Venus tuskfish with everted gut, a classic sign of barotrauma

A research project into the survival of released undersized fish continues to seek the help of recreational fishers to find out how well some popular reef species survive after they´re returned to the water. Fisheries Queensland scientist Dr Ian Brown and his research team from the Southern Fisheries Centre, Deception Bay, and the Rainforest and Reef Research Centre (RRRC), Townsville, have been examining the survival of key coral reef species after being tagged and released.

The results obtained so far have been very informative, but there is a continuing need for anglers to report data on recaptured tagged reef fish to help answer some important questions about post-release survival.

Many fish caught by recreational fishers are released because they are below the minimum legal size, above the bag limit, or caught and often tagged by catch-and-release anglers. It is estimated that in Queensland, between 20% and 90% of targeted reef fish are returned to the water, depending upon the species and fishing sector (see the table below). The expectation is that released fish will survive and breed, otherwise there wouldn´t be much point in having minimum legal sizes. However until now there's been very little hard evidence about the number of reef fish that actually do survive being caught and released.

The project, Survival of teleased line-caught tropical and subtropical reef fish, developed collaboratively by Fisheries Queensland and the RRRC, is investigating ways to help improve the survival chances of fish that are released because they´re below minimum legal size or for other reasons.

The project focuses on a group of the most important reef fish species: coral trout, red emperor, redthroat emperor, spangled emperor, grassy emperor, and crimson and saddletail snapper (previously known as small-mouth and large-mouth nannygai respectively). The work is funded by the Fisheries Research and Development Corporation (FRDC), and is one of a suite of projects Australia-wide coming under the banner of the National strategy for increasing the survival of released line-caught fish.

Estimating damage due to hooking

Opinions differ as to which of the great variety of fish hooks currently available in tackle shops are most effective. Research elsewhere has shown that hook size and design can affect fish survival, and while anglers are typically keen to ensure that fish they return to the water have the maximum chance of survival, they also want to use gear that gives them the best catch rates.

Project staff have tested small and large J-hooks, offset circle and non-offset circle hooks to see whether there are measurable differences in the amount of damage the hooks cause to the fish, and whether this might have an impact on their survival after release. Circle hooks are used widely in commercial long-line fisheries and by recreational game fishers, and are becoming increasingly popular among anglers because of the often reduced rate of gut-hooking and related internal tissue damage caused to the fish. The main difference between J-hooks and circle hooks is in the orientation of the tip or point. On a circle hook the point faces towards the shank, while on a J-hook it is parallel to the shank. The point of a circle hook may be offset (bent away from the line of the shank) to a greater or lesser degree. Those used in our experiment had either a substantial offset (about 15°) or none at all. During the hook damage experiments, experienced anglers targeted the key reef fish species and collected information on catch rates, hooking location and hook-related injuries.

Results showed that, amongst the target species individually or as a group, hook design had no significant effect on catch rates or the numbers of fish that were bleeding. Only in one of the non-target species (blackspot emperor) was there a significant effect, with higher catch rates for circle hooks than J-hooks. However hook design did affect the proportions of some species exhibiting hook-related injuries, but the trends were not always the same. Interestingly, in crimson snapper non-offset circle hooks resulted in the fewest injuries, while among the closely related saddletail snapper the same type of hook caused the greatest number of injuries.   

Hook size affected the catch rate of crimson snapper and blackspot emperor, with small hooks producing higher catch rates than large hooks. This is probably related to the small mouth size of these species; but in none of the other species was there any clear trend.

The size of the fish itself did not appear to be a major factor in determining the degree of injury sustained or the numbers of fish showing soft-tissue bleeding.

Do barotrauma-relief procedures improve survival?

Fish brought to the surface from a reasonable depth often suffer from injuries resulting from a rapid decrease in pressure, referred to as barotrauma. Common symptoms include body swelling, bulging eyes, parts of the gut protruding from the fish´s mouth (see photograph above), and an inability to swim down from the surface because of the expanded swim bladder´s increased buoyancy.

There are several ways to relieve these symptoms, including deflating the swim bladder with a clean hollow needle (venting) or forcing the fish back to its capture depth with a shot-line. The latter involves attaching a weighted barbless hook to the fish and lowering it with a heavy line to a suitable depth where it regains neutral buoyancy. A slight jerk of the line releases the fish and allows it to swim free.

Our project set out to determine whether venting or shot-line releasing improves fish survival, and if so, by how much. To do this we set up field experiments, one directly estimating survival over a period of days (short term), and the other indirectly estimating survival, over a time scale of months to years (long term). 

Enclosure (short-term) experiment

In the first set of experiments, line-caught fish were treated by venting (with a hypodermic needle) or shot-lining, and released into large floating enclosures or socks 15 m deep x 2 m diameter. In addition some control fish were released into the enclosures without any treatment. The enclosures were lifted three days later, and the number of fish surviving from the treatment and control groups counted and compared.

Barotrauma treatment appeared to significantly improve the survival rate of saddletail snapper, which of all the species tested showed the poorest short-term survival. The treatment which had the greatest impact on survival in this species was venting, which increased survival from the base rate of 44.4% to 60.2%, an improvement of more than 35%. While this seems a very large amount there was a lot of variability in the data, and from the statistical point of view the difference was of very marginal significance. Surprisingly, in all the key species other than saddletail snapper, shotlining appeared to have a slight (but not statistically significant) negative effect on survival.

Survival differed markedly between species (see table below). Red emperor were particularly hardy, with virtually all of the fish surviving, regardless of treatment, while at the other extreme only half of the saddletail snapper survived capture and release into the experimental enclosures. The survival rates of coral trout, redthroat emperor and crimson snapper were between these extremes, ranging from about 80% to 90%.

Further studies are needed to determine why there was such a large difference in survival rate between crimson and saddletail snapper, which are very closely related and similar in appearance.

Recapture rate (long-term) experiment

As the enclosures protect the captive fish from natural predators, they may not reflect what actually happens in reality. It is therefore critical that the enclosure experiments are complemented by parallel experiments in which treated fish are released into the wild.

The success of this second set of experiments depends on the important contribution made by recreational anglers who greatly assist the project by tagging and releasing fish back into the ocean after applying the release treatments. Differences in the recapture rates of fish between each treatment group can then provide an indication of the effectiveness of the various release methods on survival. 

At the end of October 2006, the number of logged releases in the project stood at 9700 fish, more than researchers needed to ensure a statistically meaningful result. However, closer inspection revealed that most of these had neither been vented nor shotline-released, and so contributed little to determining the best release method.

Our results so far indicate that barotrauma relief procedures seem to work best for members of the reds group (red emperor, crimson snapper and saddletail snapper). Of the two procedures, venting had the greater positive effect on survival (see below); a significant 27% more vented being recaptured than untreated fish. Coral trout released on a shotline appeared to survive better than those that had been vented (9.6% vs. 6.7%), but the change was minor and not statistically significant.

Among the emperor group we have seen only negligible differences in recapture rates between untreated, vented and shotline-released fish, suggesting that barotrauma relief treatment has little effect on improving the survival of these species.

The higher recapture rate for reds than for either of the other groups should not be taken to mean that the others are necessarily poorer survivors. We believe the differences are largely due to different behavioural patterns of the various species. For example, many of the red emperor, saddletail snapper and crimson snapper were caught and released at sites where the fish remained geographically concentrated and vulnerable to intense fishing pressure, increasing the likelihood that they would be recaptured.

Nevertheless, the number of multiple recaptures reported for red emperor demonstrates that this species is surprisingly resilient, even when caught in depths greater than 40 m. At this stage red emperor takes the record for multiple recaptures, with one individual tagged fish having been recaptured seven times.

Conclusions

The results of our hook damage experiments indicate that to maximise survival of released coral reef fish, anglers should consider using smaller rather than larger hooks, and circle (preferably non-offset) rather than J-hooks. However the observed differences were slight and by no means consistent across species. In addition, the recommendations assume a direct relationship between hook-related injury and subsequent death, which will be tested using the long-term tagging results. Our data also indicates that among the key species examined, J-hooks and circle hooks appear equally effective in terms of catch rate.

While there was a lot of variability in the data, the enclosure experiments suggested that barotrauma treatment - particularly venting - has a positive effect on the short-term survival of saddletail snapper (largemouth nannygai). The size of the treatment effect on the other four species tested was much less, indicating that if venting or shotlining are effective treatments, their impact on the survival of these species does not become apparent until weeks or maybe months after release.

Relieving the symptoms of barotrauma enables the fish to swim down from the sea surface, where it might otherwise drift about and be preyed on by seabirds, sharks and large pelagic fish. The enclosure experiments give an indication of the proportion of untreated fish that float at the surface, but nothing about predation rates, because these depend on the number and type of predators in the area where the tagged fish are released. Of course predators are not restricted to the surface, and most of us have had the experience of hauling a fish to the surface, only to find that all that´s left is a head. So releasing a fish with a shotline or after venting does not necessarily mean that it will be immune from predation. However barotrauma relief does benefit the fish in another way, by reducing the physical pressure exerted on its internal organs by the over-inflated swim bladder. Soft tissue compaction, twisting and lesions are common signs of barotrauma, but they are rarely seen except by careful dissection and inspection of internal organs. This type of damage may take some time - perhaps weeks or months - to have an effect on the fishes´ survival, which makes the results of the long-term tagging experiment even more important.

If a fish that´s about to be released shows any signs of barotrauma, we recommend that it be carefully vented using a clean hollow needle to release the gas from its swim bladder, or recompressed using a weighted releasing shotline. Both techniques seem to work well, and neither significantly increases post-release mortality. For some key reef species the data already points to a longer term benefit from these procedures, either because of reduced surface predation or because soft-tissue damage is minimised.

The tag-release-recapture results in this report are provisional and more detailed analyses still need to be done. The project has several more months during which to accumulate more release and recapture data. To ensure the best chance of achieving a statistically reliable outcome from this work, we urge all anglers to be on the lookout for tagged reef fish, and to report any recaptures as quickly and accurately as possible using the free-call number printed on the tag.

For further information on the project please contact:

Dr Ian Brown (Principal Investigator)
Phone 07 3817 9580 or email ian.brown@deedi.qld.gov.au 

Dr Wayne Sumpton
Phone 07 3817 9584 or email wayne.sumpton@deedi.qld.gov.au

Mr Mark McLennan
Phone 07 3817 9596 or email mark.mclennan@deedi.qld.gov.au