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CHAPTER5Marine Fishes:Introduction & MethodsMark Anderson, Arlene Olivero, Geoffrey Smith, Jennifer Greene, Jay Odell, and Caroly ShumwayIntroductionThe North Atlantic region is known for itshighly productive waters, a result of its strongtidal flows, complex circulation patterns, variedseafloor topography and diverse sedimenttypes. Accordingly, large and sustained catchesof demersal and pelagic fish have fueled regionaleconomies for centuries. The diversityof fishes in the region may be explained by thevariety of available habitats combined with theextraordinary adaptability of these creatures- the most diverse class of living vertebrates.Distinctive fish habitats are places where singularoceanographic processes occur on a regionalor local scale. Often, these correlate with physicalor structural features such as anomalies of temperature, areas of high primary productivity, regions of diverse seafloortopography, or geographically isolated settings. For demersal fish, species abundance has been found to be associated withdepth, temperature, sediment type, sediment diversity, and habitat complexity (Mahon et al. 1998; Stevenson et al. 2004;Auster et al. 2001; Lough et al. 1989; Charton and Perez-Ruzafa 1998; DeLong and Collie2004; Lindholm et al. 1999.)Similarly, the abundance of pelagic fish is correlated with thermal fronts (Etnoyer et al. 2004).This assessment focuses on identifying those places in the region that have been consistently important to fish productivityand diversity over decades. The deep basins, shallow banks, and major channels of the Gulf of Maine, for example,are tied to water masses with distinct layering and corresponding diversity. Farther south, the broad continental margin,large estuaries, and deep submarine canyons, function as nursery areas for estuary dependent fishes and migratory pathwaysfor large pelagic species. The extremely heterogeneous aspect of the region ensures that not all areas are equivalentlyimportant with respect to fish productivity. In the chapters that follow - demersal fish, diadromous fish, small andlarge pelagic fish - we use a single consistent methodology, based on the persistence of individual species over decades, toidentify areas that may be particularly important for the conservation of each species. We chose to focus on persistence,weighted by abundance, because the latter varies greatly from year to year, reflecting temporal variation in populationsizes, fluctuating prey bases, and other factors unrelated to the physical structure of the region. In contrast, places wherea species persists over decades are more likely to correlate with perennial factors important to productivity and diversity.When possible, however, we weighted the persistence score by the abundance of the species in each decade studied, toidentify areas where a species not only persisted, but persisted at high abundance.© Andrew J. Martinez, NOAANorthwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 5-