Alternative Method of Removing Otoliths from Sturgeon

Summary

The goal of the protocol is to show an effective method to extract the otoliths from sturgeon carcasses.

Abstract

Extracting the otoliths (ear bones) from fish that have very thick skulls can be difficult and very time consuming. The common practice of making a transverse vertical incision on the top of the skull with a hand or electrical saw may damage the otolith if not performed correctly. Sturgeons (Acipenseridae) are one family in particular that have a very large and thick skull. A new laboratory method entering the brain cavity from the ventral side of the fish to expose the otoliths was easier than other otolith extraction methods found in the literature. Methods reviewed in the literature are designed for the field and are more efficient at processing large quantities of fish quickly. However, this new technique was designed to be more suited for a laboratory setting when time is not pressing and successful extraction from each specimen is critical. The success of finding and removing otoliths using this technique is very high and does not compromise the structure in any manner. This alternative technique is applicable to other similar fish species for extracting the otoliths.

Introduction

Sturgeon (Acipenseridae) populations throughout the world have been declining for over a century due to impacts including habitat loss, population fragmentation, and overfishing such that many populations are protected by state and federal laws^1,2. Management agencies throughout the USA and the world have identified sturgeon as a target species for restoration and recovery¹. Population characteristics (growth, recruitment, diet) of most sturgeon populations have been studied to better understand basic biology and life history characteristics¹. Due to the protected status of most sturgeon populations, age evaluation is difficult in small threatened populations where no sacrifice or harm to individuals is warranted.

The extraction of bony structures for the purpose of age estimation and growth analysis has been a common technique³.The use of fin ray sections to determine age of sturgeon has become a practical nonlethal method, but verification of the technique has been limited and age can be very difficult to decipher in older individuals (> 15 years)⁴. Researchers evaluating the use of multiple bony structures in sturgeon for their use in validating the age of Gulf Sturgeon (Acipenser oxyrinchus desoti)⁵, Atlantic Sturgeon (A. o. oxyrinchus)⁶, White Sturgeon (A. transmontanus)⁷, and Lake Sturgeon (A. fluvescens)⁸ concluded that the first marginal pectoral fin ray provided the most accurate age but concluded this invasive procedure to be deleterious to larger individuals.  Currently, there is the development of using otolith shape metrics to distinguish between North American sturgeon species with a very high rate of success⁹. Details on the process of removing the otolith with a dorsal transverse incision at the base of the skull is a common otolith extraction method found throughout the literature and clearly detailed by Secor et al. (1991)¹⁰. However, this method is difficult to implement on sturgeon that have very thick skulls with otoliths that are small and delicate and would tend to break. A further review of the literature shows no other detailed methodology of the otolith removal process for sturgeon. The objective of this paper was to detail an alternative method of extracting otoliths from sturgeon by entering the brain cavity from the ventral side.

Protocol

Important Note: Due to the protected status of all species of sturgeon in the United States, permits to acquire and handle fish were obtained prior to laboratory procedure. All specimens were acquired from hatchery sources post mortem due to natural causes and disposed of following standard procedure.

1. Specimen Preparation

1. Before any incisions lay sturgeon carcass on its side and using a measuring tape to measure total length from the snout to the tip of the tail to the nearest 1 mm. (Figure 1A)
2. Weigh sturgeon carcass on a tared scale to the nearest 1 g. Evaluate carcass for any external tags or marks and record.

2. Otolith Extraction

1. Make a vertical incision with a large fillet knife between the base of the skull and the first dorsal scute through the body to separate the head from the body (Figure 1B). After removing the head, turn the head so the mouth is facing up (Figure 1C).
2. Remove soft tissue of the mouth and gills by using the fillet knife to cut along the ventral side of the skull (Figure 1D). Make an incision with an electric bone saw from the tip of the snout to the base of the skull to completely bisect the head (Figure 1E). Separate the two halves by hand to expose the brain cavity (Figure 2A).
3. Remove and discard brain matter (Red box left) with forceps to view semicircular canals (Red box right) within the brain cavity (Figure 2B).
4. Remove semicircular canal cartilage from brain cavity with forceps (Figure 2C). Carefully puncture semicircular canal cartilage with the tip of the forceps and remove otolith (Figure 2D). Remove the otolith (hard bony structure inside the cartilage) with forceps from the semicircular canal cartilage (Figure 2E).
5. Repeat steps 2.3 - 2.4 to remove otolith from the other half of the skull.
6. Dip the otoliths in 92% Ethanol for 5 sec to remove any cartilage. Place otoliths in an open 25 ml scintillation vial to dry for 24 hr.

Results

The adult size of sturgeon across the world varies greatly but the location of the otoliths is consistent. The use of a sharp fillet knife was found to remove the soft tissue of the mouth easily shown in Figure 1D. An electric bone saw is the preferred tool for the lateral incision to expose the brain cavity in Figure 1E. However, attention to bisecting the midline of the skull is needed otherwise the otoliths may be crushed during this process. If this p...

Discussion

The overview of an alternative method of extracting otoliths from sturgeon carcasses has been detailed. It is important to note that special attention is needed to the lateral incision placement on the midline of skull in order to bisect the skull evenly to ensure no harm to the otoliths during dissection. If the incision is not deep enough to completely bisect the skull it will be very difficult to expose the brain cavity where the location of the otoliths is clearly visible. With experience, the overall technique is re...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Measuring Board			Metric meter stick is preferred
Weighing scale
Filleting Knife
Scalpel
Forceps
Sample vials			25 ml scintillation vials work well