Discrimination between bycatch and other causes of cetacean and pinniped stranding

Abstract

The challenge of identifying cause of death in discarded bycaught marine mammals stems from a combination of the non-specific nature of the lesions of drowning, the complex physiologic adaptations unique to breath-holding marine mammals, lack of case histories, and the diverse nature of fishing gear. While no pathognomonic lesions are recognized, signs of acute external entanglement, bulging or reddened eyes, recently ingested gastric contents, pulmonary changes, and decompression-associated gas bubbles have been identified in the condition of peracute underwater entrapment (PUE) syndrome in previous studies of marine mammals. We reviewed the gross necropsy and histopathology reports of 36 cetaceans and pinnipeds including 20 directly observed bycaught and 16 live stranded animals that were euthanized between 2005 and 2011 for lesions consistent with PUE. We identified 5 criteria which present at significantly higher rates in bycaught marine mammals: external signs of acute entanglement, red or bulging eyes, recently ingested gastric contents, multi-organ congestion, and disseminated gas bubbles detected grossly during the necropsy and histologically. In contrast, froth in the trachea or primary bronchi, and lung changes (i.e. wet, heavy, froth, edema, congestion, and hemorrhage) were poor indicators of PUE. This is the first study that provides insight into the different published parameters for PUE in bycatch. For regions frequently confronted by stranded marine mammals with non-specific lesions, this could potentially aid in the investigation and quantification of marine fisheries interactions.

Retrospective Study of Fishery Interactions in Stranded Cetaceans, Canary Islands

“…Different necropsy findings might be observed in cases of fishing net entrapment, depending on the fishery and the affected cetacean species (5,21,22,44). Our results are consistent with those of Bernaldo de Quirós et al (6), who reported external net marks, evidence of recent feeding (fresh undigested gastric contents), and disseminated intravascular gas bubbles.…”

“…In contrast with the findings of Bernaldo de Quirós et al (6), only one case had reddish eyes (case 29), and another case had bubbles within the posterior chamber of the eye (case 23). In the present study, hyperinflated and hemorrhagic lungs were also identified, with occasional froth and rib impressions, corresponding histologically with areas of marked emphysema and alveolar edema.…”

“…Bycaught dolphins in gillnets or trawls are often reportedly healthy individuals in good body condition, with pathological findings that are usually consistent with peracute underwater entrapment (PUE) (5). The most common findings in this type of bycatch are net cuts and impressions on the skin (mainly over the head, but also affecting the flippers and body), changes in the lung (edema, multifocal emphysema, and atelectasis), recently ingested food, reddish or bulging eyes, congestion, and disseminated gas bubbles (6). Other bycatch findings include those produced by fishermen, such as gunshots, stabs over the body, or amputations to disentangle the animal from fishing nets, and abdominal cuts to sink the carcasses (5,7).…”

Pathologic findings and causes of death of stranded cetaceans in the Canary Islands (2006-2012)

“…Very few studies have successfully evaluated and quantified real fishing impact on cetacean populations [ 165 , 166 ]; thus, bycatch is largely believed to be under-reported. A considerable number of publications have reviewed the gross, and to a lesser extent, the histologic findings associated with bycatch in cetaceans [ 105 , 167 , 168 , 169 , 170 ]. The greater the number of indicator criteria identified in an individual case, the greater the likelihood of an accurate diagnosis of bycatch [ 170 ].…”

Vaquitas (Phocoena sinus) continue to die from bycatch not pollutants

“…The stomachs with recently ingested prey observed in six of these seven are one of the most consistent findings in cetacea bycaught in fishing nets. 12 No lesions were observed in the fetus which was markedly autolysed, so whether it was aborted or died because its mother also died and her carcase was not found is unknown. Although decomposition precluded useful microbiological analyses, no gross lesions suggestive of fatal infectious disease were evident.…”