Sea lion bycatch in New Zealand subantarctic trawl fisheries, 2006-07

The New Zealand sea lion

The New Zealand sea lion (Phocarctos hookeri) population consists of large colonies on the Auckland Islands and smaller colonies on Campbell Island, The Snares, and the South Island near the Otago peninsula . An endemic species, New Zealand sea lions are considered to be vulnerable due to an almost 50% decline in pup production since 1998.

Observed sea lion captures

NZ Sea lion

The New Zealand Ministry of Fisheries runs an observer programme that monitors the capture of New Zealand sea lions by commercial fishers. Between 1995 and 2007 squid trawlers in the Auckland Islands fishery accounted for 83% of all observed sea lion captures . There were 7 observed sea lion captures in this fishery in 2006-07. A further 6 sea lions were caught by trawlers targeting southern blue whiting near Campbell Island , with another 2 observed captures in all other non-squid trawl fisheries near the Auckland Islands and on the southern end of the Stewart-Snares shelf.

Reducing sea lion captures

In 2001 a new bycatch mitigation method, the sea lion exclusion device (SLED), was introduced in the Auckland Islands squid fishery (see figure). 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.

Sea lion exclusion device (SLED)

The sea lion model

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 (termed 'captures') but also the number that would have been caught if no SLEDs were used (termed 'interactions'). Estimating both of these values was important for allowing comparability between the fisheries and years.

Estimated sea lion captures

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.

More ...