Cruise Number: MF00-11
FOCI Number: 8MF00
Ship:NOAA Ship Miller Freeman
Area of Operations:Gulf of Alaska
Itinerary:Date depart / port: September 2, 2000 / Kodiak, AK
Date arrive / port: September 20, 2000 / Kodiak, AK
Participating organizations:NOAA - Alaska Fisheries Science Center (AFSC)
NOAA - Pacific Marine Environmental Laboratory (PMEL)
Chief Scientist:Matt Wilson M / AFSC
Cruise Objectives:The late-summer FOCI cruise was designed to address biological and methodological questions regarding age-0 walleye pollock in the western Gulf of Alaska. The primary cruise objective was to collect data on age-0 pollock and their environment in the area between Shelikof Strait and the Shumagin Islands, from nearshore to the outer shelf. Many juvenile pollock, spawned in Shelikof Strait, are carried to the study area of interest by the prevailing current. Complex topography in this area creates a habitat mosaic that probably affects pollock nursery suitability and may lead to accumulations of fish in favorable areas. This is relevant to FOCI's goal because accumulation of age-0 pollock in favorable areas may be an important factor in subsequent recruitment to the GOA stock. The Chirikof Island area was of particular interest because the abundance of yearling pollock in that area during the late winter is a proven indicator of year-class strength.
A second cruise objective was to determine the effect of day versus night sampling on estimates of age-0 density and size composition. Trawl catches are likely to be highest at night after tight aggregations of fish that are found near bottom during the day disperse to feed in the upper water column. Thus, the fish are likely to be more vulnerable to mid-water gear during the night; relevant considerations include a diel effect on vertical position, light-related net avoidance, and aggregation patterns. Acoustic estimates of age-0 density might also be highest at night when individuals are off bottom and dispersed thereby minimizing acoustic shadowing. Nighttime dispersal of fish may provide better target strength data by reducing the noise threshold effect and multiple-target detection; target strengths are important because they indicate sound scatterer size (target strengths for age-0 pollock 4.5-10.0 cm are reasonably well predicted by TS (dB) ) =20log(L)-66). Acoustic data collected during this cruise will be useful for studying fine scale horizontal and vertical distributions of age-0 pollock (38 kHz) and their prey (120 kHz, 1-m Tucker trawl).
Tertiary objectives that were accomplished include (1) CTD sampling
along Line 8, (2) sampling with the anchovy trawl along east Kodiak Island,
and (3) sampling for Atka mackeral presence off southeast Kodiak Island
using surface tows of the Tucker trawl.
Summary of Operations:
Summary of Cruise:Narrative:
The NOAA ship Miller Freeman departed Kodiak on 2 September (15:00 ADT) and returned 20 September (09:00 ADT). All objectives were completed. The primary and secondary objectives, to occupy 43 locations situated on 10 cross-shelf transects between southwest Shelikof Strait and the Shumagin Islands at both day and night, was accomplished during 3-19 September. Opportunistic sampling along these transects resulted in sampling at 16 additional locations. All tertiary objectives, involving operations at 12 additional locations, were accomplished before and after occupation of the 10 transects. The FOCI station designator (Table 1) groups temporally consecutive operations or hauls at one location; thus, day and night sampling at one location usually resulted in multiple station numbers. Therefore, the sampling activity at 71 different locations was divided into a total of 111 stations (Figure 1). A chronological list of all samples collected and a sample tally are given in Tables 1 and in the Summary of Operations, respectively.
The primary sampling occurred along 10 transects. Each of 43 predetermined locations along these transects was occupied twice, once during daylight and once at night, using a 1-m Tucker trawl and an anchovy trawl. Thus, 86 successful Tucker trawl tows were conducted at all predetermined sampling locations. Because we arrived at grid location "A5" too late to obtain a daytime sample, only 85 successful anchovy trawl tows were conducted. Additional samples were obtained when location "3D" was occupied at twilight. Nighttime was observed to occur from 21:45 - 07:00 ADT (daytime was observed to occur from 0:800 - 21:00) (ADT=GMT-8).
Occupation of the grid began at the southernmost location ("A1") just after nightfall on 3 September. Sampling proceeded shoreward along Transect 1 until daybreak when we backtracked in order to resample during the day. Moving back and forth along each transect was preferable to moving across transects so that the amount of acoustic data collected along each transect could be maximized.
Sampling at each predetermined location, day or night, involved two net-sampling operations. First, a 1-m Tucker trawl (0.333 mm mesh net) was used to collect samples of potential age-0 pollock prey. This net was fished obliquely from 200 m or 10 m above bottom, whichever was shallowest, to the surface. The Tucker trawl was used to obtain depth-discrete samples by opening and closing two nets. The first net was opened at depth and fished up to the bottom of the thermocline. When the thermocline was either indistinct or extended deeper than about 70 m, the net was fished up to 40 m. Closing the first net simultaneously opened the second net, which was then fished to the surface. Second, an anchovy trawl (3-mm codend liner) was fished over a similar net path to sample age-0 pollock, associated nekton, and scyphozoans. The anchovy trawl was deployed at about 50 m/min, allowed to settle at 200 m or 20 m (headrope depth) above bottom, whichever was shallower, and then retrieved at about 10 m/min. Towing speed was about 2.7 kts during deployment and retrieval. On 20 anchovy tows, Scanmar net mensuration equipment was used on some tows to measure the vertical and horizontal opening of the anchovy trawl mouth. The Tucker trawl was fished with a SeaCat CTD profiler, and the anchovy trawl was equipped with a SBE microbathythermograph. Thus, temperature and salinity profiles were collected at each location, and the net path was recorded. The SeaBird 911+ CTD profiler was used at six locations to collect additional CTD data (and water samples) to verify data accuracy.
A Simrad EK-500 (38 and 120 kHz) system was used to collect acoustic data throughout the duration of the cruise, but most importantly along each transect and simultaneously with net tows. Transits between transects were minimized in order to maximize acoustic coverage along each transect during each diel period (Figure 2). Acoustic data were also used to direct opportunistic sampling to boost the number of samples from locations where age-0 pollock were relatively abundant.
Opportunistic sampling involved fishing at locations where echo sign
indicated a relative abundance of sound scatterers. This sampling was mostly
restricted to daylight since most of the relatively short nighttime was
needed to occupy the predetermined grid locations. A total of 15 Tucker
tows and 14 anchovy tows were successfully obtained by fishing the gear
over an oblique path similar to that used at the predetermined stations.
However, the echo layers were often concentrated near-bottom and were inadequately
sampled. Therefore, an additional 13 anchovy trawl tows and 4 shrimp trawl
tows were used to target specific sound scattering layers. Some of these
tows targeted midwater scattering layers in order to identify species composition.
A Methot trawl was also used opportunistically, but it was used to ensure
that the smallest age-0 pollock were being adequately collected with the
anchovy net. The Methot was only used at night and only at four locations.
Acknowledgments:The scientific party would like to acknowledge the hard work and support of the officers and crew of the NOAA ship Miller Freeman. All operations were completed with an admirable degree of efficiency and professionalism that is to be commended, particularly given that the crew is minimally staffed and has not enjoyed their usual mid-summer inport.
Attachments; Tables, Pictures & Figures:Table 1
Cruise summary statistics from the cruise MF00-11 (FOCI 8MF00) aboard the NOAA ship Miller Freeman, 2-20 September 2000, in the western Gulf of Alaska.
1 - Tucker trawl being rinsed as it was retrieved aboard the
NOAA ship Miller Freeman during the cruise MF00-11. Note, the
Image 2 - Anchovy trawl coming up the stern ramp of the NOAA ship Miller Freeman during the cruise MF00-11. (JPEG Image, 183 KB)
Image 3 - Small-mesh (3 mm) liner installed in the codend of the anchovy trawl. (JPEG Image, 174 KB)
Image 4 - One sample of various species collected in the anchovy trawl as it was fished obliquely through the water column during the cruise MF00-11 (clockwise from left: age-0 pollock, age-1 pollock, squid, shrimps, capelin, eulachon, isopod). (JPEG Image, 155 KB)
5 - Fish species typically collected in the anchovy trawl during MF00-11
(age-2+ pollock are around the tub, inside it are
Figure 1. Sampling locations occupied during the cruise MF00-11
(FOCI 8MF00) aboard the NOAA ship Miller Freeman, 2-20 September 2000.
FOCI station numbers (see Table 1) are offset from their actual location
along each transect according to diel sampling period, day stations are
shifted slightly northeast of their true position, night stations are southwest,
and twilight stations are not shifted.
Figure 2. Ten transects along which acoustic and net sampling was concentrated during the cruise MF00-11 (FOCI 8MF00) aboard the NOAA ship Miller Freeman, 2-20 September 2000. The thin solid line indicates transect location, the thick offset lines indicate day (dashed line) and night (solid line) coverage.