| Abstract |
The vertical migration of sardine (Sardina pilchardus) in relation to light. The vertical migration of sardine (Sardina pilchardus Walb.) was investigated as a dynamic movement in wild conditions by a Langrancian approach. The data were collected during two hydroacoustic surveys in North Aegean sea during June 1995 and 1996. The hourly position of the centre of sardine density (WMD) was estimated by dual beam echo-sounder. The luminance and temperature profile of the water column was hourly measured by an extended net of 152 CTD stations. The pattern of sardine diurnal movements was examined in relation to surface luminence (LIS), bottom depth (B) and light intensity changes at the mean depth of sardine (LIMD). The analysis was performed separately in two sub-areas: Thracian sea and Thermaikos gulf, due to the different bottom depths that predominated each area. Three different models were estimated, one for Thracian sea and one for each sampling period in Thermaikos gulf. In each sub-area cross-correlation analysis was used to determine the periodicity and phase differences of variations in WMD and LIS. The series of WMD measurements was further analyzed by means of time series. In addition a) the means WMD and LIMD were hourly averaged in Thracian sea for both sampling periods, b) the mean hourly change in the LIMD was estimated, c) the light intensity in which sardines tend to be concentrated (PLI) was estimated, d) an area light intensity (ALI) was estimated and e) the estimations were compared by analysis of variance.Finally, a cosine model was fitted to the available data in order to trace the diel movement of sardine. The cross-corelation analysis between WMD and LIS estimated a lag of 1 h in Thracian sea and no lag in Thermaikos gulf in 1995, while no significant correlation was estimated in Thermaikos gulf in 1996. The analysis revealed that the main factors which affected the vertical migration of sardine in the field were light intensity and bottom depth. Specifically, sardine tended to regulate its depth with light intensity when it is not restricted by bottom depth. Sardine changed depth with a different rate when there was small or no bottom effect (Thracian sea) and when they moved to deeper water across the bottom slope (Thermaikos gulf). Three phases of sardine movements during day-light were discriminated: a) diving at dawn, b) resting at greater depths and c) emerging at dusk. The WMD tended to stabilize when the ambient light intensity was increased from 52 mc to 166 mc. The reverse was observed during emerging, as sardines began to ascend when the ambient light was decreased from 200 mc to 84 mc. In the mean time sardine remained at about the same depth, regulating its position according to the light intensity. According to the cosine analysis sardine was found in the maximum depth at »12:00 o'clock. It seems that the background intensity was more important than the LIMD for the vertical migrations of sardine. At sunrise there was a sudden increase in light over the dark background of the sea. This stimulus activates a negative phototaxis and the fish dived. Afterwards the light becomes diffuse, the background light and the sardine remain in diffferent light intensity. The reverse effect activates a positive phototaxis at dusk. The positive photaxis seems to be related to feeding behaviour. There was a wide range of prefered light intensity during day-time while the measurements in the twilight were always above the different thresholds estimated in the laboratory. A percentage of 70 το 80% of the total abundnce of sardine was concentrated at a range of temperature between 120C- 170C. The sampling unit limited the accuracy of our estimations. In conclusion, continuously measurement of light intensity is necessary in order to estimate field threshold.
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H κάθετη μετανάστευση της σαρδέλας (Sardina pilchardus) σε συνάρτηση με το φως με τη χρήση ηχοβολιστικών μεθόδων
