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Time-series patterns and dynamics of species richness, diversity, and community composition of fishes at Stellwagen Bank National Marine Sanctuary (1970-2017)
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    Here we present analyses conducted in support of the most recent Stellwagen Bank National Marine Sanctuary (SBNMS) Condition Report for 2008-2017. Our focus was on patterns and trends in species richness, diversity, and community composition of fishes in SBNMS and surrounding waters over a nearly 50-year period. These analyses of the larger Gulf of Maine, in which SBNMS is nested, are used to compare regional and local scale patterns and trends. The NOAA Northeast Fisheries Science Center bottom trawl survey data, based on a stratified random sampling design, was used to represent the fish fauna of SBNMS (sampling stratum 26) along with 19 additional strata (i.e., 21-40) for the larger Gulf of Maine. Our results demonstrate that fish communities in SBNMS have changed substantially during the past decade. Following a long period of slowly rising species richness through 2006, richness rose rapidly over the last decade. This change coincided with changes in composition and patterns of numerical dominance for both local (i.e., SBNMS) and large-scale (i.e., Gulf of Maine) fish communities as well. Depth was the most significant correlate of fish community structure, but the threshold between shallow and deep communities has moved from 52.5 to 75.5 m over time. Further, composition and distribution of communities were influenced by temperature. For example, Acadian redfish were more common when bottom temperature was <5.7°C while American plaice, longhorn sculpin, yellowtail and witch flounder, and silver and white hake were associated with warmer bottom conditions. Over the past decade, shallow communities south of 42°N were characterized by higher abundances of warmtolerant species, like Atlantic mackerel and little skate, while the cold-associated species like haddock are much more abundant north of this latitude. Based on related studies, these community scale changes are attributed to changes in fisheries management, changes in species interactions mediated by changes in species and trophic guild abundance, and shifts in the distributions and abundances of fish species due to climate change as a direct or indirect driver. While maintaining and enhancing diversity is a central mission of the sanctuary, the structural changes to its communities is concerning and deserves additional investigation. Identifying the drivers of these changes is important and may provide some insight on what policies might mitigate adverse changes while not sacrificing the benefits of diversity.

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