Trawl fishing grounds in the central Adriatic
Abstract
The investigations of the ediable demersal resources in the central Adriatic were carried out during 1963/71. year by research vessels: »Bios« (300 hp) and »Predvodnik« (200 hp) on the 24 permanent stations (Figure 1). Besides of the, mostly biocoenological, informations of quantitative and qualitative aspects of the demersal »trawl« populations, statistical catch and fishing effort data were considered as well.
Biocoenological data of the relationship between edible demersal resources and biocoenoses (muddy-channel, sandy-shall and claylomay biocoenoses) that occupies studied region pointed a fairly great quantitative and qualitative changes of the trawl catches. It was found that the average values of density of the edible populations (kg/1 hour) and biomass (kg/1 ha) are higher for shallower stations distributed in the channel region and Yougoslav territorial waters (Table 27). Diminution of biomass of the macro-zoobenthos with a sea depth increment was stated too (Table 28).
Structural composition of edible demersal groups: Chondrichthyes, Oste- ichthyes, Crustacea Decapoda and Cephalopoda is changed within analysed stations, biocoenoses (Figures 3, 4). Data of abundance (kg/1 hour) of the fish populations showed that Hake (Merluccius merlucis L.)* population dominate in the whole area. On the channel stations beside of Hake dominate the following fish species: Maena smaris, Diplodus annularis, Mullus barbatus, Serranus hepatus. On the stations with mostly sandy-shall bottom sediments dominate: Scyliorhinus canicula, Lepidotrigla cavillone, Argentina sphyraena and Trisopterus minutus capelanus. Open stations of the central Adriatic area characterised almos only with three populations: Merluccius merlucius, Trisopterus minutus capelanus and Trichurus trachurus.
Trophic studies of food and feeding habits of the fish species: Hake (Merluccius merlucius L.), Striped Mullet (Mullus barbatus L.) and Pandora (Pagellus erythrinus L.) were intensively carried out in the central Adriatic during 1963/71 year. Obtained results showed a close relationship between behaviour of the populations (horizontal migrations of the Hake population, distribution of the Striped Mullet and Pandora populations) and food components in the environment. As far as Hake population is concerned it is stated that young speciments (first year class) almost the entirely year retain in the deeper sea waters of the central open Adriatic (Jabuka walley) where its fed intensively on euphausied and mysid speciments: Nyctiphanes couchi, Meganyctiphanes norvegica and Lophogaster typicus. This region is considered as a spawning ground of the Adriatic Hake (Ž u p a n o v i ć, 1968). Older Hake year classes (second and third), changing feeding habits, from zooplancton to fish speciments, leave Jabuka walley and make migrations toward central Adriatic islands and channels region, intensivelly being fed with small pelagic fishes: sardins (Sardina pilchardus Walb.) and anchovys (Engraulis encrasicholus L.) In the regions, second and third Hake year classes retain until the first maturity period (about 30 cm total body lenght) when they leave shallower waters for spawning purposes, which takes a place in the colder waters of the Jabuka walley (figure 5; tables 28, 29 and 33).
Other two, benthic species: Striped Mullet and Pandora are fed dominantly with following specimens: Sternaspis scutata, Nephthys hystricis, Tellina nitida, Tellina distorta, Stylocherion sp., Processa canaliculata, Anchialina agilis, Paramysis helleri, Alpheus ruber and Alpheus megacheles, e.i. with those individuals which in the central Adriatic, mostly in the channel region (stations 1, 3, 4, 83), make characteristic bottom communities. These food components in the environment, e. i. its distribution and concetration determine horizontal distribution of the Striped Mullet and Pandora populations (figures 6, 7 and tables 34, 35). Because of the fairly great simillarity in the feeding habits between Striped Mullet and Pandora populations concerning the dominate avertebrate groups: Polychaeta, Crustacea Decapoda and Bivalvia, its seems that inter-specie competition exist (table 32).
Studies on stock assessment for the edible demersal resources (fishes, cephalopods, crustaceans) of the commercial trawl fishing ground (Blitvenica) in the central open Adriatic were carried out on the base of commercial statistical data for the period 1960/70 year. Yearly catch and effort data for the trawlers of the 240 hp were considered (table 36). Fishing effort is taken as number of effective fishing days per year (effective fishing day is 8-9 hours towing period). It is stated that in then years period catch per day trawler has been decreased from 530 kg (boat/day) with exception 1965 year (600 kg/boat/day), to 360 kg/trawler/day in 1969 year (graph 1). Slope of the regression line has negative value (b = -16.22x). Yield regression line (graph 2) with positive slope (= 0.026x) does not show overfishing of the edible resources, though a great yearly yields fluctuations are evident. It seems that the fishing ground is influenced by seasonal and yearly horizontal migrations of the semi-pelagic populations; Hake and Horse mackarel (Trachurus trachurus L.) which in this region make a great percentage of the trawl catches.
Hake cod-end selectivity experiments that were carried out in two regions, Blitvenica and Neretva channel (figure 8), by means of alternating trawl hauls and covered cod-end, showed the following:
1. in the case of alternating trawl hauls with a cotton cod-end with a knots of streched mesh-size of 400 mm, a 50% retained point is (lc = 10.6 cm) and selectivity coefficient (b = 2.6). Cotton codend with a knots of streched mesh-size 60 mm (lc = 21.4 cm) and (b = 3.6); graphs 3, 4; tables 37, 38.
2. in covered cod end experiments (figure 9) of the synthetic cod-ends (polyamid 6) with a knots with regard to Hake population the following results were obtained: cod-end of 41.4 mm streched mesh-size, a 50% retained point is (lc = 13.2 cm) and selectivity coefficient (b = 3.2). Cod-end of 64.6 mm (lc = 30.2 cm) and (b = 4.7). Cod-end of 55.2 mm (lc = 19.3 cm) and (b = 3.5). Graphs 5, 6 and 7; table 39.
Preliminar investigation of the economic aspects of commercial trawl trawl operations in the fishing region Blitvenica, for a 240 hp trawler for 1972 year, showed that 180 fishing days a year with average daily yield of 322 kg can not realize significant finnancial profit to organization. A charged average market price per kilogram of 9,88 dinars can not efficiently cover the costs (fixed and changeable expenses), e. i. they are more or less in a balance (tables 41, 42). Because of such statement, taking into the consideration quantitative-qualitative composition of the demersal resources over the Adriatic continental shelf, the following might be suggested: firstly, modernization of the Yugoslav trawlers, involving a quite new fishing technique (hidraulic drum technique) for a crew number reduction, and intensification of the trawl operations, within no yet exploited areas, by day-night fishing. With regard to this, a simplication in fishing organization should be done, so that a catches from trawlers to fish markets would be directly sold without being charged, e. i. different prices, in fishing and selling departements of the same organization. This structure of the fishing organization, it seems, a greatly limits producers-fishermen salaries, and besides of the other costs in fishing operations, personnel interest.
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* Latine names of the fishes are taken after G. Bini-Atlante dei pesci delle coste Italiane.
