Tagging of sardine (Clupea pilchardus Walb.) in the Adriatic in 1950 and 1951

Abstract

      The tagging of sardine, which was carried out in one section of the Mid-Dalmatian region and in the vicinity of Palagruž Island during the interval of time extending between May 7th and September 14th 1950, involving a total of 16,613 individuals, disclosed limited movements of fish.

     A movement was noticed starting from the coast of the west section of Brač Island towards the south coast of Čiovo Island, in the direction of Kaštela Bay and the Poljica region respectively, viz. towards the mainland. That movement took place towards the end of May, partially perhaps at the beginning of June. The movement of sardine, occuring in July and partially perhaps in August, from the west coast of Brač Island in the direction of Kaštela Bay, was evident from the 1948 tagging. It is not known, however, whether such a movement occured also before July 1948.

      There was a movement of sardine between May 15th and September 11th from the coast of the north-west section of Vis Island in the direction of Hvar Channel, towards the coast of Brač Island. A movement of sar­dine in the same direction was noticed in 1949 also, by the end of May, in the course of July, and by the end of August.

      During the interval of time extending between June 6th and Septem­ber 9th a movement of sardine occured from the south coast of Biševo Island in the direction of the coast of south-west section of Brač Island.

      By the middle of July a movement of sardine was noticed from the south coast of Biševo Island towards the coast of the south-east section of Vis Island.

      The movements, noticed in 1950, agreed, then, with those recorded in 1949, and partially also with the 1948 ones.

      No recovery was yielded by the release of tagged specimens which took place between August 12th and September 14th, 1950, along the coast of the south-west section of Brač Island, along the coast of the north-west section of Hvar Island, along the coast of the west and north-west sections of Vis Island, and in the vicinity of Biševo Island.

      The most of the tagged specimens were recaptured shortly after their release. The latest recovery occured 119 days after the date of tagging.

     The recapture of the most part of tagged fish took place not very far from the tagging locality. The greatest distance between the tagging locality and the locality of recapture amounted to thirty nautical miles.

      Only 27 specimens, i.e. 0.16%, were recaptured out of a total of 16,613 tagged ones in 1950. This percentage is six times lower than that one achieved by the 1949 taggings. It is also almost as much lower than the percentage recorded in 1948, when metallic tags with red celluloid discs were applied. We have no explanation to offer as to this fact, at least for the time being. There was no alteration of the tagging methods. Only the tag was altered. A little longer and more narrow tag was used, which was at the same time also lighter than the 1949 one. Both kinds of tags were equally conspicuous, owing to the red celluloid disc of the same dia­meter. The tag applied in 1950 could not, then, have been less efficacious than the 1949 one. The following comparison may confirm that. Out of 265 tag recoveries in 1949 with known dates of recapture, as many as 261 occured within the first month after the date of release. The rema­ining four recoveries were recorded 35, 36, 59 and 89 days after tagging. Out of 27 recoveries in 1950, as many as 25 occured within the first three weeks after the date of release. One of the remaining recoveries was recorded after 95, and the other after 119 days following the tagging. This comparison might point out to the fact that the tag, used in 1950, remained a little longer on the opercle of the tagged individuals.

      The low percentage of recovered sardine tags in 1950 could have been, to some extent, a consequence of the higher mortality of fish after tagging, which was noticed in July, and particularly during August and September. A relatively high temperature of the sea surface in 1950 might have caused a higher post-tagging mortality during those months, as well as a considerable perishing of fish from the moment of its taking over till the beginning of tagging. It was noticed that the maximal mean temperature of the sea surface in the course of taggings during a dark of the moon in 1950 was by 2.8°C higher than that in 1949. The low percen­tage of recovered specimens, tagged during the first dark of the fishing season, in the course of which no significant perishing of fish after tagging was noticed, proves that the higher mortality of sardine after release cannot be considered as the only cause of the low percentage of recovered tags in 1950. This being so, we leave still open the question of the low percentage of recoveries of specimens tagged in 1950.

      It cannot be excluded, however, that the export of fresh sardine to Italy, grown higher in 1950, might have unfavourably influenced the recovery of tagged specimens. It was not possible to have the publicity with regard to tagging of sardine as successful in Italy as it was in Yugo­slavia. It seemed, however, that the publicity in 1950 was somewhat less intensive in Yugoslavia too than it happened to be in 1949. This factor might have also played a part in the decrease of the percentage of recovered tags.

      A movement of sardine of a small extent was shown also by the results of tagging carried out during first half of July of the 1951 sardine fishing season. That movement started from Pakleni Islands in the direc­tion of the north-west section of Hvar Island and towards the south coast of Brač Island.

     A movement was also noticed starting from the east coast of Biševo Island in the direction of the north section of Vis Island and towards the south coast of Pakleni Islands. The noticed movements of this clupeoid species were, then, in accordance with those recorded during the previous fishing seasons. It is significant that all these movements were directed towards the inside channels and the mainland coast.

      We should mention here that the data on movements of sardine, resulting from tagging of this clupeoid species, are also confirmed by the (as yet unpublished) results which we have obtained in the course of some other researches into Adriatic sardine, relative to analysis of the compo­sition of samples of catches as to the total length and to the number of vertebrae.

       The recoveries of specimens tagged in 1951 followed 2-7 days after tagging. The recorded movements occured during the first half of July.

       The percentage of recoveries of tagged individuals amounted to 0.39%, being thus two and a half times higher than the 1950 one, and two and a half times lower than the 1949 one. The opercular tags and flaglets, which were fixed on the back of fish, yielded an almost equal percentage of recoveries.

       A certain perishing of fish both preceding the tagging and after it was noticed also in the course of 1951. The mean temperature of the sea surface on the July days, during which the tagging took place, was relati­vely high. A thorough comparison between the new and the earlier data on temperature is, however, not possible, as the measurements were carried out closer to the coasts of the islands than it was the case in previous years.