Nutrient Loading Increases Red Snapper Production in the Gulf of Mexico

A large, annually recurring region of hypoxia in the northern Gulf of Mexico has been attributed to water stratification and nutrient loading of nitrogen and phosphorus delivered by the Mississippi and Atchafalaya rivers. This nutrient loading increased nearly 300% since 1950, primarily due to increased use of agricultural fertilizers. Over this same time period, the red snapper (Lutjanus campechanus) population in the Gulf of Mexico has shifted strongly from being dominated by the eastern Gulf of Mexico to being dominated by the northern and western Gulf of Mexico, with the bulk of the current population in the same regions with significant nutrient loading from the Mississippi and Atchafalaya rivers and in or near areas with development of mid-summer hypoxic zones. The population decline of red snapper in the eastern Gulf is almost certainly attributable to overfishing, but the cause of the population increase in the northern and western Gulf is subject to broad debate, with the impact of artificial reefs (primarily oil platforms which have increased greatly since the 1960s) being the most contentious point. Nutrient loading has been shown to positively impact secondary production of fish in many marine systems. The present paper offers the hypothesis that increased nutrient loading has contributed significantly to increased red snapper population in the northern and western Gulf of Mexico. Nutrient loading may be working in synergy with the abundant oil platforms both increasing primary production and providing structure encouraging red snapper to feed throughout the water column. Corresponding Author Michael W. Courtney ·United States Air Force Academy, 2354 Fairchild Drive, USAF Academy, CO, 80840 Email: [email protected] Introduction The authors have been following the literature on nutrient loading and hypoxia in the Gulf of Mexico [13] for several years. Characterizations of these areas as “dead zones” perplexed us, because we had personal experience with the Louisiana-Texas shelf teeming with life, and we knew that Louisiana harvests tremendous quantities of seafood and also that Louisiana supports one of the best sport fisheries in the Gulf of Mexico. Our anecdotal observations and review of available data seemed to suggest that several sport species of fish (red drum, spotted sea trout, red snapper) are both plumper and more plentiful in Louisiana waters, and we have often considered hypothetical explanations for these observations. At the same time, we became aware of the attraction vs. production debate regarding whether oil platforms and other artificial reefs only attract red snapper for more efficient harvest or whether they make significant contributions to increased production of red snapper in the Gulf of Mexico [4-6]. While reviewing Melissa Monk's doctoral thesis [7] (Monk 2012, Figures 1.14 – 1.22), the apparent decrease in biomass with increasing distance from the Mississippi River discharge as well as the apparent greater biomass in the areas with higher nutrient loading and lower oxygen levels caught our attention. Monk's suggestion that Mississippi River outflow and nutrient loading should be considered 1 Distribution A. Approved for public release. Distribution unlimited. The views expressed in this paper are those of the authors and do not necessarily represent those of the U.S. Air Force Academy, the U.S. Air Force, the Department of Defense, or the U.S. Government.