The New Zealand sea lion (Phocarctos hookeri) population consists of large colonies on the Auckland Islands, with smaller colonies on Campbell Island, The Snares and New Zealand's South Island, near the Otago Peninsula. The species is endemic and considered vulnerable due to a decline in pup production of almost 50% since 1998. The reasons for this decline are still unknown.
In 2001 a new bycatch mitigation method, the sea lion exclusion device or SLED, was introduced in the Auckland Islands squid fishery. The SLED is a grid fitted in the net before the cod end, with the spacing between the bars designed to prevent sea lions passing through. The grid is angled so that the sea lions are directed upward towards a hole in the top of the net, and they are able to escape from the net. Since 2004-05 almost all vessels operating in this fishery have used SLEDs.
The Ministry of Fisheries runs an observer programme that monitors the capture of New Zealand sea lions by commercial fishers. Between 1995 and 2009, 3 sea lion captures in the Auckland Islands fishery were observed - the lowest in the period covered by the data. Trawls targeting southern blue whiting near Campbell Island accounted for an estimated 16 captures in 2007, but this fell to 1 in 2009.
Using statistical models and ratio estimation methods we used observer data and associated fishing effort data to estimate the number of sea lions captured in subantarctic fisheries. The Auckland Islands squid fishery is the only fishery in which SLEDs are used and so the model was necessarily more complicated than for the other fisheries, because it needed to estimate not only the number of sea lions that would be caught on trawls that used SLEDs but also the number that would have been caught if no SLEDs were used. Estimating both these values was important for allowing comparability between the fisheries and years.
In the 2006-07 fishing year there were 15 sea lion captures on observed trawls in subantarctic fisheries. The model estimated 52 (95% c.i.: 33-74) sea lion captures across all trawls (observed and unobserved) and 105 (95% c.i.: 60-169) sea lion interactions (the number that would have been caught had SLEDs not been used). Of this total, there were 20 estimated captures and 74 estimated interactions (95% c.i.: 32-136) for the Auckland Islands squid fishery (95% c.i.: 11-33); a decrease from the previous two fishing years.
In other fisheries the estimated captures have been generally declining over time. In 2006-07 there were 14 estimated captures of sea lions in the Campbell Island southern blue whiting fishery, 12 in other (non-squid) trawl fisheries near the Auckland Islands, and 5 in all trawl fisheries on the southern end of the Stewart-snares shelf.
Thompson, F. N., & Abraham, E. R. (2011). Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2008–09. New Zealand Aquatic Environment and Biodiversity Report No. 66. 25 pages.
Thompson, F. N., Oliver, M. D., & Abraham, E. R. (2010). Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2007–08. New Zealand Aquatic Environment and Biodiversity Report No. 52.
Thompson, F. N., & Abraham, E. R. (2009). Estimation of the capture of New Zealand sea lions (Phocarctos hookeri) in trawl fisheries, from 1995–96 to 2006–07. New Zealand Aquatic Environment and Biodiversity Report No. 41.
Abraham, E. R. (2008). Evaluating methods for estimating the incidental capture of New Zealand sealions. New Zealand Aquatic Environment and Biodiversity Report No. 15.