The health of future fish populations depends on today’s responsible fishing practices. Catching fish when they are the right age is one of these important practices. The right age to harvest a fish is after it has had a chance to spawn. If fish are caught before they have a chance to reproduce, then fish populations will eventually decrease.
Scientists have learned that using only a fish’s size or length is not a good way to determine a fish's age. Why? Fish stop growing, like humans, at a certain point in their lives. Fish can also grow at different rates depending on the quantity and quality of food available to them, as well as from being exposed to poor or favorable conditions in the environment. When food is plentiful and highly nutritious, fish can grow faster and bigger. If food is scarce and poorer in quality, fish growth can be stunted. So how do scientists tell how old fish are? They look in the most unlikely of places—inside a fish’s ear.
How to Age a Fish
Inside a fish’s ear, tiny calcium carbonate stones called otoliths can be found. Otoliths enable a fish to detect sound and to help it balance as it swims. As a fish grows, new layers of calcium carbonate are deposited in layers, much like the growth patterns seen in tree rings. These layers can tell us a lot about the fish’s health and environmental conditions that it experienced during its life, but more importantly, these layers tell us the age of a fish.
During annual or biennial research surveys in Alaska or aboard fishing boats during the fishing seasons, scientists and fishery observers collect otoliths. Data collected from research surveys provides a general view of the entire fish population, whereas fishery observer data tells us more specific information about fish targeted by a fishery. Both sources of data are important to seeing what is happening in the ecosystem.
Otoliths are found near the brain, and most are extracted by cutting halfway through the middle of a fish’s head to reveal two small chambers each containing an otolith. After the otoliths are collected they are brought back to the laboratory where they will be read using microscopes.
Why Are Age Data Important to Sustainable Fisheries?
“Age data help us determine the health of commercial fish populations. For instance, if there is a greater range of ages represented in a fish population and an abundance of mature fish, we know the stock is better able to generate more fish to offset what is taken out by fishing. Fishery managers will be able to set higher quotas for a stock like this” says Tom Helser, Age and Growth Program, Alaska Fisheries Science Center.
Age data tells scientists how old fish get, and can be used to determine the health of a fish population. The following questions can be answered with age data:
- How many fish are at a certain age (age distribution of a population)
- The age of the youngest fish seen in a fishery (recruitment)
- The age of fish caught by the fishery (age-specific exploitation rates)
- The age of fish when they reproduce (maturity at age)
- The age distribution of a fish population
- If fish grow differently when something in its environment changes
“An otolith is the ‘black box’ of information for a fish and its environment. Why? The chemical composition of each new layer can indicate things like water temperature, oceanographic events, water chemistry, and a fish’s previous locations” says Craig Kastelle, Age and Growth Program, Alaska Fisheries Science Center.
Using age data, scientists have learned that some fish species grow fast and have short life spans. For instance, walleye pollock can live to about 14 years, and they first spawn at around age 3 or 4. In contrast, rockfish grow slow and have long lifespans. For example, shortraker rockfish can live to be over 100 years old and are around 30 years old when they first spawn. Fishery managers must account for these differences when they set fishery quotas for species.
Nature’s Data Recorders
Conditions in the environment are constantly changing, and some fish populations do better or worse than others in dealing with these changes. Using annual age data collected from otoliths plus environmental and prey availability data, scientists can tell if a certain year class (age group) of a fish’s population is doing better than another and if that year class is increasing or decreasing in number. For example, in years when the water is cold and lipid-rich zooplankton prey are plentiful, young walleye pollock are robust with ample fat reserves and survive the winter well, but in years when the water is warm and different zooplankton dominate the ecosystem that are less lipid-rich, young pollock juveniles may be frail with meager fat reserves and subsequently are in poor shape for surviving the winter. By identifying these environmental cues and how various age fish respond, fishery scientists could potentially forecast future fish populations, giving fishery managers and fishermen a chance to adapt and plan for increases or decreases in fish abundance.
Managing our Nation’s fisheries is a tough job that takes a lot of data to ensure we manage responsibly. Our first story described how biological data are an important cornerstone to determining the health of fish populations. In this story, we learn that determining the age of a fish takes multiple steps and the data from a fish’s ear stone can tell us more than just a fish’s age.