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AbstractCoastal ecosystems along the northern Gulf of Mexico are highly productive and are affected by fishing and petroleum industries in different, sometimes contrasting, ways. As a result of the Deepwater Horizon (DWH) oil spill in April 2010, oil and oil dispersants were introduced into the northern Gulf of Mexico. At the same time, large portions of the Gulf of Mexico were closed to commercial and recreational fishing for most of the 2010 summer. This presented a unique opportunity to study the dynamics of a fish assemblage exposed to changes in two disparate types of anthropogenic disturbances. We compared assemblage data for near-coastal fish (456 samples representing over 45,000 individuals and 109 species) from pre-DWH (before 2010) and post-DWH (2011–2014) to assess potential changes in abundance, diversity (alpha, beta, and gamma), and assemblage structure. In contrast to predicted oil-induced mortality, post-DWH assemblages were characterized by high abundance in 2011 (CPUE across all species pooled was 2.5 times higher than in any other year). This high abundance was most pronounced in medium-sized fish species that are direct targets of commercial fishing or likely bycatch. Abundances returned to levels similar to pre-DWH in 2012, 2013, and 2014. There were no differences in pre- and post-DWH levels of diversity, and shifts in assemblage structure may be consistent with increases driven by reduced fishing pressure. While other assemblages and ecosystems may respond differently, our data for the near-coastal fish assemblage indicated the effects of the DWH were minimal. Rigorous empirical data from other systems are required to assess potential DWH impacts across the range of ecosystems potentially affected.Received March 6, 2015; accepted October 9, 2015

Coastal ecosystems along the northern Gulf of Mexico are highly productive and are affected by fishing and petroleum industries in different, sometimes contrasting, ways. As a result of the Deepwater Horizon (DWH) oil spill in April 2010, oil and oil dispersants were introduced into the northern Gulf of Mexico. At the same time, large portions of the Gulf of Mexico were closed to commercial and recreational fishing for most of the 2010 summer. This presented a unique opportunity to study the dynamics of a fish assemblage exposed to changes in two disparate types of anthropogenic disturbances. We compared assemblage data for near-coastal fish (456 samples representing over 45,000 individuals and 109 species) from pre-DWH (before 2010) and post-DWH (2011–2014) to assess potential changes in abundance, diversity (alpha, beta, and gamma), and assemblage structure. In contrast to predicted oil-induced mortality, post-DWH assemblages were characterized by high abundance in 2011 (CPUE across all species pooled was 2.5 times higher than in any other year). This high abundance was most pronounced in medium-sized fish species that are direct targets of commercial fishing or likely bycatch. Abundances returned to levels similar to pre-DWH in 2012, 2013, and 2014. There were no differences in pre- and post-DWH levels of diversity, and shifts in assemblage structure may be consistent with increases driven by reduced fishing pressure. While other assemblages and ecosystems may respond differently, our data for the near-coastal fish assemblage indicated the effects of the DWH were minimal. Rigorous empirical data from other systems are required to assess potential DWH impacts across the range of ecosystems potentially affected.