Nonlethal Sampling of Fish Caudal Fins Yields Valuable Stable Isotope Data for Threatened and Endangered Fishes

Abstract

Fin clipping is gaining momentum for stable isotope analysis in fish as a nonlethal alternative to lethally collecting dorsal white muscle tissue. The main advantage of fin clipping is the elimination of lethal collection, which thus increases the potential for stable isotope research involving threatened or endangered species. To make comparisons across studies using different methods, the degree to which fin clips and dorsal muscle are correlated for both δ¹⁵N and δ¹³C across time and space must be quantified. We evaluated the efficacy of using caudal fin clips by comparing tissues collected across time (2003-2005), space (21 streams), and body size (fork length, 40-140 mm) for juvenile Chinook salmon Oncorhynchus tshawytscha and O. mykiss (rainbow trout and steelhead [anadromous rainbow trout]). We also addressed several analytical and sampling issues to assess the potential benefits and limitations of using fin clips for δ¹⁵N and δ¹³C determination. We found that the relationship between fin and muscle tissue was consistent across years and streams. On average, δ¹⁵N was lower in fin tissue than in muscle tissue, whereas δ¹³C was greater in fin tissue. Although the magnitude of the difference between fin and muscle tissue is lower than the thresholds typically used to define trophic level increases, the biological significance of these differences is context dependent. Power analyses indicated that fin and muscle tissue were equal in their ability to detect differences in δ¹⁵N and δ¹³C in juvenile fish and that larger sample sizes are needed to statistically quantify these differences. Finally, the use of fin clips is not a viable option for smaller fish (<50 mm) because the majority of the caudal fin is necessary for analysis. Nonetheless, fin clipping does represent a promising nonlethal sampling technique for quantifying δ¹⁵N and δ¹³C in juvenile salmonids.