The Effects of Fishing on Chondrichthyans

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.