Drawing blood from a shark for chemical change analysis
Drawing blood from a shark for chemical change analysis

Contacts: Gregory Skomal, Brad Chase

With the growing interest in catch and release, especially for large pelagic species like tunas, sharks, and marlins, MarineFisheries biologists have been studying the effects of the "fight". These highly energetic species may suffer terminal fatigue and die after being released. The answer may lie in the fish's blood chemistry. MarineFisheries biologists are using blood samples taken from fish to determine the effects of increased anaerobic activity, muscular fatigue, and time-out-of-water caused by angling. Catch data generated by MarineFisheries tournament sampling program show that a high percentage of these big game fish are released. For bluefin tuna, the number of fish released has increased greatly since federal regulations imposed a commercial minimum size and decreased the recreational bag limit in 1992.

We use a two-pronged approach to quantify and characterize the effects of angling. First, the relative health of the fish is determined by taking a blood sample after the fight. High muscular activity and stress induced by angling causes changes and disturbances in fish tissues and organs. These changes, manifested in the blood, may be severe enough to alter normal physiology and behavior, and ultimately reduce survival. In some cases, fish may die, either on the line or more likely after release. As with evaluations in human and veterinary medicine, various chemical constituents of the blood can be used to profile the condition of the fish before it is released. For each species, changes in blood chemistry can be linked to several variables associated with the fight such as tackle type, fight time, water temperature, and fish size.

Fish that have been blood-sampled include: bluefin, yellowfin, bigeye, albacore, and skipjack tunas; blue, mako, tiger, dusky, sandbar, blacktip, and bull sharks; white and blue marlin; Atlantic bonito, little tunny; and dolphin (mahi-mahi). Preliminary findings show that these fish exhibit significant fluctuations in blood pH and blood levels of some hormones, electrolytes, and metabolites due to the fight associated with rod and reel angling. Each species was found to have a different physiological response to angling.

Is a tuna or shark capable of recovering from the internal disturbances caused by the angling event? The second part of the study involves the actual assessment of survival with acoustic telemetry. Sharks and tunas fought for extended periods have been outfitted with acoustic "pingers", which allow the MarineFisheries scientists to follow the fish, tracing its location and depth over several hours. This provides direct evidence of short-term post-release survival. In addition, since most of the fish sampled to date have been tagged and released, tag-recaptured fish provide direct evidence of long term survival. The levels of exhaustion that tunas and sharks can withstand comes from the correlation of blood chemistry data, fight data, and survival information.

Tracking large pelagic fish tagged with acoustic transmitters. Tracking large pelagic fish that were carrying acoustic transmitters.

Tracking large pelagic fish tagged with acoustic transmitters
Tracking large pelagic fish tagged with acoustic transmitters

During the second phase of the study, MarineFisheries biologists have tracked bluefin tuna, blue sharks, yellowfin tuna, and white marlin for periods ranging from 2 to 48 hours after release. All of those fish fought on rod and reel, blood sampled, and released carrying acoustic transmitters were determined to be completely exhausted by the event, and all but one bluefin tuna survived for the duration of the tracks. In addition, tag recaptures of blue sharks, yellowfin tuna, and bluefin tuna that were previously blood sampled by the study provided long-term evidence that these fish were not physiologically compromised by the angling experience or the tagging.
In some fisheries, it is likely that the cumulative effects of physiological and physical trauma associated with catch and release results in unacceptable levels of species-specific mortality. The rough handling of fish, the use of gaffs, internal hook damage, poor tagging, and excessive time out of water can cause irreparable damage to a fish that is released. The good news is that a little education can go a long way toward reducing these impacts at boatside. Physiological stress can be minimized by reducing fight and handling time. However, physical trauma can only be reduced through the conscious efforts of anglers when choosing to release a fish. Hook design, handling methods, tagging tools, and experience all play a major role in the proper release of all gamefish.

In a related study, MarineFisheries biologists teamed up with NMFS to evaluate hook design in offshore tuna "chunk" fisheries. This research compared the efficiency and hook site of standard straight shank hooks to circle hooks. The study concentrated on bluefin tuna taken during bait fisheries where hooks are more likely to be swallowed, causing considerable damage to a fish that tuna caught on circle hooks were consistently hooked in the jaw while only 52% caught on straight hooks were is to be released. The results of this study showed that 95% of bluefin hooked in the jaw and 34% were hooked in the more sensitive pharynx or esophagus. This comparison indicates that circle hooks cause less physical damage than straight hooks while catching juvenile bluefin tuna using natural bait and can be a valuable conservation tool in these recreational fisheries.