2000 Groundfish Assessment Survey of Aleutian Islands Region
The seventh comprehensive bottom trawl survey of Aleutian Island region groundfish resources was conducted from 17 May through 25 July 2000 by the Center’s Resource Assessment and Conservation Engineering (RACE) Division. Previous surveys have been conducted on a more or less triennial basis in 1980, 1983, 1986, 1991, 1994, and 1997. Future surveys will be conducted on a biennial basis.
The 70-day survey period was divided into three legs of 23-24 days each. Sampling operations began on the north side of the Aleutian Islands between Unimak Pass (long. 165°W) and the Islands of Four Mountains (long. 170°W) and extended westward throughout the remainder of the Aleutian Archipelago to Stalemate Bank (long. 170°E) (Fig. 1 above).
The primary focus of these ongoing groundfish
surveys is to build a standardized time series of
data that are designed to assess, describe, and
monitor the distribution, abundance, and biological
condition of various Aleutian groundfish stocks.
Specific objectives of the 2000 survey were to:
Define the distribution and relative abundance of the principal groundfish and commercially important invertebrate species that inhabit the Aleutian region
Obtain catch and effort data from which to estimate the absolute abundance of the principal groundfish species
Collect data to define various species-specific biological parameters i.e., age, sex, size, growth rates, length-weight relationships, feeding habits, spawning condition, and taxonomy
Collect integrated net configuration and position data for all trawl hauls to obtain precise area-swept estimates
Perform special collections as requested by other researchers or research groups.
Vessels and Gear
Survey sampling was conducted aboard the chartered commercial trawlers Vesteraalen and Dominator. Both vessels are house-forward stern trawlers with stern ramps, aft net storage reels (mounted over the stern ramp), telescoping deck cranes, propeller nozzles, and paired, controlled-tension hydraulic trawl winches with 1,280 m to 1,460 m of 2.54-cm diameter steel cable. The Vesteraalen is 38 m in overall length (LOA) and is powered by a single, 1,700 continuous horsepower (HP) main engine. The Dominator is also 38 m LOA with a 2,000 HP main engine.
Standard RACE Division Poly-Nor’Eastern high opening bottom trawls, rigged with roller gear, were utilized by both vessels. Gear specifications included: a 27.2-m headrope with twenty-one 30 cm diameter floats, and a 24.3-m, 1.3-cm diameter longlink alloy chain “fishing line” attached to a 24.9-m, 0.95-cm diameter 6 x 19 galvanized steel wire footrope. The roller gear was 24.2 m long and constructed of 1.9-cm diameter 6 x 19 galvanized steel wire rope and 36-cm rubber bobbins separated by a solid string of 10-cm rubber disks. In addition, 5.9-m wire rope extensions with 10-cm and 20-cm rubber disks were used to span each lower flying wing section. The trawls were constructed of 12.7-cm stretched-mesh polyethylene web with a 3.2-cm stretched-mesh nylon liner in the codend. Net rigging consisted of triple 54.9-m, 1.6-cm diameter galvanized wire rope dandylines. Chain extensions to the dandylines were 46 cm and 23 cm at the headrope and side panel attachments, respectively. Steel V-doors with dimensions of 1.83 x 2.74 m, and weighing approximately 800 kg each were used to open the net.
The Aleutian Islands region is an extensive archipelago of volcanic origin typified by a relatively narrow continental shelf that is crossed by numerous deep passes. Very strong currents flow through the passes and across the shelf, sometimes making productive fishing operations difficult or impossible.
Commercially valuable groundfish such as Atka mackerel (Pleurogrammus monopterygius), Pacific cod (Gadus macrocephalus), walleye pollock (Theragra chalcogramma), sablefish (Anoplopoma fimbria); flatfish, most notably, Pacific halibut (Hippoglossus stenolepis) and Greenland turbot (Reinhardtius hippoglossoides); rockfish species including Pacific ocean perch (Sebastes alutus), northern rockfish (S. polyspinis), rougheye and shortraker rockfishes (S. aleutianus and S. borealis); and invertebrates including golden king crab (Lithodes aequispina) and scallops (Chlamys spp) inhabit the area. The rough, rocky bottom conditions provide abundant substrate for many species of bryozoans, hydroids, sponges, and corals.
The Aleutian survey region is divided into four major sections based on geographic features, and North Pacific Fishery Management Council (NPFMC) regulatory areas. Those sections are further divided into 45 area-depth strata to a depth of 500 m. A Neyman optimum allocation strategy based on data from previous surveys was used to develop a stratified random sampling distribution among the 45 strata. As a result, proportionally more sampling effort was expended in the eastern, central and western Aleutian areas this year (Fig.1 above).
Tow tracklines and start and end positions were recorded using GPS output. Standard trawl hauls were 15 minutes in actual on-bottom duration. Trawl time on bottom was determined using real-time net configuration data transmitted to the vessel by acoustic net mensuration equipment which were verified posteriorly by time/depth recordings from a bathythermograph and a bottom contact sensor (tilt sensor). The acoustic devices continuously measured wing spread and headrope height above the bottom. Efforts were made to maintain constant depth during a tow, but when depths changed trawl warp length was adjusted accordingly. At most stations, tilt sensors attached to the fishing line were used to record how well the net maintained contact with the bottom.
Catches of fishes and most invertebrates were sorted to species, weighed, and enumerated according to standard AFSC and RACE Division protocol. Extensive size composition data were collected with barcode based recording devices and downloaded to computer database files after each tow. A variety of biological data including age structures (otoliths), lengths, and weights of individual specimens were collected and entered in the computer database. Special collections included sexual maturity indices for Pacific ocean perch, extensive stomach contents samples, heart tissue samples, corals, sponges and other invertebrates, sculpin eggs, and many whole fishes of various species.
Surface-to-bottom seawater temperature profiles were recorded at most sampling sites using a headrope-mounted bathythermograph. After each tow, temperature profile data were downloaded and stored in computer files, then integrated with net mensuration data to help verify actual time on bottom. Additional sea surface temperature observations were taken with bucket thermometers.
Time lost to bad weather and gear repair was
generally minimal, but during periods of extreme
tidal flow, heavy currents at specific stations
sometimes caused work to be postponed. Sampling
generally proceeded from east to west. Some
preassigned stations were not sampled due to
unsuitable bottom conditions. In cases where
trawlable bottom could not be found at a given
station, a preselected alternate location, or in
some cases a newly found location within the proper
area-depth stratum was sampled. Successful
tows were performed at 428 of 456 assigned
assessment sites. Stations ranged in depth
from 20 to 471 m. Sea surface temperatures and
successful bathythermograph recordings were
collected at 441 stations. Biomasses were
estimated for the major groundfish species using the
|Species||Western Aleutians||Central Aleutians||Eastern Aleutians||S. Bering Sea||All Areas|
|Pacific ocean perch||222,584||129,740||140,512||18,870||511,706|
|Northern rock sole||10,645||18,041||8,175||7,216||44,078|
Total biomass estimates indicate that Atka
mackerel and Pacific ocean perch were, by far, the
dominant species in the Aleutian region as a whole,
but Atka mackerel was the predominant species in the
Central regulatory area (Table 1 above). Pacific
ocean perch ranked second in overall total biomass
in the region, but was the most abundant species in
the western Aleutian area. Northern rockfish
was fourth overall in total biomass following giant
grenadiers. In the southern Bering Sea area
walleye pollock was the most abundant species.
|Pacific ocean perch||351,093||383,618||625,273||511,706|
|Northern rock sole||*37,326||*54,786||49,897||44,078|
Table 2 above contains biomass estimates from the 1991, 1994, 1997, and 2000 surveys. Most striking is the high degree of variability among some of the survey estimates, especially for those species with highly contagious distributions, i.e. Atka mackerel, Pacific ocean perch and northern rockfish. In general, sampling in the rough, hard-bottom areas inhabited by Atka mackerel, giant grenadiers, and the rockfishes has improved over the past decade, largely due to the expertise of our charter vessel operators. Some variability might be due to differences in survey timing. This survey was approximately 3 weeks earlier than the 1994 and 1997 surveys, which were, in turn, 7 weeks earlier than the 1991 survey.
Length-stratified otolith collections were taken for a number of species. Generally, samples were collected from species with high commercial value or those of special scientific interest. Length and weight measurements were recorded from individual fish of many species to update length-weight relationships used by AFSC scientists. Length measurements were the most common biological data collected. Over 3,600 stomach samples were collected from a wide variety of species with over 1,900 samples coming from four major predator species: Arrowtooth flounder (Atheresthes stomias), Pacific cod, Pacific halibut, and walleye pollock. Another 785 stomach samples came from the two major semipelagic species, Pacific ocean perch and Atka mackerel. Small flatfish such as northern rock sole (Lepidopsetta polyxystra), flathead sole (Hippoglossoides elassodon), rex sole (Glyptocephalus zachirus), and various species of skates (Bathyraja spp.) formed the majority of the remaining stomach collections.
Heart tissue samples were collected from 150
Pacific ocean perch and 50 northern rockfish as part
of a genetic stock identification analysis. Extensive
samples of invertebrate fauna were collected by
researchers from the California Academy of Sciences
in San Francisco. The AFSC has contracted the
Academy to identify and catalog invertebrates which
will allow the AFSC to collect more accurate catch
data records in the future. Numerous whole
fish were collected for later identification. At
least one probable new species of snailfish was
collected. Other special collections included
external parasites of rock sole, Greenland turbot
ovaries, and bigmouth sculpin egg clutches.
Groundfish assessment surveys conducted in the Aleutian region have occurred on a more or less triennial basis since 1980. Survey periods have ranged from early May to late September, with no fixed sampling pattern or time schedule. Generally, sampling progresses from east to west. Bottom temperatures have been routinely collected in conjunction with bottom trawl hauls. Of the seven survey years cited in the figure below, all except 1991 had temperature samples from throughout the entire Aleutian region.
Tidal currents that flow across the relatively narrow Aleutian shelf and upper slope are often very strong, as vast amounts of water are exchanged between the north Pacific Ocean and the Bering Sea. Bottom temperatures are most influenced by those large water masses.
In the Aleutian Islands region, the year 2000 was the coldest year yet detected during AFSC groundfish surveys. The warmest years tend to lag about a year behind El Niño events. The coldest years thus far detected have occurred within the same decade, 6 years apart. Generally, temperatures at shallower depths vary more than at depths greater than 300 m where they are within a range of about 0.5°C or less. Perhaps the year 2000 temperatures are not as anomalous as they appear, but many individual fish were visibly thinner than during other surveys. Unfortunately, we have no data to compare for the intervening years.
By Harold Zenger.
Impact of Transducer Motion on Echo Integration
In the summer of 1999, an inertial navigation and motion measurement system was made operational on the NOAA ship Miller Freeman. The Miller Freeman serves as the primary platform for fisheries acoustics surveys conducted by the AFSC. A 38-kHz survey transducer is mounted on the bottom of the vessel’s centerboard - approximately 4 m from the hull. Previous research has demonstrated that under certain conditions, excessive transducer motion can bias echo integration measurements – resulting in an underestimate of biomass. If the transducer is moving, the acoustic signal will be transmitted with the transducer in one position and the echo received with the transducer in a different position. The transmit and receive beam patterns are misaligned. This misalignment results in a reduction in transducer sensitivity. The error (negative bias) is a function of the beam width of the transducer, the degree of motion and the depth of the scatterers – being more severe for narrow beam transducers, large vessel motion, and deep scatterers.
Transducer pitch and roll data were collected during winter (severe weather) and summer (mild weather) field seasons in the Bering Sea and Gulf of Alaska. For this analysis, walleye pollock were assumed to be at an average depth of 125 m on the eastern Bering Sea (EBS) shelf, 230 m in Shelikof Strait, and 400 m in the Bogoslof Island region. The mean reduction in transducer sensitivity observed on the EBS shelf was about 1% and not of significant concern in either the summer 1999 or the winter 2000. However, in Shelikof Strait and especially the Bogoslof area in winter, this bias occasionally exceeded 6% and 8%, respectively. This can be a significant problem if the times of excessive transducer motion coincide with areas of high abundance. Several approaches to minimize this motion error have been proposed. The most practical is to monitor the motion in real time and stop data collection when conditions deteriorate.
Work in this area is continuing and results presented here should be considered preliminary.
By Neal Williamson.