Mitigating the discard mortality of non-target, threatened elasmobranchs in bather-protection gillnets
Globally, many gillnet fisheries have excessive discards which can comprise charismatic megafauna threatened with extinction, including numerous elasmobranchs. Very few discarded elasmobranch species have been assessed for their associated fates. Here we describe the immediate mortality of several threatened species discarded from gillnets deployed off eastern Australia to target key carcharhinids and white sharks, Carcharodon carcharias (within an objective of protecting bathers) and causal factors for abundant elasmobranchs. In total, 420 animals comprising at least 22 species were gillnetted with a total immediate mortality of 49 % and group-specific estimates of 100 % for dolphins (n = 8), 100 % for teleosts (n = 16), 86 % for sharks (n = 75), 45 % for turtles (n = 20) and 36 % for rays (n = 301). Among elasmobranchs, species that were obligate ram-ventilating (e.g. great hammerhead, Sphyrna mokarran and common blacktip shark, Carcharhinus limbatus) had greater mortality (>95 %) than those species with spiracles, and likely capable of some stationary respiration (e.g. whitespotted guitar fish, Rhynchobatus australiae, whitespotted eagle ray, Aetobatus ocellatus, Australian cownose ray, Rhinoptera neglecta and to a lesser extent, pygmy devilray, Mobula kuhlii cf. eregoodootenkee) (16-74 % mortality). Mortalities among Australian cownose rays increased across longer soaks. The effect of soak time was further assessed to determine an optimal to maintain gillnet efficiency for target sharks, but minimise the absolute mortality of abundant rays and was estimated at up to three or four fishing nights (72-96 h). Other potential operational modifications, including using light and better retroactive deterrents to reduce turtle and dolphin bycatches and mortality, warrant investigation.