Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

Large-scale exploitation of higher trophic levels by humans, together with global-scale nutrient enrichment, highlights the need to explore interactions between predator loss and resource availability. The hypothesis of exploitation ecosystems suggests that top-down and bottom-up control alternate between trophic levels, resulting in a positive relationship between primary production and the abundance of every second trophic level. Specifically, in food webs with three effective trophic levels, primary producers and predators should increase with primary production, while in food webs with two trophic levels, only herbivores should increase. We provided short-term experimental support for these model predictions in a natural benthic community with three effective trophic levels, where the number of algal recruits, but not the biomass of gastropod grazers, increased with algal production. In contrast, when the food web was reduced to two trophic levels by removing larger predators, the number of algal recruits was unchanged while gastropod grazer biomass increased with algal production. Predator removal only affected the consumer-controlled early life-stages of algae, indicating that both the number of trophic levels and the life-stage development of the producer trophic level determine the propagation of trophic cascades in benthic systems. Our results support the hypothesis that predators interact with resource availability to determine food-web structure

Best practices II : spatial distribution of the discards of the Dutch beam trawler fleet

This study aims at describing the spatial distribution – and its temporal variations - of discarding intensity (i.e. expected weight of discards for a standard trawl haul) for the 6 main species discarded by the Dutch beam trawl fisheries. For each species, the spatial distribution (quarterly maps for the period 2013 to 2017) is estimated using statistical models that take spatial and temporal correlation into account, which also allowed to test for the effect of a number of factors related to geography, environment, fishing practices and operational aspects on discarding. The data used to fit those models came from the observer trips and self-sampling program conducted at Wageningen Marine Research and from discards sampling trips conducted by the fishing industry. As by-product, the models provide descriptors of the temporal and spatial scales at which the discards of a given species are structured. The distribution of the expected discards per haul for dab was highly variable from quarter to quarter, with generally high discarding intensity in front of the southern coast of the Netherlands in quarter 1, a discarding intensity which is high on the German bight and low in front of the Dutch coast in quarter 3, and variable distributions for quarter 2 and 4. For plaice, the distribution was more stable, with high values consistently observed in the south of the area (between the south of the Netherlands and England), with occasional hot spots on the German bight. For sole, discards were not observed on the north-western part of the area, and a hotspot of sole discarding was found consistently in front of the southern coast of the Netherlands, occasionally expanding towards England or to the northern coast of the Netherlands. Discarding of turbot first occurred with a low intensity along the coast from Belgium to Germany. After the fourth quarter of 2015, high discarding started to occur, first limited to the small area in the southern North Sea, but progressively expanding to a larger area in the southern and central part of the North Sea, while discarding intensity remained low in the northern part of the area and in front of England. The distribution discarding intensity for whiting was highly variably, characterised by hotspots suddenly appearing for most years in the fourth quarter, and disappear in the following first quarter. Discarding of rays occurred mainly in the western part of the area, especially in front of southern England, with an increasing level since the fourth quarter of 2016. The distributions observed and their variability were further discussed in the light the available information on the distribution and migration of the species and on the management measures potentially influencing discarding

Best practices II : Effect on future development of sole and plaice of changing mesh size from 80mm to 90mm in the beam trawl fishery

This study investigates the consequence for future development of stock size, catches, landings and discards of sole and plaice of changing the mesh size of the cod-end from 80mm to 90mm for the Dutch beam trawlers in theTBB 70-99 fleet currently fishing with 80 mm.This study investigates the consequence for future development of stock size, catches, landings and discards of sole and plaice of changing the mesh size of the cod-end from 80mm to 90mm for the Dutch beam trawlers in theTBB 70-99 fleet currently fishing with 80 mm. The question is addressed by means of long term stochastic simulations. Using the simulation framework developed to test the effect of implementing the landing obligation, the future fishery selection pattern (how the fishing mortality is distributed across ages) is modified based on the results of the selectivity experiment to represent the consequence of changing mesh size. Simulations were then run for the next 50 years for different assumptions on the survival rate for both stocks: a 0% survival rate, and the lower and upper bounds of the current estimates of survival for each species. The differences in the effect on sole and plaice of using a 90mm net are related to both the direct effect of exploiting the stock with a different selection pattern and of applying different Fmsy values. The effects of changing mesh size are larger for sole than for plaice, because the share of the landings taken by the Dutch beam trawlers currently fishing with 80 mm is much larger for sole than for plaice. For sole, fishing with the 90mm net results in lower discards (10 to 16%). Landings are also lower (up to 4%) in the short term, but the situation reverses and landings become higher in the medium and long term (up to 3% after 5 years). These results are explained by the fact that when the 90mm net is used, the cohorts are exploited at a slightly later age combined with a stronger targeting of the older ages. This exploitation patterns leads in the medium and long term to a larger stock (by 3 to 13%), which explains the higher landings. Those benefits (in the medium and long term) of using the 90mm net are largest for the 0% and 10% survival assumptions, but are smaller (especially for the landings) for the assumption with 30% survival: the higher the chance for a discarded fish to survive, the less it pays to increase the selectivity of the gear because fish caught and discarded have still a chance to join the stock and further grow and reproduce. For plaice, in the scenarios with 0% and 10% survival, the Fmsy value for the 90mm net is higher than for the 80mm net. As a result, stock size is lower and catches, landings and (despite the improved selectivity of the net) discards are higher if the 90mm net is used. For the scenario with 20% survival rate, Fmsy values are similar for the 80mm and 90mm mesh size and the improved selectivity of the 90mm net indeed results in slightly lower discards, which in the medium and long term result in a slightly larger stock with slightly higher landings. One important assumption in these simulations is that the stocks are exploited at Fmsy in the future. However, if the beam trawl fleet switches to the 90mm net, its catchability (at least for sole) will decrease, meaning that a higher fishing effort will be necessary to achieve a same fishing mortality on the stock. The present study does not model explicitly catchability and effort, and therefore cannot quantify the change in effort implied if the stocks were to be exploited at Fmsy with the 90mm net

WORKSHOP TO REVIEW AND UPDATE OSPAR STATUS ASSESSMENTS FOR STOCKS OF LISTED SHARK, SKATES AND RAYS IN SUPPORT OF OSPAR (WKSTATUS)

publishedVersio

Report of the JRC’s Descriptor 1 workshop to support the review of the Commission Decision 2010/477/EU concerning MSFD criteria for assessing Good Environmental Status

The MSFD workshop on biodiversity (MSFD D1), held in Ispra JRC (7th-9th of September 2015) aimed to provide clear proposals and conclusions on some of the outstanding issues identified in the D1 review manual (May 2015 consultation version: https://circabc.europa.eu/w/browse/46d2b7ba-d2fd-4b3c-9eaf-18c7cb702b53) in the broader context of support to the review of Commission Decision 2010/477/EU. This report is complementing the Commission Decision 2010/477/EU review manual (JRC96521) and presents the result of the scientific and technical review concluding phase 1 of the review of the Commission Decision 2010/477/EU in relation to Descriptor 1. The review has been carried out by the EC JRC together with experts nominated by EU Member States, and has considered contributions from the GES Working Group in accordance with the roadmap set out in the MSFD implementation strategy (agreed on at the 11th CIS MSCG meeting). The main issues addressed and tackled in this workshop’s report are: - Common lists of elements for the biodiversity assessments (species & habitats) o Review of the “Biological Features” in Table 1 in the MSFD Annex III in relation to D1 requirements o Review of the “Habitat Types” entries in Table 1 in the MSFD Annex III in relation to D1 requirements - Selection/deselection criteria for the inclusion of species and habitats in a group - Updated criteria and indicators for D1 - Habitat/Bird Directives, WFD, Common Fisheries Policy and D1 o Use of species and habitats for the MSFD needs that are already included in other legislation and agreements o Links between status classification approaches (FCS vs GES, GEcS vs GES) - Streamlining of assessments, including scales of assessments - Cross-cutting issues related to D1 implementation o Aggregation rules within D1 criteria/indicators o Final GES integration across descriptors assessments Steps forward and technical needs for D1.JRC.H.1-Water Resource

Best practice II: effect of discard survival on North Sea sole and plaice

This report investigates the effects of discard survival on the current stock assessment and perception of the North Sea sole and plaice stocks. By recalculating the discard fraction of the catches and rerunning the assessment model, the stock assessment of sole and plaice is corrected for discard survivability. Secondly, all discard survival corrected assessments of both stocks are forecasted over 50 years under a landing obligation and discarding (business as usual) scenario. This simulation shows the effect of discard survival under a landing obligation and under the discarding scenario. The trend and perception of both stocks do not change when discard survivability is taken into account. But the fishing mortality, stock biomass, and recruitment are overestimated. The effect of taking into account discard survivability is a scaling depending on the characteristics of the stock (such as maturity at age) and the extent to which the part of the stock is being discarded. The effect of discard survival is greater in North Sea plaice than in North Sea sole, since the plaice is discarded more. The Fmsy reference points increase with increasing discard survivability. However, the “F-targets”, the F corresponding to the maximal yield under the landing obligation, that are calculated to simulate the “landing obligation-scenario” do not show the same trend with increasing discard survivability. The forecast simulation of North Sea sole and plaice was performed by projecting the stocks with targets for fishing mortality that maximise the yield of both stocks. This method gives insight in the effects of the discarding and landing obligation scenario on the catches, recruitment, spawning stock biomass, and fishing mortality. Differences between scenarios increase with increasing discard survivability, although differences are marginal in the simulation of sole (compared to the differences between scenarios in plaice). Mainly the catches are effected by discard survivability under the landing obligation scenario

Comparisons of landings to scientific advice indicate overshooting within the common TAC for skates and rays in the Northeast Atlantic

The International Council for the Exploration of the Sea (ICES) typically provides advice on fishing opportunities on a stock-by-stock basis. Nevertheless, levels of total allowable catch (TAC) are sometimes set for a collection of stocks and species (i.e. a common TAC). An explicit expectation of these is that landings will scale with ICES advice, especially when ICES advice is used to calculate the common TAC. This expectation is tested for skates and rays in the Northeast Atlantic, spanning 26 stocks, 8 species, and 3 ecoregions. Using ICES landings and ICES advice data from 2016 to 2022, we show that landings of several stocks and species have overshot their respective ICES advice, whereas others have undershot. Specifically, some stocks of blonde ray (Raja brachyura) in North Sea and Celtic Seas ecoregions are being landed at a rate that often exceeds double its ICES advice. By collating species based on their ICES assessment category and life-history traits, we find that those considered data-poor and potentially most vulnerable to fishing are consistently landed at higher-than-expected rates in the Celtic Seas. This study questions the appropriateness of a common TAC for skates and rays and calls for shifts towards the use of single-stock catch allocations and the application of advanced stock assessment methodologies

Evidence of difference in landings and discards patterns in the English Channel and North Sea Rajidae complex fishery

Bycatch species such as skates and rays are for most of them not subject to analytical stock assessment. However, their life history characteristics increased their vulnerability to fisheries. In the English Channel and North Sea area, the three main landed Rajidae species are Raja clavata, Raja brachyura and Raja montagui. The current management measure is a global TAC, common for all Rajidae species. Data to process analytical stock assessment are not available for these species, particularly discards data. A Bayesian multispecies biomass production model, following separately the landings and discards was applied to these stocks. This model provided proxies of reference points (MSY and BMSY) per species. All stocks were depleted in 1990 and are now rebuilding. However, rebuilding speeds are different within the complex, R. clavata being the fastest and R. brachyura the slowest. Furthermore, the proportion of the discards and landings to biomass differ between species, highlighting species specific fishing strategies. Differences in vulnerabilities within the Rajidae complex might be caused by the variability of life history parameters between species as well as landings and discards pattern differences. This second factor, usually not considered for data limited stock assessment, is particularly relevant for highly discarded chondrichthyans species, and might be considered when choosing new methodology to asses Rajidae stocks

Developing a multispecific stocks MSE framework : English Channel and North Sea Rajidae complex

Six Rajidae stocks (Raja clavata, Raja brachyura, Raja montagui, Raja microocellata, Leucoraja naevus and Raja undulata) are currently exploited in the English Channel and North Sea area (ICES divisions 3a, 4 and 7d) and subject to a TAC regulation. This TAC is set based on monospecific approaches (abundance indices) when mangement is complex based. However, stock status disparities exist between these species. In this context a multispecific MSE (Management Strategy evaluation) framework has been developped based on DLMTool package from R. The main objective was to estimate the complex management impact at the different steps of stocks management (data collect, assessment, management measures implementation). The species species assessement and  management scenario was the best performing one, in term of long term stock biomass as well as yield. However, species specific assessement associated to complex management performances were more contrasted than the performance metrics of a complex assessment and complex management scenario. Six espèces de Rajidae (Raja clavata, Raja brachyura, Raja montagui, Raja microocellata, Leucoraja naevus et Raja undulata) sont actuellement exploitées en Manche et Mer du Nord (divisions CIEM 3a, 4 et 7d) et sujettent à une régulation par TAC. La méthode pour établir ce TAC est monospécifique (indices d'abondances) tandis que leur gestion est faîte au niveau du complexe. Cependant des disparités entre états des stocks ont été mises en évidence. Dans ce cadre une approche MSE (Management Strategy evaluation) multispécifique a été développé à l'aide du package DLMTool de R.  L'objectif était d'estimer les impacts d'une gestion par complexe au cours des différentes étapes de gestion des stocks (collecte de données, évaluation, mise en place de mesure de gestion). Le scenario d'évaluation et gestion par espèce est celui ayant mené aux meilleurs résultats, tant en terme de biomasse à long terme que de débarquement. Les performances d'une évaluation par espèce associée à une gestion par complexe semblent cependant plus contrasté qu'une évaluation par complexe associée à une gestion par complexe