Abstract

Entanglements in 8–8.5” mesh monofilament gillnet is the most commonly observed fisheries entanglement in California sea lions that present to the Pacific Marine Mammal Center (PMMC) in Orange County, California.^1,2 This mesh is large enough for pinnipeds to become entrapped. If they break free from the gillnet before drowning, some animals will retain the monofilament, resulting in progressive entanglement injuries. The monofilament can sever vital soft-tissue structures (musculature, trachea, esophagus) and bone, leading to pneumonia, sepsis, starvation, and death.

Interventions often require remote sedation via dart to facilitate capture of subadult/adult pinnipeds. All rescue attempts are performed under NOAA/NMFS permit (#18786) and oversight. Due to the inherent risk of pinnipeds drowning under sedation, deployment of this technique cannot be approved until the entanglement is deemed life threatening. Here we present three cases with different approaches and outcomes.

A subadult male sea lion, “Stubby,” had a circumferential monofilament gillnet entanglement at the caudal mandible, around the skull cranial to the ear flaps. The patient was observed intermittently with progressive soft-tissue damage on the dorsal head without evidence of ventral cervical defects. Risk/benefit analysis concluded this was not a life-threatening entanglement, and remote sedation rescue was not indicated. Rescue attempts using a net and/or pole syringe for sedation delivery were made, but the patient evaded rescue. One year following the initial report, the animal stranded dead. Necropsy showed the monofilament transected through the dorsal skull, exposing the brain, causing death.

Two additional cases, one adult male sea lion, “Bolt,” and one subadult male sea lion, “Loki,” were monitored for several months. Both had circumferential monofilament gillnet entanglements at the level of the mid-cervical region (“Bolt”) and caudal to the mandible (“Loki”). As the animals were monitored, the injuries rapidly progressed and were deemed life threatening; approval to deploy remote sedation was granted. Both cases were rescued using remote sedation and brought to PMMC for treatment.

In “Bolt’s” case, complications associated with remote sedation resulted in a pneumothorax which progressed during rehabilitation. The decision was made to anesthetize the patient; he quickly decompensated upon induction, and resuscitation with thoracentesis was unsuccessful. During necropsy, the tip of the darting needle was found embedded in the rib, having penetrated the thoracic cavity, causing the persistent pneumothorax.

In “Loki’s” case, the entanglement cut into the soft tissue of the caudoventral mandible, completely detaching the esophagus from the base of the tongue, forming a fistula from the neck into the oropharynx. A Board Certified Small Animal Surgeon surgically closed the defect, reattaching the tongue to the esophagus. “Loki” recovered well, was released, and has been resighted multiple times.

California is one of the few remaining states that allows gillnet fisheries. Additional interactions with gillnets may be occurring in Mexican waters, contributing to the high number of these cases observed in Southern California. Efforts to regulate bycatch or ban gillnet fisheries should be considered, particularly since it was recently identified as the fishery with the highest amount of bycatch observed. These cases highlight outcomes that can occur with monofilament gillnet entanglements.

Acknowledgements

The authors wish to thank the staff and volunteers at Pacific Marine Mammal Center. Rescue and rehabilitation activities were conducted under a stranding agreement between NOAA and PMMC; and remote sedation was conducted under NOAA permit number 18786. Special thanks to the Sonace Foundation for providing financial and resource support that enables remote pinniped disentanglement efforts at the Pacific Marine Mammal Center.

Literature Cited

1.  Greig DJ, Gulland FMD, Kreuder C. A decade of live California sea lion (Zalophus californianus) strandings along the central California coast: causes and trends, 1991–2000. Aquat Mamm. 2005;31:11–22.

2.  Gerber JA, Roletto J, Morgan LE, Smith DM, Gage LJ. Findings in pinnipeds stranded along the Central and Northern California coast, 1984–1990. J Wildlife Dis. 1993;29:423–433.