300 
Fishery Bulletin 11 6(3-4) 
difference, but for the selectivity curve for the canary 
rockfish assessment, all fishes larger than ~15 cm TL 
are assumed to be 100% available to the gear. It is im¬ 
portant to note that the assessments are based on data 
from years and areas not represented in this analysis, 
which may be the reason for the differences in length 
composition observed in our study. These differences 
would imply that selectivity varies over time or space 
(or both). Time-varying selectivity is commonly assumed 
in rockfish assessments, although spatial variability in 
survey selectivity is considered less often, despite known 
latitudinal dines in size for many rockfishes (Fraiden- 
burg, 1980; Gertseva et al., 2010; Keller et al., 2012). 
The differences we observed between surveys also could 
be due to a reduction in availability of large fish to the 
submersible survey, but that seems unlikely given that 
we found greater proportions of large sizes for 3 species 
on untrawlable habitat patches (Fig. 3, Table 3). With 
regard to survey efficiency, the probability of detection 
of fish in submersible surveys increases with fish size, 
and the reaction of large rockfishes to the Delta has 
been found to be minimal (Yoklavich et ah, 2007; Laidig 
and Yoklavich, 2016). 
A number of studies have compared other aspects of 
data collected during underwater visual surveys and 
trawl surveys of rockfishes, including fish density on 
trawlable habitat (Adams et ah, 1995), trawl catch ef¬ 
ficiency (Krieger, 1993), fish frequency of occurrence 
and weights on trawlable and untrawlable habitat 
(Starr et al., 2016), and species composition and den¬ 
sities on trawled and untrawlable habitat (Jagielo et 
al., 2003). Lauth et al. (2004) estimated size-specific 
selectivity for a trawl survey of thornyheads ( Sebastol- 
obus spp.) off Oregon, using independent estimates of 
density and lengths obtained with a video camera sled 
on trawlable habitat. Lauth et al. (2004) calculated 
much lower selectivity values for fish >30 cm TL than 
the most recent stock assessment (which was based on 
data from California, Oregon, and Washington), rais¬ 
ing the question of spatial variability in trawl survey 
selectivity for thornyheads. 
As far as we know, ours is the first study to com¬ 
pare length distributions of rockfishes from trawl sur¬ 
veys with those from submersible surveys conducted 
in nearby areas inaccessible to trawls. Additional 
comparisons can be made for other species from these 
central California data sets, and from existing sub¬ 
mersible and trawl data sets from southern California. 
Similar comparisons of rockfish lengths estimated from 
submersible and trawl surveys from other regions of 
the west coast could help address spatial variability in 
trawl survey selectivity (Sampson, 2014) and assump¬ 
tions about the trawl selectivity functions used in stock 
assessments for rockfishes. 
Acknowledgments 
We thank M. Yoklavich, principal investigator for sub¬ 
mersible surveys; R. Starr, co-principal investigator in 
2007-2008; T. Laidig, L. Snook, M. Love, M. Nishimo- 
to, and D. Schroeder for field data collection and video 
processing; the Delta and R/V Velero crews; and the 
California Ocean Protection Council for partial fund¬ 
ing. We appreciate T. Hay’s assistance with the FRAM 
Data Warehouse, and the many NWFSC personnel 
and vessel crews that conduct the West Coast Bottom 
Trawl Survey. Comments from J. Field, A. Keller, M. 
Yoklavich, and three anonymous reviewers improved 
the manuscript. 
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