Distribution and Population Structure of Greater Weever , Trachinus draco ( Linnaeus , 1758 . ) , in the Northern and Central Adriatic Sea

INTRODUCTION / Uvod The greater weever (Trachinus draco Linnaeus, 1758) is demersal marine fish widely distributed across the Mediterranean, Black Sea and Eastern Atlantic [1], [2] and notoriously known by its venomous spines which can inflict serious human injuries by accidental sting. The main toxin is single peptide protein called dracotoxin with hemolytic and membrane depolarizing activities [3]. Because of that, it is usually classified as one of the most venomous fish in the Mediterranean [4]. In the Adriatic it is mostly distributed in the channel areas preferring muddy and sandy sediments where usually rests on the bottom or buried in the sediment exposing eyes and first dorsal fins [5]. Although greater weever is widely distributed in the Mediterranean area, population density is not so high compared to the total catch of other commercially important demersal species and therefore has a minor commercial importance [6]. However, its venom glands makes greater weever protected from the most predators giving it the possibility to freely inhabit the area and preying other organisms. These biological and behavioral characteristics, from the the ecological point of view, raised this smaller body sized species higher in the trophic levels [7] and gives it a significant role in marine demersal ecosystem. Because of its minor commercial importance greater weever usually hasn’t been considered as a priority in fisheries biology. The lack of more detailed data in recent scientific literature about distribution patterns, population structure, dynamics and trends, especially for the Adriatic Sea, is evident. The aim of this article is to provide new data which could be used as an input in more complex fisheries management process, based on ecological approach, in order to maintain responsible exploitation of demersal resources. Igor Isajlović Institut za oceanografiju i ribarstvo Split e-mail: igor@izor.hr


INTRODUCTION / Uvod
The greater weever (Trachinus draco Linnaeus, 1758) is demersal marine fish widely distributed across the Mediterranean, Black Sea and Eastern Atlantic [1], [2] and notoriously known by its venomous spines which can inflict serious human injuries by accidental sting.The main toxin is single peptide protein called dracotoxin with hemolytic and membrane depolarizing activities [3].Because of that, it is usually classified as one of the most venomous fish in the Mediterranean [4].In the Adriatic it is mostly distributed in the channel areas preferring muddy and sandy sediments where usually rests on the bottom or buried in the sediment exposing eyes and first dorsal fins [5].Although greater weever is widely distributed in the Mediterranean area, population density is not so high compared to the total catch of other commercially important demersal species and therefore has a minor commercial importance [6].However, its venom glands makes greater weever protected from the most predators giving it the possibility to freely inhabit the area and preying other organisms.These biological and behavioral characteristics, from the the ecological point of view, raised this smaller body sized species higher in the trophic levels [7] and gives it a significant role in marine demersal ecosystem.Because of its minor commercial importance greater weever usually hasn't been considered as a priority in fisheries biology.The lack of more detailed data in recent scientific literature about distribution patterns, population structure, dynamics and trends, especially for the Adriatic Sea, is evident.The aim of this article is to provide new data which could be used as an input in more complex fisheries management process, based on ecological approach, in order to maintain responsible exploitation of demersal resources.

MATERIALS AND METHODS / Materijali i metode
The samples of greater weever from the Northern and Central Adriatic Sea were obtained during MEDITS Program ("Mediterranean International Trawl Survey") which were held in spring -summer period from 1996 to 2013 Samples were collected, based on MEDITS protocol [8] using specially designed bottom trawl net GOC 73.Sampling stations were randomly distributed according to the depth strata (10-50; 50-100; 100-200; 200-500; 500-800 m) and the number of stations was proportional to the surface of each stratum (Figure 1).The duration of tow in the area shallower than 200 m was 30 min, while in the area deeper than 200 m was 60 min.The population density was expressed as indices of abundance (Nkm -2 ) and biomass (kgkm -2 ) per square kilometer calculated according to Souplet (1996) [9].Ordinary kriging interpolation method was used for modeling spatial distribution based on population density as input variable [10].In order to evaluate spatial homogeneity of greater weaver data, the semi-variance was calculated, as a measure of the degree of spatial dependence between samples.Laboratory analysis was performed on 509 specimens, mostly collected from the eastern side of the Adriatic Sea.A total length was measured for all specimens to the nearest mm in length, and weighed to the nearest 0.01 g of wet mass.Sex and maturity were determined by macroscopic examination of gonads following MEDITS protocol.The hypothetical isometric growth and statistical differences between mean lengths of females and males were tested using Student's t test and differences between their distributions by x 2 test.The length-weight relationship was determined using the power function W=aLt b , where W is the somatic fish weight in g, Lt is total length of specimen in cm, a is a proportionality constant and b a regression coefficient [11].

RESULTS AND DISCUSSION / Rezultati i diskusija DISTRIBUTION / Rasprostranjenost
The data obtained during MEDITS surveys (1996 -2013) in the Central and Northern Adriatic Sea during spring-summer period shows that T. draco has stratified spatial distribution by the area and depth (Figure 2).The average population density of 15.75 Nkm -2 and 1.27 kgkm -2 for a total surveyed area varies between 31.02Nkm -2 and 2.58 kgkm -2 on the eastern side (Croatian territorial waters) to 7.31 Nkm -2 and 0.37 kgkm -2 on the western side (Italian territorial waters), while in the extraterritorial waters population density was 10.57Nkm -2 and 0.91 kgkm -2 (Table 1).Distribution patterns and population density shows that this species is more abundant in the eastern side of the Adriatic, mainly in the channel area and in the northern part along eastern coast.It is scarcely distributed on the western side, mostly in a shallower area of the Central Adriatic.The vertical distribution also differs between eastern and western side, but generally greater weever prefers area shallower than 100 m (Table 1).Decreasing of population density by the bathymetric gradient follows general rule of depth related differences in distribution for demersal species due to the changes of environmental factors [12], [13], [14].Greater weever has a higher population density on fine-grained sand and silty sand sediments (Table 1) Detailed analysis of greater weever's distribution showed strong variation in population density between different parts of the Adriatic covered by the same sediment inside the same depth strata.The Kruskal -Wallis test could not confirm significant differences in the distribution of greater weever, according to the bottom sediments.Some authors confirmed that greater weever is more abundant in shallower area, where sandy sediments dominate [12], [15], [16].Contrary to that, Gaertner et al., (1999) [17] found that distribution of greater weever is not so strongly associated with sediment type.Also, Bagge (2004) [15] furthermore describes seasonal migration of this species between areas with different sediment type.Moreover, if distribution of greater weever in the Northern and Central Adriatic is compared with the distribution of benthic biocenosis which were previously described [18], [19], it could be noticed that the area with a highest density of greater  weever's population overlaps with the areas where biocenosis of detritic bottoms prevails.Therefore, it is more likely that distribution pattern of greater weever is driven by complex association between specific bottom communities, together with combination of abiotic (temperature, salinity, etc.) and biotic factors (mostly intra and inter species relationship, food availability, etc).Similar species relationship between abiotic factors, demersal communities and trophic food weeb has been described for the Mediterranean [20], [13], [14] and Eastern Atlantic [15].

POPULATION DENSITY TRENDS / Trendovi gustoće populacije
The average values of population density fluctuated during the period from 1996 to 2013, but generally negative trend exists since 2008 with some evidence of recovery in 2013 (Figure 3).Demersal organisms in the Adriatic Sea are mainly exploited by bottom trawlers and it is well known that intense fishing effort can lead to negative changes in distribution and demographic structure of demersal species, especially those ones which are on the higher trophic level.This situation has already been described in the Adriatic for a large fish like Zeus faber [21] and Raja clavata [22].Observed negative trend of greater weever population and fluctuation of population density between years are probably not mainly caused by intensive fishing effort of bottom trawlers because this species, characterized as a smaller body sized predator which spent most of the time buried in the sediment, has a better survival rate than other demersal fish.Also a significant part of its population is not so exploited, because the channel areas of the eastern side of the Adriatic area  under strict fisheries regulation measures which significantly reduce fishing effort of bottom trawlers.Changes in a population dynamics of greater weever are probably more affected by climatic and hydrographic regime shifts of the Adriatic Sea rather then fisheries effort.Following these regimes shifts, which are closely correlated with changes of primary production described in several studies for the open Central Adriatic [23], [24], [25], it can be observed that positive trends of greater weever follow the positive trends of primary production and vice versa.Obtained value for parameter b in this study is slightly higher from previous result described in the Adriatic Sea [26].These differences are probably associated with inter-annual changes in the nutritional condition of the organisms, the different size composition of samples included in the analysis, small number of individuals in certain studies and different sampling season.

CONCLUSION / Zaključak
Although greater weever is not abundant species in the Adriatic Sea, it is nevertheless widely distributed and it can be found in all depth strata, except the deepest parts.Generally, it prefers the area shallower than 100 m depth.The population density is mostly stable, taking into consideration some negative trends in the last years, which have been observed in the most commercially important stocks.The population structure shows normal distribution without the indication of overexploitation both for juveniles and spawners.
[5] 4.11; p<0.05).Most of the mature females (48.7 %) were in spawning phase confirming that spawning period mainly occurred during spring-summer period[2],[5].The length-weight relationship for total sample (N= 509) was W = 0.006 Lt 3.0383 (r 2 = 0.965), and the isometric growth of total sample was confirmed by Student's t test (p<0.05).When calculated for each sex, L-W relationship for females was W = 0.0048