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Fishery Bulletin 106(3) 
and negative if catch decreases over the season. For 
example, a 8 of -0.50 represents a 50% decrease in catch, 
and a <3 of +1.00 represents a 100% increase. 
Sampling considerations and gear deployment 
There are two important concerns in using catch from 
fixed-gear such as pots as an index of fish abundance. 
The relationship between catch and abundance can 
break down at either very low or very high fish densities 
(Hubert, 1996). During our pilot studies, soak times and 
sampling procedures were developed to ensure that the 
research fishing did not result in either excessive num- 
bers of empty pots or gear saturation (where the number 
of fish in the pot became so high that catchability of 
additional fish was reduced). To guard against gear satu- 
ration and to provide detail on the timing of fish catch 
within the soak period, instruments referred to as trig- 
ger timers were developed. These instruments involved 
a magnetic reed switch mounted on the triggers of the 
pot and an electronic event timer that recorded when the 
triggers were pushed open. A modular trigger assembly 
allowed instruments to be mounted and dismounted in 
pots without slowing down the pace of fishing. 
In order to avoid gear saturation and to increase 
the number of observations, pot soak times were ini- 
tially kept short (4-8 h), and fishing was conducted 
mainly during daylight hours. It was not feasible to 
fully standardize the soak time or the timing of the 
launch within the diel and tidal cycles over the 40-50 
pots fished each day. In order to compensate for varia- 
tion from these sources, each day’s sampling included 
approximately equal numbers of stations inside and 
outside the notrawl zone. Difficulty in retrieving pots 
during strong tidal currents led to a change in proce- 
dure between 2004 and 2005. In 2005 a slightly longer 
overnight soak was used, with pots being launched at 
new locations in the afternoon or evening and retrieved 
the next morning. 
Results of a pilot study conducted in 2002 showed 
short-term temporal variation (day-to-day variability in 
catch rates at a given station) as a larger component of 
variability than small-scale spatial variation (variabil- 
ity in catch between stations). In order to smooth over 
short-term temporal variation, we attempted to fish 
each station on at least three different days during each 
survey. Each day’s fishing was balanced with stations 
in both the treatment and control areas, so that any 
short-term influences on abundance would affect both 
treatment and control groups. The goal was to apply 
fishing methods in such a way that variation in catch 
due to soak time, diel and tidal cycles, weather, and 
current patterns would be minimized and distributed 
evenly between trawled and untrawled areas. 
Our pilot study results also indicated that pots lo- 
cated at least 0.11 km apart functioned as independent 
sampling units (no correlation between catch for pairs 
of pots at 0.11 km or further distances). Stations for 
the experiment were spaced 0.11 km apart within each 
zone (trawled or untrawled), and 1.8 km apart across 
the notrawl zone boundary. The same layout of study 
stations was used for all three years. Examination of 
both pilot study data and pot fishing data collected by 
fisheries observers indicated that there was not a strong 
relationship between length of pot soak and catch over 
a time span of 4-24 hours. For this reason, catch rates 
were expressed not as CPUE in fish/hour, but simply 
as total number or weight of Pacific cod caught per 
standardized pot deployment. The catch measure at 
each station was the average catch over all of the days 
that a station was fished during a survey. The use of 
an average over several days as the measure at each 
station provided smoothing over day-to-day variation 
and reduced the likelihood of zero catches in the final 
data set. 
Pots were baited with chopped Pacific herring ( Clupea 
pallasi) contained in meshed bait bags. Bait for each 
cruise was purchased as a bulk lot so that the same lot 
of bait was used for the entire cruise. Filled bait bags 
were weighed and the amount of bait adjusted to within 
0.1 kg of the target weight of 5.0 kg. Procedures for se- 
curing bait bags and triggers, launching, and retrieving 
pots were consistent in all cruises. 
Upon retrieval, all catch was sorted, identified, and 
weighed. A systematic subsample of pots (every sec- 
ond, third, or fourth pot retrieved) was selected with 
a random starting point each day; all Pacific cod from 
selected pots were processed for length frequency, by 
sex. The sampling interval was adjusted according to 
average catch rates so that at least 100 fish were mea- 
sured each day. The condition of the gonad of measured 
fish was also examined visually and coded according to 
a 5-stage system, in order to record the approximate 
frequency of different stages of reproductive maturity 
of Pacific cod in the catch 
Data analysis 
After every study year, average catch rates for each 
station and cruise were calculated for all valid fishing 
days at a station. Average catches from the two cruises 
were used to compute the d for each station. Spatial 
mapping of both raw catch data and <3s was performed 
to look for spatial patterns and verify the assumption 
of independence between study stations. Distance-based 
correlograms (both anisotropic and isotropic) were plot- 
ted to check for spatial dependence in catch data. 
Linear models were also used to look for patterns in 
untransformed catch data; effects of year, season, treat- 
ment versus control, station, and fishing day within 
each cruise were examined. After examination of the 
distributional characteristics and independence of the 
6’s, the nonparametric, rank-based Wilcoxon rank sum 
test (Ott, 1984) was used to test for a difference in dis- 
tribution of 8 between stations in the trawled and un- 
trawled areas. The nonparametric test was selected over 
the parametric t-test because 8 is a ratio of counts and 
may have a strongly non-normal statistical distribution. 
All modeling was conducted in S-Plus (Math Soft Inc., 
Seattle WA; Venables and Ripley, 2002). 
