Scientific trawl survey data
Scientific bottom trawl surveys, primarily sampling demersal commercial
species, were obtained from the Northeast Pacific and North Atlantic
shelf regions. All data used are publicly available and were downloaded
in July 2021 (see Appendix S1 for details on data processing).
We selected all scientific
surveys that sampled the fish community with otter trawls. For each tow
in each survey, we selected all demersal teleost and elasmobranch
species and obtained species
weight. We corrected these weights for differences in sampling area (in
km2) and trawl gear catchability to obtain a
standardized fish biomass across hauls and surveys. We estimated
sampling area using information on wingspread, speed of vessel and tow
duration. Weights were corrected for trawl gear catchability using
information for 80 species in the Northwest Atlantic (Link et al.2008) and 128 species and 7 functional groups in the North Sea (Walkeret al. 2017). The adjustments resulted in biomass estimates per
unit area in metric tonnes (1000 kg) per km2.
We compared the corrected trawl survey biomasses with available
fisheries stock assessment biomasses to validate the range and
distribution of the biomass estimates. To this end, we calculated
spatial overlap between the surveyed area and the bounding region of all
fisheries assessment areas from the RAM Legacy database (Ricard et
al. 2012). For each area that overlapped at least 50% with the
surveyed area, we compared biomass of each assessed stock with the
gear-corrected trawl survey biomass for the corresponding species. The
comparison shows that the corrected biomass has a reasonable match with
the stock assessment biomass and no apparent bias, for most of the 120
stocks in the Atlantic and Pacific (Figure S1.3). This finding improves
confidence that the gear-corrected trawl survey estimates, hereafter
termed demersal fish biomass and/or demersal community biomass, are
representative and comparable
across areas and surveys.
Using the individual haul coordinates, we estimated an average demersal
community biomass, in tonnes per km2, per equal area
grid cell (6000 km2) and surveyed year.
To reduce the effect of potential
outlying biomass estimates, we removed all individual observations 1.5
times less/greater than the interquantile range per survey and year
based on log-transformed biomass values (but note that the overall
conclusions are robust with or without such a data removal, not shown).