I'm not a survey biologist, but it seems to me that if you want to account for
the side bias you need to identify the factors causing it. Several come to
mind. Were the tows done in a straight line? If not, being on the inside vs.
outside of the turn would make a difference. Also, the direction of the
current. If you were headed directly into or away from the current you'd be
fine, but if it was hitting the boat at an angle or directly on one side it
could affect how each net performs. Time of day is another factor. If the sun
was visible and hitting one side of the vessel vs. the other, that could have
an effect on net performance as well. Of course, you would have had to collect
data on these factors in order to test them, say in a standardization model. I
would start by looking at what you have data on and using those factors to
attempt a standardization of the net affect (say using a glm).
From: qfish-bounce@xxxxxxxxxxxxx [mailto:qfish-bounce@xxxxxxxxxxxxx] On Behalf ;
Of Lee, Laura
Sent: Monday, January 23, 2017 11:48 AM
Subject: [qfish] correcting for bias in gear characterization study
This is not a stock assessment question, but I felt the audience of this
listserv would have the answer.
Some background...the study is a gear characterization project looking at
paired hauls from a trawling vessel to compare catches in the experimental net
to those of the control net. We are looking at three different experimental
gears. Ideally, the control and experimental nets are swapped between the port
and starboard sides so that each gear (control or experimental) tows on each
side the same number of times as the other gear. This was not always the case.
For example, in testing of one of the gears, the control gear was on the port
side 20 times and on the starboard side 4 times. If there is a side bias, this
unequal towing could magnify differences in results.
Another analyst I'm working with "solved" this problem by randomly identifying
and eliminating 16 of the tows in which the control gear was on the port side
so the analysis was restricted to tows for which each gear was on each side 4
times (for the example above). This does not seem correct to me.
I've run randomization tests to test for side bias (and day/night bias) in the
catches of select species. There is a side bias (and sometimes a time of day
bias) in some cases. What is the best way to adjust for this bias so I can
determine whether the difference between the control and experimental gear
catches is due to the gear modification alone.
I hope this makes sense. Thanks so much for your time!
Laura M. Lee
Senior Stock Assessment Scientist
Division of Marine Fisheries
Department of Environmental Quality
252 808 8094 office
252 726 6062 fax
3441 Arendell Street
P.O. Box 769
Morehead City, NC 28557-0769
Email correspondence to and from this address is subject to the
North Carolina Public Records Law and may be disclosed to third parties.