on Growth of Submerged Macrophytes during Lake Restoration by Biomanipulation

Increased recruitment of small-sized fish following biomanipulation by reducing the biomass of plankti-benthivorous fish, not least in (sub)tropical lakes, may deteriorate water quality and thereby potentially hamper the recovery of submerged macrophytes. Filter-feeding bivalves remove suspended particles from the water and may, thereby, somewhat or fully counteract this negative effect of the increasing abundance of small-sized fish. So far, only few studies have investigated the interactive effects of fish and bivalves on water clarity and macrophyte growth. We conducted a 2 × 2 factorial designed outdoor mesocosm experiment with two densities of small crucian carp Carassius carassius (low 10 g m−2 and high 40 g m−2) and two densities of bivalves Corbicula fluminea (low 204 g m−2 and high 816 g m−2). We found significant interactive effect of fish and bivalves on the growth of the macrophyte Vallisneria natans. In the low density bivalve regime, the relative growth rates, root mass, root:shoot ratio and number of tubers were 30.3%, 30.8%, 21.6% and 27.8% lower in the high than in the low density fish treatments, while the decrease was less pronounced in the high density bivalve regime: 1.2%, 8.7%, 2.1% and 13.3%, respectively. Thus, bivalves reduced the negative effects of fish, not least when bivalve density was high. The weaker effects of small fish on plants in the high- than in the low-density C. fluminea regime can be attributed to lower total suspended solids (TSS) and Chl a in the first week of the experiment. Better light conditions further stimulated the growth of benthic algae, potentially increasing the removal of nutrients from the water and reducing fish-driven resuspension of the sediment. In addition, high densities of C. fluminea also enriched the sediment total nitrogen (TN) and total phosphorus (TP) content, favouring plant growth as indicated by an increase in leaf tissue TN and TP contents. Our results demonstrate that filter-feeding bivalves can alleviate harmful effects of small fish by prolonging a clear-water state that facilitates submerged macrophyte growth. Addition of the bivalve C. fluminea can be a promising tool for the restoration of submerged macrophytes in shallow eutrophic lakes, in particular lakes containing small, rapidly reproducing fish that due to their small sizes are not capable of controlling the bivalves.

Comparative toxicological effects of planktonic Microcystis and benthic Oscillatoria on zebrafish embryonic development: Implications for cyanobacteria risk assessment.

Planktonic and benthic cyanobacteria blooms are increasing in frequency in recent years. Although many studies have focused on the effects of purified toxins or cyanobacteria extracts on fish developments, the more complex impacts of cyanobacteria cells on fish populations are still considered insufficient. This study compared the toxicological effects of harmful planktonic Microcystis and benthic Oscillatoria on zebrafish (Danio rerio) early stages of development. Zebrafish embryos, at 1-2h post fertilization (hpf), were exposed to 5, 10, and 20×105cells/mL Microcystis (producing microcystins) or Oscillatoria (producing cylindrospermopsins) until 96 hpf. The results indicated that the effects of benthic Oscillatoria on embryonic development of zebrafish were different from those of planktonic Microcystis. Reduced hatching rates, increased mortality, depressed heart rates and elevated malformation rates were observed following exposures to increased concentrations of Microcystis, whilst Oscillatoria exposures only caused yolk sac edemas. Exposure to a high concentration of Microcystis induced severe oxidative damage, growth inhibition and transcriptional downregulations of genes (GH, GHR1, IGF1, IGF1rb) associated with the growth hormone/insulin-like growth factor (GH/IGF) axis. Although Oscillatoria exposure did not affect the body growth, it obviously enhanced the antioxidant enzyme activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) and up-regulated the expressions of several oxidative stress-related genes. Discrepancies in the developmental toxicity caused by Microcystis and Oscillatoria may not only attributed to the different secondary metabolites they secrete, but also to the different adhesion behaviors of algal cells on embryonic chorion. These results suggested that harmful cyanobacteria cells could influence the successful recruitment of fish, while the effects of benthic cyanobacteria should not be ignored. It also highlighted that the necessity for further investigating the ecotoxicity of intact cyanobacterial samples when assessing the risk of cyanobacterial blooms.