Report for April-May-June 2001)
2001 Winter Bottom Trawl Surveys of Steller Sea Lion Critical Habitat in the Southeastern Bering Sea and Gulf of Alaska
The Alaska Fisheries Science Center (AFSC) conducted a series of bottom trawl surveys within selected areas of Steller sea lion critical habitat (SSLCH) in the southeastern Bering Sea and Gulf of Alaska during February and March 2001. Three fishing vessels, the Northwest Explorer, Ocean Harvester, and Sea Storm, were chartered to conduct survey operations within three selected areas. From 16 February to 1 March the Northwest Explorer and Ocean Harvester surveyed the Slime Bank area north of Unimak Island. After exchanging scientists in Dutch Harbor, the Ocean Harvester transited to the Shumagin Islands region in the Gulf of Alaska and conducted survey operations 2-15 March. The Sea Storm sampled stations on the east side of Kodiak Island from 9 February to 8 March.
The primary objective of the winter surveys was to provide information on the relationship between Pacific cod, (Gadus macrocephalus), walleye pollock, (Theragra chalcogramma), and Steller sea lions within the SSLCH. The initial 2001 survey effort resulted in distribution and absolute abundance estimates for Pacific cod and walleye pollock within selected areas of SSLCH and provided an assessment of the feasibility of conducting future winter bottom trawl surveys.
Survey Design and Methods
The survey areas included portions of SSLCH in the Slime Bank area of the southeastern Bering Sea and in the Shumagin Islands region and Kodiak Island waters in the Gulf of Alaska. The survey design incorporated a stratified random sampling scheme. The Bering Sea and Shumagin Islands survey areas were divided into high and low density sampling strata based on the expected distribution and abundance of Pacific cod, while the entire Kodiak survey area was sampled at a uniform sampling density. In the Bering Sea, 22 primary stations were scheduled for sampling in the nearshore or highest sampling density stratum (7,765 km2) and an additional 11 primary stations were selected for sampling in the offshore or low sampling density stratum (12,112 km2). In the Shumagin Islands region, seven primary stations were assigned within the relatively small high density sampling stratum (2,631 km2) and another nine primary stations to the remaining low density sampling stratum (10,279 km2). Thirty-six primary stations were assigned in the Kodiak Island area (13,639 km2). Additional secondary sampling locations were also selected for sampling with the final number dependent on the time remaining after completing the primary stations.
All regularly scheduled survey tows were conducted during daylight hours. Each vessel conducted 15-minute trawl hauls at preassigned stations. All fish and commercially important invertebrates were sorted, weighed, and enumerated by species. Biological information including lengths, individual weights, and maturity observations were taken for Pacific cod and walleye pollock. Stomach samples were collected for target species including Pacific cod.
Temperature and depth were recorded at each station with a micro-bathythermograph attached to the trawl headrope. Net mensuration devices and a bottom contact sensor were used to monitor and record net configuration and performance parameters during each tow.
Vessels and Gear
All three charter vessels are house-forward trawlers with stern ramps, multiple net storage reels (mounted forward of the working deck and/or aft over the stern ramp), telescoping deck cranes, propeller nozzles, and paired, hydraulic trawl winches with 1,280-2,190 m of 2.54-cm diameter steel cable. The Sea Storm is 38 m in overall length (LOA) and powered by a single, 1,710 continuous horsepower (HP) main engine. The 33-m Ocean Harvester is propelled by a 1,250 HP main engine; the 49-m Northwest Explorer is equipped with two 1,800 HP engines. Each vessel carried a full complement of navigation and fishing electronics including global positioning systems (GPS), video position plotters, radars, and depth sounders.
Standard RACE Division Poly-Nor’eastern high-opening bottom trawls, rigged with roller gear, were used exclusively for sampling the selected survey stations. Survey trawls were constructed of 12.7-cm stretched-mesh polyethylene web with a 3.2-cm stretched-mesh nylon liner in the codend. Accessory gear for the Nor’eastern trawl includes triple 54.9-m, 1.6-cm diameter galvanized wire rope bridles, and 1.8 X 2.7-m steel V-doors weighing approximately 850 kg each.
The February-March sampling period was marked by generally good weather which allowed successful completion of all primary and many secondary stations in each of the three survey areas. In the Bering Sea, nearly 2 days were lost to bad weather along with another 3 days to vessel loading/setup and vessel unloading/crew change, leaving 9 days for sampling. Bad weather at the beginning of the Shumagin Islands survey resulted in a longer running time from Dutch Harbor to the survey area and several lost sampling days. With the run back to Dutch Harbor and vessel unloading, sampling occurred on only 8 of 14 available days. In the Kodiak area, sampling occurred on 20 of 28 charter days, which included approximately 3 days of loading and unloading, a 1-day midtrip break to exchange crews, and 5 days of bad weather.
All primary stations were completed in each of the three survey areas, including 58 good performance tows in the Slime Bank area of the southeastern Bering Sea, 24 stations in the Shumagin Islands region, and another 72 stations off the east side of Kodiak Island. Some pre-assigned stations were not sampled due to unsuitable bottom conditions. In cases where an assigned station could not be sampled due to untrawlable bottom conditions, a preselected alternate location was sampled.
Sea surface temperatures and bathythermograph recordings were collected at nearly every trawl site. In general, bottom temperatures were higher than sea surface temperatures, and both surface and bottom temperatures increased from west to east with the lowest temperatures occurring in the Bering Sea and the highest in the Kodiak area. In the Bering Sea survey area, sea surface temperatures ranged from 1.9°C to 4.8°C and averaged 3.5°C, while bottom temperatures ranged from 3.6°C to 4.6°C and averaged 4.1°C. Temperatures were higher in the Shumagin Islands area with sea surface temperatures ranging from 4.1°C to 4.9°C (averaging 4.4°C) and bottom temperatures from 4.4°C to 5.7°C (averaging 4.8°C). In the Kodiak area, temperatures were the warmest, with sea surface temperatures ranging from 2.4°C to 5.7°C (averaging 4.6°C) and bottom temperatures from 4.4°C to 6.1°C (averaging 5.2°C).
Bering Sea Survey Area
|Table 1. Total catch (kg) of selected species or species groups encountered during the 2001 winter bottom trawl survey of Steller Sea Lion Critical Habitat in the Bering Sea, Shumagin Islands and Kodiak Island regions.|
|Species or species group||Bering Sea||Shumagin||Kodiak|
|Northern rock sole||2,451||0||272|
|Southern rock sole||0||210||750|
Pacific cod, the second most abundant species in the survey area, appeared in 57 of 58 catches and accumulated a total catch of 5,086 kg (Table 1 above). The largest catch of Pacific cod was 549 kg, with 16 catches exceeding 100 kg. The estimated cod biomass within the Bering Sea survey area is 49,309 metric tons (t). The mean weight and length of cod averaged 1.89 kg and 49.3 cm (Table 2 below).
|Table 2. Number of survey hauls, number of hauls with catch, mean CPUE, biomass, mean weight and mean length based on the 2001 winter bottom trawl survey of selected areas of Steller Seal Lion Critical Habitat in the Bering Sea and Gulf of Alaska.|
Walleye pollock accounted for
60,268 kg (79%) of the 76,724 kg total Bering Sea
catch and had the largest single catch, 12,507 kg,
of any species. Pollock occurred in 57 of 58
tows, with 16 catches exceeding 1,000 kg. The
estimated biomass of pollock is 794,743 t. Pollock
averaged 0.75 kg in weight and 45.1 cm in length.
Other relatively abundant species or groups included arrowtooth flounder (4,090 kg), northern rock sole (2,451 kg), flathead sole (1,065 kg), Pacific halibut (932 kg), and rex sole (261 kg).
Shumagin Survey Area
Pacific cod were the most abundant species in this region, accounting for 5,548 kg or 45% of the total catch. Cod were captured in 14 of 24 successful tows with two catches (2,303 kg and 1,212 kg) representing 63% of the total Shumagin cod catch. Within the Shumagin survey area, the estimated cod biomass is 142,863 t. Pacific cod were consistently larger than those from the Bering Sea, averaging 3.67 kg in weight and 63.7 cm in length.
Walleye pollock were taken in 18 of 24 trawl hauls for a total catch of 1,280 kg. Only four catches exceeded 100 kg. The estimated pollock biomass for this region is 22,755 t. Pollock from this region were similar to those from the Bering Sea, averaging .84 kg in weight and 44.4 cm in length.
Other important components of the Shumagin catch included arrowtooth flounder (3,299 kg), flathead sole (689 kg), Pacific halibut (464 kg), southern rock sole (210 kg), rex sole (137 kg) and eulachon (94 kg).
Kodiak Survey Area
Pacific cod accounted for approximately 6% of the total species catch. Cod were taken in 57 of 72 hauls with the five largest catches ranging from 100 kg to 300 kg. The estimated cod biomass is 14,167 t. Mean weight and length were considerably less than observed in the Bering Sea and Shumagin regions, averaging 0.78 kg and 31.7 cm.
Appearing in 66 of 72 hauls, pollock were widely distributed throughout the survey area but represented less than 10% of the total species catch. Only one haul exceeded 100 kg, a 1,838 kg catch consisting predominantly of adults, averaging 45.4 cm in length. Most catches however consisted primarily of smaller age-1 fish, resulting in an overall mean weight of 0.23 kg and mean length of 21.1 cm. The estimated pollock biomass within the Kodiak survey area is 22,035 t.
Flatfish represented 78% of the 29,234 kg of total catch from this region. Arrowtooth flounder with 14,150 kg accounted for 62% of the flatfish catch and 48% of the total species catch. Other important components of the flatfish catch include flathead sole (5,143 kg), Pacific halibut (1,272 kg), and southern rock sole (750 kg). Eulachon (325 kg) and pandalid shrimp (116 kg) were also relatively abundant in this region.
By Eric Brown.
Groundfish Bottom Trawl Surveys Underway:
Annual Bering Sea Bottom Trawl Survey
The annual Bering Sea bottom trawl survey for crab and groundfish began 28 May aboard the chartered fishing vessels Arcturus and Aldebaran. (Prior to the beginning of the survey the Alderbaran conducted a bottom trawl footrope escapement study 20-25 May.) The survey began operations at the eastern end of Bristol Bay and covered the eastern Bering Sea continental shelf from inner Bristol Bay to the shelf break, and between Unimak Pass to north of St. Matthew Island. (Upon completion of the standard Bering Sea survey, the Arcturus conducted a shrimp survey in Pavlof Bay for 5 days.) The primary objective of the survey is to continue the annual series of assessment surveys of crab and groundfish of the eastern Bering Sea to determine the distribution, abundance, and biological condition of the important groundfish and crab resources.
Biennial Gulf of Alaska Groundfish Bottom Trawl Survey
The fishing vessels Vesteraalen and Morning Star were chartered for 70 days each to conduct the 2001 biennial groundfish bottom trawl survey. The survey began 17 May and will be completed 25 July. The survey extends from the Islands of Four Mountains (long. 170°W) to roughly the center of the Gulf of Alaska (long. 147°W ). Normally the biennial survey extends to Dixon Entrance (long. 134°40’W), but due to limited vessel time, only the western and central regions of the Gulf of Alaska will be sampled this year. The primary survey objective is to continue the time series to monitor trends in distribution and abundance of important groundfish species and to describe and measure various biological and environmental parameters.
Triennial West Coast Shelf Bottom Trawl Survey
The ninth in the series of triennial bottom trawl surveys of groundfish off the Pacific Coast of the continental United States began on 1 June aboard the chartered fishing vessels Sea Storm and Frosti for 65 days each. Sampling will begin in the vincinity of lat. 34°30’N (Point Conception, California) and proceed in a northerly direction as far north as approximately lat. 49°30’, off the southwest coast of Vancouver Island, British Columbia. The primary objectives of the survey are to assess the distribution and abundance of the demersal component of Pacific whiting (Merluccius productus); to describe and assess the shallow component of the sablefish (Anoplopoma fimbria)resource (specifically those under 2 years old); to monitor and assess the distribution, abundance, and biological characteristics of the principal rockfish species; and to monitor the status of other groundfish species.
By Russ Nelson.
MIDWATER ASSESSMENT AND CONSERVATION ENGINEERING (MACE):
Echo Integration-trawl Survey of Walleye Pollock on the Southeastern Bering Sea Shelf and in the Aleutian Basin
The winter 2001 echo integration-trawl (EIT) survey of walleye pollock (Theragra chalcogramma), conducted from 19 February to 3 March (leg 1, the southeastern Bering Sea shelf and beginning of Bogoslof area) to 5-11 March (leg 2, Bogoslof area), covered an area of the southeastern Bering Sea shelf north of Cold Bay, westward to the Aleutian Basin west of the Islands of Four Mountains. Acoustic data were collected with a Simrad EK 500 quantitative echo-sounding system on the NOAA ship Miller Freeman. Trawl hauls were made to identify echosign and to provide biological samples. The cruise track for the shelf portion of the survey consisted of 22 north-south parallel transects spaced 8 nautical miles (nmi) apart covering a 14,200 nmi2 area, while that of the Bogoslof portion consisted of 22 north-south transects spaced 5 nmi apart covering a 3,000 nmi2 area. Echo integration and trawl data were collected 24 hours a day. Pollock length data from 35 hauls were aggregated into 6 analytical strata based on echosign type, geographic proximity of hauls, and size composition data from the catch. Estimates of pollock backscattering strength in the area represented by each stratum were calculated. These data were used to estimate numbers and biomass of pollock by size for the entire area surveyed, and separately for the eastern shelf, the entire (historical) Bogoslof area, the Central Bering Sea (CBS) specific area, and the Steller sea lion conservation area (SCA). Error bounds on the acoustic data were derived using a 1D geostatistical method.
On the Bering Sea shelf, pollock
were relatively continuously distributed across the
first 11 transects, and were most concentrated north
of the northeast end of Unimak Island (Figure 1
above). Echosign appearance differed between day,
when pollock were densely aggregated either on or
off bottom, and night, when pollock were more evenly
distributed and loosely aggregated from the bottom
into midwater. Juvenile pollock tended to form
medium-density scattering layers or schools about
5-50 m off bottom. In the Bogoslof area (west
of long. 166°W and south and west of the 500-m
depth contour) pollock were aggregated between 400
and 500 m in depth. In the area of highest
abundance, Samalga Pass, pollock aggregations were
about 300 m thick and were typically observed along
about 5-10 nmi of transect. Abundance estimates for
pollock in the Bering Sea shelf survey area were
1.42 billion fish and 0.825 million t. The 95%
confidence interval (from the 1D geostatistical
analysis) for the biomass estimate was 0.691-0.959
million t. Abundance estimates for pollock in
the Bogoslof area were 171 million fish, and 0.231
(0.185-0.278) million t. Abundance of pollock
estimated for the subset of Bogoslof pollock inside
U.S. management area 518/CBS specific area were 149
million fish and 0.208 million t, about 90% of the
estimated population for the whole area. Population
estimates for the SCA, obtained by adding together
estimates from the shelf area excluding transects
100 and half of 101, and the Bogoslof area excluding
transects 221, 222, and half of 220, were 1.467
billion pollock and 0.968 million t. Population
estimates for the entire area surveyed were 1.594
billion pollock and 1.06 million t.
Estimated pollock abundance on the eastern shelf in 2001 (0.825 million t) was about the same as in 2000 (0.816 million t). Modal lengths of the adult pollock increased from about 42 cm in 2000 to about 46 cm in 2001. Aggregations of age-1 pollock were observed near the 200 m contour in 2001 in an area not surveyed in 2000. Among adults, maturity was similar for both sexes between years, and the length at 50% maturity for females was 43 cm for both years.
In the Bogoslof survey in 2001, as in recent years (1998, 1999, and 2000), pollock were highly concentrated in Samalga Pass. There was no significant change in population biomass between 2000 and 2001. However, Bogoslof population estimates from EIT surveys have decreased over a longer time period; no significant recruitment from younger year classes has occurred since the 1989 year class began recruiting in about 1994. Bogoslof prespawning pollock appeared to remain distinct from those prespawning pollock inhabiting the Bering Sea shelf in winter. The Bogoslof spawning population’s contribution to the overall Bering Sea pollock gene pool is not known. However, these unique, deep-water spawning aggregations composed of large, relatively older fish, with corresponding higher egg production than pollock found on the shelf, may make a more significant contribution than their numbers would indicate.
By Taina Honkalehto, Paul Walline, and Denise McKelvey.
Triennial Echo Integration Trawl Survey of Pacific Whiting
Scientists from the MACE program began the ninth triennial echo integration-trawl survey of Pacific whiting off the West Coast from Monterey, California (lat. 36°5'N) to northern British Columbia (lat. 54°N) on 15 June 2001 aboard the Miller Freeman. The portion of this joint U.S.-Canada collaborative survey conducted by AFSC scientists aboard the Miller Freeman extends from Monterey to about the U.S.-Canada border (lat. 48°30'N). Canadian researchers will continue the survey northward from the area where the Miller Freeman completes operations. Approximately 3,500 nmi miles of acoustic trackline are expected to be covered during the 6-week survey by U.S. scientists. Acoustic data as well as midwater and bottom trawl catch information will be used to determine the coastwide distribution, biomass, and biological composition of Pacific whiting.
By Christopher Wilson.
Testing of Halibut Excluders for Cod Trawls
In early April 2001, a halibut excluder system for cod trawls was tested in the southeast Bering Sea aboard the fishing vessel Northwest Explorer. This was a cooperative research effort between the Groundfish Forum, Trident Seafoods, and scientists from the RACE Division.
The excluder system was developed and tested during three studies in 2000 (See AFSC Quarterly Report for Oct-Dec 2000). The resulting excluder included three sections; 1) a large mesh panel that excluded skates and other large fish, preventing them from clogging the subsequent sections; 2) rectangular panels of rings, which were sized to allow large cod to pass while blocking large halibut and forcing them toward an escape slot; and 3) side panels of horizontal slots, which allowed small halibut to escape while retaining all but the smallest cod. In contrast to previous studies, no recapture net was used, and tows with matched nets were alternated with and without the excluder. This allowed testing under actual commercial fishing conditions. Other differences included: codend meshes (176-mm stretch square mesh) used by the fishery, larger meshes in the intermediate section containing the ring excluder (195-mm diamond), size composition of halibut (see below), and due to the construction of the trawls, moving the excluders aft so that the skate excluder was in an untapered section and the slot section connected directly to the codend. While the original excluder included skate, ring, and slot sections, the ring section was removed after three excluder tows because:
1. Very few halibut greater than 60 cm were encountered, most were in the 30-40 cm range;
2. Apparent significant loss of all sizes of cod was occurring, and the large meshes in the ring section were considered a likely site of such escapes; and
3. Minor damage was occurring to fiberglass rods in the ring section.
Seven excluder tows were completed using the skate and slot excluders (Figure 1 above) as well as seven control tows. Comparisons of the catch-per-hour from those tows showed that: the average cod catch with the excluders was 11% lower than the control tows, while decreases of halibut, pollock and rock sole were 86%, 97%, and 99%, respectively (Figure 2 below). Only the cod difference was not statistically significant. Analysis of the length data indicates that cod escapes were mostly fish smaller than 55 cm. The vessel captain felt that the cod loss may have been significantly higher, based on echo sounder information during the tow and catch reports from nearby vessels. There was agreement that excluder performance would likely be improved by moving the device forward, farther away from the codend.
Presence of the excluders did not
significantly increase handling time, and the
excluders were not damaged by normal fishing
By Craig Rose
Fisheries Acoustics Science and Technology and Fishing Technology and Fish Behavior Working Group Meetings
The 2001 annual meetings of the International Council for the Exploration of the Sea (ICES) Fisheries Acoustics Science and Technology (FAST) and Fishing Technology and Fish Behavior (FTFB) working groups were held in Seattle 23-27 April under the sponsorship of the RACE Division. The FAST meeting with 65 participants and FTFB meeting with 57 participants met separately at the Best Western University Tower Hotel on all days, except 25 April, when a joint meeting was held at the NOAA Sand Point campus. Meeting organizers Bill Karp, Dave Somerton, and Peter Munro were greatly assisted by Erika Acuna, Mike Brown, and Sarah Stiennessen. Peter Munro and Taina Honkalehto were rapporteurs.
Acoustics scientists and fisheries biologists from around the world presented and discussed a wide range of recent research topics and problems in fisheries acoustics and technology. Topics addressed in FAST meeting sessions included acoustic methods of species identification, ecosystem studies based on acoustic survey data, and evaluation of effects of fish avoidance during surveys. Also included was a special session, attended by members of both working groups, on acoustic seabed classification, which included several invited speakers from academia and commercial software companies. AFSC scientists presenting papers at the FAST meeting were Chris Wilson, Neal Williamson, Mike Guttormsen, Denise McKelvey, John Horne, and Paul Walline.
Topics addressed in the FTFB meeting sessions included methods to reduce the variance of abundance indices obtained from assessment surveys using fixed and mobile fishing gears, evaluation of the selections properties of Baltic cod trawls, and methods to reduce bycatch and seabed impact of fisheries. AFSC scientists presenting papers at the FTFB meeting were Ken Weinberg, Mark Zimmermann, Michael Martin, Jeff Fujioka, Jack Turnock, Craig Rose, and Dan Nichol.
The joint meeting of the two working groups focused on 1) the impact of fish behavior on accuracy and precision of stock assessment surveys and 2) the potential for acoustic techniques to provide information about fish behavior in a wider sense for management and assessment, as well as biological and ecological interests. The joint meeting also included a poster session and a small trade show, where attendees were able to interact with vendors of various high technology sampling or analysis tools. Attendees of the annual working group banquet were entertained by the guest speaker, Jim Coe, acting director of the AFSC. Meeting reports from each working group are available on the World Wide Web at http://www.ices.dk.
By David Somerton.
Cooperation in Fisheries and Related Research Agreement Signed
In 1986, an agreement for cooperation in fisheries and related research was signed by the directors of the AFSC, the Norwegian Institute for Marine Research (IMR), and the Pacific Biological Station (PBS) located in Nanaimo, British Columbia. The agreement recognized similarities in the types of research performed and the overall level of scientific expertise at each institution and encouraged participation by universities in the Pacific Northwest and Alaska, British Columbia, and western Norway. On 20 April 2001, Dr. Åsmund Bjordal, Director of the Marine Resources Department at IMR visited the AFSC, where he and AFSC Acting Director Jim Coe signed an updated version of the agreement. It is hoped that a representative of PBS will soon sign the new agreement.
The new agreement is designed to encourage cooperation among scientists working in areas of marine science that relate to our understanding of marine organisms and the environments they inhabit. There is particularly interest in encouraging cooperation and exchange in multispecies and ecosystem-based management, responsible fishing, and associated areas such as assessment methodology and fishing gear technology. However, the importance of encouraging and supporting cooperation in a broader range of disciplines is recognized, particularly in relation to the Arctic, boreal, and sub-boreal ecosystems of the Northeast Pacific and the Northeast Atlantic. As was the case under the original agreement, these goals will be accomplished through joint sponsorship of workshops and symposia, exchange of expertise and information, extended visits of scientists, and cooperative research on common scientific issues and methodological problems.
Coordination and communication are emphasized in the new agreement. A special Web site will be developed and each signatory institution will nominate a coordinator for the joint program. Coordinators will generally serve for 2-year periods and will meet periodically to draft cooperative plans. Bill Karp of the RACE Division and Åsmund Bjordal will be the first coordinators under the new agreement. Bill is now working on the AFSC’s contribution to the initial 2-year plan, so please contact him with suggestions and questions.
By Bill Karp.