Abstract

Fish capture and immobilization often posses a real handicap especially in large volume aquaria. In these circumstances, the addition of waterborne anesthetics to the entire tank is impractical and confinement through nets and capture of fish in a smaller volume of water could be problematic. This problem magnifies when working at the open sea.

As a result of the daily work requirements at L'Oceanogràfic, we present an easy approach to this problem through the use of an injectable anesthetic combination of tiletamine and zolazepam (Zoletil®) through a DIY remote delivery system. One part of this presentation will explain the procedure followed to convert a pneumatic fishing spear gun in a remote injection delivery system while in the second part we will focus in the evaluation of Zoletil® as fish anesthetic.

There are several methods described in the literature for fish injection: by hand syringe, pole syringe or underwater darting (remote injection). A few publications describe the use of an adapted Hawaiian sling or a laser-aimed underwater dart gun Aquadart Marine Technology Ltd (Harvey et al, 1987) for this purpose. However these devices are not always easily available in the market. We describe the use of a homemade device, simple to prepare, easy to handle based on commercial pneumatic fishing spear guns.

A blowpipe dart is attached to the end of an arrow using an adapter. The threaded steel shaft is substituted by a thin carbon fiber arrow designed for longbow shooting. This is important to minimize the weight of the projectile and reduce trauma as much as possible. At the tip of this arrow a plastic cylinder is firmly attached with resin in order to make a receptacle for the insertion of the pneumatic dart. Commercial blowpipe/rifle pneumatic darts and needles were used for this purpose. Barbed needles can be used, as well, mainly for large volume injections although they are much more traumatic for fish skin. The length and thickness of the needle must be adjusted according to the fish size and species. A small silicone disk not wider in diameter than the dart is inserted at the end of the needle to diminish even more the damage caused by the impact of the dart on the injection site. This arrow-dart could be used with different terminals (all of them already commercially available) with different applications not only for immobilization and capture of animals, but also for the injection of antibiotics and vaccinations, as well as tele-implantation of transponders and collecting biopsies. Depending on the drug volume to inject and the size of the dart, the whole projectile once adapted to the arrow would be more or less buoyant. It is crucial to equilibrate a very buoyant arrow-dart with some stainless steel (avoiding lead) pellets or any other dense non toxic material behind the dart where it is attached to the arrow. Otherwise the trajectory of the dart would be severely affected, especially in long distance shots.

The less powerful pneumatic spear gun commercially available should be enough for the presented purpose. We use the SL Star 40® model (40 cm long) from Cressi®. It comes with a hand pump that fits into the rear part of the gun, compresses the atmospheric air inside and charges de gun. One of the most crucial points is to precisely adjust the pressure inside the gun to avoid major damage in the animals but reach the target accurately. This pressure must be determined according to the distance, the size of the dart and hydrodynamics, the size of the fish and kind of the scales of the animal. Ideally the spear gun should include a manometer. However, it is also possible to estimate the pressure reached considering the number of insufflations made with the hand inflator and the resistance to load the arrow-dart into the barrel of the speargun. It is advised to shoot the gun a couple of times inside the water before shooting at the patient in order to evaluate the power, trajectory and distance reached. Best results with this device have been obtained with targets between 1,25m and 2m from the tip of the dart.

Although very popular with other species, Zoletil® has not been used very often in fish anesthesia. Our main interest was to study the effect of this combination, in this case intramuscularly and in several species, to evaluate its application in the daily practice to get access to fish in large tanks.

During this study, some individuals were injected by hand under controlled situations, and weighted afterwards to be sure about the exact dose received while some others were injected with the adapted spear gun. The purpose of these two methodologies was to be able to discriminate between the effect obtained with the drug itself and the efficiency achieved with the darting system (in case it failed administering the full dose).

The particularities of this experience will be discussed during this presentation comparing this commercial combination with other anesthetic protocols used at the aquarium in the same groups of animals under the same circumstances. No mortalities occurred during the experiment. More than 150 fish of more than 20 species were anesthetized.

As a summary we concluded that tiletamine-zolazepam combination seems to be an effective and hence useful drug to sedate-anesthetize fish when injected, although its effect varied remarkably in the different species tested. As an orientation but not applicable in all cases, we found out that most fish got sedated with doses between 5 to 10mg/kg, getting anesthetized and completely not responsive with doses around 20mg/kg. Although we experienced no mortalities (even in trials with doses over 60mg/kg), we observed really long recovery times, sometimes up to 72h with the higher dosages. All the fish injected with Zoletil® during this study recovered from anesthesia on its own without requiring any specific assistance or emergency therapy. However, certain species such as eels or some cold water species seemed to be really sensitive to the drug and even with the lower dose range the animals took some days to fully recover. Large fish (over 3kg) seem to require remarkably smaller doses than small ones in relation to weight and low water temperatures seemed to prolong significantly recovery times.

In our experience, although other drug combinations seemed to be also effective and maybe more predictable, Zoletil® has the advantage of the small volume required to effect. This is really important when you need to guarantee that the animal received the full dose as the main limitation we found with this remote injection system was the volume of drug that had to be injected to get the desired effect. Large volumes of anesthetics posed a real problem in several senses: trajectory was more unpredictable, damage caused at the impact and injection was greater and time to full injection was longer so quiet often it failed to administer the full dose. This is the case of combinations with ketamine, diazepam or medetomidine. However, Zoletil® has the great advantage that being more powerful as a fish anesthetic, it also comes lyophilized giving the chance to concentrate it as much as you need.

Acknowledgements

We thank the curator of fishes and invertebrates, Alfredo Terrón, aquarists from the quarantine, tropical and ocean-pelagic areas as well as divers and veterinary students at L'Oceanogràfic of Valencia. They all collaborate in several ways with this project.