posted on 2016-12-14, 03:13authored byLeonardo Guida
Global
fishing activity is major threat to chondrichthyan (sharks, rays and chimaeras)
populations. The effects of fishing are predominantly estimated by quantifying
immediate and post-capture mortality rates. However, the true effect of fishing
is likely to be underestimated, because measuring the mortality rates of
released animals is logistically challenging and little is known about the sub-lethal
effects of capture on life-processes such as reproduction. The physiological
stress response to capture driving lethal and sub-lethal outcomes in
chondrichthyans is not fully understood. The overall aim of this study is to
better understand the post-capture fate of chondrichthyans for the purpose of
developing and improving sustainable fishing practices. This aim is addressed
by investigating how the behaviour during capture influences the stress
response; improving the estimation of mortality risk by developing an
additional physiological stress indicator; and investigating the sub-lethal
effects of capture by examining reproductive consequences.
To determine how behaviour during longline capture influenced
the stress response of the gummy shark (Mustelus antarcticus),
haematologically derived stress indicators were correlated with animal movement
and water temperature data obtained from time-depth recorders (TDR) attached to
individual hooks. In water temperatures ranging 12–20°C, higher temperatures
increased metabolic rate but variable capture duration (32–241 min) did not
change the stress response. Animal movement occurred for an average of 10% of
the time spent captured. Limited movement of M. antarcticus for the
majority of capture and the ability to respire whilst stationary probably
mitigated the influence of increased temperature and capture duration on the
stress response.
TDRs were also evaluated for their ability to measure capture
behaviour in eight species caught in either surface or demersal longline
configurations. Three analytical methods were developed to interpret TDR data;
the Visual Assessment Method (VAM), Gangion Extension Method (GEM) and Vertical
Excursion Method (VEM). With respect to each method, movement indicative of struggling
during capture was identified as: visually-determined erratic changes in depth
in the TDR trace (VAM); periods when captured animals altered their depth by
more than 50% of the gangion length (GEM); and periods when the absolute depth
change between successive data points exceeded a threshold determined from the
maximum depth change in the TDR data prior to capture of the animal (VEM). All
analytical methods identified initial capture at the same point in time. VAM
estimated significantly more movement than both GEM and VEM. There was no
significant difference in movement between GEM and VEM; however, GEM could only
be applied in demersal configurations. VEM was determined as the best method of
analysis because it could be applied in all longline configurations and was
more conservative in estimating movement. Differences in movement observed
between species were consistent across all methods making it possible to
quantify species-specific behavioural responses to capture.
To improve the estimation of mortality risk and develop an
additional physiological stress indicator, the adenylate energy charge (AEC; a
measure of available metabolic energy) was quantified in liver, muscle, heart,
brain and blood tissues of M. antarcticus immediately following gillnet
capture and after 3 h recovery under laboratory conditions. Immediately after
capture, liver, muscle and blood exhibited significant declines in the AEC of
38%, 20% and 7%, respectively. The heart and brain did not show significant
declines. Only liver and blood returned to ‘unstressed’ levels following
recovery, whereas muscle remained at capture levels. In liver and muscle the
AEC declined with worsening animal condition and the use of muscle biopsies
suggests that the AEC is a practical and non-lethal indicator of capture
stress.
Pregnant southern fiddler rays (Trygnorrhina dumerilii)
were subjected to trawling and air exposure under laboratory conditions to
investigate the reproductive consequences of capture. Pregnant females were
routinely monitored for changes to body mass (BM), sex steroid concentrations
(17-β estradiol, progesterone, testosterone) and granulocyte to lymphocyte
(G:L) ratio for up to 28 days following trawling. At parturition, neonates were
measured for total length (TL), BM and G:L ratio. For mothers, capture during
pregnancy significantly elevated the G:L ratio for up to 28 days and reduced
post-partum BM. Concentrations of all sex steroids were unaffected by capture.
Neonates from trawled mothers were reduced in size (TL and BM) and also
exhibited an elevated G:L ratio at birth.
The results of this study suggest that assessing species’
resilience to capture under different environmental (e.g. temperature) and
operational (e.g. surface vs demersal longline) conditions can be improved by
incorporating behavioural responses to capture with existing physiological
measurements of stress. Inclusion of the AEC to the suite of existing stress
indicators will also provide a more comprehensive assessment of animal
condition and thus, the likelihood of mortality. For pregnant females which
survive capture, sublethal stress may manifest itself in reduced maternal
condition and smaller offspring, potentially placing populations under
increased strain from fishing activity. Increasingly comprehensive and accurate
assessments of the lethal and sub-lethal effects of capture stress will better
inform fisheries management strategies, aiding the continual development of
sustainable fishing practices for the conservation of chondrichthyan species.