Helping Local Agencies in Guam and Saipan Understand the Life Histories of Their Fishes

June 14, 2016

By Dr. Allen H. Andrews

Research |

Pacific Islands

Understanding the age of a fish helps managers understand the population dynamics of a fish stock. For example, at what age is a fish species able to reproduce? Is its natural life span long or short? These basic facts are often poorly understood and all contribute to understanding the life history—the characteristics that make up the biological life cycle—of various fishes, from birth to reproduction to death.

Estimating the age of fish usually relies on examining the hard body parts that grow as the fish grows. The most commonly used of which are otoliths. Otoliths are stony, calcium carbonate structures that are functionally analogous to parts of the human ear. These “ear stones” help fish live in a three-dimensional environment by providing balance, orientation, and sound detection. They’re found in the skull next to the brain, and they lay down growth rings (akin to tree rings) as the fish grows older.

All fish otoliths are not created equal.

The size of a fish has little bearing on the size or structure of its otolith. Some fish can have regularly shaped otoliths, while some may have numerously ridged or oddly protruded otoliths. Some can be quite small relative to the fish’s size (marlin have some of the smallest otoliths), while otoliths of smaller fishes can be massive and look like a small white rock. And though otolith rings are analogous to tree rings, they’re not as regular as we would like them to be—they can be influenced by a variety of environmental conditions, such as water chemistry, temperature, or stress levels.

Otolith cross section from a deep-water snapper called "onaga" (Etelis coruscans). Age interpretation is quite difficult, but the regular banding pattern is there for counting with proper illumination under a microscope. This onaga was estimated to have lived for 53 years.

Using otoliths to estimate fish age is not new but it can still be technically challenging. To help fisheries life-history scientists in Guam and Saipan better understand the techniques involved in this process, I held two otolith age estimation workshops—functioning in my capacity as a life history scientist with NOAA's Pacific Islands Fisheries Science Center in Honolulu, Hawaiʻi—using regional fishes as test subjects. Through the workshops, we were able to share technology and skills that focus on using otoliths as a means of describing the age and growth of each fish species.

Step One: Preparation

Building new skills isn’t just about getting in the lab and doing the work; the preparations are especially important. As the cliché says, “Proper Planning and Preparation Prevents Poor Performance.” Therefore, we began with an assessment of the equipment and supplies available to do the work, along with some instructions on how the equipment is used and maintained. The primary pieces of necessary equipment were:

Low-speed saw for otolith sectioning
Lapping wheel used for polishing otolith sections
A dissecting microscope with both transmitted and reflected light for viewing and interpreting proper sectioning, as well as the growth zones visible in good otolith sections

Step Two: Otolith Selection and Orientation

Properly selecting and orienting otoliths for sectioning is an important skill. We examined whole otoliths of various species to understand the variations between species and how to mount them for the best chance at getting the necessary age data. Proper alignment of the otolith relative to the sectioning blades is crucial — the blades must be perpendicular to the ridges associated with the sulcus acusticus (a groove on the proximal side of the whole otolith). These ridges tend to have the best-defined growth zones for age estimation and the section angle can make the difference between a readable and unreadable section. The workshop participants each practiced sectioning otoliths from various species from each region. Eric Cruz (PIFSC-Guam) and Jane Boyer (University of Guam) prepared a range of onaga (Etelis coruscans) otoliths ahead of time, which the participants used for age estimation while waiting for their mounted workshop otoliths to harden. I critiqued the work of each participant during the course of the day, and by the end of the day the participants had all successfully mounted otolith sections, along with a select set of whole otoliths for sectioning later.

Step Three: Otolith Section Interpretation

To determine the age of their otoliths, the participants microscopically examined their sections from the day before. As they did this, I displayed example images of good and poor onaga otolith sections. We discussed, at length, what factors made the sections good or poor, what could have been done to prevent poor sectioning, and the various length-at-age scenarios we saw. Participants learned that the better the section, the more objective decisions they could make on what to count and what not to count. In addition, we focused a considerable amount of time on discriminating young (fast) growth from older (slow) growth structure in the older adult fish otoliths. Understanding the nuances of otolith age-reading requires practice and a keen eye, and it does not come easily.

In Guam, our emphasis was on the onaga otoliths, with discussions on the best species to pursue in the near future. In Saipan, we investigated two emperor species (Family Lethrinidae). The Saipan team described the defining characteristics of growth zones and found a common pattern among the samples of the orange-striped emperor (Lethrinus obsoletus). We preserved sectioned samples (good and poor) to have a lasting example of the defining characteristics for the continued efforts to age this species.

During the week in Saipan, the local media highlighted the workshops I provided, as well as my additional seminars on the longevity of fishes. For further information, please see:

1) Article titled “Age Matters Even for Fish” in the Marianas Variety newspaper at: http://www.mvariety.com/cnmi/cnmi-news/local/81577-age-matters-even-for-fish

2) A televised interview with Tori Youngblood of KSPN2 News (minute 11:00): http://www.saipantv.com/video.asp?vidID=1232&secID=36

I would like to thank the participants for their time and attention during each week. I look forward to continuing to work with them to build on their new skills and to further investigate the life history of regional fishes.