The report by McLeay et al. (2002) (“National strategy for the survival of line-caught fish: a review of research and fishery information”) summarises the need for this research as follows:
“The commercial and recreational line fisheries are the most highly participatory of all Australia’s fisheries. They are managed by a complex array of regulations, including size and catch limits, which create a high potential for captured fish to be released. The growing interest of recreational and charter fishers in catch-and-release practices has also increased release rates of line-caught fish. The susceptibility of line-caught fish to post-release mortality (PRM) is largely unknown, and is not taken into account in most current stock assessments”.
Perhaps half of the fish caught by line in Australia are released, for a variety of reasons including minimum legal sizes, bag limits, catch-release philosophy etc. However, we have little idea of how many of these die as a result of hook damage, inappropriate handling, barotrauma or capture stress, nor what effect this source of ‘cryptic’ mortality will have on the long-term sustainability of the various fisheries.
While there is good information on release rates in the recreational line fishing sector there is also a need to test the supposition that the commercial sector catches few undersized fish and also establish any differences between the general recreational community and the charter sector.
To more realistically appreciate the full effect of line fishing on the various species, the released catch needs to be described and quantified, and an attempt made to estimate post-release survival (PRS) rates. Alternative capture methods (hook designs) need to be tested, to determine whether a change in apparatus (via regulation or a Code of Practice) could reduce the catch of undersized fish. Pre-release handling and barotrauma relief procedures need to be evaluated to determine whether any changes may increase survival of fish returned to the water.
Final report
Experiments were conducted in northern, central and southern Queensland to investigate the effects of hook design and size on the incidence of hooking injury, and the effects of a number of factors, including barotrauma-treatment method, on post-release survival rates of a suite of key reef-associated demersal fish species of particular importance to Queensland’s reef line fishery. The key species examined were common coral trout (Plectropomus leopardus), redthroat emperor (Lethrinus miniatus), crimson snapper (Lutjanus erythropterus), saddletail snapper (Lutjanus malabaricus), red emperor (Lutjanus sebae) and spangled emperor (Lethrinus nebulosus).
The work comprised four components:
(i) Analysis of existing datasets in terms of discarding/release rates in the commercial, recreational and charter sectors and estimation of barotrauma treatment effects;
(ii) Field trials to determine experimentally the effects of three hook patterns (J-hooks, offset circles and non-offset circles) and two sizes (small: 4/0 or 5/0, and large: 8/0) on hooking injury, location of hook lodgment and catch rate;
(iii) Field experiments using specially-developed vertical floating enclosures to test short-term (3-day) survival rates of fish treated to relieve barotrauma by venting or shotline releasing;
(iv) A community-based tag-release-recapture experiment involving recreational anglers to test the effects of barotrauma treatment and other covariates on long-term (months to years) post-release survival.
Discarding rates of some species have increased since 1997 largely as a result of increases in legislated minimum legal size limits and the introduction of maximum size limits. In December 2003 the minimum legal size (MLS) limit for red emperor was raised significantly, from 45 cm (TL) to 55 cm. Concurrent increases in MLS of bluespot coral trout, redthroat emperor and spangled emperor did not result in an observable change in discard rate. By 2005 the reported recreational discarding rates for coral trout, redthroat emperor, spangled emperor and saddletail snapper ranged between 42% and 55%, but for crimson snapper the rate was 69%, and for red emperor it was 83%. Between 1989 and 2003 some 300–620 t of coral trout and 33–95 t of redthroat emperor were discarded annually by the commercial reef line fishery on the GBR. Modelling of potential high-grading after the introduction of a (competitive) total allowable commercial catch for coral trout indicated that discarding of this species could increase to as much as 3,900 t. Spatial (but not temporal) differences in discarding rates were significant, and modelling indicated a potential for large increases in discarding rates and subsequent cryptic mortality as a result of changes in management arrangements.
The effects of hook pattern varied between species, with no consistent significant trends. Across all species only a relatively small proportion of fish (< 4%) were deep-hooked (in the throat or gut). Small hooks (5/0 circle and 4/0 J-hooks) were more likely to lodge in the lip or mouth than large hooks, although the effect was weak. Crimson snapper were significantly less prone to damage from non-offset circle hooks than either of the other patterns, but the opposite trend occurred with saddletail snapper. There was also a weak tendency for coral trout to sustain more injuries when captured on J-hooks or offset circle hooks than on non-offset circles. Hook size showed a more consistent trend, with large (8/0) circle or J-hooks being more frequently associated with injury than small hooks in all species, but this was statistically significant only in coral trout and blackblotch emperor.
Our controlled short-term (3-day) field experiments revealed that hook location was a major determinant of short-term survival in coral trout, crimson snapper and saddletail snapper. Even when the hooks were left in place according to best practice procedures, survival rates among deep-hooked fish were considerably reduced compared to those hooked in the mouth or lip. The modelled survival rates of shallow and deep-hooked fish repectively were as follows: coral trout 81 and 50%, redthroat emperor 86 and 59%, crimson snapper 96 and 35%, and saddletail snapper 73 and 38%. This represents an overall reduction across species in survival rate of around 50% as a result of deep hooking, even when the hooks were left in place. The results of our hooking damage experiment showed that the incidence of deep hooking was generally low across the species examined (< 4%), so that the predicted added mortality due to deep hooking would be in the order of 0.04 × 0.50, or 2%. However this figure probably underestimates the actual value, as not all anglers are prepared to cut their hooks off if the hooks have become lodged in the gullet or gut. It is also likely that the higher level of angling skill among researchers conducting the hooking trials accounted for a lower incidence of deep-hooking than might be expected across the general angling community.
Keywords: Released fish, Hook design, hooking injury, barotrauma, post-release survival, demersal fish species, reef line fishery, Coral Trout (Plectropomus leopardus), Redthroat Emperor (Lethrinus miniatus), Crimson Snapper (Lutjanus erythropterus), Saddletail Snapper (Lutjanus malabaricus), Red Emperor (Lutjanus sebae) and Spangled Emperor (Lethrinus nebulosus)
