Decadal scale changes in seasonal patterns of oyster recruitment in the Virginia sub estuaries of the Chesapeake Bay

Journal of Shellfisheries Research, August, 2004 by Melissa Southworth, Roger Mann

ABSTRACT Reproductive periodicity of sessile estuarine invertebrates reflects local seasonality of environmental (temperature, salinity) and biologic (food) parameters. Estuaries are ephemeral features in geologic time but considered somewhat constant in the course of recent human history (decadal time scales). Analyses of long-term trends in eastern oyster (Crassostrea virginica) settlement periodicity since 1960 in three major Chesapeake Bay rivers (James, Piankatank and Great Wicomico Rivers) of the Chesapeake Bay show marked changes within the 4-decade time frame. The 50th percentile of cumulative recruitment occurs between day 194 and 250 of the year depending on year and location. Significant coherence in interannual variation is observed across a wide range of sites. These patterns are related to pre and post disease (both Haplosporidium nelsoni and Perkinsus marinus) events, periods characterized by high and low river flow, varying harvest pressure, and trends arguably associated with directed climate change.

KEY WORDS: eastern oyster. Crassostrea virginica, settlement, recruitment, climate change, Chesapeake Bay

INTRODUCTION

Sessile marine invertebrates are suitable candidates to examine long-term changes in climate and anthropogenically induced changes in local environments. Recruitment intensity and periodicity are annual signals of the integrated impact of local perturbations superimposed on long term (geological scale) changes. Commercially valuable species have been the focus of quantitative annual monitoring programs in support of fishery management, but have been examined in a limited manner with respect to combined impacts of climate change and fishing pressure (Allen & Turner 1989, Kim & Powell 1998). Estuarine environments are particularly susceptible to stress from cyclical changes on time scales ranging from tidal to annual. Long-term data on marine invertebrate communities in estuaries are limited, especially so in regions subject to increasing watershed development, water quality degradation, habitat destruction and/or diseases, and parasites. Temperate estuaries are natural laboratories where cumulative impacts of human societal growth are highly visible. Eastern oysters, Crassostrea virginica, are considered sentinel organisms in estuaries on the North American Atlantic seaboard in terms of biologic and geologic (habitat) function. Their loss in such environments predicates significant changes in ecosystem function and food chain dynamics with trickle down effects on nutrient cycling, species richness and complexity, stability of food webs, and production to support commercial fisheries.

The eastern oyster has long been recognized for its ecologic and commercial importance in the Chesapeake Bay, but the species has suffered numerous insults over the past century. Over fishing of oysters in the Chesapeake Bay has long been recognized. Maryland's harvests have been in decline since about 1885 and Virginia's since about 1904 (Hargis & Haven 1995). Recent catches are less than 1% of what they were 100 years ago. In addition to the continuous removal of market and seed oysters, uncounted millions of tons of shell have been removed for use in road building, chemical processing, and poultry husbandry. This essential habitat loss has resulted in the gradual replacement of 3-dimensional intertidal reefs, with 2-dimensional, subtidal reefs that are highly susceptible to siltation and burial (Hargis & Haven 1995). The onset of Perkinsus marinus in 1950 (Andrews 1996) and the arrival of the non-native disease Haplosporidium nelsoni (MSX) in 1959 (Burreson et al. 2000) caused further decline in the already seriously depleted oyster populations. Despite efforts of replenishment, beginning as early as 1924 in Maryland and 1928 in Virginia, oyster stocks have continuously declined. In response to these accumulating problems, monitoring efforts increased and became routine starting in the late 1940s and early 1950s in both Maryland and Virginia (Andrews 1982).

Early studies on oyster settlement (spat or young of the year oysters undergoing metamorphosis and attaching to the bottom) and recruitment (those oysters that survive post settlement to become part of the population) in Virginia focused on seasonal patterns in onset, duration, intensity, and cessation of oyster settlement (Andrews 1951, Andrews 1954). With the onset of the diseases, P. marinus and H. nelsoni, in the late 1950s and 1960s these patterns changed. Population studies from 1946 to 1967 in the James River showed that post H. nelsoni settlement was of lower intensity and occurred during a shorter period when compared with pre H. nelsoni settlement (Andrews 1982).

There have been two other long-term studies of oyster settlement and recruitment in the Virginia portion of the Chesapeake Bay. Haven and Fritz (1985) focused on the temporal and spatial distribution of oyster settlement. They examined weekly settlement in the James River from 1963 to 1980. They separated the rivet" into three distinct settlement zones related to water circulation and found that settlement was synchronous at stations within a zone, but occurred 1 to 2 weeks earlier at stations in the upriver zones compared with the downriver zones. They also found that post H. nelsoni settlement intensity was lower and occurred in discrete pulses, 1 to 2 weeks in duration, instead of the continuous settlement pattern seen in pre H. nelsoni conditions. Austin et al. (1996) performed a time series analysis of recruitment from 1946 to 1993 in the tour major sub estuaries of the Virginia portion of the Chesapeake Bay. The data used were from the Virginia Institute of Marine Science's (VIMS) annual fall dredge survey. They found a relationship between spat (young of the year, recently settled oysters) and subsequent seed at 2- and 3-years post settlement, but no relationship between recruitment numbers and spring and summer water temperatures and river discharge. Their study provided an overall picture of interannual variation in recruitment, but did not provide any information on interannual variation in the onset and duration of oyster settlement. Whereas both of these studies provide insight into oyster settlement and recruitment in the larger rivers of the Virginia portion of the Bay, relatively little effort has been devoted to critical examination of changes in settlement patterns in some of the smaller rivers. VIMS provides a descriptive monitoring report of settlement and recruitment in both small and large rivers in Virginia (summaries available at http//: www.vims.edu/mollusc) on an annual basis. Oyster settlement in these smaller systems and the overall health of resident oyster populations has become increasingly important over the past decade, primarily due to increasing restoration efforts in these smaller systems. The Piankatank and Great Wicomico Rivers in particular have served as important building blocks in a long-term plan for oyster restoration in Virginia (Bartol & Mann 1997, Southworth & Mann 1998).