CRUISE REPORT INTERPELACUS 0414

An intercalibration survey aiming at to verify if the PELACUS survey time series could have been affected by the change from R/V Thalassa to R/V Miguel Oliver has been carried out off the Garonne mouth. To do that, the inter-ship variability in some sampler devices (mainly acoustics, CUFES and fishing gears) have been compared with the intra-ship variability in order to give coherence to the time series (i.e. small vessel effect). Accordingly, the null hypothesis the characterisation of the pelagic ecosystem by means of an acoustic-trawl survey would give significant differences on account the vessel effects was tested.
Acoustic sampling consisted in two tracks 2.5 espaced and divided in two areas, shallower, with 22 nmi from 40 to 60 m, and deeper , with 10 nmi, from 110 m to the slope. These four track were surveyed three times for each vessel, first in parallel and the others with one of the vessels leading. In the same way 15 parallel fishing station were also perfomed. However, the intercalibration has been made in a small area and only during the light hours of four days and a half of effective work. It is, therefore, complicateto extract conclusions for a large scale survey such as PELACUS time series.
In spite no significant diffences in mean backscattering energy was found, it seems that R/V Miguel Oliver, although showed worse noise spectra than R/V Thalassa specially at higher frequencies (200 kHz), consistently accounted higher cumulated backscattering energy values than Thalassa. In addition. Contradictory, the high level of cavitation showed by R/V Miguel Oliver would not result in a higher fish avoidance and the low cumulated backscattering energy values ahchieved by R/V Thalassa could be related with a higher fish avoidance or more presumably with a higher diving response to R/V Thalassa. This kind of response changes the tilt angle and TS becomes lower than expected, which in turn results in an underestimation of the fish abundance. This contradictory response has been already observed in other ship comparisons. Fish reactions cannot be explained only by considering noise spectra but also sound pressure fields and particle acceleration. This later feature would explain the results obtained.
In the same way the different fishing gear used for both vessels would not result in significant differences between catch composition and length structure as the intra-ship variability was similar to the inter-ship one. However it should be mention that R/V Miguel Oliver has had higher accessibility to horse mackerel and hake than R/V Thalassa.
Finally CUFES performance was similar for both vessels as the intra-ship variability is of the same order as the inter-ship one.
Given these results, it seems that the PELACUS time series would not be affected by the change from R/V Thalassa to R/V Miguel Oliver.