ISMAR-CNR Ancona

1. IOF Split ISMAR-CNR Ancona M. Azzali1, V. Ticina2, A. De Felice1, I. Leonori1, E. Paschini1, M. Marini1, B. Grbec2, O. Vidjak2, L. Grubisic2, A. Pallaoro2, F. Matic2 1Institute of Marine Sciences (ISMAR-CNR), Ancona, Italy 2Institute of Oceanography and Fisheries (IOF), Split, Croatia Inter - ship calibration to compare acoustic estimations of small pelagic fish in the Adriatic Sea GFCM – SAC – SCSA, Rome: 26-30 September, 2005

2. INTRODUCTION Acoustic surveys with the aim to estimate abundance and spatial distributions of small pelagic fish in the Adriatic Sea recently are performed by two research vessels: R/V “Dallaporta” (operating in western part) and R/V “BIOS” (operating in eastern part). With the aim to ensure that the acoustic systems on those research vessels are working correctly and to enable data comparison, an inter-calibration exercise between the vessels was needed. Within framework of FAO AdriaMed Project, intercalibration exercise between R/V “Dallaporta” and R/V “BIOS” was realized in May-June 2005.

3. MATERIALS AND METHODS R/V BIOS R/V G. Dallaporta

4. SAMPLING STRATEGY The study area chosen for this intercalibration exercise in the Adriatic Sea was Neretva channel (i.e. area within geographic boundaries: 43°00'-43°10' N and 16°57'-17°25' E). In this area, the two research vessels (R/V BIOS and R/V G. Dallaporta) performed four mini-surveys with identical routes, twice during daytime and twice during the night. During each of these mini-surveys, two biological samplings and correspondent CTD samplings were performed. Effective routes of the first mini-survey

5. ACOUSTIC DATA RESULTS 1ST MINISURVEY 2ND MINISURVEY 3RD MINISURVEY 4TH MINISURVEY

6. For each vessel, mean Sa-values over all the mini-surveys, presented non-significant variations (Kruskal-Wallis test: P > 0.05 for each vessel). This uniformity confirms the validity of the acoustic data in a situation where there was a constant layer (with a presence of Aphia minuta) day and night. Also small but no significant variations have been noticed comparing day and night average Sa data. • Trends of measured acoustic data by two research vessels are quite similar, but in general Sa-values measured by R/V BIOS were a bit higher than those of R/V Dallaporta. According to statistical analyses, performed for each of four mini-surveys respectively, these differences in all cases were found not to be significant (t-test; P > 0.05).

7. Linear regression analyses showed that correlation of acoustic data collected by these two research vessels can be very well described by equation: Sa(BIOS) = 1.0646 * Sa(Dallaporta) + 37.339 (r2 = 0.7526). In the regression model the results of ANOVA (F = 168.698, p < 0.01) confirmed that the linear relation is highly significant between the two vessels acoustic data sets (Sa-values).

8. NOISE DATA RESULTS The level of noise could restrict the detection of fish. For quantitative measurements a signal-to-noise ratio >10dB is necessary so that single fish can be detected up to the maximum depth (around 200 m for small pelagic fish). Using the maximum Noise Level measured on each vessel, we can calculate the signal-to-noise ratio for a fish with TS = -50dB (i.e for an anchovy of around 14 cm) at a range of 200 m. The Echo Level (dB re 1μPa) produced by a fish of TS = -50dB can be calculated using the equation (Johannensson & Mitson, 1983): where: SL (Source Level) is calculated from the Acoustic-Electrical Calibration (Urick, 1975) of the systems installed on the two vessels. The results are: DALLAPORTA: SL = 228.36 (dB re 1μPa at 1m) BIOS: SL = 227.94 (dB re 1μPa at 1m) The two-way loss for single targets at 200 m is equal: -95.52 (dB) Assuming a TS = -50 (dB) we found: EL(DALLAPORTA)=82.84 (dB re 1μPa) EL(BIOS)= 82.42 (dB re 1μPa) Therefore the single fish signal can be detected by both vessel if: NL (max) < 72 (dB re 1μPa) This condition is satisfied by the acoustic systems of both vessels: DALLAPORTA: EL-NL(max) = 82.84-65.9 = 16.94 (dB re 1μPa) > 10dB BIOS: EL-NL(max) = 82.42-61.7 = 20.72 (dB re 1μPa) > 10dB

10. BIOLOGICAL DATA RESULTS Total catch per m3 for each analysed haul obtained by R/V Dallaporta and R/V BIOS Due to the fact that net mouth of pelagic trawl of R/V Dallaporta (in average 86.2 m2) was larger than the net mouth of Bios’ trawl (about 29.2 m2), sample quantities usually were higher on R/V Dallaporta than on R/V BIOS. Consequently, the total amount of the catches obtained by the two vessels for correspondent hauls presented significant differences (Wilcoxon test; P = 0.046), but if the catches were compared regarding to calculated volume of filtrated water these differences were not significant (Wilcoxon test; P > 0.05). However, given the fact that principal aim of fish sampling is identification of acoustic targets, the most important thing is that the composition of the samples collected, concerning the percentage in weight of the species, showed non-significant differences (Wilcoxon test; P > 0.05).

11. Mean size of anchovy sampled by two research vessels during the six hauls analysed was significantly different (T-test, p < 0.01). Anchovy mean size was 12.19 cm for BIOS and 12.88 cm for Dallaporta. These differences could be due not only to the different amounts of catches by the two vessels but also to the bigger presence of higher-sized individuals in the catches made by R/V Dallaporta. However, given the simple fact that exactly the same fish cannot be sampled by two pelagic trawls, it is not quite clear if this difference could be related to larger trawl and higher towing power of R/V Dallaporta than R/V BIOS has (i.e. systematic error), or it should be considered as normal consequence of random sampling (i.e. random error). Length frequency distribution for anchovies by R/V Dallaporta and R/V BIOS

12. The conversion factors were derived for anchovies and sardines respectively by their length frequency distributions for all the hauls carried out by each vessel. The CF coefficients (kg/nm2) were obtained experimentally at 38 kHz: CF = for anchovies for sardines where The final expression of CF was calculated by summing over the length frequency distribution: = For anchovies CF = 148.677 for BIOS and 161.770 for Dallaporta. For sardines CF = 235.376 for BIOS and 249.541 for Dallaporta. The difference of CF between the two vessels results 8.1% for anchovy and 5.7% for sardines, highest factors corresponding to Dallaporta catch. However the sardines samples were not sufficient to do a reliable estimation, and these findings should be understood as preliminary.

13. ENVIRONMENTAL DATA RESULTS Comparison of temperature profile measurements made by R/V BIOS and R/V Dallaporta in 8 stations of the Neretva Channel Comparison of salinity profile measurements made by R/V BIOS and R/V Dallaporta in 8 stations of the Neretva Channel Upon comparison of all data, an appreciable correlation was found in the deeper layer data, whilst data from the superficial most layer could not be compared because of high variability of these parameters in the studied area.

14. Comparison of ∆ salinity (37.9-38.6 interval) measurements made by R/V BIOS and R/V Dallaporta in 8 station of the Neretva Channel Comparison of ∆ temperature (14.0-17.5 interval) measurements made by R/V BIOS and R/V Dallaporta in 8 station of the Neretva Channel The uniform distribution of the data around zero indicates that variability was not due to differences between the two instruments, but rather to the high variability due to environmental conditions, thermo-haline stratification and strong gradient.

15. CONCLUSIONS • It was noticed that acoustic data (mean Sa-values) for all mini-surveys, measured by R/V BIOS and R/V Dallaporta are very similar and there were no significant differences among them. Also, all data collected by these research vessels showed highly significant correlation between them. Relationship between Sa-values measured by these research vessels is described by equation: Sa(BIOS) = 1.0646 x Sa(Dallaporta) + 37.339 , with high determination coefficient (r2 = 0.7526, p < 0.01). • Concerning to biological sampling, it was observed that, due to bigger net size, R/V Dallaporta collected significantly higher amount of biological samples than R/V BIOS. However, no significant differences in the qualitative composition of biological samples between the research vessels were found; also total catch per volume unit didn’t show significant differences. Both vessels observed a predominance of anchovies of similar sizes in the samples but the difference in length frequency distribution of anchovy has been noticed. This species was rather stable for its distribution and abundance over the entire study area.

16. Noise measurements made on both research vessels confirmed that noise generated by R/V BIOS and R/V Dallaporta does not present an obstacle for carrying out acoustic surveys. • The comparison between the CTD temperature and salinity sensors have shown that, even if the oceanographic conditions were not good due to the stratification of the water column and to the presence of a gradient, the comparison of each sensor was reasonable, although not optimal. Therefore, the in situ tests recorded a relatively low variability with discrepancies between the two instruments within interval of approximately ±0.08 for salinity and ±0.1 for temperature.