Cages as tools for in situ bioassays have a proven track record in monitoring the effects of effluent and sediment toxicity. Application to biomonitoring, however, has received comparably little attention. With the increasing threat of harmful algal blooms to both humans and aquatic organisms, their use as sentinels for early warning and for the examination of organism response in situ is evident. During efforts to monitor and describe the response of juvenile menhaden (Brevoortia tyrannus) to the reportedly toxic dinoflagellates of the genus Pfiesteria, we devised a simple, inexpensive cage for the conduct of in situ bioassays. This design, in combination with fish husbandry and transport techniques specific for menhaden, allows for rapid, inexpensive deployment of cages in at least a 4-h radius of the holding facility with an acceptable level of mortality and minimal caging effects. Here we describe their application in Middle River, Maryland, where a high prevalence of menhaden with ulcerative lesions was detected in the presence of Pfiesteria-like organisms in August 1999. In all cages, no mortalities occurred that were attributable to anything other than transport stress, and physiological and neurological variables investigated proved to be in the normal range for the species. This approach demonstrates the feasibility of using inexpensive cages for biomonitoring with a sensitive piscine species.
