Original publication: January 2016
Authors: Giuseppe Scarcella, Alicia Mosteiro Cabanelas
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Fishing for clams (Chamelea gallina) by means of hydraulic dredges is of paramount importance in the Adriatic. The fishery is divided into Districts (Compartimenti Marittimi) and for each District a fixed number of dredgers are licensed to operate.
Annual landings reached a peak value of about 100,000 tonnes in the early 1980s and subsequently declined because such high fishing pressure was not sustainable (Froglia, 1989). The fishery experienced a drop in catches and at present annual landings are roughly of the order of 14,000 tonnes with an approximate first sale value of about €32M (EU Member States for the 2015 DCF fishing fleet economic data call). Such reduction in catches spurred the Italian government to impose a number of management regulations on gear characteristics (technical measures), the number of fishing days per year (fishing effort or inputs) and individual catch quota per day (catches or outputs).
The scientific advice for such regulations is based mainly on resource assessment surveys conducted once per year by scientific institutions (e.g., CNR – IRPEM, etc.). These surveys provide an estimate of relative abundance of the resource at sea in terms of biomass and an estimate of the size-distribution of the clam populations.
The fishery is therefore managed by a number of general regulations issued by the Italian Directorate for Fisheries. Within the scope of these general regulations the local Consortium of Clam Fishermen can add additional rules such as diminishing the daily individual quota or imposition of a rotation of the fishing areas inside the District. Scientific advice on these decisions is required from an official scientific institution.
Summarizing, it seems clear, that in its early years the fishery that is the subject of this work recorded high fishing yields, but they soon started declining at a steady rate in spite of various measures adopted to limit the fishing effort and such a negative trend progressively reduced the commercial output to approximately one sixth of that from initial times.
Causes of decline
Causes of the decline in clam production remain uncertain and negative impacts both by the high exploitation rate (Froglia, 1989a; Morello et al., 2005a) and unknown environmental factors (Froglia, 2000) have been assumed. Moreover, the possible impact on C. gallina eggs and larvae by the large phytoplanktonic aggregates recorded in the Adriatic during the summer of 1989 was tentatively invoked to explain an exceptionally low level of landings in 1991 (Stachowitz et al., 1990; Del Piero et al., 1998; Froglia, 2000). According to Romanelli et al., (2009) the progressive reduction of freshwater flow into the Adriatic Sea, as well as of its phosphate content, have been playing a relevant role in the clam reduction.
Morello et al., 2011 presented the outcomes of fishery-independent surveys conducted in the Ancona (AN) and S. Benedetto (SB) Maritime Districts from 1984 to 2001 to assess the C. gallina stock. The study revealed a considerable year-to-year fluctuations and the results are indicative of a resource and fleet heavily dependent on stochastic substantial recruitment events. Large recruitment events followed by significant natural mortality episodes and the paucity of older individuals may suggest a shift in the allocation of energy, from growth to reproduction.
From an Ecosystem Approach to Fisheries Management (EAFM) perspective, the hydraulic dredging determines strong physical disturbances of the sea bed. As a matter of fact the impact of hydraulic dredges not only scrapes the surface of the substratum but also dig into it, re-suspending significant amounts of sediment. It has been found that hydraulic dredging contributes to destabilisation and partial modification of sediment conditions, resulting in an overall decrease in habitat complexity with direct implications for the benthic community (Godcharles 1971; Meyer et al. 1981; Brambati and Fontolan 1990; Pranovi and Giovanardi 1994; Tuck et al. 2000; Kaiser et al. 2002). Drastic reductions in abundances of infaunal organisms are widely reported as a consequence of mechanical, suction and hydraulic dredging (Hall et al. 1990; Hall and Harding 1997; Pranovi and Giovanardi 1994; Tuck et al. 2000). Shifts in benthic community structure in favour of a few dominant opportunistic species have been observed (Dayton et al. 1995; Pranovi and Giovanardi 1994). This is a condition that Warwick (1986) associated with disturbance.
Short-term effects of inshore fisheries such as hydraulic dredging are widely reported in the literature (Godcharles 1971; Hall et al. 1990; Hall and Harding 1997; Tuck et al. 2000). Communities living in high-energy environments and constantly subjected to natural environmental stress will be less susceptible to fishing disturbance (Currie and Parry 1996; Kaiser et al. 1996; Ball et al. 2000; Craeymeersch et al. 2000; Jennings and Reynolds 2000; Zajac and Whitlatch 2003). This may be the case for the microbenthic community inhabiting the shallow sandy bottoms of the area considered in this study. Frequent small- scale disturbances, such as dredging operations, may thus be masked by large-scale environmental perturbations, such as storms, and prevailing hydrodynamic processes may be among the key factors determining the extent to which an area will be resilient to fishing disturbance.
Morello et al. (2005b) has highlighted important aspects of medium-term effects of hydraulic dredging in the Central Adriatic Sea. The results of this study have emphasised the fact that, despite intensive fishing that has been going on for decades and a benthic community that is typical of a moderately disturbed environment, above threshold intensity, the effects of fishing on community structure were still discernible over and above natural variation and this was particularly true for the shallower assemblages. Furthermore, whilst the results described indicate a relatively rapid recovery of the system (within 6 months), longer-term recovery studies are necessary to understand the complexity of benthic successional dynamics in the area.
The new Common Fisheries Policy (CFP) includes the application of the ‘precautionary principle’ to fisheries management, and the progressive implementation of an ecosystem approach. Consequently, the Commission defined objectives for CFP management decisions to be based on the best available knowledge about the interactions between fishing and ecosystems, and that both direct and indirect impacts on the marine environment are minimised, in particular by reducing the overall fishing pressure.
Taking into consideration such objectives, the clam fishery historically carried out in the Adriatic sea needs to be examined in the light of the European Union (EU) approach to protect the ecological balance of our oceans as a sustainable source of wealth and well- being or future generations. In particular the present study focuses on the high fishing pressure on the commercial bivalve resources, on the socio-economic cascade due to the decrease of the resources and on the impact on the environment caused by the fishing gear employed.
Link to the full study: http://bit.ly/573-412
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