Chemical analysis can be used to tell spatial history
Knowledge of fish movement can be important — in some cases essential — for their management. Differences in water chemistry can offer conclusive evidence of where young stages of fish were reared.
History of fish movement studies
For decades, fish were marked with some type of visible mark or a tag. Recapture at a later time provided information about fish movement. This approach provided a lot of information that is still useful today, but it also provided some “misinformation” because it does not tell you where a fish was between the time it was marked and the time it was recaptured. Recaptures may occur years later, and fish are only recaptured where the biologist is actively sampling.
Telemetry — detecting the location of a fish from signals emitted from sonic (sound) or radio (radio-frequency) tags attached to or implanted into a fish — has been used to monitor fish movement and habitat use since the 1970s. Telemetry allows a fish to be tracked continuously and in real time. Sample sizes are usually small, because tags are expensive and implantation and tracking is time consuming; however, technological advances allow detection of tagged fish passing fixed-location receivers.
Natural chemical signatures
Telemetry is restricted to larger fish, so the size and weight of the tag does not interfere with movement. Although advances in electronic and battery technology are allowing production of smaller tags, it still is not possible to tag very young fish.
Solution: use a natural tag. The chemical make up of a calcified (bony) structure that is present and grows throughout the life of the fish reflects the chemistry of the water in which they lived at the time that part of the bony structure was deposited. The otoliths — ear bones — are most commonly used, but fin spines also have been used. These structures contain annual growth marks called annuli and are also used to age fish.
Tools are available that can obtain a microscopic sample at a specific location on a spine or otolith. Chemical analysis of the sample reveals the chemical environment at that point in the fish’s life. For example, analysis of a sample collected near the center of an otolith would describe the chemical environment of the fish during its first few months of life; a sample collected just beyond the fourth annual mark would reveal the chemical environment during the fish’s fourth year of life.
Examples of questions answered
Walleye in the Missouri River. The middle reaches of the Missouri River in North and South Dakota are impounded by a series of hydroelectric dams to create four large mainstem reservoirs. Walleye are the principal sportfish, and walleye migrate up tributary creeks to spawn. Chemical analysis of otoliths collected from adult walleye revealed most young walleye were produced in only a few tributaries. The same chemical analyses also revealed whether walleye moved upstream or downstream within reservoirs and that substantial movement of walleye occurred through the dams to add to the walleye population in the downstream reservoir. This entrainment increased during a record flood in 2011. This information can be used to identify tributary habitats to conserve, tributary habitats to improve to increase spawning, and to account for changes in walleye abundance among the four reservoirs.
Spotted bass in the Ohio River. Otolith chemistry revealed that most spotted bass spawned in the tributary streams remained in them and most spotted bass spawned in Ohio River embayments remained in the Ohio River. Spotted bass in the Ohio River grew faster and attained larger size than those in tributaries. These results suggest spotted bass in tributaries should be managed differently from those in the Ohio River.
Sturgeon in the Missouri and Mississippi rivers. Shovelnose and pallid sturgeon are native to the Missouri and Mississippi rivers. Declining numbers have made these species a significant conservation concern. Fin ray chemistry demonstrated that about one-third of sturgeon collected in the middle Mississippi River and all fish collected in the lower Missouri River were spawned in the Missouri River. A substantial portion of sturgeon in both rivers was spawned in an upstream reach of the Missouri River suggesting the importance of this reach for sturgeon recruitment.
Obviously, movement of fish and the origin of fish can only be detected if different habitats have different chemical signatures. While this probably would not be the case in small, confined waters, chemical differences are common in larger systems and where geology and soil types are variable. Anthropogenic chemicals, such as mercury, can also be useful to detect habitats used by fish.
Nevertheless, the ability to assess the environmental history of a fish is a tool that can provide important information used to better manage fisheries, such as resolving the debate about whether flathead catfish remain in relatively small sections of a river or make extensive movements into tributary streams to spawn.