Original article
Economic modelling as a tool to support macroalgal bloom management:
a case study (Sacca di Goro, Po river delta)
Un modŁle Øconomique comme outil de gestion d une oraison
macroalgale. Un cas d Øcole : la Sacca di Goro dans le delta du P
Francesca Cellina
a
, Guilio A. De Leo
a,b,
*, Andrea E. Rizzoli
c
,
Pierluigi Viaroli
a
, Marco Bartoli
a
a
Dipartimento di Scienze Ambientali, Universit degli Studi di Parma, Parco Area delle Scienze 33/A, 43100 Parma, Italy
b
Agenzia Regionale per la Protezione Ambientale, Via Restelli 1, 20124 Milano, Italy
c
Istituto Dalle Molle di Studi sull Intelligenza Arti ciale, Galleria 2, 6928 Manno, Switzerland
Received 31 July 2002; accepted 30 September 2002
Abstract
During the last 20 years, intensive mollusk farming has been developed in coastal waters, mostly in sheltered bays and lagoons. Often,
mollusk stocks are threatened by frequent anoxic events from macroalgal blooms. Here, a decision support tool is described to select the
optimal short-term strategy to control algal biomasses. Even though long-term and detailed studies of the lagoon systems are required to
provide reliable, biologically based policies, we have here developed a simpli ed analysis that overlooks most of the ecological complexity,
but explicitly includes environmental variability and uncertainty in parameter estimation in the economic assessment of the performances of
different management strategies. The aim is to quickly screen management alternatives of harvesting Ulva rigida biomass in terms of the
control measures considered, i.e. the number of vessels used (the harvesting effort) and the U. rigida density at which the vessels start to
operate. The decision support tool was applied to the Sacca di Goro, the southernmost coastal lagoon of the Italian Po river delta, where
intensive farming of the clam Tapes philippinarum occurs. The results of the analysis indicate that an intermediate number of vessels should
start operating as soon as U. rigida biomass exceeds low thresholds.
' 2003 ditions scienti ques et mØdicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.
RØsumØ
Ces vingt derniŁres annØes, l aquaculture des mollusques s est intensi Øe dans les eaux c tiŁres, particuliŁrement dans des baies abritØes
et des lagunes. Souvent, les mollusques sont perturbØs par des crises anoxiques liØes des oraisons macroalgales. Nous dØcrivons un outil de
dØcision permettant de choisir la meilleure stratØgie court terme pour contr ler la biomasse des algues. MŒme si des Øtudes dØtaillØes long
terme des systŁmes lagunaires sont nØcessaires pour dØ nir une politique durable basØe sur la biologie, nous prØsentons ici une analyse
simpli Øe qui s affranchit d une bonne part de la complexitØ Øcologique, mais qui inclut la variabilitØ de l environnement et l incertitude de
l estimation des paramŁtres dans l approche Øconomique des performances des diffØrentes stratØgies de gestion. L objectif est d examiner
rapidement les scØnarios de ramassage d Ulva rigida en termes de nombre de navires utilisØs (effort de pŒche) et de densitØ d U. rigida
laquelle les navires cessent de travailler. Cet outil de dØcision a ØtØ appliquØ au Sacca di Goro, la lagune la plus au sud du delta du P , siŁge
d une aquaculture intensive de Tapes philippinarum. Les rØsultats montrent qu un nombre intermØdiaire de navires doit commencer
l opØration aussit t que la biomasse d U. rigida dØpasse les seuils bas.
' 2003 ditions scienti ques et mØdicales Elsevier SAS and Ifremer/CNRS/IRD. Tous droits rØservØs.
Keywords: Algal bloom management; Bioeconomic analysis; Stochastic modelling; Ulva rigida; Tapes philippinarum
Mots clØs :Gestion d une oraison macroalgale ; Analyse bio-Øconomique ; ModØlisation stochastique ;Ulva rigida ; Tapes philippinarum
* Corresponding author.
E-mail address: deleo@dsa.unipr.it (G.A. De Leo).
Oceanologica Acta 26 (2003) 139 147
www.elsevier.com/locate/oceact
' 2003 ditions scienti ques et mØdicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.
DOI:10.1016/S0399-1784(02)01238-0

1. Introduction
Increasing pressure on coastal areas from land use change,
sewage discharge and agricultural chemicals are con icting
uses with the preservation of ecological integrity, the conser-
vation of recreational use and the development of economic
activities such as sheries and mollusk farming (Charlier and
Lonhienne, 1996). Eutrophication and intensive aquaculture
in coastal lagoons created suitable conditions for the blooms
of macroalgae in several coastal lagoons and bays in the last
20 years (Sfriso et al., 1992; Borum, 1996; Castel et al., 1996;
Flechter, 1996; Morand and Briand, 1996; Solidoro et al.,
1997; Valiela et al., 1997; Raffaelli et al., 1998). Algal
blooms are particularly noxious, as the consequent anoxic
crises lead to massive mortality of clam with resultant eco-
nomic damages to the mollusk shery.
Lagoon managers have a suite of different strategies to
manage these problems (Charlier and Lonhienne, 1996;
Scheffer, 1998). Long-term, comprehensive policies that tar-
get reduction of nutrient loading from agriculture and sewage
systems require large investments, long implementation
times and a coordinated land planning strategy with different
public bodies governing the territory.
On the contrary, lagoon managers often rely on medium-
to short-term policies that focus on the limitation of the
damages from blooms rather than on the causes that trigger
them. For instance, channels can be dredged in a lagoon to
improve hydrodynamics and thus reduce the risk of anoxia.
However, while dredging must be planned far in advance, the
direct removal of algal beds with special vessels can be
performed almost in emergency conditions with little or no
planning and a fairly limited cost and, thus, it has become the
preferred short-term strategy for algal bloom control. This
strategy can be considered effective if it is able to avoid algal
collapses and the consequent economic damages to aquacul-
ture. The actual ability to control algal blooms depends upon
the number of the vessels used, their harvesting ef ciency
and the threshold of macroalgal biomass above which the
vessels start to operate. Other variables, such as macroalgal
growth rate and nutrient availability in the water column, and
several physical factors, water temperature, light intensity,
wind, currents, waves which may be important determi-
nants of algal growth cannot be controlled by the lagoon
managers. Harvesting costs obviously increase with the num-
ber of vessel-days and their actual value depends also upon
the rental rate of vessels, man-power and the costs of disposal
of the harvested biomass. These costs should be compared to
the bene ts measured in terms of avoided economic losses of
commercial yield that would be otherwise caused by algal
blooms and the consequent anoxic crises. Unfortunately,
despite the widespread use of these short-term strategies, the
effectiveness of this policy has been rarely assessed in a
quantitative framework of a cost revenue analysis.
The Sacca di Goro a large shallow coastal lagoon (sur-
face: 26 km
2
; average depth: 1.5 m) located on the southern
Po river delta in Italy (Fig. 1 ) is an interesting testing
ground to assess the economic ef ciency and the ef cacy of
harvesting Ulva to control algal blooms. The Sacca di Goro
has four nutrient-rich freshwater inlets (Po di Volano and Po
di Goro rivers and Bianco and Giralda channels) and is
separated from the sea by a narrow sandy barrier. There are
two openings to the Adriatic Sea of ca. 1 km in width. Since
the late 1980s, the introduction of the exotic clam species
Tapes philippinarum has locally enhanced the mollusk sh-
ery, which still has not recovered since the great economic
turndown of the late 1970s and the resultant market collapse
(Carrieri et al., 1992). Today clam shery covers an area of
10 km
2
and is concentrated in the south-central part of the
lagoon. Average income from shell sh production is ca.
50 million euros, distributed among a 1000 local people, who
depend directly or indirectly on the shery (Bencivelli, per-
sonal communication).
As for many eutrophic coastal environments, nutrient
loads have led in the Sacca di Goro to the disappearance of
rooted macrophytes and the appearance of opportunistic
oating macroalgae. During winter and early spring, due to
low biomass, these algae drift within the lagoon, whilst
above 3 400gDWm
2
, they stand and accumulate in the
shallower sites. Water stagnation due to Ulva beds has dra-
matic implications for clams, leading to extended mortality
and considerable economic losses.
Lagoon managers attempts of direct intervention have
centred on harvesting Ulva with two vessels rented from a
third part company. The number of vessels and operational
mode are usually constrained by budget limitation on a year
to year basis. The aim of this work is to assess the effective-
ness and the ef cacy of this policy, namely to: (i) evaluate its
ability to control algal blooms; (ii) assess whether this policy
is also economically effective, namely whether on average
the bene ts exceed the costs, and with what frequency; (iii)
identify possibly more effective policies of Ulva harvesting,
if any, in terms of number of vessels used and their harvesting
intensity, under a variety of scenarios, including lower rent-
ing costs of vessel-days and the acquisition of a own eet
instead of renting vessels from third part companies.
Fig. 1. Sacca di Goro map and its position in Northern Italy.
140 F. Cellina et al. / Oceanologica Acta 26 (2003) 139 147