Acoustic Surveys

Alex De Robertis, Chris Wilson, and Kevin Taylor

Objectives – To determine the abundance, vertical distribution, and environmental associations of the dominant midwater organisms over the northeastern Bering Sea and eastern Chukchi Sea.

Final Reports
Preliminary 2012 and 2013 Results

To the best of our knowledge, no quantitative assessment of the vertical distribution of midwater fishes in the Chukchi Sea has been conducted to date. Hence the abundance of midwater fish, jellyfish, and other large zooplankton (e.g., euphausiids) were measured using acoustics. Acoustic backscatter was continuously measured along the vessel track from the F/V Bristol Explorer chartered for the 2012 and 2013 surface trawl surveys. The identity of acoustic scatterers was confirmed using two pelagic trawls (CanTrawl and Marinovich Trawl) and optical measurements from a drop camera. The underway measurements were supplemented with multi-frequency acoustic measurements to increase taxonomic information (for example, zooplankton abundance) from the acoustic measurements.

Preliminary analysis of opportunistically collected acoustic records for past surveys indicates that there may be a large biomass of midwater fish in the study area, although identity of the organisms remains unknown. The abundance of midwater organisms will be classified using echosign characteristics, trawl catches, drop camera images, and the relative frequency response (Korneliussen and Ona 2003). These acoustic results will be directly comparable to pelagic surveys conducted by NOAA’s Alaska Fisheries Science Center over the eastern Bering Sea shelf (e.g. Honkalehto et al. 2005).

Final Reports

Final Reports to CIAP and BOEM:

De Robertis, A., Taylor, K., Williams, K., Wilson, C. D. 2015. Species and size selectivity of two midwater trawls used in an acoustic survey of the Alaska Arctic. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 48 pp. – DRAFT REPORT

Abstract:
Acoustic-trawl (AT) survey methods are widely used to estimate the abundance and distribution of pelagic organisms. This technique relies on estimates of size and species composition from trawl catches along with estimates of the acoustic properties of these animals to convert measurements of acoustic backscatter into animal abundance. However, trawls are selective samplers, and if the catch does not represent the size and species composition of the animals in the acoustic beam the resulting abundance estimates will be biased. We conducted an experiment to quantify trawl selectivity for species encountered during an AT survey of the Alaska Arctic. The pelagic assemblage in this environment was dominated by small young-of-the-year (age-0) fishes and jellyfish, which may be poorly retained in trawls. A large midwater trawl (Cantrawl) and a smaller midwater trawl (modified Marinovich) were used during the survey. The Marinovich was equipped with 8 small-mesh recapture nets which were used to estimate the probability that an individual that enters the trawl is retained. In addition, paired hauls were made with the Cantrawl and Marinovich to estimate the difference in selectivity between the two trawls. A statistical model was developed to combine the catches of the recapture nets and the paired hauls to estimate the length-dependent selectivity of the trawls for the most abundant species (e.g., age-0 fishes and jellyfish). The analysis indicated that there was substantial size and species selectivity: although the modified Marinovich generally had a higher catch per unit effort, many of the animals encountered in this environment were poorly retained by both trawls. The observed size and species selectivity of the trawls can be used to select appropriate nets for sampling pelagic fishes, and correct survey estimates for the biases introduced in the trawl capture process.

De Robertis, A., Wilson, C. D., Taylor, K., Farley, E. 2016. Abundance and Distribution of Arctic cod (Boreogadus saida) and other Pelagic Fishes over the U.S. Continental Shelf of the Northern Bering and Chukchi Seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 68 pp. – DRAFT REPORT

Abstract:
We conducted acoustic-trawl (AT) surveys of the northern Bering and Chukchi Seas during ice-free periods in 2012 and 2013. The mixed species assemblages in the study area required refinement of standard AT survey methods, and adjustment of trawl catches for the effects of trawl selectivity. Sensitivity analyses indicate that the AT abundance estimates are relatively robust to the assumptions of the analysis. These surveys indicate that midwater fishes are dominated by age-0 Arctic cod (Boreogadus saida) age-0, saffron cod (Eleginus gracilis), capelin (Mallotus villosus), and Pacific herring (Clupea pallasii). In both years, age-0 Arctic cod were distributed principally ≥ 69.5° N, age-0 saffron cod were abundant in coastal areas between 66.5-69.5 °N, and Pacific herring were distributed south of 67 °N. These three fishes exhibited consistent associations with temperature, salinity and bottom depth: e.g., age-0 Arctic cod were abundant at lower mean water column temperatures than saffron cod. In contrast, capelin were distributed throughout the study area, and were not consistently associated with environmental measures. There was a geographic trend in body length, with smaller Arctic cod, saffron cod and capelin in northern areas, but smaller herring in the south. Arctic cod, saffron cod, herring and capelin were all > 2 times more abundant in 2013 than 2012. Sizeable populations of age-0 Arctic cod were observed in the northern Chukchi Sea, which suggests that this area is an important nursery ground. However, relatively few older Arctic cod were observed in this and other surveys of the area, which suggests that either overwinter mortality of age-0 Arctic cod is high, and/or these fish are not retained on the Chukchi shelf.

Manuscripts:

    De Robertis, A., and Taylor, K. 2014. In situ target strength measurements of the scyphomedusa Chrysaora melanaster. Fish. Res. 153:18-23.
    De Robertis, A., Taylor, K., Williams, K., Wilson, C.D. 2015. Species and size selectivity of two midwater trawls used in an acoustic survey of the Alaskan Arctic. Deep Sea Research II. http://dx.doi.org/10.1016/j.dsr2.2015.11.014.
    De Robertis, A., Taylor, K., Wilson, C., Farley, E. in press. Abundance and Distribution of Arctic cod (Boreogadus saida) and other Pelagic Fishes over the U.S. Continental Shelf of the Northern Bering and Chukchi Seas. Deep-Sea Research II: Arctic Eis Special Issue.

Preliminary 2012 and 2013 Results
Click on figures to enlarge.

Figure 1. Example of the volumetric abundance (i.e. fish per 1000 m-3 sampled) of age-0 arctic cod in 12 sites sampled by both the Marinovich and Cantrawl trawls. The black line shows the estimated abundance of fish estimated by combining the catch in the codend of the Marinovich net (red line) along and estimates of the fish that escape through the meshes of this net derived from 8 small recapture nets mounted on the trawl. The blue line shows the number of fish in each size class captured per unit volume sampled by the Cantrawl. The figure indicates that both nets under-estimate the abundance of the age-0 Arctic Cod, and that they are more likely to capture larger specimens.


Arctic Cod

Figure 2. Arctic Cod (Age-0) density.


Figure 3. Arctic Cod (Age-0) biomass.


Saffron Cod

Figure 4. Saffron Cod (Age-0) density.


Figure 5. Saffron Cod (Age-0) biomass.


Capelin

Figure 6. Juvenile Capelin density.


Figure 7. Juvenile Capelin biomass.


Figure 8. Adult Capelin density.


Figure 9. Adult Capelin biomass.


Herring

Figure 10. Juvenile Pacific Herring density.


Figure 11. Juvenile Pacific Herring biomass.


Figure 12. Adult Pacific Herring density.


Figure 13. Adult Pacific Herring biomass.

Acoustic-trawl surveys were conducted as part of the 2012 and 2013 Arctic Eis surveys. Standard sphere calibrations of the acoustic system were accomplished to assure quality control. Then, acoustic backscatter data at 38 and 120 kHz was near-continuously monitored and collected throughout the surveys, where large or interesting backscatter areas were followed by targeted Marinovich trawl hauls to verify species present in the acoustic signals. In addition, to test net selectivity of the normal surface CanTrawls, Marinovich hauls were conducted to compare the species composition and size distributions across the different net types (Figure 1). Preliminary conclusions support size selectivity of catch by both nets not favoring retention of small fishes.

Arctic Cod
Arctic cod (age0) appear to make up the majority of pelagic fish catch in the Chukchi Sea (Figure 2). They are ubiquitous, however large congregations appear in the northern portion of the study area. The largest numbers and biomass were caught in areas surrounding the yearly marginal ice zone, an example backscatter image can be seen below.

An example acoustic backscatter signal throughout the water column in an area of high Arctic Cod (age0) densities. The red line indicates the sea floor.


Overall, acoustics supports biomass estimates (Figure 3) nearly two orders of magnitude higher than those estimated from the surface trawl catch alone. Further comparison of this discrepancy will lead to better estimation of actual fish biomass and net selectivity. Arctic cod were about 3X more abundant in 2013 than in 2012. It should also be noted, the absence of Arctic Cod age1+ is indicative of life history, or ontogenetic shift, where adults are found more closely associated to the sea floor and their larger benthic prey source. Evidence of age1+ adults can be found in the 2012 Bottom Trawl catch and the exploratory acoustic survey from 2013 in the Barrow Canyon region (see below).

Saffron Cod
Saffron cod (age0) are ~1/50th as abundant as Arctic cod in the pelagic system in the Chukchi Sea (Figure 4, 5). Their habitat is mostly in the nearshore areas of the Alaska Coastal Water and brackish systems, encompassing the southern Chukchi Sea, northeastern Bering Sea and Norton Sound. Abundances were nearly 4X greater in 2013 than 2012. Interestingly, age1+ adults were fairly similar in abundance to age1+ Arctic cod.

Capelin
Juvenile and adult capelin abundances are closely linked throughout our study region, i.e. the northeast Bering to the northern Chukchi. Large abundances of capelin are generally associated with cooler water temperatures in the Bering Sea (stronger cold pool regimes), which then carry through to the Chukchi with decreasing abundance and size heading northward. The largest abundances are generally seen in the southern region however. Information from trawls and acoustics suggests juveniles were nearly 6X more abundance in the northern Chukchi in 2013 (Figures 6, 7) as compared to 2012, where warmer (near average) water temperatures as compared to 2012 (historically cold) could have supported better larval survival. Adults, however, were ~40% less abundant in 2013 (Figures 8, 9). This dichotomy in ontegenetic success is being explored further.

Herring
Juvenile and adult herring abundances are closely linked throughout our study region as well, i.e. the northeast Bering to the northern Chukchi. However in contrast to capelin, large abundances of herring are generally associated with warmer water temperatures in the Bering Sea (weaker cold pool regimes), which then carry through to the Chukchi with decreasing abundance and size heading northward. The largest abundances are still generally seen in the southern region. Information from trawls and acoustics suggests juveniles were nearly 11X more abundant in the northern Bering and Norton Sound in 2013 (Figures 10, 11) as compared to 2012 and were nearly absent from the Chukchi in both years. Adults were ~4X more abundant in 2013 (Figures 12, 13). Interestingly, the 2013 juvenile catch was the largest on record over the entire BASIS time series in the region!

Supplementary Information
In addition for 2013, a two-day exploratory acoustic survey was conducted at the end of leg 2 in the northern Chukchi Sea and Beaufort Sea to determine whether larger (age-1+) Arctic cod are abundant at the shelf break at the northeastern part of the survey area (below). No significant aggregations of fish were observed acoustically during the exploratory survey in deeper water (to ~1500m depth) at the northeastern boundary of the survey grid. A trawl targeted at low backscatter in Barrow canyon yielded a few age-1+ arctic cod suggesting that larger arctic cod were present, but at low densities.

Exploratory acoustic survey track lines.


Oceanography and Arctic Cod. From Oceanography Catch Results page.

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