Food-web changes in the Adriatic Sea over the last three decades

Abstract

We used a process-oriented model (Ecosim) to characterize changes in marine resources in the north-central (NC) Adriatic Sea (Mediterranean) from 1975 to 2002, and to explore the extent to which these changes were driven by trophic interactions, environment and fishing. Fishing efforts and fishing mortalities were used to drive the ecosystem model, and available biomass and catch data were compared to the model predictions. We calibrated and assessed the fit of the model using the sum of the squared deviations of the observed and predicted biomass values. Trophodynamic indicators were then calculated and used to analyse changes to the ecosystem. Trophic interactions, fishing, and environmental conditions were important driving factors of the ecosystem. Predictions from the model adequately matched observed biomass and catch data for each year. Observed trends and predicted data for biomasses decreased with time for several species, including Norway lobster, hake, red mullets, flatfish, and anchovy. The environmental functions resulting from the fitting procedure predicted that primary production and nutrients increased beyond the baseline from 1975 level, and then showed a decrease. They were negatively correlated with mean sea surface temperature and positively correlated with the Mediterranean Oscillation Index. Our results suggested that the NC Adriatic ecosystem became more degraded over time due to expanding fishing effort, bottom-up effects, and climatic anomalies. Possible mechanisms involved are discussed. Comparing our findings with descriptions of the south Catalan Sea ecosystem showed certain similarities between these 2 Mediterranean ecosystems and revealed unique features of the Adriatic Sea.

Changes to Marine Trophic Networks Caused by Fishing

“…This phenomenon has been observed in a large number of studies that use mass balance trophic models as analytical tools (e.g. Albouy et al, 2010;Barausse et al, 2009;Coll et al, 2008aColl et al, , 2008bColl et al, , 2009aColl et al, , 2009bHinke et al, 2004;Jones et al, 2009;Savenkoff et al, 2007aSavenkoff et al, , 2007b.…”

“…Although very few studies explicitly mention structural simplification of networks as a result of fishing (Coll et al, 2008a(Coll et al, , 2009a(Coll et al, , 2009b, or changes in "species scaling laws", they can be inferred in studies reporting changes in the distribution of biomass among different trophic levels through time. This phenomenon has been observed in a large number of studies that use mass balance trophic models as analytical tools (e.g.…”

“…These authors also found that highly impacted environments, not only in terms of intensity but also of time, show stronger overexploitation effects at high trophic levels, larger network simplification, and high reductions in productivity and biomass. As a result of this drastic structural simplification, Mediterranean ecosystems showed less robustness to secondary extinctions than other less impacted ecosystems (Coll et al, 2009a(Coll et al, , 2009b.…”

“…Using this approach, Coll et al (2008aColl et al ( , 2009aColl et al ( , 2009b found significant differences in the composition of a trophic network subjected to different fishing levels through time. Their results showed a change in the biomass proportions (equivalent to species scaling laws) among trophic levels between the ecosystem without fishing pressure and the three scenarios with increasing fishing pressure.…”

“…More specifically, generalized effects are reductions in biomass of top predators and an increase in the proportion of species at intermediate trophic levels and basal species, suggesting that fishing is shifting ecosystem structure from large species with low abundance and slow developmental cycles to small species with high abundance and faster developmental cycles (Albouy et al, 2010;AriasGonzález et al, 2004;Barausse et al, 2009;Chen et al, 2008;Coll et al, 2007Coll et al, , 2008aColl et al, , 2009aColl et al, , 2009bColl et al, , 2010Duan et al, 2009;Lotze et al, 2011;Savenkoff et al, 2007aSavenkoff et al, , 2007b. This generalized effect is reducing the naturally occurring competition among the original species in the network and facilitating the onset of indirect effects that generate competition among species that did not strongly interact before.…”