The body of European hake is long and cylindrical. The widest part is behind the head. The mouth is large. There are two dorsal fins. The first one is short and triangular and the second one is long. The anal fin is similar in shape and size to the second dorsal fin. The ventral fins are placed before the pectoral ones. The caudal fin is cut in a straight line.

Morphology data:
The number of rays in particular fins is as follows: D1: 8-10, D2: 35-40, A: 36-40, P: 12-14, V: 7 (Fisher et al., 1987; Jardas, 1996). The colour is slate grey above and lighter on sides, the belly is whitish (Relini et al., 1999).

World:
European hake inhabit the north-eastern Atlantic from Norway to Mauritania and the entire Mediterranean; in the Black Sea the species lives only along the southern coasts (Jardas, 1996, Relini et al., 1999).

Adriatic Sea:
According to available data, European hakes are distributed throughout the Adriatic. It is a distinctively euritopic species. Bathymetric distribution of the species in the Adriatic is from only several meters in the coastal area to 800 m in the South Adriatic Pit (Kirinčić and Lepetić, 1955; Županović and Jardas, 1986; Ungaro et al., 1993; Jukić et al., 1999). There are only limited areas to the north of the Po delta in which it is not caught (Jukić and Arneri, 1984; Frattini and Paolini, 1995; Frattini and Casali, 1998).

Habitat:
This nectobenthonic species is most abundant at depths between 100 and 200 m, where the catches are mainly composed of juveniles (Ghirardelli, 1959b; Županović, 1968; Jukić and Arneri, 1984; Flamigni, 1983; Giovanardi and Rizzoli, 1984 Bello et al., 1986; Županović and Jardas, 1989; Ungaro et al., 1993; Vrgoč, 2000).

In daylight, the European hake stay on the bottom and move vertically to higher strata at night (Jardas, 1996). In addition to circadian migrations, there are also horizontal migrations as a consequence of searching for food.

Seasonal migration:
Spring:
In the spring months, there are local movements of sexually immature adolescent hakes into the more shallow channel waters of the central Adriatic between Croatian islands. Adult European hake are mainly caught at depths of 100 to 150 m. In the spring, adult hakes migrate to more shallow coastal waters for spawning. The juveniles display migration patterns in search of food.
Winter:
In the winter period, after spawning, adult fish migrate towards the deeper water, wintering with the juveniles (Županović and Jardas, 1989). In the southern Adriatic the largest individuals are caught in waters deeper than 200 m, whereas medium-sized fish appear in the stratum not deeper than 100 m (Ungaro et al., 1993).

Bottom preferences:
European hake prefer muddy bottoms, but are well distributed on other types of bottom as well (muddy-sandy and sandy bottoms). It is most abundant in the open central Adriatic (the Pomo/Jabuka Pit) and further southwards (Županović, 1961a, Županović and Jardas, 1986).

Distribution map of Merluccius merluccius in the Adriatic Sea: indicator kriging representation (Sabatella and Piccinetti, 2005).
Data: Medits Programme (CLICK TO ENLARGE):

Some examples on the distribution of the total stock, recruits and spawner fractions in the GSA N° 18
Abundance data coming from Medits survey 1996-2004 in the GSA 18 were processed by means of geostatistic techniques. The Indicator Kriging procedure was utilised in order to best manage all the available data. The indicator kriging permits to correct the effect of spatial clustering of sample points that sometimes occurs in the theoretical random sampling design. Several threshold were used to best fit the probability that value at any location exceed the limit value (e.g. number per Km2>1) and resulting maps were combined to better describe the spatial distribution of the abundance index values. In order to perform semivariogram analysis and subsequent Kriging Interpolation, GSTAT Software was used. Map representations for the GSA 18 were obtained by means of ESRI ArcView GIS. The examples are related to the total stocks, recruits and spawner fractions. The cut-off values and R/ Tot – A/Tot ratio weren’t considered.

Size:
According to Jardas (1996), European hake can grow to 130 cm of total length. However, its usual length in trawl catches is from 10 to 60 cm. This is a long-lived species, it can live more than 20 years. In the Adriatic, however, the exploited stock is mainly composed in number of 0+, 1+ and 2+ year-old individuals. On the basis of the vertebral counts of European hake from the northern and central Adriatic, Piccinetti and Piccinetti Manfrin (1971b) found that all specimens analysed belonged to the same population. Similarly, the Adriatic population has the same number of vertebrae as the European hake from the rest of the Mediterranean (Maurin, 1965).

Length-weight relationship:
The data about the length-weight relationship are summarised in the table Jardas (1976) found out that the length-weight relationship could be divided into three phases according to the coefficient b: juvenile, adolescent and adult.

Total Length (TL, cm) – weight (g) relationship.

Reproduction:
In the Adriatic, European hake spawn throughout the year, but with different intensities. The spawning peaks are in the summer and winter periods (Karlovac, 1965; Županović, 1968; Županović and Jardas, 1986, Županović and Jardas, 1989; Jukić and Piccinetti, 1981; Ungaro et al., 1993). Hake are partial spawners. Females spawn usually four or five times without ovarian rests. In females in the pre-spawning stage, fish 70 cm long can contain more than 400 000 oocytes (Sarano, 1986). The earliest spawning in the Pomo/Jabuka Pit occurs in winter in deeper water, (up to 200 m). As the season progresses into the spring-summer period, spawning occurs in more shallow water. The recruitment of young individuals into the breeding stock has two different maxima. The first one is in the spring and the second one in the autumn.

In the Pomo/Jabuka Pit, both of these maxima can be linked to hake's more intense summer and winter spawning period in the central Adriatic (Županović and Jardas, 1989). The recruitment peaks are in the spring and autumn (Karlovac, 1965). Recruitment does not seem to be related to the parental stock size (Alegria Hernandez and Jukić, 1992). Nursery areas are located close to the Pomo/Jabuka Pit, between 150 and 200 m, on the upper part of the slope, and off the Gargano Cape (Županović, 1968; Jukić and Arneri, 1984; Županović and Jardas, 1986, Županović and Jardas, 1989; Frattini and Paolini, 1995; Frattini and Casali, 1998). Karlovac (1965) recorded young hake larvae from October to June, the highest numbers were recorded in January and February. Larvae and postlarvae were mainly distributed between 40 and 200 m; the highest number of individuals was caught mainly between 50 and 100 m.

Length at the first sexual maturity:
Different data about the size at first sexual maturity of European hake in the Adriatic Sea, given by different authors, are shown in table.

Total Length (L_m, cm) at the first sexual maturity.

Differences in the growth dynamics between males and females can be seen in the following Tables. Females attain larger size than males, who grow more slowly after maturation at the age of three or four years. Consequently, the proportion of males in the population is higher in lower length classes and proportion of females is higher at greater lengths. In the central and northern Adriatic, females already start dominating the population at lengths of about 30 to 33 cm. In trawl catches over 38 to 40 cm, almost all the specimens are females (Vrgoč, 2000).

Total Length and age data:

Total Length (TL, cm) and age (year) data.

Von Bertalanffy Growth Function (VBGF).
Parameters of the Von Bertalanffy Growth Function (VBGF).

Feeding behaviour:
Until they are about 16 cm long (first year of life), European hake feed mostly on crustaceans (Euphasiacea, Mysidacea and Amphipoda). During that period, they live predominantly in the Pomo/Jabuka Pit and in the southern Adriatic pit region.

Their migration to the channel regions of the eastern Adriatic coast is linked to the changes of feeding patterns as they start feeding on fish, primarily Sardina pilchardus, Sprattus sprattus and Engraulis encrasicolus. Other fish prey of European hake are Scomber scomber, Trachurus spp. and Merluccius merluccius. Cephalopods were also found in hake stomachs (Kirinčić and Lepetić, 1955, Karlovac, 1959; Županović, 1968; Piccinetti and Piccinetti Manfrin, 1971a; Jukić, 1972; Froglia 1973; Jardas, 1976; Ungaro et al., 1993).

By comparing the catch of European hake during the expeditions “Hvar” and “Pipeta” (1982), in the northern and central Adriatic, Jukić and Arneri (1984) found that the highest catches were during the “Hvar” expedition in depths over 200 m (6.05 kg/hr).

CPUE:
During the “Pipeta” expedition, the highest catches were in the 50 to 100 m stratum (2.96 kg/hr). Jukić and Piccinetti (1981) found that, in the 1970s, catches were about 6 kg/hr. In the southern Adriatic, the CPUE varied from 1985 to 1997 in the range of 1.4 to 9.9 kg/hr (Marano et al., 1998b Marano et al., 1998c).

Biomass/Abundance Indexes:
During the MEDITS expedition (1996-98), in the central and northern Adriatic, the European hake's biomass index was 47.45 kg/km2 (the average value for the entire region). The highest catches were in the 100 to 200 m stratum (71.80 kg/km2) and the smallest in the 50 m stratum (14.65 kg/km2). The proportion of hake in the total demersal fish catch was 16.41% (Vrgoč, 2000).

In the Adriatic, small specimens dominate the catches. Most specimens are under 20 cm TL (Županović, 1968; Jukić and Piccinetti, 1981; Flamigni and Giovanardi, 1984; Jukić and Arneri, 1984; Bello et al., 1986; Giovanardi et al., 1986; Županović and Jardas, 1986 Županović and Jardas, 1989; Alegria Hernandez and Jukić, 1992; Ungaro et al., 1993; Marano et al., 1998b Marano et al., 1998c; Ungaro et al., 1998, Vrgoč, 2000). The proportion of juveniles in the catch of European hake in the Adriatic during the expedition “Hvar” was 72.3% and during the MEDITS expedition, 81.52% (Vrgoč, 2000). In 1972-73, a maximum production (MSY) of 3000-4000 tonnes/year was estimated for the Adriatic Sea (Jukić and Piccinetti, 1981). From the management point of view, an increase of the mesh size at first capture will increase the hake yield (Giovanardi et al., 1986; Jukić and Piccinetti, 1987). Kirinčić and Lepetić (1955) and De Zio et al. (1998) investigated the catch size structure from the bottom long-line fishery in the Southern Adriatic. The average total length of the European hake was 58.6 cm (Kirinčić and Lepetić, 1955), while De Zio et al. (1998) found a median total length of 70 cm. The average catch was 5.6 specimens per 100 hooks.

In the Adriatic, the species is mainly fished with bottom trawl nets, but long-lines and trammel-net are also used. According to the FAO statistics, in the 1980s and 1990s the annual European hake landings in the Adriatic were estimated at 2000 – 4000 tonnes, and this species was the most abundant within the demersal fish group.

Results from global models underlined the overexploitation of the European hake stock since the 1960s (Levi and Giannetti, 1972, Alegria Hernandez et al., 1982).

Mortality:
The mortality parameters of the European hake population in the Adriatic are shown in the following table.

Mortality rate coefficients for European hake in the Adriatic.

Recent time-series studies carried out in the southern and central Adriatic showed an apparent increasing trend of the survey catch rates from 1985 to 1995 and a decreasing trend during the second half of the 1990s (Piccinetti and Piccinetti Manfrin, 1994, Manfrin et al., 1998). In the southern Adriatic, recent time-series showed an apparent increasing trend from 1985 to 1993, and a decreasing trend from 1994 to 1997. Italian landings reached the maximum in the first half of the 1990s. In the eastern Adriatic, where the demersal fishery appeared to have developed quickly during the 1990s, a positive yield trend could be observed starting from the 1980s. However, in general a marked decrease could be observed after the relatively high landing of 1993 (Mannini and Massa, 2000).

Summary Table of European hake (Merluccius merluccius) life history for the Mediterranean Sea and North East Atlantic Ocean. Associations and interactions with environmental and habitat variables are listed in the 2 following tables: