 Crangon nigromaculata
 Distribution of Crangon nigromaculata in San
Francisco Bay
Crangon nigromaculata (the blackspotted bay shrimp) ranges from the Farallon
Islands, California to Turtle Bay, Baja California and is more common in the nearshore
ocean area than in estuaries. Based on sampling by the City of San Francisco in the
vicinity of their ocean outfall, it is the most abundant species of Crangon in the
Gulf of the Farallones. There is sparse information on the life history of C.
nigromaculata in the Bay or the coastal area. The period of peak abundance of
juveniles in our collections has been highly variable, ranging from late spring to
winter. We have hypothesized that the Bay functions primarily as an expansion of
the nearshore nursery area of C. nigromaculata.
Abundance of Crangon nigromaculata steadily increased since the mid-1980's. From
1991 through 1993, it was the second most commonly collected shrimp species in the
Bay. As for C. nigricauda, this species probably benefited from the higher
salinities associated with the drought, especially during the winter and spring.
Additionally, abundance may have increased in response to the 1992-93 El Niņo event.
The northern limit of C. nigromaculata is normally the Gulf of the Farallones, but the
center of their coastal distribution may have shifted northward with increased ocean
temperatures. But not all warm-water or El Niņo events have resulted in increased
abundance of C. nigromaculata, as abundance increased in 1983, but not in 1982 or 1987.
Annual abundance indices for the
juveniles is generated by the San Francisco Bay Monitoring Program
using otter trawl data.
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