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In the past two decades, occurrences of summertime upwelling-driven low dissolved oxygen (DO) events, or hypoxia, have increased along the northeast Pacific coast. If hypoxic events are severe enough to cause marine species mortality, the areas affected are often called “dead zones.” In 2002 and again in 2006, the events were lethal, resulting in mass invertebrate and fish kills. Many organisms are impacted by these events, but less mobile juvenile fishes and crabs are potentially more vulnerable to the long-term effects of these stressful conditions than adults. Little is known about how low
DO impacts coastal fishes and invertebrates, especially during the juvenile phase. Nursery habitats in the nearshore open-coast and enclosed estuaries may be differentially affected by hypoxia by prevailing summertime oceanographic conditions. Both ecosystems are critical for the early development of many ecologically and commercially important species. While one habitat may be compromised by stressful environmental conditions, the other may provide refuge. In upwelling driven areas, low dissolved oxygen is typically coupled with colder water, while in estuarine or semi-enclosed systems, dissolved oxygen diminishes with increasing temperatures. Temperature and dissolved oxygen are known to have interactive effects on fish behavior and physiology, which underscores the importance of studying hypoxia within different systems and in relation to other co-occurring environmental variables. In order to isolate the effects of these parameters on growth, I conducted three experiments over the course of two years on juvenile Dungeness crab (Cancer magister) and English sole (Parophrys vetulus). During each experiment, I held English sole in the lab in fixed oxygen and temperature conditions for at least thirty days, and Dungeness crab for approximately fifty. I tested a total of 15 oxygen and temperature treatment combinations, with at least two replicates of each treatment. These treatments simulated summertime oceanographic conditions that are typically found in two nursery habitats used by these species: the Yaquina Bay estuary and Moolack Beach, Oregon USA. English sole specific growth (g d-1) increased with higher temperatures (13 and 15°C) and oxygen concentrations (2.0 and 6.0 mL L-1) and was up to four times lower in all low temperature and oxygen treatments (5 and 7°C; < 1.4 mL L-1). Dungeness crab intermolt period decreased with temperature from 48 days at 5°C to 12 days at 15°C. Crab specific growth (mm d-1) increased with temperature, with crabs in the highest temperatures growing nearly five times as much as crabs in the coldest temperatures. Finally, if people have heard of hypoxia, it is commonly in relation to the condition found in the Gulf of Mexico, which is nutrient driven and can be traced to anthropogenic input. Instead, Oregon hypoxia is more difficult to trace directly to human-related activity, making West Coast hypoxia a relatively lesser understood occurrence. In order to share the results of my research and improve general public understanding of hypoxia, I designed a ~3:00 minute video exhibit that was displayed at the Hatfield Marine Science Center Visitor Center. Our intent was to create a video that could be distributed to different Visitor Centers and museums that are interested in educating their visitors about the occurrence, causes and frequency of hypoxia on the Oregon Coast.
Available online from the National Sea Grant Library