Pelagic Fish Biomass Research Articles

Distribution of pelagic and demersal fish assemblages by video prospection in the archipelago of Fernando de Noronha

This study presents an approach that complements previously identified trends in the distribution of acoustic biomass of demersal and pelagic fishes in Fernando de Noronha. Using underwater video as the main method of analysis, patterns of relative fish abundance were evaluated as indicators of biomass, richness, and trophic levels of demersal and pelagic assemblages. Thus, depth stratum, Marine Protected Area (MPA) including the National Marine Park and the Sustainable Fishing Zone, and oceanographic/abiotic factors (e.g., winds, currents, and substrate complexity) were considered. The results confirm that complex substrates and shallow areas (0–20 m) are characterised by greater abundance and species richness. However, it is important to note the limitation of video in detecting fish in deeper areas. Wind exposure also positively influences the abundance and diversity of demersal fish, demonstrating the impact of the "island effect" on species distribution. The research also highlights the relevance of the MPA in Fernando de Noronha for both biodiversity conservation and the sustainability of fishing activities. However, the analysis of trophic levels revealed an increased prevalence of higher trophic levels outside the MPA, mainly due to the abundance of the species Melichthys niger. The presence of complex substrates and lower trophic competition may explain this observation. Therefore, the study provides important insights for the planning of MPAs and the need to include the spatial distribution of higher trophic level species for marine conservation.

Repeated large declines in the Barents Sea capelin population are associated with different ecosystem conditions

Abstract Although fluctuations in the biomass of small pelagic fish are strong and rapid, they are largely investigated as individual events or generalized to what is common across several species and populations. The Barents Sea capelin population, which experienced four large decreases in biomass between 1980 and 2016, can be used to compare the similarity of causes and sequence of events leading up to large biomass decline in the same population. Here, we used an age-structured state-space model to investigate the possible contributions from somatic growth, food availability, temperature, predation by Atlantic cod, herring interactions, and/or fisheries on the capelin biomass declines. We recalculated total biomass while perturbing these effects and assessed the change in total biomass. We did not find a single, common cause for the dramatic biomass declines. Although median biomass increased in several of the scenarios, a large decline in total biomass was mostly retained. Atlantic cod predation and food availability influenced adult capelin biomass. Food availability mainly impacted the first period, while the importance of Atlantic cod predation increased from the first period to the third period. Reducing larval mortality from herring interactions or temperature did not suppress the biomass declines. Recruitment failures were maintained due to a combination of process errors and uncertainty in the effect of spawning biomass, and thus fisheries, on age 0 abundance. The impact of each perturbation depended on the specific ecosystem context at the time of the biomass decline.

Estimation of The Potential of Large Pelagic Fish Fisheries Using Satellite Data in The Banda and Arafura Seas

The Banda and Arafura Sea possesses significant potential for the fisheries and fishing industry in the country. The large pelagic fish production data studied were derived from satellite imagery data in the Banda and Arafura Seas (2002-2022). The data of chlorophyll-a content, sea surface temperature and Photosynthetically Available Radiation (PAR) derived from AQUA MODIS satellite images were used to estimate the value of primary productivity with the Vertically Generalised Production Model (VGPM). The results showed that the biomass of large pelagic fish fluctuated seasonally, with the Banda Sea and Arafura Sea having high average fish biomass values in the eastern season. With the highest fish biomass peak in August and the lowest in December. The high value of fish biomass in the Banda Sea and Arafura Sea is caused by upwelling events where sea surface temperatures are low but chlorophyll is high. in the Arafura Sea the value of fish biomass is higher than the Banda Sea, this is due to the large amount of nutrient input from large rivers in the area.

Summer Biomass Variability and Spatial Interactions between European Sprat (Sprattus sprattus) and Moon Jellyfish (Aurelia aurita) in the Western Part of the Black Sea

Over the past few decades, various causal connections between commercial small pelagic fish species and gelatinous zooplankton have been reported in the Black Sea, which affect the pelagic ecosystem. Recently, moon jellyfish regained dominance among gelatinous plankton; however, biomass fluctuations and interactions with small pelagic fish remain poorly understood. During the summers of 2019–2022, scientific pelagic trawl surveys in the Western Black Sea enabled simultaneous monitoring of small pelagic fish biomass, with sprat as the key species and moon jellyfish as an incidental catch. In total, 153 trawl hauls were conducted across four depth strata from 15 to 100 m, and a “swept area” method was used for biomass estimation. The sprat stock biomass ranged from 10,698 to 29,177 t, with an average value of 19,432 ± 4834 t. The total biomass of moon jellyfish was 2002 ± 868.73 t, and dense aggregations were observed in the coastal waters during certain years. Two scenarios of spatial interactions between planktivorous species were identified and linked to the formation of A. aurita aggregations. We found that changes in jellyfish density were associated with weak-to-moderate effects on the spatial distribution of sprat assemblages in coastal areas.

Exploring fishing impacts on the structure and functioning of the Yellow Sea ecosystem using an individual-based modeling approach

The Yellow Sea is a marginal sea in the Northwestern Pacific where the fishery resources have been overfished and the community structure has greatly changed over the past six decades. Ecosystem modeling approaches are valuable tools to uncover potential mechanisms behind the ecosystem changes. Here, we developed ‘OSMOSE-YS’, an individual-based multi-species OSMOSE model that includes important commercial pelagic and demersal fish and invertebrates in the Yellow Sea. Simulations were carried out under three fishing scenarios to investigate how different levels of fishing pressure may have impacted the Yellow Sea ecosystem. Results indicate that the biomass of demersal fish continued to decline during 1970–2014, while the biomass of pelagic fish and invertebrates fluctuated periodically. Long-term fishing pressure has led to the reduction of total biomass, body sizes, and longevity of the modelled species. Under low-fishing condition, the ecosystem biomass is restored and the proportion of elder and larger individuals increases. On the contrary, high-fishing condition further decreases the proportion of high-trophic-level species. OSMOSE-YS serves as a baseline model to investigate ecosystem responses to different fishing strategies, in support of ecosystem-based fisheries management in the Yellow Sea.

Extreme and compound ocean events are key drivers of projected low pelagic fish biomass.

Ocean extreme events, such as marine heatwaves, can have harmful impacts on marine ecosystems. Understanding the risks posed by such extreme events is key to develop strategies to predict and mitigate their effects. However, the underlying ocean conditions driving severe impacts on marine ecosystems are complex and often unknown as risks to marine ecosystems arise not only from hazards but also from the interactions between hazards, exposure and vulnerability. Marine ecosystems may not be impacted by extreme events in single drivers but rather by the compounding effects of moderate ocean anomalies. Here, we employ an ensemble climate-impact modeling approach that combines a global marine fish model with output from a large ensemble simulation of an Earth system model, to identify the key ocean ecosystem drivers associated with the most severe impacts on the total biomass of 326 pelagic fish species. We show that low net primary productivity is the most influential driver of extremely low fish biomass over 68% of the ocean area considered by the model, especially in the subtropics and the mid-latitudes, followed by high temperature and low oxygen in the eastern equatorial Pacific and the high latitudes. Severe biomass loss is generally driven by extreme anomalies in at least one ocean ecosystem driver, except in the tropics, where a combination of moderate ocean anomalies is sufficient to drive extreme impacts. Single moderate anomalies never drive extremely low fish biomass. Compound events with either moderate or extreme ocean conditions are a necessary condition for extremely low fish biomass over 78% of the global ocean, and compound events with at least one extreme variable are a necessary condition over 61% of the global ocean. Overall, our model results highlight the crucial role of extreme and compound events in driving severe impacts on pelagic marine ecosystems.

The nocturnal distribution of deep-pelagic fish on the continental slope of the Bay of Biscay

The biomass of deep-sea pelagic fishes could represent more than 90% of the total fish biomass on earth, which represents an important potential for exploitation. However, this community plays multiple key ecological roles in biogeochemical cycles and food webs. Information on their ecology and function is needed to plan effective sustainable conservation measures. In particular, the distribution of deep-sea pelagic fish biomass and the environmental factors that control it remain poorly understood on slope areas at the interface of coastal and oceanic habitats. The combined use of biological data collected by pelagic trawling between 20 and 2000 m depth at night and 16 environmental variables allowed us to study the distribution of this community on the continental slope of the Bay of Biscay, NE Atlantic. Multivariate regression tree analysis suggested that immersion depth was the most important variable structuring this community by night, defining four depth assemblages with different indicator species (i.e. organisms whose presence, absence, or biomass reflects specific environmental conditions): the epipelagic (20–175 m), the upper mesopelagic (175–700 m), the lower mesopelagic (700–1000 m) and the bathypelagic assemblage (1000–2000 m). The use of generalized additive models indicated a positive relationship between bottom proximity and biomass for three of the four assemblages. This contrasts with the paradigm in open ocean areas where the biomass of meso- and bathypelagic fishes decreases with depth. The echograms also showed low acoustic detection at the surface at night, which differs from the open ocean models where nocturnal migration results in a high density of mesopelagic organisms in the epipelagic layer. Different mechanisms could explain this relationship, such as the concentration of food resources in the benthic boundary layer, an ontogenetic change of some species, active horizontal migration, or a demersal spawning behavior. This specific distribution on the continental slope may influence the transition of carbon and energy flows within this ecosystem located in a Special Area of Conservation (SAC).

Climate-driven zooplankton shifts cause large-scale declines in food quality for fish

Zooplankton are the primary energy pathway from phytoplankton to fish. Yet, there is limited understanding about how climate change will modify zooplankton communities and the implications for marine food webs globally. Using a trait-based marine ecosystem model resolving key zooplankton groups, we find that future oceans, particularly in tropical regions, favour food webs increasingly dominated by carnivorous (chaetognaths, jellyfish and carnivorous copepods) and gelatinous filter-feeding zooplankton (larvaceans and salps) at the expense of omnivorous copepods and euphausiids. By providing a direct energetic pathway from small phytoplankton to fish, the rise of gelatinous filter feeders partially offsets the increase in trophic steps between primary producers and fish from declining phytoplankton biomass and increases in carnivorous zooplankton. However, future fish communities experience reduced carrying capacity from falling phytoplankton biomass and less nutritious food as environmental conditions increasingly favour gelatinous zooplankton, slightly exacerbating projected declines in small pelagic fish biomass in tropical regions by 2100.

Patterns in the plankton – Spatial distribution and long-term variability of copepods on the Agulhas Bank

Copepods dominate the zooplankton community on the broad Agulhas Bank off southern Africa, where they provide an important food resource for pelagic fish and other biota. Previous studies have shown the dominant copepod Calanus agulhensis, which may comprise up to 80% of copepod biomass, to be strongly associated with the productive cold ridge of upwelled water on the central (CAB) and eastern (EAB) Agulhas Bank. However, there is little information available on other copepod taxa, and whether the Agulhas Bank community has changed over time in response to environmental variability or other ecosystem changes, such as the recent eastward shift in pelagic fish distribution. We use in situ temperature, chlorophyll a (chl a), and zooplankton data collected annually in late spring to explore spatio-temporal variability in copepod biomass and species composition on the Agulhas Bank over a 24-year period, from 1988 to 2011. Functional traits were used to interpret the observed patterns. Total copepod abundance and biomass were concentrated on the outer central and eastern shelf (>100 m), coincident with the region of elevated chl a at 30 m, and largely downstream from cooler subsurface water linked to the cold ridge and coastal upwelling. Current and cruise-feeding herbivores C. agulhensis and the smaller Para- and Clausocalanidae collectively accounted for 73% and 54% of total copepod biomass and abundance respectively, driving the main patterns for total biomass. C. agulhensis copepodites showed an ontogenetic shift in distribution with distance offshore and downstream from the cold ridge, and accumulation of older stages near the southern tip of the bank. The upwelling specialist Calanoides natalis was closely associated with shelf-edge upwelling, particularly near the Agulhas Bight, but was low in biomass compared to the Benguela upwelling system. The current/cruise-feeding detritivores Metridia lucens and the Oncaeidae were largely associated with the western sector of the bank but showed different niche preferences. M. lucens inhabited the chl-rich outer bank of the western Agulhas Bank (WAB), likely a continuation of the southern Benguela upwelling system community, while oncaeid biomass was associated with deeper thermoclines across the WAB and western part of the CAB that often feature accumulations of the gelatinous zooplankton they feed on. Centropages spp. and the Oithonidae, mainly omnivorous ambush feeders, were associated with the inner to mid-shelf region of the WAB and CAB, away from the main influence of the Agulhas Current. Centropages biomass aligned well with elevated chl a on the inner WAB, likely an extension of the Benguela community (C. brachiatus), and with the cold ridge over the inner CAB shelf, matching earlier patterns linked to C. chierchiae. Oithonidae were widespread but low in biomass, and scarce over the Agulhas Current-influenced outer shelf, suggesting a suboptimal environment overall for this group compared to the cooler Benguela upwelling region. Current-feeding and omnivorous Pleuromamma spp., known to be deep vertical migrators, were concentrated beyond the WAB shelf edge. Total copepod biomass showed a significant decline over the entire Agulhas Bank during the 24-year time series, as did all stages of C. agulhensis, including large nauplii, and the small calanoids (Paracalanidae and Clausocalanidae). No long-term trends were observed for the other copepod taxa. We found no trends in temperature, but a significant but weak (1.5% yr−1) increase in chl a at 30 m. Although there was no obvious shift in copepod biomass in response to the eastward shift in pelagic fish in ca. 1996, there were significant negative relationships between copepod biomass (for total copepods and C. agulhensis) and pelagic fish biomass (total, anchovy and redeye) from 1988 to 2011, with significantly lower copepod biomass after 1998 compared to before. This suggests predation pressure has an important top-down influence on copepod biomass on the Agulhas Bank. Bottom-up forcing from environmental drivers cannot be ruled out, however, despite ambiguous results from in situ and remote data series, and long-term warming due to climate change is expected to negatively impact ecosystem productivity at the large scale.

Exploring the role of fishing in a heavily bioinvaded shelf ecosystem

We used Ecopath with Ecosim to describe the structure and functioning of the shelf ecosystem of the island of Cyprus (eastern Mediterranean) and simulate a series of scenarios related to fisheries and ecosystem management strategies. Analysis was based on a previous 2015–17 model of the same ecosystem, updated with recent data on recreational catches and diets. Two new models were built based on that model, one for 2005 and one for 2021, which along with time series of effort, biomass and catches were used to fit model predictions to observations and simulate the evolution of the ecosystem in the next 30 years, under different fishing scenarios. A total of 32 scenarios were tested related to either changes in fishing effort by fishing fleet, or changes in fishing mortality of selected predatory and invasive groups, along with the business as usual scenario. Ecosystem responses were expressed through a series of ecological and fisheries-related indicators. A 25% increase or decrease in overall fishing mortality, especially coming from recreational fisheries, was predicted to have a dramatic impact on the biomass (and consequently catch) of large pelagic fishes, which seemed to act as regulators in the ecosystem impacting most groups directly or indirectly via trophic interactions and competition. Instead, large demersal fishes were predicted to respond differently to such fishing changes, with less intense biomass fluctuations and a tendency to quickly return to the initial state. Invasive lionfish and pufferfishes were also greatly affected by targeted increased fishing mortalities but seemed to be able to return to initial biomasses in only a few years when mortalities went back to normal. Predatory biomass and catch, total catch, native species catch and catch value, all increased in scenarios with reduced fishing mortality and decreased in the opposite case. Kempton’s Q diversity index decreased in most scenarios, however, most key ecosystem indicators like the total system throughput (TST), Finn's cycling index (FCI) and mean path length (PL), as well as mean transfer efficiency (TE) remained more or less constant in all simulations, indicating an ecosystem stability in terms of its fundamental traits, such as its ecological size (i.e., the sum of flows), trophic complexity and maturity. Simulations revealed the magnitude of uncertainties that ecosystem’s complexity can introduce to fisheries management and highlighted the importance of holistic scientific advice for managers.

Long‐Term Variation in Mesozooplankton Biomass Caused by Top‐Down Effects: A Case Study in the Coastal Sea of Japan

AbstractMesozooplankton is a key group for the recruitment of fish and global biogeochemical cycles. Four decadal observations in the coastal Sea of Japan, the marginal sea of the North Pacific, indicate that wet weight‐based mesozooplankton biomass is controlled by both environment‐induced bottom‐up and predatory‐induced top‐down processes. Interannual variations in mesozooplankton biomass using a generalized linear model approach showed a decrease in biomass during the 1980s, followed by a rapid increase in the early 1990s, and a gradual decrease in the 2010s. These interannual variations were the mirror image of those seen small pelagic planktivorous fish biomass. The difference in zooplankton biomass from the previous year was positively correlated with that of the Pacific decadal oscillation index and negatively with that of fish biomass. The latter was supported by a Granger causality analysis. Therefore, top‐down control is one of the main causes of long‐term variations of zooplankton biomass in the ocean.

Acoustic correction factor estimate for compensating vertical diel migration of small pelagics

Differences in acoustic estimates of small pelagic fish biomass, due to data acquisition during daytime and nighttime surveys, have been recognized for many years as a problem in acoustic surveys. In the absence of a single rule for all species and for all locations, some expert groups identified specific time intervals for acoustic data acquisition in relation to the schooling behavior of the target species. In the Mediterranean Sea, the research groups working in the MEDIAS (Mediterranean International Acoustic Survey) agreed on the importance that acoustic sampling are conducted only during day-time. Only when available time does not permit to complete the survey during daytime, data collection might be extended. In this case, working on data collected during both daytime and nighttime, a bias may occur in the biomass estimates. In order to evaluate and correct such bias, specific experiments were performed in some geographical sub-areas of the Mediterranean Sea. The data analysis allowed the estimation of a mean correction factor for the Strait of Sicily, where five surveys were carried out in different years. The correction factor was estimated also for the Adriatic Sea, Tyrrhenian Sea and Northern Spain; the observed variability among areas highlighted the importance of the spatial and temporal coverage of the survey area in order to obtain reliable estimates of the correction factor. Further studies are necessary to improve the interpretation of the obtained estimates in relation to area-related peculiarities such as zooplankton composition and abundance along with small pelagic fish community structure.

Modeling small-pelagic fish biomass in the Indonesian seas: climate variability and climate change impacts

The present study uses the Community Earth System Model, version 1– Biogeochemistry [CESM1(BGC)] to examine the influence of climate variability and climate change on small pelagic fish biomass in the Indonesian seas. The fish biomass was calculated based on a fish production model according to primary production and energy transfer at the tropic level. The primary production data were obtained from results of CESM1(BGC) model from 1850 to 2015. Empirical orthogonal function (EOF) analysis of the calculated fish biomass identifies three regions in the Indonesian seas that are associated with coastal upwelling. These regions are located in (1) southern coast of Central Java Province until west-coast of West Sumatra Province, (2) southern coast of Central Java Province until the southern coast of Bali Province, and (3) Banda-Arafura Seas. Fish production variability in these regions exhibits semi-annual, annual, and IOD-ENSO related signals. Climate change impact for RCP 4.5 scenario produces ‘fish stock increase status’ in 2025 for the three regions, while the ‘fish stock current status’ will reoccur in 2050, except for the western part of Sumatra (part of region-1) which alters to ‘fish stock decrease status’.

Estimating Pelagic Fish Biomass in a Tropical Seascape Using Echosounding and Baited Stereo-Videography

The pelagic ecosystem is the ocean’s largest by volume and of major importance for food provision and carbon cycling. The high fish species diversity common in the tropics presents a major challenge for biomass estimation using fisheries acoustics, the traditional approach for evaluating mid-water biomass. Converting echo intensities to biomass density requires information on species identity and size, which are typically obtained by lethal means, and thus unsuitable in the portion of the ocean that is ‘no take’. To improve conservation and ecosystem-based management, we present a procedure for determining fish biomass density, using data on species identity, relative abundance, and lengths obtained from stereo baited remote underwater video systems (stereo-BRUVS) to inform the scaling of echosounder survey data (at 38 kHz). We apply the procedure in the British Indian Ocean Territory marine protected area, using acoustic data from 3025 km of survey transects and 546 BRUVS deployments recording relative abundance and size of 12,335 individual fish. Using a Generalised Additive Model of biomass density (GAM, adjR2 = 0.61) we predict, on the basis of oceanographic conditions and bathymetry, that the top 200 m pelagic ecosystem in the Chagos Archipelago, some 118,324 km2, held 3.84 (2.66, 5.62, 95% CI), 33.09 (23.41, 47.35) and 4.08 (3.1, 5.44) million tonnes of fish in November 2012, January 2015, and February 2016, respectively. Our non-extractive procedure yields ecologically credible patterns in biomass across multiple temporal (hours and years) and spatial (metres and kilometres) scales, and marks an improvement on the use of echo intensity alone as a biomass proxy. High seasonal and interannual variability has implication for pelagic fish monitoring.

Effects of Nutrient Management Scenarios on Marine Food Webs: A Pan-European Assessment in Support of the Marine Strategy Framework Directive

Eutrophication is one of the most important anthropogenic pressures impacting coastal seas. In Europe, several legislations and management measures have been implemented to halt nutrient overloading in marine ecosystems. This study evaluates the impact of freshwater nutrient control measures on higher trophic levels (HTL) in European marine ecosystems following descriptors and criteria as defined by the Marine Strategy Framework Directive (MSFD). We used a novel pan-European marine modeling ensemble of fourteen HTL models, covering almost all the EU seas, under two nutrient management scenarios. Results from our projections suggest that the proposed nutrient reduction measures may not have a significant impact on the structure and function of European marine ecosystems. Among the assessed criteria, the spawning stock biomass of commercially important fish stocks and the biomass of small pelagic fishes would be the most impacted, albeit with values lower than 2.5%. For the other criteria/indicators, such as species diversity and trophic level indicators, the impact was lower. The Black Sea and the North-East Atlantic were the most negatively impacted regions, while the Baltic Sea was the only region showing signs of improvement. Coastal and shelf areas were more sensitive to environmental changes than large regional and sub-regional ecosystems that also include open seas. This is the first pan-European multi-model comparison study used to assess the impacts of land-based measures on marine and coastal European ecosystems through a set of selected ecological indicators. Since anthropogenic pressures are expanding apace in the marine environment and policy makers need to use rapid and effective policy measures for fast-changing environments, this modeling framework is an essential asset in supporting and guiding EU policy needs and decisions.

Changes in Pelagic Fish Community Composition, Abundance, and Biomass along a Productivity Gradient in Subtropical Lakes

How fish communities change with eutrophication in temperate lakes is well documented, while only a few studies are available from subtropical lakes. We investigate the fish community structure in 36 lakes located in the Yangtze River basin, covering a wide nutrient gradient. We found that fish species richness and total fish catch per unit effort (CPUE) increased significantly with chlorophyll a (Chla). Among the different feeding types, the proportion of zooplanktivores increased significantly with Chla, while the percentage of omnibenthivores showed no obvious changes; the CPUE of piscivorous Culter spp. increased with Chla, while their proportion of total catch decreased pronouncedly. Based on the index of relative importance (IRI), the most important and dominant fish species was the zooplanktivorous Sijiao (Toxabramis swinhonis), followed by the omniplanktivorous sharpbelly (Hemiculter leucisculus) and the omnibenthivorous crucian carp (Carassius carassius), a small-sized species belonging to the Cyprinidae family. The CPUE of these three species increased significantly with Chla. The focus has, so far, been directed at large fish, but as emphasized by our results, the abundant small fish species were dominant in our subtropical study lakes even in terms of biomass, and, accordingly, we recommend that more attention be paid to the population dynamics of these species in the future.

Spatio-temporal drivers of microphytoplankton community in the Bay of Biscay: Do species ecological niches matter?

From 2000 to 2016, substantial changes in biomass and community structure of small pelagic fish and mesozooplankton have been reported in the Bay of Biscay. Since significant relationships have been found between phytoplankton chlorophyll a and mesozooplankton as well as between phytoplankton chlorophyll a and shifts in sardine body condition, it was hypothesized that phytoplankton communities may have also been affected during this period and may have played a role in these changes. However, the available data were insufficient to validate this hypothesis and the causes of these changes remained unexplained. The present study analyzed a spatio-temporal marine microphytoplankton dataset collected during the annual PELGAS (PELagiques GAScogne) surveys from 2003 to 2014. The thorough analysis of microphytoplankton taxonomic composition, with an approach integrating the relative role of environmental conditions as well as biotic interactions and applying the concept of ecological niche, confirmed that significant modifications in microphytoplankton community structure occurred during this period. Temporal changes were stronger than spatial differences at these sampling scales. Three main periods, 2003–2005, 2006 and 2007–2014, showing different community structure, diversity and dominant taxonomic units were highlighted. Twenty eight taxonomic units were involved in these community changes. Among them, five were identified as the protagonists (Pseudo-nitzschia spp., Gymnodinium spp. + Gyrodinium spp., Leptocylindrus danicus, Leptocylindrus minimus and Chaetoceros sp.). Variations in water temperature and equivalent freshwater depth constrained the realized ecological niches of these species and explained, at least in part, changes in community structure. This study stresses the need to improve our knowledge of phytoplankton species ecological niches and to take into account biotic interactions for a thorough understanding of the processes shaping plankton communities and the resulting diversity patterns.

Climate-related changes in the biomass and distribution of small pelagic fishes in the eastern Bering Sea during late summer, 2002–2018

Climate change is altering the distribution and biomass of marine species in Arctic and sub-Arctic waters. In this study, we investigate the influence of sea temperature on the annual distribution and biomass of pelagic fishes in the eastern Bering Sea during late summer, 2002–2018. The distribution (easting, northing, and area occupied) and biomass of capelin, Pacific herring, juvenile chum salmon, juvenile pink salmon, juvenile sockeye salmon, and age-0 walleye pollock collected by surface trawl were estimated using a standardized geostatistical delta-generalized linear mixed modeling approach. Species showed varied responses to warming on a temporal scale. Warming corresponded with more northerly distributions for capelin and juvenile sockeye salmon, a more westerly distribution of juvenile sockeye salmon, and range expansions for juvenile chum and sockeye salmon. Warming corresponded to a decrease in the annual biomass of capelin and an increase in the biomass of herring, age-0 pollock, and juvenile sockeye salmon. The spatio-temporal covariation in sea temperature and the distribution was nonlinear for juvenile pink salmon and age-0 pollock, positive for juvenile chum salmon and juvenile pink salmon, and negative for capelin indicating different responses of the distribution of pelagic fishes to warming in the eastern Bering Sea during late summer. In warmer areas, we found that the catch rates were higher for juvenile pink salmon, lower for capelin, and not significantly different for juvenile chum salmon, herring, age-0 pollock, and juvenile sockeye salmon. Juvenile sockeye salmon, a southerly distributed species in the survey area, appeared most responsive to warming. In this study, sockeye salmon and pollock are the most commercially important species while chum salmon are important for subsistence fishing. These temperature related changes during early life history stages for survival may have impacts on the numbers of these fishes recruiting to the fisheries.

Ecological response to changes in the structure of water masses in the pelagic of the Canary Current Marine Ecosystem

Based on the results of sea expeditions, interannual changes in structure of the ichthyocenosis in the pelagic zone of the southern shelf of Morocco in the Atlantic Ocean were studied in connection with changes in the habitat. Mass pelagic fish were selected as reference biological species: European and West African horse mackerel, European sardine, Eastern mackerel, sea bream, sea snipe, myctophids, as well as non-commercial fish species. Field data were obtained during the implementation of complex oceanographic surveys in the fall seasons (October-November) 2015–2018. The results of control trawls were used, in which any species amounted to at least 70 % of the catch. The analysis of the species composition of the control trawls, data of hydroacoustic tracking of trawls, thermohaline water characteristics is carried out. Data pairing was carried out by phased extraction and transformation using standard computer programs for processing various types of data (acoustic, hydrological, ichthyological), as well as specially designed programs that allow you to create a single database. It has been established that recent changes in the number of commercial and by-catch species observed in the ARZ of Morocco in recent years are associated with the presence or absence of so-called “settlement windows”, which appear in the T, S‑diagram field by regions of different configurations, which indicates changes in the ratios of water masses. Based on the identified relationships between changes in the ratio of water masses and changes in the species composition of the control trawls of accounting replenishment surveys, estimates of the expected changes in the biomass of mass pelagic fish in the coming years are made.

Responses of the fish community in a eutrophicated lake to long-term food web management assessed by multiple sampling methods

Fish community responses to long-term mass removal of planktivorous fish from eutrophicated Lake Tuusulanjärvi were examined during 1996–2018 by gill net test fishing, hydroacoustics with simultaneous trawling, virtual population analysis (VPA) and fish growth measurements. The gillnet catches of white bream and roach increased first while bream and bleak decreased but recovered in the early 2000s. Perch and pikeperch increased but ruffe decreased and remained low. According to hydroacoustics, pelagic fish biomass decreased. Smelt was the most abundant species at the beginning and the end of the monitoring period but bream dominated the fish biomass in most years. VPA calculations indicated a five- and threefold increase in the biomass of bream and white bream, respectively, during 2005–2011. Significant increases appeared in the growth of perch, bream and roach. Overall, the responses in the fish community structure and fish abundance to the food web management were slight and mostly masked by changes in reproduction and growth of fish and annual variability in environmental conditions such as temperature and water turbidity. Gillnet test fishing and echo sounding complemented each other well in monitoring the effects of management fishing. Where bream is one of the dominant species VPA is also recommended.