Salmonid Field Protocols Handbook: Techniques for Assessing Status and Trends in Salmon and Trout

Carcass Counts

Bruce Crawford, Thaddeus R. Mosey, and David H. Johnson

doi: https://doi.org/10.47886/9781888569926.ch6

Most adult anadromous salmon die shortly after returning from the sea to spawn in their natal streams. Their bodies contribute significantly to the nutrients needed by the next generation of salmonids (Wipfli et al. 2003), a wide array of wildlife species (Cederholm et al. 2001), and the larger watershed ecosystems in which they play an integral part (Wipfli et al. 1999; Bilby et al. 2001; Stockner 2003) (see Figure 1). This protocol deals with the counting and biological sampling of recently spawned anadromous salmonid carcasses. We have drawn heavily from Heindl (1989) in the development of this protocol.

Salmon carcasses provide important information to scientists, including scales, tissue samples, length measurements, and population sex composition data. Scales and otoliths are used to determine age and offer insights into the population’s age characteristics. Fish lengths (i.e., length from postorbital to hypural plate—POHL) provide length–frequency information. Although snout to fork (fork length) is one measurement option, POHL is more useful because the tails of spawners, especially females, are frequently so worn that the fork is difficult or impossible to locate. It is also important to use POHL to avoid length distortion caused by jaw development (kype). Some scientists have taken postorbital to fork length, which we consider less useful because of the need for subsequent conversion to either POHL or fork length (S. Young, Washington Department of Fish and Wildlife, personal communication). Marks (fin clips and/or tags) provide key information on the fish’s origins. A clipped adipose fin indicates a hatchery-origin fish, which many also have coded wire tags (CWT) implanted in their snouts. In the field, a coded wire wand or detector can be used to determine the presence of a CWT in the snout of the fish; without field detection, it is recommended to cut off the snout to assess the CWT information at a later point. Other types of markers and tags as well as radio telemetry units may be collected from carcasses where specific studies are involved. In addition to scales, tissue samples from hard body parts (e.g., fin rays, vertebrae, otoliths) may be collected for age determination. Soft tissue samples (e.g., liver, heart, eyes) may be collected for genetic analysis. Field staff use both foot and boat surveys to locate carcasses.

Combined with redd counts and live fish counts, carcass counts can be used to assess escapement as well as carcass-derived nutrient contributions to the ecosystem. Although data from this protocol can be used to support other assessments, this document does not focus on counts of salmonids that have undergone mass die-offs due to other causes (e.g., water temperature or oxygen level issues, strandings, poisons).