White Hake Catch Rates
White hake were fished by all four gear types.
Longline and gillnets were the dominant gears, but there were enough
purchase slips for analysis of the otter trawl and seine catch rates
too. However, data were not available from all gears in each year.
There were no data at all in 1995 presumably because this was the
first year of the moratorium. Data for 1996 and 1997 came from longline
and gillnet vessels involved in the sentinel surveys. Otter trawl
data were available for the period 1985 -1993, and for seines from
1986 -1991.
Figure 18: Comparison of overall
mean catch rates by fishing gear for white hake in St. Georges Bay,
1985-1997. The vertical bars give the approximate 95% confidence interval
of the average.
The analysis method used allows
for comparisons between fishing gears and seasons. The highest white
hake catch rates were recorded by seines. Gillnets had the lowest
catch rates, about half the seine catch rates, while longline and
otter trawl catch rates were intermediate (Fig. 18). There was little
variation in white hake catch rates by month. There were a low number
of observations from May and June, and these two months had the lowest
catch rates. But from July to October, catch rates remained relatively
high with little variation.
Annual average white hake catch
rates declined from 1985 -1994, from 0.5 t/slip to 0.25 t/slip (Fig.
19). There was a substantial increase in catch rates in 1996 and 1997
in the sentinel surveys. These values were about twice as high as
the average annual catch rate in the 1985 -1994 period. This is very
perplexing given the low estimate of stock size from the DFO research
vessel survey in this same area in the same years.
Figure 19: Average annual catch
rates for white hake in St. Georges Bay. The estimates are from an
analysis of purchase slip data from longlines, gillnets, otter trawls,
and seines. The high catch rates in 1996 and 1997 were observed in
the sentinel survey. The error bars indicate the approximate 95% confidence
intervals of the averages.
It is possible that the catch
rates from 1996 and 1997 were so much higher than the other years
because of difference in the level of fishing. The catch rates calculated
for 1985 -1994 were taken from commercial fishing trips when the total
fishing effort was very high, with over 500 fishing trips being made
annually. The 1996 and 1997 points came from the sentinel surveys
when only 4 vessels were fishing and the total number of fishing trips
per year was less than 50 (Fig. 20). It is possible that when a commercial
fishery is occurring there is competition for the best fishing grounds
and disturbance of the fish, and this in turn may reduce the average
catch rate. In the case of the sentinel survey, there was rarely more
than 4 vessels fishing white hake at any one time, and their fishing
locations were dispersed around the Bay. Fishermen could also choose
the best fishing locations for their trips, at least within a 2.5
km radius.
Figure 20: Comparison of the average
annual catch rate for white hake in St. Georges Bay with the annual
total fishing effort directed at white hake, as indicated by the number
of purchase slips.
We examined the available data
to try and determine if competition among vessels and disturbance
of the fish may have an effect on catch rates. The approach was to
examine the weekly catch and effort data for individual years to see
if catch rates were lower in weeks with high fishing effort. An example
for 1992 is shown in Fig. 21. In this year, catch rates increased
in the initial weeks of the fishery, then remained relatively high
but variable throughout the rest of the year. Fishing effort increased
from low initial values in week 27 to a peak of about 225 slips in
week 34. There was a drop of more than 50% in effort in week 35 to
about 100 slips, and thereafter effort declined for the rest of the
year. If interference was an important factor in catch rates, then
one would have expected a decline in catch rate during the initial
weeks when effort increased, followed by an increase in week 35 and
thereafter when effort declined. Instead, catch rates remained relatively
constant. Another possibly complicating factor is the effect of stock
depletion. One would expect catch rates to decline as the cumulative
catch from the population increases and the local abundance of fish
becomes depleted. It is possible that the depletion effect offset
the interference effect.
Figure 21: Weekly average catch
rate (top panel), fishing effort (number of purchase slips), and cumulative
landings of white hake in the 1992 longline fishery in St. Georges
Bay.
Unfortunately, the annual pattern
of fishing effort in other years was similar to that in 1992. In order
to test whether the depletion or interference effects are occurring,
it might be necessary to conduct an experiment. This could consist
of conducting a sentinel survey and recording catch rates throughout
the fishing season, as is currently being done. Then, there could
be intense open fishing for a period long enough to allow approximately
100-150 vessel-fishing days. The test would be whether or not the
sentinel survey catch rates decline during the period of open fishing,
and recover in the ensuing period. More than one open period could
be used. However, the amount of fishing allowed would have to account
for the status of the resource. An experiment should only be conducted
if the resource is assessed to be on the road to recovery. Another
possible explanation for the high catch rates in the sentinel survey
is that the fishermen who participated were also among the best in
the area. There were 284 different fishing vessels identified as having
directed for white hake during the study period. Of the four vessels
in the sentinel surveys, one had catch rates close to the average
while the other 3 were in the top 80% of the entire fleet. We attempted
to account for this factor by adjusting the annual estimates for the
overall average performance of the vessels. At this point it is not
clear whether this was enough.
