Dolphin Dental Pathology

Marine mammal strandings have shown a marked increase in the Southwest of the UK over the last decade. The Cornwall Wildlife Trust Strandings Network received over 72 reports of stranded cetaceans in Cornwall alone over the first three months of 2006. The carcass of a female common dolphin from a recent stranding at Seaton in the Southwest of the UK showed evidence of entanglement in fishing gear on at least one occasion previous to the animals’ death (Figures 1 and 2). Along with recent cut marks on the beak and fins, scarring was found on the fins, beak and head which were consistent with a previous entanglement. Due to the length of time between the death of the animal and it stranding (at least 10 days), differentiating old injuries from new was not instantly straightforward due to the effects of decay and scavengers etc.

Figure 1.a Healed injury on inside of beak showing teeth repositioned in gum and deformation of beak tip 1.b Deformation of the beak and lower jaw (anterior dorsal view)

(Link to stranding report for the Seaton dolphin)

However, a dental examination of the carcass showed that the fractured teeth were either sharp and jagged (recent injury), or worn smooth (old injury), giving concise evidence of repeat capture. Net entanglement has serious implications in issues such as animal welfare, as it implies that wild cetaceans are not only sustaining and surviving serious injuries, they may also be repeatedly feeding from fishing gear. The bite mark or closed contact point between the remaining beak and lower jaw shows that this animal has been living with this injury for a considerable length of time, a finding confirmed by the dental examination.

2. Materials and methods

Brief examination of the carcass

Age of carcass - approximately 10 days post-mortem

Condition cut marks from a recent injury and over 15 cm of the beak was absent, though the injury had healed. Injury sustained to the skin covering the beak and lower jaw. Healed injury on inside of beak showing teeth repositioned in gum and deformation of beak tip; scarring consistent with previous net entanglement was also found on the flipper, along with an injury on the head below the left eye. This animal had obviously suffered serious trauma in the past (possibly from net entanglement) from which it survived until it was caught again, this time with fatal consequences. The lower jaw was removed from the carcass and examples of teeth showing recent and old injuries were extracted and cleaned with ethanol and a fine brush, then left to dry naturally. The teeth were mounted on a specimen stub using a carbon tab, and coated with c. 8 nm of gold in an Emitech K 550 sputter coater (working at approximately 5 x 10-6 Torr). The processed specimens were investigated and photographed using a JEOL JSM 5600 scanning electron microscope operated at 15 kv, and a 15 mm working distance.

3. Results

The position of the broken teeth in Figure 2 shows that most of the damage was caused to the middle teeth in the right lower jaw and the middle teeth in the left upper jaw, a finding consistent with the animal twisting to the left whilst pulling back from a tangle net or other ensnaring fishing device.

Figure 2. Position of the severely broken teeth (most of the teeth showed some evidence of old damage).

Figure 3.a presents a SEM image of tooth number 149 side view (jagged edge indicates a recent injury), whilst Figure 3.b presents a top view of the tooth (rough surface also indicates a recent injury).

Figure 3.a Tooth number 149 side view (jagged edge indicates a recent injury). 3.b Top view (rough surface also indicates a recent injury)

Figure 4. Tooth number 160 (rough surface indicating a recent injury)

Figure 5. Tooth number 166. The damaged area has been worn smooth indicating an old injury

Figure 6. Tooth number 172 presenting with a vertical fracture and missing section though the damaged area has been worn smooth (similar damage continued to Tooth number 181)

4. Discussion

Evidence of repeated entanglement has serious implications in issues of animal welfare, as it implies that wild animals are not only sustaining serious injuries caused by net entanglement, they may also be repeatedly feeding from fishing gear, indicating the possible need for restrictions in the use of tangle and gill nets during certain times of the year. In addition, disabilities that may reduce the dolphins’ ability to catch food (e.g. an old injury or deafness) may give the animals little choice other than to feed from easy sources such as a static tangle net. Figure 7 is a Tooth from a second bycaught D. delphis (similar injury to tooth number 172 in Figure 5 but with the fractured section in situe), presenting with the enamel chipped and a vertical fracture running over halfway down the body of the tooth. The chipped enamel and underlying dentin is rough and the fractured section is still in place, indicating that the tooth has not been used since the injury was sustained, as the animal was most probably dead.

Figure 7. Tooth from a second bycaught D. delphis (Link to Fort Bovisand) (similar injury to tooth number 172 in Figure 6), presenting with the enamel chipped and a vertical fracture running over halfway down the body of the tooth. The chipped enamel and underlying dentin is rough and the fractured section is still in place, showing that the tooth has not been used since the injury was sustained

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