Fabio Pranovi

Ecosystem Overfishing in the Ocean

Fisheries catches represent a net export of mass and energy that can no longer be used by trophic levels higher than those fished. Thus, exploitation implies a depletion of secondary production of higher trophic levels (here the production of mass and energy by herbivores and carnivores in the ecosystem) due to the removal of prey. The depletion of secondary production due to the export of biomass and energy through catches was recently formulated as a proxy for evaluating the ecosystem impacts of fishing–i.e., the level of ecosystem overfishing. Here we evaluate the historical and current risk of ecosystem overfishing at a global scale by quantifying the depletion of secondary production using the best available fisheries and ecological data (i.e., catch and primary production). Our results highlight an increasing trend in the number of unsustainable fisheries (i.e., an increase in the risk of ecosystem overfishing) from the 1950s to the 2000s, and illustrate the worldwide geographic expansion of overfishing. These results enable to assess when and where fishing became unsustainable at the ecosystem level. At present, total catch per capita from Large Marine Ecosystems is at least twice the value estimated to ensure fishing at moderate sustainable levels.

An Ecological Imbalance Induced by a Non-Native Species: The Manila Clam in the Venice Lagoon

Among the 19 non-native species of marine invertebrates which have invaded the Venice Lagoon and have established populations, Ruditapes philippinarum, deliberately introduced in 1983, is surely the most successful species. According to the hypothesis that alien species invasion could be favoured by an altered ecological, chemical or physical state of the system induced by anthropogenic disturbance, R. philippinarum turned out to be ‘the right species at the right moment’. By comparing historical data (1968, 1985, 1990) with 1999 data, changes in macrobenthic community, in particular bivalve molluscs, of the lagoon induced by R. philippinarum introduction and subsequent clam exploiting activity were assessed. It has been possible to describe a sharp reduction, both in terms of distribution area and density, of all other filter feeder bivalves. Moreover, by using the clearance rate of the most abundant bivalve species in 1990 and 1999 (Cerastoderma glaucum and R. philippinarum, respectively), it was possible to estimate that the filtration capacity, expressed as l h−1 m−2, has more than doubled. This has altered the functioning of the ecosystem, resulting in a stronger benthic–pelagic coupling. In this context, R. philippinarum attains control of the system. Considering all this, it is possible to state that the Venice Lagoon ecosystem has entered into a new state, probably more resistant but less resilient, with implications for future management choices.

Need for restricting bivalve culture in the southern basin of the Lagoon of Venice

At present, one of the environmental emergencies in the Lagoon of Venice is the impact of short-necked clam (Tapes philippinarum) fishery, which is practically an unregulated fishery. Although one of the proposed solutions would be the restriction of Tapes fishery to licensed areas, high seeding density can cause undesired effects on the environment. In this study several hydrobiological variables are compared between small areas of the Lagoon of Venice traditionally used for bivalve culture (clam, T. philippinarum and mussel, Mytilus galloprovincialis), and areas in the southern basin with seagrass meadows. Labile and suspended organic matter in the water was higher in areas with bivalve farming than in Zostera areas (undisturbed control). The same pattern was recorded for contents of total organic matter and acid volatile sulphides. The biomass of microplankton in farming areas was quite high (0.8–2.7 g m−3). Mesozooplankton was extremely abundant, particularly at night, when its biomass was 1–2 orders of magnitude higher than during the day. Its composition was different in the culture areas and in Zostera areas. The biomass of Tapes in culture beds and their filtering capacity were also estimated.

Recolonization dynamics in areas disturbed by bottom fishing gears

Results of two investigations on the effects of disturbance on benthic communities in lagoon and coastal areas, caused by bottom fishing-gears (‘hydraulic dredge’ for clams and ‘rapido’, a kind of beam-trawl for soles and scallops employed in the Northern Adriatic sea), are given. Such gears, although characterised by different features and targets, have similar effects on the sea bottom: both produce deep furrows (7–13 cm for the ‘rapido’, up to 20 cm for the ‘hydraulic dredge’), thus affecting the texture of the bottom. In 1992 (‘hydraulic dredge’) and in 1995 (‘rapido’) two different research projects were carried out; samples of benthos were collected immediately after the passage of the gears and at fortnightly-intervals, in treated and control areas. This allowed study of the modifications of the macrobenthic communities and investigation of the short and medium-term (dredge: 60 days, ‘rapido’: 15 days) progression of the recolonization processes in the disturbed areas. These dynamics have been analysed by giving emphasis to the species and to their time-space fluctuations. It has been found that characteristically ‘non-opportunistic’ species can assume an opportunistic behaviour during the initial phase of the recolonization processes of the disturbed areas.

Male Courtship Sounds in a Teleost with Alternative Reproductive Tactics, the Grass Goby, Zosterisessor ophiocephalus

Male grass gobies show two alternative breeding tactics, territorial and sneaker, distinguished by body size and difference in ray elongation on the second dorsal fin. The larger males, with elongated fins, are territorial and emit sounds during courtship. Smaller males, without elongated fins, act as sneakers. Both large and small males produce sounds in the presence of a ripe female. Males produce a grunt, lasting about 300 ms, made up of pulses repeated at a low rate (22–68 pps). Pulse duration, number, and repetition rate, did not differ between the two male types, but dominant frequency and sound amplitude did. Dominant frequency had a strong, inverse relationship with body size, whereas sound amplitude showed a weak positive relation to body size. Male size, and not the particular reproductive male tactic employed, is the most important correlate of sound properties in this species.

Ethoxyresorufin O-deethylase (EROD) activity and fluctuating asymmetry (FA) in Zosterisessor ophiocephalus (Teleostei, Gobiidae) as indicators of environmental stress in the Venice lagoon

The impact of environmental stress on the grassgoby, Zosterisessor ophiocephalus, wasstudied at both biochemical and morphologicallevels, by comparing three sites in the Venicelagoon (the Northern, Central and Southernbasins) which differ in terms of the degree ofanthropogenic disturbance, in particular withregard to organic sediment contamination. At abiochemical level, the hepatic ethoxyresorufinO-deethylase (EROD) enzyme activity wasmeasured as a biomarker of the degree oforganic pollution (in particular regardingcontamination by PCBs and Dioxins). At amorphological level, the amount of fluctuatingasymmetry (FA) with regard to four bilateraltraits (postorbital length; otolith area;minimum otolith diameter; maximum and minimumotolith radii) was measured as a long termindicator of developmental stress. The ERODactivity was higher in grass gobies from theCentral basin, whereas the amount of FA washigher in the gobies collected from theNorthern basin. Both measurements indicatedthat the Southern basin was affected by lowlevels of environmental stress (low ERODinduction and a lower amount of FA). Theresults were discussed in the light of both theenvironmental differences among the threebasins and the different levels of biologicalorganisation at which the two indicators act.

Elongation of fin rays in parental males of Zosterisessor ophiocephalus (Pisces: Gobiidae)

The first description of the elongation of rays of the second dorsal fin in the parental male of the grass goby, Zosterisessor ophiocephalus, is reported. The exhibition of fin ray elongation in the male grass goby may be a useful diagnostic character to identify individuals adopting alternative styles (i.e. parental males vs. parasitic spawners). The possible role played by this secondary sexual trait in the context of alternative reproductive styles adopted by smaller males of this species to gain fertilization is also discussed.

Modeling species invasions using thermal and trophic niche dynamics under climate change

Changing marine temperatures modify the distributional ranges of natural populations, but the success of invasion of new areas depends on local physical and ecological conditions. We explore the invasion by thermophilic species and their ecosystem effects by simulating a sea surface temperature increase using a trophodynamic model for the northern Adriatic Sea (NAS), in which thermal and trophic niches are explicitly represented for each thermophilic non-indigenous species and native species. The NAS acts as a cul-de-sac for local species, preventing a further poleward migration as a response to temperature rise. In this situation, model results showed that effects of warming and invasion produced complex, non-linear changes on biomasses but never resulted in a complete overturn of a group of native species and/or a bloom of invasive ones. Despite this, the diversity index stabilizes at increased values after simulating invasion, possibly indicating that in such enclosed systems the establishment of invasive species could represent enrichment in ecosystem structure. In addition, the absence of complete species substitution clearly showed the contribution of resident species towards increasing the resilience, i.e. the capability of the system to cope with invasion without changing substantially. Contrasting scenarios highlighted that changes in ecosystem primary production and species adaptation had secondary effects in ecosystem structure, while results for scenarios with different exploitation levels indicated that fishing can destabilize community structure in these change contexts, e.g. reducing community resilience. The results confirmed the importance of an ecological niche approach to analyze possible effects of invasion and highlighted the complexity of dynamics linked to temperature-driven species invasion’, in terms of both the predicted strength of impacts and the direction of biomass change.

Vulnerability of artisanal fisheries to climate change in the Venice lagoon

Within the context of global warming, the western coast of the northern Adriatic Sea can be regarded as an extremely vulnerable area. Owing to the local geographic features, this area has been described as the Venetian lacuna, where Mediterranean Sea climatic conditions are replaced by Atlantic Ocean ones, supporting the presence of glacial relicts, such as sprat Sprattus sprattus, flounder Platichthys flesus and brown shrimp Crangon crangon. Nektonic assemblage therefore represents a good candidate in terms of an early proxy for thermal regime alterations. It represents a dynamic component of the lagoon ecosystem, changing in space and time, actively moving through the entire system, and dynamically exchanging with the open sea. Here, the first signals of the change have been already detected, such as the presence of alien thermophilic species. Within this context, since the beginning of the century, sampling of the nektonic assemblage has been carried out, integrating them with landings data from the fish market. Vulnerabilities to thermal regime changes have been tested by (1) categorizing species according to the mean distribution area in terms of latitudinal range (over 45°, 30°-45° and below 30°), and (2) analysing both spatial and temporal variations within fishing grounds. Results indicated a high potential vulnerability of the artisanal fishery to climate change, as the commercial catch is entirely composed of species from cold (>45° N) and temperate (between 45° and 30° N) latitudes. At present no alien thermophilic species have been recorded within the lagoon, which is possibly a sign of good resilience of the assemblage. Finally, abundance of species from cold latitudes has decreased during the past decade. All of this has been discussed in the context of the mean annual temperature trend.

Linking food web functioning and habitat diversity for an ecosystem based management : a Mediterranean lagoon case-study

We propose a modelling approach relating the functioning of a transitional ecosystem with the spatial extension of its habitats. A test case is presented for the lagoon of Venice, discussing the results in the context of the application of current EU directives. The effects on food web functioning due to changes related to manageable and unmanageable drivers were investigated. The modelling procedure involved the use of steady-state food web models and network analysis, respectively applied to estimate the fluxes of energy associated with trophic interactions, and to compute indices of food web functioning. On the long term (hundred years) temporal scale, the model indicated that the expected loss of salt marshes will produce further changes at the system level, with a lagoon showing a decrease in the energy processing efficiency. On the short term scale, simulation results indicated that fishery management accompanied by seagrass restoration measures would produce a slight transition towards a more healthy system, with higher energy cycling, and maintaining a good balance between processing efficiency and resilience. Scenarios presented suggest that the effectiveness of short term management strategies can be better evaluated when contextualized in the long term trends of evolution of a system. We also remark the need for further studying the relationship between habitat diversity and indicators of food web functioning.