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Chapter 5 - Marine Fishes: Introduction & MethodsMethodsIn each of the following fish chapters we discuss how thetarget species were selected and examine their distribution,abundance trends, and areas of persistence in theregion. Specifically, three questions concerning the distributionof fish species in the Northwest Atlantic were addressedwith this analysis:• What is the general distribution of the species inthe region? (distribution)• Where in the region has the abundance beenincreasing or decreasing? (trends in abundance)• Where in the region has the species beenconsistently found over time at the highestabundances? (weighted persistence)DataTo answer these questions, data from the NationalMarine Fisheries Service (NMFS) spring and fall bottomtrawl surveys (1968 – 2006) were analyzed (Figure5-1). All analyses were conducted on a species by speciesbasis to account for differences in the catchability of eachspecies. Comparisons among species were not performed.We limited the data to valid records collected in the fallor spring as these two seasons were surveyed using similargear and methods over a similar geographic area. Toensure that each record was comparable, the number offish per tow was adjusted based on correction factorsdeveloped by NOAA’s Northeast Fisheries Science Centerto account for changes in survey vessels, trawl net design,and trawl doors over time.Binning data by decadesIndividual trawl survey points do not overlap from yearto year. Thus, in order to calculate temporal trends inabundance and persistence the region was partitionedinto a grid of ten minute squares (TMS), with each squarecontaining multiple survey points covering a range ofyears. The binned data set was examined to determinethe smallest time interval (annual, biannual, 5-yrs, 10-yrs)for which consistent values could be calculated for mostsquares. The 10-yr decadal period was selected becauseit allowed for a robust analysis that included almost all ofthe TMS in the region. In other words, most squares containedat least one survey point from each decade (1970-1979, 1980-1989, 1990-1999, and 2000-2006). TMSthat did not have survey points in at least three or fourdecades were excluded from the analysis. For the remainingTMS, each one was scored based on the presence ofthe species of interest within each decade.Data limitationsA limitation of these surveys is that different species demonstratevarying degrees of susceptibility to being caughtby the survey gear (i.e., catch coefficients for cod are muchhigher than those for wolffish or other species). Ottertrawl systems like the one utilized to conduct survey samplingare specifically designed to capture a variety of demersalfish species, including many of the species analyzedin this assessment. It is important to note, however, thatthe catch rates for various species within the group arevariable. Catchability coefficients are generally higher fordemersal, round-bodied species (e.g., Atlantic cod, haddock,pollock), and lower for flat-bodied fish (e.g., Atlantichalibut, summer flounder) and pelagic species (e.g., bluefintuna, Atlantic herring). In additional, catch rates atany given location can be heavily influenced by day/nightdifferences in species distribution within in the watercolumn, and by seasonal variations in species distributionwithin their geographic range. Some species are also ableto avoid capture in trawls by using sensory or behavioralcapabilities.Additionally, trawl samples are particularly difficult toconduct in areas of high habitat complexity, such as boulderfields, canyons, or the seamounts just outside theNorthwest Atlantic region. The survey also may miss keynearshore areas and some offshore areas (e.g., NantucketSound) due to survey vessel depth limitations. Many ofthese coastal areas, especially bays and estuaries, are criticalfor earlier life stages of fish. For future analyses, a goalis to merge inshore trawl sampling conducted by individualstates with the results presented here. As such, it shouldbe recognized that while analyses derived from the trawlsurvey database are indeed informative, results obtainedfrom other data sources should also be considered. Finally,Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 5-