Assessing whether oyster reef habitat functions as fish foraging grounds

Assessing whether oyster reef habitat functions as fish foraging grounds

The decline of the oyster fishery in the eastern United States continues even though oyster reefs are beginning to receive recognition for the important ecosystem services they perform. In addition to production of oysters for harvest, oyster reefs stabilize other critical habitats, filter water, and create habitat for economically valuable finfish and mobile invertebrates. Oyster reefs typical harbor greater densities of prey fishes, crustaceans, polychaetes, and mollusks than surrounding unstructured bottom; however, biogenic structure created by oyster reef habitat may limit predator access to reef-associated prey. We conducted experiments in 1-m² mesocosms located within a laboratory settling-tank to determine whether oyster reef habitat enhances fish foraging rates. We manipulated oyster cover and conducted a series of predator-prey experiments in which we subjected common estuarine predator fish to mesocosms with reef habitat vs. ones with mud bottom. Each experiment consisted of one predator species (gag Mycteroperca microlepis or flounder Paralichthys spp.) and one prey species (mummichog Fundulus heteroclitus and white shrimp Penaeus setiferus). Prey survivorship was quantified to estimate predator foraging rates. Gag consumed significantly more mummichog on reefs than on mud bottom, whereas reef habitat did not affect gag foraging rates on white shrimp. Conversely, flounder consumed more shrimp on mud bottom than on reef habitat. Flounder did not forage effectively on mummichogs in either habitat. Habitat structure can influence predator foraging efficiency; therefore, predator-prey interactions on oyster reefs are dependent on predator foraging behavior.

Michael H. Graham.* Center for Population Biology, University of California, Davis, California, USA, 95616, USA; e-mail mhgraham@ucdavis.edu.

Kelp forest distributions are constrained by the availability of rocky substrate within the depth range tolerable for growth and reproduction. The size and distribution of such reefs may vary over relatively short geological timescales (100’s of yrs) due to interactions between coastal bathymetry and climate-driven changes in eustatic sea level. I developed a digital bathymetric map for the southern California coast. Using a Geographic Information System (GIS) and published sea level curves and kelp depth tolerances for the same region, I reconstructed changes in the maximum size, distribution, and productivity of insular giant kelp (Macrocystis pyrifera) forests in the Southern California Bight since the last glacial maximum. Comparing the known size and distribution of present-day giant kelp forests to GIS predictions based on current sea level validated reconstructions. Reconstructions predicted that the total area of available kelp forest habitat was approximately 200% greater at 16.5 kyr BP (1130 square km), than at present (382 square km). Available kelp forest habitat during the last glacial maximum (18.5 kyr BP; 628 square km) was approximately 64% greater than at present, the difference between 16.5 and 18.5 kyr BP being due to greater exposure of the steep shelf slope during lowest sea levels. Coupled with area-specific biomass and productivity estimates from present-day kelp forests, these reconstructions suggested more productive and spatially extensive kelp forests in the Southern California Bight near the last glacial maximum than at present. The large climate-driven (i.e. sea level) decreases in kelp forest distribution and productivity since the last glacial maximum likely had important historical impacts on the ecology and evolution of the present-day kelp ecosystem, as well as the exploitation of marine resources by the early human inhabitants of southern California.


Mortality estimates of the southern bay scallop (Argopecten irradians concentricus), in the Gulf of Mexico

Jaime M. Greenawalt,* Thomas K. Frazer, and Stephanie R. Keller. University of Florida, Department of Fisheries and Aquatic Sciences, 7922 NW 71st St., Gainesville, FL 32653, USA.

Historically, the southern bay scallop (Argopecten irradians concentricus) has been an important recreational fisheries species along the Gulf coast of Florida. However, a decline in scallop abundance resulted in regulation changes in 1995, including a closure to harvest south of the Suwannee River. Although the cause of the population decline is unknown, it is potentially a result of over fishing. Populations in some areas of the closed regions have since rebounded to "healthy" densities of 13 and 33 per 50m², in 2000 and 2001 respectively. Quantitative estimates of fishing mortality and natural mortality are required to develop an effective management plan for the bay scallop fishery. This study provides the necessary estimates through a comparison of the mortality rate of a fished and unfished population. Two local populations of bay scallops (one subject to harvest and another in an area closed to harvest) were sampled by visual benthic surveys prior to the fishing season, and at monthly intervals until after the close of the fishing season for two years. Catch curves were used to estimate the total mortality for both populations, and subsequently to estimate fishing mortality and natural mortality rates for two years. The decrease in density thoughout the season in the area open to harvest (Steinhatchee) can be attributed to both fishing mortality and natural mortality. Whereas, the seasonal density declines in the closed region (Homosassa) represent only natural mortality. Simulations of increased exploitation were run to determine the effect that increased harvest pressure may have on the sustainability of bay scallop populations. Results from this study may play an important role as closed areas are reopened to harvest in 2002.


Characterization of the development of fish cage fouling communities

Jennifer Greene* and Raymond Grizzle. University of New Hampshire, Jackson Estuarine Laboratory, 85 Adams Point Road, Durham, NH 03824, USA; e-mail jenn.greene@unh.edu or ray.grizzle@unh.edu.

A major problem with open ocean aquaculture of finfish in the Gulf of Maine is the growth of algal and invertebrate communities on the surface of the cages. These organisms add weight and drag to the cage, affecting its behavior under different wave and current conditions. In 2000, an estimated $40,000 was spent to routinely remove the organisms from the two cages. This project will characterize successional development of these communities to better understand the dynamics of the populations that inhabit this unique, floating habitat. A six-month preliminary experiment was started in December 2001 to provide data for the design of a year-long experiment commencing in June 2002. Three experimental net panels, each consisting of a 30x30 cm CPVC square frame with knotless nylon net stretched and cable-tied evenly over it, were attached to each cage. One cage is at a water depth of 15 m and the other is at the surface. Two of the panels will stay out for the entire six months. Two more panels will be deployed each month with one being collected monthly and the other staying out for the duration of the preliminary experiment. Hence, monthly recruits as well as successional sequences of different durations will be monitored. Retrieved panels were photographed, plastic-wrapped and stored on ice until returned to the lab for processing. The first month (December) panel from the surface cage showed an initial colonization by filamentous diatoms (Melosira, Biddulphia, and Licmophora), skeleton shrimp (Caprellidae),and mussels (Mytilidae). The first month panel from 15 m showed little to no algal colonization while skeleton shrimp and mussel spat dominated. During the second month (January), the surface panel showed similar abundant diatom growth but no invertebrates, while on the 15 m panel, skeleton shrimp remained abundant but mussel spat and diatoms were absent.


Is dissolution of gypsum stone a sufficient proxy for water flow in ecological studies?

Kevan Gregalis,* Sean P. Powers, and Melissa Boykin. University of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, NC 28557, USA.

For field ecologists, determining mechanistic explanations for patterns in nature requires that differences in physical/chemical parameters be determined. When investigations move from small-scale, single-site manipulations to larger field studies, the need for cost-effective methods to estimate environmental differences becomes critical to the success of the study. However, the high-unit cost of unattended sampling instrumentation prevents the use of high-resolution sampling equipment at multiple sites. In many cases a time-integrated proxy of the parameter in question may be sufficient to evaluate differences within or among sites. In this vein, aquatic ecologists have relied on the dissolution of gypsum stone as a means of measuring spatial heterogeneity in water flow patterns; however, the efficacy of using the dissolution rate of gypsum molds as a proxy for water motion has been questioned. We present the results of a relatively simple set of field experiments designed to determine if the dissolution of gypsum molds corresponded to manipulated patterns of water motion. Water flow was manipulated in situ by use of large plywood channels. Water velocity was increased or decreased compared to ambient levels by varying the width of the opening relative to the width of the center, working section of the channels. Comparisons between velocity measurements collected within the channels using current meters and dissolution rates of gypsum molds demonstrated a high degree of correspondence (R² > 0.75; p < 0.05). Additional experiments were performed to compare dissolution rates under similar current speeds, but dissimilar flow patterns. This was achieved by comparing gypsum dissolution in channeled flows to dissolution rates in areas where flow was unrestricted. The results of both sets of experiments demonstrate the usefulness of dissolution of gypsum molds as proxy for differences in water flow.


Long-term changes in intertidal oyster reefs and the potential effects of boating activities

Raymond Grizzle,¹* Jamie Adams,¹ and Linda Walters.^{2 1}Jackson Estuarine Laboratory, University of New Hampshire, Durham, NH 03824, USA; ray.grizzle@unh.edu; ²Department of Biology, University of Central Florida, Orlando, FL 32816, USA.

Previous research had shown that some intertidal oyster (Crassostrea virginica) reefs in the Mosquito Lagoon within the Canaveral National Seashore, Florida had dead margins consisting of mounded up, disarticulated shells. It was hypothesized that boating activities were the cause of the damage because all the reefs were adjacent to major boating channels. We characterized the history of the appearance of dead margins using aerial photographs taken between 1943 and 2000. Imagery analyzed included prints (black & white, color, or color infrared) from 1943, 1951, 1963, 1975, 1988, and 1995, and digital imagery from 2000 (USGS 1:12,000 digital ortho-quarterquads), at scales from 1:6,000 to 1:24,000. Prints were scanned at a resolution sufficient to yield 1-m pixels and referenced to the year 2000 imagery using ArcView and ArcInfo GIS software. All reefs with dead margins on the 2000 aerials were visited in November 2001 to confirm the presence and extent of dead areas. This provided a general ground-truthing for the "signature" (a highly reflective, light-colored area adjacent to darker-colored live reef) to be used to detect the appearance of dead margins in the historical aerials. The earliest appearance of dead margins was in 1975 on reefs adjacent to the Atlantic Intracoastal Waterway (ICW). The total number of reefs with dead margins and the total areal extent of dead margins increased over time. Our preliminary estimate is that about 15% of the total number of reefs in the Seashore have been damaged, with the most dramatic changes occurring in reefs along the ICW. This historical analysis provides strong (although only correlative) evidence that boating activity has detrimentally affected some reefs in the study area. Ongoing studies are aimed at further testing this hypothesis and elucidating the actual mechanisms involved.

Digestive symbionts of the fiddler crab Uca pugnax: who, where, and why?

Lara K. Gulmann* and Lauren S. Mullineaux. Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.

We are studying the gut microbiota of the fiddler crab Uca pugnax to determine the role of resident microbes in the digestive physiology of the host. U. pugnax consumes a diet of marsh detritus and benthic unicellular algae and could utilize microbial enzymes and/or fermentation products to maximize nutritional and energetic yield from its diet.

Our first objective was to examine the gut lining for associated microbiota. Using scanning electron microscopy, we found that while the fore- and mid-gut sections were free from microbial attachment, the hindgut was densely colonized by bacteria and filamentous fungi. The majority of bacteria observed were curved rod-shaped bacteria (2 μm x 0.5 μm). The fungi appear to be of the class Trichomycetes and order Eccrinales and produce hyphae approximately 10 μm wide and 5-20 mm long. To determine the consistency of the association and resolve any seasonal changes, we have made monthly direct counts of the resident hindgut microbiota. We found that bacterial densities are significantly higher (p<0.05) in the summer months, averaging 2.7 x 10⁶ bacteria per mm gut length, as compared with winter months, when densities average 5.0 x 10⁵ per mm. We expect that this seasonal difference reflects a reduction in food consumption during the crabs’ winter dormancy and highlights the microbiota’s dependence on host food supply. These results offer support for our expectation that the gut microbiota has a mutualistic role, but do not discount the possibility of a commensal association.

Scott Hagins¹* and Robert Feller.^{1,2 1}Marine Science Program, ²Department of Biological Sciences, University of South Carolina, USA.