Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Protected species: Sea lions
SQU6T trawl fishery (Chilvers 2008). If this limit is reached, the fishery may be mandatorily closed
for the remainder of the season. This has happened seven times (1996 to1998, 2000, and 2002 to
2004) since this plan was first adopted in 1993 (Table 3.5; Robertson and Chilvers 2011).
Figure 3.2: Diagram of a NZ sea lion exclusion device (SLED) inside a trawl net. Image courtesy of the Deepwater
Group.
Before the widespread use of SLEDs, NZ sea lions incidentally caught during fishing were usually
retained in trawl nets and hauled on board, allowing observers to gain an accurate assessment of the
number of NZ sea lions being captured on observed tows in a given fishery. This enabled a relatively
simple estimation of the total number of NZ sea lions killed. However, following the introduction of
SLEDs, the number of NZ sea lions interacting with SLEDs and the proportion of those surviving are
much more difficult to estimate. Since the introduction of SLEDs, therefore, it has become necessary
to estimate the number of NZ sea lions interacting with trawls using a predetermined strike rate to
monitor performance against any bycatch limits set. Using a predetermined strike rate enables the
FRML to be converted into a number of tows for management purposes. The rate of 5.65% assumed
by MPI for the SQU6T fishery is based on rates observed on vessels without SLEDs from 2003/04 to
2005/06 and is also assumed as part of the fishery implementation within an integrated management
procedure evaluation model (named the BFG model after its authors, see section 3.3.3). A strike rate
of 5.89 will be assumed for the 2012-13 season, reflecting a slight increase in the long-term average.
The most recent strike rates are given in Table 3.4 (Thompson et al. 2012).
The current management regime for the SQU6T fishery provides for a “discounted” strike rate to
apply to all tows when an approved SLED is used (because SLEDs allow some NZ sea lions to escape
and survive their encounters with trawl nets; Thompson and Abraham 2010, see Table 3.5). The
SLED discount rate is a fisheries management setting and should not be confused with the actual
survival of NZ sea lions that encounter a trawl equipped with a SLED, but the discount mechanism is
31