274 
Fishery Bulletin 1 13(3) 
and Erofeeva, 2002). To address current-dependent 
catchability of the bottom trawl, predicted tidal veloc- 
ity was decomposed into component vectors with veloci- 
ties parallel or perpendicular to the ship’s direction of 
travel. The difference between the ship’s bearing and 
the direction of tidal flow (Aq) was calculated as 
A o~if(G T - 9 S + 360°) < 360°, 
then(0 T -d s + 360°), (1) 
else ( — 
where dy = the direction of tidal flow; and 
6>s = the direction of the ship’s travel during the 
trawl survey. 
Parallel current (Cp) was then calculated as 
Cp~if (90° < A h <270°, 
then 
1 + (tan(A H(r) )) 
( 2 ) 
else 
Vt 
' l + (hxn(A 0(r) )) 
where Ae( r ) = the angular difference between the direc- 
tion of tide flow and ship’s travel. 
Note that C p carries a negative sign if it is opposite the 
ship’s direction of travel. Because the speed of water 
flowing through the mouth of the trawl is an impor- 
tant factor that could affect the catch efficiency of the 
bottom trawl, we estimated trawl velocity at the net 
mouth (iVg) as 
Ns~v$ - C p , (3) 
where vq = the ship’s velocity. 
Finally, cross current (C x ) can be estimated as 
C x ~C p tan(Au( r )). (4) 
Its absolute value ( | C x | ) was included in the model. 
Invertebrate data 
The precision of sponge identification by scientists 
during the Aleutian Islands bottom trawl surveys 
has varied over time and remains challenging (Stone 
et ah, 2011). We chose to examine the Aleutian 
Islands surveys between 1997 and 2010 because, 
with the advent of waterproof invertebrate field guides 
for use on deck in 1996 (Kessler 5 ), 1997 (Clark 6 ), 
1999 (Clark 7 ), and 2006 (Clark 8 ), our ability to consis- 
tently identify sponges and other invertebrates to high- 
er taxonomic levels in the field was enhanced. As AFSC 
field biologists have become more familiar with these 
identification tools, the number of sponge taxa reported 
from the Aleutian Islands bottom trawl survey has also 
increased, rising from 7 in 1994 to around 70 in 2006 
and 2010. Recently, Stone et al. (2011) recognized 125 
unique sponge taxa from the Central Aleutian Islands 
and indicated that there were likely many more species 
yet to be described from this region. 
Sponges were identified on Aleutian Islands bottom 
trawl surveys to the lowest possible taxonomic level 
on the basis of the existing field guides, but identifica- 
tions depended on the identifier’s expertise as well as 
on the minimum level of identification required that 
year, both of which varied within and between survey 
years. The uncertainty in our taxonomic identifications 
of sponges coupled with our primary aim of considering 
their contribution to structural heterogeneity in the 
environment led us to employ Bell and Barnes’ (2000) 
method of grouping sponges by body form into morpho- 
logical groups (i.e., morphogroups). By combining this 
technique with a modified version of the list of macro- 
scopical features in Boury-Esnault and Rtitzler’s (1997) 
thesaurus of sponge morphology, we created 15 mor- 
phogroups representing local Aleutian sponge fauna 
(Fig. 2) and assigned field identified sponges to these 
groups ex post facto. A group composed of identifiable 
sponges that did not fit into 1 of the 15 morphogroups 
(e.g., Plakina tanaga ) and unidentifiable fragments of 
sponges were assigned to a general category entitled 
“Porifera unidentified.” 
Corals and bryozoans are epibenthic invertebrates 
that also provide biogenic structure, which may serve 
as habitat resources for Pacific ocean perch in this part 
of their range (Rooper and Boldt, 2005; Rooper et ah, 
2007; Boldt and Rooper, 2009). To allow comparison 
with these and similar studies (e.g., Rooper and Mar- 
tin, 2012), we grouped the coral and bryozoan taxa into 
single presence-absence composite “corals” and “bryo- 
zoans” factors, respectively. Wing and Barnard (2004) 
included 105 coral species from Alaska waters in their 
revised field guide. Heifetz et al. (2005) documented 69 
taxa of corals in the Aleutian Islands; 25 of them were 
endemic to the region. For this study, the composite 
corals factor was composed of members of the orders 
Alcyonacea, Anthoathecata, Antipatharia, and Sclerac- 
tinia. From these orders, 99 unique taxa were reported 
from our catches during the Aleutian Islands bottom 
trawl surveys conducted between 1997 and 2010. The 
5 Kessler, D. W. 1996. Alaska’s saltwater fishes and other 
sea life. Unpubl. manuscript. Revision by staff of the Alas- 
ka Fish. Sci. Cent. Resource Assessment and Conservation 
Engineering (RACE) Division (for use at sea) of Kessler, D. 
W. 1985. Alaska’s saltwater fishes and other sea life, 358 p. 
Alaska Northwest Publishing Co., Anchorage, AK. 
6 Clark, R. N. 1997. Invertebrates of the Aleutian Islands, 
169 p. Unpubl. manuscript. Alaska Fish. Sci. Cent., Natl. 
Mar. Fish. Serv., 7600 Sand Point Way NE, Seattle WA 98115. 
7 Clark , R. N. 1999. Gulf of Alaska invertebrates, 100-1000 
m, 179 p. Unpubl. manuscript. Alaska Fish. Sci. Cent., 
Natl. Mar. Fish. Serv., 7600 Sand Point Way NE, Seattle WA 
98115. 
8 Clark, R. N. 2006. Field guide to the benthic marine inver- 
tebrates of Alaska’s shelf and upper slope taken by NOAA7 
NMFS/AFSC/RACE Division trawl surveys, 305 p. Unpubl. 
manuscript. Alaska Fish. Sci. Cent., Natl. Mar. Fish. Serv., 
7600 Sand Point Way NE, Seattle WA 98115. 
