Modeling sardine and anchovy population dynamics

11 
 
Chapter 1 
 
Introduction 
 
1. Introduction 
 
The North Central Adriatic Sea is the widest shelf of the Mediterranean Sea. This region 
features different habitats and high marine biodiversity (Barausse et all. 2009, Coll et all. 
2010).  
The North and Central (NC) Adriatic Sea, in principle, provides good conditions for sardine 
and anchovy populations, given the productivity of the ecosystem. Nevertheless the past four 
decades show, for this area, a notable decrease in abundance and in biodiversity of pelagic as 
well as of benthic fish. The cause of these changes has been ascribed to fishing pressure, 
environmental factors and other anthropogenic impacts such as nutrient enrichment and the 
resulting eutrophication (Coll et al. 2010). 
Recent scientific surveys (Santojanni et all. 2010) report that the sardine population crashed 
from an estimated average of more than 620000 tonnes of fish in the water before 1993 to less 
than 130000 tonnes in the years after 2000  and anchovy population declined from 300000 
tonnes to 150000 during the early 1980s and then declined again between 1995 and 2001.  
Anchovy and sardine, known locally as ‘blue gold’, are of major importance for Adriatic 
fisheries and ecology. Indeed together these two species account for approximately 40% of 
total Adriatic marine catches (FAO, 2007). 
The economical importance of these pelagic fish in Adriatic has led to many scientific surveys 
for providing a good management of fishery. However their dependence on environmental 
and climatic factors and fishery is not completely understood yet (Morello et al. 2009).  
Therefore, a need for ecosystem-based studies to inform human marine resources 
management is still present. This is underlined by the recent Marine Strategy Framework 
Directive (MSFD) 2008/56/EC. The directive asks for an ecosystem approach to fishery, 
based on indicators on the state of the ecosystem, the fishery impact and the socio-economic 
consequences determined by the management policy. Interactions between the ecological 
components of the ecosystem (e.g. predator-prey interactions), as well as the feedbacks 
between those ecological components and coastal socio-economic systems should be taken 
into account to achieve a truly sustainable management of the marine environment. The
12 
 
Chapter 1 
directive requires that the Common Fishery Policy takes into consideration the environmental 
impacts of fishing and anthropogenic activities to achieve a good environmental status of 
marine ecosystems (for example controlling eutrophication) and to ensure that populations of 
fish are within safe biological limits. In other word the directive aims to coupling sustainable 
use, conservation and socio-economic issues of marine ecosystems. 
In the present study fishery and environmental impacts are combined to describe the 
dynamics of the NC Adriatic sardine and anchovy populations, with the goal of identifying 
the key processes driving the changes over time in the biomass of these fishes. This 
information is crucial to help defining an ecosystem approach to the management of anchovy 
and sardine based on sound ecological knowledge, something that has rarely been done in the 
past scientific literature so far (but see Barausse et al. 2009, Coll et al. 2007, 2010). This 
research is based on various single stage production models represented by modified logistic 
growth differential equations, which are fitted to observations represented by anchovy and 
sardine population biomass time series estimated through virtual population analysis (VPA) 
stock assessment done by the AdriaMed (MIPAAF-FAO project) working group for small 
pelagic. The biotic (including human) and abiotic factors, which interact with pelagic fish, 
considered in this work are the sea water temperature, phosphates load, Po river  freshwater 
flow, predators of pelagic fish biomass (due to mackerel), competitors biomass (due to 
anchovy upon sardine and vice versa), and fishing mortality.  
In order to achieve an ecosystem-based management of the NC Adriatic Sea these work’s 
goals are the improvement of the scarce understanding of what happens when the many 
different pressures are acting together on this ecosystems. Synergies between fishing, climate 
and pollution are studied to provide answers on which can be the causes of anchovy and 
sardine collapses and which are, between them, the major drivers factors to consider for a 
sustainable management of the resource. 
The aims of this work are reached thanks to the development of various, simple production 
models used to explore the dynamics of pelagic fish in the North Central Adriatic Sea from 
1975 to 2010 and understand which environmental/anthropogenic factors affect mostly 
sardine/anchovy populations. This last feature is achieved by confronting the obtained results 
through goodness of model fit indicators such as the Root Sum of Squared Error RSSE, Nash 
- Sutcliffe Efficiency and Akaiake Information Criterion. The models outputs are obtained 
through to the implementation of a Matlab code. The code manages to solve the logistic 
differential equation, to calibrate the model parameters to the values that best fit to VPA data 
and to simulate the effect of forcing functions(temperature, fishing mortality etc.). The “best” 
(according to the abovementioned indicators of goodness of fit) models are then used to 
provide some predictions regarding pelagic fish biomass in the next decade (2011 – 2020) 
based on possible future scenarios of the forcing functions of the model.
13 
 
Introduction 
By providing a better understanding of the effect of the Po river nutrients discharge on pelagic 
fish dynamics, this research has also the foal of investigating a potential mismatch between 
the MSFD and the Water Framework Directive (WFD). Indeed, the Po river nutrients 
discharge is regulated by the WFD, whose aim is (also) to reduce human-induced nutrient 
loadings to coastal zones, while the pelagic fish dynamics are regulated by the MSFD, which 
demands productive and healthy stocks; yet, the biomass of fish in the Adriatic Sea is well-
known to depend positively on the amount of nutrient inputs from coasts which enhances the 
primary productivity of the system and, consequently, that of the whole marine food web 
(Barausse et al. 2011). The examination of such potential mismatch is key to achieve the 
goals of the MSFD, as a successful implementation of the MSFD requires the integration of 
sectoral policies, whether dealing with marine sectors or with activities on land affecting the 
marine environment. 
This research constitutes the first attempt to describe the population dynamics of pelagic fish 
in the NC Adriatic Sea by accounting for environmental changes, anthropogenic impacts and 
fishing pressure through the construction of continuous production models.
15 
 
Chapter 2 
 
Materials and Methods 
 
2. Materials and Methods 
2.1 Study area 
 
The dynamics of sardine and anchovy biomass is analyzed in the North and Central Adriatic 
Sea (FAO Geographical Sub Area (GSA) 17- southern limit: Gargano Promontory). The 
northern and central Adriatic, as a part of the Adriatic Sea, is a basin semi-enclosed by Italy, 
Slovenia and Croatia. The North Central (NC) Adriatic Sea is characterized by high 
freshwater inflows and nutrient inputs in its northern area, mostly from Po river, which 
crosses the intensely populated, industrialized and cultivated northern Italian plains. These 
features make the northern Adriatic Sea eutrophic and productive. Figure 1 shows the 
bathymetry of the Adriatic Sea. The northern Adriatic is a shallow basin with an average 
depth of 50 meters, which rarely exceed the 100 meters depth. The central Adriatic is deeper 
and it is characterized by three pits. Among them, the Mid Adriatic pit, known as Jabunka pit 
which deep reaches 270 meters, is the deepest. Riverine nutrient loads represent, together with 
fishing pressure, a major driver of variations in fish and invertebrate biomass and community 
composition in the basin over the past decades (Barausse et al. 2011). The most abundant and 
exploited pelagic fish populations are sardine (Sardina pilchardus) and anchovy (Engraulis 
encrasicolus). 
In the Adriatic Sea, the pelagic food web is dominated by low-medium trophic levels which 
comprise mainly zooplankton, anchovy and sardine. The high pelagic production sustains an 
intense fishing effort which causes low maturity and stress in the NC Adriatic Sea (fig. 2). 
Along the years these features have led to a low abundance of top predators. Nowadays most 
of top predators do not influence the trophic network having low biomass and impact on the 
Adriatic ecosystem (see Barausse et al. 2009 for the northern basin). 
The small pelagic fishery has developed on both East and West sides of the Adriatic; 
however, more than 90% of the anchovy catches are landed by the Italian fleet, while the 
pelagic fleets from Slovenia and Croatia have concentrated primarily on sardines. In fact the 
eastern Adriatic sardines catches are almost equal to the Italian Adriatic catches (Cingolani et 
al. 2004).
16 
 
Chapter 2 
Fig. 1 Adriatic Sea bathymetry. North and Central Adriatic Sea. Source: Image from Eoearth.org
17 
 
Materials and  methods 
2.2 Background of sardine biology and fishery 
2.2.1 Characteristics 
 
Sardine (Sardina pilchardus) is a pelagic fish whose length can reach, if allowed to grow to 
maturity, 21 cm (or 86 g of mass). Sardines reach sexual maturity between the first and the 
second year when individual length is around 8 cm. Sardine can live up to 8 years (Morello et 
al. 2009). Their legal size limit has been set at 11 cm. Recent research showed that only 
around 5% of sardines landed were classified as ‘large’ adults confirming that the number of 
large sardines caught decreased with respect to the past (fig. 2), a possible sign of overfishing. 
 
Fig. 2 Average length of sardine in the NC Adriatic. Data from the public institute for marine research 
(ICRAM), published in 1997and the Italian Fishery Research and Studies Center CIRSPE 
 
2.2.2 Reproduction  
 
In the Adriatic Sea spawning takes place mainly in winter, generally between October and 
May, near the coast of Croatia. In particular two main spawning grounds was discovered in 
the northern Adriatic off the Dugi Otok Island and the central-southern Adriatic around the 
mid-Dalmatian Islands. Spawning timing and success rely on environmental biotic and abiotic 
factors. The most important factors are temperature, salinity and food availability (Regner et 
al. 1987). Spawning, in the Adriatic Sea, has been reported to take place between 9 and 20°C 
at salinities ranging from 35.2 to 38.8 psu. Spawning peaks occur between 11 and 16°C.
18 
 
Chapter 2 
2.2.3 Migrations 
 
As it appears clear in Fig. 3, in the northern area migration is longitudinal. In winter the adults 
population migrates southwards from the Gulfs of Trieste and Venice and the Istrian coast 
towards Dugi Otok for spawning (Tičina et al. 2000). This happens due to spawning strategy 
of sardine, which consist of a search for a favorable environment for eggs and larvae. This 
species of pelagic fish reproduce in a environment represented by hydrographically stable 
waters, which are poor in food availability, but also in predators. In spring, at the end of 
spawning, the spawners follow the reverse route and migrate northwards towards the 
productive northern Adriatic waters in search of food (Regner et al. 1987).  
Migration to and from the central Adriatic spawning grounds is, on the other hand, transverse 
(Regner et al. 1987). During the last stages of sexual maturation, in late autumn/winter, adult 
sardines migrate offshore towards the deeper, colder waters of the outer Dalmatian Islands. 
The inverse happens in early spring when adults, larvae and post-larvae turn towards the 
inshore waters searching for food (Mužinić 1973).  
 
Fig. 3 The migrations of sardine in the Adriatic Sea. Image from Morello et al. 2009
19 
 
Materials and  methods 
2.2.4 Feeding 
 
Adult sardine feed on both zooplankton (mainly adults and juvenile copepods) and 
phytoplankton, although some authors support the thesis that phytoplankton dominates in 
stomach contents (Oliver et al. 1952) but others have found the opposite (Cunha et al. 2005). 
Larval sardine, in contrast, feed primarily on zooplankton (Fernandez et al. 2006) and 
copepod. 
 
2.2.5 Fishery 
 
Sardines are fished by purse seiners (attracting fish by light) and pelagic trawlers belonging to 
Italy, Croatia and Slovenia. The fishery takes place during all year. A closure period is 
observed from the Italian pelagic trawlers on August, while from 15th December to 15th 
January in Croatia. Exploitation is based on all the age classes from 0 to 6+. The Croatian 
catches of sardine represent the largest fraction of the total catches, while the Italian small 
pelagic fishery concentrate mainly on anchovy (though high amounts of sardine were also 
caught by the Italian fleet in the past). The Italian fleet is composed of about 65 pairs of mid-
water trawlers and about 45 purse seiners (with quite different tonnage), with the former 
being predominant on the latter ones. In Croatia, small pelagic (mainly sardine) are fished by 
purse seiners (Santojanni et al. 2010). 
 
2.2.6 Predation 
 
Very few data are available on predation of sardine in the Adriatic Sea. Some authors report 
that a significant amount of sardine larvae are eaten by mackerel post-larvae, while the 
copepod Candacia feed on both sardine larvae and post-larvae (Karlovac 1967). Predators of 
adult sardine are primarily hake (Merluccius merluccius), mackerel (Scomber scombrus), 
tunnids (e.g., Thunnus thynnus) and other large pelagic fish species, marine birds and 
dolphins (Coll et al. 2007). 
 
2.3 Background of anchovy biology and fishery 
2.3.1 Characteristics 
 
Anchovy (Engraulis encrasicolus) is a small pelagic fish. Adults individuals length can grow 
up at maximum approximately 20 cm. Similarly to sardine, anchovy reach sexual maturity at
20 
 
Chapter 2 
the end of first year when individuals length is around 8 - 9 cm. Anchovy can live at 
maximum 6 years (Morello et al. 2009). The law requires anchovies to be at least 9 cm when 
they are landed. The high fishing  pressure leads to the disappearance of large, mature 
individuals and results in a population that is dominated by small and juvenile fish. The 
accompanying figure 4 show graphically how the size of anchovy in the region have shrunk in 
recent years. 
 
Fig. 4 Average length of anchovy in the NC Adriatic. Data from the public institute for marine research 
(ICRAM), published in 1997and the Italian Fishery Research and Studies Center CIRSPE 
 
2.3.2 Reproduction  
 
In contrast to sardine, anchovy spawning takes place in the warmer months and in highly 
productive waters. Generally the spawning season occurs between April and October (Regner 
1972). The presence of anchovy eggs in the Adriatic has been reported at temperatures 
between 11.6 and 28°C and at salinities ranging from 35.2 to 38.8 psu (Morello et al. 2009). 
Spawning peaks occur at the higher temperatures in the range between 18 and 28°C whereas 
egg density is inversely proportional to salinity (Zavodnik 1970).  
It is believed that the onset and duration of spawning are heavily influenced by temperature 
and most of all by food availability that appears to govern the spawning dynamics of anchovy 
in both spatial and temporal terms (Regner 1987). 
Vučetić (1975) found that variations in egg densities in the central Adriatic were closely 
correlated with variations in zooplankton and increases in egg numbers coincided with 
enhanced ingressions of nutrient-rich Mediterranean water that are known to enhance