An improved technique for estimating short-term survival of released line-caught fish, and an application comparing barotrauma-relief methods in red emperor (Lutjanus sebae Cuvier 1816)

Abstract

Promotion of better procedures for releasing undersize fish, advocacy of catch-and-release angling, and changing minimum legal sizes are increasingly being used as tools for sustainable management of fish stocks. However without knowing the proportion of released fish that survive, the conservation value of any of these measures is uncertain. We developed a floating vertical enclosure to estimate short-term survival of released line-caught tropical and subtropical reef-associated species, and used it to compare the effectiveness of two barotrauma-relief procedures (venting and shotline releasing) on red emperor (Lutjanus sebae). Barotrauma signs varied with capture depth, but not with the size of the fish. Fish from the greatest depths (40-52 m) exhibited extreme signs less frequently than did those from intermediate depths (30-40 m), possibly as a result of swim bladder gas being vented externally through a rupture in the body wall. All but two fish survived the experiment, and as neither release technique significantly improved short-term survival of the red emperor over non-treatment we see little benefit in promoting either venting or shotline releasing for this comparatively resilient species. Floating vertical enclosures can improve short-term post-release mortality estimates as they overcome many problems encountered when constraining fish in submerged cages.

Short-term survival of discarded pearl perch (Glaucosoma scapulare Ramsay, 1881) caught by hook-and-line in Queensland, Australia

“…Contrasting post-release survival rates in congeneric species was also reported by Brown et al (2010), who found that two Lutjanids, L. sebae and Lutjanus campechanus, had post-release survival rates of 98.4% and 77% (as reported by Rummer and Bennett, 2005), respectively. Brown et al (2010) offer no explanation as to the cause of the difference in mortality rates.…”

“…These authors reported 49% survival for G. hebraicum caught at depths to 59 m, with survival as low as 14% for fish caught between 45 m and 59 m. However, these authors used much smaller, circular (75 cm diameter) cages compared to those used in the current study. Brown et al (2010) found that the post-release survival of red emperor (Lutjanus sebae) was 0% in small cages compared to 98.4% using the larger cages deployed in the current study. St.…”

“…Post-release survival was assessed using the vertical enclosures described by Brown et al (2010). Three cylindrical enclosures (hereafter referred to as cages) were constructed of #36 ply, 4-inch (101-mm) trawl mesh sewn into a cylinder 1.9 m in diameter and 15 m deep (see Brown et al, 2010 for an illustration).…”

“…Three cylindrical enclosures (hereafter referred to as cages) were constructed of #36 ply, 4-inch (101-mm) trawl mesh sewn into a cylinder 1.9 m in diameter and 15 m deep (see Brown et al, 2010 for an illustration). Seven galvanised steel (12 mm rod) hoops, 1.9 m in diameter, were used to maintain the cylindrical shape of the cages and were separated by 27.5 meshes (∼2.5 m).…”

“…depth), as do dissolved gases found in the blood and tissue cells, which leave solution and form bubbles (St. John and Syers, 2005). The expansion of gases results in symptoms of barotrauma such as the distension of the body cavity, eversion of the stomach and bulging eyes or exophthalmia (Brown et al, 2010). Prior to rupture, the expanding swim bladder may cause injuries to the adjacent organs (Rummer and Bennett, 2005;Wilde, 2009).…”