RESOURCE ASSESSMENT & CONSERVATION ENGINEERING (RACE) DIVISION

MIDWATER ASSESSMENT & CONSERVATION ENGINEERING PROGRAM

Personnel from the Midwater Assessment and Conservation Engineering (MACE) Program began testing the Fisheries Scientific Computer System (FSCS), designed and developed by NOAA's Office of Marine and Aviation Operations, to digitally collect fisheries-independent data aboard research vessels. The system will be installed on the NOAA ship Miller Freeman prior to the 2003 field season. The system's components include LCD touch-screen displays, digital scales, and electronic fish measuring boards, which are used to collect catch composition information and biological data from individual fish, including lengths, weights, sex, and maturity stage. It is expected that the PC-based FSCS will speed up data collection operations and reduce the transcription errors inherent in the historical paper-based system.

MACE scientists tested an acoustic communication link system during gear trials aboard the fishing vessel Sea Storm in Puget Sound, Washington, on 9 and 11 December 2002. The system controls an opening and closing device installed on a frame fixed in the mouth of the trawl codend. The opening-closing net will be used in difficult sampling situations, such as when juvenile fish form continuous midwater layers and adult fish are present lower in the water column and it is impossible to sample the lower layer without contamination from the upper. With an opening and closing codend, these layers can be sampled discretely, with catches from each layer retained in different codends. The fully integrated communication system is comprised of software, a computer-controlled acoustic transmitter/receiver unit, transducers, acoustic releases, and associated cables, switches, and batteries. Work on this system is still in progress. The next phase of testing will be conducted aboard the NOAA ship Miller Freeman in January 2003.

By Mike Guttormsen.
 

SHELLFISH ASSESSMENT PROGRAM:

Estimating Selectivity for Bristol Bay Red King Crab

An experiment designed to estimate Bristol Bay red king crab (RKC), Paralithodes camtschaticus, selectivity for the 83/112 Eastern bottom trawl, the trawl used in the annual Bering Sea shelf survey of crab and groundfish resources, was conducted following the 2002 survey. Stock assessments for RKC utilize relative abundance estimates generated from the most recent survey for input into a length-based population model (LBA) to compute Guideline Harvest Levels (GHL). This modeling approach assumes the survey trawl has 100% capture efficiency (i.e., the proportion of RKC in the path of the survey trawl captured by the trawl). However, this assumption is likely untrue as evidenced by video footage taken during a crab resampling exercise in 2000 in which a number of adult RKC were observed passing under the approximate center of the survey trawl's footrope.

In this experiment red king crab escaping beneath the survey trawl were captured with an auxiliary net that was attached behind the footrope of the trawl. Video of survey trawl footrope performance and crab interaction with the footrope was taken with a SIT (silicon intensified tube) camera on several tows where the ambient light level met the camera's threshold.

Operations extended from 21 to 29 July aboard the chartered fishing vessel Arcturus, one of two vessels chartered since 1993 to conduct annual Bering Sea surveys. Trawling took place in Bristol Bay at depths ranging from 41 to 77 m following standardized survey protocol that included towing during daylight hours at a 3 knot vessel speed using standardized tow direction, trawl wire scope ratios, and locked winches. A bottom contact sensor was placed at the center of both the survey trawl and the auxiliary net's footrope to monitor bottom contact performance. Two deviations from standardized survey protocol were necessary. First, 15-fathom bridles were used instead of 30-fathom bridles to offset the loss of wingspread caused by the added drag of the auxiliary net. Bridles are known to have a herding effect on many fish species but not for the slow-moving king crab. Second, tow length was shortened to 20 minutes from the survey standard 30 minute duration. Shorter tows were required as we observed during gear trials a trend of decreasing path width over time as the nets filled and the drag increased. Wingspreads over a 20 minute tow with the experimental gear were nearly the same as the standard gear for a 30 minute tow.

Towing sites were selected based upon carapace length (CL) frequency distributions obtained from the recently completed survey. Because selectivity curves are most useful when they cover a broad portion of the entire size range of an animal, emphasis was placed on searching for sites where the smallest and largest RKC had occurred. Tows were made in pairs, one tow in a northerly direction the other heading to the south in order to mitigate any data bias due to a current flow effect which could potentially cause the footrope to lift. The direction of the first tow in each pair was randomly determined. Increased effort was given to those sites producing favorable numbers and crab lengths by adding additional towing pairs.

Figure 7.  Total number (circles) of male red king crab caught and grouped into 5-mm intervals, and computed capture efficiency (triangles) over 43 experimental tows using the 83/112 Eastern survey trawl fitted with an auxiliary net.

Length measurements from 43 successful tows (21 pairs plus 1 tow using the appropriate combination of experimental trawl gear made during gear trials) made in the vicinity of 14 standard Bering Sea survey stations were included for selectivity analyses. A total of 3,233 RKC were measured. Male samples numbering 1,667 individuals ranged in size from 20 to 180 mm CL. Females numbering 1,566 individuals ranged in size from 50 to 160 mm CL. Figures 7 (above) and 8 (below) show capture efficiencies (triangles) and capture frequencies (circles) for male and female RKC binned into 5 mm length groups. The y-axis scale extends 0-200 for total numbers caught and 0%-100% for capture efficiency data. For the purpose of this preliminary report, the capture efficiency data has been fit using simple linear regression.

Figure 8.  Total number (circles) of female red king crab caught, grouped into 5-mm intervals, and computed capture efficiency (triangles) over 43 experimental tows using the 83/112 Eastern survey trawl fitted with an auxiliary net.

The 2002 experiment confirmed that escapement of adult Bristol Bay RKC beneath the footrope of our survey trawl was occurring as evidenced by our videos taken in 2000 and that while capture efficiency increases with size, it is not 100% as assumed in current stock assessments. Furthermore, trawl selectivity, although similar between sexes, was generally 10% higher for males than females.

Input included into the stock assessment modeling process for Bristol Bay RKC are survey catch statistics for legal-sized males (≥135 mm CL), male prerecruits (94-134 mm CL), the estimated mature biomass of males (≥120 mm CL), and the estimated mature biomass of spawning females as determined from the size at 50% maturity (≥90 mm CL). Our preliminary regression-based estimates of selectivity for legal-sized males up to 180 mm CL ranged from 83% to 87%. Selectivity of prerecruit males included in the modeling process ranged from 78% to 83% and for the mature portion of the male spawning population from 81% to 87%. Survey trawl efficiency for the female portion of the spawning RKC population ranged from 68% to 76% for animals up to 160 mm. More detailed analyses are to be applied to this data to assist the stock assessment process in the near future.

By Ken Weinberg and Bob Otto.

AFSC Quarterly
Research Reports
Oct.-Dec. 2002