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Chapter 5 - Marine Fishes: Introduction & Methodsany shifts in movement due to changes in temperaturecaused by climate change may not be reflected in thesesnapshots.DistributionA basic distribution map was created for each species thatshows the trawl survey points where the species was capturedweighted by its relative abundance (Figure 5-2a).All spring and fall trawl data from the years 1968 through2006 were used and the maps were produced by season.Because the data were skewed toward low abundances, theraw catch values were transformed into a cumulative percentile.Tows in which the target species was caught werethen divided into four quartiles based on percentage of thetotal catch of that species per season. This transformationallows the abundance patterns to be displayed in meaningfulunits.Trends in AbundanceUsing the binned data described above, trends in averageabundance over four decades were calculated for eachTMS for each species. Only squares with four decades ofsampling were used. For this analysis, a linear regressionline was fit to the average abundance values for each of thefour successive decades. Regression lines with a p-valueless than 0.1 (90% probability) were considered to show asignificant trend. Positive slopes indicated an increasingtrend in abundance, negative slopes indicated a decreasingtrend, and insignificant regressions indicated no trend. Bymapping these results for each species, the spatial locationswhere changes in abundance were detected werehighlighted (for an example, see Figure 5-3).Regressions were also used to analyze overall trends inspecies abundances based on the individual (unbinned)samples. From these analyses, significant changes in abundanceof a given species in a given season across the full36-year period were detected for many species. Note thatalthough sometimes there is a significant change in theabundance of a species when spatial location is not considered,for some species changes in abundance were onlyrevealed by studying the spatially linked regression mapresults. By using both of these trend results, both overallpopulation trends and the distinct changes in the spatiallocations of abundance over time were explored.PersistencePersistence refers to the consistency with which a specieswas caught in the same general area over time. To beincluded in this analysis, a TMS had to have data from atleast one survey point from each of three or four decades.Those TMS that did not meet these criteria were excludedfrom the analysis. For the remaining TMS, we scored eachone based on the presence of the species of interest withineach decade.Score 1 = The species was present in 1 out of thesampled decadesScore 2 = The species was present in 2 out of thesampled decadesScore 3 = The species was present in 3 out of thesampled decadesScore 4 = The species was present in 4 out of thesampled decadesFor example, a TMS with a persistence score of 4.00 indicatedthat the species was caught in the trawl survey atleast once in each of the 4 decades sampled (Figure 5-2c).Weighted PersistenceThe weighted persistence score is a variation of the persistencescore in which each decade is weighted by the averageabundance of the species over the decades it was present.Abundance was measured as the numbers of individualsof a given species caught per sampling tow. Because theabundance data were skewed toward low abundances witha few very high abundances, values were log-transformedand mean log abundance were calculated for each decadewithin each TMS. These decadal mean scores were averagedacross all decades to obtain a grand average for eachTMS. The grand average was then normalized across allTMS for the species of interest to create a metric of abundanceranging between 0.0 and 1.00 for each TMS, withlow abundance defined as 0-0.49 and high abundancedefined as 0.50 – 0.99 (Figure 5-2b)5-Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report