Stuart A. Ludsin - Ohio State University and Stephen B. Brandt
This research program, funded by the Sloan Foundation (Census of Marine Life, CoML), has been integrating spatially extensive databases comprised of traditional, optical, and acoustical information to demonstrate the benefits of quantifying marine populations over a range of sizes and trophic levels for a large ecosystem. High-resolution sampling was conducted during 1995-2000 in Chesapeake Bay, as part of an NSF-funded Land Margin Ecosystem Research, and will allow us to define spatiotemporal variability in fish and zooplankton populations, and thus obtain a better census of these marine resources on a variety of scales, including the entire Chesapeake Bay.
GLERL scientists were involved with the six-year Chesapeake Bay sampling effort, focusing on the use of acoustics to survey fish populations. As a part of the CoML project, we are integrating our fish data with other physical (temperature, dissolved oxygen) and biological (phytoplankton, zooplankton) data collected by our colleagues. This project concluded in 2004.
We have made much progress on both Chesapeake Bay projects during FY2004 in terms of data processing, analysis, and synthesis. We have completed processing all axial transects from Chesapeake Bay during spring, summer, and fall 1996, 1997, and 2000, as well as all lateral transects conducted during summer and fall in these years (some spring transects also have been processed). In addition, all acoustics data associated with midwater trawling trawls conducted during 1995-2000 have been processed. All physical and lower trophic level data associated with acoustics data have been compiled as well. Using this suite of information, we have generated surface and vertical-profile maps of distributions of relative fish biomass across the bay, as well as have begun to use multivariate statistics and spatially-explicit bioenergetics modeling to explore how habitat availability (e.g., temperature, oxygen, food) influences the distribution and growth rate potential of Chesapeake Bay fishes. To date, a full bioenergetics-based growth rate potential model has been developed for Bay Anchovy (results presented at the annual American Fisheries Society meeting during August 2004), as well as for striped bass. We are in the midst of writing publications to present these modeling efforts.
Below we briefly discuss results from our Bay Anchovy modeling exercise and multivariate work. Co-investigators on this project: Stuart Ludsin, Doran Mason, Steve Brandt, Xinsheng Zhang, Mike Roman, and Bill Boicourt.
Bay Anchovy growth rate potential (GRP) modeling. Bay anchovy (Anchoa mitchilli) is the most abundant pelagic prey fish and the dominant zooplanktivore in Chesapeake Bay. Thus, an understanding of factors that influence this species growth, abundance, and distribution is critical for fisheries management and ecological evaluation of the Chesapeake Bay. To assess the influence of both physical (e.g., water temperature, dissolved oxygen) and biological (e.g., zooplankton prey availability) factors on Bay Anchovy demographics, we developed a spatially explicit, bioenergetics-based model to predict the physiological growth response (i.e., growth rate potential) and distribution of biomass of Bay Anchovy across a 350-km north-south transect that spanned the entire bay. Physical and zooplankton data used to predict metabolic expenditures, consumption, and ultimately growth were collected continuously along this transect during spring, summer, and fall 1996-2000 with an undulating, towed sensor package. Bay anchovy biomass, used to test model predictions, was measured simultaneously with surface-towed split-beam hydroacoustics.
Modeling results demonstrate that anoxia had an overwhelming impact on the distribution of potential growth rates of Bay Anchovy. Overall, the potential for growth was typically greater during spring and fall than during summer (Figure 1).
The predicted distribution of Bay Anchovy GRP closely mimicked that of oxygen availability such that lowest GRPs were found in waters with low oxygen (Figure 2).
In large part, this result was due to the fact that zooplankton availability during summer tended to be higher in low-oxygen waters relative to high-oxygen waters. Thus, given that zooplankton have a higher tolerance of hypoxia than most fishes, we hypothesize that zooplankton use low oxygen waters as a refuge from predation by planktivorous fishes in Chesapeake Bay. Ultimately, reduced oxygen availability associated with eutrophication might be negatively affecting Bay Anchovy growth and survival by limiting this species ability to migrate into deep waters during daylight hours, where it can forage on zooplankton and hide from predators (e.g., striped bass). Certainly, the negative effect of low oxygen availability on Bay Anchovy habitat and behavior may help to explain the decline in Bay Anchovy recruitment in recent years.
We have begun synthesis of the fish acoustic data and refined methodologies for assessing fish abundance and biomass from the hydroacoustics data. Improvements have been made in the way our software handles the fish acoustic data, and the entire analysis process has been streamlined.
Ludsin, S.A., X. Zhang, L.W. Florence, M.R. Roman, and S.B. Brandt. 2003. A multi-scale analysis of factors that influence spatial distributions of Bay Anchovy in Chesapeake Bay. American Fisheries Society, Quebec City, Canada.
Florence, L.W., S.A. Ludsin, W.C. Boicourt, M.R. Roman, and S.B. Brandt. 2003. Effects of the Chesapeake Bay hydraulic control point on zooplankton and Bay Anchovy distributions. American Fisheries Society, Quebec City, Canada.