RESOURCE ASSESSMENT &
CONSERVATION ENGINEERING (RACE) DIVISION (cont.)

Midwater Assessment & Conservation Engineering (MACE)

Fishery Interaction Study
Field work for the third year of a fishery interaction experiment was completed between 14  August and 5 September off the east side of Kodiak Island in the Gulf of Alaska as a collaborative effort between AFSC scientists. The work is part of a larger program designed to evaluate the effect of commercial fishing activity on the prey availability of walleye pollock and other forage fish species to endangered Steller sea lions.

The principal objective of the experiment was to use standard acoustic survey methods to describe the spatio-temporal variability in pollock abundance and distribution patterns in two troughs over a period of several weeks before and during the commercial pollock fishery.  The study area consisted of a treatment site, Barnabas Trough where commercial fishing was allowed, and a control site, Chiniak Trough where commercial fishing was prohibited.  Repeated survey passes (Fig. 1) were conducted within each trough before and during the fishery to document whether a fishery-induced perturbation occurred in the fish distribution.  Oceanographic data to characterize the physical environment were also collected with seven current meter moorings, drifters, conductivity-temperature-depth profilers (CTDs), expendable bathythermographs (XBTs), a vessel-mounted thermosalinograph, and acoustic Doppler current profiler.

Most of the acoustic backscattering was attributed to three fish groups: adult pollock, juvenile pollock, and capelin (Mallotus villosus). The adults generally formed loose, near-bottom aggregations during the day, whereas the juvenile pollock and capelin formed more discrete aggregations that were higher in the water column.  Preliminary analyses suggested that adult pollock were distributed in the northern portion of Barnabas Trough and throughout Chiniak Trough.  Juvenile pollock were distributed throughout both troughs.  Capelin were often broadly distributed over the shallower edges of Chiniak Trough, whereas in Barnabas Trough they were found over a wide range of bottom depths and mostly in the southern portion of the trough.

The adult pollock size distributions in both troughs were variable with length modes ranging between 45 and 65 cm fork length (FL), and probably related to depth.  The juvenile pollock size distributions were often characterized with prominent modes near 30-32 and 35 cm FL.

Analyses are currently under way to determine whether significant changes in fish abundance and distribution patterns were detectable in response to the commercial fishing activities.  Pollock and capelin echosign spatial patterns will be explored with variography and by comparison of other descriptive parameter estimates of the fish aggregations to evaluate whether the scale of patchiness changed during the study period.   In subsequent years, repeated surveys will also be conducted following closure of the 1- and 2-week August fishery to describe the duration that a potential fishery-induced perturbation may persist in the area.

By Chris Wilson.

MACE: Preliminary Results of the Eastern Bering Sea EIT Pollock Survey
The MACE Program conducted an echo integration-trawl (EIT) survey of walleye pollock between 4 June and 30 July 2002 on the eastern Bering Sea shelf.  The principal objective of the survey was to collect echo integration and trawl data to estimate midwater pollock abundance and distribution.   Acoustic data were collected with Simrad EK500 and EK60 quantitative echo-sounding systems on the NOAA ship Miller Freeman.

The complete survey report is available in pdf format

Three split-beam transducers (38 kHz, 120 kHz, and 200 kHz) were used to collect acoustic data.  Results presented here are based on Simrad EK500 38-kHz data.  Midwater and near-bottom echosign was sampled using an Aleutian Wing 30/26 Trawl (midwater trawl).  On or near bottom echosign was sampled with an 83-112 bottom trawl without roller gear.  A Methot trawl was used to target age-0 pollock and macrozooplankton.  The survey design consisted of 28 north-south transects spaced 20 nmi apart over the Bering Sea shelf from Port Moller, Alaska, to the U.S./Russia Convention Line  (Fig. 2).  Echo integration and trawl data were collected during daylight hours (typically between 0600 and 2400, depending on calendar date and location).  Nighttime operations included additional trawling, target strength data collection, and acoustic system testing.

Walleye pollock was the dominant species captured in midwater and bottom trawl hauls.  Jellyfish (Scyphozoa) were the next most abundant species group sampled in midwater trawl hauls by weight, and Pacific cod were the next most abundant by weight in bottom trawl hauls.  Methot trawl hauls caught mainly jellyfish and euphausiids.  Preliminary abundance estimates for pollock indicate that approximately one-third of the total biomass was found east of 170°W and two-thirds west of 170°W.  The predominant length mode east of 170°W was 27 cm, with additional modes at 49 cm, 37 cm, and 15 cm (Fig. 3).  West of 170°W the predominant length modes for pollock were 24 cm and 31 cm, with additional modes at 39 cm, 45 cm and 15 cm.  Pollock were observed on all transects (Fig. 2).  They were most dense north of Unimak Island, southeast of St. Matthew Island, and west of St. Matthew Island.  During daylight hours, pollock were usually observed in aggregations within 20 m of the seafloor or as discrete schools located throughout the water column (Fig. 4).  During the night, pollock tended to disperse throughout the water column.

In addition to the EIT survey work, scientists from the National Marine Mammal Laboratory (NMML) conducted a marine mammal sighting survey along the EIT survey track (see NMML Cetacean Assessment and Ecology Program report in this issue).  NMML biologists surveyed during daylight hours when visibility permitted.  If they observed large whales in situations where they thought identification of individual whales was possible, they requested time to go off trackline and use the vessel to approach the animals for photographs.  These photo sessions resulted in several exceptional humpback and orca displays for all of the scientists and crew.  The AFSC survey was also conducted in cooperation with the research vessel TINRO from TINRO-Centre, Vladivostok, Russia.

By Taina Honkalehto.

Recruitment Processess

Distribution of Age-1 and Age-2 Walleye Pollock in the Gulf of Alaska and Eastern Bering Sea: Sources of Variation and Implications for Higher Trophic Levels
Walleye pollock is the predominant groundfish species in the North Pacific Ocean and is a focal point in the ecology of the region.  The immature stages of this species constitute an important link in the food chain, transferring energy from zooplankton to higher trophic levels.  However, there is only limited knowledge of the distribution of the juveniles of this species, particularly the later immature stages (age-1 and age-2).

We examined the horizontal and vertical distribution of age-1 and age-2 walleye pollock in the eastern Bering Sea and the Gulf of Alaska and related observed patterns to predominant physical (temperature, latitude, longitude, bathymetry) and biological (diet, physiology) characteristics.  We addressed these objectives using data collected from three sources: a field survey conducted in the Gulf of Alaska (2001); field data collected from a series of hydroacoustic surveys of the eastern Bering Sea (1997, 1999, 2000); and a series of laboratory experiments investigating the behavior and physiology of age-1 and age-2 individuals under various thermal conditions.   Results suggest differences in the ecology of walleye pollock between the Bering Sea and the Gulf of Alaska.

It appears that age-1 and age-2 cohorts in the Bering Sea may be vertically separated in the water column, with age-1 pollock located near bottom during the day and age-2 pollock schooling higher in the water column.  However, we found no evidence of vertical separation among these cohorts in the Gulf of Alaska.  Adult pollock (age 4+) appear to be demersal during the day in both systems.  Analyses of the diet of pollock collected in the Bering Sea indicate a high degree of cannibalism of age-0 fish by age-1 and age-2 individuals, though there is no evidence of intra-cohort cannibalism in samples collected from the Gulf of Alaska.  Additionally, laboratory experiments show that the thermal range of pollock decreases with age, suggesting that younger fish may be able to exploit more of the vertical water column than older fish because they have greater thermal tolerances.

These results imply that vertical stratification of cohorts in the Bering Sea could be related to a combination of temperature tolerance and intensive intraspecific predation pressure, while the lack of predation pressure in the Gulf of Alaska permits greater comingling of age classes.  Further studies on diel patterns, the effects of prey distribution, and factors motivating schooling are necessary to further test this hypothesis.

By Janet Duffy-Anderson.

Age-0 Walleye Pollock and Capelin in the Western Gulf of Alaska:
Potential Competitive Interaction?
The potential for competitive interaction between age-0 walleye pollock and capelin is currently being examined using data from a small-mesh trawl study conducted in the western Gulf of Alaska (GOA) during September 2000 and 2001.  The immediate objective is to determine how similar capelin are to age-0 pollock with regard to resource utilization so that we may begin to separate environmental effects from species interaction effects when considering geographic variation in predator-prey relations. Ultimately, our goal is to understand geographic variation in the suitability of coastal areas for the production of forage fishes. Geographic variation is likely to be superimposed upon, and thereby complicate our understanding of climate-related alterations in the flow of trophic energy through the Gulf ecosystem.  Pollock and capelin are prevalent members of the coastal pelagic community whose relative abundance is associated with climate variability, and these fishes are particularly relevant to current issues involving sustainable fisheries and endangered Steller sea lions.

To measure ecological similarity among age-0 pollock and capelin, we are currently comparing geographic distributions, sizes, and diets.  Rigorous analysis of the data is planned, but preliminary results indicate very high similarity among these fishes:

Geographic distributions were similar (Fig. 1).  The percent frequency of co-occurrence (2000, 63%; 2001, 59%) was similar to the percent occurrence of capelin (2000, 65%; 2001, 59%).  In other words, capelin almost always occurred with pollock.

Fish sizes were similar (Fig. 2), at least during September 2000.  Both species were abundant over the 60 – 100 mm SL interval.

Fish diets were similar (Fig. 3), at least during September 2000.  Both predators mostly ate copepods and euphausiids.  We used Horn’s index, which ranges from 0 (no overlap) to 1 (complete overlap), to quantify dietary overlap.  Considering all fish together, overlap was very high whether by abundance (R_o=0.97) or biomass (R_o=0.96).  Interestingly, Horn’s index greatly varied among samples probably due to differences in species-specific feeding chronologies. (Fig. 4).

Due to ecological similarity, there was very high potential for competitive interaction between age-0 pollock and capelin.  There were, however, no obvious, severe adverse effects due to competition (e.g., disjunct distributions or diets).  High overlap may therefore indicate an abundance of space and food resources in comparison to demand.  Thus, major ecological features of these populations, at least at the spatial and temporal scale examined, may be governed more by underlying environmental conditions rather than competitive interaction.

By Matt Wilson and Janet Duffy-Anderson.

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AFSC Quarterly
Research Reports
July-Sept 2002