The behaviour, distribution and quantity of sardines in the Bay of Kaštela
Abstract
The observations about the behaviour, distribution and quantity of sardine were carried out in the bay of Kaštela in the period from May 1962. up to the end of the year 1966. These observations were carried out by means of echo sounder SIMRAD type 580-4, frequency 30 kHz.
At first the observations were carried out three times a day in the morning, in the afternoon and at night, and later only during the day (in the morning). During the observation elementary hydrographic and meteorological data were taken.
The traces of fish concentration recorded on the echo-gram are put into three categories according to their dimension or their surface area (fig. 1), in order to be able to follow the behaviour and estimation of the quantity of fish.
It was found out that the traces of fish concentration do not extend to an area larger than those mentioned in the categories i. e. 25 sq. mm.
A considerably greater number of traces were recorded during the morning then in the afternoon. The number of traces recorded at night were very small (table 2 and 3). Compared with the traces recorded during the day the traces recorded at night were very few. The small number and minute dimensions of the night traces we tried to explain in the light of observations carried out of sardines under experimental conditions (M u - ž i n i ć, 1964).
The traces recorded during the morning were a greater distance from the surface then those recorded during the later afternoon hours. Because of this it was supposed that the fish during the evening hours started to move verticaly towards the surface. Similar behaviour has been seen with some clupeids and it was assumed that the recorded traces showed the concentration of sardine in which anchovies were present in a considerable quantity.
The observations were carried out during the dark on moonless and also on moonlight. In both cases the fish behave in a similar manner.
The observations carried out in the vicinity of artificial light has shown at the traces of the fish were minute and of »dotted« shape, and that they were near to the sea surface.
Observations about fish behaviour were done during bright and cloudy weather, but any difference on the fish behaviour could not be noticed.
The small number of traces recorded during the winter one tried to explain with the help of results obtained by other authors. So it was found that there were recorded a smaller number of traces of fish concentration during the period of the most intensive spawning of the sardine in the bay of Kaštela. It was noticed that fewer traces were recorded as the temperature of the water fell and as the temperature of the water rose, so the greater number of traces were recorded (fig. 7). It was assumed, on the basis of these observations, that the sardine behaves differently at the time of spawning when the sea temperature are low, and very likely a part of sardine population leaves the bay. This supposition is also held by other authors.
It was not possible to find any relationship between sea salinity and the behaviour of fish or the recording of bigger or smaller number of traces in winter there are very few (fig. 9).
The distribution of fish in the bay is not equal. The greatest number of traces of fish concentration were recorded in the shallowest part of the bay i.e. in its west and northwest parts and on the areas which are near the coast. In the deeper part of the bay the traces of fish are met very rarely (table 4, 5, 6, fig. 10).
On this basis of the recorded traces one tries to estimate the approximate quantity of fish in the each parf of the year and from year to year. In the warmer part of the year the amount of fish increase from March till April, and the greatest quantity are registered in July, September and October. In the colder part of the year the smallest quantity of fish are recorded in February and March (table 7).
The largest number of traces and with them the greatest quantity of fish were registered in the warmer part of 1963. and the smallest in the warmer part of 1964. (tab. 8).
The data about the catch of sardine in the bay of Kaštela collected by fishermen show many similarities with the research carried out by echo sounder.
