Fish Kills Research Articles

Extreme events and ichthyofauna: case report of a neotropical river

The increasing frequency and intensity of extreme weather events and their impact on aquatic ecosystems and fish species is concerning. This manuscript reports and analyzes two events, a drought in 2019 and a flood in 2024, providing valuable insights into the consequences of these events. The rescue of 269 individuals of 19 fish species during the 2019 drought highlights the significant impact of the extreme conditions on the ichthyofauna. Rising temperatures, isolation, and habitat fragmentation must have posed severe challenges for the fish species during this period. On the other hand, the observation of 53 dead individuals of 5 species during the 2024 flood indicates the detrimental effects of this event on the fish population. The association of mortality and dispersal of several species with the existing dam further underscores the complexity of the challenges faced by fish during extreme flood events. The correlation between rainfall, river regime, and the impact of land use and occupation provides an important context for understanding the dynamics of these extreme events. The low rainfall during the 2019 drought and the gradual increase in rainfall between 2022 and 2024 leading to the flood demonstrate the significant role of precipitation patterns in shaping the outcomes for fish species. Additionally, the analysis of land use and occupation, particularly the substantial increase in urbanization in the drainage basin over the last 30 years, highlights the human-induced factors contributing to the stress on aquatic ecosystems. Both extreme floods and droughts have significant implications for fish species. Extreme floods can alter riverbeds, cause direct mortality, and disrupt isolated populations, while extreme droughts can lead to fish kills, and habitat fragmentation, and exacerbate environmental stressors. This analysis emphasizes the urgent need for comprehensive measures to mitigate the impact of extreme weather events on aquatic ecosystems and fish species. Addressing factors such as land use and occupation, water management, and climate change adaptation strategies is crucial for promoting the resilience of fish populations in the face of ongoing environmental challenges. Keywords: drought, Ipanema National Forest, rainfall.

The Effect of Climate Change on Indicator Wetland Insects: Predicting the Current and Future Distribution of Two Giant Water Bugs (Hemiptera: Belostomatidae) in South Korea.

Giant water bugs (Hemiptera: Belostomatidae) are top predators in wetland ecosystems, serving as biological indicators of the health of lentic ecosystems and as effective biological control agents for freshwater snails and mosquitoes. This study aimed to predict the current and future distribution of two Korean giant water bugs, Appasus japonicus and Diplonychus esakii, under three climate change scenarios, contributing to the sustainable management of wetland ecosystems in South Korea. Using MaxEnt models, we employed seven climatic and three non-climatic variables to investigate the habitat preferences and distribution patterns of the species. The results revealed that A. japonicus is likely to experience a northward range contraction due to climate change, while D. esakii is predicted to expand its distribution northward without losing its current range. These responses may lead to occupancy turnover between the two species, potentially driving reassembly in aquatic organism community. Elevation was the primary factor influencing the distribution of A. japonicus, whereas annual mean temperature was the most informative variable for D. esakii, both factors derived under the current climate conditions. These findings suggest that both species are highly sensitive to climate change, with potential range shifts toward higher latitudes and elevations. This study provides insights into how climate change could impact two giant water bugs, thereby supporting future efforts to manage and conserve wetland ecosystems in this country.

INFLUENCE OF MIXOTROPHY ON CELL CYCLE PHASE DURATION AND CORRELATION OF KARLOTOXIN SYNTHESIS WITH LIGHT AND G1 PHASE IN KARLODINIUM VENEFICUM: Influence of mixotrophy on cell cycle and toxicity in Karlodinium veneficum

Karlodinium veneficum forms fish killing blooms in estuaries worldwide. The toxicity of these blooms is variable and thought to be connected to bloom stage and in situ growth rates. Methods for measuring in situ growth rates rely on the assumption that cell cycle progression is phased to the diel photocycle, which is true for phototrophically-growing K. veneficum cultures where G1 phase occurs during light and S and G2 + M phases occur during darkness. However, K. veneficum is a facultative mixotroph that also phagocytizes microalgal prey, and the effects of this mixotrophy on its cell cycle synchrony are unknown. Furthermore, toxicity in laboratory cultures is inversely related to growth rate and is light dependent, suggesting synchrony between the cell cycle (G1 phase) and karlotoxin synthesis. To test this, the cell cycle phase distribution and cellular toxin content for phototrophic and mixotrophic cultures of K. veneficum were monitored hourly for a full diel cycle. The results demonstrated that mixotrophic cultures maintained a synchronized cell cycle, despite increased growth rates. The faster growth rates were attributed to a shortened duration of G1 phase in mixotrophic cultures compared to phototrophic cultures (30.8 ± 9.2 hours vs 69.4 ± 21.5 hours, respectively). Meanwhile, toxin production was observed only during light hours, consistent with synthesis initiating with the photorespiratory byproduct glycolate. Cellular toxin content had a significant positive correlation with the percentage of G1 phase cells and a significant negative correlation with the percentage of S phase cells. These results indicate a clear role in mixotrophy increasing growth rates of K. veneficum and of the diel photocycle in synchronizing the cell and karlotoxin synthetic cycles.

First record of water mite larvae, Hydrachna sp. (Acari, Hydrachnidae) parasitism as quiescent nymphophan on two major aquatic insects of Coleoptera and Hemiptera from West Bengal and Odisha, India

Occurrence of quiescent nymphophan of Hydrachna sp. attaching to the sternites and tergites of thorax and abdomen of water beetle (Hydrophilus sp.) and giant water bug (Lethocerus sp.) is reported for the first time from West Bengal and Odisha of India. After feeding from one to five weeks as a parasitic larva on its host, Hydrachna sp., stops feeding and enters a quiescent nymphophan (nymphochrysalid) stage of development in which the larva squeezes into its exoskeleton and forms a sac-like structure where metamorphosis occurs. By the means of gnathoma, it remains attached to the host body casts off its exoskeleton, and within a short time, the developing nymph can be seen within it. The nymph comes out of a slit in the exoskeleton and assumes a free-living existence. These nymphochrysalids ranged in length from 682.17 to 2112.45?m with lateral stripes adorning their external integuments. Body appeared to be bottle shaped with pointed or rounded posterior end. Preferences of water mites for insect host body parts and seasons, infection intensity and prevalence were reported.

Genetic Attributes and Conservation of an Endangered Giant Water Bug Species, Diplonychus esakii Miyamoto and Lee, 1966 (Hemiptera: Belostomatidae).

Diplonychus esakii, a water bug from the family Belostomatidae, plays an important role in freshwater ecosystems as one of the top predators. In this study, we investigated the genetic diversity and population structure of D. esakii by analyzing 318 specimens across 27 sites in South Korea. We found that the populations of D. esakii possess 11 haplotypes with a haplotype diversity of 0.623. This represents a relatively low level of genetic diversity compared to other known belostomatids and endangered species. AMOVA and FST analyses revealed significant genetic differentiation among populations, with most populations harboring only 1-2 haplotypes, suggesting restricted gene flow between populations and a low level of genetic diversity. This low genetic diversity and limited gene flow suggest a potential vulnerability to environmental changes and an increased risk of extinction, indicating that D. esakii should be designated as a protected species in South Korea as part of future conservation efforts. Based on the results of this study, Upo Wetland, which maintains relatively high levels of genetic diversity and Jeju Island, which, despite its lower genetic diversity compared to the mainland, does not share haplotypes with other regions, should be considered key conservation units for this species. This study highlights the importance of incorporating genetic information into conservation status assessments under the Red List Categories and Criteria and also emphasizes the need to evaluate this species on the Korean Red List. The data provided here will serve as essential baseline information and valuable resources for the development of effective conservation strategies.

Life history of different giant water bug, Appasus japonicus (Belostomatidae: Heteroptera), populations throughout the Japanese Archipelago

AbstractAs temperatures in the Japanese Archipelago vary, species widely distributed in Japan should have different active periods, ecologies, and life histories. Information on developmental zero and effective accumulated temperature is necessary to estimate life history. The giant water bug, Appasus japonicus, found in rice paddies, ponds, and other lentic waters, is distributed throughout Japan except for the Ryukyu Islands, and is a representative taxon in which the male cares for the eggs. In this study, we investigated the life history, including the length of the breeding season, of different A. japonicus populations from Hokkaido, Ibaraki, Hyogo, and Nagasaki. The developmental period, developmental zero, and effective accumulated temperature of the eggs and nymphs showed no significant differences among the populations. The life history for each population was estimated based on climate data. This showed that the Hokkaido population had a shorter and more extremely limited breeding season than the other populations. The estimated number of generations during one breeding season was approximately one in Hokkaido, two in Ibaraki and Hyogo, and three in Nagasaki. The estimated number of times males cared for eggs during the breeding season was less than one in Hokkaido, eight in Ibaraki and Hyogo, and twelve in Nagasaki. Although Belostomatidae may carry multiple egg masses during the breeding season, this study shows that the male of the Hokkaido population does not care for multiple egg masses.

Allelopathic Effect of a Chilean Strain of Karenia selliformis (Gymnodiniales, Dinoflagellata) on Phytoplankton Species.

Blooms of the dinoflagellate Karenia selliformis in Chile, often associated with massive fish kills, have been noted alongside other species from the Kareniaceae family, such as Karenia spp. and Karlodinium spp. However, the potential allelopathy impact of Chilean K. selliformis on other phytoplankton species remains unexplored. Here, we assessed the allelopathic effects of cell-free exudates from a Chilean K. selliformis strain on six phytoplankton strains representing diverse microalgal groups. The findings of these experiments offer valuable insights into the varied responses of both non-toxic and toxic microalgae to allelochemicals produced by a toxic microalga, showcasing the intricate and multifaceted nature of allelopathic interactions in microalgal communities. The study revealed species-dependent effects, with variable response in cell growth, photosynthetic efficiency (i.e., Fv/Fm), and intracellular reactive oxygen species (ROS) production. While certain strains exhibited significant growth inhibition in response to the allelochemicals, others demonstrated no apparent effect on cell proliferation, indicating varying sensitivity to specific allelochemicals or potentially distinct detoxification mechanisms. Similarly, the diverse effects on Fv/Fm highlight the complexity of allelopathic interactions, with some species showing reduced efficiency without alterations in intracellular ROS production, while others displayed increased ROS production alongside impaired photosynthesis.

If a fish comes out of the river and speaks, we should believe it: South African perspective on fish kills

AbstractIn South Africa, fish kill events are increasing in frequency because of multiple stressors associated with managing natural resources. Despite the ecosystem services associated with fish, South Africa's progressive legislation towards environmental protection seems to negate the management of fish kills. In this study, we provide an overview of reports and regulations associated with acute pollution spills resulting in fish kill events in South Africa. In addition, we highlight the implication of these using a fish kill event from 2019 on the Msunduzi River, South Africa, as a case study. The fish kill on the Msunduzi River showed a decline in relative abundance by up to 41%, and its ecological condition was already impaired by poor management. The poor condition of the Msunduzi River has jeopardised the recovery of the fish populations, and intervention is needed to restore the fish population that includes species red‐listed as vulnerable by the International Union for the Conservation of Nature. We found that despite the severity of fish kills and the detriment to the fish fauna, including near‐threatened species, there is a general apathy, incapacity and lack of knowledge on managing fish kills in South Africa. In addition, although several legislated Acts have good intentions for protecting nature, they do not align and show that a fish kill is an afterthought rather than a need to prevent. Poor management practices have exacerbated this in an already stressed state from excessive use of environmental water. As a way forward, the alignment of the various Acts associated with various ministerial departments in South Africa is needed so that better protection of the environment may occur. Finally, we argue that the African proverb ‘If the fish comes out of the river to tell you that the crocodile has one eye, you should believe it’ is not considered when managing water resources and that with the present ecological state and water resource use there may be no fish to ‘come out of the water’ warning us that our water is unsafe even for human consumption and use.

Insights into the nature of ichthyotoxins from the Chrysochromulina leadbeateri blooms in Northern Norwegian fjords

In May–June 2019, the microalga Chrysochromulina leadbeateri caused a massive fish-killing event in several fjords in Northern Norway, resulting in the largest direct impact ever on aquaculture in northern Europe due to toxic algae. Motivated by the fact that no algal toxins have previously been described from C. leadbeateri, we set out to investigate the chemical nature and toxicity of secondary metabolites in extracts of two strains (UIO 393, UIO 394) isolated from the 2019 bloom, as well as one older strain (UIO 035) isolated during a bloom in Northern Norway in 1991. Initial LC–DAD–MS/MS-based molecular networking analysis of the crude MeOH extracts of the cultivated strains showed that their profiles of small organic molecules, including a large number of known lipids, were very similar, suggesting that the same class of toxin(s) were likely the causative agents of the two harmful algal bloom (HAB) events. Next, bioassay-guided fractionation using the RTgill-W1 cell line and metabolomics analysis pointed to a major compound affording [M + H]+ ions at m/z 1399.8333 as a possible toxin, corresponding to a compound with the formula C67H127ClO27. Moreover, our study unveiled a series of minor analogues exhibiting distinct patterns of chlorination and sulfation, together defining a new family of compounds, which we propose to name leadbeaterins. Remarkably, these suspected toxins were detected in situ in samples collected during the 2019 bloom close to Tromsø, thereby consistent with a role in fish kills. The elemental compositions of the putative C. leadbeateri ichthyotoxins strongly indicate them to be long linear polyhydroxylated polyketides, structurally similar to sterolysins reported from a number of dinoflagellates.

The battle against the introduced pathogenic monogenean Gyrodactylus salaris in Norwegian Atlantic salmon rivers and fish farms.

The introduced salmonid ectoparasite Gyrodactylus salaris has been detected on Atlantic salmon in 53 Norwegian rivers and in 39 Norwegian fish farms. In affected rivers, the mortality of Atlantic salmon juveniles is very high, estimated to a mean of 86%. G. salaris has been considered one of the biggest threats to wild Norwegian Atlantic salmon stocks. With various measures, the authorities have reduced the potential for further spread of the parasite to new rivers and fish farms, and G. salaris has been eradicated from 43 rivers and all fish farms. Furthermore, the eradication process is almost completed in five affected rivers located at the Norwegian west coast, while preparations for the eradication in the remaining five rivers in the southeastern part of Norway have begun. The goal of Norwegian management is to eradicate the introduced pathogenic G. salaris strains from all occurrences in Norway. In fish farms, the parasite has been removed by mandatory slaughter of infected fish. In rivers, G. salaris has mostly been removed by killing all the fish hosts with rotenone. The indigenous genetic Atlantic salmon stocks are re-established after eradication of the parasite. New methods are developed using chemicals that kill the parasite without killing fish in the rivers. Norwegian authorities have so far used more than NOK 1.5 billion on research, monitoring and combating G. salaris. However, the benefits are considered many times greater than the spending. Without control measures, G. salaris would likely have spread to new Atlantic salmon rivers where the same catastrophic outcome had to be expected. The Norwegian authorities seem to meet the goal in their long-term work to halt the spread of G. salaris and to eradicate the parasite in affected rivers.

Belostomatidae Leach, 1815 (Insecta: Hemiptera: Heteroptera: Nepomorpha) of northeastern Brazil

Belostomatidae Leach, 1815 (Insecta: Hemiptera: Heteroptera: Nepomorpha), also known as giant water bugs, is a family with 11 genera and about 160 described species, most of which are recorded from the Neotropical region. Knowledge about these bugs in northeastern Brazil is relatively poor, with 16 previously recorded species. Here, we present new records for five additional species based on material from the states of Ceará, Maranhão, and Piauí deposited in the Coleção Zoológica do Maranhão, Caxias, Brazil. This increases to 21 the number of species recorded from the region. In addition, we provide photographs, distribution maps, and a key to the fauna of Belostomatidae from northeastern Brazil.

Spatial and Temporal Changes in the Fish Fauna of a Low-Inflow Estuary following a Mass Mortality Event and Natural and Artificial Bar Breaches

Estuaries that become closed from the ocean through the formation of a sand bar are particularly susceptible to degradation and often experience poor water quality and fish kills. Sampling was conducted seasonally for two years in the deeper waters of the Vasse–Wonnerup (southwestern Australia) after a large fish mortality event to identify the fish species present and determine if richness, abundance, diversity and faunal composition differed among regions and over time. Species richness, abundance and diversity were far greater in the downstream regions than in the upstream regions, reflecting patterns in salinity. Catch rates of two marine estuarine-opportunist mugilids (Mugil cephalus and Aldrichetta forsteri) and an estuarine-resident sparid (Acanthopagrus butcheri), which were the species most affected by the mortality event, were relatively stable. It is suggested that before sampling commenced, mugilids had entered the estuary following an artificial sand bar breach, while the loss of 0+ (new recruits) A. butcheri in the mortality event and subsequent recruitment failure prevented stocks from increasing. Temporal shifts in fish composition were driven by less abundant species, which utilized the estuary more opportunistically. Increases in the occurrences and abundances of these species coincided with an open bar and salinities close to those of seawater. The data also show how the estuary responds to differing hydrodynamic phases and artificial breaches.

Perspectives of fishers on the incident of fish kills in Rivers State Coastal Waters, Nigeria

This study was conducted to assess fishermen's perspectives on the massive fish kill incident in coastal waters in Rivers State, Nigeria. Primary data were collected using questionnaires from 101 randomly selected fishermen and the data were evaluated using descriptive statistics. The results showed that fishing was the main occupation of the respondents and the interviewed fishermen have fishing experience in the region studied and knowledge of their fishery. The results showed that 53.3 percent of fishermen attributed the strange phenomenon to pollution, while 19.8 percent cited infection and disease as the cause of the fish kills. The majority of respondents (94%) reported that croaker fish species were particularly affected in the study area. Other observations include fish swimming on the surface or jumping onto shore and affected fish washing up on shore. The unusual appearances of the dead fish commonly reported by respondents were as follows: fin rot, open mouth, disease spots and deep blue gills. About half of the respondents (50.5%) gave a range of 1,000 - 5000 dead fish and a size class of the dead fish of more than 30 cm. Most respondents (79.2%) indicated that the government has not taken any action. Few of those surveyed reported government action on sea clean-up exercises to remove dead fish and end oil spillage and bunkering as measures put in place by the government. The study recommended partnerships between the government and fishermen and fishermen's groups as stakeholders are required to address the concerns about marine waters.Keyword: Coastal waters; Fishers; Mass fish mortality; Nigeria

Effects of dietary garlic (Allium sativum) oil on growth performance, haemato-biochemical and histopathology of cypermethrin-intoxicated Nile tilapia (Oreochromis niloticus).

When pesticides are introduced into wetlands by agriculture, fish quickly absorb them through their gills. Pesticides reduce hatchability, impede growth, and antioxidant response, killing fish. Therefore, it's crucial to find effective pesticide mitigation methods for fish. In this study, the effects of garlic (Allium sativum) oil on the growth, haematology, biochemistry and histopathology parameters of Nile tilapia (Oreochromis niloticus) exposed to cypermethrin toxicity were investigated. In the research, cypermethrin was added to the water of the experimental groups at a rate of 1:20 of the LC50 value, and 1.00% garlic oil was added to the fish feed. Fish with an initial weight of 30.26 ± 0.26g were fed for 45 days. At the end of feeding, the final weights were determined as 69.39 ± 0.41 (G1), 61.81 ± 0.65 (G2), 82.25 ± 0.36 (G3), and 75.04 ± 0.68 (G4) grams, respectively. Histopathological examinations revealed serious lesions in the gill, liver, brain, and muscle tissues in the cypermethrin group, whereas these lesions were minimal or absent in the garlic oil group. Garlic oil supplementation had positive effects on growth, haematology, blood biochemistry, hepatosomatic index and histopathological parameters. These findings suggest that garlic oil is a potential protective agent against cypermethrin toxicity.

Evaluation and Refinement of Chlorophyll-a Algorithms for High-Biomass Blooms in San Francisco Bay (USA)

A massive bloom of the raphidophyte Heterosigma akashiwo occurred in summer 2022 in San Francisco Bay, causing widespread ecological impacts including events of low dissolved oxygen and mass fish kills. The rapidly evolving bloom required equally rapid management response, leading to the use of near-real-time image analysis of chlorophyll from the Ocean and Land Colour Instrument (OLCI) aboard Sentinel-3. Standard algorithms failed to adequately capture the bloom, signifying a need to refine a two-band algorithm developed for coastal and inland waters that relates the red-edge part of the remote sensing reflectance spectrum to chlorophyll. While the bloom was the initial motivation for optimizing this algorithm, an extensive dataset of in-water validation measurements from both bloom and non-bloom periods was used to evaluate performance over a range of concentrations and community composition. The modified red-edge algorithm with a simplified atmospheric correction scheme outperformed existing standard products across diverse conditions, and given the modest computational requirements, was found suitable for operational use and near-real-time product generation. The final version of the algorithm successfully minimizes error for non-bloom periods when chlorophyll a is typically <30 mg m−3, while also capturing bloom periods of >100 mg m−3 chlorophyll a.

Variaciones fisicoquímicas del agua de una gran presa Hidroeléctrica Mexicana, Región Centro semi-árida

The Zimapan hydropower plant built in 1995 was the first project to take advantage of Mexico's City wastewater for electric power generation. This tropical reservoir receives pollutants that affect water quality from municipal, agricultural, and industrial wastewater through the Tula and San Juan rivers. The study considered the changes in the physicochemical parameters (surface to 20 m depth) for May (wet warm), November (wet cold-after rains), and February (dry cold) in seven dam sites: two lotic, two transitional, and three lentic sites. Mean values ranges were temperature 20.71-24.38 °C, 1.59-7.56 mg/L dissolved oxygen, Secchi disk transparency 2.26-3.98 m, pH 7.12-8.40, electrical conductivity 48-1334 µS/cm, total nitrogen 14.20-30.74 mg/L, total phosphorus 10.45-15.83 mg/L, and dissolved organic carbon 9.11-20.63 mg/L. In November, all the reservoir surface water was hypoxic due to increased dissolved organic carbon by river input. Even though dissolved oxygen has a seasonal variation, this reservoir has a sustainable fishery of tilapia and black bass (~400 tons/y). The dam is meromictic and hypereutrophic, with seasonal water quality variations with no significant spatial variations. Water quality variation observed in this study can be useful to Mexican decision-makers on water and fisheries management, prevent massive fish kills, and be a support guide for commercial and sport fishers.

Ecological Implications on Aquatic Food Webs Due to Effects of Pesticides on Invertebrate Predators in a Neotropical Region.

Predation presents specific behavioral characteristics for each species, and the interaction between prey and predator influences the structuring of the food web. Concerning insects, predation can be affected in different ways, such as exposure to chemical stressors, e.g., pesticides. Therefore, analyses were carried out of the effects of exposure to insecticide fipronil and the herbicide 2,4-D on predation, parameters of food selectivity, and the swimming behavior of two neotropical predatory aquatic insects of the families Belostomatidae (giant water bugs) and larvae of Libellulidae (dragonfly). These predatory insects were exposed for 24 h to a commercial formulation of the chlorophenoxy herbicide, 2,4-D at nominal concentrations of 200, 300, 700, and 1400μgL-1, and to a commercial formulation of the phenylpyrazole insecticide, fipronil at nominal concentrations of 10, 70, 140, and 250µgL-1. In a control treatment, the insects were placed in clean, unspiked water. At the end of the exposure, the maximum swimming speed of the predators was evaluated. Afterward, the predators were placed in clean water in a shared environment for 24 h with several prey species, including the cladoceran Ceriodaphnia silvestrii, larvae of the insect Chironomus sancticaroli, the amphipod Hyalella meinerti, the ostracod Strandesia trispinosa, and the oligochaete Allonais inaequalis for 24 h. After this period, the consumed prey was counted. The results reveal that predators from both families changed prey consumption compared with organisms from the control treatment, marked by a decrease after exposure to fipronil and an increase in consumption caused by 2,4-D. In addition, there were changes in the food preferences of both predators, especially when exposed to the insecticide. Exposure to fipronil decreased the swimming speed of Belostomatidae individuals, possibly due to its neurotoxic effect. Exposure to the insecticide and the herbicide altered prey intake by predators, which could negatively influence the complex prey-predator relationship and the functioning of aquatic ecosystems in contaminated areas.

Recovery of a Population of Slimy Sculpin (Uranidea cognata) after an Autumn Fish Kill in the Headwaters of a Minnesota Trout Stream

Toxic runoff from heavy rains on 26 September 2019 caused a complete fish kill on the 2 km long headwater reach of Garvin Brook, Winona County, Minnesota, USA. This project examined the recovery of the slimy sculpin (Uranidea cognata) population within the lower 900 m of the kill zone, comparing relative abundance estimates and size structures between the kill zone and a downstream, unimpacted reference section. Electrofishing surveys were conducted at 24 sites (12 within both kill and reference zones) at 6, 11, 18, 28, and 42 months post-kill to assess relative abundance (catch-per-effort (CPE), fish/min) and population age structures (based on total length). At six months post-kill, sculpin were present throughout the kill zone. However, adult CPE declined significantly with upstream distance, which was suggestive of the ongoing immigration of adult sculpin from downstream. Age structures were similar in kill and reference zones with all size/age groups present in both zones after six months, while CPE was twice as high in the reference (6.3 fish/min) versus the kill zone (3.0 fish/min). After 11 months, CPE did not differ between zones (14 fish/min) and remained that way for the remainder of the study. However, age structures differed dramatically between zones at both 11 and 18 months post-kill; adult sculpin were significantly more abundant in the reference zone, whereas juvenile fish dominated the kill zone. By 28 months post-kill, both sculpin abundance and age structure within the kill zone had fully recovered from the kill. Sculpin recovery was accomplished mostly through enhanced reproduction within the kill zone following initial post-kill movements of both adult and juvenile fish into the kill zone from downstream. Low predation on juvenile sculpin due to a reduced abundance of trout and adult sculpin post-kill likely allowed more rapid recovery of the sculpin population within the kill zone.

Karenia mikimotoi induced adverse impacts on abalone Haliotis discus hannai in Fujian coastal areas, China

Abstract Large-scale outbreaks of the dinoflagellate Karenia mikimotoi caused substantial mortality of abalone, Haliotis discus hannai in Fujian, China in 2012, resulting in 20 billion in economic losses to abalone industries. However, the mechanism behind the mortality, especially the reaction of abalone to this microalgal toxicity, which possibly differed significantly from the former ‘fish killer’ strain in the South China Sea (SCS). Our study revealed that K. mikimotoi FJ-strain exhibited a four-fold higher haemolytic toxicity than the SCS-strain during the late exponential phase. At the microalgal cell density of 3 × 107 cell L−1, the FJ-strain caused abalone mortality of 67% in 48 h, with decreased granulocyte–hyalinocyts ratio and phagocytic activity by 58.96% and 75.64%, respectively, increased haemocyte viability by 4.8-fold and severe gill damage. The toxic effect only worked for the haemolytic toxicity from active algal cells, which were probably produced under the contact of algal cells and abalone gills. However, under exposure to the SCS-strain, more than 80% of individuals survived under aeration. The results indicated that FJ-strain was a new K. mikimotoi ecotype with stronger toxicity. It evoked severe effects, with complete abalone mortality within 24 h under the cascading effect of non-aeration (dissolved oxygen declined to 2.0 mg L−1), when exposed to K. mikimotoi FJ-strain at the above density. Thus, apart from the microalgal toxicity, DO depletion exacerbated the mortality of abalone in the experiment. The massive abalone mortalities in Fujian were probably caused by the combination of microalgal toxic effects and oxygen depletion, leading to immunological depression and histopathological disruption.

Distributional and behavioral responses of the wintering Oriental Storks to drought in China's largest freshwater lake

Extreme droughts are increasing in frequency and severity globally as a result of climate change. Developing understanding of species' responses to drought is crucial for their conservation, especially in regions experiencing increased aridity. Although numerous studies have investigated birds' responses to drought, the emphasis has primarily been on landbirds. Drought can significantly alter the wetland environments that waterbirds inhabit, but the response of waterbirds to drought remains understudied. In this study, we surveyed the distribution and behavior of Oriental Storks (Ciconia boyciana) in Poyang Lake, which is the largest freshwater lake in China. Results indicate that drought-induced catchment areas at the lowest water level limited the total population size of Oriental Storks in the sub-lakes. Sub-lakes with large catchment areas at the lowest water level demonstrated a capacity to support a larger population of wintering Oriental Storks. Over time, Oriental Storks exhibited a gradual concentration in Changhu Lake, characterized by larger catchments, after resource depletion in sub-lakes with smaller catchments. Additionally, the duration of Oriental Storks’ vigilance and moving behaviors decreased significantly compared with that observed before the drought. After the drought, Oriental Storks increased their foraging efforts, as evidenced by increased presence in deeper water and reaching their heads and necks into deeper water to forage, higher search rates, but lower foraging rates. In accordance with area-restricted search theory, reductions in habitat quality resulting from drought, including extensive fish die-offs, forced Oriental Storks to increase their foraging efforts. Sustaining a specific water area in sub-lakes during droughts can preserve resource availability, which is crucial for the conservation of Oriental Storks. Implementing measures such as water level control and micro-modification of lake bottoms in sub-lakes might mitigate the impact of drought on the piscivorous Oriental Storks.