Mitigating discard mortality from dusky flathead Platycephalus fuscus gillnets

Abstract

The mortalities and contributing parameters were estimated for key species discarded during commercial gillnetting (80 mm mesh) targeting dusky flathead Platycephalus fuscus in a southeastern Australian estuary. Bycatches (1470 individuals from 16 species over 11 deployments) were assessed for their immediate mortalities onboard the gillnetter, before subsamples (570 individuals from 11 species) were discarded into cages and monitored for their short-term fate over 4 d. Appropriate controls were concurrently caged and monitored. Blood samples were taken from some live meshed-and-discarded yellowfin bream Acanthopagrus australis and luderick Girella tricuspidata and analysed for plasma cortisol and glucose. Concomitantly angled fish were similarly sampled (to provide baseline estimates of blood physiology). The immediate mortalities of the abundant species ranged between 0 (undersize blue swimmer crab Portunus pelagicus < 6 cm carapace length) and 70% (undersize P. fuscus < 36 cm total length [TL]). Water temperature had a statistically significant positive relationship with the immediate mortality of G. tricuspidata and large-tooth flounder Pseudorhombus arsius, and TL had a significant negative relationship with the immediate mortality of black sole Synaptura nigra. Compared to baseline estimates, mean plasma cortsiol concentrations in meshed-and-discarded G. tricuspidata and A. australis were significantly greater, and approached levels comparable to most teleosts after peak stress. Mean glucose concentrations were not concomitantly elevated, possibly reflecting limited time between stress and sampling for some individuals. Short-term mortalities occurred throughout the entire 4 d monitoring period for most species and ranged from 0 (yellowfin leatherjacket Meuschenia trachylepis) to 29% (A. australis). Water temperature and TL were identified as having significant impacts similar to those described above on the delayed fate of A. australis and G. tricuspidata. The partitioned mortalities were combined to provide estimates of overall mortality (± SE) for the main species that ranged between 5.9 ± 3.3% (P. pelagicus) and 76.9 ± 7.8% (undersize P. fuscus). Discard mortality in this fishery could be mitigated by allowing fishers to retain a small percentage of undersize P. fuscus, restricting the deployment of nets in water temperatures >16 to 17°C, and encouraging the careful removal of catches from meshes.KEY WORDS: Gillnet · Bycatch · Discards · Mortality · Unaccounted fishing mortality Resale or republication not permitted without written consent of the publisherDis Aquat Org 85: [157][158][159][160][161][162][163][164][165][166] 2009 factors (such as twine diameter -∅, material and hanging ratios) according to the species being targeted (Gray 2002, Gray et al. 2004, 2005a, 2005b. While there are considerable differences among the types and designs of gillnets used in NSW, most can nevertheless be separated into 4 broad categories according to their deployment and the main species sought. These c...

Evaluating the flexibility of a reflex action mortality predictor to determine bycatch mortality rates: A case study of Tanner crab (Chionoecetes bairdi) bycaught in Alaska bottom trawls

“…These tools include direct observation: mark-recapture (Kruse et al, 1994;Trumble et al, 2000;Watson and Pengilly, 1994), acoustic telemetry (Pepperell and Davis, 1999), and captive holding studies Kennelly et al, 1990;Parker et al, 2003). Researchers have also quantified impairment attributed to fishing stressors based on physiological parameters, including metabolic, biochemical, and immune responses (Aparicio-Simón et al, 2010;Broadhurst et al, 2009;Leland et al, 2013;Mercier et al, 2006; http://dx.doi.org/10.1016/j.fishres.2014.07.012 0165-7836/© 2014 The Authors. Published by Elsevier B.V.…”

Understanding social sustainability of capture fisheries

“…Moreover, more research is needed on the relation between mortality and most gear characteristics, species composition in the net and various landing procedures. In addition to the variables considered in this review, research is needed to determine the influence of other variables, such as fish' reproductive cycle, fish morphology, changes in salinity (Broadhurst et al, 2009), light intensity (i.e. visibility) and towing speeds (Olla et al, 1997) on injuries and mortality.…”

“…Fish length explains mortality from gillnets, with higher mortality in smaller than in larger individuals of black sole (Broadhurst et al, 2009), Japanese whiting (Purbayanto et al, 2001) and southern flounder (Smith and Scharf, 2011). No relation between fish length and mortality was identified in paddlefish (Bettoli and Scholten, 2006), black bream, surf bream, large-tooth flounder or dusky flathead (Broadhurst et al, 2009).…”

“…No relation between fish length and mortality was identified in paddlefish (Bettoli and Scholten, 2006), black bream, surf bream, large-tooth flounder or dusky flathead (Broadhurst et al, 2009). Species also explains mortality from gillnets, with higher mortality of surf bream, largetooth flounder and dusky flathead than of black bream, black sole and yellowfin leather jacket (Broadhurst et al, 2009).…”

“…Greater capture depth resulted in higher mortality of Black Sea turbot (Basaran and Samsun, 2004). A larger change in water temperature resulted in higher mortality of paddlefish (Bettoli and Scholten, 2006), black bream and large-tooth flounder, but not dusky flathead or surf bream (Broadhurst et al, 2009).…”