REPORT OF THE WORKSHOP ON TRAWL SURVEY ... - FAO.org

variation of catch rates did not vary with tow duration, except possibly for species with very
low catch rates. For ocean pout on the George’s Bank, optimum tow length for maximum
precision was around 10-15 minutes, allowing 30% more tows by reducing tow length from
30 to 10 minutes. Comparative towing in the Bering Sea also showed no significant difference
in length compositions for a range of species taken in 30-min and 15-min tows.
An example of a comparative tow exercise to examine effects of tow duration was given in a
Working Document to WKSAD(2005) by Wieland and Storr-Paulsen (“Effect of tow duration
on catch rates and mean length of Northern shrimp (Pandalus borealis) and Greenland halibut
(Reinhardtius hippoglossoides) in the Greenland Bottom Trawl Survey, 1999-2004”). Tows
in this survey had been progressively reduced from 60 mins down to a mixture of 30min and
15min tows, and an analysis of the effect of reducing all tows to 15mins was needed. A GLM
approach where log (CPUE + 1) was modelled as a linear response to stratum, year, depth and
tow duration, or to log (mean CPUE) and tow duration, indicated no significant effect of tow
duration on catch rates or maximum size of shrimp or halibut.
The WKSAD (2005) also presented results from an experiment to determine the catch rate of
trawls during the periods before and after the official haul duration, and the potential for a
“surprise” effect noted in some camera studies. This was done by carrying out “zero-duration”
tows – i.e. where the net was retrieved as soon as it had settled adequately on the seabed at the
official start of the tow. The study indicated that for less mobile species such as sole, the catch
before and after the nominal haul duration may be proportional to tow duration during these
brief periods, whereas for highly mobile species such as hake, the initial surprise effect may
be more important, casting doubts on swept-area based abundance indices for such species. A
significant surprise effect at the start of a tow could imply that surveys with many short tows
of equal duration would perform better than having fewer longer tows that in some areas have
to be reduced in duration for various operational reasons.
Estimating biological parameters
WKSAD(2005) examined the statistical basis for estimating biological parameters, treating
the samples from individual tows as cluster samples. It was demonstrated that effective
sample size to determine parameters such as length composition can be much smaller than the
number of samples taken. This implies the need for more but smaller independent samples
(another benefit of short tows). Spatial mapping and interpolating of biological parameters on
acoustic surveys is also examined in the WKSAD report.
Practical aspects of survey design and implementation: IBTSWG and SGSTS
Trawl surveillance
Trawl standardisation and inter-calibration of vessels and gears is a major topic addressed by
IBTSWG and by SGSTS and its predecessor the SGSTG. Each national survey is expected to
fully implement the strict protocols to ensure that their trawl gear fishes as consistently as
possible, and to monitor trawl performance using suitable sensors. Good trawl surveillance
data can clearly indicate the effects of changes in vessel or gear construction and deployment.
The ICES SGSTS gives an example of the change in vessel by Cefas (Lowestoft) from the
Cirolana to the Endeavour (Fig. 5) showing a significant increase in door spread and
reduction in headline height of the GOV trawl when towed by the more powerful Endeavour.
The 2005 and 2006 SGSTS reports devote considerable space to issues of trawl surveillance
including the types of sensor that are available, the deployment, testing and calibration of
47