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Interactions Between Commercial Fishing and Walleye Pollock (cont.)

Feature: Part 4
Geographical Distribution and Abundance

Figure 4. Acoustic backscatter attributed to adult pollock along transects during a representative pass from the prefishery period and fishery period in August 2001. See text for explanation.

The geographical distribution of pollock and capelin were similar between years although some notable differences existed for juvenile pollock.  Adult pollock were distributed throughout Chiniak Trough, whereas in Barnabas Trough they tended to concentrate more towards the northern half of the trough during both years (Fig. 4 above).  Age-1 pollock were broadly distributed in Chiniak but virtually absent in Barnabas during the first year.  They occurred in both troughs during the second year, with distributions similar to the adults (Fig. 5 below).  The mix of age-0 pollock and capelin were broadly distributed in both troughs during the August 2000 survey.  The following year, age-0 pollock were only detected at the east end of one transect in Chiniak during the second survey pass.  Capelin were often present over the shallower edges of Chiniak Trough, but were concentrated in the deeper waters within the southern half of Barnabas Trough during the second year.  These geographical patterns generally persisted for all species and age groups during all passes for each year.  This suggests that the distributional patterns were quite stable, at least during the several week study period and, further, that no major disruptions to these patterns occurred in response to the commercial fishing activities in Barnabas during the second year.

Although capelin contributed substantially to the total fish abundance based on the acoustic data for both years, results of these analyses are not yet available because questions remain regarding the TS to fish length relationship for capelin.  Thus only biomass estimates for pollock are presented (Table 1).  Estimates for August 2000 indicated that adults were about twice as abundant in Barnabas as Chiniak.  Juvenile (age-1) pollock were scarce in Barnabas but present in quantities similar to adults in Chiniak.  Differences in estimates between passes were slight and ranged from 7% to 36%.  These differences were not considered significant because of the overlap in associated confidence intervals.  The results suggest that the amount of adult pollock was relatively stable over a period of 1-2 weeks in either trough and offer support for using the two troughs as treatment and control sites.  This trend was corroborated for juvenile pollock during the second year field season, although results were mixed for the adults (see below).

Figure 5. Acoustic backscatter mainly attributed to age-1 pollock along transects during a representative pass from the prefishery period and fishery period in August 2001.

Estimates of adult pollock biomass for the August 2001 field season decreased for adults and increased for juveniles when compared to the previous year (Table 1).  These differences highlight the value of a multiyear research effort.  As was the case in 2000, in 2001 the first two survey passes in each trough were conducted prior to the fishery to evaluate the temporal variability in fish abundance in the absence of the fishery.  Similar biomass estimates, based on overlapping confidence intervals, were generated for each trough with differences between estimates ranging from 14% to 20% except for the adults in Barnabas (Fig. 6 below).  For these adults, the pass 1 estimate (12,733 t) was over 2.5 times greater than the estimate for pass 2 (4,829 t).  This large difference likely occurred because an extremely dense but relatively small aggregation of adult pollock accounted for nearly 50% of the total biomass estimate during pass 1.  The fact that this dense aggregation was not observed during subsequent survey passes raises questions regarding the ability to discern the effects of fishing on adult pollock biomass using the current experimental design, or, perhaps suggests the need to reconsider survey design parameters in future years.

Figure 6. Pollock biomass estimates for August 2001 in Barnabas and Chiniak Troughs.

Significant differences among either the juvenile or adult pollock biomass estimates in response to commercial fishing activities were not detected (Table 1, Fig. 6).  For example, confidence bounds for juvenile estimates overlapped for all passes before (passes 1-2) and during fishing (passes 3-4).   Adult biomass estimates showed greater variability among passes than those for juveniles.  For example, confidence intervals did not overlap between the two adult estimates during the prefishery period (passes 1-2) or between the two during the fishery period (passes 3-4).  Furthermore, adult estimates from the latter three passes (prefishery and fishery) were less than half the value of the first pass (prefishery).   Note that large differences did not occur between adult estimates in Chiniak during year 2 or in either trough during the first year.  However, future work will indicate whether the variability in adult biomass estimates, particularly from the prefishery period in August 2001, is representative of “typical”conditions.  If so, it will be difficult to conclude that changes in biomass are the result of commercial fishing activity unless differences in estimates between the prefishery and fishery periods are quite dramatic (e.g., > 65% reduction) given the large natural variability observed in the existing estimates.

Vertical Distribution

The daytime vertical distribution patterns of pollock and capelin were remarkably similar between years in Chiniak and Barnabas Troughs.  Adult pollock generally formed loose near-bottom aggregations, whereas the juvenile pollock and capelin formed more discrete aggregations, higher in the water column.  Adult pollock were slightly deeper in the water column yet further off the bottom in Chiniak than in Barnabas (Fig. 7 below).   Juveniles were at similar depths in both troughs, although those in Chiniak were further off the bottom.  No differences were detected, based on overlapping confidence bounds, in mean depths of either adult or juvenile pollock when prefishing estimates were compared with the fishing estimates.

Figure 7. Weighted mean fish depth (A) and mean distance off bottom (B) estimates for pollock in Barnabas and Chiniak Troughs for each pass during August 2000 and for passes combined into prefishery and fishery periods during August 2001. Ninety-five percent confidence intervals are shown.

Several diel comparisons were made to assess whether there was a possibility of conducting the survey 24 hours per day.  Adult pollock exhibited relatively little dispersion and typically did not rise in the water column, remaining within about 30 m of the bottom at night.  In 2001, however, the adult pollock layers were sometimes difficult to distinguish from the juvenile pollock layers.  Echosign attributed to age-1 pollock or capelin typically dispersed from aggregated daytime layers and rose in the water column during darkness.   It was usually not possible to distinguish these layers from one another during the night due to the high degree of dispersal and mixing.  On occasions when these layers were recognizable at night, however, the capelin generally rose to within about 20 m of the surface, and the age-1 pollock moved to depths immediately below the capelin layers.  Small amounts of plankton echosign which were visible during the day were also often indistinguishable from other echosign during the night.   These results suggest that it will be necessary to conduct the EIT surveys only during daylight hours in subsequent years.


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