WILKINS: ABUNDANCE OF WIDOW ROCKFISH 



the night to provide information on variability of 

 abundance and distribution within a given night. 

 Selected study areas were again surveyed after 

 an interlude of several days to study variability 

 over longer periods. 

 3) Fish aggregations noted during transecting were 

 sampled with midwater trawls for species iden- 

 tification. This was done on alternate nights so 

 as not to impede the progress of the acoustic 

 assessment portion of the survey. Biological data 

 (eg, size composition, maturity, stomach con- 

 tents) were collected from widow rockfish in the 

 catches. 



Results 



About 725 km of transects were covered in the five 

 study areas during the 12 nights of hydroacoustic 

 data collection. Ten midwater trawl hauls were made 

 to identify species present in various schools. Widow 

 rockfish schools were seen in all areas, but were 

 sparse on the Cape Blanco, Heceta Bank, and The 

 Fingers grounds. The Halibut Hill ground, only 

 recently exploited, contained the highest density of 

 widow rockfish schools and also the largest average 

 school size After editing videotaped sonar records, 

 127 schools were identified as widow rockfish; data 

 from 37 of these were integrated on the echo sounder 

 system and used to calculate school biomass esti- 

 mates. Ideally, a mean school biomass would have 

 been derived for each ground, but because few 

 schools were observed there, school biomass 

 estimates were pooled and averaged for the Nelson 

 Island, The Fingers, and Heceta Bank grounds. No 

 measurable widow rockfish schools were seen dur- 

 ing surveys of the Cape Blanco ground. School abun- 



dance was estimated for each area by treating each 

 pass through the area as a replicate and pooling data 

 from all replicates within the area. School abundance 

 (excepting Cape Blanco) ranged from 0.6035 schools/ 

 km 2 on The Fingers ground to 1.4810 schools/km 2 

 on the Halibut Hill ground. Area biomass estimates 

 are summarized in Table 7. The total estimated 

 biomass for the five survey areas was about 830 t; 

 50% at Halibut Hill ground, 30% at Heceta Bank, 

 11% at The Fingers, and 9% at Nelson Island. 



Sampling was concentrated on the Halibut Hill 

 ground, where widow rockfish schools were largest 

 and most plentiful, in order to investigate the diel 

 and night-to-night variability in school abundance 

 The survey of this ground was repeated seven times; 

 three times each night on 26-27 March and 31 

 March-1 April and once on 30 March. Separate 

 sighting functions for each night were estimated by 

 pooling observations. Corresponding school abun- 

 dance and mean school biomass estimates were then 

 calculated for each night. School abundance ranged 

 from 0.39 schools/km 2 on 26-27 March to 4.50 

 schools/km 2 on 30 March. Mean school biomass 

 tended to decline as school abundance increased, 

 however, so biomass estimates for each of the sam- 

 pling periods changed less than either school abun- 

 dance or mean school biomass (Table 8). It was not 

 possible to analyze the Halibut Hill data on a 

 replicate-by-replicate basis because few schools were 

 sighted during any single replicate The number of 

 sightings per replicate ranged from 4 to 34. Burn- 

 ham et al. (1980) cautioned that such stratification 

 procedures for line transect surveys should be 

 "severely limited to those few surveys where the 

 number of objects seen on replicate lines is fairly 

 large (perhaps at least in the 20 to 30 range)". 



Table 7— Summary of estimates of school abundance (D), mean school biomass (MSB), and 

 biomass (fl) in each of four study areas covered during the 1982 widow rockfish assessment 

 feasibility survey. 



Study area 



, D 

 schools 



km 2 j 



Var0) 



MSB 



Area B 



school / Var(M$8 (km 2 ) (t) Var(£) 



CV(6) 

 [Var(g)] 1/2 



6 



Heceta Bank 0.9490 0.0305 1.968 0.656 68.60 245.76 6,758.37 0.335 



(6 schools) 



The Fingers 0.6035 0.0711 6.409 5.486 40.47 92.20 2,212.03 0.510 



(4 schools) 



Nelson Island 0.7587 0.3010 3.924 9.514 27.44 78.59 3,467.75 0.749 



(2 schools) 

 3 above areas 



pooled 1 3.775 1.151 



Halibut Hill 1.4810 0.2028 9.639 21.668 28.81411.27 51,758.47 0.553 



(25 schools) 



'School biomass data from Heceta, The Fingers, and Nelson Island were pooled to provide a mean school 

 biomass which was used to calculate total biomass in each area. 



303 



