GLERL 2005 Milestone Reports

< GLERL 2005 Milestone main page

GOAL: Ecosystem

Scientist: Dr. Stuart Ludsin (GLERL)

NOAA Performance Objective: Increase number of regional coastal and marine ecosystems delineated with approved indicators of ecological health and socio-economic benefits that are monitored and understood

NOAA Corporate Measure: TBD

Ecosystem Research Program Performance Measure: PM1: Extent of ocean, coastal and Great Lakes ecosystems explored, mapped, characterized, and inventoried and publicly available

OAR Performance Measure: Research to improve our understanding of the factors affecting ecosystems and the success of ecosystem approaches to management.

NOAA Program: Ecosystem Research

Milestone: Develop spatially explicit maps of bay anchovy growth rate potential for Chesapeake Bay.

Purpose: The magnitude and duration of hypoxia/anoxia has increased in the Chesapeake Bay estuary during recent decades, owing primarily to cultural eutrophication. For example, during summer, hypoxia can encompass more than two-thirds of the water column in mesohaline portions of the bay. Because all metazoans require oxygen to function, reduced oxygen availability theoretically could limit suitable habitat for economically and ecologically important species (e.g., fish), which in turn, might cause direct or indirect reductions in population size or production. At present, however, our understanding of whether reduced oxygen availability has influenced the fisheries of Chesapeake Bay remains largely enigmatic. Herein, we used bioenergetics-based, spatially-explicit growth rate potential (GRP) modeling to develop detailed maps of habitat suitability for bay anchovy Anchoa mitchilli in Chesapeake Bay. These maps provide a means to evaluate whether reduced oxygen availability might limit bay anchovy production by reducing access to other important habitat attributes such as preferred temperature and prey (zooplankton) resources. Assuming that bay anchovy seeks out habitats that maximize growth, modeled areas of high GRP are reflective of high habitat quality. By contrast areas of reduced GRP indicate areas of poor habitat quality. We focused on bay anchovy because 1) it is the most abundant fish in Chesapeake Bay, and 2) it serves as a major prey item for numerous commercially and recreationally important Chesapeake Bay fishes (e.g., striped bass, bluefish, and weakfish).

Efforts and Results (to date): Maps of adult bay anchovy GRP have been generated for Chesapeake Bay during Spring, Summer, and Fall 1996, 1999, and 2000 along the entire latitudinal extent of the Bay. Although some interannual variation in the distribution of habitat quality (as measured by GRP) was evident both spatially and temporally, some striking generalizations could be drawn:

1. Spring (Figure 1, top four panels): Growth rate potential (habitat quality) was greatest during spring across the entire Bay, owing primarily to high zooplankton biomass that was accessible to bay anchovy because of well-oxygenated waters (> 3 mg/l) throughout most of the water column. In fact, >90% of the modeled cells (cell dimensions: 0.001 latitude x 1-m depth) had a positive habitat suitability value (i.e., a potential for positive growth; GRP > 0) during spring 1996, 1999, and 2000.

2. Summer (Figure 1, middle four panels): Growth rate potential (habitat quality) was typically lowest during summer, especially in the deep, mesohaline (mid-portion) of the Bay, owing to reduced oxygen availability (< 3 mg/l). This reduction in habitat quality was evident despite optimal temperatures for bay anchovy growth. Although ample zooplankton (prey) resources existed in the mesohaline region of the bay, they were typically unavailable to bay anchovy; the highest levels of zooplankton prey occurred in deep, hypoxic waters, which were not suitable for high levels of bay anchovy consumption (dissolved oxygen levels < 3 mg/l are stressful to bay anchovy). Thus, reduced oxygen availability may indirectly limit bay anchovy growth by reducing access to prey resources during summer. Indeed, observed distributions of bay anchovy during summer (as measured by hydroacoustics) closely matched the observed distribution of dissolved oxygen (and hence, GRP), thereby supporting our model's predictions (i.e., our maps). Further, when the model was run without any oxygen effects on bay anchovy foraging during summer, average bay anchovy GRPs were significantly higher because of heightened access to zooplankton prey. Ultimately, these results indicate that dissolved oxygen availability may severely limit production potential of bay anchovy during summer months, which in turn, may have ramifications for higher predators (e.g., striped bass, bluefish).

Importantly, habitat quality for bay anchovy during summer can be variable. For example, during 1996 and 2000, the number of cells with positive GRP was typically low during summer (< 51%), whereas the clear majority of cells (88%) had a positive GRP during 1999 (Table 1). Thus, factors (e.g., nutrient loading, temperature, wind) that influence the spatial extent of oxygen availability also can indirectly influence habitat quality for bay anchovy growth.

3. Fall (Figure 1, bottom four panels): Habitat quality for bay anchovy tended to be intermediate during fall months as a result of enhanced oxygen availability in bottom waters relative to summer, but lower zooplankton biomass relative to spring. Similar to summer observations, inter-annual variability in GRP was evident during fall. For example, owing to delayed fall mixing, anoxia was still prevalent during fall 2000 sampling, which in turn, caused ~30% of the modeled cells to have a negative growth potential (Table 1).

1996

Spring

Summer

Fall

Figure 1. Maps of temperature, dissolved oxygen, and zooplankton biovolume (biomass), and growth rate potential (GRP), a measure of habitat quality, in Chesapeake Bay during spring (top four panels), summer (middle four panels), and fall (bottom four panels) of 1996. Only results from daytime cruises are shown because 1) bay anchovy feed primarily during daytime, and 2) zooplankton distributions during nighttime are not reflective of actual availability, owing to diel vertical migration behavior.

Table 1. Summary statistics of adult (age-1+) bay anchovy growth rate potential, a measure of habitat quality, in Chesapeake Bay during spring, summer, and fall, 1996, 1999, and 2000. The sample size (N), mean (X), standard deviation of the mean (SDx), and median (M) are presented for each cruise, as is the percentage of grid cells with a positive growth rate potential for each cruise (%pos).

In summary, it appears that reduced oxygen availability can limit habitat quality (as indexed by GRP) for bay anchovy, primarily during summer and occasionally during fall. Most affected are deep, mesohaline waters of the mid-Bay. Habitat quality in the shallower upper and lower reaches of the bay, as well as the surface waters of the mid-Bay, tended to be high (positive GRP), regardless of season. Ultimately, these results suggest that efforts to minimize hypoxia might have a positive effect on important zooplanktivorous prey species such as bay anchovy.

Customer(s): ): There is a diverse community of potential users of the maps we are developing. Environmental management (including fisheries) within the Chesapeake Bay is a partnership among federal, state, and local authorities. In addition, the Chesapeake Bay region is rich in environmental advocacy groups. These NGOs play an important role in regional management bodies and are a second important audience for these products.

Cause Factors (if milestone not met): N/A

Revised Completion Date (if milestone not met): N/A

Last updated: 2005-04-28 mbl

Home | Research | Data | Products & Services | Outreach | About GLERL | News & Events
Privacy Policy | Disclaimer | Accessibility Statement | GLERL only | Contact Us | DOC

GLERL 2205 Commonwealth Blvd., Ann Arbor, MI 48105-2945     (734) 741-2235
Lake Michigan Field Station, 1431 Beach St., Muskegon, MI 49441-1098     (231) 759-7824