Pelagic Resources Research Articles

Environmental and ecological drivers of eye size variation in a freshwater predator: A trade‐off between foraging and predation risk

Abstract Variations in the size and shape of the eye have been observed in many species of fish. As eye size is positively related to visual acuity, larger eyes should favour foraging and detection of predators. However, few studies have examined the variation in eye morphology in relation to the complexity of lake conditions, including environmental perturbation and spatial variation in predation and competition. Such tests are especially important as the degrading of the visual climate is expected due to climate change, where browning, turbidity and variations in structural complexity should set different demands for visual acuity of foraging fish under predation risk. In this study, we tested the variation of the eye size among 667 individuals of an aquatic predator perch, Perca fluviatilis, from littoral and pelagic habitats of 14 lakes. We used Secchi depth to assess the effects of the visual climate of our lake systems, as fish foraging is highly related to visual conditions, and studied eye size variation in relation to the contribution of the pelagic resources to an individual's diet and the risk of predation. Secchi depth, the pelagic contribution to the diet and the percentage of piscivores had significant effects on eye size. These variable outcomes suggest that the lake environment in terms of visual climate, predation landscape and diet are major factors of eye size variation in this generalist predator. As many fish species trade off foraging against predation risk, future studies will show whether the complexity of intra‐ and interspecific interactions contribute to the variation in eye size in freshwater fish. Read the free Plain Language Summary for this article on the Journal blog.

Towards Cross-Border Fisheries Management: An Analysis of Fleet Structures and Species-Specific Regulatory Measures in the Aegean Sea

Intrinsic differences in the applied management measures of shared stocks in transboundary waters inhibit the ability of either state to benefit from such measures, as one state may benefit in lieu of the other, thus reducing the efficacy of both. This study compares for the first time the fleet structure, specific management measures applied to species-specific regulations for commercial fishing, species listed in official monitoring schemes, and protected taxa between Greece and Türkiye for the Aegean Sea. A comparative analysis utilizing the official data was sourced from both countries. Large-scale Greek and Turkish fishing fleets have been modernized within the last 30 years (1991-2021). Greek and Turkish large-scale fishing fleets included smaller trawlers with lower tonnage and slightly higher engine horsepower, while purse seines of both states included larger vessels with higher horsepower and tonnage. This indicates that the fishing pressure on the demersal resources in the Aegean Sea has partially decreased, but the fishing pressure on pelagic resources has increased despite the decrease in the number of vessels. Only 11 out of 74 minimum conservation reference sizes (MCRS) are sufficiently set above the Lm50 sizes they should be based on, whereas 18 species need reproductive studies, and 22 are set below the Lm50, and could benefit from an increase. The application of specific closed seasons for commercial species was used by both states with the intent to protect the reproductive periods of some stocks. However, several of these closure periods did not fully or even partially cover the spawning periods of the respective species. Species-specific closures were applied to 13 species in Greece and 23 species in Türkiye, with only two fish species (Xiphias gladius and Thunnus thynnus) listed for both countries. Only 14 species (out of 34 protected by Greece and 46 by Türkiye) are protected by both states, most of which are listed as critically endangered by the International Union for Conservation of Nature and Natural Resources (IUCN). Harmonization of fisheries management measures currently does not align between the two neighbouring states, and the General Fisheries Commission of the Mediterranean is the competent authority that would be able to restructure such measures, especially as Türkiye has been aligning their measures with that of the EU for the accession process. This contribution highlights the clear differences between Greece and Türkiye, and provides advice for developing a unified management regime for the Aegean Sea.

Mesocosm evidence for sandprawn-mediated shifts in pelagic resource ratios and phytoplankton traits

Bentho-pelagic coupling is a key determinant of multifunctionality in marine ecosystems, influencing critical processes such as production and trophic dynamics. However, knowledge gaps regarding biotically mediated bentho-pelagic coupling limit our understanding of ecosystem responses to global change, and the functional significance of biota nested therein. This is especially relevant for endobenthic ecosystem engineers, which rank amongst the most dynamic of sediment and porewater manipulators in marine systems. Based on this rationale, we tested the consequences of ecosystem engineering by South African endobenthic deposit-feeding crustaceans (sandprawns) for pelagic microbial assemblages, to make inferences about bentho-pelagic coupling. Findings from a factorial 18-day mesocosm experiment indicated a density-dependent, sandprawn-induced decline in phytoplankton relative abundance, but negligible effects on bacterial groups. Picophytoplankton appeared more susceptible than nanophytoplankton to sandprawn density, since relative abundance of the former group declined more in the presence of sandprawns than the latter. By the end of the experiment, there was evidence of a trait shift in phytoplankton assemblages in response to sandprawn presence, with nanophytoplankton being more dominant in the presence of sandprawns. Our findings suggest that pelagic microbes may have differential susceptibilities to endobenthic deposit-feeders and provide useful information on pathways by which these engineers influence bentho-pelagic coupling. These pathways need further evaluation to better understand secondary food-web ramifications and ecosystem responses to global change.

Productivity of the eelgrass biotope and the resilience of fisher-hunter-gatherers in prehistoric Denmark

In the shallow coastal waters of the Danish archipelago, eelgrass (Zostera marina) forms extensive and extremely productive stands, which support food webs that are isotopically distinct from those based on the primary productivity of marine phytoplankton. The isotopic signatures of eelgrass and phytoplankton food webs explain much of the variation in δ13C, δ15N and δ34S values in marine vertebrate remains recovered from prehistoric coastal sites in Denmark. Isotope data from human and dog remains reflect the overwhelming importance of the eelgrass biotope to the fisher-hunter-gatherer subsistence during the Late Mesolithic Ertebølle epoch (∼7400-5800 cal BP). By recognising eelgrass and pelagic resources as isotopically distinct, we obtain more accurate diet reconstructions and dietary 14C reservoir effect corrections for Ertebølle individuals. Longstanding research issues, including technology, palaeodemography, and the resilience and sustainability of the Ertebølle economy need to be reassessed in view of the isotopic signature and productivity of the eelgrass biotope in Danish waters. Eelgrass productivity is also pertinent to comparisons between Ertebølle and Neolithic economies in Denmark, and between fisher-hunter-gatherer adaptations in Denmark and neighbouring regions, and may be key to understanding the much-discussed delay in the transition to farming.

Pelagic Fisheries of India – An overview

The annual pelagic fish landings of the country comprising diverse species, increased from0.3 million t in 1950 to >3 million t in 2021, marking a tenfold increase. The pelagic resourcesconsistently constituted over half of the total marine fish landings. Major contributors tothe pelagic landings are sardines, mackerels, tunas, carangids, ribbonfishes, Bombayduck,codlets, billfishes and barracudas. The west coast of India accounts for 60% of total pelagiclandings with the south-west coast comprising of Kerala, Karnataka and Goa sustainingvast shoals of small pelagic fishes thriving in the nutrient-rich upwelled waters. The pelagicfishes play a crucial role in the marine ecosystem, functioning as both prey and predator.Furthermore, the small pelagic fishes serve as sources of protein-rich food for coastalpopulations, and the large pelagic species command lucrative market prices both in thedomestic and export markets. Continuous monitoring of the landings of pelagic fishes isessential for recommending suitable management plans for the optimal and sustainableexploitation of the available pelagic resources in Indian waters. This paper attempts toprovide an overview of the marine pelagic fisheries of India, based on analysis of the fishlandings data across various sectors spanning the period from 1985 to 2021. Keywords:Marine fish landings, Pelagic fishes, Sustainableexploitation, State-wise contribution, Trends

Resource partitioning in hammerhead shark species out-migrating from coastal ecosystems in the Gulf of California

Juveniles of large hammerhead shark species occupy coastal nurseries before migrating offshore to reproduce. In the central Gulf of California, artisanal elasmobranch fisheries have reported catches of juvenile scalloped Sphyrna lewini and smooth S. zygaena hammerhead sharks, but their local foraging habits are yet to be fully understood. In this study, the trophic niches of both hammerhead species as well as of sympatric Pacific sharpnose sharks Rhizoprionodon longurio were investigated using stable isotope values (δ13C, δ34S and δ15N) and fatty acid compositions in whole blood and muscle tissues. Despite interspecific similarities among trophic niches, smooth hammerheads were characterized by lower δ13C, higher δ34S and greater proportion of docosahexaenoic acid (DHA) in both tissues, suggesting they were already partly relying on offshore pelagic resources. For scalloped hammerheads, muscle reflected coastal dietary resources, while offshore trophic markers were detected in blood integrating prey signal over shorter time periods, indicating their more recent initiation of ontogenetic migration. Multidimensional niche calculation revealed low overlap between hammerhead shark trophic niches, implying that potential fine-scale differences in habitat use could reduce competition between these morphologically and ecologically similar species. In the meantime, the isotopic niches of juvenile scalloped and smooth hammerheads were smaller than that of Pacific sharpnose sharks, suggesting they could be more specialized consumers. Overall, the identification of foraging grounds for juvenile hammerhead sharks calls for a future characterization of their residency time in coastal ecosystems to further understand their interactions with fishing pressure in the Gulf of California.

Projections of winter polynyas and their biophysical impacts in the Ross Sea Antarctica

This study investigates winter polynyas in the southern Ross Sea, Antarctica where several polynyas are known to form. Coastal polynyas are areas of lower sea ice concentration and/or thickness along the coast that are otherwise surrounded by more extensive, thicker sea ice pack. Polynyas are also locations where organisms can exploit both the ice substrate and pelagic resources. Using a self organizing map algorithm, we identify polynya events in the Community Earth System Model Version 2 Large Ensemble (CESM2-LE). The neural network algorithm is able to identify polynya events without imposing an ice concentration or thickness threshold, as is often done when identifying polynyas. The CESM2-LE produces a wintertime polynya feature comparable in size and location to the Ross Sea polynya, and during polynya events there are large turbulent heat fluxes and export of sea ice from the Ross Sea. In the CESM2-LE polynya event frequency is projected to decrease sharply in the later twentyfirst century, leading to increasing sea ice concentrations and thicknesses in the region. The drivers of the polynya frequency decline are likely both large scale circulation changes and local atmosphere and ocean feedbacks. If declines in wintertime polynya frequency over the twentyfirst century do occur they may impact Antarctic Bottom Water formation and local net primary productivity. Thus, better understanding potential local and unexpected sea ice changes in the Ross Sea is important for both assessing climate system impacts and ecological impacts on the Ross Sea ecosystem, which is currently protected by an internationally recognized marine protected area.

Changing environmental conditions have altered the feeding ecology of two keystone Arctic marine predators

Environmental change in the Arctic has impacted the composition and structure of marine food webs. Tracking feeding ecology changes of culturally-valued Arctic char (Salvelinus alpinus) and ringed seals (Pusa hispida) can provide an indication of the ecological significance of climate change in a vulnerable region. We characterized how changes in sea ice conditions, sea surface temperature (SST), and primary productivity affected the feeding ecology of these two keystone species over a 13- and 18-year period, respectively, in northern Labrador, Canada. Arctic char fed consistently on pelagic resources (δ13C) but shifted over time to feeding at a higher trophic level (δ15N) and on more marine/offshore resources (δ34S), which correlated with decreases in chlorophyll a concentration. A reduction in Arctic char condition factor and lipid content was associated with higher trophic position. Ringed seals also shifted to feeding at a higher trophic level, but on more pelagic resources, which was associated with lower SST and higher chlorophyll a concentrations. Years with abnormally high SSTs and reduced sea ice concentrations resulted in large isotopic niche sizes for both species, suggesting abrupt change can result in more variable feeding. Changes in abundance and distribution of species long valued by the Inuit of Labrador could diminish food security.

Estimation of the sustainable potential of scads fish (Decapterus sp.) landed at the Pusong Fish Landing Base, Lhokseumawe

Scad (Decapterus sp.) is one of the small pelagic resources that play a big role in the fishermen’s economic sector in PPI Pusong, Lhokseumawe. Based on time series data within a period of 5 years (2016-2020) scad production tends to fluctuate every year, while the scad fishing grounds are farther than the previous 5 years, namely 4-20 miles to 80-200 miles. This is one of the indications of the disruption of fish resources. Therefore, it is necessary to conduct research on the analysis of sustainable potential (MSY), and the utilization rate of scad landed at the PPI Pusong. This research was conducted in March-April 2022. The data used in this study used primary and secondary data with a sample number of 25 purse seine. Based on the results of data analysis, an MSY value of 1,482 tons/year was obtained, with optimum efforts (Fopt) of 1,930 trips/year and the number of catches allowed (TAC) of 80% MSY of 1,186 tons/year. While the average utilization rate is 70.95%, which is in the optimum level category. However, the precautionary approach to the use of scad must still be controlled in its fishing, so that scad resources remain sustainable.

Seasonality can affect ecological interactions between fishes of different thermal guilds

Seasonality could play a crucial role in structuring species interactions. For example, many ectotherms alter their activity, habitat, and diet in response to seasonal temperature variation. Species also vary widely in physiological traits, like thermal preference, which may mediate their response to seasonal variation. How behavioral responses to seasonality differ between competing species and alter their overlap along multiple niche axes in space and time, remains understudied. Here, we used bulk carbon and nitrogen stable isotopes combined with stomach content analysis to determine the seasonal diet overlap between a native cold-water species [lake trout (Salvelinus namaycush)] and a range-expanding warm-water species [smallmouth bass (Micropterus dolomieu)] in two north-temperate lakes over 2 years. We coupled these analyses with fine-scale acoustic telemetry from one of the lakes to determine seasonal overlap in habitat use and activity levels. We found that dietary niche overlap was higher in the spring, when both species were active and using more littoral resources, compared to the summer, when the cold-water lake trout increased their reliance on pelagic resources. Telemetry data revealed that activity rates diverged in the winter, when lake trout remained active, but the warm-water smallmouth bass reduced their activity. Combining stable isotopes and stomach contents with acoustic telemetry was a powerful approach for demonstrating that species interactions are temporally and spatially dynamic. In our case, the study species diverged in their diet, habitat, and activity more strongly during certain times of the year than others, in ways that were consistent with their thermal preferences. Despite large differences in thermal preference, however, there were times of year when both species were active and sharing a common habitat and prey source (i.e., resource overlap was greater in spring than summer). Based on our findings, important ecological processes are occurring during all seasons, which would be missed by summer sampling alone. Our study stresses that quantifying multiple niche axes in both space and time is important for understanding the possible outcomes of altered seasonal conditions, including shorter winters, already arising under a changing climate.

Constraint analysis of Fishermen and Market Intermediaries of Marine Fish Markets in Ratnagiri, Maharashtra, India

Maharashtra ranks fifth in total marine fish landing in India, and the significant share contributed by pelagic resources (38 %), followed by demersal (28.7 %), crustaceans (24.8 %), and mollusks (9 %). The marine fish landings of Maharashtra during 2020 were 398511 tons, having an annual growth rate of -10.61%. (Fisheries Dept. report 2021), mainly because of the loss of fishing days and lack of fishing labour due to the Covid-19 pandemic. The constraint analysis through rank-based quotient (RBQ) is done to analyse the various problems stakeholders face during marketing in the Ratnagiri District of Maharashtra. RBQ analysis revealed that the high transportation cost, uncertain catch, non-availability of high-grade fish catch, marketing cost (transportation/labour etc.), poor infrastructure facilities at the market yard, non -availability of insulated boxes are the significant constraints faced by the actors involved in the marketing. The policymakers should consider the challenges faced by the marketers of this region to encourage fish marketing by building organized markets.

Characteristics of anchoveta (Engraulis ringens) schools in the optimum zone and the physiological stress zone of its distribution between 2011 and 2021

AbstractThis study has considered anchoveta acoustic data from the Pelagic Resources Hydroacoustic Assessment Surveys carried out by the Peruvian Sea Institute (IMARPE) and free access satellite data (HYCOM and MODIS from the USA) of some oceanographic variables between the period 2011 and 2021, to know the oceanographic conditions, characteristics of schools and size structure in the optimum zone (OZ) and in the physiological stress zone (PSZ), of their total distribution. The OZ is determined by the extent of the Cold Coastal Waters (CCW), and the PSZ is determined by the mixing of waters, to the north by the CCW‐ESW (Equatorial Surface Waters) and to the west between the CCW‐SSW (Subtropical Surface Waters). The range of sea surface temperature and salinity in the PSZ (16.1–25.0°C and 34.65–35.29, respectively) was wider than in the OZ (14.5–24.0°C and 34.65–35.20, respectively), where a higher number of schools were recorded. Our analyses showed that the morphometric and energetic acoustic descriptors of anchoveta schools in the summer were higher during summers compared to springs. In terms of positional characteristics, the mean depth of the schools in the PSZ was slightly greater than in the OZ. The total size structure of anchoveta in the OZ consisted mainly of juvenile specimens (less than 12 cm total length) in the summer of 2011 and 2013–2014 and in the spring between 2014–2017 and 2021. In the PSZ, adult specimens (greater than 12 cm total length) predominated in summer between 2011–2013, 2015, 2018, and 2021 and in spring between 2011–2013, 2015, and between 2018–2021. However, specimens greater than 10 cm in total length were found in the PSZ. Knowledge of the location of this zone away from the coast will allow a high probability of anchoveta capture of adult specimens during fishing periods.

Comparative trophic ecology of microhabitat-associated guilds of reef fishes in the Adriatic Sea.

Reef ecosystems are characterized by highly heterogenous habitats and functionally diverse fish communities. Few studies have examined how functional diversity differs among habitats within these communities, i.e., species associated with a specific habitat may have similar trophic ecologies meaning that the functional diversity within the community is driven by habitat diversity or, conversely, high functional diversity within each habitat would indicate that resource segregation also occurs at the habitat level. We used stable isotope ratios of carbon and nitrogen to estimate trophic position, resource use and ontogenetic niche shifts of 15 reef fishes associated with four distinct habitat types (cryptobenthic, epibenthic sand, epibenthic rock and hyperbenthic) on the Croatian coast of the Adriatic Sea. Trophic ecology was quite similar across fish assemblages, but there was strong evidence of niche segregation among fish species within each assemblage showing high functional diversity within each microhabitat. The sampled fish community contained benthic and pelagic resource users, along with multiple intermediate generalists. Consumer stable isotope ratios revealed considerable interspecific variation in resource use among fishes within each habitat type. The cryptobenthic fishes were a notable exception to this trend with the narrow range of resource use values, indicating reliance of these species on a single resource. The greatest diversity of trophic positions within a guild was observed in cryptobenthic and rock-associated epibenthic fishes. The majority of observed ontogenetic variation in studied fish species reflected an increase in benthic resource use and trophic position. However, the degree of ontogenetic variation in trophic ecology of studied species, if present, was generally low, showing no dramatic change in the ecology of any species. The size structuring among guilds was considerable, with cryptobenthic fishes the smallest on average and hyperbenthic fishes the largest, despite guilds having similar ranges of trophic positions.

SUSTAINABLE MANAGEMENT OF DOGTOOTH TUNA FISHERIES IN ECOLOGICAL DIMENSIONS IN EAST SIMEULUE, ACEH, INDONESIA

Simeulue Regency is surrounded by small islands rich in diverse fish resources, namely dogtooth tuna. The interaction (reciprocity) between groups of organisms and their environment plays an important role in ecological systems. The determination of ecological status can be used to monitor, manage, and sustain waters in tuna fishing areas. Fishing activities will affect environmental changes that will impact marine biota's sustainability, existence, and diversity. This study aimed to analyze the ecological sustainability status of tuna fishing areas in East Simeulue waters. The research was conducted in May-July 2021. The method used in this research was observing and measuring the characteristics of the aquatic environment and conducting interviews. The attributes seen include the condition of the aquatic environment, conservation of pelagic resources, fishing activities, utilization of pelagic resources, environmental carrying capacity, type of fishing gear, the volume of fishing gear, level of suitability for fishing. The sustainability status of the ecological dimension obtained a value of 79.74. The results obtained are classified as well sustainable.

Characteristics of purse seine fisheries in the Sulawesi Sea (case study in Tumumpa fishing port)

The purse seine is the dominant fishing gear operating at the Tumumpa Fishing Port with a vessel of >90%. The purse seines that have developed are large pelagic targets, with the main catch targets being skipjack, mackerel tuna, and tuna. Tuna, mackerel tuna and skipjack are important commodities for fisheries in Indonesia. This study aims to determine the characteristics of purse seine fishing gear and its catch in the Sulawesi Sea. Data collection was carried out for two years from 2019-2020 at Tumumpa Fishery Port. Indications for the use of fish resources are calculated by comparing the length-at-first-capture (Lc) with the length-at-first-mature (Lm). The results showed that purse seines are very dominant in exploiting large pelagic resources (skipjack, mackerel tuna and tuna) with the proportion reaching 78%, small pelagic (21% scads and 1% other fish). The large pelagic fishing season using the purse seine vessel occurs in May – November, with 2 peak seasons in July and September. While the small pelagic occurs in May – September, with the peak occurring in July. The size when the frigate tuna was first caught was at a fork length of 25.15 cm (Lc) and Lm in a size of 28.52 cm for females and 28.29 cm for males. Meanwhile, the first scads was caught (Lc) at a length of 21.4 cm with the length-at-first-mature (Lm) being 20 cm for the female and 19 cm for the male. Based on this analysis, it was shown that the purse seines is a selective fishing gear for small pelagic fish but not selective for large pelagic fish.

Assessing the small pelagic fishery status based on the bioeconomic approach in the Prigi Bay and surrounding waters

Given the importance of small pelagic resources for the livelihood of a coastal community, the present study aimed to determine its stock status to evaluate fishery sustainability. The catch of pelagic fish and fishing trips from multi-fishing gears were collected based on a daily-catch monitoring program by Archipelagic Fishing Port of Prigi during 2005 and 2018. Five surplus production models and the Fishing Power Index (FPI) also were applied to develop the catch per unit of effort as a relative abundance index. As a result, the Walter and Hilborn model (WH) was chosen as the best fit model relied on the highest coefficient of determination compared to other models (R2 = 0.637). The estimated intrinsic growth rate (r), catchability coefficient (q), and carrying capacity (K) were 5.89 tons per year, 0.0003 ton per unit, and 9,920 tons per year, respectively. The CMSY and FMSY were estimated as 14,609 tons per year and 7,873 trips per year, respectively. For economical sustainability, the CMSY and FMSY level were estimated at 14,307 tons per year and 6,742 trips per year, respectively. The average of actual trips for the last five years is greater than the optimum fishing effort, hence, an overfishing status was defined for small pelagic fishery. Therefore, some precaution approaches can be initiated in maintaining the resource sustainability and generating optimum economic benefits around the coastal area.

Marine fisheries and biodiversity management in Maharashtra: Status, challenges and opportunities

Fisheries plays a pivotal role in the economy, food security, foreign exchange earnings and employment of the coastal population. Maharashtra is bestowed with a coastline of 720 km spread over 7 coastal districts, viz. Thane, Palghar, Mumbai city, Mumbai suburban, Raigad, Ratnagiri and Sindhudurg along the Arabian sea with rich marine fishery resources. There are 25 fishing zones in the seven districts with 173 fish landing centres. The total marine fish production from the state during 2018-19 was 4.6 MT valued at `6298 crores (DOF, Government of Maharashtra). The state offers huge potential for the sustainable utilization of the marine bioresources for fisheries, mariculture, coastal aquaculture and post -harvest development and exports. Maharashtra falls under the North- West coast which contributes the highest (32.8%) in the total catch during 2019-20. The major share is contributed by pelagic resources (39%), followed by crustaceans (31%), demersal comprising 22% and molluscs (7%). The prominent species/groupwise landings include non-penaeid shrimp (21%), penaeid shrimp (9%), Bombay duck and croakers (8.2%), Indian mackeral (6.9%), threadfin breams (5.9%), squids (5.1%), ribbon fishes (4.1%), golden anchovy (4%), horse mackerel (3.5%), cuttle fish (2.1%), silver pomfret (2.0%). The trawl fishing accounted for 55%, set bagnet (SBN/Dolnet) 23%, purse seines (15%) and gillnet (7%) (CMFRI Annual Report 2019).

Pre-catch and discard mortality in Northeast Atlantic herring and mackerel fisheries: consequences for stock estimates and advice

Abstract Unaccounted mortality caused by discarding or pre-catch losses is a major challenge for fisheries management. In pelagic fisheries, a considerable proportion of catches may be lost due to intentional release of unwanted catch (slipping) or net bursts (fishing net tears due to the weight of the catch). Here we review and estimate ranges of discard and pre-catch mortality for two important pelagic fisheries, the Northeast Atlantic (NEA) mackerel and Norwegian spring spawning (NSS) herring, and explore the effects on stock estimates and catch advice. We show that mortality caused by discarding, slipping, and net bursts is unknown but probably corresponds to a considerable percentage of total registered catches. Including estimated unaccounted mortality into assessment models leads to underestimation of the stock levels by 3.7–19.5% and 2.8–6.8% for NEA mackerel and NSS herring, respectively, corresponding to up to several million tonnes of fish that die annually due to fishing without being landed. If discard and pre-catch mortality were eliminated, allowed catches could increase by 10–20%. We demonstrate that unaccounted mortality in pelagic fisheries may be substantial, affecting stock estimates and catch advice. This may undermine the sustainable management and efficient use of pelagic resources.

Trophic niche segregation among native whitefish and invasive vendace in a north Norwegian lake system

AbstractIntroductions and invasions of non‐native species alter nutrient cycling and trophic dynamics resulting in significant ecological disturbance. Stable isotope data were used to test for evidence of invader‐induced trophic niche differences in a north Norwegian lake system differentially dominated by native European whitefish (Coregonus lavaretus) morphotypes and invasive vendace (Coregonus albula). Aspects of both realised trophic niche position and trophic niche width were affected by the invader, with the effects varying by whitefish morphotype. Densely rakered pelagic whitefish demonstrated a relatively lower reliance on pelagic resources in the presence of the invader and the isotopic niche size was relatively larger in conjunction with the broadening of the prey base. Within the benthic‐dwelling sparsely rakered whitefish morphotype, the trophic impacts of invading vendace were size‐dependent, with larger individuals experiencing niche compression. Smaller sparsely rakered whitefish increased, contrary to our hypothesis, reliance on pelagic‐based energy in the face of invasion. Our findings demonstrate that the trophic ecology of invaded systems can differ in multiple and subtle ways that have consequences for community‐ and ecosystem‐level energy flows, which if persisting over time are likely to have implications for the recruitment, growth and reproductive rate of the native fishes.

Microbial response to the presence of invasive ctenophore Mnemiopsis leidyi in the coastal waters of the Northeastern Adriatic

Seasonal blooms of the invasive ctenophore Mnemiopsis leidyi have been recorded throughout the Adriatic Sea, affecting the ecosystem and its pelagic resources. Gelatinous organisms such as M. leidyi release significant quantities of excreta as labile and surface active dissolved organic carbon (DOC) that supports the microbial activity.The effects of M. leidyi's presence on heterotrophic prokaryotes (HP), cyanobacteria (CB) and heterotrophic nanoflagellates (HNF) were investigated at five locations in the coastal waters of Rovinj-Rovigno (northern Adriatic) during intensive blooms in September 2019. Additionally, circulation in the study area was examined using available current measurements and a hydrodynamic model. In September, low-pass filtered currents near the surface were mostly directed toward S-SE reaching 25 cm/s, indicating the presence of the Istrian Coastal Countercurrent (ICCC).Ctenophores displayed a patchy spatial distribution from 0.05 to 100 M. leidyi individuals per m3. When ctenophore density in the swarms surpassed 1 individual per m3, the change in the abundances of microbial groups became evident. The lowest microbial densities and lowest concentrations of dissolved and particulate organic carbon (DOC and POC) and surface-active substances occurred in the absence of M. leidyi, while the highest were within the swarms.We hypothesise that HP utilises the available DOC enhancing the remineralisation of organic matter. Finally, greater availability of HNF's prey (HP and CB) leads to an increase in their abundance.This study illustrates how the release of surface-active DOC during intensive M. leidyi's blooms can stimulate the microbial loop. The current system with the ICCC prevailing in September enables the spreading of M. leidyi from its hypothetic origin in an open sea gyre towards the coast. Bora wind episodes may initiate and favour this type of circulation. In these conditions, M. leidyi agglomerates in large numbers along the coast, enabling the accumulation of organic matter and altering the abundance of pico- and nanoplankton.