Modeling the Population Effects of Hypoxia on Atlantic Croaker (Micropogonias undulatus) in the Northwestern Gulf of Mexico: Part 1—Model Description and Idealized Hypoxia

Abstract

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%.

Modeling the Population Effects of Hypoxia on Atlantic Croaker (Micropogonias undulatus) in the Northwestern Gulf of Mexico: Part 2—Realistic Hypoxia and Eutrophication

“…A model year was 365 days long and began on September 1. The model is described in detail in Rose et al (2017a).…”

“…Hourly temperature and surface chlorophyll-a concentrations were specified for each cell for a year and that year's set of hourly values were repeated every year (see ESM-1 in Rose et al 2017a). Temperature was specified from estimated climatological values and we used bottom values for cells until 100-m depth was reached, after which the values for 100-m depth were used for cells.…”

“…We then built up the population model, using a spatially explicit individual-based approach, to allow for simulation of realistic exposures and to use laboratory information to clearly link the time-varying exposures to growth, mortality, and reproduction effects on individuals. We described the individual-based population model in Rose et al (2017a) and the exposure-effects model specifically designed to deal with fluctuating DO exposures in Neilan and Rose (2014). As part of the Rose et al (2017a) analysis, we used idealized (climatological) environmental conditions repeated yearafter-year so that long-term population responses to hypoxia could be easily identified and the reasons for the responses within the model understood.…”

“…We described the individual-based population model in Rose et al (2017a) and the exposure-effects model specifically designed to deal with fluctuating DO exposures in Neilan and Rose (2014). As part of the Rose et al (2017a) analysis, we used idealized (climatological) environmental conditions repeated yearafter-year so that long-term population responses to hypoxia could be easily identified and the reasons for the responses within the model understood. In this companion paper, we focus on expanding that analysis to include more realistic spatial and temporal variation in DO concentrations and to include possible changes in croaker food (river-related production; mortality of benthos) that can occur concomitantly with the severity of hypoxia.…”

“…In this paper, we use the individual-based croaker population model described in Rose et al (2017a) and simulate long-term population responses under realistic variation in DO and with croaker food coupled to the severity of the hypoxia. We use DO concentrations from a 3-dimensional (3-D) coupled hydrodynamics-water quality model (Justić and Wang 2014) for the NWGOM as input to the croaker model.…”