Many marine populations exhibit complex life histories in which larval and juvenile stages are spatially separated from adults. This is the case for many coastal-spawning, estuarine-dependent fishes which utilize multiple estuaries as nursery grounds to ensure that recruitment failure in any single estuary does not translate to total recruitment failure at the population level. For these species, the location and timing of spawning is believed to regulate the pattern of supply of larvae to potential estuarine nursery areas. Furthermore, many of these species exhibit age-dependent coastal migrations which increase in amplitude with age. Thus, there is the potential that changes in the age structure in the population can affect the pattern of supply of larvae to nursery areas and structure the pattern of recruitment. The investigators will carry out an integrated empirical and simulation approach to study the sources, patterns and consequences of larval supply to estuarine nursery areas for Atlantic menhaden (Brevoortia tyrannus) along the East Coast of the US. The first goal will be to quantify the contribution of these nursery areas to coast wide recruitment. Juvenile menhaden from nursery areas from Massachusetts to Georgia will be sampled and the microchemical constituents of their otoliths will be characterized. These chemical signatures will be used to assign the nursery affinities of adult menhaden in the coastwide population. The investigators will test the null hypothesis that the Chesapeake Bay remains the most important source of recruits to the population. By determining the nursery affinities of adults from different year classes in the population they will assess whether the contribution of nurseries varies or has shifted over time. The second goal is use a population model linked to an individual-based coupled physical-biological model of recruitment to evaluate whether the known age-dependent migrations of adult menhaden are sufficient to cause the observed shifts in the distribution of larval menhaden that seed potential nursery areas. The simulation model will assist in evaluating mechanisms behind observed changes in the distribution of juvenile menhaden.
This work will contribute to the fundamental understanding of the regulation of spatially-structured marine populations. The last decade has seen the range extension of several estuarine-dependent marine species with dispersive larvae and the long-term recruitment decline of others. This integrated research program seeks to explore the effects of population demography, oceanographic circulation, and nursery site diversity on subsequent population dynamics. Given the documented changes in habitat quality in many estuarine nursery areas, and the anticipated impacts of climate change on oceanographic circulation, distributional changes in individual species are likely to become more common. Moreover, given the pivotal role that many estuarine-dependent species play in many marine ecosystems, understanding distributional changes will have direct consequences for the structure and function of the ecosystems to which they belong. The project will also train young scientists in areas of research (quantitative fisheries ecology, physical oceanography) for which there is current a national need.
Many coastal-spawning fish utilize multiple estuaries as nursery grounds, which guards against the population as a whole being adversely affected by recruitment failure in any single estuary. Some such species also undertake coastal migrations that extend further afield each year as the adults age. In these species, the location and timing of spawning associated with age-dependent behavior, coupled with natural variability in ocean currents that transport eggs and young larvae, can affect the supply of larvae to nursery areas and ultimately structure the pattern of overall population recruitment. Atlantic menhaden (Brevoortia tyrannus), sometimes called "bunker" in Mid-Atlantic States, is one such species. In this collaborative study, physical oceanographers at Rutgers University developed a hydrodynamic computer simulation model of ocean circulation in the Mid-Atlantic Bight that was utlized by fisheries science colleagues at the University of Maryland to underpin a coupled physical-biological model of menhaden larval dispersal and survival post-spawning. The hydrodynamic model ROMS (Regional Ocean Modeling System) is used by an international user community of some 4000 ocean scientists, and has been widely applied to studies of coastal ocean biogeochemistry, geomorphology, and ecosystems to deduce transport pathways for nutrients, sediments and pollutants, in addition to larvae. The ROMS computer software is open source, and distributed and maintained through a registered user portal www.myroms.org hosted at Rutgers University. Simulation models for limited geographic sub-regions of the ocean – here, the Mid-Atlantic Bight continental shelf – must be constrained at their perimeter by open boundary conditions. When this information (ocean temperature, salinity, velocity and sea level) is derived from larger domain basin or global model, as we do here using NOAA’s Integrated Ocean Observing System (IOOS) HYCOM model, this procedure is typically referred to as "downscaling". Whilst downscaled hydrodynamic models are generally skillful at reproducing the statistics of ocean current variability, much greater skill at reproducing individual circulation events is achieved if direct ocean observations are formally assimilated into the model in the same fashion that atmospheric data are incorporated into Numerical Weather Prediction (NWP) models. This project achieved significant improvements in the methodology of 4-Dimensional Variational data assimilation (4DVAR) in ROMS to incorporate observations of sea level anomaly (SLA) from radar altimeter satellites, sea surface temperature (SST) from infrared and passive microwave radiometer satellites, and surface currents from land-based high-frequency radar (CODAR) in the model-based analysis. Furthermore, a procedure was developed by which 4DVAR could be applied to estimate the mean climatological state of the ocean constrained by data from an historical archive of moored current-meters, more than 10 years of shipboard acoustic current meter data acquired by a commercial vessel that regularly traverses the route from New York to Bermuda, and more that 600,000 vertical profiles of temperature and salinity held in archives at the National Ocean Data Center and National Marine Fisheries Service. This estimate of the true ocean mean state was used to remove regional biases in the global HYCOM model that would otherwise have introduced errors into the downscaling process, and to define an accurate mean sea surface height map to sum with the anomaly sea level data from satellites. With these aspects of the 4DVAR assimilation improved, a retrospective reanalysis of the period 2006-2012 was conducted assimilating all available satellite SLA and SST data, and CODAR currents. The outcome is the most accurate reanalysis of coastal ocean circulation thus far achieved for this region, as assessed by a comparison of the model results to those from 6 other models. We therefore have confidence that the coupled physical-biological model of menhaden larvae dispersal is the most comprehensive analysis presently achievable. The data from this historical reanalysis of ocean circulation is openly accessible by other researchers through a THREDDS Data Server (Thematic Real-time Environmental Distributed Data Services) hosted at Rutgers University A further broader impact of the work is that the data-assimilative ROMS ocean circulation model is also run each day, in real-time, delivering 72-hour forecasts of ocean surface currents to the U.S. Coast Guard through the Environmental Data Server operated by the Mid-Atlantic Regional Association of Coastal Ocean Observing Systems (maracoos.org).
