Are Antarctic and sub-Antarctic marine food webs different?

Aim: Food web structure plays an important role in determining ecosystem stability to perturbations. High latitude marine ecosystems are being affected by environmental stressors and ecological shifts. In the West Antarctic Peninsula these transformations are driven by climate change, and in the sub-Antarctic region by anthropogenic activities. Understanding the differences between these areas is necessary to monitor the changes that are expected to occur in the upcoming decades. Here, we compared the structure and stability of Antarctic and sub-Antarctic marine food webs. Location: Antarctic (Potter Cove, 25 de Mayo/King George Island, West Antarctic Peninsula) and sub-Antarctic (Beagle Channel, Tierra del Fuego, South America) regions. Time period: 1965 - 2019. Major taxa studied: from phytoplankton to fish. Methods: We compiled species trophic (predator-prey) interactions and calculated complexity (number of species and interactions, connectance), structure (mean trophic level, omnivory, degree distribution, modularity, species roles and traits) and stability (QSS) metrics. To be able to make statistical comparisons, we used a randomization algorithm (Strona Curveball) maintaining the number of prey and predators for each species and calculated metrics for each simulation. Results: The Beagle Channel food web presented higher values for complexity metrics (number of species and interactions), structure (mean trophic level, omnivory, modularity) but lower stability (QSS). Potter Cove fitted the exponential degree distribution, while Beagle Channel the power-law with exponential cutoff model. Both food webs presented the same connectance value (0.05), similar distribution of species in top, intermediate and top positions and topological roles, with only one network connector each. Main conclusions: Our results showed that Beagle Channel food web is more complex, but less stable and sensitive to the loss of its most connected species. While the Potter Cove food web presented less complexity and greater stability to perturbations.

Identifying assessment scales for food web criteria in the NE Atlantic: implications for the Marine Strategy Framework Directive

The implementation of food web criteria in the Marine Strategy Framework Directive context faces several difficulties, namely the lack of data for relevant taxa, the absence of operational indicators, and spatially and temporally limited datasets. This work aims to identify ecologically relevant scales in the Celtic Seas (CS) and the Bay of Biscay and Iberian Coast (BBIC). Four food web criteria—mean trophic level (MTL), mean trophic level with cut-offs (MTL_3.25 and MTL_4), large fish indicator (LFI) and mean abundance across trophic guild (MATG)—were assessed using groundfish data and tested using generalized additive models, for six spatial scales and four temporal scales. In both subregions, MTL required yearly and locally defined assessment scales. As for MTL_3.25, it improved significantly when downsizing spatial scales but was temporally consistent. In the CS, locally defined scales and yearly data explained MTL_4 and LFI. While in BBIC, MTL_4 and LFI patterns were defined spatially by region and depth and temporally by year. MATG variability was unaffected by scales. Using the scales identified, food web criteriawere assessed for the Portuguese continental waters. Criteria failed to achieve Good Environmental Status in areas of the Southwest and South of Portugal. Although downsizing scales revealed that criteria were below the threshold at local/regional level, differences in classification are expected to be limited if spatial assessments are aggregated.

China’s bottom trawl fisheries and their global impact

China dominates the world’s highly destructive bottom trawl fisheries (BTF), active in 30 countries and landing 28% of BTF catch. We created the first time series for China’s BTF, from 1950 to 2018, and examined their national and global impacts. Between 1978 and 1997, China’s BTF fleet increased 47 fold (in numbers) and 26 fold (in engine power), driving enormous acceleration in Asia’s fishing capacity. China embarked on BTF globally from 1985, particularly in East Asia and Africa. Such distant water fisheries (DWF) absorb about 20% of China’s BTF capacity. China’s rampant BTF raises significant concerns: fish availability plummeted wherever China fished, domestically and in DWF; there are strong indications of fishing through the food web and fishing indiscriminately in China’s EEZ; and the mean trophic level of catch eaten by humans has declined. Urgent management interventions are needed to stem such ‘slash-and-burn’ fishing practices in China and worldwide.

Investigation of spatiotemporal patterns in mean temperature and mean trophic level of MEDITS survey catches in the Mediterranean Sea

Mean temperature (MTC) and mean trophic level (MTL) spatiotemporal patterns of MEDITS survey catches were examined in 13 geographic statistical areas (GSAs) of the Mediterranean between 1994 and 2016. The study aimed to detect changes in the demersal community structure related to anthropogenic impacts. A generalized additive modelling approach was used to examine the effects of year and GSA on the MTC and MTL indexes and on bottom temperature by haul. For the MTC index, the year was significant only in 4 GSAs, while for MTL it was significant in 5. Higher MTC values were observed in central and eastern areas. Bottom temperature increased after 2010, and also from west to east and from north to south. Our results indicate that the recently observed increase in bottom sea temperature has not resulted in an immediate response by demersal marine communities, but areas with higher warming rates or shallow depths were found to be more susceptible to sea warming. For MTL, decreasing trends were observed in only 2 GSAs, while the temporal trends observed in 5 GSAs may have reflected changes in fishing activity patterns. However, higher MTL values were observed in GSAs with generally higher exploitation rates, indicating that factors other than fishing play an important structuring role in marine communities. The present results indicate differences among Mediterranean subareas in regard to changes in the community structure attributed to environmental conditions and exploitation patterns and have implications for the ecology and dynamics of the stocks.

Towards coherent GES assessments at sub-regional level: signs of fisheries expansion processes in the Bay of Biscay using an OSPAR food web indicator, the mean trophic level

Using the Bay of Biscay (BoB) as a case study, we conducted a transnational assessment of the mean trophic level (MTL, Ospar FW4) indicator at sub-regional level, over the last three decades. Our results confirm the apparent recovery of BoB’s bentho-demersal system, as shown by trends in the MTL indicator based on survey data. However, they also point at a concomitant “fishing through” process where the apparent stability revealed by the MTL indicator based on landed catch data may be masking the expansion of demersal fisheries to deeper waters, and an over-exploitation of resources (particularly abundant pelagic species). Moreover, they show how the combined examination of independent surveys and fishery landings allows the identification of ecological trends in ecosystem studies. In addition, our results confirm that analysing MTL at various threshold levels helps discerning the causality of trends in this indicator, especially if analyses for pelagic and demersal species are run independently. Further studies, at smaller (i.e. local) spatial scales, need to be conducted to ascertain our results and suggest appropriate management strategies aimed at regulating fisheries expansions in the area.

Reconciling Trends of Mean Trophic Index and Mean Temperature of the Catch in the Eastern Mediterranean and Black Seas

The mean trophic level, i.e., the Mean Trophic Index (MTI) and Mean Temperature of the Catch (MTC) were estimated for Turkish waters in the Eastern Mediterranean and the Black Sea based on the reconstructed marine fisheries catches (consisting of N = 88 species and/or higher taxa) of in Turkish waters for the year 1950 to 2010. The MTI decreased in both regions of Turkey, following expectations, but the MTC showed different trends in these two regions. It increased (by 0.48 °C·decade-1), along with the SST in the Eastern Mediterranean, while it fluctuated in the Black Sea. This fluctuating behaviour, however, was similar to the fluctuating tendency of SST in the Black Sea. This suggests that the MTI and MTC can be relied upon as indicators of fisheries impacts on ecosystems, and of the impact of changing temperatures on fisheries, respectively.