Temporal Variability of Co-Occurring Calanus finmarchicus and C. helgolandicus in Skagerrak

The congeneric copepods Calanus finmarchicus and C. helgolandicus co-occur in the northern North Sea and Skagerrak where they play important roles as prey for higher trophic levels. This study analyses a 26-year time series (1994-2019) with a ~ two-week temporal resolution for Calanus spp. at a fixed monitoring station in Skagerrak, off southern Norway. Seasonal variation, inter-annual variability and long-term trends for the two species were examined. Strong differences in the species-specific seasonality were revealed, with C. finmarchicus dominating in spring and C. helgolandicus in autumn. The seasonal peak of C. finmarchicus was associated with relatively low temperatures (6-8°C) and high chlorophyll a concentration, while C. helgolandicus displayed its seasonal maximum at higher temperatures (11-16°C). C. finmarchicus was found to produce one dominant annual generation (in spring), but two or more generations are considered likely. Contrasting long-term trends in abundances were found for the two species, suggesting that their population sizes were affected by different mechanisms. The abundance of C. helgolandicus showed an increasing trend over the time period studied, apart from the last years. The abundance of C. finmarchicus was more variable and displayed no unidirectional long-term trends over the time series. The study revealed a shift in the phenology of Calanus spp. over the last 25 years at this site. Since 1994 the timing of the annual peaks in both C. finmarchicus and C. helgolandicus have advanced by about one month. Moreover, the seasonal pattern of C. helgolandicus, switched from a unimodal to a bimodal pattern around 2002, with a small additional peak also appearing in spring. The results suggest that the proximity to the Norwegian Trench influences the demography and abundance of C. finmarchicus in this coastal area, both as a gateway for the advective supply, as well as a habitat for local overwintering.publishedVersio

Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

publishedVersio

Achieving Reliable Estimates of the Spatial Distribution of Kelp Biomass

Kelp forests are highly productive systems that are important ecologically and commercially as well as in a blue carbon perspective. Given their importance, there is an urgent need to achieve reliable estimates of the spatial distribution of their biomass. Species distribution modelling is a powerful tool for producing such estimates, but it requires a solid framework, including important environmental covariates that have a direct effect on their biomass, a proper sampling strategy, and an independent evaluation dataset. Using Laminaria hyperborea as a model species, we developed a modelling framework considering these requirements and necessary steps to produce reliable predictions. Our modelling framework included proportion of hard substrate and bottom wave exposure, both crucial covariates that have a direct effect on the biomass of L. hyperborea, but rarely included in modelling studies. Furthermore, we devised a sampling strategy with field observations covering the whole environmental covariate space present in the study area. Subsequently, we fitted GAMs relating the field observations of the biomass of L. hyperborea to relevant environmental covariates. The best model containing the predictors bottom wave exposure, depth, and proportion hard substrate explained most of the variance in the dataset (83.1% deviance explained). This model was then used to predict the spatial distribution of biomass across the whole study area. To assess the reliability of the biomass predictions, we used an independent dataset of L. hyperborea biomass observations from the same area. This independent dataset correlated very well with spatial predictions of biomass based on our best model (R = 0.85). In total, we predicted a biomass of 457,000 tonnes in a 1,150 km2 study area on the West coast of Norway. Our modelling framework provides the means for developing a biomass model on a broader geographical scale. Such a model will be invaluable in improving kelp management regimes as well as for estimating the contribution of kelp forests to ecosystem services such as carbon sequestration and climate budgets.publishedVersio

Changing fish distributions challenge the effective management of European fisheries.

Changes in fish distribution are being observed across the globe. In Europe's Common Fisheries Policy, the share of the catch of each fish stock is split among management areas using a fixed allocation key known as ‘Relative Stability’: in each management area, member states get the same proportion of the total catch each year. That proportion is largely based on catches made by those member states in the 1970s. Changes in distribution can, therefore, result in a mismatch between quota shares and regional abundances within management areas, with potential repercussions for the status of fish stocks and the fisheries that depend on them. Assessing distribution changes is crucial to ensure adequate management and sustainable exploitation of our fish resources. We analysed scientific survey data using a three‐tiered analytical approach to provide, for the first time, an overview of changes in distribution for 19 northeast Atlantic fish species encompassing 73 commercial stocks over 30 yr. All species have experienced changes in distribution, five of which did so across management areas. A cross‐species analysis suggested that shifts in areas of suitable thermal habitat, and density‐dependent use of these areas, are at least partly responsible for the observed changes. These findings challenge the current use of relative stability to allocate quotas.acceptedVersio

Biomassemodell for stortare - Ressursmodell for fremtidens forvaltning

En romlig biomassemodell for stortare er utviklet i et pilotområde med gode data for bunn-, dyp- og miljøforhold og med høy oppløsning. Modellen predikerer en total stortarebiomasse på 457 000 tonn innenfor området. Korrelasjonen mellom modellen og uavhengige data var på hele 0,85 og stemte dermed svært godt overens med virkeligheten. Pilotmodellen er første skritt i utviklingen av en ressursmodell for hele Norge som kan gi forvaltning og næring en oversikt over stortareressursene og legge til rette for bærekraftig høsting av denne rike ressursen.publishedVersio

The Seascape of Demersal Fish Nursery Areas in the North Mediterranean Sea, a First Step Towards the Implementation of Spatial Planning for Trawl Fisheries

The identification of nursery grounds and other essential fish habitats of exploited stocks is a key requirement for the development of spatial conservation planning aimed at reducing the adverse impact of fishing on the exploited populations and ecosystems. The reduction in juvenile mortality is particularly relevant in the Mediterranean and is considered as one of the main prerequisites for the future sustainability of trawl fisheries. The distribution of nursery areas of 11 important commercial species of demersal fish and shellfish was analysed in the European Union Mediterranean waters using time series of bottom trawl survey data with the aim of identifying the most persistent recruitment areas. A high interspecific spatial overlap between nursery areas was mainly found along the shelf break of many different sectors of the Northern Mediterranean indicating a high potential for the implementation of conservation measures. Overlap of the nursery grounds with existing spatial fisheries management measures and trawl fisheries restricted areas was also investigated. Spatial analyses revealed considerable variation depending on species and associated habitat/depth preferences with increased protection seen in coastal nurseries and minimal protection seen for deeper nurseries (e.g. Parapenaeus longirostris 6%). This is partly attributed to existing environmental policy instruments (e.g. Habitats Directive and Mediterranean Regulation EC 1967/2006) aiming at minimising impacts on coastal priority habitats such as seagrass, coralligenous and maerl beds. The new knowledge on the distribution and persistence of demersal nurseries provided in this study can support the application of spatial conservation measures, such as the designation of no-take Marine Protected Areas in EU Mediterranean waters and their inclusion in a conservation network. The establishment of no-take zones will be consistent with the objectives of the Common Fisheries Policy applying the ecosystem approach to fisheries management and with the requirements of the Marine Strategy Framework Directive to maintain or achieve seafloor integrity and good environmental status.Versión del editor4,411

Trophic ecology of small pelagic fish

The main aim of the present dissertation was to study the trophic ecology of two of the most abundant small pelagic fish species in the North Aegean Sea, i.e., the European anchovy (Engraulis encrasicolus) and the European sardine (Sardina pilchardus). The North Aegean Sea is one of the most important fishing grounds for small pelagic fishes in the Mediterranean Sea, largely owed to the enhanced productivity of this area compared to the generally oligotrophic Eastern Mediterranean. Regarding the trophic ecology of the two species, the feeding periodicity, feeding intensity and daily ration was studied in fish collected during four oceanographic cruises (July 2007, December 2007, July 2008 and February 2009). The prey composition of the stomach contents was also analyzed in an attempt to define dietary preferences for each species and subsequently relate them with the structure and function of the pelagic food web. To achieve this, concurrent measurements of environmental and plankton variables (from bacteria to mesozooplankton) were also carried out in an effort to infer major trophic pathways in the pelagic food web and how these are related with prey selection in sardine and anchovy. The trophic ecology of the two species studied was quite similar. Specifically, the feeding periodicity and intensity was similar for the two species in each sampling period, irrespectively of the ontogenetic stage (juveniles or adults). During summer, both species fed consistently during daytime, with a prominent peak at around sunset, i.e. when the vertical migration towards surface is taking place. In winter, high feeding rates were recorded in the early night (first 6 h after sunset) but not in the second half of the night. Daily rations were estimated by applying the Elliott-Persson and the Eggers models and varied from 2.02 to 3.67% total weight (TW) in adult sardines and 4.18 to 5.36% TW in juveniles. For anchovy, the respective rations were 1.50 to 4.30% TW for adults and 1.84 to 4.74% TW for juveniles. The instantaneous gastric evacuation rate (R) was similar in those periods that juveniles and adults of each species co-existed. In summer, anchovy juveniles and adults had almost double R values compared with sardine (Ranchovy ~0.4 h-1 ; Rsardine ~0.2 h-1). The same pattern was noted in December 2007 (Ranchovy ~0.2 h-1 ; Rsardine ~0.1 h-1). In February 2009, when temperature was lower, R was the similar for the two species (R ~ 0.13 h-1). A significant positive relationship emerged when daily ration was regressed against the ratio of mesozooplankton biomass to fish catch per unit effort (CPUE), implying a density-dependent rate of food consumption in both anchovy and sardine. Regarding the dietary preferences of each species, adult sardines consumed large amounts of phytoplankton, in contrast to both juveniles of the same species and anchovy. However, for both species, the bulk of the dietary carbon derived from calanoid copepods, >1 mm total length (Acartia, Temora, Centropages), most likely captured by particulate feeding. Diet composition and prey selection seemed to be driven by the availability of large/suitable prey. During summer, microbial processes prevailed (ratio of autotrophs 20 μm = 13-15, ratio of heterotrophic:autotrophic pico- and nanoplankton biomass 20 μm dominated the carbon budget and the abundance of larger sized copepods (e.g. Centropages) was high. In December 2007, when waters were mixed but yet relatively warm, both the ‘microbial’ and ‘classical’ (herbivorous) trophic pathways seemed to be important. The mean size of mesozooplankton (copepods+cladocerans) in fish stomachs was highly correlated with their mean size in the field, the latter being in turn highly and positively correlated with the concentration of diatoms. When larger-sized preys were less abundant in the field, both species broadened their trophic niche, turning towards other zooplankton preys as well. On the contrary, when larger-sized preys were dominating the plankton (mixing periods) both species preferred such preys, with anchovy consuming the largest available copepods (e.g., Candacia, Eucalanus). This size-based partitioning of food resources was confirmed by examining the size frequency distributions of preys in both the stomachs and the field. The study of the trophic ecology of small pelagic fish is an expanding research field, given the increasing effort worldwide to couple Lower Trophic Level (LTL) models with fish bioenergetic models. The linking of LTL with higher trophic level models aims at accurately simulating the functional relationships in marine ecosystems, providing a powerful tool for fisheries management.Στόχος της παρούσας διατριβής ήταν να μελετηθεί η τροφική οικολογία των δύο κυρίαρχων ειδών μικρών πελαγικών ψαριών στο Βόρειο Αιγαίο, του Ευρωπαϊκού γαύρου (Engraulis encrasicolus) και της Ευρωπαϊκής σαρδέλας (Sardina pilchardus). Το Βόρειο Αιγαίο αποτελεί ένα από τα σημαντικότερα αλιευτικά πεδία της Μεσογείου Θάλασσας όσον αφορά στα μικρά πελαγικά ψάρια, δεδομένης της υψηλότερης παραγωγικότητας που παρατηρείται στην περιοχή αυτή συγκριτικά με την ολιγοτροφική Ανατολική Μεσόγειο. Στο πλαίσιο της μελέτης της τροφικής οικολογίας των δύο ειδών εξετάστηκε η περιοδικότητα, η ένταση και η κατανάλωση τροφής από δείγματα που συλλέχθηκαν κατά τη διάρκεια τεσσάρων ωκεανογραφικών αποστολών (Ιούλιος 2007, Δεκέμβριος 2007, Ιούλιος 2008, Φεβρουάριος 2009). Μελετήθηκε επίσης η σύνθεση των στομαχικών περιεχομένων κάθε είδους, με σκοπό να αναλυθούν οι διατροφικές τους προτιμήσεις και να εξεταστεί αν η επιλογή θηραμάτων σχετίζεται με την δομή και οργάνωση του πελαγικού τροφικού πλέγματος. Για το σκοπό αυτό, συλλέχθηκε στο πεδίο μεγάλος αριθμός παραμέτρων, τόσο περιβαλλοντικών όσο και σχετιζόμενων με το πλαγκτό (από βακτήρια έως μεσοζωοπλαγκτό). Τα αποτελέσματα της μελέτης έδειξαν ότι τα δύο είδη έχουν πολλά κοινά χαρακτηριστικά σχετικά με την τροφική οικολογία τους. Συγκεκριμένα, παρουσίασαν κοινό πρότυπο στην ημερήσια διατροφή τους, το οποίο ήταν ανεξάρτητο του οντογενετικού σταδίου. Στις καλοκαιρινές περιόδους θερμο-στρωμάτωσης, τα ψάρια τρέφονταν κατά τη διάρκεια της ημέρας, με την ένταση της πρόσληψης τροφής να κορυφώνεται κατά τη διάρκεια της ανόδου των ψαριών προς τα επιφανειακά στρώματα στο πλαίσιο της ημερήσιας κατακόρυφης μετανάστευσης. Αντίθετα, κατά τη διάρκεια των χειμερινών περιόδων ανάμειξης, η εντονότερη κατανάλωση τροφής καταγράφηκε κατά την διάρκεια της μετανάστευσης προς την επιφάνεια και συνεχίστηκε σε υψηλά επίπεδα μέχρι τις πρώτες μεταμεσονύκτιες ώρες. Η ημερήσια κατανάλωση τροφής όπως εκτιμήθηκε από την εφαρμογή του μοντέλου των Elliott - Persson και του μοντέλου του Eggers για την ενήλικη σαρδέλα κυμάνθηκε από 2.02 - 3.67% του σωματικού βάρους (TW), ενώ του ενήλικου γαύρου από 1.50 - 4.30% TW. Αντίθετα, τα ιχθύδια του κάθε είδους είχαν υψηλότερες ημερήσιες καταναλώσεις από τα ενήλικα σε κάθε περίοδο δειγματοληψίας, με τιμές που κυμάνθηκαν από 4.18 - 5.36% TW για τη σαρδέλα και 1.84 - 4.74% TW για το γαύρο. Ο ρυθμός γαστρικής κένωσης (R) μεταξύ ιχθυδίων και ενηλίκων κάθε είδους ήταν παρόμοιος, όποτε συνυπήρχαν. Κατά τις θερινές περιόδους δειγματοληψίας, ο γαύρος (τόσο τα ιχθύδια όσο και τα ενήλικα) είχε περίπου διπλάσιο ρυθμό γαστρικής κένωσης από τη σαρδέλα, ~0.4 h-1 και ~0.2 h-1 αντίστοιχα. Η ίδια τάση παρατηρήθηκε και σε ενδιάμεσες θερμοκρασίες (Δεκέμβριος 2007: Rγαύρου ~ 0.2 h-1, Rσαρδέλας ~ 0.1 h-1, ενώ στις χαμηλότερες θερμοκρασίες του χειμώνα (Φεβρουάριος 2009) οι ρυθμοί γαστρικής κένωσης ήταν ίδιοι και για τα δύο είδη (R ~ 0.13). Επιπλέον, καταγράφηκε μια σημαντική σχέση μεταξύ της ημερήσιας κατανάλωσης τροφής και της κατά κεφαλήν διαθέσιμης τροφής (μεσοζωοπλαγκτού) στο πεδίο και για τα δύο είδη, η οποία υποδηλώνει την ύπαρξη ενός μηχανισμού πυκνο-εξάρτησης στην κατανάλωση τροφής. Σχετικά με τις διατροφικές προτιμήσεις των δύο ειδών, βρέθηκε ότι η ενήλικη σαρδέλα καταναλώνει υψηλές ποσότητες φυτοπλαγκτού, σε αντίθεση τόσο με τα ιχθύδια του ίδιου είδους όσο και με το γαύρο. Ωστόσο, τόσο για τη σαρδέλα όσο και το γαύρο, σχεδόν το σύνολο του διατροφικού άνθρακα προερχόταν από την κατανάλωση ζωοπλαγκτού και συγκεκριμένα σχετικά μεγάλων (>1 mm) καλανοειδών κωπηπόδων (Acartia, Temora και Centropages), τα οποία συλλαμβάνονταν κατά πάσα πιθανότητα ατομικά και όχι με ηθμοφαγία («φιλτράρισμα»). Η σύνθεση της δίαιτας φάνηκε να καθορίζεται από την διαθεσιμότητα/καταλληλότητα των μεγάλων θηραμάτων. Κατά τις περιόδους θερμο-στρωμάτωσης, στο πελαγικό τροφικό πλέγμα επικρατούσαν οι μικροβιακές διεργασίες (αναλογία αυτότροφων κυττάρων 20 μm = 13-15, αναλογία βιομάζας ετερότροφων : αυτότροφων πικο- και νανοπλαγκτικής κυττάρων 20 μm (κυρίως διάτομα) κυριαρχούσαν και η αφθονία των μεγαλύτερων κωπηπόδων (π.χ. Centropages) ήταν υψηλή. Το Δεκέμβριο του 2007, όταν η στήλη του νερού είχε αναμειχθεί αλλά η θερμοκρασία ήταν ακόμα σχετικά υψηλή, τόσο οι μικροβιακές διεργασίες όσο και η κλασσική τροφική αλυσίδα συμμετείχαν στη ροή ενέργειας. Η αφθονία των διατόμων, φάνηκε να συσχετίζεται σημαντικά και θετικά με το μέσο μέγεθος του μεσοζωοπλαγκτού (κωπήποδα και κλαδοκεραιωτά) στο πεδίο. Με τη σειρά του, η αύξηση του μέσου μεγέθους του μεσοζωοπλαγκτού στο πεδίο αντικατοπτρίστηκε στα στομάχια του γαύρου και της σαρδέλας. Όταν τα μεγαλύτερα μεγέθους θηράματα ήταν λιγότερο άφθονα στο πεδίο (περίοδοι θερμο-στρωμάτωσης), τότε και τα δύο είδη διεύρυναν τη δίαιτά τους, καταναλώνοντας και άλλες ζωοπλαγκτικές ομάδες. Αντίθετα όταν στο πεδίο κυριαρχούσαν μεγαλύτερα θηράματα (περίοδοι ανάμειξης) και τα δύο είδη έδειξαν μια σαφή προτίμηση προς αυτά τα μεγέθη, με το γαύρο ωστόσο να στρέφεται προς τα μεγαλύτερα διαθέσιμα θηράματα (π.χ. Candacia, Eucalanus) σε σχέση με τη σαρδέλα. Αυτός ο κατά μέγεθος διαχωρισμός των διαθέσιμων τροφικών πόρων επιβεβαιώθηκε από τις κατανομές μεγεθών των θηραμάτων στο πεδίο και στα στομάχια του κάθε είδους. Η μελέτη της τροφικής οικολογίας των μικρών πελαγικών ψαριών αποτελεί σημαντικό πεδίο έρευνας, δεδομένης της προσπάθειας που γίνεται σήμερα σε παγκόσμιο επίπεδο, να συνδεθούν μοντέλα κατώτερων τροφικών επιπέδων (Lower Trophic Level models) με βιοενεργητικά μοντέλα ψαριών. Η σύνδεση αυτή αποσκοπεί στο να επιτύχει την ακριβή προσομοίωση των σχέσεων και διεργασιών που λαμβάνουν χώρα στο οικοσύστημα, η οποία, με τη σειρά της, μπορεί να χρησιμοποιηθεί ως εργαλείο πρόβλεψης και διαχείρισης

Temporal Variability of Co-Occurring Calanus finmarchicus and C. helgolandicus in Skagerrak

The congeneric copepods Calanus finmarchicus and C. helgolandicus co-occur in the northern North Sea and Skagerrak where they play important roles as prey for higher trophic levels. This study analyses a 26-year time series (1994-2019) with a ~ two-week temporal resolution for Calanus spp. at a fixed monitoring station in Skagerrak, off southern Norway. Seasonal variation, inter-annual variability and long-term trends for the two species were examined. Strong differences in the species-specific seasonality were revealed, with C. finmarchicus dominating in spring and C. helgolandicus in autumn. The seasonal peak of C. finmarchicus was associated with relatively low temperatures (6-8°C) and high chlorophyll a concentration, while C. helgolandicus displayed its seasonal maximum at higher temperatures (11-16°C). C. finmarchicus was found to produce one dominant annual generation (in spring), but two or more generations are considered likely. Contrasting long-term trends in abundances were found for the two species, suggesting that their population sizes were affected by different mechanisms. The abundance of C. helgolandicus showed an increasing trend over the time period studied, apart from the last years. The abundance of C. finmarchicus was more variable and displayed no unidirectional long-term trends over the time series. The study revealed a shift in the phenology of Calanus spp. over the last 25 years at this site. Since 1994 the timing of the annual peaks in both C. finmarchicus and C. helgolandicus have advanced by about one month. Moreover, the seasonal pattern of C. helgolandicus, switched from a unimodal to a bimodal pattern around 2002, with a small additional peak also appearing in spring. The results suggest that the proximity to the Norwegian Trench influences the demography and abundance of C. finmarchicus in this coastal area, both as a gateway for the advective supply, as well as a habitat for local overwintering

Viability and Management Targets of Mediterranean Demersal Fisheries: The Case of the Aegean Sea

<div><p>Management of the Mediterranean demersal stocks has proven challenging mainly due to the multi-species character of the fisheries. In the present work, we focus on the multi-species demersal fisheries of the Aegean Sea (eastern Mediterranean) aiming to study the effects of different management measures on the main commercial stocks, as well as to explore the economic viability of the fisheries depending upon these resources, by means of simulated projections. Utilizing the limited available data, our results demonstrated that, under the current exploitation pattern, the economic viability of the fleets is threatened, particularly if fuel prices increase. Additionally, the biological targets set for the most exploited species, such as hake, will not be met under the current management regime. The projections also showed that the only management scenario under which both resource sustainability and economic viability of the fisheries are ensured is the decrease of fleet capacity in terms of vessel numbers. In this case, however, measures to support the fisheries-dependent communities need to be implemented to prevent the collapse of local economies due to employment decrease. Scenarios assuming selectivity improvements would be also beneficial for the stocks but they showed low economic performance and their application would threaten the viability of the fleets, particularly that of the trawlers.</p></div

Spawning Stock Biomass by species (in 000’s tons) under the four simulated scenarios.

<p>Dotted lines indicate the median estimates and the upper and lower limits of each ribbon represent the 0.1 and 0.9 quantiles.</p