Atlantic salmon are produced in two very different ways and reared (raised) under several distinctly different conditions. For instance, some are the result of natural reproduction between adults that have returned to Maine's rivers to spawn (right). These individuals, along with those that have been stocked as fry as part of the conservation hatchery program, are reared in a natural environment (i.e. naturally reared), spending all or most of their juvenile life in Maine's rivers.
Others are produced under Maine's conservation hatchery program (right) from river-specific broodstock (the adults from which gametes are obtained for production). Still others are produced by Maine's commercial aquaculture industry from broodstock that, until recently, could be of any origin, including European. Both are reared in an artificial environment for at least a portion of their life history.
Although a portion of hatchery reared parr are given identifiable fin clips and many smolts are marked with Visual Implant Elastomer (VIE) tags specific to stocking date and site, not all are marked and these marks can be lost over time (retention of VIE tags is only 50%). Additionally, stocked fry are not administered a distinguishing mark or clip prior to release.
Examples of different VIE tags.
Luckily, all Atlantic salmon have natural marks that can also inform fishery scientists and managers about their background. For instance, the parasitic load of an individual and the isotopic signature of its otoliths (the inner ear bones) correspond to its region of origin. And its unique genetic makeup provides a natural identifier as well. Because a fish's fins can become deformed when constantly in contact with the concrete hatchery environment, fin deformities provide an indication of what type of rearing conditions it experienced. Scales are particularly informative, providing a natural record of age, rearing and growth-related information. Although each of these natural marks have limitations (some require lethal sampling or currently unavailable baseline data), they are all useful in learning something about the identity of individuals captured in Maine's rivers or at sea.
When NEST researchers capture an unmarked fish, it is important to identify its origin and the conditions under which it was reared to answer questions such as:
- How much are stocked fry, parr and smolts contributing to adult returns?
- Do fish reared under natural conditions experience less mortality?
- Which rearing and stocking strategies (e.g. location and timing of release) under the conservation hatchery program are best?
- Are there aquaculture escapees present in the population?
- What is the age structure of the population?
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Scale pattern analysis is one technique NEST uses to address these questions. Much like the rings of a tree, the number & pattern of rings on a scale can tell a story about a fish's history.
For instance, counting a scale's annuli, identified by patterns of widely and narrowly spaced circuli (rings), enables NEST researchers to effectively age Atlantic salmon captured in the wild. Knowing the age structure of populations is an important aspect of managing any species.
Scales can also tell us about a fish's growth and origin. Using computer image analysis and statistics software, the patterns found on a captured fish's scales can be analyzed and compared to known scale patterns from hatchery and naturally reared specimens. This allows researchers to determine what kind of rearing conditions an individual likely experienced.
Taking a cursory look at the scale pictured on the left, you may note that its circuli are all relatively evenly spaced, while the spacing between the circuli of the scale on the right is more uneven. Evenly spaced circuli indicate that growth was consistent—and that food was consistently available. Since hatchery fish are fed a regimented diet, you would predict that this particular individual was reared in a hatchery. These predictions can be further verified by observed dorsal fin condition/deformities (below) or other distinguishing marks.
Advancements in scale pattern analysis techniques may soon allow NEST to better understand the finer details of a captured fish’s history—for instance, if a naturally reared individual was stocked as a fry or spawned by naturally reproducing adults; or if a hatchery reared individual was stocked in the fall as a parr or in the spring as a smolt; and which adults make it back to Maine rivers to spawn. Scale pattern analysis, in conjunction with a variety of other tools, continues to play an important role informing NEST’s scientists and managers about the status of Atlantic salmon and success of recovery efforts. |
natural reproduction
hatchery production
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Atlantic salmon reared in hatchery tanks (bottom) live under very different conditions than those experienced by naturally reared individuals (top). |
Observed dorsal fin condition and deformities are noted.
Much like the rings of a tree, the number & pattern of rings on this scale can provide information about the fish's age, origin, & growth...
Cross section of a tree showing growth rings.
An Atlantic salmon scale.
 NEST researcher, Ruth Haas-Castro, analyzes an Atlantic salmon scale using computer software.
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hatchery suspect |
naturally reared |
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