What caused the UK's largest common dolphin (Delphinus delphis) mass stranding event?

Abstract

On 9 June 2008, the UK's largest mass stranding event (MSE) of short-beaked common dolphins (Delphinus delphis) occurred in Falmouth Bay, Cornwall. At least 26 dolphins died, and a similar number was refloated/herded back to sea. On necropsy, all dolphins were in good nutritive status with empty stomachs and no evidence of known infectious disease or acute physical injury. Auditory tissues were grossly normal (26/26) but had microscopic haemorrhages (5/5) and mild otitis media (1/5) in the freshest cases. Five lactating adult dolphins, one immature male, and one immature female tested were free of harmful algal toxins and had low chemical pollutant levels. Pathological evidence of mud/seawater inhalation (11/26), local tide cycle, and the relative lack of renal myoglobinuria (26/26) suggested MSE onset on a rising tide between 06:30 and 08∶21 hrs (9 June). Potential causes excluded or considered highly unlikely included infectious disease, gas/fat embolism, boat strike, by-catch, predator attack, foraging unusually close to shore, chemical or algal toxin exposure, abnormal weather/climatic conditions, and high-intensity acoustic inputs from seismic airgun arrays or natural sources (e.g., earthquakes). International naval exercises did occur in close proximity to the MSE with the most intense part of the exercises (including mid-frequency sonars) occurring four days before the MSE and resuming with helicopter exercises on the morning of the MSE. The MSE may therefore have been a "two-stage process" where a group of normally pelagic dolphins entered Falmouth Bay and, after 3-4 days in/around the Bay, a second acoustic/disturbance event occurred causing them to strand en masse. This spatial and temporal association with the MSE, previous associations between naval activities and cetacean MSEs, and an absence of other identifiable factors known to cause cetacean MSEs, indicates naval activity to be the most probable cause of the Falmouth Bay MSE.

Figure 1. Locations of live sightings/strandings and dead/euthanized common dolphins in Falmouth Bay, Cornwall on 9 June 2008 (inset map shows location of Falmouth Bay in UK).

Red box show location of 25/26 dead and necropsied animals found in Porth Creek, Cornwall (see Figure 2). Data sourced from Cornwall Wildlife Trust Marine Strandings Network.

Figure 2. Locations of live sightings/strandings and dead/euthanized common dolphins found in Porth Creek, Percuil River, Cornwall on 9 June 2008.

Solid line  =  high water mark. Dotted line  =  low water mark. Data sourced from Cornwall Wildlife Trust Marine Strandings Network.

Figure 3. All common dolphin strandings (live and dead) in Cornwall (by week) Jan-Dec 2008 (Data: CWT Marine Strandings Network).

Figure 4. Gross pathology (lung) and anti-fibrinogen immuno-histochemistry (IHC) of cardiac and skeletal muscle and anti-myoglobin IHC of kidney.

A (lung) – watery fluid and small clumps of mud in bronchi (gross); B (lung) – copious muddy substrate in bronchi (gross); C – acute myocardial degeneration – cardiac muscle (anti-fibrinogen IHC); Bar  = 50 µm; D – rhabdomyolysis – skeletal muscle (anti-fibrinogen IHC) Bar  = 50 µm; E – live stranded common dolphin from MSE negative for myoglobinuria (anti-myoglobin IHC) Bar  = 50 µm; F – myoglobinuria in live stranded striped dolphin from Canary Islands, Spain (positive control – anti-myoglobin (IHC); Bar  = 50 µm.

Figure 5. Histopathology of the tympanoperiotic complex.

A – congestion of posterior spiral vein and venule (ref: SW2008/94.14) Bar  = 200 µm; B – perilymph deposits, vascular congestion and aggregates of erythrocytes surrounded by a fine network of fibroblasts (ref: SW2008/10) Bar  = 200 µm; C – anterior canal of semicircular ducts filled with blood (ref: SW2008/94.2) Bar  = 200 µm; D – anterior canal of semi-lunar ducts filled with blood (ref: SW2008/94.10) Bar  = 200 µm.

Figure 6. Mean summed 25CBs levels in UK-stranded adult female common dolphins from 1990–1992 (n = 8) and from the MSE in 2008 (n = 6). Error bar = 2 S.E.

Figure 7. Distribution of high-intensity acoustic activities nearest to stranding location.

Figure 8. Temporal distribution (5-9 June 2008) of naval acoustic activities and possible bottlenose dolphin (Tursiops truncatus) sighting in western part of South Coast Exercise Area.

Figure 9. Tidal cycle at Froe (Porth Creek) from Sunday evening (8 June) to Monday afternoon (9 June) in 2008.

(A) There are two scenarios for when the dolphins may have entered (Scenario 1 on an ebbing tide and Scenario 2 on a rising tide). The pathological evidence of aspiration of mud/seawater in the lungs of 11/26 dolphins was most consistent with death occurring on a rising tide (Scenario 2). Image courtesy of CWT Marine Strandings Network (B) Photo of Froe (Porth Creek) at/near high water (photo credit: C. Curtis). (C) Photo of Froe (Porth Creek) at low water showing gently shelving bank in the distance (photo credit: D. Wallis).