Sampling Times and Frequency
Sampling time describes when the samples are collected (e.g., season, day, hour). Sampling frequency is how often the site is revisited to collect samples (e.g., every 5 years). The sampling time selected should align with the timing of peak harvesting of the fish as well as reflect the life history of the fish and conditions in the water body. Sampling frequency is largely dependent on program resources as well as on the prioritization of waterbodies based on the type of contaminant present and the potential health risk to local fish and shellfish consumers.
The actual sampling time and frequency, as well as the rationale for their selection should be documented, and the final data report should include consideration of sampling period effects on the result if data are available.
Sampling Times
Sampling should largely be conducted during the period when the target species is most frequently harvested. Additional factors including local conditions, budget and staffing constraints, and the target species’ life histories can also impact sampling times.
Some stages in the target species’ life histories are not ideal for sampling because the tissue samples may not always be representative of the normal population. Sampling should be avoided during the spawning period including 1 month before and 1 month after due to natural differences in lipid content. Species specific information on spawning periods and other life history factors is widely available in numerous sources (e.g., Carlander, 1969; Emmett et al., 1991; Phillips, 1980). In addition, life history information is online at FishBase.
The most desirable sampling period for freshwater ecosystems is from late summer to early fall (i.e., August to October) (Phillips, 1980). During this period, the lipid content of many species (which represents an important reservoir for many organic pollutants) is generally highest and water levels are typically lower. If the legal harvest season of the target species is outside of the late summer to early fall window or if spawning takes place as this time, a different sampling time should be selected. For regions that have ice fishing, sampling during this season may be considered as well.
The most appropriate sampling time for estuarine and coastal ecosystems is during the period when most fish are caught and consumed (usually summer for recreational and subsistence fishers). For shellfish (bivalve mollusks and crustaceans), if the legal harvesting season is strictly controlled for fisheries resource management purposes, shellfish contaminant monitoring should be conducted during the legal harvest period. If harvesting is open year-round, monitoring should be conducted to correspond to the period when the majority of harvesting is conducted. Sampling staff may have to consider different sampling times for target shellfish species if differences in the commercial and recreational harvesting period exist.
Sampling Frequency
State and local authorities should base sampling frequency on assessment of risk to consumers of local fish and shellfish and on sampling capacity/program resources. Use best professional judgement to determine whether program resources should be applied to a waterbody that has been monitored in the past versus a waterbody that has never been monitored. Based on what is known of contaminant levels and use of waterbody by anglers, select waterbodies that have the potential to pose the greatest risk to fish or shellfish consumers.
A five-year sampling frequency may be adequate for waterbodies of concern. This cycle may be considered too frequent for some waterbodies where fish consumption advisories are based on mercury or PCB contaminant levels; in such cases, a ten-year sampling frequency may be sufficient. For new or emerging contaminants, sampling more frequently than 5 years (if resources are available) may be needed to understand the contaminant fate and transport and the risk to fish and shellfish consumers.
References
- Carlander, K.D. 1969. Handbook of Freshwater Fishes of the United States and Canada. 3rd Ed. Iowa State University Press, Ames, IA.
- Emmett, R.L., S.A. Hinton, S.L. Stone, and M.E. Monaco. 1991. Distribution and Abundance of Fishes and Invertebrates in West Coast Estuaries. Volume II:Life History Summaries. ELMR Report No. 8. Strategic Assessment Division, National Oceanic and Atmospheric Administration, Rockville, MD.
- Phillips, D.J.H. 1980. Quantitative Aquatic Biological Indicators. Pollution Monitoring Series. Applied Science Publishers Ltd, London, England.