Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Protected species: Sea lions
Table 3.6: Tow duration in the SQU6T fishery (i.e. for trawl fishers targeting SQU in statistical areas 602, 603, 617
and 618). Years are calendar years. Data from MPI databases.
No. of Mean tow duration
Percentage of tows
Year
tows
(hours) Less than 4 hours Between 4 & 8 hours More than 8 hours
1995 4 014 3.7 64.2 33.5 2.2
1996 4 474 3.6 64.3 34.2 1.5
1997 3 719 3.8 62.7 33.7 3.7
1998 1 446 3.2 74.4 24.7 0.9
1999 403 3.5 73.0 24.3 2.7
2000 1 213 3.5 70.3 27.0 2.7
2001 583 3.3 72.9 26.6 0.5
2002 1 647 3.8 59.8 38.8 1.4
2003 1 467 4.1 52.4 44.0 3.6
2004 2 598 5.0 36.7 53.6 9.7
2005 2 693 4.7 43.7 48.6 7.7
2006 2 462 6.3 26.0 49.6 24.3
2007 1 317 7.3 18.9 46.3 34.8
2008 1 265 6.2 20.4 58.7 20.9
2009 1 925 6.5 21.1 51.4 27.5
2010 1 190 7.9 16.4 37.4 46.2
2011 1 585 6.8 24.7 42.8 32.4
2012* 1 283 6.6 23.5 49.3 27.3
* Includes data up to November 30, 2012.
There are a number of possible sources of uncertainty relating to the biomechanical modelling (Ponte
et al. 2010, 2011, Abraham 2011, Lyle 2011). The use of linear acceleration, as opposed to rotational
(angular) acceleration, in the biomechanical modelling may underestimate the risk of MTBI, although
this was thought to be accounted for at least in part by sensitivity analysis of the scaling of HIC
values. The testing used an artificial “head form” based on human anatomy, so the effect of NZ sea
lion scalp thickness and skull morphology is unknown, although differences in head and brain masses
are accounted for. Potential effects of differences in the angle of the head on impact (relative to the
neck) were not tested. Impact speeds, locations and orientations of NZ sea lions may differ from those
of Australian fur seals, although the fur seal data were considered to be a reasonable proxy by a
Research Advisory Group. The head mass values used may be lower than average for NZ sea lions;
this would mean risk is likely to be overestimated. This approach assesses risk associated with
collisions with the grid of a SLED and cannot be used to assess other sources of mortality resulting,
for example, from an animal being retained in a net long enough for them to exceed their dive limit
before reaching the surface after escaping from either the SLED or the front of the net. Such sources
of cryptic mortality have always existed, are presently unquantified and are not reflected in the
estimated overall survival rate of encounters with trawls.
3.4.5. Potential indirect threats
In addition to sources of uncertainty associated with direct fisheries interactions, there is the
possibility that indirect fisheries effects may have population-level consequences for NZ sea lions.
Such indirect effects may include competition for food resources between various fisheries and NZ
sea lions (Robertson and Chilvers 2011). In order to determine whether resource competition is
present and is having a population-level effect on NZ sea lions, research must identify if there are
resources in common for NZ sea lions and the various fisheries within the range of NZ sea lions, and
if those resources are limiting. Diet studies have demonstrated overlap in the species consumed by NZ
sea lions and those caught in fisheries within the range of NZ sea lions, particularly hoki and arrow
squid (Cawthorn et al. 1985, Childerhouse et al. 2001, Meynier et al. 2009). A recent study focused
on energy and amino acid content of prey determined that the selected prey species contained all
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