tion to support exposure assessment. The two stages correspond to an 

 individual organism and edible tissue. Samples of individual organisms 

 may or may not be composited depending on specific study objectives 

 (see below, Kinds of Samples, Composite Sampling). Sampling strata 

 may include harvest areas, species, and size classes. Other sampling 

 strategies may be either too simple or inappropriate to meet the typical 

 objectives of exposure assessment studies. 



The timing of bioaccumulation surveys should be based on the tem- 

 poral distribution of harvest seasons and inherent biological cycles of 

 target species. The timing of harvest periods depends on the availability 

 of fishery resources, which may be influenced by the migratory patterns 

 and feeding cycles of target species. Biological cycles that influence an 

 organism's susceptibility to bioaccumulation should also be considered 

 when choosing a sampling period. The most important of these is the 

 reproductive cycle, which is discussed further below. In crustaceans 

 (e.g., crab and shrimp), the molting cycle also determines the potential 

 for bioaccumulation of toxic chemicals. The rate of uptake of con- 

 taminants by crustaceans is highest just after molting, before hardening 

 of the integument limits its permeability. 



The reproductive cycles of aquatic organisms may exert a major in- 

 fluence on tissue concentrations of many contaminants, especially 

 organic compounds (Phillips 1980). If a worst-case analysis is desired, 

 the target species should be sampled at a time during the harvest period 

 when tissue contaminant concentrations are expected to be at their 

 highest levels. In some species, contaminant content of edible tissues 

 may reach a seasonal maximum at or just before the peak of reproduc- 

 tive ripeness, before gametes or offspring are released. This may be 

 especially characteristic of species that are consumed whole (e.g., 

 clams and oysters). In other species (e.g., salmonids), lipid and as- 

 sociated contaminants may be mobilized and transferred from muscle 

 tissue to eggs before they are released. In such species, the peak of 

 contamination may occur in edible tissue (muscle) well before spawn- 

 ing. Because the time of sampling should be tailored to the reproduc- 

 tive characteristics of the target species, sampling periods may vary 

 among species. However, once a sampling period is chosen, it should 

 remain constant over time if an ongoing monitoring program is 

 planned. 



An alternative approach is to sample throughout the harvest season for 

 each target species. In this way, representative values can be obtained 

 for estimating means within sampling periods and for detecting 

 seasonal or long-term trends. In most cases, exposure assessments will 

 be performed over relatively short periods of time (e.g., a year), and 

 multiyear sampling may not be required. Within a harvest season, 

 however, sufficient samples should be collected to estimate the mean 

 concentrations of contaminants during the harvest period. To estimate 

 temporal variation or to obtain worst-case estimates, replicate samples 

 will be needed at several times within the harvest season. The frequency 

 of sampling should be related to the expected rate of change in tissue 

 concentrations of contaminants. For an extensive review of temporal 

 changes in bioaccumulation and body burdens of contaminants in 

 aquatic organisms, the reader should consult Phillips (1980). 



Time of Sampling 



43 



