|Bottom Trawl Gear Comparisons|
Objectives – Compare size, species composition, and catch-per-unit-effort of common demersal fishes and crabs between a 3-m plumb-staff beam trawl (PSBT) and a standard NMFS 83-112 eastern bottom trawl.
Two common trawl gears used to investigate the community structure of epibenthic fishes, crabs, and other invertebrates in the Chukchi and Beaufort Seas are a small-mesh plumb staff beam trawl and a large-mesh standard 83-112 eastern bottom trawl. Data gathered by both trawls are valuable for monitoring the health and status of the Arctic ecosystem and a comparison study is necessary for understanding how the standardized results from the two different gears compare in terms of species composition and standardized abundance-at-length by species.
Paired tows using both gears will be conducted during the 2012 Bottom Trawl Survey of the Chukchi Sea (see above). Collections will be made with both a PSBT and an 83-112 bottom trawl at the same stations. Paired comparison tows will be well-distributed across the bottom trawl sampling grid to compare the gears across different depths and bottom types. Small demersal fishes and crabs will be collected with a PSBT with 7 mm mesh and 4 mm codend liner (after Gunderson and Ellis 1986, Abookire and Rose 2005). This trawl is towed for 2 – 5 minutes on the sea floor while the vessel is moving at 1 – 1.5 kt; a typical tow is 100 – 300 m in distance. A rigid 3 m pipe forward of the net holds the mouth open for an effective swath of 2.26 m, allowing for accurate quantifications of trawl effort by area swept. The vertical opening of the net is approximately 1.2 m. A typical beam trawl catch contains less than 100 lbs in the codend; however mud substrate in the Chukchi Sea can increase the weight 5-fold. We have used this net design successfully during 10 research cruises in the Chukchi Sea; it is very effective at capturing juvenile and small adult demersal fishes and other epibenthic fauna.
Catches from both gear types will be sorted to the lowest taxonomic level and processed onboard the ship for comparisons of species composition and catch-per-unit-effort (CPUE). Percentage of species will be calculated for each trawl type and will be compared between hauls at the same stations. Percentage of species also will be calculated over the same bottom type for each trawl type and compared. Additionally, species composition will be compared over an aggregate of all hauls. Size of individual species (range, mean, standard error) will be compared between PSBT and 83-112. The relatively low number of comparison samples anticipated and the big differences between the two gear types in terms of area swept and mesh size will preclude making any statistically valid comparisons.
A geo-referenced database of haul events for area swept calculations in addition to numbers, weights, and length-frequencies of all major taxa by haul, station, and gear type for 2012. A report will be produced that details the comparisons between the two types of bottom trawls.
|Acoustics Trawl Comparison|
Alex De Robertis, Chris Wilson, and Kevin Taylor
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
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.