We developed a spatially explicit, individual-based model to analyze how hypoxia effects on reproduction, growth, and mortality of Atlantic croaker in the northwestern Gulf of Mexico lead to population-level responses. The model follows the hourly growth, mortality, reproduction, and movement of individuals on a 300 × 800 spatial grid of 1-km 2 cells for 140 years. Chlorophyll-a concentration, water temperature, and dissolved oxygen (DO) were specified daily for each grid cell and repeated for each year of the simulation. A bioenergetics model was used to represent growth, mortality was assumed stage-and age-dependent, and the movement behavior of juveniles and adults was modeled based on temperature and avoidance of low DO. Hypoxia effects were imposed using exposure effect submodels that converted time-varying exposures to low DO to reduced hourly growth, increased hourly mortality, and reduced annual fecundity. Results showed that 100 years of either mild or intermediate hypoxia produced small reductions in population abundance, while repeated severe hypoxia caused a 19% reduction in long-term population abundance. Relatively few individuals were exposed to low DO each hour, but many individuals experienced some exposure. The response was dominated by a 5% average reduction in annual fecundity of individuals. Under conditions of random sequences of mild, intermediate, and severe hypoxia years occurring in proportion to their historical frequency, the model predicted a 10% decrease in the long-term population abundance of croaker. A companion paper substitutes hourly DO values from a threedimensional water quality model for the idealized hypoxia and results in a more realistic population reduction of about 25%.
Quantifying the population-level effects of hypoxia on coastal fish species has been challenging. In the companion paper (part 1), we described an individual-based population model (IBM) for Atlantic croaker in the northwestern Gulf of Mexico (NWGOM) designed to quantify the long-term population responses to low dissolved oxygen (DO) concentrations during the summer. Here in part 2, we replace the idealized hypoxia conditions with realistic DO concentrations generated from a 3-dimensional water quality model. Three years were used and randomly arranged into a time series based on the historical occurrence of mild, intermediate, and severe hypoxia year types. We also used another water quality model to generate multipliers of the chlorophyll concentrations to reflect that croaker food can be correlated to the severity of hypoxia. Simulations used 100 years under normoxia and hypoxia conditions to examine croaker population responses to the following: (1) hypoxia with food uncoupled and coupled to the severity of hypoxia, (2) hypoxia reducing benthos due to direct mortality, (3) how much hypoxia would need to be reduced to offset decreased croaker food expected under 25 and 50% reduction in nutrient loadings, and (4) key assumptions about avoidance movement. Direct mortality on benthos had no effect on long-term simulated croaker abundance, and the effect of hypoxia (about a 25% reduction in abundance) was consistent whether chlorophyll (food) varied with hypoxia or not. Reductions in hypoxia needed with a 25% reduction in nutrient loadings to result in minimal loss of croaker is feasible, and the croaker population will likely do as well as possible (approach abundance under normoxia) under the 50% reduction in nutrient loadings. We conclude with a discussion of why we consider our simulation-based estimates of hypoxia causing a 25% reduction the long-term population abundance of croaker in the NWGOM to be realistic and robust.
Declines in dissolved oxygen (DO) concentrations in aquatic environments can lead to conditions of hypoxia (DO ≤ 2 mg/L), which can directly and indirectly affect aquatic organisms. Direct effects include changes in growth and mortality; indirect effects include changes in distribution, movement, and interactions with other species. For mobile species, such as the pelagic filter‐feeding Gulf Menhaden Brevoortia patronus, indirect effects are more prevalent than direct effects. The northern Gulf of Mexico experiences one of the largest areas of seasonal hypoxia in the world; this area overlaps spatially and temporally with the Gulf Menhaden commercial purse‐seine fishery, which is among the largest fisheries by weight in the United States. Harvest records from the Gulf Menhaden fishery in 2006–2009 and fine‐scale spatial and temporal predictions from a physical–biogeochemical model were used with spatially varying regression models to examine the effects of bottom DO concentration, spatial location, depth, week, and year on four response variables: probability of fishing, total Gulf Menhaden catch, total fishery effort, and CPUE. We found nearshore shifts in the probability of fishing as DO concentration declined, and we detected a general westward shift in all response variables. We also found increases in CPUE as DO concentration declined in the Louisiana Bight, an area that experiences chronic, severe hypoxia. The overall effects of environmental conditions on fishing response variables appeared to be moderate. Nevertheless, movement of either Gulf Menhaden or the purse‐seine fishery in response to environmental conditions could potentially affect the susceptibility of Gulf Menhaden to harvest and could therefore influence assessment of the stock and associated stock status indicators.
Received March 4, 2014; accepted June 26, 2014
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