Report for Jan-Feb-Mar 2000)
The Effect of Trawl Speed on Footrope Contact of a Survey Trawl
Groundfish and crab assessment surveys conducted by the Groundfish Assessment Program standardize towing speed to 3.0 knots speed over ground (SOG). However, the catchability of our survey trawls may change with towing speed because of its effect on trawl dimensions and fish swimming speed. The between-tow variability in catch per unit effort (CPUE) caused by changes in trawl geometry might be reduced if we were to instead standardize towing speed to speed through water (STW). In an experiment conducted off the coast of Washington in September 1999 (described in July-September 1999 Quarterly Report), we examined whether changing towing speeds (STW) had an effect on footrope contact with the bottom, a situation which could potentially affect fish escapement beneath the trawl.
The fishing vessel Sea Storm was chartered for 8 days to tow the RACE Division’s standard Poly Nor’Eastern bottom trawl fitted with roller gear at half-knot increments in speed, ranging from 2.0 to 5.0 knots SOG as determined by differential GPS. In addition to a locally moored current meter set 3 m from the bottom, two current meters were mounted to the trawl to monitor STW. Bottom contact was measured by means of a pivoting tilt meter mounted to a sled that attached to the center of the footrope permitting continuous contact with the seabed, even when the centermost bobbins lifted. Tilt measurements were later calibrated to distance off bottom. A video camera was used to verify whether the sled and tilt meter were forced off bottom due to excessive water pressure and to describe the varying levels of footrope contact at the different towing speeds. Observations were classified as: 5) best contact, center bobbins rolling; 4) center bobbins not rolling but close enough to create a mud cloud; 3) center bobbins off bottom but tilt meter still in contact; 2) center bobbins and tilt meter not in contact and the bottom still in view; 1) bottom no longer in view.
Bottom contact and STW data were collected for 19 tows, each spanning a 2.5 - 5.0 knot range of towing speeds. Door collapse occurred at several of the 2.0 knot speed increments as evidenced by a rapid decline in wing spread suggesting that the lowest STWs used in the experiment were at the operational limit of the trawl. The relationship between distance off bottom (cm) and trawl speed through water was found to be described by the equation: distance = 5.60 - 5.90 STW +1.55 STW2. At speeds under 2.7 knots STW the footrope made hard contact with the seafloor. At 3.0 knots STW, our survey target speed, the center bobbins came off bottom 2 cm on average. At 4.0, 4.5, and 5.0 knots STW the centermost bobbins lifted by 7, 10, and 15 cm, respectively. However, at speeds greater than 4.5 knots the predicted distances off bottom are likely underestimated because the tilt meter was often observed losing contact with the bottom, and in many cases the bottom could no longer be seen.
The evidence that footrope contact and trawl fishing dimensions vary over this range of towing speeds is clear and makes a strong case for monitoring STW during AFSC groundfish assessment surveys. What is not certain, however, is whether fish escapement beneath the footrope is any different when the footrope is in contact with the seafloor, or is 2 cm, or even 7 cm off bottom. A study aimed at quantifying the escapement underneath the trawl at varying trawl speeds using an underbag is scheduled for this summer.
In preparation for this summer’s planned catchability study, preliminary testing of the gear took place during March 7-10 off Blaine, Washington, aboard the fishing vessel Larkin.
By Ken Weinberg.
Midwater Assessment and Conservation Engineering Program
Echo Integration/Trawl Surveys of Prespawning Pollock in the Eastern Bering Sea Shelf, Bogoslof Island, and Shelikof Strait
Scientists from the Midwater Assessment and Conservation Engineering (MACE) Program completed echo integration-trawl (EIT) surveys of walleye pollock (Theragra chalcogramma) on the southeastern Bering Sea shelf and in the Aleutian Basin near Bogoslof Island aboard the NOAA ship Miller Freeman between 27 February and 13 March 2000. Following this cruise, a survey of the distribution and abundance of spawning walleye pollock within the Shelikof Strait area of the Gulf of Alaska was conducted between Chirikof Island and Cape Chiniak between 15 and 28 March 2000. The objectives of the surveys were to obtain echo integration and trawl data for determination of the distribution, biomass, and biological composition of spawning pollock.
The Bering Sea shelf survey was the sixth such winter survey conducted since 1989. Originally scheduled to last 2 weeks and cover an area between Cape Rozhnof on the Alaska Peninsula and St. George Island, the survey was scaled down to 4 days because of heavy January sea ice cover and forecasts of continued severe ice conditions. Data collected during the survey will be used to estimate the abundance of pollock inhabiting the eastern portion of the area designated as Steller sea lion Conservation Area (SCA). Surveys of the Bogoslof area have been conducted annually (except 1990 and 1999) since 1988. They are designed to monitor pollock spawning over deep water in the southeastern Aleutian Basin. The biomass estimate for pollock inside U.S. management area 518 obtained during these surveys provides an index of Aleutian Basin pollock abundance for each year’s Central Bering Sea Convention meeting. The Shelikof Strait survey was the eighteenth spawning stock survey of walleye pollock in this area area since 1980 (surveys were not conducted in 1982 and 1999).
A calibrated, scientific-quality echo sounder/echo integrator (Simrad EK500/BI500) operating at 38 and 120 kHz was used to collect acoustic data during these surveys. Two series of parallel transects were surveyed on the Bering Sea shelf and in the Bogoslof area (Figure 1). Transect spacing was 12.5 nmi on the Bering Sea shelf and transects were oriented east-west. The survey covered about 500 nmi of trackline. In the Aleutian Basin-Bogoslof Island area, 10-, 5-, or 2.5-nmi spaced transects (depending on fish density) were oriented north-south. The survey began in the east at about 166°W and proceeded westward to about 170°15W, covering 2,000 nmi of trackline. Northern boundaries for transects 1-8 were about 54°40N, and about 54°N for transects 9-16. About 1,100 nmi of transect trackline and 31 trawl hauls were completed during the survey of Shelikof Strait. Opportunistic trawl hauls targeting pollock and other fish echosign were made with an Aleutian wing trawl (AWT) and a Poly Nor’Eastern bottom trawl with roller gear. Species composition, and for pollock, sex composition, length frequencies, whole fish and ovary weights, maturities, and otoliths were collected from each haul. Pollock tissue samples were taken from selected hauls for fecundity and genetic studies. Video recordings of pollock behavior within the midwater trawl were collected during the Shelikof Strait survey and will be used to evaluate sampling gear performance.
On the southeastern Bering Sea shelf, pollock were observed from near the start of transect 101 to near the end of transect 108. On the first several transects, pollock formed dense, near-bottom, aggregations between 95- and 100-m bottom depths. These aggregations often extended for several miles. Dense pollock schools were found adjacent to Unimak Island beginning at about 50-m bottom depth; some continued westward to >150-m bottom depths. Highest densities were observed on transects 103, 104, and 106 (Figure 1). Fork lengths (FL) of pollock from eight trawl hauls ranged between 30 and 73 cm and averaged 44 cm. Smaller pollock were encountered in the final shelf-area trawl haul that targeted isolated, dense, schools over bottom depths of around 160 m. Most males and females were found in prespawning (74% and 48%, respectively) condition, although 43% of the females had small ovaries and were categorized as developing. The preliminary estimate of biomass for the eastern Bering Sea shelf is 816,000 metric tons (t).
In the Bogoslof area, pollock were observed in the first 2 nmi of transect 1 in relatively shallow water (Figure 1). Farther to the west, pollock aggregations were sparse. Between 1 and 2 nmi of pollock echo sign were observed on transects 3 and 5. On the south end of transect 7, a large pollock school was encountered in nearly the same location northeast of Umnak Island as in previous years. Very few pollock were observed between transects 8 and 11.5 (168E-169oW). Much of the remainder of the cruise was spent surveying and trawling between 169o-170oW, north of Samalga Pass and east of the Islands of Four Mountains, where relatively large pollock spawning aggregations were observed. During the Bogoslof survey, ten trawls were conducted and catches contained pollock of 31-68 cm FL. On average, fish were largest in the Samalga Pass area. Percent female ranged from 35 to 79, with more females caught overall. The vast majority of fish were in prespawning (95% and 94% among males and females, respectively) condition. Numbers and biomass for the Bogoslof area appeared to be somewhat lower than observed in 1998 and the 1999 Japanese survey. The preliminary estimate of biomass for the Bogoslof area is 321,000 t.
In the Gulf of Alaska most of the mature pollock were distributed along the western side of Shelikof Strait, with the greatest densities occurring from Cape Kekurnoi to Cape Nuskhak; a similar pattern of distribution has been observed in previous years. Fish were most abundant within 50-150 m of the bottom. The size distributions of pollock from hauls within the strait generally exhibited dominant modes around 10-14 cm, 20-24 cm, 30-36 cm, and 43-57 cm FL. Seventy percent of the females greater than 34 cm FL were mature, with 67% in a prespawning condition, 1% in spawning condition, and 2% spent. Pollock from the 1999 year class (mode 10-14 cm FL) formed a strong, well-defined midwater layer (150-200 m depth) from about Chirikof Island to Sitkinak Strait and off Cape Kekurnoi. The amount of eulachon, Thaleichthys pacificus, in trawl catches in the 2000 survey was substantially higher than in recent years. Eulachon is considered to be a contaminate of acoustic returns from pollock. Analysis of the effect of increased eulachon abundance on pollock abundance estimates is currently in progress.
By Taina Honkalehto and Michael Guttormsen.
Fisheries Oceanography Coordinated Investigations
Kevin Bailey and Susan Picquelle presented the poster “Larval distribution patterns of offshore spawning flatfish in the Gulf of Alaska: Sea valleys as transport pathways and enhanced inshore transport during ENSO events” (.pdf, 2.9 MB) at the PICES symposium, “Beyond El Niño,” in San Diego, California, during March 2000.
Offshore and deep-water, spawning flatfish species in the Gulf of Alaska, such as arrowtooth flounder and Pacific halibut, have juvenile nurseries that are inshore, in bays, or at the mouths of bays. Larvae must emigrate from their spawning areas along the continental slope and outer shelf towards inshore bays, in a direction normal to the prevailing Alaskan Stream. Using a 20-year time series of data from ichthyoplankton surveys in the Gulf of Alaska, we examined patterns of variability in larval halibut and flounder distributions that may reflect processes resulting in successful recruitment to nursery areas. Several patterns can be observed in these data. Eggs and the smallest sized larvae are located along the outer shelf and slope. Larger larvae tend to be located farther inshore over the continental shelf. Larger larvae are also associated with deep-sea valleys and troughs that penetrate the shelf. Larvae of these flatfish species live in deep water, and literature reports indicate that bottom water flows up the sea valleys. Thus, these topographic features may serve as transport pathways to juvenile nursery grounds. ENSO-conditions and warm year anomalies are linked to recruitment strength of Pacific halibut. Variability in larval transport as related to ENSO and other events that enhance onshore advection, may play an important role in recruitment of flatfishes to their nursery grounds.