A contribution to the investigation of relations of sardine (Sardina pilchardus Walb.) to the environment
Abstract
Some observations dealing with the size distribution of sardines in the Mid-Dalmatian area were made in the 1954 fishing season. At the same time tagging experiments were carried out, when possible. At the same time observations on temperature, salinity, zooplankton and on gut contents of fish were also made.*) These simultaneous observations were made in an attempt to explain the size distribution of the fish and its changes. It was hoped that they would lead to a better understanding of the movements of sardines and their relations to the environment.
The samples of commercial catches were taken in the area of Vis and Biševo Islands, in Hvar Channel and in Kaštela Bay. The data on temperature and salinity were collected at seven stations (Fig. 1).
The data on total length (Fig. 2) showed once more that in the area of Vis and Biševo Islands only big sardines were to be found.**) Their length even exceeded that in 1946-1952 (Tab. I) and equalled the length observed by S. M u ž i n i ć (1936) in 1931 in the same area. The fluctuations in length from year to year and the data on age composition shown earlier (M u ž i - n i ć, R., 1954) led us to suppose that the poor catch in the fishing seasons 1952-1954 was not due to an over-exploitation of the stock but to fluctuations in year class strength.
The changes in the size distribution of sardines in the Mid-Dalmatian area and previous data on their movements (M u ž i n i ć, R., op. cit.) could indicate that during the fishing season 1954 too a movement of fish took place from Vis and Biševo Islands to the coast of mainland. This could lead us to suppose that the poor catch in the 1954 fishing season was no caused by a change in the direction of movement of the fish.
A definite relation of sardine distribution to the temperature and salinity could not be shown from the analysis of these factors (Tab. II). The fish were subject, during the fishing season, to rather wide fluctuations of temperature. The widest fluctuations were found at the surface, but they were also important at 10 m, although only in the area of Vis and Biševo Islands. Moreover, considerable differences were found in the same column of water, especially at the end of June, in July and in first days of August, i.e. in the period of sexual inactivity in sardines.
The sardines could be caught also when there was a temperature in the upper layers and at a steep temperature gradient shown by some of the samples analysed. It seems, however, that a reduced sardine catch coincides with the highest annual temperatures.
The salinity distribution also did not help in explaining what determined sardine movements. The fish seemed to move, at the beginning of the fishing season, in the direction of lower salinities and greater vertical salinity gradients, but these were not necessarily factors determining the movement of the fish. Further direct observations are needed in order to determine the relations of sardine to both factors, temperature and salinity.
A hypothesis is put forward concerning the influence of the difference in the year class strength on the movement of sardines. It is assumed that the spreading of the habitat of a numerous recruit year class due probably to an intra-class competition, could disturb, to a certain extent, the movement of big sardines from the open sea to the coast of mainland. This spreading of the habitat of these recruits appeared in the investigated area in 1947 and 1948, while the movement of big sardines did not take place in the great part of the fishing season in 1948 (for 1947 the data on movement are missing). In the case of a more intensive recruitment of younger fish in the area of Vis and Biševo Islands and a disturbance in the movement of big sardines from this area to the mainland coast it would be possible to presume a rich year class in Mid-Dalmatia and an imminent increase in the catch. The eventual use of rough indicators of the state of stocks could perhaps be of some interest for the sardine fisheries in the Mediterranean where the age determinations are still uncertain and fisheries statistics inadequate.
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*) The observations on zooplankton and on gut contents were not included in the author’s work and they will be published separately.
**) In the length frequency curves the lower class limit was used and the means were corrected.
