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Research and Analysis

Histological sections are being prepared from Pacific cod ovaries collected in the Bering Sea, and arrowtooth flounder ovaries collected in the Gulf of Alaska are being processed. Additional cod ovaries will be collected this winter from small cod encountered during the Gulf of Alaska “A” fishing season. The additional samples will augment previous collections, providing ovaries from the entire length range of mature Pacific cod. These histological samples will be used to classify the maturity of the specimens from which they were collected. The study aims to delineate the length and age at maturity and the growth of these species, essential factors for assessing the status of these stocks.

Progress in the RACE Division’s Systematics Laboratory includes further work on a revision of the family Bathymasteridae, a review of rougheye rockfish, and a synopsis of skate catches in Alaska, as well as descriptions of new species and records of skates, snailfishes, and eelpouts of Alaska.

By Mark Wilkins.

NOAA Multibeam Sonar/ Ocean Mapping Workshops

RACE Division staff are participating in ongoing efforts to develop a coordinated ocean mapping program within NOAA. The primary focus is on multibeam echosounders which measure water depth (bathymetry) and acoustic characteristics of the seabed (backscatter) across broad swaths of seabed using transmitted pulses of sound. Data from these systems are typically used for construction of nautical charts and benthic habitat maps. They may also be useful for quantitative assessments of pelagic fish populations. Recognizing the need to bring all potential users together in order to gain a better understanding of NOAA-wide requirements, NOAA Marine and Aviation Operations and NOAA’s Office of Coast Survey sponsored two Multibeam Sonar/Ocean Mapping Workshops in February and October 2003. The most recent workshop was hosted by the Center for Coastal Ocean Mapping/Joint Hydrographic Center at the University of New Hampshire on 28-29 October 2003.

Participants at the October workshop agreed that ocean mapping should be coordinated between the various NOAA line offices and mission areas. It was also agreed that NOAA would benefit from the availability of additional multibeam mapping systems on NOAA ships. Presently, NOAA’s multibeam data are collected by NOAA ships (Ronald H. Brown, Thomas Jefferson, Rude, and Rainier) and through private sector contracts. Despite growing needs, it was acknowledged that only a few employees outside of the NOAA Commissioned Corps, the hydrographic survey ships, and the Office of Coast Survey have expertise or training in multibeam systems and data processing. As such, training and retention of trained personnel was identified as a significant requirement in an expanded NOAA-wide effort.

It was further agreed that use of competing technologies would encourage technology improvements to the benefit of both NOAA and the marine acoustics industry. Many participants stressed the need to determine a common data acquisition standard to facilitate the sharing of data. Participants also expressed a need for collaborative research to further investigate the suitability of multibeam systems for water column applications. Above all, it was concluded that a coordinated approach to ocean mapping cannot take place without addressing the full process from data acquisition and processing to completion of products and data archiving. Participants agreed that a follow-up workshop should be scheduled in March 2004 to discuss data acquisition standards, training, data processing, and data archival issues.

By Bob McConnaughey.

Estimating Capture Probability of Skates for Survey Trawl

Stock assessments relying upon NMFS summer bottom trawl surveys as a primary source of information on the biomass and distribution of the major skate species (Bathyraja spp.) in the eastern Bering Sea and Gulf of Alaska currently assume that our survey trawls are 100% efficient at capturing these bottom dwellers. However, in recent years, RACE experiments have demonstrated that the capture efficiency of our survey trawls for some benthic fauna (crab and flatfish) varies by size and species due to individuals escaping beneath the trawl footrope. As part of an effort to improve skate stock assessments, we conducted an experiment to estimate skate capture efficiency following the 2003 Bering Sea survey of shelf groundfish resources for the bottom trawl used in that survey.

The experimental design was similar to our previous catchability studies and followed standard survey protocols using standard survey gear. An auxiliary net was attached under the standard 83/112 Eastern survey bottom trawl to capture skates passing underneath the footrope. The extra drag associated with the auxiliary net necessitated the use of shorter bridles (27.5 m) from those typically used on the survey (55 m) in order to achieve similar average wingspreads. Tows were made in pairs, one heading north the other heading south, to reduce the effect of current flow on the bottom tending performance of the trawl. Bottom contact sensors were deployed on both the trawl and the auxiliary net to monitor footrope contact with the sea floor.

Figure 1, see caption
Figure 1.  Length frequency distribution of skates caught in the survey trawl (a) and the auxiliary net (b).


Figure 2, see caption
Figure 2.  Survey trawl capture efficiency as a function of skate length.  Circle size is proportional to the number of individuals (ranging from 1 to 202) caught in each 5 cm length category.

A total of 1,011 skates (849 B. permifera, 142 B. interrupta, 14 B. taranetzi, and 2 B. maculata) were measured (total length) to the nearest centimenter from 46 successful tows. These four species, which have similar body morphology, were grouped together to compute capture efficiency. Length frequency distributions from both the captures in the survey trawl and the auxiliary net were generally bimodal with considerably higher numbers of skates taken in the 20-40 cm and 90-110 cm ranges (Fig. 1).

Preliminary results indicate that skate capture efficiency (expressed as a percentage and calculated as the numbers caught at length in the standard trawl divided by the sum of the numbers caught at length in the standard trawl and the auxiliary net) increases with increasing lengths (Fig. 2) and that, for all sizes sampled, efficiency was less than 100%, confirming the need to account for skate escapement in the stock assessment process. More detailed analyses and model-fitting are in progress.

By Stan Kotwicki.


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