When adult salmon start returning to the freshwater tributaries of Cook Inlet its cause for excitement. Fishermen are ready to get their hands on some “chrome”: ocean-bright salmon. Bears and other wildlife are also ready to stock up for the winter. Meanwhile, our staff begin the next phase of field work—adult salmon sampling and counting.
By mid-July, our seasonal fisheries crews switch gears from counting smolt to enumerating adult salmon. This is when we collect biological information on the fish that allows biologists to tell a story about the population we observe.
Sampling for age, sex, and length
We generally refer to adult salmon sampling as “ASL,” short for age, sex, and length.
We can tell the age by collecting any of the fish’s bony structure—usually scales or otoliths. These are read by a trained technician.
We determine sex by looking at the external features of the fish. Female salmon tend to be more football shaped with a bulging belly, whereas males get a humped back and a distinctive large, hook shape on the lower jaw, called a “kype.” We also measure salmon from the middle of their eye to the fork of their tail. This measurement avoids including the large kypes.
Length, of course, is easy. All you need is a ruler or tape measure.
Scales versus otoliths
Salmon scales protect the fish from dangers in their environment like predators and pathogens. They also help the fish glide through the water. In climates where the seasons change, fish grow more during the warmer summer months and less during the colder winter.
Because of these seasonal changes, as the fish grows, the scales form rings similar to the rings of a tree, which allows us to read them.
Let’s say we find that a salmon is four years old; that age often noted as 1.3 or 2.2 with the first number noting how much time they spent in the freshwater and the second number noting its time in the ocean.
A 2.2 fish spent two years in freshwater before moving out to the ocean and returning after two years, and a 1.3 fish spent one year in freshwater and three years in the ocean before returning to spawn. The growth rings from fresh water are smaller than those formed in the ocean because freshwater ecosystems are generally less productive than the ocean.
Otoliths are hard bony structures in the network of tubes attached to the salmon’s ears. When we collect otolith samples, we have to sacrifice salmon. This raises the question: Why would resource managers want to kill salmon to determine their age?
This happens when projects assess a mixed stock fishery that contains both naturally and hatchery produced fish—and we need to determine the percentage of hatchery to wild fish. Nearly all hatchery salmon released in Alaska have a unique mark in order to identify what hatchery they come from.
That unique mark is actually found on their otolith! Otoliths develop these marks by changing the water temperature or by dry marking, which lets the eggs go almost dry for a period of time. These changes create “artificial seasons” early in the life stage that stand out when we compare the otoliths to other hatchery or naturally spawned fish.
To read a scale sample, we first need to press the scales onto a sheet of thin, clear acetate that is read on a microfiche, a machine similar to a slide projector. To read otoliths, trained technicians must first mount them on a glass slide, then grind them down. Once this happens, those technicians can use a microscope to read the otoliths.
Collecting the samples
We conduct adult salmon sampling at our weirs, which are structures used to divert fish into a holding area or through a specific gap where we can more easily see each fish. Once a certain number of fish have passed through the weir, the crew prepares for sampling.
At this point, the crew closes the weir, and opens a live box where fish can move in and be sampled. To do this, the crew catches the fish in a net and then places it into a padded measuring board. The crew collects 3–4 scales and puts them on a labeled scale card. Before the fish is released, the sex is determined. When otolith samples are collected, the fish is first humanely dispatched so the otolith can be collected from the cranium.
We use all of the ASL data we collected from a representative proportion of a given salmon population to describe the overall population demographics of that group of fish. By knowing the size and ages of the salmon we are counting, biologists are able to tell the freshwater and saltwater rearing conditions for that particular group as well as make comparisons between naturally and hatchery produced fish. All the data the field crews collect are available for review in our year end project reports.
Go deeper
Check out some project reports that summarize the data field crews collect.
