As with tree rings, which add a layer each growing season, fish scales have annual rings. Year marks, called "annuli", can be detected on many scales by skilled scale-readers. Annuli occur where circuli are clustered together. The spacing between annuli gives an indication of the particular conditions the fish encountered during a period of growth. For this reason, it was once thought that growth records as shown in scales would reveal how environmental conditions have changed over time. But it is now clear that fish scale rings are unsatisfactory indicators of climate change. First, so many factors influence ring width (e.g., food availability, fish migration, varying growth responses to temperature) that it is nearly impossible to attribute changes solely to climate change. In addition, individual scales represent relatively brief amounts of time; they generally show years rather than decades or centuries of growth. Therefore, any given scale shows scientists only a partial picture of changing environmental conditions. Collections of many scales, such as those that settle to the seafloor over decades or centuries, are far more useful.
A fish keeps most of its scales for its entire life, but some scales are lost and replaced. Discarded scales drift downward and accumulate in sediments. These collections of scales serve as records of environmental change. By identifying the origin and number of scales in the layers of a sediment core, scientists can infer which fishes lived in a specific area at particular times. Changes in fish populations can be very revealing because different species of fish prefer particular ranges of water temperature in the same way many land animals prefer cooler or warmer habitats. Shifts in scale deposits suggest changing conditions. Off the Southern California coast, for instance, sediments deposited in colder times tend to have greater abundances of anchovy scales.
Recently, some scientists have used fishes' otoliths (ear bones) rather than scales to search for indications of changing environmental conditions. Fish develop daily as well as annual layers in their otoliths, so layers in fossil otoliths can be used to obtain a detailed development record. (It is possible to see otolith layers under a dissecting microscope, particularly in still partially-transparent otoliths from young fish.)
b. Many living things respond to climatic conditions. Some crops,
such as mangos, pineapples, and bananas, grow best in warmer conditions.
Other crops, including cabbage, potatos, and squash, thrive in cooler conditions.
Check newspaper ads, visit local supermarkets, or interview a greengrocer
to obtain the prices of selected produce over the course of a year. How
and why does the abundance of fruits and vegetables fluctuate in response
to seasonal and other conditions?
b. Place a scale under the dissecting microscope and look for the following features: circuli, annuli, radii, focus.
c. Draw and label an illustration of the scale that you see under the dissecting microscope.
d. Look for a scale that has very large growth increments. This might be a replacement scale - a scale which grew very quickly to replace a lost scale.
e. On the diagram above, measure the distance between the annuli. The circuli are not shown, to make the diagram clearer. Measure distances by drawing a line from the focus to the outside of the scale and measuring the width of each increment (i.e., each gap between annuli). Given the number of annuli, calculate the approximate age of the fish. Speculate as to the conditions the fish might have encountered in each year of its life.
The shape of this scale suggests that it comes from a fish that grows
best when the water is warmer. Assume the fish was caught in 1995, when
the average water temperature in this fish's habitat was 11°C. Also
assume that one degree of temperature leads to 1 mm of growth difference.
What temperature conditions might the fish have encountered in earlier
years? The outer ring is 6 mm wide, so the average temperature was 11°
C. The next ring is 3 mm wide, so the average temperature was 8° C,
and so forth.
b. If a fish grows throughout its life, each increment can be compared to any other. How would the ring widths appear if growth slowed as the fish aged?
c. Scales' orientation and the irregularity of their rings affect
measurement. Calculating ring width, for instance, depends on scales' position
and the direction of measurements made. What standard means of measurement
can students devise so that comparisons among scales are uniform? Devise
a standard way in which to measure the scales for comparison between scales.