Science Serving Maryland's Coasts

R/FISH-99

Large-scale blue crab recruitment limitation in upper Chesapeake Bay nurseries: Dispersal and post-settlement processes

Principal Investigator: 

Eric Johnson

Start/End Year: 

2009 to 2012

Institution: 

Smithsonian Environmental Research Center

Co-Principal investigator: 

Denise L. Breitburg, Anson Hines, Smithsonian Environmental Research Center

Topic(s): 

Strategic focus area: 

Sustainable natural resources of coastal Maryland

Description: 

*This project is a continuing project initially funded through award NA05OAR4171042 (year one) and currently funded through award NA10OAR4170072 (year two)*

OBJECTIVES: The Chesapeake blue crab population has declined markedly in the past 15 years and our data indicate that blue crabs may suffer large-scale recruitment limitation in upper Chesapeake Bay nurseries. Our overall goal is to develop a mechanistic understanding of blue crab recruitment, focusing on juvenile dispersal to, and post-settlement survival and growth within Maryland subestuaries. Objectives over 2 years are: • To quantify spatial and annual variation in juvenile blue crab recruitment into Maryland subestuaries. • To quantify spatial and annual variation in post-settlement survival and growth of blue crabs among Maryland subestuaries. • To discover the route and behavioral mechanism(s) by which juvenile crabs disperse up-estuary from lower Bay settlement habitats to subestuaries in Maryland which serve as secondary nursery habitats. • To identify key watershed and estuarine attributes of blue crab nurseries.

METHODOLOGY: This project will integrate field surveys and experiments with spatial modeling to understand patterns of juvenile blue crab abundance within Maryland nurseries. To investigate the spatio-temporal patterns of juvenile recruitment, we will conduct standardized tow and seine sampling in nearshore shallow habitats within subestuaries along the mainstem of upper Chesapeake Bay. Tethering experiments will quantify relative differences in juvenile crab survival among subestuaries and field caging will quantify differences in growth. The route and mechanisms underlying up-estuary transport will be determined by sampling with trawls on various tidal phases during both night and day, testing for vertical (depth) distribution and horizontal position of dispersing juveniles. Spatial modeling using multivariate and GIS approaches will be used to identify key characteristics (or surrogates) of watersheds and subestuaries that correlate with juvenile crab abundance.

RATIONALE: The major, persistent decline in the Chesapeake blue crab population appears to be related to reductions in the female spawning stock and resulting limitation in juvenile recruitment. Spatial variation in blue crab abundance reflects differences in both initial densities established by annual recruitment of juveniles into nursery habitats as well as in post-settlement processes affecting survival and growth. However, we know very little about juvenile dispersal into these nurseries, or how survival and growth vary spatially among subestuaries. Our initial data indicate a marked gradient in juvenile abundance among subestuaries with very low recruitment into upper Bay sites. This project will employ integrated field sampling and spatial modeling as a first step to assessing the relative importance of recruitment and post-settlement processes to local fishery production and how they vary spatially to identify key upper Bay nurseries; information that can inform fishery management to help facilitate stock restoration.

Impact/Outcome: 

This section describes how this project has advanced scientific knowledge and/or made a difference in the lives of coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

RECAP: This project has provided supporting evidence for continuing a ban on harvesting female blue crabs as a means to help increase the population of blue crabs in the Chesapeake Bay.

RELEVANCE: The Chesapeake Bay’s blue crab population declined during the 1990s, a trend that has correlated with reductions in spawning females and, as a result, juveniles. This project used field sampling and spatial modeling to understand the dispersal of juvenile blue crabs in key nurseries in the upper Bay -- information that can inform fishery management to help facilitate stock restoration.

RESPONSE: The principal investigators on this completed project were Eric Johnson, Denise Breitburg, and Anson Hines, all of the Smithsonian Environmental Research Center.

RESULTS: In this completed study, the first of its kind, the researchers’ findings supported the hypothesis that important nursery habitats were recruitment-limited (receiving relatively few juvenile crabs), which contributed to the lack of population recovery. These findings supported an earlier series of studies by these researchers, who found evidence that tended to rule out other alternative explanations -- such as predation, inadequate food supply, and poor water quality -- for the low numbers of juveniles in the nurseries.

Based on their earlier studies, the investigators had advised natural-resource managers in Maryland and Virginia that restricting the winter harvest of female blue crabs would tend to increase numbers of juveniles reaching nursery grounds in the upper Chesapeake Bay and so would contribute to the population’s growth. One of the principal investigators, Eric Johnson, served on the Chesapeake Bay Stock Assessment Committee that made this recommendation formally. As a result, in 2008 the states of Maryland and Virginia jointly banned the winter harvest of female blue crabs. By 2012, the Bay’s total estimated population of blue crabs had more than doubled. This research project provided continuing supporting evidence after 2008 for the efficacy of the harvest ban as a means to help increase the Chesapeake Bay’s population of blue crabs. This was an important contribution given that the post-2008 population growth led to calls for reversing the ban.

 

Related Publications: