Periodic Reporting for period 1 - MIRROR (Through the looking-glass: investigating the effectiveness of fish condition in mirroring trawling disturbance)
Reporting period: 2020-09-01 to 2022-08-31
The study of fishing disturbances at relevant spatial and temporal scales to inform and build realistic management strategies represents one of the main challenges to the next generation of fisheries scientists, stakeholders and policy makers. The evaluation of trawling impact on the productivity of demersal fish species is a crucial knowledge to maintain the yield from wild-capture fisheries and to cope with the global increasing demand of sea proteins. The idnirect effects of trawling disturbance (TD) are still understudied. It remains still largely unknown how the alteration of benthic dynamics due to chronic TD can trigger unpredictable cascade effects involving most ecological components impairing the system to cope with the natural environmental variability. MIRROR aimed at investigating the TD effects on the diet and condition of a target species (Mullus barbatus – red mullet) in the Central Mediterranean Sea. MIRROR explored the effects that changes in fish condition can have on life-history traits (e.g. growth and reproductive potential). Understanding the link between TD, condition and LH traits represents a steppingstone for the inclusion of condition as an indicator for the management of exploited species. The red mullet, Mullus barbatus, was chosen as model species being one of the most commercially important benthivorous species in the Mediterranean Sea (95% of landing composition, 6.73 €/kg mean EU countries values). The red mullet is a good indicator species with respect to habitat quality, a defined income breeder whose number of eggs depend on food intake and whose energy reserves are not mobilized from other tissues during reproduction. These reproduction and energy storage strategies allow examination of the secondary effects of TD on diet, condition and LH traits (growth and reproductive potential). The indirect monitoring of benthic prey communities though the proxy of body condition of selected fish species may represent the only feasible way to integrate the benthic system into the ecosystem-based fisheries management. The monitoring of entire marine communities at spatial scales relevant to a fishery is too costly and time consuming, thus justifying the need for new indicators and the importance of MIRROR’s contribution towards an ecosystem approach, in the Mediterranean Sea where evident knowledge gaps on the ecosystem effects of TD remain. MIRROR has encompassed 3 main objectives: 1) Investigate diet and condition changes along TD gradients, by comparing areas with different trawling histories and environmental conditions to provide an indication at what trawling intensity fish condition is maximised and an indirect measure of benthic prey status; 2) Investigate the consequences of condition changes on selected LH traits such as growth, onset of maturity, reproductive potential and survival (link established by only few studies); 3) Investigate the effect of TD on the condition of red mullet through space and identify trawling intensity threshold (vulnerability maps) with ramifications on population parameters that are the focus of fisheries management.
MIRROR investigated the secondary effects of TD on the diet and body condition of the red mullet in the Central Mediterranean (CMED) basin through the mechanisms of competitor and benthic prey reduction by the fishing activity. Something particularly topical, but still scarcely implemented, in the Mediterranean Sea, a sensitive ecosystem characterised by low production levels coupled with intense and highly aggregated fishing effort where the ecosystem-based fisheries management is a true challenge. MIRROR explored the consequences of TD on changes to fish LH traits such as growth and reproductive potential. The relationship between TD, fish condition and LH traits has been examined across different areas subject to different trawling histories and environmental conditions. Four main areas have been surveyed off the Sicilian coast, across the continental shelf, encompassing two highly exploited commercial fishing grounds, an Exclusion Fishery Zone - where the otter trawl activity has been banned since nineteen - and a recently established Fishery Restriction Area. Red mullets’ fish condition has been measured at each area and used to explore the effects of trawling allowing to evaluate the consequences of changes due to TD on fish growth. Additional fish Life History (LH) traits, such as maximum body size, maximum growth rate, age-at-maturation, size-at-maturation, age structure, individual fecundity, lifespan and foraging behaviour have been empirically measured. The total catch of benthivorous species, as well as the total biomass and size distribution of the catch, was also recorded at each sampling area to help disentangling red mullet responses by taking into account any potential competitors and predators’ effect. Regarding the results achieved so far, the integrated methodological approach allowed to depict fish feeding changes induced by trawling disturbance, with the composition of ingested prey reflecting it (i.e. a more poor diet, less diversified with a lower number of item and higher occurrence of empty stomachs) mirroring the overlooked phenomena of habitat degradation and species vulnerability and resilience (i.e. diet items significant differences among fished and unfished areas). The integration of growth increments analysis with dietary and condition allowed to investigate the relationship between organismal condition and growth: with fish showing a lower condition, associated to a lower growth rate on highly fished fishing grounds. MIRROR efforts and evidence have been synthesized thankfully to a modelling exercise that allowed to spatially identify trawling intensity thresholds to achieve a desired mean optimal condition, salient information to support a sustainable exploitation of the fish stock and ecosystem integrity. Spatially contextualised vulnerability maps represent a powerful indicator tool that supports a proactive management of trawling effort across different spatial scales integrating theoretical knowledge with practical problem solving.
MIRROR’s outcomesallowed to compare and extent the range of observed TD responses. MIRROR’s integrated approach - of interdependence between fishing effort, fish density, benthic prey status and condition - based on the integration of different methodologies, allowed the examination of the relationship between fishing effort-fish condition on a regional spatial scale, that experience different environmental and anthropogenic conditions. Analysing changes in fish condition through space allowed to determine the TD levels at which local populations have a condition that does not negatively affect LH traits. These maps may represent a credible, effective and salient tool to support the decisional process into the future, being easy-to-read from stakeholders while still based on robust scientific evidence. The outcomes may help in visualise (current) and predict (forecast) spatial shift in both population structure and seabed integrity, being exploitable to inform future sustainable management measures. Outcomes might be further implemented in a context of sustainability. MIRROR outcomes support the need to tune spatially explicit Mediterranean tailored measures into European policies.