Matlock et al.: Estimating deepwater fish populations 



93 



of the sediment probably caused the frequently ob- 

 served cave-ins and sloughing of sediment around the 

 mouths of burrows. Extensive secondary bioerosion by 

 galatheid crabs and other burrowers (similar to the 

 mechanism described by Able et al. 1982 and Grimes 

 et al. 1986) further weakens the structure, contributing 

 to cave-ins of the burrow roofs dug at an angle. Con- 

 sequently, large areas of up to 6 x 9m appeared to be 

 plowed or cratered. It is not known if these broad areas 

 were caused by one or several tilefish within each of 

 the "plowed areas," or if one or more generations of 

 tilefish were responsible. 



Other tilefish burrows found in the study site were 

 more like the typical "vertical" burrows known for 

 these animals (Able et al. 1982, Grimes et al. 1986). 

 Vertical burrows are apparently more stable than 

 oblique ones, requiring less constant re- excavation. 

 Some burrows that had become inactive were filled 

 with sediment at the shaft entrance, but they showed 

 evidence of recent bioerosion around their margins 

 from secondary burrowers. There were also extensive 

 areas that contained numerous depressions, apparently 

 remains of old structures that were 1-2 m across and 

 0.6-1. 5m deep. Concentrations of 15-20 such depres- 

 sions, 6-8 filled-in burrows, and 2-5 active burrows 

 were common in the study area. 



Yello wedge grouper A study area (1.28 x 1.28km) 

 slightly inshore of the tilefish site was selected at 

 27°41.3'N lat. and 94°23.6'W long. (Fig. 2). Depth 

 ranged from 267 m along a central ridge to 311m at 

 the outer edge of the study area. The area was char- 

 acterized by isolated boulders and scattered low rock 

 ridges concentrated in depths of less than 283 m. The 

 bottom was comprised of a sand-clay mixture inter- 

 spersed with rubble and shell. Patches of avalanche 

 anemones (Bolocera sp.) and sea pens (Penatula sp.) 

 were frequently attached in the vicinity of rubble and 

 "hard bottom." Bottom temperature fluctuated little 

 at the study site (12.0-12.9°C). Fishing activities were 

 confined to a 640 x 640m (409,600 m 2 ) quadrant of the 

 study area because time available was shortened by bad 

 weather. 



Study activities 



Submersible observations Burrow and fish counts 

 were made from the submersible Johnson-Sea-Link 

 during morning. Accordian-type transects with ran- 

 domly selected starting points and alternating 366 

 (east- west) and 91-m (north-south) legs (up to 2652 m 

 total distance per dive) were run with the submersible 

 within lm of the bottom and traveling at 1.9km/h. At 

 the end of the east-west portions of each transect leg, 

 the submersible would maintain position while the RV 



Figure 2 



Yellowedge grouper study site (center point at 27°41.3'N lat. 

 and 94°23.6'W long.) showing depth contours in meters, 

 submersible transect tracks (dashed lines), and distribution 

 of longline sets (solid lines) within the northeast part of the 

 study area. Chart represents one square nautical mile. 



Johnson maneuvered directly above and recorded the 

 position on a LORAN plot of the study area. Five 

 transect dives were made on each of the study areas 

 (Figs. 1, 2). Two of these transects were located com- 

 pletely within the portions of the tilefish area fished 

 with longlines; one transect was completely within the 

 fished portion of the grouper area. 



The number of adult and juvenile burrows that were 

 "filled-in" (denoting previous occupancy) or were 

 "depressions" (characteristic of long-abandoned bur- 

 rows that had been gradually filled in and smoothed 

 over) were counted as inactive burrows. All others were 

 counted as "active" (currently used by fish); only 

 "active" burrow counts were used in estimating pop- 

 ulations. Burrows within 7.3m on either side of the 

 submersible in the tilefish area and 11.0m in the 

 yellowedge grouper area were counted. These viewing 

 distances were based on the range of visibility and 

 viewing angles and were different in the two study 

 areas because grouper and their burrows were general- 

 ly larger than tilefish. So, grouper could be seen far- 

 ther away than tilefish. All tilefish and yellowedge 

 grouper seen were counted. All other fish seen were 

 identified to species, if possible (names follow Robins 

 et al. 1980), and these identifications were verified 



