On species composition and length characteristics in mixed species catches of pelagic fishes in the Adriatic

Abstract

      Mixed species samples of pelagic fish catches obtained by seine and light in the shallow coastal waters of the eastern central Adriatic (Kaštela Bay), in the 1969-1974 period, usually contained sardine and anchovy, but in very variable proportions. In most of these samples sardine was the dominant species. The proportions of horse mackerel and bogue only rarely exceeded those of the sardine.

     The proportions of sardine and anchovy in mixed samples of catches by light (1971-1974) also varied from year to year. The proportions of sardine were highest in 1972 and those of anchovy in 1973 and 1974.

      Mixed pelagic fishes were not uncommon in the bottom trawl catches taken in the channel Mid-Dalmatian waters, in 1957 and 1958, their proportion amounting to 50.9%. The proportion was smaller (40.0%) in the bottom trawl catches from the Fishery Biological Expedition »Hvar« which worked mainly in the open Adriatic, in 1948 and 1949. Such proportions might point out to the fact that mixed pelagic fish species were not in part accidentally taken by trawls, i. e. during their hauling.

      The mixed pelagic fishes were more frequent in the demersal catches from shallow waters due to a preference of lesser depths in three (sardine, sprat and anchovy) out of the five species.

      In shallow waters of the central and in part southern Adriatic sardine often appeared in the bottom trawl catches with anchovy. On the other hand, he catches from more open areas rarely contained sardine and anchovy together. These sardine occurred more frequently with mackerel and this was even more true for anchovy. In the bottom trawl catches anchovy rarely appeared alone.

       Judging from the mixed catches of the pelagic fish species caught by bottom trawl, the highest overlapping of the horizontal distribution of sardine and anchovy seems to occur on their common nursery grounds which show the least depths. However, mixed pelagic fishes in the bottom trawl catches also frequently occurred north of the Jabuka Pit and in the Palagruž Island area.

      The proportions of component species from mixed pelagic fish catches obtained by bottom trawl also greatly varied. In some of the more abundant catches of pelagic fishes the dominant species represented more than 95% of the catch. In such catches from deeper waters mackerel dominated and in those from shallower waters it was mostly sardine.

      Mean total lengths of the mixed species samples of pelagic fish catches obtained by light in the coastal eastern Adriatic, in the 1969-1974 period, ranged from 10.9 to 17.6 cm, and of sardine and anchovy from 10.6 to 17.6 cm and from 9.3 to 15.7 cm respectively.

      Mean lengths of sardine from mixed species samples were mostly larger than those of anchovy from the same samples.

     The largest mean lengths of sardine were observed in 1971 and those of anchovy in 1972, i. e. in the years of their low proportions in mixed species samples.

     The range of length variation in the mixed species samples of pelagic fish catches carried out by light was rather large (standard deviations: 0.51-2.56 cm). Some of them showed small standard deviations of length (< 0.80 cm), similar to those of component species, and highly overlapping length distributions in these species.

      Contrasting sharply to these samples were those with large standard deviations of length (mainly > 1.50 cm), mostly very different from those of component species and, consequently, with none or very little overlap in the length distributions of these species.

     On the basis of aquarium observations on sardine and, to some extent, on sardine and anchovy kept together, it was supposed that the species from the first samples formed coherent common groups in natural conditions and those from the latter samples did not shoal together but only aggregated. Some of the latter samples were probably taken from coherent shoals of the species which became mixed during the capture operation.

      Of course, there were mixed species samples representing transitions between the mentioned two groups of samples. Such samples show moderate length variation and partially overlapping length distributions of their component species. If these species shoal together it might be only temporarily. Such groups would easily break with an increase of swimming speed.

       It was also supposed that species from mixed samples of the pelagic fish catches represented by larger individuals, but in low proportions may coexist with small well-represented individuals of other species in coherent groups. Conversely, such coexistence could hardly be realized by small badly represented individuals and it would, moreover, not be advantageous to them. Two-sized monospecific individuals would probably behave similarly.

       Finally, on the basis of some aquarium experiences it was presumed that common shoaling of sardine and horse mackerel (Trachurus sp.) would be only temporary. 

       It would follow that a relatively small number out of numerous mixed samples of pelagic fish catches by light was taken from permanently coherent mixed shoals, while many others originated from aggregations or temporary, labile shoals. Some of mixed samples were probably taken from separate stabile shoals of the species which became mixed during capture.

      Individual total lengths of mixed pelagic fishes and component species trawled during the »Hvar« Expedition, in 1948 and 1949, were somewhat greater than those of the fish trawled in the central and in part southern Adriatic, during the 1954-1955 season.

      Mean total lengths of the mixed catches of pelagic fishes from the 1954 -1955 trawling season ranged from 11.1 to 17.6 cm, those of sardine from 10.9 to 16.6 cm and those of anchovy from 11.3 to 16.4 cm. These catches mainly showed moderate length variation (standard deviations from 0.75 to 1.58 cm with one exception). The largest standard deviations of the length recorded in them were similar to those of the one-species catches. On the basis of the standard deviation values and distributions of the length shown by these mixed pelagic fishes a common shoaling for the species from most of these cathes was presumed in so much more as the catches were taken in the colder part of the year, when the fish is generally less active.

      Sincere thanks are due to Prof. Dr. Otmar  K a r l o v a c  for his kind supplying the data collected during the »Hvar« Fishery Biological Expedition (1948 and 1949).

      I express my best thanlks to Mr. Ivica  Š a š k o r, my technical assistant, who kindly helped me in preparing the documentation and drew the graphs.

      I am also very indebted to Mr. Vlado   D a d i ć,  B.  Sc., who also kindly helped me with the documentation.