Review of research and assessments on the efficacy of sea lion exclusion devices in reducing the incidental mortality of New Zealand sea lions Phocarctos hookeri in the Auckland Islands squid trawl fishery

Abstract

New Zealand sea lions are incidentally killed in the Auckland Islands squid trawl fishery. Sea lion exclusion devices (SLEDs) that allow animals to escape from the trawl net have received considerable development and assessment attention. Nonetheless, there are claims that some animals could suffer head trauma when colliding with the hard grid that forms part of the SLED and this may compromise post-escape survival. We reviewed published and unpublished research that assessed the effectiveness of SLEDs in reducing the incidental capture (i.e. bycatch) of sea lions, including assessments on the likelihood of post-SLED survival. The available evidence shows that SLEDs are effective in reducing sea lion bycatch in trawl nets and contribute to reduced rates of observed sea lion mortality in the Auckland Islands squid fishery. Efforts to test SLED efficacy have shown that most sea lions are likely to survive following their escape via a SLED, despite the shortage of verifying video evidence due to poor visibility at fishing depths. Laboratory necropsies of incidentally caught sea lions have been unable to reliably evaluate post-SLED survivability of sea lions due to the effects of on-board handling of carcasses, including the logistical necessity of freezing them. Some lesions initially considered to be evidence of trauma were subsequently deemed to be artefacts of freezing. Nonetheless, there was no clear difference in the trauma assessments between sea lions caught in nets with and without SLEDs. Biomechanical modelling suggested it was unlikely that impact with a SLED would cause fatal brain trauma and the probability of concussion that could result in post-SLED drowning was probably less than 10%. As fisheries bycatch has been reduced to levels that are unlikely to be driving continued decline of New Zealand sea lions at the Auckland Islands, future work may be better focussed on alternative research and management areas that may be more effective in addressing and reversing New Zealand sea lion population decline.