512 



Fishery Bulletin 103(3) 



In addition, the survey biomass estimate for the eastern 

 two-thirds of the HSE area is within or close to the 

 upper 95% confidence bounds of the Leslie analyses of 

 trawl and longline Pacific cod fishery data (Table 5). 



One possible explanation for the lower fishery-derived 

 estimates in the eastern portion of the HSE area is that 

 emigration of fish after spawning contributed to the low 

 CPUEs observed near the end of the fishery time series. 

 If this emigration occurred, however, it went largely 

 undetected in the neighboring areas. Emigration over 

 the course of the fishery would decrease CPUEs fast- 

 er than what would be attributable to fisheries alone, 

 which would, in turn, decrease the estimate of initial 

 biomass. 



111 



a. 



o 



5.000 



10,000 



2.5 



2.0 



1.5 



1.0 



0.5 



0.0 



B East of survey area 



B East of 

 Survey Area 



A 

 A 



A A 



D □ 



*F? A 

 -^ 

 CD ' 



2.500 5.000 



Cumulative cod catch (t) 



Figure 6 



Daily average catch per unit of effort (CPUE) of Pacific cod (Gadus 

 macrocephalus) by the observed Pacific cod fishery by gear type plot- 

 ted against the estimated cumulative catch of cod by the groundfish 

 fishery in the high sampling-effort area (A) and in the area east 

 of the survey area (B; Fig. 1). For the trawl fishery (at least 20% 

 of the haul catch was cod). CPUE = t/h; for the longline fishery, 

 CPUE = t/1000 hooks. Lines are shown for those regressions whose 

 slope was significantly different from (P<0.05; Table 5). 



Plots of fishery CPUEs of Pacific cod were very simi- 

 lar for all gears used in each area. This finding indi- 

 cates that these time series are useful as indices of 

 relative cod abundance. Similarly, inferences can be 

 made through analyses of fishery CPUE data regard- 

 ing fish movement from area to area (or lack thereof) 

 to a possible cause in the observed declines in CPUE 

 (or local abundance). For instance, the lack of fish- 

 ery CPUE increases in areas to the north, east, and 

 south of the HSE survey area in March indicates that 

 emigration was not a significant factor in the CPUE 

 decline observed in both the longline and trawl fishery 

 CPUE data from early February through 24 March. In 

 fact, in the AE of the survey area through 2 March, 

 longline CPUE declined, indicating that ei- 

 ther fish left this area (to the north) or were 

 reduced in abundance by fishing and were 

 not replenished. Although the time series 

 from the AS of the survey area is short, 

 there is no indication that cod moved there 

 in early March. There is also no evidence 

 that cod moved north to the LSE survey 

 area because the longline or pot fleets tar- 

 geting cod did not move there, nor did the 

 proportion of cod in trawl hauls increase 

 (otherwise they would have been labeled 

 as a cod-target fishery). It is possible that 

 cod emigrating from the HSE area were so 

 dispersed or their catchabilities were much 

 lower than those for residents in other ar- 

 eas that their presence went undetected, 

 but there is no evidence to suggest that ei- 

 ther of these were any more likely than the 

 more simple assumption that changes in 

 CPUE within the fished area represented 

 real changes in local abundance even after 

 accounting for some level of emigration. If 

 cod immigration exceeded emigration for the 

 HSE area during early March as CPUEs 

 were declining, then fishery-derived esti- 

 mates of initial biomass calculated in our 

 study are biased high. 



Pot fishery CPUE data in the AS of the 

 sampling area and in the HSE area indicated 

 that there was an influx of Pacific cod from 

 the south in mid-March. This was evident 

 from the increase in pot fishery CPUE on 

 18 March in the AS of the survey area and 

 beginning on 24 March in the HSE area. 

 Cod may have moved into nearshore sections 

 of the HSE area where they would be more 

 vulnerable to pot gear than to trawlers. How- 

 ever, on 25-26 March, trawl CPUE on the 

 border of the HSE area and the AE of the 

 survey area increased substantially, indicat- 

 ing that these fish had moved offshore to 

 areas worked by trawlers, or that they be- 

 came highly aggregated (perhaps just prior to 

 spawning). The late-March "pulse" of Pacific 

 cod biomass was probably smaller than the 



15,000 



7,500 



