The following is an excerpt from the Maryland Dept. of Natural Resources website. Virginia has been very slow in doing research on predation. Read Full Article

A review of predation on blue crabs in Chesapeake Bay
By: Harley Speir

In recent years the recovery of striped bass populations in the Chesapeake Bay has fueled concern about the effect of their predation on blue crab abundance. Fishermen reported finding large numbers of small crabs in the stomachs of striped bass and worry that high populations of stripers may be responsible for the decline of crabs.

Orth, et al. (1999), on two days in 1998, sampled striped bass, croaker and red drum in seagrass beds in the York River where juvenile blue crabs were abundant and found that 60% of the striped bass, 100% of the channel bass and 34% of the Atlantic croaker had consumed juvenile blue crabs.

From preliminary analysis of the 2000 data, the average number of crabs in striped bass varied from 1.9 to 15.1 and averaged 3.5 crabs per stomach. The proportion of striped bass eating crabs appears to be similar to previous years.

***NOTE: That if the average was 3.5 crabs per stomach then how many crabs is that when you figure every fish in the bay. And remember the fish don’t just eat the day they were sample. KS

These studies illustrate the large difference in striped bass predation on crabs between striped bass sampled in grass beds and striped bass sampled in open waters of the Bay. The grass beds in Virginia waters are where the crab larvae, which are moving back into Bay waters from offshore, settle to molt into young crabs. The grass bed offers food and shelter for the growing crabs and densities of young crabs have been found to average 30 per square meter in grass beds and only one per square meter on adjacent unvegetated habitat (Orth et al. 1996). Predators in the grass beds would find greatly increased opportunities and increased striped bass feeding on crabs is a not surprising consequence. Crab size in the stomachs of striped bass averages about 0.85 inch and is double the 0.45 inch average size of crabs measured from the grass bed (Orth et al.1999). This difference indicates that striped bass are selecting for the larger young crabs. However, Orth et al.(1996) found that survival increased significantly in vegetated habitats with increasing crab size until about two inches when survival in vegetated and unvegetated habitats did not differ.

Seagrass is a very important habitat for young crabs as evidenced by their abundance in grass beds. However, consumption rates of predators and the magnitude of Baywide effects of predation cannot be based solely on studies of crab consumption in these areas. Striped bass and crabs are distributed throughout the tidal Bay and tributaries which covers some 2,816,000 acres (Lippson 1973). Densities of the various sizes and ages of both species vary seasonally by area. Grass beds covered only 2.3% of the Bay bottom in both 1990 and 1999 (60,300 and 64,700 acres respectively. Orth et al. 1999). The acreage available for foraging is much greater outside the grass beds.

As crabs grow they move progressively northward into Maryland waters. They utilize grass beds, when available, for shedding and hardening but they are not limited to grass beds as they forage throughout the Bay. Hines and Ruiz (1995) identified shallow water and the associated structural features as an important habitat for juvenile crabs. Predation mortality rates for juveniles were lowest in shallow (less than three feet) water. They determined cannibalism by large crabs was the major cause of juvenile mortality and accounted for 75 to 97% of the loss of the age 0 year class (2-3 inches). Predation mortality is highest for smaller crabs in waters deeper than three feet.