Fishery-aspects of eutrophication

Information on possible effects of pollution on fish life is reviewed with special emphasis to the North Sea. Concentrations of heavy metals are high in onshore waters, sediments and organisms. Offshore regions are considered to be not endangered. In estuaries concentrations of certain heavy metals are often surpassing thresholds, which have been experimentally demonstrated to produce sublethal effects on marine organisms. Also mussel culture areas had to be closed due to excess of cadmium contamination. Mercury in fillets of cod in the southern North Sea sometimes reaches 0.3 ppm representing 1/3 of the legal limit. Reductions of inputs are necessary due to the bioaccumulative and toxic potential of heavy metals. Organochlorine contamination of water, sediments and organisms is higher in the southern than in the northern North Sea. In offshore areas clearly elevated levels of PCB's, which are the dominating organochlorine compound in marine organisms, have been analyzed. At present only a minor fraction of total organochlorine residues accumulated in marine biota is analyzed. Margins between experimentally derived lethal levels and those occurring in marine biota are narrow suggesting that sublethal effects might occur. One of the special concerns in the context of organochlorine contamination has to be seen in the potential of these substances affecting the reproductive capacity of marine organisms. Reduction measures are urgently necessary and should include substances with bioaccumulative and toxicological potential, which are not presently included in Marine Monitoring Programmes. Impact of oil pollution is considered to be a regional problem, although especially in estuaries concentrations in the water column might be high. Input through oil platforms has led to local reductions in benthic communities. Measures have to be taken, to further reduce these inputs also in view to avoid contamination of fish in the vicinity of oil fields. Oxygen depletion in German and Danish coastal waters has been shown to be a severe degradation in coastal water quality resulting in significant impact on fish populations. Indications for large-scale avoidance reactions of low DO-areas and mortalities of fish and benthos organisms have been demonstrated. It is suspected that the low DO-situations are related to increased nutrients input and high phytoplankton production. Improvement of the situation can only be expected, if input of nutrients is reduced. Reasons for the occurrence of toxic plankton blooms are not understood. An intensified research is necessary, to avoid damage to fisheries and mariculture. Although activity of radionucleids in North Sea waters is increased due to input of radioactive material from La Hague and Sellafield reprocessing units, the present levels of activity are not considered to be a threat for marine life. The same holds for thermal pollution, which is, depending on the recipient area, expected to have only local effects. Sewage sludge dumping and discharge of untreated sewage introduce significant amounts of accompanying heavy metals, which can be expected to have a negative influence on the total metal budget of recipient waters. Nutrient input through sewage will exhibit only local effects. Dumping of wastes from titanium dioxide industry are suspected to be causally linked to the occurrence of increased prevalences of certain diseases of dab in the German Bight. Present management strategies should take account of the fact that through dumping of waste from titanium dioxide production significant quantities of heavy metal are introduced into the southern North Sea. Also the incineration of chemical wastes at sea should be reduced due to uncertainties concerning quantities and toxicity of incineration products and their potential impact on marine life. It is concluded that clear evidence exists for pollution impact in estuarine regions. Most of the major estuaries at the North Sea coast show benthos reductions, which can be related to water quality and it has to be assumed that areas of a sublethal effect are located in the vicinity of those areas of well demonstrated effects. They are of unknown size and attempts have to be undertaken to quantify areas of sublethal impact. The occurrence of fish diseases is interpreted as an expression of degraded water quality. Some of the diseases detected can produce mortality, thereby having an as yet unquantified impact on fish populations. The presence of fish diseases also reduces the marketability of catches. It cannot be excluded that changes in species composition of fish populations in German Wadden Sea areas are related to pollution. Concentrations of organochlorine substances in fishes of the southern North Sea give rise to the concern that their reproductive capability might be impaired. Future work has to be directed to this problem. Impact of pollution on populations of marine organisms has so far not been demonstrated.

The Yangtze River has experienced severe ecological degradation due to intensive human activities, including dam construction, land reclamation, and overfishing. These disturbances have disrupted the natural habitats of the Yangtze River, leading to a sharp decline in fish biodiversity and fishery resources. To address this ecological crisis, the Chinese government implemented a 10-year fishing ban in January 2021 to mitigate pressures on fish populations, restore aquatic habitats, and promote biodiversity recovery. The middle reaches of the Yangtze River are characterized by diverse fish species and a critical habitat for aquatic life, this study seeks to assess the effects of the fishing ban on fish diversity, body structure, population and community dynamics in this region. Fish monitoring data collected from 2017–2019 (pre-ban) and 2021–2023 (post-ban) were analyzed to evaluate changes in fish body size, species diversity, and community structure. The analysis results using the PSD method indicate that fish body size has increased following the fishing ban, suggesting the improvement of population structures, and a change in the complexity of food web structure. Species diversity indices showed partial recovery, but the recovery was uneven across different sampling sites. While fish populations showed signs of improvement, particularly in terms of body size and community stability, species diversity remained at relatively low levels in some areas, indicating that full recovery in biodiversity and resource levels may require extended conservation efforts. These findings suggest that while the fishing ban has had a positive initial impact on fish populations and ecological conditions, continued and long-term conservation measures are essential for fully recovering the river’s biodiversity and restoring its fishery resources. The study also highlights the importance of monitoring fish species diversity, body structure, and community dynamics as part of ongoing efforts to evaluate the effectiveness of the fishing ban and refine resource management strategies for the middle reaches of the Yangtze River.

Model-based versus model-free feeding control and water-quality monitoring for fish-growth tracking in aquaculture systems

Higher temperatures exacerbate drought conditions by increasing evaporation rates, reducing soil moisture and altering precipitation patterns. As global temperatures rise as a result of climate change, these effects intensify, leading to more frequent and severe droughts. This link between higher temperatures and drought is particularly evident in sensitive ecosystems like the Amazon rainforest, where reduced rainfall and higher evaporation rates result in significantly lower water levels, threatening biodiversity and human livelihoods. As an example, the serious drought experienced in the Amazon basin in 2023 resulted in a significant decline in fish populations. Elevated water temperatures, reaching up to 38°C, led to mass mortality events, because these temperatures surpass the thermal tolerance of many Amazonian fish species. We know this because our group has collected data on critical thermal maxima (CTmax) for various fish species over multiple years. Additionally, warmer waters can cause hypoxia, further exacerbating fish mortality. Thus, even Amazon fish species, which have relatively high thermal tolerance, are being impacted by climate change. The Amazon drought experienced in 2023 underscores the urgent need for climate action to mitigate the devastating effects on Amazonian biodiversity. The fact that we have been able to link fish mortality events to data on the thermal tolerance of fishes emphasizes the important role of experimental biology in elucidating the mechanisms behind these events, a link that we aim to highlight in this Perspective.

An invasive species is defined as an alien (or introduced or non-native) species whose establishment and spread threaten ecosystems, habitats or species with harm. Such threats to UK lake fish communities have long been appreciated and this review assembles case histories, including new data, from the largest lakes of Scotland, Northern Ireland, Wales and England to examine the hypothesis that at least some of these introductions have become invasive. Loch Lomond in Scotland has experienced six introductions [chub (Leuciscus cephalus), common bream (Abramis brama), crucian carp (Carassius carassius), dace (Leuciscus leuciscus), gudgeon (Gobio gobio) and ruffe (Gymnocephalus cernuus)], of which the most significant has been that of the percid ruffe, which has been implicated in a recent decline of the native coregonid whitefish (Coregonus lavaretus). In Northern Ireland, the introduction of the cyprinid roach (Rutilus rutilus) to Lough Neagh has apparently had a negative impact on some overwintering waterfowl, although the native coregonid pollan (Coregonus autumnalis) remains abundant. Llyn Tegid in Wales has received three introductions [rudd (Scardinius erythrophthalmus), ruffe and silver bream (Blicca bjoerkna)], although no impacts on the native whitefish or other fish populations have been observed. In England, individuals of at least 12 native and non-native fish species have been brought to Windermere for the purpose of live-baiting, although only those of the cyprinids roach and common bream have established abundant populations. At the same time, the native salmonid Arctic charr (Salvelinus alpinus) has declined markedly while the native esocid pike (Esox lucius) has shown changes in abundance, distribution and individual condition, although these developments have not been shown to be causally linked. None of these introductions were sanctioned by appropriate fisheries or other regulatory bodies and almost all of them probably arose from the release or escape of live-bait used by pike anglers. Of the 10 species introductions documented here, four (common bream, gudgeon, roach and ruffe) have established abundant populations and two of these (roach and ruffe) have apparently caused or currently threaten harm, supporting the hypothesis that at least some of these introductions have become invasive.

Evaluating the cascade dam construction effects on endemic fish habitat and population status in spawning sites of Lancang River (in Tibet), China

The population dynamics of fishes are strongly affected by habitat characteristics that are constantly changing due to seasonal and interannual variations in climatic, hydrological and geographical processes. Seasonal and interannual variations in hydrological events directly affect the movement, growth and mortality of stream fishes, and indirectly affect their population density and standing crop through changes in local‐scale habitat structures. Characteristics of local‐scale habitats directly affect availability of foods, shelters, spawning sites and interspecific interactions of stream fishes. Therefore, long‐term data regarding the local‐scale dynamics of habitat characteristics and their use by fish species over a few generations or longer would be needed for understanding the mechanism of the determinant process of fish population dynamics. When investigating the effects of habitat changes due to natural processes on the long‐term dynamics of fish populations, removal of artificial influences such as changes in land use, dam constructions and channelization of riverines is often problematic. Thereby, sampling sites where long‐term observation of nonartificial changes in stream fish populations and habitats are very valuable. Here, I report the long‐term seasonal and annual changes in stream fish populations and their habitat characteristics based on 75 snorkeling observations of fishes and environmental measurements in the upper reaches of the Yura River at Ashiu research forest, which has a 36.5 km2 catchment area with very little artificial disturbance, from May 2007 to June 2018. Snorkeling observations were conducted using the line‐transect method during the day and at night, and microhabitat characteristics (water depth, current velocity, substratum characteristics and the presence/absence of cover) were determined for all individuals observed. A total of 56,042 individuals were observed, belonging to 17 fish and 7 other aquatic vertebrate species. These data may be used to examine the effects of habitat change on the processes that determine fish populations.

Impacts of mesomycetozoean parasites on amphibian and freshwater fish populations

Catch equations: restoring the missing terms in the nominally generalized Baranov catch equation

Introduction to Water Use From Arctic Lakes: Identification, Impacts, and Decision Support¹

Shallow coastal and estuarine habitats play an essential role in the life cycles of many fish species, providing spawning, nursery, feeding, and migration areas. However, these ecologically valuable habitats are increasingly threatened by anthropogenic activities, causing substantial changes in both habitat availability and quality. Fish species use these shallow coastal habitats and estuaries during various life stages, leading to their categorization into guilds based on how and when they rely on these areas. This differential functional use of estuaries means that changes to these habitats may affect each guild differently. To understand the impact of estuarine habitat degradation on fish populations, it is therefore necessary to consider the full life cycle of fish and when they rely on these coastal habitats. Here, we use conceptual size-structured population models to study how estuarine habitat degradation affects two functionally different guilds. We use these models to predict how reduced food productivity in the estuary affects the demographic rates and population dynamics of these groups. Specifically, we model estuarine residents, which complete their entire life cycle in estuaries, and marine estuarine-dependent species, which inhabit estuaries during early life before transitioning offshore. We find that total fish biomass for both guilds decreases with decreasing food productivity. However, the density of juveniles of the marine estuarine-dependent guild can, under certain conditions, increase in the estuary. This occurs due to a shift in the population biomass distribution over different life stages and a simultaneous shift in which life stage is most limited by food. At the individual level, somatic growth of juveniles belonging to the estuarine-dependent guild decreased with lower food supply in the estuary, due to increased competition for food. The somatic growth rates of fish belonging to the resident guild were largely unaffected by low food supply, as the total fish density decreased at the same time and therefore the per-capita food availability was similar. These outcomes challenge the assumption that responses to habitat degradation are similar between fish guilds. Our study highlights the need to assess not only fish biomass but also size distributions, survival, and somatic growth rates for a comprehensive understanding of the effects of habitat degradation on fish populations. This understanding is crucial not only for estuary fish communities but also for successful conservation and management of commercially harvested offshore population components.

Summary From the years 1997 to 2000, fish assemblages from 15 groyne fields and training walls (a current guiding dyke, protecting the groyne field) of the Middle Elbe River (Germany) were examined by electrofishing. The aim of the study was to detect abiotic environmental key variables which determine preadult and adult fish assemblage in typical riverbank structures using multivariate ordination techniques (canonical correspondence analysis) and univariate methods (logistic regression analysis). Habitat preferences of preadult and adult stages of selected fish species were also estimated. Fish sampling was carried out at 190 river stretches; a further 1615 samples were taken by the point-abundance sampling method. In total, 21 732 preadult and adult individuals belonging to 30 fish species were caught; perch, eel, ide, roach, chub, gudgeon, bleak and white bream were the most frequent. Eurytopic species clearly dominated the fish assemblage with 72% total abundance, followed by rheophilic species (26%). Limnophilic fish species (rudd, crucian carp, stickleback and tench) were relatively rare (2% of total abundance), with frequencies of occurrence between 1 and 10%. The highest species diversities were estimated in structurally diverse training walls and groyne fields with defective groynes, whereas low species diversity was found to occur in poorly structured, strongly silted groyne fields with intact groynes. Additionally, typical seasonal and annual changes of the fish assemblage were observed. From the mesoscalic point of view, the structure of the preadult and adult fish assemblage was affected by both spatial (type of groyne field, river stretch) and temporal factors (year, season). Regarding the microhabitat, hydromorphological parameters (slope, current velocity, water level, predominant and secondary substrate) followed by physicochemical water values [dissolved oxygen (DO), temperature and pH-value] are of significance for the structure of the fish community. Furthermore, hiding places (shelter) are of subordinate but significant importance for the fish assemblage in the poorly structured main channel of the Elbe River. Importance of hydromorphological factors on the microhabitat scale was confirmed by multiple logistic regression in 12 of 14 preference models, conducted for different ontogenetic stages of roach, bream, white bream, bleak, ide, chub, gudgeon, asp, perch and eel. Habitat preferences of adult bleak and preadult roach were first determined by physicochemical water values and seasonal influences. Univariate models were generated to describe specific habitat preferences of different species and age-stages regarding the most important environmental variables (depth, velocity, predominant substrate, slope and shelter). Intraspecific differences were noticed between preadult and adult chub regarding preferences for water velocity, or between preadult and adult perch concerning slope of shore. Interspecific differences were observed, e.g. between adult roach and white bream, with regard to the use of shelters.

A 1 yr study was conducted to determine which fish species may play a role in the life cycle of Anguillicola crassus in various habitats of Lake Balaton, Hungary. The prevalence and intensity of the larval infection of fish species acting as paratenic hosts was studied, and observations were made on the types of paratenic host reactions against larvae. With the exception of 1 species, all 20 fish species were infected by A. crassus larvae; however, the prevalence and intensity of infection varied widely. Six species (asp, white bream, Chinese rasbora, pike, river goby, European catfish), hitherto unreported as paratenic hosts, also proved to be infected by larvae in Lake Balaton. Of the 13 fish species examined in large numbers, ruffe and European catfish showed the highest prevalence of infection (100 %), followed by river goby (83 %), white bream (79 %) and bleak (68 %). Of these 13 fish species, ruffe showed the highest intensity of infection by live larvae (mean intensity: 39.3 3rd stage larvae, L3), followed by European catfish (mean number of live larvae: 26.9) and river goby (mean number of live larvae: 9.1). The mean number of live L3 in bleak, a species regarded as the principal food source for eels, was 4.1. Specimens containing only dead or both dead and live larvae were much more common in cyprinid fishes than in species belonging to other taxonomical entities. In these fish, the process of encapsulation and subsequent necrosis of live larvae could also be observed. With knowledge of the feeding habits of eels, it appears that bleak play the most important role in the transmission of anguillicolosis. Other intensively infected fish species (e.g. ruffe) may also contribute to massive infection of individual eels, even if they have a lower share in the eels' food structure.

The presence of microplastic (MP) in different fish species taken from stations in Erzurum, Erzincan and Bingöl was examined. The obtained data were classified and shared with the scientific world as the first record made in this region. In the obtained results, the most dominant color was black (39–58%) and the most prevalent forms were fragment and fiber. The sizes (0–50, 50–100 µm) of microplastics differed according to the region and species. When the number of MPs in the gastrointestinal systems of different fish species in the Bingöl, Erzurum and Erzincan provinces was evaluated, the most microplastics were found in Squalius squalus (20.7%) and Blicca bjoerkna (18.2%) in Bingöl province from among six different species. In Erzincan province, four fish species were sampled, and the rates were (29.7%) in Capoeta umbla and (26.6%) in Blicca bjoerkna. The highest abundance in Erzurum province was determined in Cyprinus carpio (53.0%). In the analyses performed on liver tissues, the highest ROS, which is the indicator of oxidative damage, was listed as Bingöl > Erzincan > Erzurum, while MDA levels were recorded as Bingöl > Erzurum > Erzincan, from high to low. When the differences between species were examined, the highest SOD and CAT activity was determined in the Mugil cephalus species. Considering the total MP numbers in fish samples, 47 MP was determined in this species. On the other hand, in the Squalius squalus species, where the highest total MP was determined, SOD and CAT activities were found to be low in Bingöl province. Therewithal, the high levels of ROS and MDA in this species can be said to induce oxidative stress due to the presence of microplastics on the one hand and to reduce antioxidant levels on the other hand. When the findings were evaluated, it was concluded that MPs in freshwater are a potential stressor, and freshwater environments may represent a critical target habitat for future MP removal and remediation strategies.

Fish has a vital role in the socioeconomic structure of South Asian nations and is a valuable source of protein. India is one of the twelve countries in the world with the highest level of biological diversity due to its vast biological heritage. Since the turn of the century, researchers have focused on the 2,546 species of fish that make up the fish population as well as the fish in the Indian subcontinent's inland waterways. The ichthyofauna's lack of knowledge is a major barrier to the popularization of lesser-known fish species in a given environment. Therefore, it is necessary to assess the fish species linked with various freshwater habitats in order to plan production and successful exploitation strategies. The goal of the current study is to better understand the fish diversity in Khudiya Dam between 2023 and 2024. The distance between this dam and Lormi Village in Mungeli District is 20 kilometers. Thirty fish species from five orders and ten families were observed in the current study.