Effects of anthropogenic shoreline hardening and invasion by Phragmites australis on habitat quality for juvenile blue crabs (<I>Callinectes sapidus</I>)

Abstract

Unvegetated, shallow water habitats adjacent to marshes are an important nursery for juvenile blue crabs, Callinectes sapidus, in Chesapeake Bay. Alteration of the shoreline, either through the replacement of marshes with anthropogenic structures, such as riprap and bulkheads, or through the replacement of the native marsh grass Spartina sp. (Spartina) with the invasive Phragmites australis (Phragmites), may affect the value of this habitat as a nursery. In this study, we compared the effects of four common shoreline types, bulkheads, riprap, Phragmites marshes, and Spartina marshes, on food availability, feeding, growth, and survival of juvenile blue crabs in adjacent subtidal areas, as well as on the abundance and size of predators in the South River, Maryland. Sites with each shoreline type were randomly selected. We used benthic cores to sample macrobenthic prey and performed gut content analysis on caged crabs to examine food availability and feeding. Growth was estimated using caged crabs. Survival was assayed with a tethering experiment and predators were sampled with a seine net. Riprap had a lower abundance of macrofaunal prey, and the macrofaunal community differed from both marsh types in that it had it had smaller an more opportunistic species such as nematodes and small polychaetes compared to more bivalves and larger polychaetes at the marsh sites; however, gut contents and growth did not vary among shoreline types. Predation pressure on juvenile blue crabs was highest at bulkhead sites and lowest at riprap. Predator abundance did not vary among the shoreline types, though piscine predators were smaller in size near Spartina marshes compared to the other shorelines. We conclude that shoreline hardening substantially reduced the value of shoreline habitats for juvenile blue crabs, but that Spartina and Phragmites are functionally equivalent.