Cryptic Mortality and Its Effects

Florida's resident population of 14.7 million increases daily by about 1,200, and 40 million tourists visit the state annually. Because of our robust fishery resources, fishing is a favorite pastime of both residents and visitors.  In 2001, marine anglers made about 27.5 million trips, caught 145 million marine fishes, and released 78 million of them.  Even though fishery resources are renewable and more than half of the total catch may be released, future increases in the human population and the accompanying increase in fishing mortality mandate that Florida's fisheries be managed prudently to ensure long-term quality angling for residents and visitors alike. Regulations can serve to maximize stock abundance, but only anglers who practice proper catch-and-release techniques can minimize the intrinsic loss of marine fishes to 'cryptic mortality,' which refers to  those fishes that die unobserved after having been  caught and improperly released.

Dynamics of the snook fishery illustrate the magnitude and consequences of these unseen losses and reveal why we need to carefully release any snook that is caught but not destined for the creel. A review of the statistics from the latest stock assessment  indicates that we are precipitously close to a situation that has no palatable solution. On the Gulf coast, of the 1.335 million snook that were caught, more than 97% of them - 1.299 million - were released. Of the total number of fish harvested - 63.9 thousand - however, only 36.3 thousand were landed or put in the cooler. What happened to the other 27.7 thousand snook? How can that many fish be unaccounted for and how did biologists arrive at that number? A controlled study conducted within the fishery estimated the catch-and-release mortality rate for snook to be 2.13%. Simply multiply the number of snook released by .0213 (1.299 x .0213 = 27.7) and you arrive at the number of snook that died after being released - fish that will never make it to a cooler. Wasted!! In other words, 43% (27.6/63.9) of the total number of snook harvested were lost to cryptic mortality.  Now, the total number of fish lost to cryptic mortality is an estimate from a single iteration, which means that we assume a single snook is caught only one time in the year. But what really happens is not that simple.

Another look at the latest stock assessment describes what really happened. The estimated number of snook in the total population that are at least age 3 or about 20" total length is 329,000. Last year, anglers caught 1.335 million snook - far more snook than are in the total population. If we divide the number of snook caught by the number of snook in the estimated population (1.335/329 = 4.06), we discover that each snook may have been caught as many as 4 times each year. This means that in this real-life situation, the real loss to improperly dehooking and releasing snook was not the 2.13% previously estimated, but was in fact much higher:  the percentage of snook 'wasted' to cryptic mortality in this example was greater than the estimated  43%. The real loss is not well understood because we don't know how many snook less than 20 inches are caught and released, but it is fair odds that perhaps half of the total harvest of snook die each year from that unseen but pervasive problem of cryptic mortality.     

What can anglers do to reduce the number of deaths due to cryptic mortality? You've heard it before and it has become a litany - be as careful as possible when releasing any snook! Practice proper catch-and-release techniques! So what are the proper techniques for dehooking and returning snook, or any fish for that matter? What follow are guidelines, not rules, but remember that the future of our fish stocks depends in part on how carefully and closely we follow them.

• "Limit your kill, don't kill your limit!"   Perhaps the most important guideline!
  Decide beforehand which fish are to be kept and immediately release all others.
• Hook and land as quickly as possible fish you want to bring onboard for a short time and then release- extended struggle may lead to the fish's death.
• Try barbless or circle hooks. Catch rates increase, and physical damage decreases.
• Avoid the use of gaffs or 'hard' landing nets.
• Leave the snook in the water, if possible, while dehooking it.
• Cut the leader close if the fish is gut-hooked or if the hook is difficult to remove.
• Wet your hands or gloves to handle fish. Remove as little slime as possible.
• Control the fish as best you can. Dropping the fish could kill it.
• Use a dehooking tool if the hook is deep in the mouth.
• Do not hold large fish vertically by the lower jaw. In a study, 50 of 50 barramundi (a related species) died after being held this way.
• If your fish is in good shape, release it immediately back to the water head first. If it is exhausted, attempt to revive it.  Move the fish into the shade, either alongside the boat, under the edge of a dock, or to the bottom. Cooler water contains more oxygen. If the fish is in good shape, merely hold it headfirst into the current. If it is severely lethargic, hold the bottom jaw agape and gently move the fish forward. No back and forth movements! (Have you ever seen a fish swim backwards?) Severely exhausted fish may require several minutes to revive.  At the first sign of attempting to swim away, let it go!  Some fish will swim a short distance, become disoriented, sink to the bottom and die, snook especially, so be observant.
• If your fish dies despite your best efforts, you can add it to your creel if it meets all regulations. Otherwise, discard it.

A closely related and highly debated subject concerns the effects of catching and releasing snook during the spawning season. Should we fish for snook while they are reproductively active?  We conducted extensive research on the spawning aggregations in Jupiter and Lake Worth inlets during the summer of 1988 and 1999. Individual aggregations containing several thousand adult snook were monitored consistently from June until early September.  One hundred and forty large spawning female snook were caught on hook and line, tagged with a visible external tag, and released back into the inlet where they were captured and tagged.  Five of these fish were implanted with acoustical transmitters.  These five fish were then observed by divers and manually tracked with acoustic hydrophones. Periodically, a total of 50 of the tagged females were speared, sacrificed, and their ovaries fixed for histological observation. The time between capture and recapture ranged from 1 to 73 days. Each ovary, regardless of time lapsed after being hooked and released, showed evidence of either immediate past spawning (contained postovulatory follicles) or indications of imminent spawning (hydrated oocytes). These results show that reproductive snook are capable of spawning after being caught and released. We can't yet determine if hooking episodes reduce the quality or quantity of the gametes, but we do know that reproductive snook continue to spawn uninterrupted, despite being caught with hook and line and released. However, for obvious reasons, we should be doubly diligent in using proper catch-and-release techniques when fishing for large reproductive snook; in doing so, we will not only preserve the female, but also ensure the survival of her progeny. Hopefully, what many anglers practice - voluntarily choosing not to fish for snook during the summer spawning season - will not become a rule. If proper catch-and-release techniques are used, this practice is unnecessary; however, the ethic is commendable.