Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
CARLSBAD FWO: Study Sheds Light on Contaminant Exposure of Fish and Seabirds in San Diego Bay
California-Nevada Offices , May 1, 2009
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Aerial view of the South San Diego Bay. (photo: Ryan Hagerty/USFWS)
Aerial view of the South San Diego Bay. (photo: Ryan Hagerty/USFWS) - Photo Credit: n/a
California least tern in flight. (photo: M.Pavelka/USFWS)
California least tern in flight. (photo: M.Pavelka/USFWS) - Photo Credit: n/a

by Jane Hendron, Carlsbad FWO
A two-year study conducted by the Service’s Environmental Contaminants Division in Carlsbad, California, investigated contaminant exposure of several species of seabirds, including black skimmers, elegant terns, Caspian terns, and federally endangered California least terns.

The study was conducted on the South Bay Salt Works (Salt Works) portion of the South San Diego Bay National Wildlife Refuge (SSBNWR) where eggs that failed to hatch were collected.  A total of 35 eggs collected from the four target bird species, and composite samples of forage fish, including killifish, topsmelt, and longjaw mudsucker were submitted for analysis.

All of the samples were screened for a variety of contaminants including: DDT, PCBs, PBDEs (a byproduct of flame retardant) as well as mercury and selenium.

This investigation was prompted by evidence of high rates of egg cracking, crushing and denting; and high mortality rates reported for day-old black skimmer chicks on the Salt Works.

Overall, eggshell thickness for all four bird species appear to be thinner than eggs produced by those species before the widespread use of DDT and other organochlorine compounds.  Eggs showing the highest levels of contaminants were those of the black skimmer which had concentration of DDTs, PCBs and PBDEs approaching levels associated with adverse impacts.

The investigation also found higher levels of contaminants in longjaw mudsuckers from the Otay River compared with longjaw mudsuckers and other forage fish species sampled from the Salt Works. 

This study is an important starting point in determining the overall health of the Salt Works portion of the SSBNWR.  Further scientific investigations, including more in-depth analysis of the Otay River and adjoining upland habitat, additional analyses of randomly collected eggs, and other types of studies will be needed to more specifically understand and characterize potential sources of organochlorine pesticides and PCBs in the Otay River and the Salt Works, and their potential effects on wildlife. 

The SSBNWR was dedicated in June 1999.  This 3,940 acre unit of the San Diego National Wildlife Refuge Complex was established to conserve and restore wetlands, intertidal mudflats and eelgrass beds that are utilized by thousands of migrating and resident shorebirds and waterfowl.

The Service’s Environmental Contaminants Division works to identify, minimize and remediate sources of pollution that may adversely impact the health of fish and wildlife species.  Pollutants that adversely impact our native fish and wildlife can have implications for the health of people.

Additional Information:

Selenium – A naturally occurring trace element, selenium can be found in meat, fish, nuts, and eggs and is a component of a balanced diet.  However, it can be toxic to humans and wildlife in higher concentrations resulting from its use in industrial processes such as mining, electronics, copper, aluminum and other metals, coal-burning power plants and metal recycling.

PBDE – Polybrominated diphenyl ether (PBDE) is a component of flame retardants that are used in many aspects of everyday living, including electronics, foam, and textiles.  PBDEs are now widely found in the environment where studies of wildlife have found PBDEs can affect neurodevelopment, and can be harmful to thyroid and liver functioning.

DDT – This was once heavily used as an insecticide; particularly against mosquitoes.  Over time, it became apparent that DDT was adversely impacting fish and other wildlife species further up the food chain.  Some species of birds such as bald eagles and brown pelicans were almost wiped out because DDT accumulated in their systems and caused the females to lay eggs with exceptionally thin shells that would crack during incubation.  Since the ban on DDT was implemented in the United States in 1973, populations of many of these affected wildlife species have rebounded.

PCBsPolychlorinated biphenyls (PCBs) refers to a class of organic compounds.  PCBs were once used in many common industrial and household products, including transformers, hydraulic fluids, paints, floor finishes, electronic components and sealants.  Use of PCBs was discontinued in the 1970s because of their high level of toxicity and persistence in the environment.


More on the Web:

Species Profile: California least tern

About the California least tern (for kids)


Contact Info: jane hendron, , jane_hendron@fws.gov
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