Effects of Environmental Changes on Adriatic Small Pelagic Stocks

The Adriatic Sea is an elongated basin located in the northernmost part of the Mediterranean Sea, between the Italian peninsula and the Balkans. Anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) account for approximately 41% of total Adriatic marine catches. These species play an important ecological role in the ecosystem through the capture of energy from the lower trophic levels, and transfer of energy to the higher trophic levels. Since the late 1970s the Research National Council of Ancona has systematically collected data of fishing effort, catch data and biometric data, as well as length and weight at age for both the species. Echo-surveys using acoustic methods have started in the same years providing population abundance indices. Besides that, electronic logbooks have been installed onboard since 2003 to collect daily catch data with the relative GPS position and the in situ water temperature. It is well known that to better understand the meaning of the population dynamics it is necessary to consider the fishery as well as other abiotic and biotic factors. A decrease in the abundance of sardine was revealed by acoustic data and population dynamic models since the late 1980s. By contrast, anchovy catches collapsed in 1987, and after that the stock showed continuous fluctuations that are not associated with variations in the fishing effort. Fishery cannot have been the main reason for the collapse because the decrease in biomass started well before fluctuations in fishing effort and fishing mortality were of any relevance. Furthermore, the absence of a clear relationship between spawning biomass and recruits implies that environmental factors may have played a predominant role in determining recruitment levels. Many hypotheses have been put forward and it is likely that the truth is to be sought in a combination of all of them. A preliminary study has been carried out to search for correlations between recruitment and some environmental variables (e.g. Po river flow rate and NAO index). These parameters seem to have an influence on anchovy and a relatively weak correlation for sardine. More environmental parameters are going to be investigated (temperature, river flow rate, MOI index, nutrients, winds…) with the support of an oceanographic model (ROMS), as well as possible interspecific correlations between the two species. Virtual Population Analysis and Integrated Catch Analysis have been used for population modelling. Generalized Additive Models will then be applied for the relationships with environmental variables.